cacat pansat
DESCRIPTION
CACATRANSCRIPT
Two-Stroke Marine Diesel Engine Variable Injection Timing System Performance Evaluation And Optimum Setting For Minimum Fuel Consumption At Acceptable
NOx Levels
Dimitrios T. Hountalas*1
E-mail: [email protected]
Spiridon Raptotasios1
E-mail: [email protected]
Antonis Antonopoulos1
E-mail: [email protected]
Stavros Daniolos2
E-mail:
Iosif Dolaptzis2
E-mail:
Maria Tsobanoglou2
E-mail:
m.tsobanoglou@ minervamarine.com
1 National Technical University of Athens, School of Mechanical Engineering, Internal Combustion Engines Laboratory, Heroon Polytechniou 9, Zografou Campus 15780 Athens, Greece
2 Minerva Marine Inc, 141-143 Vouliagmenis Avenue Voula, 16673 Athens, Greece
ABSTRACT Currently the most promising solution for marine propulsion is
the two-stroke low-speed diesel engine. Start of Injection (SOI) is of
significant importance for these engines due to its effect on firing
pressure and specific fuel consumption. Therefore these engines are
usually equipped with Variable Injection Timing (VIT) systems for
variation of SOI with load. Proper operation of these systems is
essential for both safe engine operation and performance since they
are also used to control peak firing pressure.
However, it is rather difficult to evaluate the operation of VIT
system and determine the required rack settings for a specific SOI
angle without using experimental techniques, which are extremely
expensive and time consuming. For this reason in the present work it
is examined the use of on-board monitoring and diagnosis techniques
to overcome this difficulty. The application is conducted on a
commercial vessel equipped with a two-stroke engine from which
cylinder pressure measurements were acquired. From the processing
of measurements acquired at various operating conditions it is
determined the relation between VIT rack position and start of
injection angle. This is used to evaluate the VIT system condition and
determine the required settings to achieve the desired SOI angle.
After VIT system tuning, new measurements were acquired from the
processing of which results were derived for various operating
parameters, i.e. brake power, specific fuel consumption, heat release
rate, start of combustion etc. From the comparative evaluation of
results before and after VIT adjustment it is revealed an improvement
of specific fuel consumption while firing pressure remains within
limits. It is thus revealed that the proposed method has the potential
to overcome the disadvantages of purely experimental trial and error
methods and that its use can result to fuel saving with minimum effort
and time.
To evaluate the corresponding effect on NOx emissions, as
required by Marpol Annex-VI regulation a theoretical investigation is
conducted using a multi-zone combustion model. Shop-test and NOx-
file data are used to evaluate its ability to predict engine performance
and NOx emissions before conducting the investigation. Moreover,
the results derived from the on-board cylinder pressure
measurements, after VIT system tuning, are used to evaluate the
model’s ability to predict the effect of SOI variation on engine
performance. Then the simulation model is applied to estimate the
impact of SOI advance on NOx emissions. As revealed NOx
emissions remain within limits despite the SOI variation (increase).
Keywords: Two-stroke marine diesel engine, Diagnostic technique,
Variable Injection Timing (VIT), Multi-zone combustion model, NOx
emissions, engine performance, brake specific fuel consumption.
NOMENCLATURE A Area (m2)
adel Ignition delay constant
cr Radiation constant (W/m2K4)
cv Specific heat capacity under constant volume (J/kg K)
D Cylinder bore (m)
f Number of cycles per second
hc Heat transfer coefficient (W/m2K)
k Turbulent kinetic energy (J)
kith Forward reaction rate constant for the “ith” reaction
ṁ Mass flow rate (kg/s)
m Mass (kg)
p Pressure (N/m2)
Q Heat (J)
Ri One way reaction rate for the “ith” reaction
Spr Integral value in ignition delay correlation
t Time (s)
T Temperature (K)
V Volume (m3)
Wi Indicated power (W)
Proceedings of the ASME 2014 12th Biennial Conference on Engineering Systems Design and Analysis ESDA2014
June 25-27, 2014, Copenhagen, Denmark
ESDA2014-20528
1 Copyright © 2014 by ASME
z Number of cylinders (-)
Abbreviations
CA Crank angle
IMEP Indicated Mean Effective Pressure
IMO International Maritime Organization
LHV Lower Heating Value
MCR Maximum Continuous Rating
rpm revolutions per minute
SMD Sauter Mean Diameter
SOI Start of Injection
TDC Top dead center
VIT Variable Injection Timing
Greek Letters
δrc Equivalent cylinder ring clearance (m)
εt Viscous dissipation rate per unit mass (W/kg)
θ Crank angle (deg)
φeq Equivalence ratio
Subscripts
cumul Cumulative
f Fuel
g Gas
gros Gross
hl Heat loss
net Net
w Wall
INTRODUCTION The major issues that have to be addressed for the large-scale
two-stroke diesel engines, which are the primary solutions for marine
vessel propulsion and for power generation (in specific applications),
are reduction of fuel consumption and NOx emissions. A common
technique, which is extensively used in these engines, to achieve
greater fuel economy is Variable Injection Timing (VIT). The VIT
system is used to modify Start of Injection (SOI) in order to control
the maximum combustion pressure and achieve optimum
performance. Thus, proper operation of VIT system, is of significant
importance for two-stroke diesel engines, since it can result to
reduced bsfc and optimized engine operation at a wide operating
range, while maintaining at the same time NOx emissions at
acceptable levels [1].Variable Injection timing is also used for small
adjustments to consider for the effect of fuel oil quality [2].
