home - webthesis - politecnico di torinoadvisor: student: prof. lorenzo peroni leandro lasciato 1...
TRANSCRIPT
Politecnico di Torino
Automotive Engineering
MASTER DEGREE THESIS
Feasibility study of an Electric Racing Motorcycle
for the Motostudent Competition
Advisor: Student:
Prof. Lorenzo Peroni Leandro Lasciato
1
TABLE OF CONTENTS
2
ABSTRACT
The team 2WheelsPoliTO of Politecnico di Torino decided to concur in the
Motostudent Competition, in βElectric Categoryβ. It will be the first experience in
Electric Racing Motorcycle Design and for this reason a Feasibility Study is required
in order to define the design approach and the quantity of components necessary to
the correct functioning of the vehicle. In order to fulfil the request, an analysis of
Regulations and how scoring assignment works is required. This first step helps to
understand which the focus will be to reach best results. Old Regulations will be
confronted with new ones to understand how the other teams worked in the past.
Then a benchmark analysis becomes useful to define best practices, design
particularities and best in class motorcycles. Next step will be the analysis of the most
important necessary components. Particular attention will be given to Battery Pack
and all its auxiliaries, because its design will hardly affect the dynamic behaviour of
the vehicle, the power transferred to wheels and maximum range at racing working
conditions. This last argument is crucial: will be analysed creating a script based on
best time lap performance and requires temporary assumption from previous
experiences and benchmark analysis. It will follow the design of the Transmission. It
depends not only by electrical and physical characteristics of Electric Motor but also
by dynamic necessities, the most important one is the squat ratio. The design will not
be definitive and is done trying to create a lightweight system, the simplest and most
effective possible, looking both to theory and past experiences of other teams, not
only participating to Motostudent. The thesis will end with the Bill of Material, and
the setup of an efficient strategy to understand which ones should be designed
internally by the team, which ones requires design outsourcing and collaborations,
and the ones worthless to design and bought by external suppliers.
3
AKNOLEDGEMENTS
4
1ST CHAPTER:
MOTOSTUDENT EVENT AND TEAMS
OVERVIEW
1.1: MOTOSTUDENT EVENT
MotoStudent is a university challenge between student teams, that must design and
develop a competition motorbike project (βElectricβ or βPetrolβ) which will be
evaluated and tested in a Final Event to be held at the MotorLand AragΓ³n (in MotoGP
World Championship version) facilities in AlcaΓ±iz (Teruel), Spain [RB49].
5
This is the 6th edition and the Final Event will be on October 2020, with sign-up
opened from mid-2019.
The two different classes compete in two different categories. For each category
there are different prizes not only symbolic but of economic and material nature,
always remembering that the main objective of the competition is didactical:
During MS1 teams will summarize the evolution of the project, showing the design
process, the theoretical process of Racing Team creation and all aspects related to
real or theoretical business activities that the Team faced during all the event. During
this phase the minor prizes of βBest Design Projectβ (best documentation related to
structural, thermodynamic, manufacturing/purchasing and design validation) and
βBest Innovationβ (most creative solutions, formal aspects of solution and feasibility
of manufacturing) are assigned. The remaining part of score is mostly related to
Business Plan: it requires a summary, Market analysis, Internal analysis and SWOT
analysis. The presentations in front of the jury will have a moderate influence on the
final score.
MS2 is characterized by a list of dynamic tests: Braking (from a minimum speed of 80
km/h, there are two attempts, the lowest score is the result), Gymkhana (description
6
of the circuit is showed below), Acceleration (from 0 km/h in 150 m, there are two
attempts), Maximum Speed at Speed Trap, Regularity (measured in a given sector)
and warm up sessions. At the end there will be a race with a qualifying session. All of
this (except warm ups) give a score. Criteria are fixed by the organization and comes
from tables:
7
The full participation to Main Event requires the accomplishment of defined
milestones and a Scrutineering phase. Milestones are set during development
process, donβt assign points but can be a source of penalties if not fully respected.
Scrutineering consists in an Administrative check (verification of enrolment
conditions), a Static check (application of Static Forces on saddle and front wheel,
plus a braking test and specific tests depending on category) and a series of dynamic
tests (in which is included a full Lap by the tester). The lack of relative approval
stickers means the exclusion from MS2 phase.
8
1.2: 2WHEELSPOLITO AND MOTOSTUDENT
2wheelsPolito Team participated at all editions except the 5th, achieving through
these years very satisfying results. At the 1st edition team was awarded with βBest
Designβ prize, two years later won the MS1 event and reached an impressive 3rd place
at MS2 event. At 3rd edition the motorcycle presented by the team was very
competitive, winning MS1 Event and constrained to renounce to MS2 Win due to
9
engine failure during Race. In the following edition, the last one before hiatus, the
team ended the event at overall 2nd place winning the MS2 Event for the first time
[RB69]. All these accomplishments were reached concurring in βPetrolβ category.
The 6th edition will be the first in which the team will concur in βElectricβ Category.
The team lacks the know-how from past years that other teams have achieved: the
first step will be bringing the project at a similar level respect the opponents, from
there the brand new motorcycle will be perfectioned trying to replicate the glories of
previous editions. A Benchmark analysis becomes very useful to reach these targets
in a short time.
1.3: CONFRONTATION WITH OLD RULES
MotoStudent Regulations slightly change in each edition. On the previous, for
example, Regularity Test was not complained and replace a Mechanical Test
consisting in the rapid disassembly and reassembly of fairing and front assembly. But
10
the most important difference was on points assignment. For the new event all points
are derived from tabs and depends by the position only.
On previous edition almost all the MS2 Tests were characterized by a point
assignment strategy that took into account the differences in performance in the
specific test. An example related to Gymkhana test, replicable for almost all the other
test, is the following:
Where ππ‘ represent the best time, ππ‘ the worst and π the time scored by the
examined team. Itβs clear that the gap between most performing motorcycles and
motorcycles with good but not outstanding scores was steep. For instance, a 2.5
seconds gap between the 1st and the 4th motorcycles corresponds to more than 30
points of difference, while looking to previous tab related to Gymkhana is possible to
see a difference of 20 points between the same positions.
Race Event assignment points can be food for thought:
π =ππ‘ β 100 β ππ‘ + 99 β π
ππ‘ β ππ‘
11
What immediately jumps on eyes is that on previous edition the 2nd place and 4th
place gives the same points than the 3rd place and 9th place (respectively) on next
edition. This means that while in the past the design of a very performing motorcycle
was practically an almost mandatory requirement to reach podium results, in the new
edition the pure performances will lose importance at favour of design rationality,
innovation and functionality. The teams must be sure that their motorcycles will be
able to participate to all the tests and that MS1 documentation will be praised by
organization jury.
This is a very important point to state before that Benchmark activity starts: it will
help the team to understand where the Design Focus should be.
Another huge difference which will determine the energy installed in the Battery Pack
is the increase of Race Laps to 6 respect to 5 of last edition.
12
1.4: BENCHMARK ANALYSIS SETUP
To identify main targets of this benchmark activity and exclude from the analysis least
useful targets, a statistical approach is necessary.
Analysis starts regrouping all the data necessary for score calculation, related to each
Test of the Event [RB06].
For sake of simplicity and just for statistical study, Event is subdivided by the author
in two different Days: the first Day contains all the Static Tests, the second Day all the
Dynamic Tests. For each Static Test scores were assigned by the commission, while
for each Dynamic Test scores are calculated through different interpolation formulas,
considering performance indicators such as Time or Speed. Scores can be subject to
penalization.
In the first Day these main Awards were assigned:
-Best Design, to the team with the highest score in the Design Test;
-Best Innovation, to the team with the highest score in the Innovation Test;
-Best Industrial Project, to the team with the overall highest score of the Day.
While in the second Day main Awards assigned were:
-1Β°, 2Β° and 3Β° Best Day2, to teams with 1Β°, 2Β° and 3Β° highest score of the Day;
-Best Event, to the team with highest overall score, in practice the winner of the
Event;
-Best Rookie, to the team at its first participation with highest overall score.
Scores, their experimental Medium Value ππ₯ and Standard Deviation (transformed
in unbiased estimator) πππππ, and Awards are useful to create different Classes.
Calculation of standard deviation πππππ takes into account the limited number of
samples π, applying a correction:
πππππ =π
π4=
1
π4β β
β (π₯π β ππ₯)2ππ=1
π β 1
Theoretical value of π4 [RB01] is:
π4 = β2
π β 1β
π€(π2β )
π€[(π β 1)
2β ]
13
But it can be taken from specific tables or approximated to:
π4 =4 β (π β 1)
4π β 3
If π > 25. [RB02].
Classes are defined in the following way:
-one class (Class 1) regroups βbest teamsβ in the event. Each one scored the best
result in at least 2 different Tests, won at least 2 main Awards, and reached very high
scores in most of the tests, with few exceptions;
-one class (Class 2) regroups βunderdogsβ. Excluding βbest teamsβ, this class is
created considering teams with scores exceeding 0.5 β πππππ from ππ₯, verified for
each Day, and adding to them teams which scored best result in at least 1 Test or won
at least 1 main Award. With few exceptions, all the teams in this Class reached high
scores respect to the others in at least one Day, most of times in Day2;
-one class (Class 3) regroups βmiddle rankβ teams. Excluding the team previously
grouped, all the teams belonging to this class reached scores between ππ₯ and ππ₯ β
0.5 β πππππ, verified for each Day. Sometimes interesting scores are reached in single
Tests, and in few cases they were comparable even to Class 1, but overall scores were
farer from the optimal;
-all the remaining teams (Class 4) were βexcludedβ for statistical reasons. These
teams could be disqualified or didnβt participate to Day2 for various reasons or
reached very bad scores, especially in Day1. The majority of Rookie teams belong to
this class.
An example related to Brake Test is showed below. Team belonging to Class 1 and
Class 2 are labelled, respectively, in Green and in Yellow. Team labelled in White are
part of Class 3 while in RosΓ© and Grey are indicated Class 4 teams (the ones in Grey
didnβt participate to MS2 Event):
14
15
1.5: BENCHMARK ANALYSIS RESULTS
Once that Class 3 and Class 4 teams are excluded from the analysis, each team is
compared with the others trying to define technical datasheets for each motorcycle.
Particular attention is given to chassis design, battery technology and, when
available, to performance data declared by teams.
These are the main results of the analysis:
-Chassis: with an important exception (Multi-Tubular together with composites),
Geometry is somehow attributable to a twin spar for all the motorcycle analysed. This
is a very common choice on Motorsport because the elimination of down-tubes and
lower cradle means easy access to work on motor and saving of valuable space in that
zone [RB10]. Twin Spar layout is not particularly efficient looking to stiffness to weight
ratio, as consequence different solutions could be evaluated, not only by structural
behaviour but also looking to placement of electronic devices.
Talking about materials, the winner of MS1 declared 25CrMo45 steel as structural
material, a choice awarded maybe due to the enhanced workability of the material,
which is easy to weld, to extrude, tends to self-hardening if temperature at which
casting process starts increases and has not tempering related fragility [RB09]. In
general, excepting a Class 1 motorbike with Carbon Fibre structure, material choice
varies between Steel Alloys and Aluminium Alloys.
Talking about mounts, being a PM motor, it isnβt requested a particular tendency to
damping. All teams showed simple metallic clamp as the following showed:
16
-Caster Angle: with few exceptions at 25Β°, motorcycles are characterized by very little
caster angles, ranging from 17,5Β° to 22Β°. These values are slightly lower respect to
Moto3 Honda NSF250R with a Caster Angle equal to 22Β° 36β [RB03] but very different
weight. A good reason to decrease Caster Angle could be the necessity to reach better
placements on Gymkhana Test, because if it decreases also normal trail will decrease.
