1.0 TOPICBody design of EMoC car (GT car)

2.0 OBJECTIVETo design the body of EMoC car and analyze the body for better performance and

practicality. The objective also to fabricate, if possible, the body and shape of the car.

3.0 BACKGROUNDSupervisor had assigned me to design the new body shape of EMoC car of

Perodua Eco Challenge. This car had used by UiTM team on 2011. I have to redesign

and improve the design in order to reduce the drag and/or weight. Basically, weight of

this car is around 300 kg and the drag is 0.48. For me, I have to design GT car concept.

Grand Tourer (GT) cars are high-performance luxury car designed for long

distance driving. The term derives from the Italian phrase Gran Turismo, homage to the

tradition of the grand tour which able to make long-distance and high-speed journeys in

comfort and style. Grand Tourers differ from standard two-seat sport cars in typically

being engineered as larger, heavier and emphasizing comfort over straight-out

performance. Most GT cars have been front-engined with rear-wheel drive which

creates more space for the cabin than mid-mounted engine layouts. Softer suspensions,

greater storage and more luxurious appointments had been added to their driving

appeal. Some of very high-performance grand tourers such as Aston Martin DB9,

Ferrari 599 GTB Fiorano and Mercedes-Benz SLR McLaren make various compromises

in the opposite direction while rivaling sport cars in speed, acceleration and cornering

ability and earning them special designation, supercars. There are many variations of

GT cars such as GTO, GTS, GTB, GTV and many more. GTS (Gran Turismo Sport) is

four door saloon GT cars. GTO (Gran Turismo Omologato) are homologated cars for

racing and many more.

Parametric Study

Detail Design

Fabricate

Minor Modification

Wind Tunnel Test

Premilinary Design

4.0 METHODOLOGY

In order to complete this project, I have to do some parametric study. This

parametric study is to gather information about GT cars. I have to do some research

about GT cars that I can gather from the internet. All of the data we gathered, I should

interpret into graph form.

Then, I have to do some preliminary design. This section, there are some

calculations, estimations and approximation of the car regarding design parameter and

characteristic of the body shape and shell design.

After that, I should do the wind tunnel test. This test needs me to draw the car by

using the Computer Aided Design (CAD) program. Then convert this into machine

compatible codes to fabricate the model by using Rapid Prototyping (RP) machine.

Then, test the model with wind tunnel test and this result will compare with

Computational Fluid Dynamic (CFD).

The minor modification to the preliminary design in order to improve the design

for strength and must based on calculation and reasons. The detail design is needed to

estimate strength of certain body part for practicality. Basically, this section is for

improvement of the structures of the body part. If possible, I will fabricate this whole or

partially parts of the car.

5.0 INTRODUCTIONThroughout the history of the motor car have been individual vehicles that have

demonstrated strong aerodynamic influence upon their design. Until recently their

flowing lines primarily a statement of style and fashion with little regard for economic

benefits. It was only raising fuel prices, triggered by the fuel crisis of the early 1970s that

provided the serious drive towards fuel-efficient aerodynamic design. The three primary

influences upon fuel efficiency are the mass of the vehicle, the efficiency of the engine

and the aerodynamic drag. The aerodynamic design will be considered in this topic and

it is very important to recognize the interactions between all three since it is their

combined actions and interactions that influence the dynamic stability and the safety of

the vehicle.

The engineers and designers of the early car realize that the powerful engine are

not enough to improve car performance but they come up with the idea of lightweight

body of the car. As early as 1903 we find for example the pioneer French car

manufacturer, Panhard et Levassor, offering an optional lighter weight all-aluminium

body albeit at an extra cost to the customer. However, soon, this type of lighter weight

body construction suffer an declination due to demise of the separate steel chassis

frame on which it was built and the widespread adoption of the all-steel integral body

construction. There were also trials to extensive the use of the aluminium alloy but it

rather less commercial than technical success. Nowadays, through Audi Space Frame

(ASF) form, the situation may change. This introduced by Audi in 1994 due to

construction for their luxury high-performance A8 and it is comparable in term of both

weight and fuel economy to a mid-range saloon of conventional steel construction.