A typical two-stroke low-speed marine diesel engine is
optimised for operation in the region of 80% to 85% of full load [2-
4]. Currently, due to the global financial slowdown, the marine
industry is looking into methods for operating cost and mostly fuel
consumption reduction without negative environmental impact.
Towards this direction it has been introduced the slow steaming
concept [5]. However, marine diesel engines are not designed to
operate in this region and VIT can become a useful tool to improve
performance.
For most applications the VIT system is adjusted so that start of
injection is kept constant at low load (usually up to 40% of full load)
and then advances up to ~ 85% load from where it starts to reduce to
avoid excessive peak firing pressures [6]. As already mentioned the
load where the engine is designed to have its minimum bsfc and its
peak firing pressure (~85% of full load) is the “breakpoint” i.e. the
load where the VIT system is set up to start retarding the start of
injection. For this reason there is no significant change in the
maximum combustion pressure in the region of 85% to 100% of full
load. In Figure 1, it is depicted the typical adjustment of the VIT
system and the corresponding variation of peak firing pressure [6].
Figure 1 Variation of VIT index and maximum cylinder pressure
for the entire load operating range [6].
Variable Injection Timing can be achieved either by a
Mechanical-Pneumatic (older) system, or by an Electro Pneumatic
system which is implemented in newer engine designs. The essential
difference between the two systems is the use of the “breakpoint” for
the pressure rise control. For the mechanical system the “breakpoint”
is fixed, while for the electrical VIT system it is variable and is
controlled from the scavenge pressure. For high scavenge pressure
and as a result high compression pressure, the maximum combustion
pressure can rise above the design point. Therefore the “breakpoint”
must be shifted to a lower load point. On the contrary, for low
scavenge pressure the breakpoint can be shifted to higher load, for the
optimum peak firing pressure to be achieved [6].
The optimal tuning of the VIT system is essential to obtain the
required SOI setting for optimum performance. The use of common
experimental methods (trial and error, coupled with continuous
measurements) on large-scale diesel engines to achieve this requires
significant effort and time, which results as well to significant cost.
Towards this the combination of experimental and computational
techniques can become a useful tool that can be applied efficiently
with minimum effort.
Therefore, the results from a technique which has been
developed and applied on a two-stroke diesel engine are presented in
the present study. The SOI was adjusted through the VIT system
using cylinder pressure measurements acquired at sea, under actual
conditions. The cylinder pressure measurements were processed
using an existing, well validated, diagnostic technique [7,8], from
which -beyond diagnosis and tuning- results are generated for various
operating parameters such as brake power, specific fuel consumption,
heat release rate, start of combustion etc.
Primarily, the measurements acquired at various loads with the
existing VIT settings, were processed, to determine the condition of
the engine and the setting of the VIT system. Using this methodology
the relation between VIT rack position and start of injection angle
(referred to as VIT scale) was determined and used to make proposals
for optimum tuning and/or adjustment of individual cylinders. This is
extremely important since it is made easy to define the required VIT
rack adjustment to achieve a requested SOI angle without use of trial
and error techniques. After this the VIT setting was modified and
measurements were repeated at nearly the same operating conditions.
The results before and after the VIT system modification were
evaluated and revealed a potential fuel saving without peak firing
pressure exceeding the design limits. The investigation also revealed
improper operation of the VIT system on one engine cylinder.
However, for two-stroke marine diesel engines the reduction of
bsfc is affected by the well-established tradeoff between bsfc and
2 Copyright © 2014 by ASME
NOx [3,4]. Injection advance decreases bsfc but at the expense of
NOx increase at the engine exhaust. Considering the NOx emission
regulations which are set by Marpol Annex VI [9,10], an additional
investigation is necessary to ensure that NOx emissions after VIT
system tuning will remain within limits.
In the present work this investigation is conducted using an
existing well validated multi-zone combustion model [11-16] initially
developed for high-speed DI diesel engines and modified to properly
describe the processes of the entire two-stroke operating cycle [17-
19]. For field applications it will be obviously required to finally
conduct emission measurements onboard vessel. Towards this
direction it is first demonstrated the model’s ability to predict the
engine performance and NOx emissions at various loads, using data
acquired from the engine shop-tests and NOx file. From the analysis
of results it is demonstrated that the simulation model manages to
predict NOx emissions adequately and for this reason it is used to
estimate the corresponding effect of SOI at the test conditions. The
target is to determine if the NOx values, after SOI advance via the
VIT system, remain within the IMO Marpol Annex-VI limits foreseen
for the specific engine design.
The results derived from the investigation indicate a clear fuel
saving potential, while at the same time both engine performance
(peak firing pressure) and NOx emissions are maintained within
limits. Moreover, it is revealed that the diagnostic technique can be
successfully used for evaluation of VIT system operation and
especially for optimum tuning to achieve minimum fuel consumption
at acceptable NO values. Furthermore, having defined the relation
between VIT index and SOI it is possible to conduct investigations
for optimum SOI setting if the engine will operate in the low load
region according to the slow steaming concept. As clearly shown this
is achieved with minimum effort and low cost which is promising for
field applications. Last but not least it is demonstrated the combustion
model’s ability to predict both engine performance and NOx
emissions of a two-stoke marine diesel engine at various operating
conditions and SOI settings which is promising for its use as a tool
for engine development.