This means that also the ratio between front and rear normal trail will decrease.
Assuming ππ as front normal trail, ππ as real normal trail and the ratio between ππ
and ππ as the weight distribution:
π π =ππ
ππβ
ππ
ππ
If π π decreases, the motorcycle will have better manoeuvrability at low speeds, at
cost of lower stability at high speeds [RB04].
When the weight increases and the weight distribution is higher on front wheel,
decreasing the Caster Angle will be less impactful on stability decrease, in fact heavier
17
motorcycles have lower Caster Angle respect to the others. Going under 17,5Β° will be
probably dangerous due to higher wobble effect caused by the greater deformation
at which the fork will be subjected.
As previously stated, few motorcycles had a Caster Angle of 25Β°. In these cases,
results related to Best Lap and Race were excellent if related to the overall position
at the event. Maybe the choice was successful due to the conformation of the Circuit,
characterized by a limited number of tight curves and several curves at open throttle.
Further investigations could be useful;
-Wheelbase: data related to wheelbase of the various motorcycles are not declared
by most of the teams. The few data at disposition suggest that value could be
between 1250 mm and 1350 mm. This range of values is derived in the following way:
a) tires and rims are given by MotoStudent, in the case of the front wheel:
95 70β π 17
From here itβs possible to calculate the front wheel radius. Assuming, by definition
[RB05]:
π = 95 ππ
π» πβ = 70%
π· = 17 β 2,56
Front wheel radius will be:
ππ = (π β π» πβ ) +π·
2= 282,4 ππ
b) considering a Caster Angle range between 17,5Β° and 25Β°, and assuming null fork
offset, normal front trail will be [RB4]:
ππ = ππ β π πππ
18
c) From (FORMULA 1) itβs possible to derive the ideal ππ assuming different values for
weight distribution and π π, and then calculate wheelbase as follows:
π =ππ β ππ
πππ π
It can be applied a reverse engineering approach: if data about wheelbase are
available, itβs possible to calculate the optimal weight distribution, given π π.
The low Caster Angle of heavier motorcycles decreases the normal trail. As
consequence, if a good level of manoeuvrability is requested, π π must decrease and,
as consequence, the wheelbase will increase;
-Battery Pack Position/Technology/Energy and Inverter: Even if the Regulations allow
the possibility to use multiple battery packs [RB06], none of the examined
motorcycles presents this solution.
Battery pack takes always place in between the main beams. For Li-ion batteries with
cylindrical cells it must be inclined to fit in. Adopting pouch cells, itβs possible to see
that best in class teams opted for almost vertical battery packs positioned in a raised
19
position, raising up centre of gravity. This assured success in Braking tests for sure: in
particular, there are some videos in which is possible to see how itβs easy for these
motorbikes to stoppie. However, it helps also on Gymkhana test because it decreases
the lean angle necessary to accomplish the same turn [RB05].
Cell Types are various, from the classical Cylindrical (mainly on 18650 format), to the
Pouch format, which is more compact and more affordable thinking from a Six-Sigma
quality perception [RB07]. Most of teams bought or received battery cells from
sponsors, in few cases all the battery pack. Technology is always Li-ion. It is important
to underline that often the nomenclature Li-ion-Po is used. This is not related to the
choice of a cell with a solid electrolyte but is related to the Pouch cell polymer
packaging. The misconception, that causes often confusion, is even known and stated
in literature [RB07, RB08].
However, the choice of Pouch format requires strategies to overcome two main
problems: the swelling, which is the generation of πΆπ2 and πΆπ related to small
internal shorts happening during aging [RB08] and, due to the non-rigid construction,
cell can increase volume. An increase in volume between 8 and 10% over 500 cycles
is considered normal and must be taken into account during design phase. The
second is the so called Lithium Plating: means that lithium-ions remain stuck inside
the cell, increasing battery resistance and wearing, and happens when external
pressure is not distributed uniformly on the cell [RB07]
20
Energy installed is function of weight, both of battery pack and of motorcycle. Higher
values installed means that the battery pack can be able to deliver higher current,
allowing to the motor to produce higher power, maintaining the same autonomy.
Energy installed depends also on efficiency of each component (BMS, motor,
Inverter, etc.) and higher values allow a higher safe margin. Best in class motorcycles
show values of energy installed between 5,5 kWh and 7 kWh.
About Inverter, some teams which have Cylindrical Cells Li-ion batteries decided to
position it under the seat of the motorcycle, due to space. This changes the weight
distribution shifting it slightly to the rear. Other motorcycle with more compact
battery packs have Inverter hidden inside the fairings, maybe integrated with battery
pack itself.
-Weight: a lot of different technologies applied on chassis design and battery packs
means a wide range of different weight for all the motorcycles analysed. Considering
only motorcycles with declared weight, range goes from 116 kg to 152 kg. Must be
21
noted that the heaviest motorcycle is placed on Class 1 and obtained excellent
placements in all the dynamic tests which could be negatively affected by weight
(Gymkhana, Brake, Acceleration), while motorcycles weighting less than 130 kg
obtained overall good results, but never excellent and sometimes disappointing. In
author opinion, the choice of the rider and its adaptability and knowledge of the
motorcycle played the most important role, except in Acceleration test where power
delivered by battery pack is the main objective.
-Swing arm: none of the examined motorcycles present the heavier single arm swing
arm. Double arm design, both in high truss and low truss variants, is variable: itβs
possible to see Tubular with Rectangular section, Tubular with Cylindrical section,
Trellis design or Asymmetric design. Materials ranges from Aluminium Alloy, chosen
by all Class 1 teams, to Steel Alloy. In certain cases the production process is
underlined: in particular for Aluminium Alloy swing arms, due to Innovation or
Business reasons, very different processes are presented, which goes from Selective
Laser Melting (the alloy is AlSi7Mg0.6, characterized by high corrosion resistance and
high static/dynamic strength) if the swing arm is designed through Topological
Optimization, to machining and extrusion.
However, doesnβt seems to exist a trend between design choice and performances in
dynamic tests.
22
-Transmission: different layouts are presented by the teams, but information about
them are few. This suggests that in most cases, except when differently declared,
most of motorcycles have not a real gearbox, but a single gear directly attached to
motor shaft. It is possible and easy to actuate due to the Transversal Layout of the
Motor always chosen. To obtain better results during all the events, teams could have
at least two different final gears.
-Battery case cooling: for all motorcycles analysed, except one, seems that cooling of
battery case is accomplished by air. In certain cases cooling is forced by one or more
fans, this is true in particular for Li-ion Cylindrical cells packs, which compartments
are bulkier, longer and very crowded, so create artificially a pressure jump becomes
necessary. In other cases cooling can be facilitated by conveyors. Motorcycles
equipped with conveyors demonstrate excellent maximum speed values, so further
investigation related to Internal Aerodynamics consequent to the choice of this
solution is required.
-Brakes: All teams opted for a single brake disk on the front.
All Spanish Teams chose NG Brake Disc as producer of discs, but no one declared
dimensions of disks. Looking to the only one team that declares dimensions and uses
Brembo brake disks, and the very good result reached in Braking test, itβs possible to
state that the choice of a Γ310 for front brake and Γ220 for the rear brake is quite
conservative considering the medium weight of all the motorcycles analysed. Further
investigations and calculations are requested.
23
2ND CHAPTER:
BATTERY PACK DEVELOPMENT
2.1: PREVIEW
From the definition given by Joseph Beretta [RB23], an Electric Motorcycle with a
single EM and a single Battery Pack can be considered as a Single-Energy Chain and
Single-Energy System vehicle.
This system can be modelled as follows [RB35]:
The Recharging Module allows to recharge the battery from the power distribution
grid and can be external. The Battery System, mounted on chassis, stores the electric
energy that will be converted in mechanical power by the Electric Machine, which is
driven by the Power Converter unit. This last element converts the fixed DC voltage
24
available from the source into a variable voltage, variable frequency source
controlled to maintain the desired operating point of the vehicle [RB24]. It can also
allow regenerative braking, if appositely designed.
The aim of the thesis, as a feasibility study, is the definition of what should be bought
and, more important, what should be designed and built by the team, or outsourced;
then the integration of the system inside the actual chassis.
Cause the EM is selected by regulations, and what concerns most of Power Converter
and Recharging Module will be bought from external suppliers, main focus of the
thesis will be the definition of Battery Pack (in detail in this chapter), Transmission
and their synergy with actual chassis.
2.2: BATTERY SYSTEM BLOCKS
As general rule, Battery System (BS) can be divided in 4 principal operative blocks:
-Battery Pack (BP): the core of the system, which contains cells and their connection, defining
energy installed, power and voltage of the system;
-Battery Management System (BMS): it optimizes the usage, avoiding abuse of the pack,
detecting state of the battery cells and maximizing performance and efficiency [RB29];
-Thermal Management System (TMS): in communication with the cooling system allows to
maintain, inside the BS, a temperature range between -20 Β°C and 60 Β°C, avoiding detrimental
side reactions such as Lithium Plating and Corrosion or Gas generation [RB29];
-Power Interface (PI): it can be considered as the communication centre between the BS and
the rest of the system, allowing current flow from Recharging Module and to (or also from in
Regenerative Braking phase) Power Converter.
Functions of BMS and TMS can be managed by a single system. In this case it takes the name
of BTMS.
25
2.3: ABOUT BASIC FUNCTIONS AND LI-ION CELLS
Doesnβt matter which type of technology and materials are used, the basic
energy/power module follows the same working principles and has a common
structure.
Without going, for now, too much in depth regarding chemical reactions and
technologies, itβs possible to detect four different components inside a cell [RB24]:
-Anode: the negative electrode, through Oxidation process it releases electrons
during discharge;
-Cathode: the positive electrode, through Reduction process it acquires electrons
released by the Anode during discharge;
-Electrolyte: it is a positive ions selector, allowing passage between Cathode and
Anode;
-Separator: it physically divides Anode from Cathode.
Notice that main chemical transformations inside the cell are exothermal [RB07].
Today, Li-ion cells are the most performing and widespread in almost all utilization
fields. Advantages respect to other type of cells are related to position of Lithium in
the periodic table: it is the lightest of all metals and one of the smallest atoms, which
means highest specific energy (considering pure elements) and electrochemical
potential [RB18, RB36] equal to -3.065 V [RB24]. Other noticeable advantages related
to utilization in a racing motorcycle are low internal resistance, relatively short charge
times and high Coulombic Efficiency (CE), intended as the ratio between the
discharged capacity respect the charged capacity.
In the case of Li-ion cells, Anode and Cathode are composed by the active material (a
lot of types, which will be discussed later) and current collector material, almost
always Aluminium for Cathode side and Copper for Anode side, due to their high
conductivity and potential stability respect to active materials in each side. A BP
composed by Li-ion cells is often called, in literature, LIB.
26
The final BP will be created connecting, in series and in parallel, multiple type of cells.
Ideally, through cells connected in series will flow the same current, while single (or
grouped) cells connected in parallel will have the same voltage.
27
As we will see, due to regulations and energy/power request, this aspect will be the
core of the problem. In fact, the voltage requirements (highest as possible, but under
126 V in operating conditions) will be the primary importance condition about the
choice of the single cell and, depending on the nature of connection, will require
advanced methods to balance the system, guaranteeing highest possible
performances together with safety and efficiency of the pack itself.
2.4: POUCH FORMAT
In commerce there exist three cell main formats: Cylindrical, Prismatic and Pouch.