Engines in the 1950s and 1960s were often oversized, having many more cubic

centimeters or liters than necessary. At low speed, this type of engines resulted less

efficiency and fuel-wasted engines. Large engines do not produce as much power as

smaller engines for the amount of fuel consumed especially in urban areas. So, the

modern engines have been reduced in term of size in order to improve the efficiency.

But, in this topic, we will not discuss about the engine of the vehicle.

One of the major factors affecting fuel mileage is the amount of engine power

used to overcome wind resistance at all speed. All modern bodies have good

aerodynamic shapes which allow air to move and reduce the wind resistance or drag.

Basically, fewer horizontal surfaces and more curved and rounded surfaces will reduce

the drag of a car.

6.0 LITERATURE REVIEWThe term aerodynamic may be simply defined as science of air in motion. So, we

might expected it first serious study is connected to early airplane and airships. On the

introduction was mentioned that one of the factors that influenced the motion of a

vehicle was air drag. This assumes increasing importance the faster a vehicle is

travelling because of the force that opposing motion due to air resistance varies as the

square of the speed. So, as soon as the car began to increase the speed, the engineers

became aware that energy-consuming factor and slowly tried to changes in body

contour in order to make the air disturbed as little as possible. So, they began to adopt

less box-shaped into more curved cars to possess a certain degree of streamline which

promote a steady airflow with no additional air crossing its path. Later than, they

designed sloped hoods and windshield, rounded surface and fewer horizontal surfaces

in order to reduce drag.

Figure 1: Older vehicles had flat surfaces and sharp corners, which disrupts air flow. B-Aerodynamic body designs reduce wind resistance at high speeds, reducing the amount of power needed to move the vehicle and increasing fuel economy.

The underside of the vehicle has been improved from an aerodynamic

standpoint. In term to reduce wind resistance, they smoothened the underside surfaces

of the car as smooth as possible. To increase the stability, the shapes of the body also

designed to manipulate the air to hold down the vehicle. Nowadays, all the new body

designs will undergo testing and computer simulation to produce the wind resistance as

little as possible.

Figure 2: Schematic airflow around a passenger car

HypercarThe limited use of carbon fibre composites has become evident in actual

production car but this is regularly mentioned in guise of the Rocky Mountain Institute’s

(RMI) hypercar design concept. This combined both ultra-light and ultra-low-drag car.

From the RMI paper show afew prediction between hypercar nowadays and mid 90’s

car could achieve three times better fuel economy. They also show that the hypercar

have lower mass, aerodynamic drag and rolling resistance which are 63%, 55% and

65%, respectively. Those above attributes hypercars would employ composites that

embedded reinforcing fibres in a polymetric matrix. Materials experts from various

carmakers estimate that an all-advanced-composite autobody could be 50-67% lighter

than currently similar sized steel autobody.

In addition, the secondary weight savings result from the better performance,

allowing frugal use of those materials combined with less capital intensive

manufacturing and assembly and help overcome the cost per kilogram premium over

steel.

DragForm drag. Form drag is basically derived from shape of the vehicle body. This

represents the main source of aerodynamic drag which visually impressed through the

airflow over the streamlined body of the car. As a vehicle moves forward the motion of

the air around it gives rise to pressure that vary over the entire body surfaces. If a small

element of the surface area is considered then the force component acting along the

axis of the axis of the car depends upon the magnitude of the pressure, the area which

it acts and inclination of surface elements. So, it is possible to have a different values of

form drag when two cars each having different design but similar frontal area. The

coefficient of drag, Cd, is used to measure the drag produced by car produced.

Figure 3: Lift, drag, side force and moment axes

Surface drag. As we know, air is also categorized as fluid. In this form of drag, air

will form as a fluid which tends to adhere itself to the surface. Between the surface of

the moving body and the airflow, there’s a thin boundary layer of air and the air

molecules closest to the surface tend to adhere to the surfaces. The theory is the air

velocity is zero at the surface to its local maximum in some distance from the surface.