BRIEF DESCRIPTION OF THE DIAGNOSTIC TECHNIQUE
The proposed diagnostic technique is based on the processing of
measured cylinder pressure data using a two-zone modeling approach
for the corresponding thermodynamic calculations. The two-zone
modeling, which is based purely on thermodynamics, has been
extensively validated in the past. As mentioned, in the present study
the diagnostic technique is used as a tool to determine engine tuning
and performance at the present conditions and then investigate the
possibility for fuel saving via SOI optimum adjustment. Furthermore,
using the diagnostic technique, it is determined the relation between
VIT index and SOI angle which is important for VIT performance
evaluation and optimum SOI setting. For this reason it is required to
determine the following parameters:
To evaluate the results it is necessary to estimate and compare
the fuel consumption of the engine before and after VIT adjustment.
However, it is difficult to acquire measurements on field with the
required accuracy especially when fuel consumption differences in
the region of 1-2% are involved. Another difficulty is that it is also
impossible to have exactly the same operation between the two test
conditions. For this reason it was decided to estimate and compare
the bsfc values which are derived from the diagnostic methodology.
For this reason it is given herein, only a brief description of the
diagnosis methodology and special focus to the methodologies for
TDC position determination, cylinder power estimation and
determination of fuel oil consumption form the heat release rate.
Having determined the TDC it is also possible to estimate the ignition
angle and from this to derive an estimate for the SOI angle which is
used to determine the VIT scale and the SOI setting of engine
cylinders enabling proper VIT adjustment.
Determination of TDC Position A significant advantage of the present technique which makes it
suitable for field applications is that it does not require measurement
of TDC position, which is a time consuming procedure with specific
difficulties. The precise determination of the TDC is a crucial
parameter, since an error results to an incorrect pressure diagram and
to significant errors on several thermodynamic calculation results
such as IMEP (Indicated Mean Effective Pressure), indicated power
etc. [20 - 23]. For example an error of 0.5 CA deg on the estimated
timing of TDC, results to a fault in the calculated indicated engine
power of up to ~4%. For this reason it is employed a thermodynamic
methodology for TDC estimation developed by the authors in the past
and extensively validated by both lab and field measurements. TDC
estimation is based on the processing of the measured compression
part of the pressure diagram which is then compared to the calculated
one using the embedded simulation model mentioned above. TDC
position is estimated when the difference between the two curves is
minimized using a special constants determination methodology
(multi-parameter determination) where TDC is considered to be an
additional unknown constant together with the main parameters that
affect the compression stroke i.e. effective CR, initial pressure at
exhaust valve closure, blow-by and heat transfer. The error of the
proposed method is in the range of ±0.2 degrees of crank angle which
is adequate for accurate estimation of cylinder power and heat release
rate.
Estimation of Cylinder Brake Power The measured mean cylinder pressure trace is used to calculate
the indicated power output for each cylinder. The TDC position
determined from the aforementioned methodology is used to convert
the measured “p-t” signal into a “p-V” one from, the integration of
which it is derived the indicated power as follows [3]:
�̇�𝑖 = 𝑧 ∙ (∮𝑝𝑑𝑉) ∙ 𝑓 (1)
Then, using the mechanical efficiency, which is available from the
engine shop tests, the corresponding break power is determined.
Estimation of Cylinder Fuel Flow Rate For a multi-cylinder engine operating on the field, it is extremely
difficult to determine the fuel flow rate with the required accuracy. To
overcome this difficulty a method has been developed in the past to
estimate the fuel mass flow rate of each cylinder. The method is
based on the processing of the measured cylinder pressure diagram.
An estimate for the actual amount of fuel mass burned inside the
combustion chamber is obtained from the heat release rate analysis
procedure from the following [3,24]:
�̇�𝑓 =𝑄𝑔,𝑐𝑢𝑚𝑢𝑙
𝐿𝐻𝑉𝑓𝑢𝑒𝑙 (2)
where (LHVfuel) is the lower heating value of the fuel used and
(Qg,cumul) represents the cumulative gross heat release obtained from
3 Copyright © 2014 by ASME
the integration of the instantaneous gross heat release given from the
following relation: 𝑑𝑄𝑔𝑟𝑜𝑠
𝑑𝜃=𝑑𝑄𝑛𝑒𝑡𝑑𝜃
+𝑑𝑄ℎ𝑙𝑑𝜃
(3)
while the instantaneous heat losses to the cylinder walls are estimated
from: 𝑑𝑄ℎ𝑙𝑑𝜃
= 𝐴 ∙ [ℎ𝑐 ∙ (𝑇𝑔 − 𝑇𝑤) + 𝑐𝑟 ∙ (𝑇𝑔4 − 𝑇𝑤
4)] (4)
and the instantaneous mean gas temperature (Tg) is obtained from the
perfect gas state equation and the measured cylinder pressure value.
The mass of the cylinder charge is estimated from the simulation
model (open cycle simulation including gas exchange) using the
measured values of inlet pressure, temperature etc., while constant hc
and the corresponding cylinder wall temperature are obtained from
the model constant estimation procedure. The specific methodology
has been validated, by laboratory experiments and a great number of
field tests on both marine and stationary engines, and its accuracy is
in the region of ±1.5%. However when used on a comparative basis,
such as in the present investigation, where bsfc before and after VIT
system tuning is compared, the relative variation of values between
the two different test cases of the same engine is in the range of
±0.5%.
BRIEF DESCRIPTION OF THE COMBUSTION MODEL FOR NOx EMISSIONS ESTIMATION
As mentioned the model embedded in the diagnostic technique
is a two-zone which is adequate for performance studies. For NOx
emission studies and especially for the investigation of the SOI effect,
it is made use of a well validated multi-zone combustion model. The
specific model has been extensively applied and validated in the past
in a number of light and heavy duty DI diesel engine configurations
[11-16]. For the specific application modifications were necessary to
make its use possible on slow speed 2-stroke diesel engines [17-19].