Packs mounted on motorcycles analysed and participating at MotoStudent adopt
Cylindrical or Pouch formats. The Prismatic format, used mainly on automotive field,
is not suitable for racing motorcycles due to its bulkiness except rare cases such as
Mugen Shinden Nana.
Talking about Cylindrical cells, the most common type is 18650. Main advantages on
application are mechanical stability (the presence of metal shell simplifies the
internal geometry of the battery case, facilitating also cooling), cheapness and
calendar life. It can allow higher specific energy respect to Pouch cells [RB37]. Ideally
it should allow a lot of freedom about the shape of the pack, but the huge number of
cells requested to reach satisfying energy installed value makes difficult the
construction and the control by a Team, requiring outsourcing (together with
increased costs). Last but not least, the necessity of alleviate cell-to-cell heat
propagation, reinforced by the low heat transport from inner cell parts to the outside
[RB38], requires a minimum of 2 mm spacing between each cell [RB30]. Together
with the lower packing density of the Cylindrical cell [RB37], this means that Battery
Case will be easier to cool, but bulkier. As result, a look to photographs of concurrent
BPs adopting this layout underlines the necessity to place the pack in a lower position.
This is detrimental for the agility of the motorcycle, performance necessary in
Gymkhana test. This reason (forgetting all the problems related to optimization usage
28
of a high number of cells) is sufficient to not consider the Cylindrical format for this
thesis.
What remains is the Pouch format. The Pouch cell is in a vacuum-packed thin plate
shape in which many stacking layers of thin cathode and anode electrodes are
alternately winded by a long ribbon of polyolefin separator. Each current collector,
except the outermost stacks, is coated with electrode films [RB19]. This
separator/electrode assembly including widespread solid-state electrolyte between
each layer are sealed inside a flexible thin plastic or aluminium pouch cover. [RB33]
29
The main advantage of Pouch format is the elevate packing efficiency, between 90-
95%, and the high degree of design freedom. Thanks to its production process, the
dimensions of each cell are easy to customize. These are the main reason for which
Pouch format is widespread in EV field: they simplify the definition of the shape of
battery pack in order to find best position in the vehicle.
Exactly as other formats, talking about terminals position, a single Pouch cell can have
both electrodes on the same side (Type A), allowing for simplified electrical wiring in
the final pack structure, or can have terminals on opposite side (Type B): cells
adopting this last layout shows improvement on current and heat distribution, hence
their use on high-power applications such as modern HEVs [RB30]. Type B cells could
be more indicated for this specific project, but opponent teams often adopted Type
A cells, so this will be a secondary importance element of choice.
2.5: LI-ION POUCH CHEMISTRIES
Pouch cells are available in a wide range of technologies. Name of the technology is
related to Cathode chemistry, which is usually composed by a Transition Metal (TM)
[RB34] but exist also batteries with Cathode composed by Sulphur [RB18].
Mechanisms of deterioration are different for each chemistry, but are all amenable
to structural changes during cycling, chemical decomposition or dissolution, and
surface film modifications [RB41].
30
A panoramic of technologies is now reported:
-LCO: active material is πΏππΆππ2, with optimal hysteresis cycle, low self-discharge, high
discharge voltage (3.7 V) and good practical specific charge [RB18]. This is by far the
most common technology in consumer electronics. Cost is high, related to Cobalt
presence, and there are limits concerning thermal stability and specific power [RB40].
LCO is one of the few technologies which can allow Surface Coating, increasing
maximum voltage of the cell;
-LFP: active material is πΏππΉπππ4. Best characteristic is the safety related to thermal
runaway. Other advantages are related to very low internal resistance, the possibility
to reach high values of power density [RB12] and the excellent cycling stability. On
the other side, LFP Cells are bigger respect their counterparts, and their lower
potential (3.5 V and lower), together with lower ionic conductivity and carbon
coating, significantly reduces energy density [RB18]. For this reason, it could be a
possible choice in case of speed record attempts [RB39] but an analysis for
motorcycle racing adoption must be done;
-NCA: active material is πΏπππ0.8πΆπ0.15π΄π0.05π2. This is the technology used by Tesla
for its battery packs. It can be considered as a LNO technology stabilized by Cobalt
and Aluminium [RB32]. Pure LNO is not viable because ions ππ2++ and πΏπ+ have
similar ionic radius and the first one tends to substitute the second during synthesis
and de-lithiation, making impossible to use the technology [RB18]. Advantages are
related to high specific energy and good specific power, while cost is still high (but
lower respect LCO) and tendency to thermal runaway is sadly famous [RB40]. NCA is
the second technology that allow Surface Coating, even if more critical respect to LCO
due to Manganese presence;
-NMC: it can be considered as a βfamilyβ. Formula of the Cathode chemistry is
standard: πΏππππ₯πππ¦πΆππ§π2, where π₯ + π¦ + π§ = 1 [RB17]. The adding of Tetravalent
Manganese limits reactivity of Nickel with most of electrolytes used today, so this
technology is better respect to NCA [RB18]. NMC cells shows very high energy density
and low internal resistance. Cost is a lot lower respect LCO technology, due to the
low presence of Cobalt in most of chemistries, but performances are similar. Today,
π₯ > 0.6 [RB19] and the state of the art is represented by πππΆ622 and πππΆ811:
specific capacities are similar but the first shows a strong dependence on electrode
31
loading, worsening performances at higher loads [RB20]. πππΆ811 is considered as
the near future by most EV designers. Another member of the family is the so called
LR-NMC, characterized by very interesting values related to specific charge and lower
costs respect NMC, but a permanent loss on capacity after activation [RB18];
-LMO: active material is πΏπππ2π4. It could be viable as power buffer, combining it
with NMC as already done by EV manufacturers such as Nissan, Chevrolet and BMW.
This because LMO batteries could keep a high discharge efficiency at the high
discharging rate, showing good rate discharging performance [RB29], but specific
energy values are limited [RB40];
-LNMO: active material is πΏπππ1.5ππ0.5π4. They are still in prototype phase because
the extremely high voltage reachable (4.7 V) is too high for the electrolytes today in
commerce [RB18];
-Li-S: this is a promise for the future, thanks to the theoretical specific energy
achievable, around 900 Wh/kg, while today values are around 300 and 350 Wh/kg.
Cost is similar to LR-NMC but production process is not reliable enough [RB18]. To be
competitive, strategies to increase the Sulphur-cathode density, such as roll pressing
and high-content Sulphur design, are crucial [RB19].
Talking about Anode, in general is made in Graphite. The reason is the following: in
Anode, during each cycle, a small amount of active Li reacts to thicken a passivating
layer on the surface of the electrode, universally known as SEI [RB22]. The presence
of SEI is related to irreversible loss of active Lithium and can also disconnect,
increasing impedance of cell [RB41]. The usage of Graphite helps creating a protective
layer upon SEI [RB34]. Graphite is also cheap and has good cycle life. However,
gravimetric capacity and recharging current are limited. Usually creation of Anode is
helped by binders: historically PVDF is used, but in recent years new aqueous or
polymeric materials like SBR+CMC show better performances and reduced toxicity
[RB21].
The best material for Anode should be Lithium, together with other materials that
can increase its Specific charge. Another reason for which it shouldnβt be used alone
is the easy formation of dendrites, related to short-circuits and heavy exothermal
activity in contact with air [RB17].
32
In recent years, and only for NMC and LMO, in order to overcome limits of Graphite
anodes, there exist some cells with πΏπ4ππ5π12 as anode material. It shows wide
working temperature range and long cycle life [RB29], and very high discharge
currents, but also low specific energy, the lowest in Li-ion cells [RB40]. Cells like this
can be used as power buffers.
Also Silicon and Tin are valid candidates for the utilization for Anodes, actually the
limit is fixed by the swelling related to their utilization [RB32].
Talking about electrolyte, typical formulations are based on constituents such as
ethylene carbonate (EC), diethyl carbonate (DEC), ethyl-methyl carbonate (EMC) and
πΏππΉπ6 and are generally stable up to around 4.5 V vs. Li+/Li (or lower at elevated
temperatures). Notice that an excess of electrolyte can negatively affect internal
resistance of the cell [RB31].
In the end, separator is usually made in polymeric material, permeable to ions,
sometimes it is designed to support electrolyte. It can be also designed as a fuse
[RB26]: at high temperatures, related to an excess of current, pores in the polymer
start to melt, prohibiting motion of li-ions [RB07]
2.6: PERFORMANCES SPECIFICATIONS
One of the basics about choosing the best cells is that a battery cannot be designed
in order to deliver high electric power, still guaranteeing highest specific energy
possible. Ragoneβs Relationship can summarize this:
(ππ)π β (ππΈ) = π
Where π and π are constants [RB24].
33
Ragoneβs Relationship is a direct consequence (or, if preferred, an alternative way to
express the same concept) of Peukertβs Law: under the condition of high discharge
current, the maximum available capacity of the battery is not fully released [RB29].
In a simplified way [RB27] Peukertβs Law states that:
π‘ = π‘πππ‘ππ (πΌπππ‘ππ
πΌ)
π
Where π‘, measured in hours, is the actual discharge time at current πΌ, and π, which is
always higher than 1, has no dimension and depends on technology and chemistry.
π‘πππ‘ππ is related to πΌπππ‘ππ, the one declared in datasheet.
Once that this point is clarified, main specifications which will guide to the choice of
the correct cell for the battery are reported [RB24, RB25]:
-Capacity: it is measured in Ah or mAh and, as can be expected by Peukertβs Law, is
related to specific modalities of discharge;
-C-Rate: it is one of the most interesting parameter. It is related only to the chemistry
chosen, and represent the normalized current respect declared battery capacity. In
practice, if a datasheet reports a declared battery capacity of 100 Ah, if C-Rate is equal
to 1C the continuous discharge current is equal to 100 A. High C-Rates are welcome
in the utilization with an EM;
34
-E-rate: represent the correspondent of C-rate, but related to Electric Power;
-OCV (Open Circuit Voltage): itβs the voltage between the two electrodes when there
isnβt any load applied. It is function of SOC (which decreases with OCV) and
Temperature. Models progressively more precise were developed to define how
these parameters have influence [RB42];
-Terminal V: itβs the voltage between the two electrodes when a load is applied. It is
not equal to OCV because dynamic events have an influence on inductance of the
system. It is function of SOC, Temperature and Discharge current: in particular,
increases when the discharging current increases [RB29];
-Internal Resistance: which is different between charge and discharge, and is function
of SOC. In particular, if SOC increases, Internal Resistance decreases. It is an important
parameter to define Efficiency;
-Nominal (or Installed) Energy: it is related to the single cell and will define the total
Installed Energy of the battery pack. It is function of the C-Rate. Depending on cell
selection and usage profile, energy calculated before represents the 80-90% of the
effective Energy used [RB07] and this must be taken into account during design
phase;
-Max Continuous Discharge Current: simply C-Rate multiplied for Capacity. It will
define the maximum speed reachable by the motorcycle;
-Max 30s Discharge Pulse Current: a battery is able to discharge higher current, but
only for a limited amount of time. This will define the Acceleration performance of
the motorcycle during Acceleration Test. There are reasons to think that at least one
team used Discharge Pulse Current to reach maximum speed only during Qualifying
phase.
-Efficiency: it is related to Internal Resistance, but also on unavoidable parasitic
currents [RB22]. There are 4 main processes which cause parasitic losses: SEI growth
and repair (which causes Li-ion losses), electrolyte oxidation at positive electrode,
dissolution of TM ions from positive electrode and deposition on negative one (TM
leaching, directly related to Surface Coating and more severe in NCA cells), lithium
trapping on positive electrode. In general, the battery has high discharging efficiency
with high SOC and a high charging efficiency with low SOC. The net cycle efficiency
has a maximum in the middle range of the SOC [RB28].