This sets up a shearing action in the air near the body surfaces which known as skin

friction. As a result, energy is absorbed from the motive power of the car and dissipated

in the form of heat. This is the evident that the aerodynamic drag force is mainly

depends on the surface area of the body.

Figure 4: The force acting on a surface element

Interference drag. Some of these features individually create only small drag

forces their summative effect can increase overall drag up to 50%. The interaction

between the main flow and flows about external devices can further add to the drag.

Further to reduce interference drag attempts are also being made to provide a smoother

underbody structure of the car and other components wherever this is practical.

Lift induced drag. A consequence of the constraints imposed by realistic

passenger space and mechanical design requirements is the creation of a profile which

in most situations is found to generate a force with a vertical component. To overcome

this problem in the modern high-performance cars, laterally mounted air deflectors

known as spoilers and air dams. A sophisticated application of spoiler and air dams is

that found in the Mitsubishi called Action Aero System where automatically deflect the

air dam is deflected downward and increase angle of attack of the spoiler when the car

reach speed of 80 km/h. on the other hands, the air dam will retract and spoiler will

return to its original position when the speed falls to 50 km/h. This practically to

constantly maintain the Cd value of the car.

Excrescence drag. This type of drag is largely due to practical requirements

which disturb the smoothness of the surface of the car and which generate energy

absorbing eddies and turbulence. Obvious contributor includes wheel, wheel arches,

wing mirrors, door handles, rain gutters and windscreen wiper blades yet exhausted

system are also major drag sources.

Internal drag. The last of the major influences upon vehicle drag is that arising

from the cooling of the engine, the cooling of other mechanical components such as

brakes and from cabin ventilation flows. This is because the air passing through them is

relatively slow and its entry and exit points have been deliberately sited to take

advantage of the air flow over the body.

Drag reductionThe broad requirements for low drag design have been long understood. Recent

trends in vehicle design reflect the gradual and detailed refinements that have become

possible both as a result of increased technical understanding and of the improved

manufacturing methods that have enabled more complex shapes to be produced at an

acceptable cost. There were researched that shows that the screen rakes will affect the

drag coefficient, Cd. The research demonstrated the benefits of shallow screens but the

raked angles desired for aerodynamic efficiency lead to problem not only of reduced

cabin space and driver headroom but also to problem of internal, optical reflections from

the screen and poor light transmission. Such problem can be overcome by the use of

sophisticated optical coatings similar to those widely used on camera lenses but as yet

there has been little use of such remedies by manufacturers. Figure below demonstrate

the benefits that may achieved by changing bonnet slope and the screen rake (based

on data of Carr (1968)).

Figure 5: Typical static pressure coefficient distribution

Figure 6: Drag reduction by changes to front body shape

The airflow over the rear surfaces of the vehicle is more complex and the solution

required minimizing drag for practical shapes are less intuitive. Research found that

squareback shapes car is characterized as large low pressure wake. The airflow is

unable to follow the body surface around the sharp, rear corner. Drag which associated

with such flows is depends on the cross-sectional are of the tail, pressure acting on the

A B

body and energy absorbed by the creation of eddies. This governed largely by the

speed of the vehicle, height and width of the tail. There was also research about change

of Cd due to change of angle of the backlight. The research showed that when the

backlight angle increases from zero to 15° (from horizontal) there is slight change of the

drag coefficient. This is due to decrease of the effective area. If we increase the angle to

nearly 30°, the Cd became smaller. This sudden drop corresponds to the backlight angle

at which the upper surface flow is no longer able to remain attached around the

increasingly sharp top, rear corner and the flow reverts to a structure more akin to that

of the initial squareback.