The model is a three-dimensional multi-zone one where the fuel
jet is divided into zones using a concentric consideration as shown in
Figure 2a and Figure 2b. The number of axial direction depends on
the injection duration and the calculation time step used. In the
present work, five zones are used in the radial direction and eight in
the circumferential direction. Each zone has its own history of
temperature, composition etc., while the pressure inside the engine
cylinder is considered to be uniform. The condition inside each zone
is calculated from the first law of thermodynamics and the
conservation equations for mass and momentum. In the following
sections, it is provided a brief description of the sub-models used.
Figure 2.a Zone formation on the “r-z” plane normal to injection
direction
Figure 2.b: Zone formation on the “x-r” plane
Heat Transfer For the estimation of the characteristic velocity, necessary for
the heat transfer calculations, a turbulent kinetic energy viscous
dissipation rate k∼εt model is used [3,25,26]. Having determined the
characteristic velocity and the heat transfer coefficient the
instantaneous heat rate is obtained from eq. (4), where Tg is the bulk
temperature of the fuel jet defined by (where index k denotes the ‘kth’
zone of a total number n):
𝑇𝑔 =∑ 𝑚𝑘 ∙ 𝑐𝑣𝑘 ∙ 𝑇𝑘𝑛𝑘=1
∑ 𝑚𝑘 ∙ 𝑐𝑣𝑘𝑛𝑘=1
(5)
so that the heat exchange rate obtained from eq. (4) is then distributed
to the zones according to their mass, temperature and specific heat
capacity as follows,
𝛥�̇�𝑘 =�̇� ∙ (𝑚𝑘 ∙ 𝑐𝑣𝑘 ∙ 𝑇𝑘)
∑ 𝑚𝑘 ∙ 𝑐𝑣𝑘 ∙ 𝑇𝑘𝑛𝑘=1
(6)
The Jet Model After initiation of fuel injection, zones start to form and
penetrate inside the combustion chamber. The zone velocity along the
jet axis is obtained from correlations providing the penetration length
of the fuel jet inside the cylinder [3,27]. The zone velocity at the jet
periphery is estimated using the radial distance of the zones from the
jet axis. The effect of air swirl on zone velocity is also considered,
using the local components of air velocity in the radial and axial
directions and the momentum conservation equation on both axes.
From the previous considerations using momentum and mass
conservation it is estimated the position of each zone inside the
combustion chamber. After wall impingement the wall jet theory of
Glauert is used to determine the jet history on the cylinder walls [28].
Air Entrainment into the Zones
Momentum conservation is adopted to estimate the amount of air
entrained into the zones [11,12]. The effect of injection pressure
variation on the jet formation mechanism is taken into account by
considering the independent initial velocity of each zone, based on
the instantaneous injection and cylinder gas pressures.
Droplet Evaporation and Breakup The injected fuel is distributed to the zones according to the
instantaneous injection rate and inside each zone it is divided into
packages (groups) where the droplets have the same Sauter Mean
Diameter (SMD).Inside each zone a chi squared distribution is used
to describe the distribution of the fuel droplet diameter. For the
evaporation process the model of Borman and Johnson is followed.
z
r
(1,1)
(2,1)
(3,1)
(4,1)
(1,2)
(1,3)
(1,4)
(1,5)
(1,6)
(1,7)
(1,8)
OX
r
(1,2,3)
(1,1,3)
(1,3,3)
4 Copyright © 2014 by ASME
Ignition-Combustion Model Ignition commences after an ignition delay period which is
given by the following relation [3,29]:
𝑆𝑝𝑟 = ∫1
𝑎𝑑𝑒𝑙 ∙ 𝑃𝑔−2.5 ∙ 𝜑𝑒𝑞
−1.04 ∙ 𝑒𝑥𝑝 (5000 𝑇𝑔⁄ )
∙ 𝑑𝑡 = 1
1
0
(7)
where (φeq) is the local equivalence ratio of the fuel air mixture inside
the burnt zone, (Tg) is the local temperature in K, and (Pg) the
cylinder pressure expressed in bar. Constant (adel) depends on the
ignition quality of the fuel and mainly on its cetane number.
Having evaporated and mixed with the entrained air and the
existing combustion products the evaporated fuel is ready for
combustion after the ignition delay period. The amount of air
entering a zone mixes with the evaporated fuel and the
combustion rate, which is derived using an Arrhenius type
expression, depends strongly on local temperature and on the
concentration of O2 and evaporated fuel.
Cylinder Blow-by Blow-by has a significant effect on the compression and
combustion-expansion part of the pressure diagram [3,24]. According
to the simplified model approach, the blow-by rate is modelled
assuming an equivalent blow-by area between the cylinder rings and
the cylinder wall. The mass flow is then calculated using isentropic
compressible flow relations. The equivalent blow-by area A is equal
to:
𝐴 = 𝜋 ∙ 𝑑 ∙ 𝛿𝑟𝑐 (8)
where δrc is the equivalent cylinder-ring clearance that defines the
level of cylinder liner–ring wear.
Gas Exchange For the simulation of the inlet and exhaust manifolds and the
calculation of the mass exchange rate between them and the engine
cylinder, the method of filling and emptying is used, which provides
good results [3,27], for constant pressure turbocharging systems that
are used for large scale marine two stroke diesel engines. Provision is
taken in the present model to simulate also the operation of the
turbocharger and the air-cooler.
Scavenging Model Practically all slow-speed, marine diesel engines are two-stroke
turbocharged, thus the scavenging process is of great importance for their operation [3]. For this reason a two-zone scavenging model has been developed which during gas exchange divides the cylinder contents into two parts, one consisting of fresh entrained air and a second consisting of combustion products from the previous cycle and fresh entrained air.