35
2.7: NECESSITY OF BMS AND TMS
In such a high-voltage, high-power system, with different connection in series and in
parallel, both these systems are strictly necessary.
Values of current and voltage of a series/parallel connection of cells are defined and
measurable over time. This doesnβt mean that every cell is at the same conditions of
the others. As already stated, in a series connection will flow the same current, but
in the same series connection can happen (or better say, it is always the case) that,
at the same SOC, voltages for each cell arenβt equal due to manufacturing tolerances
and different boundary conditions. This means that, when state of charge is 100% or
0%, voltage of singular cells can be out of the boundaries allowed by the technology.
At maximum SOC some cells can show Overcharge, while at minimum SOC different
cells can show Overdischarge. Both, the second more than the first, are very
dangerous because they lead to unwanted side reactions: there are risks of internal
short circuits [RB29] and Lithium Plating (it depends also on Temperature), which will
cause permanent lithium-ions losses and increase of internal resistance [RB07].
Another aspect to take into account is the current discharged by each parallel branch.
The worst cell in the series branch will limit the functioning of the others, so, during
discharge, itβs possible to say that a cell π will have the minimum discharge capacity:
πππβ_πππ₯[π] = πππ{ππππ₯[1] β πππΆ[1]; ππππ₯[2] β πππΆ[2]; β¦ ππππ₯[π] β πππΆ[π]}
= ππππ₯[π] β πππΆ[π]
When discharge capacity reaches πππβ_πππ₯[π], cell π will finish its discharging first,
practically prohibiting to other cells on the same branch to release current. Obviously,
a similar concept can be applied on recharging [RB29]. Parallel branches can so
consistently define the BP voltage, but cannot help on define charge/discharge
properties and ageing of each series branch.
Other issues are caused by the Temperature. As already stated, temperature range
allowed by most Li-ion technologies is between -20 and 60 Β°C, with optimal
temperature between 20 and 40 Β°C [RB43]. Due to the location of Aragon circuit, low
temperatures arenβt a real issue, so attention is required in order to maintain
temperature far from the upper limit. A temperature equal to 60 Β°C is sufficient to
36
cause Thermal Runaway if it is maintained for a prolonged period of time [RB47]. High
temperatures are also related to Corrosion and Gas formation inside the cell [RB34],
which cause swelling, reinforced by the high rates [RB45]. The problem, impossible
to avoid but at least controllable, will define some challenges relating the design of
the case, argument which will be discussed later. Obviously, also Temperature
between cells will not be consistent, meaning different aging. Some manuals [RB07]
advise to maintain a difference of 2-3 Β°C between the coolest and the warmest cell.
The aim of BMS and TMS (or BTMS if the system is unique) is to solve all these
inconsistencies, allowing optimal usage: no abuse and irrational use, clear definition
of states of cells, maximize performance during driving and increase of efficiency and
battery usage.
Advanced BTMS must accomplish seven basic functions [RB29]:
-Online monitoring of battery external parameters, such as voltage, temperature,
currentβ¦
-Battery fault analysis and alarm;
-Starting the cooling fan when the battery temperature is high;
-BP SOC estimation;
-BP Estimation, SOC + SOE (Energy) + SOF (Function) + SOH (Health);
-Battery Equalization;
-Battery Safety Management.
A BTMS can be Passive or Active: a Passive BTMS tends to discharge the most charged
cells through a resistive load, in order to balance voltages (see the example in the
image showed below), while the Active BMS shows theoretically the highest
efficiency because it transport charge from the most charged cell to the least. There
have been a lot of evaluations of both systems over the past few years; however,
current studies do not show the long-term benefits of using an Active balancing
system. In short, with the current level of technology, there does not appear to be
any major system benefits that can be achieved, and those minimal benefits do not
outweigh the added cost for an Active balancing system [RB07].
37
To manage temperature, production designs will usually include two to three
temperature sensors per module, more or less mounted one of the first cells in the
module, one in the middle, and one near the end. The proper method to determine
exactly how many sensors are required and where to locate them should be based
on module characterization testing and CFD analysis. It is also good practice to install
temperature sensors to monitor the temperature of the incoming fluid (liquid or air)
as well as the temperature of the outgoing fluid (liquid or air) [RB07].
38
2.8: CASE DESIGN
The Case must be safe, compact, with good NVH, EMC and EMI performances,
lightweight and thermally efficient, still guaranteeing optimal working conditions for
every single cell.
Itβs comfortable to divide the BP in βsystem interfacesβ:
Inside the system interfaces more attention is required on βcontrol factorsβ. A rule of
thumb for identification of control factors is: any factor that lies outside the system
boundary is not regarded as a control factor [RB30]. The most critical are cells, cell
spacersβ type, number and location of exhaust gas nozzles, cooling system and
insulation coating system.
Talking about the structure of the case, the material should be lightweight and rigid:
two conflicting goals because more the Battery Case is heavy, more it is stiff. Carbon
Fiber would be the best choice, without considering the cost, and was chosen by a
Class 1 team. Other teams opted for Aluminum or Composite Cases, or a mix between
them. Pure metal cases require that the battery and all connectors are sealed and
insulated, or that design provides painting or sealing the case to render it non-
conductive [RB08]. If Polymeric material is used, the choice must be done considering
39
the Flame Retardant Rate. The case must ensure an Ingress Protection which is at
least IP69K, corresponding to βsealed against both liquid and dust intrusionβ [RB07].
Vibration frequencies should be suppressed to avoid resonances at typical natural
frequencies of the system: critical values are between 0 and 7 Hz for suspensions and
sprung masses, from 7 to 20 Hz for drivetrain, and from 20 to 40 Hz for the chassis
[RB30]. Modal Analysis can be a strong tool to test frequencies during design phase.
Another characteristic highly recommended is the fast-swap. This is related to the
entire battery pack, not only single modules or cells. Due to the short time available
for recharging between sessions and the intrinsic difficulty of fast-charging, build two
equal BPs and swap them between events will help to maintain battery healthy and
to decrease with safety the BP energy. If the same chassis is maintained, the only
viable dismount solution is vertical.
The utilization of pouch cells requires the facing of strict challenges for what concern
the design choices about cell spacers, which perform a dual role in the case of pouch
cells. Besides their primary function (providing cell-holding functionality), they
provide the binding pressure necessary to counteract the internal spring forces and
to prevent the cell windings from expanding as a result of it. Battery cell spacers
should create sufficient binding on the cell sides without covering so much of the cell
surface area that cooling becomes ineffective (according to NASA-Battery Safety
Requirements Document [RB44], cell spacing is more critical for pack designs
employing battery cells of gravimetric energy density greater than 80 Wh/kg).
Spacers should also avoid delamination of electrodes, the alternative is to choose, as
assembly tech for the cells, clamping.
Even if not convenient for what concern cooling design, BP should be compact for
two reasons: the first is the possibility to place it in an optimal position (argument
more in depth in the chapter βDesign Hypothesisβ), the second is that has an
appreciable impact on thermal performance of the battery pack. Research shows that
increasing the cell-to-cell spacing for a battery pack from 1 to 10 mm can lead to a
loss of approximately 1 Β°C in the steady-state cell core temperature, for all the three
physical formats.
An aspect extremely important for what concern safety is the presence of exhaust
nozzles. Almost all cells have a system which limits the gas pressure inside it, venting
40
it outside. Up to half of the gas venting from lithium battery cells is made up of highly
flammable gases including hydrogen, methane and ethylene gas [RB08]. In the case
of pouch cells, itβs possible to identify a weak spot across the seals close to the tabs,
which will collapse when pressure rises dramatically during, for example, an
Overcharge event [RB46]. Packing location and battery chemistry type will influence
whether a gas vent routing system to the vehicle exterior is necessary in the event of
a cell vent during a malfunction [RB30].
The choice of Forced Air Cooling for most of teams is amply justified by weight
reduction and simplicity in the design. Efficiency of the system, eventual synergies
with Aerodynamics of the motorcycle and Dimensions and position of cooling fans
must be carefully evaluated. This last process can be done at the end, analysing
pressure drop inside the pack. Only one motorcycle of Class 2 is equipped with Liquid
Cooling, in this case also the lower fairing must be chosen in order to contain at least
41
half of the cooling fluid, with a minimum value of 2.5 litres [RB06]. Natural Air Cooling
is not recommended because cells cannot have enough time to cool efficiently at
lower speeds reached in the middle part of the circuit, exacerbating the loss of range.
Efficiency of the thermal design can be further increased by using perforated battery
compartments. Effluents generated by the battery cells enter the hollow guideways
formed within the battery pack through these perforations. The guideways direct the
effluents to a gas exhaust nozzle, which releases it out of the battery pack.
2.9: BATTERY PACK DIMENSIONING
Correct choice of the technology requires the definition, in an approximate way, of
energy stored inside the pack, before to choice the best assembly method.
At disposal there are old data from telemetry of the Petrol motorcycle already
designed by the university team. This motorcycle was able to reach a speed higher
than 180 km/h in the main straight, and perform a time lap better than 2:30:000: if
compared to scores done during last race event, this motorbike would be one of the
best of Class 1.
By the way, foreseeing a performance improve of the other teams, the target time
lap is set to a value under 2:15:000.
Data from telemetry, together with altimetry data from the circuit, are sufficient to
define the energy requirements (in the next section will be see how). Once that a first
attempt on technology is done, weight of this simulated pack will be, simply:
ππ΅π΄ππ,πΈ =πΈπ΅π΄ππ
πΈπ πππ
And will be compared to weight required to satisfy maximum power requirement:
ππ΅π΄ππ,π =πππ΄π
ππ πππ
The highest value will satisfy both energy and power requirements [RB12]. If the last
one is chosen, energy stored in will be calculated again.
42
As already said, the author had at disposal old telemetry data. Inside this .xls file there
are, defined for fraction of time, instantaneous velocity and throttle conditions.
The author programmed a script, written with MATLAB 2014a, using this process:
-first of all telemetry data are cleaned up with the objective of remove redundant
data, tending to increase drastically the value of energy requested;
-the speeds from .xls file are artificially modified identifying moments in which the
motorcycle is accelerating. The total time will be reduced by 15 seconds, in this way
we can define a coefficient:
πππππππ‘πππ =ππππ β 15
ππππ
The instantaneous speed will be defined as:
πππππ =π
πππππππ‘πππ
43
Notice that with this approach speeds during cornering and braking are ignored. As
direct consequence, this time lap is created ignoring shifting of braking points, so
calculations about power will be conservative due to higher speeds than real. A speed
limit is fixed at 56 m/s, also acceleration values will have a maximum value fixed to
be sure that values after modification of the data are coherent with reality;
-total mass of the system motorbike + driver, πΆπ₯ and rolling coefficient are taken by
hypothesis and the instantaneous force is derived as:
πΉπππ π‘ = π β πππππ +1
2πΆπ₯ππππππ
2 + πππ πππΌ + πππΆπππππππ πΌ
Where π, π and πΌ are, respectively, the air density at ambient conditions, gravity
acceleration and instantaneous slope of the circuit;
44
-power is simply derived using the established formula:
ππππ π‘ = πΉπππ π‘ β πππππ
Cause the new engine has a maximum power declared of 42 kW, thanks to the higher
voltage assured by the BP it was reasonable to limit the script to a power equal to 50
kW.