Figure 7: A-Squareback large scale flow separation, B-Hatchback/fastback vortex generation

There were cars that influenced by all of the flow phenomena that have been

discussed for the form discussed above. The inclination of the screen may be sufficient

to cause the flow separate from the rear window although in many case the separation

is followed by re-attachment along the boot lid. Research has shown that in this

situation, the critical angle is not that of the screen alone but the angle made between

the rear corner of the roof and the tip of the boot. This suggests that the effect of the

separation is to re-profile the rear surface to something approximating to a hatchback

shape. In order to achieve minimum drag condition, the backlight angle of 15° it is

necessary to raise the boot lid. There some benefits to this practice which increase the

luggage space but the rearward visibility is reduce. Rear end, boot-lid spoilers have a

similar effect without the associated practical benefits. The base models produced by

AB

most manufacturers are usually designed to provide the best overall aerodynamic

performance within the constraints imposed by other design considerations and the

spoiler that feature on the upmarket models rarely provide further aerodynamic benefit.

Figure 8: A-The influence of backlight angle on drag coefficient, B-high tail, low drag design

Sides of the car also can help reduce the drag. The most effective way to reduce

drag is adoption of boat-tailing design technique. This way can reduce the cross-

sectional area of the rear section of the car and hence reduced the volume enclosed

within the wake. For more extreme condition, we can design the tail extending to the

fine point which can eliminate the wake flow but this will increase surface friction drag

and the pressure on the extended surfaces may also contribute to the overall drag. This

also will affect the aerodynamic efficiency will reduce a little.

Figure 9: Boat tailing: reduce wake

Despite the effort to smooth the visible surfaces, it is recently that serious

attempts have been made to smooth the underbody. But there are many considerations

to be made to achieve this target. As example, there is such as access to maintenance,

clearance for suspension and wheel movement and many more. Surprisingly,

sometimes, larger air dam that is fitted to the production car can help reduce the drag.

The air dams have two useful functions. Firstly, to reduce the lift force acting on the

front axle by reducing pressure beneath nose this accelerates with a corresponding

drop on pressure. Secondly, for the passenger cars a neutral desirable to maintain

stability without excessive increase in the steering forces required at high speed. For

high speed car, the air dam will create significant aerodynamic downforce to increase

adhesion of the tyres. But, there are some negative impacts which are will increase drag

and extreme steering sensitivity.

Figure 10: Simple comparison between the profile of a car and an aerofoil section for lift induced drag (R-resultant aerodynamic force, L-lift force, D-drag force).

Stability and cross-windsThe aerodynamic stability of cars addressed as two independent concerns which

are the ‘feel’ of the car and effects of steady cross-winds and transient gusts associated

to atmospheric conditions. The ‘feel’ of the cars is mean by the calm condition and

maneuverability of the car in a straight line and high speed and also when lane change.

The effect of steady cross-winds due to atmospheric condition may be exaggerated by

local topographical influences such embankments, bridges and others.

Figure 11: Rear, underbody diffusion

The most difficult thing to prove in aerodynamic influences is the source of the

instability in calm condition. This due to complex interaction between chassis dynamic

and small changes magnitude of lift force and centre of pressure. There some evident

suggested that stability degrade with increase in the overall lift and with different lift

between the front and rear axles.

Cross-winds effect is easier to quantifiable. This effect is rarely present as safety

hazard but their effect on drag and noise is considerable. Most of the new vehicles will

be model tested under yawed conditions in wind tunnel but optimization for drag and

wind noise is always based upon zero cross-wind assumptions. If we able to obtain or

estimate the average yaw angle experience of a country then there is a strong case for

optimizing the aerodynamic design for that condition.

Influence of transient cross-wind gusts often experience when you overtaking

heavy vehicle or passing bridge abutments. In order to reduce the problem, it is

desirable to design cars that able to minimize side forces, yawing moments and yaw

rate that occur as the vehicle is progressively and rapidly exposed to the cross-wind.