One part of the amount of air entering the cylinder, at a certain time instant during intake, escapes to the exhaust manifold directly, while the remaining one enters the fresh air and combustion products zone. Likewise, during scavenging, the total amount of exhausted cylinder mass to the exhaust manifold is taken partially from the fresh air zone and the combustion products one [27]. At the end of the scavenging process perfect mixing between the two zones is assumed resulting to only one zone which is a mixture of fresh entrained air and combustion products from the previous cycle.
Nitric Oxide Formation Model In order to calculate the formation of nitric oxide inside each
zone, a chemical equilibrium scheme is used to calculate the concentration of various components under equilibrium conditions. Due to the very high temperatures existing inside the zones chemical dissociation takes place. Inside each zone the following eleven species are assumed to exist: O2, N2, CO2, H2O, H, H2, N, NO, O, OH, CO. NO formation is widely assumed to be a non-equilibrium process controlled by chemical kinetics. The most commonly used scheme for NO formation is the extended Zeldovich mechanism [1,3,24,27]. This mechanism is comprised of the reactions, along with their related forward and reverse reaction rate constants that govern NO formation. Finally the NO formation rate in each zone is expressed by the following differential equation:
1
𝑉∙𝑑([𝑁𝑂]𝑉)
𝑑𝑡=2 ∙ (1 − 𝛽2) ∙ 𝑅1
(1 + 𝛽 ∙𝑅1
𝑅2+𝑅3)
(9)
β = [NO] / [NO]e, where [NO] is the actual concentration and [NO]e
the corresponding equilibrium one inside each zone.
DESCRIPTION OF THE ENGINE AND THE OPERATING POINTS CONSIDERED
The experimental investigation was conducted on a commercial
vessel powered by a large-scale, two-stroke, six-cylinder, marine
diesel engine. The characteristic technical data of the engine are
summarized inTable 1.
Table 1 Engine specifications
Cylinder Bore 700 mm
Piston Stroke 2674 mm
MCR speed 91 rpm
MCR power 16860 kW
In Table 2 are shown the operating points considered. The first
three measurements were acquired, using the reference VIT setting.
Then after the analysis of derived results the VIT rack was adjusted to
a higher value compared to normal i.e. advanced SOI (measurement
No. 14 ~ 85% load)and then to a lower value compared to normal
(measurement No. 16 ~ 60% load).
Table 2 Test cases examined
Test
Case
Engine
Speed (rpm)
Load
(%)
Engine
Power (kW)
VIT
setting
1 85.6 86.0 14494 Normal
2 84.2 80.3 13547 Normal
3 76.2 59.6 10042 Normal
14 85.5 85.2 14363 Increased
16 75.4 58.1 9795 Decreased
Cylinder pressure measurements were acquired using a highly
accurate air-cooled transducer (Kistler 6613CP). For each
measurement 50 continuous operating cycles were recorded using a
sampling rate of 0.5 deg CA. Measurements were acquired and stored
using a high speed sampling system (i.e. USB/AD Card) and a
portable PC with the installed diagnostic software.
As far as the investigation for the effect of SOI on NOx
emissions is concerned, the data provided in the official NOx
Technical file were used corresponding to 25%, 50%, 75%, 85% and
5 Copyright © 2014 by ASME
100% of full engine load. The official NOx Technical file
corresponds to the NOx emission data of the parent engine.
VIT INDEX-SOI ANGLE SCALE DETERMINATION The first set of measurements acquired for the reference engine
VIT setting was used to evaluate the engine condition and to
determine the relation between VIT index and SOI angle. The
measured cylinder pressure data are processed to estimate the TDC
and to convert the measured “p-t” signal into a crank angle based “p-
φ”. The resulting mean cylinder pressure traces for each cylinder for
the two operating test cases of ~60% load and ~85% load are
depicted in Figure 3 and Figure 4 respectively.
Figure 3 Cylinder pressure traces for ~60% load before VIT
modification
Figure 4 Cylinder pressure traces for ~85% load before VIT
modification
In Figure 3 and Figure 4, it is shown, that with the exception of
cylinder No.3, there are no significant differences between the
cylinder pressure traces revealing fairly uniform cylinder operation.
The small differences observed are mainly due to differences in
fuelling rate and SOI. However, the significant difference for cylinder
No.3 is attributed to the fact that VIT system of this cylinder does not
function properly as noted below. The processing of measured
cylinder pressure data provided information for the following
parameters: brake power, specific fuel consumption, heat release rate
and cylinder ignition and injection angle. To validate the findings fuel
consumption was monitored using the installed flow meters revealing
a good coincidence between measured and estimated data.
The estimated injection angle and the recorded VIT rack setting
were used to determine VIT scale i.e. the correlation between VIT
rack position and start of injection angle. In Figure 5, are given the
corresponding values of injection angle and VIT index (mm) for the
first three measurements using the normal VIT setting for all
cylinders. As revealed there exists a clear linear correlation with a
slope of ~ 0.4 CA deg/10mm. Using this value it is now possible to
define the required VIT setting to achieve a specific SOI angle
avoiding thus the use of trial and error methods which is extremely
difficult for such applications due to the size of the engines.
Figure 5 VIT Index - Injection Angle Interrelation as determined
from the methodology
Based on the findings it was then decided to modify the advance
of SOI by ~1deg CA and investigate the corresponding effect on peak
firing pressure and bsfc at 85% load. Following this SOI was retarded
by ~1deg CA compared to the reference value and measurements
were repeated for ~60% load. In Figure 6 and Figure 7 are given the
corresponding cylinder pressure traces for all cylinders where it is
noticeable the different behavior of No.3 cylinder. In the next section
it is described by detail the effect of SOI on the cylinder pressure
trace for the two test cases examined (i.e. advance and retard by ~1
deg CA).