-mean power is calculated and the energy spent on a lap is simply equal to:
πΈπππ = πππππ β ππππ
-total energy spent on the event is the value obtained from previous step multiplied
for a constant, taking into account that the race will consist of 6 laps plus recognition
and exiting lap, which together will consume, depending on driverβs behaviour, more
than 70% of energy of a regular lap [RB16]. For safety reason during calculation is
45
assumed that energy spent is 80%. Real value will be surely lower, especially if
regenerative braking is considered:
ππππ π‘ = 6.8
A further safety factor is taken into account, because calculation is done considering
an ideal battery. In reality, internal resistance and transient current will negatively
afflict the total energy consumed by the motor. This safety factor is in line with other
works in the same field [RB16]:
ππΉ = 1.05
Depending on cell selection, usage profile, and efficiencies of Transmission and
Inverter, energy at disposal can be the 80% of the required in the worst case scenario
[RB07]. This means that Installed Energy must be, at least:
πΈ =πΈππππ’ππ π‘
0.8
At the end, total energy necessary to end the race, calculated with this method, is
equal to 7.7622 kWh for the version without regenerative braking. Script can be
adapted to study impact of regenerative braking, estimated by previous analysis on
about the 10% of energy saved. The premises are good, but adaptation on the motor
must be carefully evaluated.
2.10: CHOICE BETWEEN SINGLE OR DOUBLE PACK
Rules [RB49] allow the possibility to create more modules, and the possibility to place
them in different places in the motorcycle. This choice can be done in order to satisfy
power request without renounce to range capability and to increase Regenerative
Braking Efficiency, creating a Series Hybrid Electric Vehicle [RB52].
46
The storage system will take the name of HESS (Hybrid Electric Storage System)
[RB53]. Usually the Power Buffer, as the name can suggest, is created connecting in
series cells with high specific power (such as LTO, LFP, LMO or Supercapacitors) that
can supply it under request, but can be created also adopting same chemistry of Main
BP but at higher voltage. It shows also excellent performances in Regenerative
Braking because such chemistries show interesting C-Charge rates.
However, no one of the other teams chose a layout like this and there arenβt
examples of commercial and racing motorcycles with such technology. The reason is
simple: as calculated in the cycle, regenerative braking will have a non negligible but
small role on BP dimensioning and, except when Pulse Discharge Current is required
(during Acceleration test and in very rare cases during Race and Qualifying events),
Battery will run almost in Continuous Discharge Current conditions. On the other side,
there is the necessity of an interface between the two packs, because they cannot be
connected directly in parallel due to safety issues [RB53]. Whatever is the
architecture (between the ones viable on Racing, so semi-active or fully active), this
corresponds to an increase of number of components (and as direct consequence,
weight) and increase of design requirements. For example, it could be necessary to
design a further case, which means more complex thermal management and analysis,
to add other BTMS boards and there is the risk to denaturalize weight distribution on
the vehicle. Furthermore, the setup of a robust control strategy is required. These
47
reasons are enough to state that, for high performance lightweight motorcycles, is
more convenient to focus on a single and compact BP.
2.11: BP LAYOUT
Once that requirements of the pack (in terms of Power, Energy and Voltage) are set,
the research of a suitable cell and layout can start. Approach will follow this scheme:
-Data about different cells (and modules) related to Electrical Characteristics
(Voltages, Capacity, C-Rates) and Physical Characteristics (Dimensions and Weight)
are collected.
-As further safety factor, Capacity is corrected as follows:
πΆππππ = 0.9 β πΆππππππππ
This takes into account that cells arenβt available for testing. Capacity declared by
manufacturers is related usually to low values of current (from less than 1C to around
2C), but itβs known by bibliography [RB48] that, at ambient temperature, battery
maintains good performances until maximum continuous C-Discharge, then losing a
huge amount of its discharge capacity until maximum pulse C-Discharge. Value
obtained is expressed in Wh.
48
-Number of cells belonging to a series branch is chosen imposing that the maximum
voltage admissible for the competition, equal to 126 V [RB49], must be lower respect
the maximum voltage of the series branch. Even if it could seems disrespectful of
rules, this strategy takes advantage of charging and discharging characteristics of
various technologies: only a little part of energy is stored at around maximum
voltage, and time required to recharge at maximum voltage is higher. This means that
itβs convenient to increase Total Capacity and decrease recharge time limiting
artificially the total voltage to regulation request, even if this means renounce to a
part of maximum energy (this is more true for NMC and LCO, looking to their very
similar discharging law, both from testing and modelling [RB51]).
Another possible trick is to charge the battery at its maximum voltage and then
unload it before utilization, being sure that voltage of the BP will be the maximum
allowed by regulation and not a lower value depending on charger characteristics
and, as consequence, decreasing current necessary to obtain maximum power. For
these, and other reasons often related to BMS and specific rules, the same tactic was
chosen by other teams participating to Motostudent [RB50] and FSAE [RB12].
49
-Number of required parallel branches is calculated with the objective of satisfy the
Installed Energy Request. This means that Installed Energy must be, at least:
πΈ = 7762 πβ
And the number of Parallel Branches must be, at least:
ππππππ =πΈ
ππππππ β πΆππππ
-If the BP virtually built satisfies the minimum power request it can be considered
valid. Minimum power is fixed at:
ππππ = 50000 π
As declared by other teams in previous edition. Notice that there is a high probability
that value of Power is related to C-Pulse current. For lack of this information, a very
safe approach is chosen:
π = ππππππ β ππππ₯ β πΆππππ β πΆπ·πΌππΆπ»
Where ππππ₯ = 126 π and πΆπ·πΌππΆπ» is the C-Rate relative to Continuous Discharge
Current.
On the following table there will be results of the cell research. Packs that donβt
satisfy minimum power request (as chosen by the author) are labelled in Red. The
packs labelled in Green show very high power and a good energy margin, together
with the lowest weight:
50
51
2.12: BP GEOMETRY AND CONNECTIONS (CASE 1)
Data related to dimensions of a single module are showed below:
Each module should be already equipped with a fuse (further investigations are
required).
Considering the 1st model, a 16S3P BP is required to satisfy Energy Request.
The total volume occupied by the cells will be:
π = ππππππ β ππππππ β π β π‘ β π€ = 0,0199 π3
For the 2nd model, with a configuration 11S3P:
π = ππππππ β ππππππ β π β π‘ β π€ = 0,0202 π3
Traducing in a very compact shape. This means that there will be enough space to
place other components such as BTMS and Inverter in the same case, decreasing
wiring length and thus increasing efficiency of the whole system. This will have also
positive effects on motorcycle dynamics and will decrease number of components
that must be designed. However, special care about EMC, EMI and electrical isolation
must be taken into account.
Now, it must be decided how to connect the batteries. Two solutions are viable: in
both cases, if a module stop to function, the maximum current available will be
limited by BTMS in order to save the others.
52
In this case, the βSolution cβ should be better for modularity because only two
terminals (positive and negative) are required for its functioning, while the βSolution
bβ requires two positive and negative terminals, plus connections between each
parallel stack for a total of 15 or 10 depending on the module chosen. However, the
same layout can be risky in the case of a broken module: it means that all the series
branch will be disconnected from the circuit, losing 1/3 of overall installed energy and
higher electrical work on the functioning branches.
It's clear that, due to the difficulty to separate the single cells of the module,
connection will be done through connectors.
53
2:13 BP GEOMETRY AND CONNECTIONS (CASE 2)
In order to have better control on the manufacturing process of the pack, looking to
experiences of other teams (competing also in old MotoE championship) and
considering performances of the Motenergy MS1718, itβs possible to decrease the
power requirements assuming that is sufficient to deliver Continuous Discharge
Power in order to satisfy the request:
ππππ = 32000 π
In this way another cell, produced by Melasta, will be a potential candidate:
Letβs see all the advantages related to this choice:
-very strong brand in EV racing;
-better chemistry;
-slightly minor battery cell volume:
π = ππππππ β ππππππ β π β π‘ β π€ = 0,0195 π3
-possibility to choose different technologies to connect cells each other;
-better BTMS management (it could be done on single cell and not on single module);
-potentially better efficiency (due to the possibility to avoid the utilization of
connectors);
-easier to cool.
Disadvantages are:
-weight increase of the whole BP (42.1 kg against 38.5 or 39.8 kg);
-increase of complexity due to the higher number of cells, this requires a very robust
approach on connections and further considerations about BTMS;
54
-range capability and reliability must be verified in case of Pulse Discharge Current
utilization (especially if is intended to set a map in order to apply during the race);
-dimensioning of fuses becomes necessary.
In this case, βSolution bβ is always better for what concern robustness of the system,
even if the increased number of parallel branches decreases the previous problem of
installed energy for βSolution cβ (1/7). Another clear advantage is that if a suitable
connecting method is chosen, like for example clamping, it will give clear advantages
on modularity because will allow a direct dismount of the broken cell. Another
advantage to consider is that balancing issues can be managed, under certain limits,
by cells themselves, because current can flow in between branches. However, this
means that parallel connections must be carefully dimensioned. Itβs worth to
consider that for βSolution cβ more boards are necessary because parallel group cells
could have different voltages but reading errors can be an issue in βSolution bβ
[RB50].
Differently respect the Case 1, the connection between the cell can be done in
different ways. In order to understand which is the most convenient, the author setup
a Pugh Matrix inspired by application studies on Automotive Field [RB54]. The Criteria
considered particularly important are grouped and a Weight is arbitrarily given,
considering the application in a racing environment:
55
Even if there isnβt an evident advantage looking to points only, the author suggests
the design of a mechanical assembly through clamping. At the cost of a slight increase
in weight and dimensions and a reduction of efficiency, safety related to null heat
transfer during assembly, strength of the joint and, in particular, the total modularity
of the system, are advantages impossible to ignore in a racing environment. To
support this idea, other teams working on way more powerful motorcycles [RB55]
used successfully this technology, obtaining a quite acceptable value of efficiency of
97% at maximum charge and 96% at minimum charge. These values can be reached
sanding and degreasing the cell tabs and the copper in order to remove oxidation and
grease.
2.14: FURTHER ANALYSIS REQUIRED
There are enough information to start the development of the pack, starting from
purchasing the cells.
56
The pack must be physically created, tested, together with its case and then placed
in a suitable position:
-behaviour of the single cell under specific current charge and discharge (suitable for
the real requirement of a single branch) can be validated with an automated tester,
interfaced through MATLAB/Simulink. The tester should guarantee a way higher
current respect the single cell capability and relays to simulate the various conditions
(rest, charge, discharge), and should be equipped with a DC/DC converter to comply
with typical portable computer direct current (DC) power supply [RB50]. Itβs
suggested to buy more cells in order that during testing the ones with lower internal
resistance can be selected for the construction of the pack;
-quality of connection between different cells must be evaluated. Once that the stack
is designed, by using a programmable DC load it is possible to control the drawn
current very accurately and indeed, when the load is enabled, it is observed that the
total voltage on the terminals of the stack drops slightly. Using simply Ohmβs law, the
internal resistance of the stack can be calculated straightforward. The linearity of this
equation even allows to extrapolate the obtained resistance to the complete battery
pack [RB55]. Itβs suggested to not go under values of 95% of Efficiency of the stack;
-itβs possible to design BTMS connections in order to have a single board for all the
BP. Usage of CAN Cell Group Modules makes a BTMS virtually scalable to an infinite
number of cells [RB56]. CAN protocol is widespread in Automotive Industry, and
already used in Formula E to the same purpose as presented here [RB57]. In case the
decision about BTMS will be towards the utilization of multiple boards, in order to
increase precision controlling directly the cells, an estimation of the number of
necessary boards must be done. The number of boards can directly affect thermal
analysis and dimensioning of the case;
Talking about the case, important requirements to consider are:
-Ingress Protection: at least IP69K. It must be considered that the case will have a
water/air filter necessary to comply both to cooling requirements and waterproof
tests required by regulations. If there are time and conditions available, waterproof
effectiveness can be tested using specific sensors like the WaterSwitch M158
produced by Kemo;
57
-Modal Analysis: natural frequencies of the case must be at least higher than natural
frequencies of the chassis. If possible, it should be higher respect shimmy, derived
from tolerances related to the front assembly. Virtual tests have to be done both in
straight and in cornering conditions. Last but not least, it should be as light as
possible;
-Thermal Analysis: the pack must be designed in order to never reach temperatures
higher than 60 Β°C, however as previously stated should be convenient to dimension
it having in mind that optimal range is between 20 and 40 Β°C. Also, temperature
between different cells should be as similar as possible. Differences of about 3 Β°C
between coolest and warmest cell are acceptable. In order to reach these results, due
to the lower heat transfer coefficient of air [RB59] can be useful the design of a Vortex
Generator, facilitating the mixing of air inside the pack. While results related to
Cylindrical cells are encouraging [RB58] the effectiveness related to Pouch cells
should be investigated. There are a lot of virtual methods useful to perform the
analysis [RB60], but one which is absolutely necessary is CFD in order to calculate the
pressure drop inside the pack, in order to dimension cooling fan and to analyze
aerodynamic interactions with side flow and wake. This is valid for each
compartment;
-Other requirements: the case must be not pierceable, acid resistant, and
compartment dedicated to Inverter should be electromagnetically shielded in order
to not interfere with functioning of the BP. It is suggested to place the Inverter in
downward position.