Basically, rounded shapes vehicles able to overcome this type of problem. It is

necessary to built and develop wind tunnel model that provide accurate and reliable

data. Engineers also have to understand the mechanism that give rise to the

aerodynamic forces and moments. Initial results from the recent developments in wind

tunnel testing suggested that the side forces and yawing moments experienced in true

transient case exceed that have been measured in steady state yaw tests.

Aerodynamic research initially focused upon drag reduction but it soon became

apparent that the lift and side forces were also of great significance in terms of vehicle

stability. An unfortunate side effect of some of the low drag shapes developed during

the early 1980s was reduced stability especially when driven cross-wind conditions.

Cross-wind effect are now routinely considered as highly complex and often unsteady

flows that associated with the airflow over passenger cars but the figure of this effects is

still remain sketchy. In order to understand this flow, the experimental techniques and

computational flow prediction methods still required some development and

improvement.

Noise and ventilation systemsSome aerodynamic noise is created by ventilation flows through the cabin the

most obtrusive noise is generally that created by the external flow around the vehicle.

The creation of aerodynamic noise is mostly associated with turbulence at or near the

body surface and moves to reduce drag and inevitably provided the additional benefits

of noise reduction. Improvements of rain gutters, positioning of windscreen wipers

reflect, improve manufacturing technique and quality control and improve panel fit have

resulted in major noise reduction. Open windows can create noise problems also.

Increased use of air condition is the best practical solution to this problem.

Figure 12: Noise sources (Piatek, 1986)

Underhood ventilation. Numerous researchers suggest that the engine cooling

system is responsible for 10 to 15% of the vehicle drag. Basically, the cooling drag has

been determined from wind tunnel drag measurements with and without the cooling

intakes blanked-off. We should know that closure of the intakes may alter the entire

flow-field around the car. Many of the sources of cooling drag are readily apparent. In

general, any smoothing of the flow path will reduce the drag, as will velocity reductions

by diffusion upstream from the cooling system. Less obvious but also significant is the

interaction between the undercar flow and the cooling flow at its exit where high

turbulence and flow separations may to occur.

The reduction of underhood drag is the greatest if the airflow can be controlled by

the use of the ducting guide the air into and out from the radiator core. The high

blockage of the radiator core has the effect of dramatically reducing the air velocity

through the radiator thus much of the air that approaches the radiator spills around it.

Small mass flow passes the core exhibit substantially non-uniformity that can reduce the

effectiveness of the cooling system. This can be solving if the flow is ducted into radiator

in controlled and efficient manner. Increase the diffusion slows the air flowing through

radiator which reduce drag and heat transfer. At low speed, large core area creates less

drag for given heat transfer rate. Too much diffusion will lead to flow separation within

the intake which may result in severe flow non-uniformities across the face of the

radiator.

Cabin ventilation. Sealing between the body panels and particularly around the

doors has achieved benefits in terms of noise and drag reduction. In order to achieve

the required ventilation flow rates, we must more focus on intake and exit locations and

velocity and path of the fresh air through the passenger compartment. The intake

should located in a zone of relatively high pressure and it should not be too close to the

road surface where particulate and pollutants level tend to be highest. The region

immediately ahead of the windscreen is perfect for the air entry to the passenger

compartment and air conditioning system. The most effective extraction is at a zone of

lower pressure to be sought which at the rear of the vehicle. Increasing the pressure

difference between the intake and exit will give high ventilation air flow rates but only at

a flow rate that is sensitive to the velocity of the vehicle. A recent trend has been to use

relatively low pressure differences coupled with a greater degree of fan assistance to

provide a more controllable and consistent internal flow whether for simple ventilation

systems or for increasingly popular air conditioning systems.

Unibody constructionOlder cars used a separate body and frame in their construction. This type of

design provided great strength but also increased the vehicle’s overall weight. Most car

today use unitized body or unibody construction in which the frame and body

assemblies are stamped from the same piece of sheet metal, welded together or by the

combination of both techniques. The design eliminates the need and weight of a

separate frame and attaching hardware while retaining the strength and rigidity of the

heavier body. Unitized bodies are less likely to develop rattles. However, there are more

prone to transmit noises from other parts of the vehicle. Light trucks, most sport utilities

vehicles and some vans continue to use separate frames and becoming smaller and

lighter than they were earlier.