Figure 6 Cylinder pressure traces for ~60% load after VIT
Decrease Compared to Reference
60 90 120 150 180 210 240 270 300
Crank Angle (deg)
0
20
40
60
80
100
120
140
Pre
ssu
re (
ba
r)
Cyl No 1
Cyl No 2
Cyl No 3
Cyl No 4
Cyl No 5
Cyl No 6
60 90 120 150 180 210 240 270 300
Crank Angle (deg)
0
20
40
60
80
100
120
140
Pre
ssu
re (
ba
r)
Cyl No 1
Cyl No 2
Cyl No 3
Cyl No 4
Cyl No 5
Cyl No 6
1 2 3 4 5 6 7
VIT Index (-)
-1
0
1
2
3
Inje
ctio
n A
ng
le (
de
g)
AT
DC
Y = -0.4 * X + 2.4
60 90 120 150 180 210 240 270 300
Crank Angle (deg)
0
20
40
60
80
100
120
140
Pre
ssu
re (
ba
r)
Cyl No 1
Cyl No 2
Cyl No 3
Cyl No 4
Cyl No 5
Cyl No 6
6 Copyright © 2014 by ASME
Figure 7 Cylinder pressure traces for ~85% load after VIT
Increase Compared to Reference
EFFECT OF VIT TUNING ON ENGINE PERFORMANCE
In the present section it is investigated the effect of SOI
variation on engine performance. For this reason a comparative
evaluation is conducted for test cases 1-14 (load ~ 85%) and 3-16
(load ~ 60%). In Figure 8 and Figure 9 it is shown the effect of VIT
variation on the ignition angle of each cylinder for the two loads
examined i.e. ~60% load and ~85% load respectively. In both cases
the variation of ignition angle is as initially estimated i.e. ~1 deg CA
which reveals the validity of the applied methodology. On the other
hand the ignition angle of cylinder No.3 remains constant, which
verifies the previous statement i.e. that the VIT system of specific
cylinder does not function properly.
Figure 8 Cylinder ignition angle before and after VIT
modification at ~60% load
Figure 9 Cylinder ignition angle before and after VIT
modification at ~85% load
To better understand the effect of VIT variation on cylinder
pressure it is compared the cylinder pressure trace of one cylinder
before and after adjustment. For this comparative evaluation, cylinder
No.4 was chosen as the representative cylinder since its power and
fuel consumption presented the lowest deviation from the mean value
of the six cylinders. In Figure 10 and Figure 11 it is given the
comparison of cylinder pressure traces before and after VIT
adjustment. As observed SOI advance (for ~85% load test case)
results to significant increase of peak firing pressure, while the
injection retard (for ~60% load test case) has the exact opposite
effect. The peak firing pressure value is acceptable since it does not
exceed the 145 bar limit.
Figure 10 Cylinder No.4 pressure trace before and after VIT
modification for ~60% load
60 90 120 150 180 210 240 270 300
Crank Angle (deg)
0
20
40
60
80
100
120
140
Pre
ssu
re (
ba
r)
Cyl No 1
Cyl No 2
Cyl No 3
Cyl No 4
Cyl No 5
Cyl No 6
1 2 3 4 5 6Cylinder Number
0
1
2
3
4
5
6
Ignitio
n A
ng
le (
de
g)
AT
DC 60% Load(normal VIT)
60% Load (decreased VIT)
VIT does not function
1 2 3 4 5 6Cylinder Number
0
1
2
3
4
5
6
Ignitio
n A
ng
le (
de
g)
AT
DC 85% Load(normal VIT)
85% Load (increased VIT)
VIT does not function
60 90 120 150 180 210 240 270 300
Crank Angle (deg)
0
20
40
60
80
100
120
140
Pre
ssu
re (
ba
r)
60% Load (normal VIT)
60% Load (decreased VIT)
7 Copyright © 2014 by ASME
Figure 11 Cylinder No.4 pressure trace before and after VIT
modification for ~85% load
In Figure 12 and Figure 13, it is summarized the effect of VIT
variation on the peak firing pressure of all cylinders. It is observed
once again that the cylinder No.3 firing pressure remains unaffected.
Figure 12 Effect of VIT modification on peak firing pressure at
~60% load
Figure 13 Effect of VIT modification on peak firing pressure at
~85% load
In Figure 14 and Figure 15 it is given the corresponding effect of
SOI variation on the net heat release for cylinder No. 4 for ~60% load
and ~85% load respectively. Obviously SOI variation affects the start
of combustion as also observed in Figure 8 and Figure 9, while the
net HRR is practically shifted towards expansion when SOI is
retarded and to the reverse direction when SOI is advanced. On the
other hand there is no noticeable effect on combustion rate and
duration for the specific SOI variation level which is most possibly
attributed to the slow speed of the engine and the relatively low
ignitions delay when expressed in deg CA. From the processing of
the heat release rate histories and the knowledge of the heat loss rate,
it is estimated the fuel consumption of each cylinder using the heating
value of the fuel used.