58
3RD CHAPTER:
HYPOTHESIS ON TRANSMISSION AND
MOTORBIKE DESIGN
3.1: TRANSMISSION BASIC DESIGN
In the past edition of Motostudent there were various solutions related to
Transmission Design. The dominant one consists on a single gear ratio without any
clutch, and final pinion often directly mounted on EM shaft, sometimes with an
intermediate transmission shaft. In general, different solutions (like for example the
utilization of a 5-gear gearbox) came from a different philosophy, oriented more on
MS1 competitiveness or derived from unique design solutions.
Ideally, a good way to understand if the EM needs a multi-gear gearbox is observe its
torque and power characteristics. In the study case of the engine that will be
mounted on the motorbike, produced by ENGIRO, characteristics are showed below.
59
60
Of particular interest is the ratio between Maximum Rotation Speed and Nominal
Rotation Speed, which takes the name of βspeed ratioβ:
π₯ = 1.4
This is a quite standard value in PMEM due to its intrinsic difficulty on flux weakening
range [RB28]. This means, as clearly shown by the graph, that constant torque region
is larger than constant power region and that maximum torque canβt be significantly
high. Looking to efficiency map, it is clear also that motor efficiency will be quite low
during all the utilization range.
61
If this would be a heavier racing car (like FormulaE) or a multi-purpose road vehicle
the utilization of a multi-gear gearbox would be advisable. However, in our case study
itβs important to notice that:
-efficiency will be higher due to higher voltage of BP;
-slopes in the circuit are steep only in few parts;
-total weight of the motorcycle has a dominant importance, especially related to BP
dimensioning;
-a lot of Road Racing Electric Motorcycles still adopt a single gear transmission even
with PMEM motors, like Energica Ego Corsa, Mugen Shinden Nana and Victory RR;
-the choice of a single gear ratio doesnβt require the compilation of a complicated
script to identify correct gear shifting patterns [RB60].
Last, but not least, the examined motor is very bulky in all reference planes (xy, xz
and yz). This means that the presence of a bulky transmission would make the
problem worse. In practice, the EM is wide enough to constrain designers to the
utilization of an intermediate transmission shaft in order to have the chain parallel to
x axis. This will give, as direct advantage, the possibility to reach a satisfying Squat
Ratio for two reasons: the secondary shaft can be positioned in a rearward and along
swingarm axis position, and the presence of an intermediate reduction stage allows
to decrease the final gear ratio [RB05].
3.2: SQUAT RATIO CONSIDERATIONS
To understand Squat Ratio and its link with dynamic behaviour of rear suspension, it
should be considered the intersection point A: it is the intersection between upper
branch of the chain and swingarm axis. The straight-line connecting contact point of
rear wheel and point A is called squat line, characterized by an angle π. The point A
represent the Instantaneous Force Centre [RB10]. With load transfer line is intended
the direction of force obtained summing load transfer force and thrust force and is
characterized by an angle π (usually π in bibliography [RB05], π is used to avoid
confusion with gear ratios).
62
Assuming π· as swingarm angle respect the x axis, πΏ as its length, and π as the angle
between upper branch of chain and same reference axis, we can define the Squat
Ratio as:
β =ππ‘π β πΏ β πππ π·
π β πΏ β π πππ· + π β πΏ β sin (π· β π)
Itβs possible to demonstrate that expressing the load transfer as a function of the
driving force, the ratio is a function of geometric characteristics only:
β =β β πππ π·
π β [π πππ· +π π
ππsin (π· β π)]
=π‘ππ
π‘ππ
Squat Ratio is of crucial importance in dynamic behaviour: it says how the swingarm
will rotate due to thrust force, and as consequence will influence shock absorber
behaviour. In particular, if β < 1 suspension will tend to extend, and this is a
detrimental result that will decrease torque utilization and stability in curve. Normally
63
the objective of designers is to obtain a neutral Squat Ratio equal to 1 or slightly
higher.
Itβs worth to say that the Squat Ratio depends also on bumps and other obstacles
which can vary these geometric values. In the following examples is possible to see
how the suspension state can condition this situation:
In Drawing A we can see that Instantaneous Force Centre can be behind the wheel.
In this case the suspension will compress if the point is below the ground and expand
in the opposite case.
The other drawings are related (exaggerating) to extended suspension (B) and
compressed suspension (C). We can see that a progressive suspension compression
will condition negatively the instantaneous Squat Ratio, while during extension Squat
Ratio will tend to values higher than 1.
Letβs return to previous formula. Notice that, considering wheel dimensions constant:
-a bigger crown gear tends to decrease the squat ratio;
-swingarm inclination angle and difference between the arm inclination angle and
the chain inclination angle have a direct influence. This last value depends on position
64
and radial dimensions of pinion gear. In particular, a smart positioning along
swingarm axis, or concentric with swingarm pivot (like in some early Bimota models
[RB10]) will help to reach Squat Ratio values near to Neutral.
3.3: POSSIBLE LAYOUT
Considering the absence of the clutch, design of intermediate stage can be done
taking as reference camshaft activations systems:
-bevel gears: this is an old-fashion solution, in the β50s was very common in Racing
motorcycles with single camshaft like Honda RC141 and Honda RC160. Later on also
Ducati used this technology as trademark and in 1971 equipped its 500 GP engine
with this system [RB64]. Today this solution is abandoned in Motorcycle industry, due
to heaviness, cost, necessity of dimension at least 2 intermediate shafts and the lower
efficiency (around 93%) respect spur gears [RB65];
-timing belt or chain: simple and widespread solutions, the first has the advantage of
easier adjustability and no lubrication requirement. There are in the market double
sided timing belts, which can be used to create a counter rotating system without the
utilization of a further intermediate shaft. Timing chain is way more resistant and can
decrease weight of the system if carefully chosen. Both assure liberties about
secondary shaft positioning;
-gear train: this solution is typical of high revving racing engines of our days due to
best power/weight ratio, high precision of the system [RB66] and very high efficiency.
Another advantage is the opportunity to position the EM in a way that it rotates in
the opposite verse. This strategy is widespread on modern SBKs motorcycles because
in dry road conditions allows to decrease roll angle (given a curvature radius) and
limit wheeling during acceleration. Handling is improved during chicanes and curve
approach but worsen during exit [RB68]. Advantages of counter rotating engine are
however limited in our specific case due to centre of gravity shifting towards the front
and low power and inertia, so the objective of make transmission compact comes
first. As disadvantages, the number of components can increase drastically with
65
number of intermediate gears required and itβs by far the most expensive solution. It
is also very noisy, even if noise is rarely a concern in Racing.
The team opted for the design of a gear train with a secondary shaft. One side of the
shaft will define the intermediate reduction gear ratio and on the other side there
will be the final pinion directly connected to the final crown gear through a chain.
3.4: GEAR TRAIN DIMENSIONING
The reduction ratio is derived as follows:
-final gear ratio is set looking to common commercial setups. Usually, the factor
which affects more negatively the squat ratio, given the position of the pinion, is the
diameter of the crown gear. However, to maintain the drivetrain compact, also the
gear connected to intermediate shaft (rotating at same speed of the pinion gear)
should be enough compact. Four possible final gear ratios are selected: 46/15, 42/17,
42/14 and 40/16. Remember that these values are not definitive, they are useful just
to understand final dimensions;
-a final speed equal to 190 km/h is requested, during the race, in order to be
competitive with Class 1 Motorcycles. Given dimensions of rear wheel and maximum
rotational speed of the EM:
ππ€βπππ = 0.298 π
πππ΄π = 8000 πππ
Final gear ratio can be calculated as follows:
ππππ = π1 β ππΉπΌππ΄πΏ =3.6
60β
2ππππ΄π β ππ€βπππ
π£ππ΄π= 4.73
Is immediately clear how it is necessary a further stage to decrease final crown gear
dimensions. This stage will have a gear ratio of:
ππ π‘πππ =ππππ
ππΉπΌππ΄πΏ
-talking about rotation verse of the motor, to obtain a synchronous motion of EM
respect wheel the intermediate stage requires an odd number of gears; if counter
66
rotating motion is required the number of gears will be even. Assuming a
synchronous motion, and assuming as hypothesis that we want to place the
intermediate shaft more or less along swingarm axis, is assumed that the
intermediate stage will have 5 gears, with a total of 7 gears considering final
reduction stage.
In this way:
ππ π‘πππ = π1β2 β π2β3 β π3β4 β π4β5
Itβs easy to demonstrate that the gear ratio depends exclusively by first gear and last
gear of the gear train;
-the dimensions of EM shaft pinion are assumed imposing as minimum primitive
diameter 40 mm. Assuming (for now) pitch equal to 12.7 mm, number of teeth is
equal to (value rounded to excess):
π§1 =2π
arcsin (πππ‘πβ
πππππ_1)
= 20
Corresponding to a primitive diameter of:
πππππ =πππ‘πβ
sin (2ππ§1
)= 41.1 ππ
-calculation of number of teeth of the last intermediate gear is trivial. The value
obtained is rounded down in order to be sure that the maximum speed requirement
is satisfied and reached at rotational speed lower than πππ΄π:
π§5 = ππ π‘πππ β π§1
πππππ_5 =πππ‘πβ
sin (2ππ§5
)
-the remaining gears are dimensioned estimating that the distance between centre
of EM Shaft and ideal final pinion position should be around 340 mm. This must be
the minimum value of the sum of all intermediate gear diameters and radius of first
and last gear of the stage. Assuming that all intermediate gears should be equal:
67
πππππ_2 = πππππ_3 = πππππ_4 =πππ π‘ππππ β
πππππ_1 + πππππ_5
23
π§2 = π§3 = π§4 =2π
arcsin (πππ‘πβ
πππππ_2)
Results of the dimensioning are showed in the following table:
Layout ππΉπΌππ΄πΏ ππ π‘πππ π§5 πππππ_5 π§2
1 46/15 1.5425 30 61.1 48
2 42/17 1.9146 38 77.2 47
3 42/14 1.5767 31 63.1 48
4 40/16 1.8921 37 75.1 47
What jumps endured to the eye is the difference between primitive diameters of last
gear. It will condition space in the swingarm area and itβs convenient to maintain it
the smallest possible. In contemporary is important to maintain final crown gear the
smallest possible to control squat ratio as discussed before. Analysing differences
between Layout 1 and Layout 3 shows that intermediate gears differ for just one
tooth, meaning that weight difference is almost negligible: itβs clear that Layout 3
should be preferable.