Figure 13: unibody construction allows a lighter overall body weight than the separate frame and body design. Putting the frame and the body into one unit makes the body more rigid, increasing its strength.

Material: AluminiumBasically, the attraction of aluminium is based on their characteristic which is low

density. The advantage of the aluminium is also can be resistant corrosion. In addition,

they got strong supply base and can be recycle. Based on consideration structures or

sub assemblies to steel, the weight of aluminium can be approximately halved but the

cost is doubled. Although the density is one-third of steel, the full down weighting

potential cannot be realized as the modulus of aluminium is considerably lower than that

of steel and as stiffness is a primary influence for the design of the most body parts

some compensation must be made and thickness increased. The cost for aluminium is

affected by commodity market and for planning purpose, some means of stabilizing

future costs by buying ahead or alternative strategy must be considered. The

characteristic of the aluminium made it to become poorer formability and less readily

welded compared to steel.

Material: Advanced compositesHigh performance advance carbon graphite epoxy. More that 95% of the

McLaren F1’s body is constructed in this material. This material consists of carbon

embedded with partially cured resin. Then, it will laminate and curing take place in an

autoclave programmed with two cure cycles. The advantages of this type of technology

is that it allows the engineer to control the properties, including stiffness and strength in

three dimension and can be develop the characteristics exactly where he wants. Plus,

the maximum efficiency is obtained from each gram. The big advantage is the strength

to weight ratio is impressive and it is claimed that the same tensile strength as steel is

obtained but at one-quarter of the mass. Conjunction of the honeycomb panels with this

material can also create a strong and stiff assembly.

SP Resin Infusion Technology (SPRINT). This material consists of two layer of

dry fiber reinforcement of a side of a precast and precatalysed resin film. This type of

material provides an alternative approach which far less labor intensive than others

materials. The nature of the product allows good consolidation and integrity and flow of

the material allows shape control even into corners without entrapment. Gel coats can

be applied to give the desired finish.

Aluminium Structured Vehicle Technology (ASVT). Stiffness of the aluminium

structure can be greatly improved by the use of a structural adhesive rather than spot

welding and it was proved in tests that torsional stiffness levels approaching those of

spot welded steel could be produced at half the weight with alloy sheets. In order to

guard against peel failures in impact it was necessary to use toughened epoxy plus

some spot welds in flanges. However, to ensure durability under service conditions it is

necessary to pretreat the aluminium sheet and the selection of a suitable formulation

followed only after extensive accelerated tests.

Ultra-Light Steel Auto Body (ULSAB). At the time of increasing worldwide interest

in aluminium bodied cars, ULSAB initiative was launched to re-emphasize the versatility

of steel and introduce new ideas to future weights saving that could be achieved. By

using the high strength steels, hydroformed sections and sheet hydroforming, tailor

welded blanks and alternative assembly methods, it was shown that the body weight

can reduce to 25%, torsional rigidity improved to 80%. The static bending and first body

mode increase by 52% and 58%, respectively.

Refferences:

1. Geoff Davies, “Materials for Automobile Bodies”, Elsevier Ltd., 1st Edition, 2003.

2. Julian Happian-Smith, “An Introduction to Modern Vehicle Design, Reed

Educational and Professional Publishing Ltd., 1st Edition, 2002.

3. M J Nunney, “Light & Heavy Vehicle Technology”, Butterworth-Heinemann, 3 rd

Edition, 2002.

4. Chris Johanson, “Auto Engine Performance and Driveability”, The Goodheart-

Willcox Company Inc., 2004.

5. SAE International, “Advances in Lightweight Materials: Casting and Aluiminium

and Achieving Lightweight Vehicles”, 2007.

6. http://en.wikipedia.org/wiki/Grand_tourer