Figure 14 Effect of VIT modification on the net heat release rate
of cylinder No.4 at ~60% load
Figure 15 Effect of VIT modification on the net heat release rate
of cylinder No.4 at ~85% load
To derive an estimate for the effect of SOI and thus VIT on fuel
consumption, it is used its effect on the bsfc as already mentioned,
which allows direct comparison. In Figure 16 and Figure 17, is
depicted the comparison of cylinder bsfc before and after VIT
adjustment for the two load cases examined. As expected, the ~1 CA
deg SOI retard results to an increase of bsfc by ~1g/kWh (mid-load
test case ~60%), while ~1 CA deg SOI advance (at ~ 85% load)
results to decrease by ~ 1.7g/kWh bsfc, which for the current power
setting (i.e. ~85% load) is reflected to a daily fuel saving potential of
~0.6 tn.
60 90 120 150 180 210 240 270 300
Crank Angle (deg)
0
20
40
60
80
100
120
140
Pre
ssu
re (
ba
r)
85% Load (normal VIT)
85% Load (increased VIT)
1 2 3 4 5 6Cylinder Number
90
95
100
105
110
115
120
125
130
Fir
ing p
ressu
re (
bar)
60% Load(normal VIT)
60% Load (decreased VIT)
VIT does not function
1 2 3 4 5 6Cylinder Number
110
115
120
125
130
135
140
145
150
Fir
ing p
ressu
re (
bar)
85% Load(normal VIT)
85% Load (increased VIT)
VIT does not function
140 160 180 200 220 240
Crank Angle (deg)
-50000
0
50000
100000
150000
200000
250000
He
at R
ele
ase
Ra
te (
J/d
eg
) 60% Load (normal VIT)
60% Load (decreased VIT)
140 160 180 200 220 240
Crank Angle (deg)
-50000
0
50000
100000
150000
200000
250000
He
at R
ele
ase
Ra
te (
J/d
eg
) 85% Load (normal VIT)
85% Load (increased VIT)
8 Copyright © 2014 by ASME
Figure 16 Cylinder bsfc before and after VIT modification at
~60% load
Figure 17: Cylinder bsfc before and after VIT modification at
~85% load
COMBUSTION MODEL CALIBRATION –VALIDATION As mentioned an important issue for Marine Diesel Engines are
NOx emissions which have to be maintained inside specific limits as
implied by Marpol Annex-VI. SOI obviously affects strongly NOx
emissions and for this reason it is necessary to investigate the
expected impact of SOI variation in the magnitude considered in the
present work. This is conducted using a simulation tool capable for
predicting performance and emissions of DI diesel engines. But
before this it is necessary to investigate and demonstrate model’s
ability to predict adequately NOx tailpipe emission values for two-
stroke marine diesel engines.
For this reason it is examined herein, the multi-zone combustion
model’s ability to predict performance and NOx emissions at various
load points. For this reason as already mentioned use is made of the
data provided in the official NOx file of the engine at 25% (57.3
rpm), 50% (72.2 rpm), 75% (82.7 rpm), 85% (86.2 rpm) and 100%
(91 rpm) of the maximum continuous rating (MCR). It is to be noted
that for 85% load NOx emissions were not available. To evaluate
models predictive ability it is given the comparison between the
calculated and measured values of: brake power output in Figure 18,
peak firing pressure and compression pressure in Figure 19, brake
specific fuel consumption in Figure 20, and NOx emissions in Figure
21.
Figure 18 Comparison between calculated and measured brake
power output vs. engine speed
Figure 19: Comparison between calculated and measured peak
firing pressure and compression pressure vs. engine load
Figure 20: Comparison between calculated and measured brake
specific fuel consumption vs. engine load
1 2 3 4 5 6Cylinder Number
195
197
199
201
203
205
bsfc
(g
/kW
h)
60% Load(normal VIT)
60% Load (decreased VIT)
1 2 3 4 5 6Cylinder Number
190
192
194
196
198
200
bsfc
(g
/kW
h)
85% Load(normal VIT)
85% Load (increased VIT)
50 60 70 80 90 100
Engine Speed (rpm)
0
3000
6000
9000
12000
15000
18000
Bra
ke
Po
we
r (k
W)
Experimental
Calculated
20 30 40 50 60 70 80 90 100 110
Load (%)
40
50
60
70
80
90
100
110
120
130
140
150
Pe
ak F
irin
g P
ressu
re (
ba
r)
Experimental
Calculated
40
50
60
70
80
90
100
110
120
130
140
150
Co
mp
ressio
n p
ressu
re (
ba
r)
20 30 40 50 60 70 80 90 100
Load (%)
160
170
180
190
200
210
220
BS
FC
(g
/kW
h)
Experimental
Calculated
9 Copyright © 2014 by ASME
Figure 21 Comparison between the calculated and measured NOx
emissions vs. engine load.
As observed from Figure 18 Figure 19 and Figure 20 the model
manages to predict with reasonable accuracy the main engine
operating parameters without constant tuning since they are
calibrated at 75% load and are then maintained constant. As far as
NOx emissions are concerned it is obvious from Figure 21 that NOx
are predicted quite well even though the relative error is higher, as
expected, when compared to the corresponding ones for engine
performance parameters. This is encouraging since the main target
when investigating emissions is basically to capture trends.
MODEL APPLICATION FOR SOI VARIATION EFFECT ON ENGINE PERFORMANCE AND NOx EMISSIONS
Having evaluated model’s ability to predict both engine
performance and NOx emissions it is then applied to estimate the
corresponding impact of SOI variation. The theoretical investigation
is conducted for the 85% load point i.e. measurements 1 and 14
respectively. In Figure 22, Figure 23 and Figure 24 it is given the
comparison between the measured values of each cylinder and
calculated values of break power, peak firing pressure and bsfc vs.
ignition angle. Use of ignition angle was preferred instead of SOI
since it is directly provided form the diagnosis methodology. On the
other hand the SOI angle is estimated form the ignition angle and an
estimate of the ignition delay using equation (7). As observed the
model predicts the effect of SOI variation on the engine performance
and most important the trend. This is an indication that the simulation
model can be used as a tool to examine the effect of SOI variation on
engine performance and NOx emissions.