The Gear Train case can be done in Aluminium alloy. Lightness of the component
should be the primary requirement, so alloys with good performances on tensile
strength (obtained by precipitation hardening) are preferable. Another welcome
characteristic is toughness, in order to obtain the component by mechanical process.
So the author suggest an analysis of 6000 series alloys to identify the best [RB73].
68
3.5: TRANSMISSION CHAIN DIMENSIONING
Chain will be chosen from the Regina Chain catalogue, long-time partner of the team
since previous incarnations. In particular it will be examined section about Regina
Chain Extra, dedicated to racing motorcycles:
69
In past editions, 2wheelsPolito chose for their Petrol motorcycles the Regina Chain
415, mounted even in more powerful motorcycle. We want to verify if this same chain
is good for the Electric Motorcycle The approach is the following:
-the final pinion is the same chosen in previous pages, with 14 teeth, which
correspond to a primitive diameter equal to:
πππππ =πππ‘πβ
sin (2ππ§6
)= 29.3 ππ
-running conditions of the chain require that power transmitted should be corrected
before utilization [RB63]:
πππππ =πππ΄π
π β π β π
Coefficients π, π and π are related to lubrication, mechanical characteristics of chain
and power, respectively. π = 1 assuming βperfect lubricationβ due to the racing
utilization, π = 0.8 as safety coefficient because chain analysed is different from
competitor catalogue and π is derived by the following table:
70
Transmission ratio is lower than 2, while π = 1.5 < 2 because it will be connected to
an EM [RB63], from here:
π = 0.92
πππππ =πππ΄π
π β π β π= 57065 π
-transmission chain must withstand a force equal to:
πΉπβπππ = πΉπππ€ππ + πΉππππ‘ππππ’πππ
Where:
πΉπππ€ππ =πππππ
π£πππππ’πππππππ‘πππ
πΉππππ‘ππππ’πππ = π β π£πππππ’πππππππ‘πππ2
With π equal to the weight of the chain per unitary meter, and:
π£πππππ’πππππππ‘πππ =πππππ β πππ΄π [πππ]
19100
At the end:
πΉπβπππ = 4745 π
The model analysed, with an average tensile strength of 21000 π, should be
sufficient to sustain chain shocks and efforts.
71
3.6: DESIGN HYPOTHESIS AND CHASSIS
CONSIDERATIONS
Last step is the placement of core components to the main chassis. In order to fulfil
requirements:
-EM is place in a low and forward position, and slightly inclined to decrease inverter
wires length but maintaining a good cooling efficiency;
-Inverter can be placed in the BP Case. This choice is done intending to maintain a
forward centre of gravity position and requires an adequate EMC shield between the
two components;
-BP Case will be inclined respect the vertical and will have an irregular shape. The
reason is that author intends to assure to the rider a comfortable position and to gain
space useful to place wirings, BTMS boards and other stuff, at expense of a lower
centre of gravity: the case previously dedicated to airbox, under that BP will find
place, will be the same of last editions.
72
73
The team should work on increase chassis length in order to gain BP space, not
sufficient to assure the Energy Requirement. Increase chassis width is detrimental for
torsional stiffness, which should be the highest possible to have good driving feeling,
as demonstrated by different testers trying chassis with different torsional stiffness
values [RB56]. Increasing length, lateral stiffness of the chassis will decrease, this
means that a list of tests and requirements must be satisfied in order to have a stable,
safe and with a good lateral stiffness value chassis. As tradition in 2wheelsPolito
team, it will be done by bonded components made in Aluminium Foam surrounded
by very thin Aluminium sheets.
-Twist Axis must be recalculated, which is defined in the following way: chassis is
constrained in steering hub, and a plane is defined drawing three points around rear
wheel hub. A force is applied perpendicularly on wheel hub, making this plane
rotating. The Twist Axis is the intersection between the plane in the new position and
the plane in the original. The axis will never be perpendicular to x axis and, together
with their intersection, will define the stiffness matrix [RB58];
74
-the new chassis will be tested through simulation [RB59]. It must perform during
Panic Brake Stop, Impulsive Wheeling Force (Safety Factor sufficient equal to one
because the rider tends to make a soft impact and is unrealistic that the wheel is
locked while in mid-air), Impulsive Stoppie Force (Safety Factor = 1) and exceptional
forces just as step impact. This last calculation is done considering this kind of loads
as static or quasi-static loads, then multiply the results for dynamic factors:
πΉππ¦πππππ = πΉπ π‘ππ‘ππ β π·πΉ
πΉπ π‘ππ‘ππ is calculated assuming, as simplification, that resultant reaction force will pass
through centre of wheel:
π = ππ΄ β π‘ππ
With:
π = ππππ ππ (1 βπ»
π )
π·πΉ will be derived confronting the quasi-static calculation with real dynamic
equivalent case and:
2.3 < π·πΉ < 3
75
Last notes are related to swingarm. It has a low truss geometry, solution usually
preferred in Racing because respect to the high truss, it is possible to lower the shear
axis, about which is applied the torsion moment to the swingarm. The movements in
a point closer to the ground, resulting in a lower movement of the contact point tire-
ground. All this is translated in terms of rider feeling. Moreover, the system results
very compact and near to the centre of gravity [RB57].
Respect to chassis, where is desired a certain grade of lateral flexibility, swingarm
should be the stiffest possible. Flexibility is related to difficult and slow manoeuvring
and is dominant respect to eventual swingarm asymmetry and neutral axis [RB56].
76
4TH CHAPTER:
βMAKE OR BUYβ STRATEGY AND
PRODUCT ARCHITECTURE
4.1: PRODUCT ARCHITECTURE
This is a very important reflexion useful both for MS1 settlement and for effective
success on design final result. Differently respect to a manufacturer, inside the
University donβt exist any machinery or tool to build any component: the Team is able
to design a huge number of components, perform FEM and CFD analyses, and
assembly the motorbike and small subparts related mainly to GLVS (like for example
the Dashboard).
In this specific case, with βmakeβ is intended a component that will be designed by
the team (or by an external supplier following teamβs specifications) before getting
manufactured by a supplier (if design is done by an external supplier, manufacturing
can be done by a different company), with few exceptions; while with βbuyβ is
intended a component already in commerce and bought βas it isβ, considering
obviously their relations with βmadeβ components. Itβs worth to underline that we
are speaking of βComponentβ Make or Buy, while βStrategicβ (investment on facilities
and manufacturing tools) and βTacticalβ (related to sudden changes on demand) are
less interesting for this specific competition [RB54].
While for certain components the choice can be trivial due to previous experiences
and MotoStudent Regulations [RB49], is still important to define a strategy in order
to obtain low purchasing and design cost (intended as money and time) remembering
that is extremely important to create a performing motorcycle in track but, after last
Regulations changes and the growing importance of MS1, also simple and cheap to
build and (possibly) simple to setup and repair intended as investment and as
77
dexterity.
First of all, is important to underline that the Team will be considered as a Company
that wants to compete in a fictitious championship that will involve the following 6
races:
Inside Regulations isnβt specified how many motorcycles of the same Company can
compete in the Championship and if itβs possible to create Satellite Teams. However,
the possibility to create more than one prototype will be taken into account. Part
related to logistics organization related to transport of the Team and their
motorcycles is out of the scope of this thesis.
At this point settlement of the strategy can start. For most companies, business
strategy starts by having a top-level definition of what the business is aiming for. This
usually takes the form of a Company Mission Statement, possibly followed by some
specific goals [RB54]. Goal statement is a job for the top management team and will
define the most crucial components on which work will be focused.
At this point is possible to set up the list of all components. Keeping in mind Company
Mission Statement it will be possible to define the Make or Buy Strategy. A common
tool useful to breaking down the product architecture is the utilization of tree
diagrams as the one showed below, built considering main design areas and then
doing the assessment of all components related to each area (as example, only the
branch related to Fairing β Front is deepened):
78
79
4.2: STRATEGY SETTLEMENT
A rule of thumb for outsourcing is described here. It prescribes that a firm outsource
all items that do not fit one of the following three categories:
1) the item is critical to the success of the product, including customer perception of
important product attributes;
2) the item requires specialized design and manufacturing skills or equipment, and
the number of capable and reliable suppliers is extremely limited;
3) the item fits well within the firm's core competencies, or within those the firm
must develop to fulfil future plans.
Items that fit under at least one of these three categories are considered strategic in
nature [RB61].
At this point is easy to see the extreme importance of Mission Statement. It gives an
answer to questions related to distinction between critical items and the others.
There are a lot of items belonging both to Category 1) and 2): examples are Chassis,
Swingarm, EM Supports and BP. At this stage it isnβt known who will design and/or
make the component. However, itβs advisable to choose suppliers with Competence
Trust (when possible), which means that each partner can actually do what they say
they can do, i.e. that they can provide the service to the specified level of capability.
Usually this is a matter of visiting the site, investigating manufacturing capability,
running trials and finally monitoring performance [RB54]. Geographic location of
Team and contacts from previous experiences, even if not the βbest in classβ respect
other teams (mainly due to location), surely helps on verify the Competence Trust.
In Regulations are reported the items which the Organization will give to the team,
and obviously all these items will be bought βas it isβ. Doubts about βwhoβ will design
and/or make the other components can be solved using a flow chart which takes the
name of Decision Tree. Utilization of this tool assumes that team already did the
strategic decisions, but often it helps more facing up the strategic decision through
the route [RB54]. For this project the author used the following Decision Tree:
80
The main problem of the Team is the impossibility to use directly machinery tools in
order to produce in house, remembering always the educative purposes of the
project. For this reason, if the item isnβt strategic at all, it should directly be bought
βas it isβ, except if it is of engineering interest and so it can be worth to be designed.
For example, design Footrests Supports could be a good opportunity to learn the
utilization of a CAD and a FEM program and it could be built by the same supplier
which machines a way more strategic item just as the Steering Plates. What concern
screws and bolt instead will be bought without doubts. An item where know-how is
stated, with good engineering interest and easy to assembly can be made in house.
An example can be the GLVS Wirings.
81
If under analysis there is a strategic component, team members must understand if
their know-how is sufficient to design. Since this is the first experience with an Electric
Racing Motorcycle, it is always better to hear what a specialist has to say about
techniques and βtricksβ in order to increase, at maximum, the necessity to be
competitive. For this reason, what concern BP (discussed in previous chapter) was
useful to understand where the team has to act to fit the system in the chassis and
which are the components necessary to make it work (with a rough estimation of Cell
Number), but itβs worth to outsource the effective design of the system (or, in this
case, allow a team member to collaborate and participate actively in this phase with
supplier). In future competitions, after that some experience is matured and network
of contacts is created and validated, the Team can decide to work with more
autonomy, buying directly cells, dimensioning HVS wirings, managing an aftermarket
BTMS (or, why not, design it directly), and so on.
Where instead the know-how is verified, if the assembly of the item can be done in
house (for example, the Dashboard) the Team should make it, otherwise the Team
should focus with attention on design and choose then a competent supplier (an easy
example is the Chassis).
4.3: BOM
Component Subsystem N. Strategy
Brake Lever Protection Accessories 1 buy
Chain Guard Accessories 1 make
Footrest Pads Accessories 2 buy
Footrest Supports Accessories 2 make
Footrests Accessories 2 buy
Handlebar supports Accessories 2 buy
Handlebars Accessories 2 buy
Knobs Accessories 2 buy
Left Protection Cap Accessories 1 make
Potentiometer for Shock Absorber Support Accessories 1 buy
Rear Brake Assembly Accessories 1 buy
Rear Brake Lever Accessories 1 buy
Right Protection Cap Accessories 1 make
Swingarm Protection Caps Accessories 2 make
Air Filter Battery 1 buy
82
Battery Case Battery 1 make
Battery Cells Battery TBA buy
Battery Cells Clamps Battery TBA make
BMS Boards Battery TBA buy
BMS Cell Sensors Battery TBA buy
BMS Control Unit Battery 1 buy
Charging Connector (IP65) Battery 1 buy
Cooling Fans Battery TBA buy
Current Fuses Battery TBA buy
Escape Valve Battery TBA buy
Inverter EMC Shield Battery 1 make
Temperature Sensors Battery TBA buy
Vortex Generator Battery 1 make
Water Sensors (Waterswitch) Battery TBA buy
Brake Levers Bearings Brakes 2 buy
Front Brake Caliper Brakes 1 buy
Front Brake Disc Brakes 1 buy
Front Brake Oil Tank Brakes 1 buy
Front Brake Oil Tank Support Brakes 1 make
Front Brake Pump Brakes 1 buy
Front Brake Tube Brakes 1 buy
Rear Brake Caliper Brakes 1 buy
Rear Brake Caliper Support Brakes 1 make
Rear Brake Disc Brakes 1 buy
Rear Brake Oil Tank Brakes 1 buy
Rear Brake Pump Brakes 1 buy
Rear Brake Tube Brakes 1 buy
Chain Tensioner Chassis 1 make
Chassis Chassis 1 make
Duct Support Chassis 1 make
Motor Spacers Chassis 2 make
Rear Frame Chassis 1 make
Rear Wheel Hub Chassis 1 make
Rear Wheel Nuts Chassis 2 make
Suspension Hub Chassis 1 make
Swingarm Chassis 1 make
Swingarm Bearings Chassis 4 buy
Swingarm Bearings Spacers Chassis 2 make
Swingarm External Spacers Chassis 2 make
Swingarm Hub Chassis 1 make
Swingarm Internal Spacers Chassis 2 make
Wire Fixing Points Chassis TBA make
Airduct Fairing 1 buy
Dashboard Support Fairing 1 buy
Fairing Plastic Washer Fairing 12 buy
83
Fairing Screws Fairing 12 buy
Fairing Support Fairing 2 make
Front Fairing Fairing 1 buy
Front Fender Support Fairing 2 make
Front Mudguard Fairing 1 buy
Left Fairing Fairing 1 buy
Low Fairing Fairing 1 buy
Rear Mudguard Fairing 1 buy
Right Fairing Fairing 1 buy
Saddle Fairing 1 buy
Seat Unit Fairing 1 buy
Tail Fairing 1 buy
Windshield Fairing 1 buy
Axial Bearings Steering Group Front Suspension 2 buy
Axial Flat Washer/Fifth Wheel Steering Group Front Suspension 4 buy
Bottom Steering Plate Front Suspension 1 make
Fork Front Suspension 1 buy
Fork Plates Ferrules Front Suspension 2 make
Front Brake Caliper Spacers Front Suspension 2 make
Front Rim Front Suspension 1 buy
Front Wheel Hub Front Suspension 1 make
Radial Bearings Steering Group Front Suspension 2 buy
Steering Damper Front Suspension 1 buy
Steering Hub Front Suspension 2 make
Top Steering Plate Front Suspension 1 make
Trasponder Zipties Front Suspension 1 make
Dashboard GLVS 1 make
Front Wheel Speed Sensor GLVS 1 buy
GLV Master Switch GLVS 1 buy
GLVS Wirings GLVS TBA make
Potentiometer for Fork Suspension GLVS 1 buy
Potentiometer for Shock Absorber GLVS 1 buy
Pressure Sensor Front Brake GLVS 1 buy
Rear Light GLVS 1 buy
Rear Wheel Speed Sensor GLVS 1 buy
Secondary Battery GLVS 1 buy
Trasponder GLVS 1 buy
Battery/Inverter Wirings HVS TBA make
Electric Ignition Button HVS 1 buy
Electric Machine HVS 1 buy
Emergency Shut-Down Button HVS 1 buy
Fast Throttle Command HVS 1 buy
IMD HVS 1 buy
Inverter HVS 1 buy
Inverter/Motor Wirings HVS TBA make
84
Motor Microcontroller HVS 1 buy
NO-line Contactor HVS 1 buy
Throttle Potentiometer HVS 1 buy
Tractive System Master Switch (TSMS) HVS 1 buy
Flexible Coupling Rear Suspension 1 make
Flexible Coupling Bearing Rear Suspension 1 buy
Rear Rim Rear Suspension 1 buy
Rear Suspension Triangle Rear Suspension 1 make
Rubber Tips Rear Suspension 3 buy
Shock Absorber Rear Suspension 1 buy
Shock Absorber Hub Rear Suspension 1 buy
Small Rod for Rear Suspension Rear Suspension 1 make
Small Rod Hub Rear Suspension 1 buy
Triangle Hub Rear Suspension 1 buy
Outer Casing Transmission 1 make
Crown Gear (acc) Transmission 1 buy
Crown Gear (race) Transmission 1 buy
Drive Shaft Transmission 1 buy
Drive Shaft Sprocket Transmission 1 buy
Final Chain Transmission 1 buy
Intermediate Gears Transmission 3 buy
Intermediate Gears Bearings Transmission 3 buy
Pinion EM Shaft Transmission 1 make
Pinion Gear (acc) Transmission 1 buy
Pinion Gear (race) Transmission 1 buy
Shaft radial bearing Transmission 1 buy
Shaft axial bearing Transmission 1 buy
Some clarifications about most important components are showed below:
-about the transmission, the Pinion EM Shaft will be created ad hoc due to the unique
internal broach. Material suggested is 18NiCrMo5 hardened and carburized, in order
to have high teeth hardness and resiliency. All the other components (with, as unique
exception, casing) can be bought or are already in house: for final crown gear is
recommended the utilization of 7075-T6 Aluminium Alloy due to market diffusion,
high teeth resistance and lightness;
-the fairing will be the same of previous editions, which gave good results in air
penetration and pressure at air intake. New studies about Internal Aerodynamics are
required. Design of an internal fairing can be required if presence of EM wirings
results in not acceptable aerodynamic performances;
85
-separation between HVS and GLVS is assured physically. For this reason, there will
be a secondary battery, mounted under tail;
-with TBA are intended all the components which surely will find a place in the
motorcycle, but for which isnβt already known the final number. Itβs related mainly to
Battery and GLVS, the first will be outsourced and the second isnβt already defined in
this design phase;
-speed sensors nature isnβt already defined. The author suggests the utilization of Hall
Sensors similar to the one mounted on Aprilia RSV4 competing in SuperStock Italian
Championship [RB55], which donβt use a magnetic exciter mounted coaxial with
wheel hub but performs using braking discs nuts without punching and flathead. The
result will be a very compact system;
4.4: COST ANALYSIS
There are different ways to classify costs related to production process: they can be
defined:
-as nature;
-depending on reference period;
-depending on attribution modality;
-depending on variability;
-evaluating their level of controllability;
-as programming modality.
In this analysis BOM will be analysed considering costs as nature. Precisely, only
production cost will be considered (ignoring distribution, administration and eventual
financial costs), and limiting the analysis on direct manpower and materials (no
indirect manpower and depreciation will be considered). These costs cannot be
definitive but can give a rough and conservative esteem during Design Phase.
First of all, MotoStudent gives to each team a standard kit composed by:
-EM;
-IMD;
-1 set of front and rear slick tires;
86
-for what concern braking system there are Front Caliper, Rear Caliper, Front Hand
Master Cylinder and Rear Foot Master Cylinder.
These components are already labelled as βbuyβ and are given by organization for
free. Thus, they will be ignored in this phase.
With TBA is intended that the system isnβt already defined so itβs difficult to state an
approximate cost, except for some parts (for example, HVS will contain Inverter,
which the cost is already known). In the price even parts given by sponsors or already
available in warehouse are considered:
-the Entry Fee is fixed on a base of 3140 β¬ sufficient to the participation of 7 members.
For each further member the Team must pay 325 β¬ [RB49];
-about Battery subsystem, itβs possible to divide the cost in 2 parts. One part is related
to cells, and itβs already derivable even if cells arenβt defined simply dividing Installed
Energy for Cost per kWh [RB70]. The other part is assumed and itβs related to all the
contour (Case, Fans, BTMS, Wirings, Testing, etc.);
-data for Brake subassembly come from older experiences in MotoStudent
competition [RB72];
-in Front (and Rear) Suspension subassembly are considered both the Fork (or Rear
Shock Absorber) [RB71] and front (or Rear) Rim;
-in Transmission subassembly the only one component for which is premature to talk
about costs is the Outer Casing. The other prices (Final Gears, Chain, Intermediate
Gears and Shaft) are fixed after some research.
Subsystem Cost [β¬]
Entry Fee 3140+
Accessories
Battery 5000
Brakes 450
Chassis
Fairing 600+
Front Suspension 2700+
GLVS TBA
HVS TBA
Rear Suspension 1200+
Transmission 500+
87
CONCLUSIONS
For the 6th edition 49 teams confirm their signup to participate in βElectricβ category,
almost the double respect previous edition. In this year the number of Rookie Teams
increased drastically and some of them have an interesting history in βPetrolβ
category. There are reasons to think that these elite teams from βPetrolβ category
will not belong to Class 4 but have the capability of design a Class 2 (or even Class 1)
motorcycle. 2wheelsPolito is in the same position and hard work is required to shine
in these conditions.
On 1st chapter, Benchmark analysis gave as result the necessity to maintain the
prototype simple, with high and forward centre of gravity, with a small caster angle
(the possibility to vary this value can be taken into account designing bushing inside
the inserts of the steering tube [RB58]), and a wheelbase higher than 1300 mm. High
consideration will be given to preparation of MS1 documentation and Innovative
solutions.
On 2nd chapter, the author introduced the BS, studying basics of battery, state of the
art of technology and control methodologies. A script was created to estimate energy
requirements of the pack, after that it was possible to choose some cells from
catalogues. The solution gives an LCO or NMC BP, potentially under 40 kilograms and
an installed energy around 8 kWh. Matching with BTMS, Inverter, and electric scheme
related to HVS and GLVS are still under definition and will be done in collaboration
with local suppliers.
3rd chapter is dedicated to Transmission Design and Design Hypothesis: Transmission
chosen consists in a gear train and an intermediate shaft and it is dimensioned to
reach speed higher than 190 km/h. The final stage is dimensioned assuming the final
ratio and calculating static and dynamic forces on chain. Design Hypothesis is done
having in mind the desire to maintain a comfortable driving for the rider, together
with high and forward centre of gravity. A central attention is required to chassis
design, which will require a lot of work in the future for what concern design,
dimensioning and validation.
88
Then, in the last chapter, Product Architecture is defined and Make or Buy strategy is
set, considering that βMakeβ capabilities of the team are limited and so it will focus
mainly on Design aspect before to develop Manufacturing in collaboration with local
suppliers. The final result is the BOM composed by 131 labels, accompanied by an
approximate cost analysis. Cost analysis requires further investigations (and the
setting of a sub-team involved in this aspect) not only to purely economic reasons,
but also for its importance on MS1 scores.
89
REFERENCES