Figure 22 Comparison between the calculated and measured
brake power of 85% load vs. ignition angle
Figure 23 Comparison between the calculated and measured
peak firing pressure of 85% load vs. ignition angle
Figure 24 Comparison between the calculated and measured bsfc
of 85% load vs. ignition angle
The corresponding estimated effect of SOI variation on NOx
emissions is depicted in Figure 25. As expected, SOI advance results
to NOx emissions increase which is approximately 4-5%/1 CA deg of
SOI variation. This is encouraging, since this percentage corresponds
20 30 40 50 60 70 80 90 100
Load (%)
10
15
20
25
Exh
au
st
NO
x (
g/k
Wh
)
Experimental
Calculated
0 1 2 3 4
Ignition Angle (deg) ATDC
10000
12000
14000
16000
18000
Bra
ke
Po
we
r (k
W)
Experimental
Calculated
0 1 2 3 4
Ignition Angle (deg) ATDC
100
110
120
130
140
150
160
Pe
ak F
irin
g P
ressu
re (
ba
r)
Exprimental
Calculated
0 1 2 3 4
Ignition Angle (deg) ATDC
150
170
190
210
230
250
BS
FC
(g
/kW
h)
Exprimental
Calculated
10 Copyright © 2014 by ASME
to a ~0.7 g/kWh increase on NOx emissions which for the specific
engine is expected to create no problem for engine NOx file. This
conclusion is valid for similar engine designs.
Thus, as revealed, there is a clear fuel saving potential using SOI
advance, without the risk of exceeding the peak firing pressure and
NOx emission limits. It is also concluded from the results that there is
a potential for application of an even higher SOI advance (more than
one degree of crank angle) for a greater reduction of bsfc, since the
forthcoming effect on NOx is not significant.
Figure 25 Estimated effect of ignition angle variation on NOx
emissions.
CONCLUSIONS In the present study, it is investigated the potential for reducing
bsfc of a two-stroke marine diesel engine using SOI advance, without
exceeding peak firing pressure and NOx emission limits. Towards
this direction, a diagnostic technique has been applied on a two-
stroke marine diesel engine on-board a commercial vessel, where SOI
was adjusted using the VIT system. At the beginning cylinder
pressure measurements were acquired at three different operating
load points and processed to derive various engine operating
parameters. The derived values for injection angle along with the
measured VIT index values were used to evaluate VIT system
behaviour and to determine the engine VIT scale (i.e. the correlation
between VIT index and start of injection angle which is: SOI = -0.4 · VIT + 2.4). For the present case the VIT scale was estimated to be
~0.4 CA deg/10 mm.
Estimation of the VIT scale enabled direct determination of the
required VIT adjustment to achieve a specific SOI value without use
of trial and error techniques. The evaluation of SOI values after VIT
system adjustment verified this. In the present application SOI has
been modified by ~1 deg CA around its nominal value. From the
analysis of performance data, the effect of injection timing variation
on bsfc was determined. As revealed the fuel saving is ~1.7 g/kWh
which corresponds to a fuel saving of ~0.6 tn/day the value of which
can be increased to ~1 tn/day if SOI is further advanced by an
additional crank angle degree. Most important it has been observed
that this is achieved without significant increase of peak firing
pressure. From the comparison of the heat release rate before and
after VIT adjustment there is no noticeable effect on combustion rate
and duration since HRR is practically shifted to the left or the right
compared to TDC. Furthermore from the analysis it was also
identified that the VIT system of Cylinder No3 does not function
properly.
For NOx emissions an existing well-validated multi-zone
combustion model (since the one embedded in the diagnostic
technique is a two-zone one) was used to evaluate the effect of SOI
variation. Towards this direction, the model was initially validated for
its ability to predict overall engine performance and NOx emissions
for various load points using the normal SOI. The data for the
validation were derived from the official NOx file of the engine. The
validation revealed model’s ability to predict accurately engine
performance and NOx emissions for various engine loads. Following
this the model was applied to predict the corresponding effect of SOI
on performance and emissions at 85% load where NOx values are
higher. As revealed the simulation predicts adequately the effect of
SOI on engine performance enhancing the reliability for NOx
predictions. As observed SOI variation in the range of 1 deg CA
results to an increase of NOx emission by ~4-5 % which corresponds
to NOx variation of ~0.7 g/kWh. This variation creates no danger for
the NOx file of the engine while it permits even higher SOI variation
increasing the fuel saving potential.
Thus the investigation conducted reveals the existence of a fuel
saving potential at acceptable peak firing pressure and NOx values.
Moreover, it is revealed that the proposed methodology can be
successfully applied as a tuning (optimum VIT setting) and diagnosis
(detection of VIT system performance and scaling) tool, with
minimum instrumentation and effort, without the disadvantages of
purely experimental trial and error methods. Last but not least it is
demonstrated the multi-zone combustion model’s ability to predict
engine performance and NOx emissions of a two-stoke marine diesel
engine at various operating conditions and the effect of SOI. This
reveals that a potential exists for its use as a tool to assist engine
development studies and for optimum tuning of two-stroke marine
diesel engines.
ACKNOWLEDGMENTS Special thanks are attributed to Minerva Marine Inc. for
supporting the on-board vessel measurement campaign and for
providing valuable data and technical comments.
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Ignition Angle (deg) ATDC
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NO
x (
g/k
Wh
)
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12 Copyright © 2014 by ASME