chapter 7 cyclic variations -...
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114
CHAPTER 7
CYCLIC VARIATIONS
7.1 INTRODUCTION
In an apparently steady running spark ignition engine, there will
be as much as 70% variation in peak pressures at certain operating
condition (Winsor 1973). This variation in cylinder pressure from cycle to
cycle, which originates from many sources, is termed as cyclic variation.
Cyclic variation in spark ignition engine is identified as a fundamental
combustion problem (Patterson 1966). It limits the use of lean mixtures, the
amount of recycled exhaust and increases the idle speed operation. By
eliminating this cyclic variation, the engine power output can be increased by
10 % for the same fuel consumption (Soltau 1961, Karim 1967). In certain
transmission types, the cyclic variation results in torque fluctuations and poor
drivability of the vehicle (Tsuchiya et al 1983). It is also identified that
reducing the cyclic variation may suppress engine noise and vibration (Andon
1964). The present work reported here analyzes the cyclic variation of a two-
stroke engine and explores the possibility of reducing cyclic variation.
7.2 TWO-STROKE ENGINE CYCLIC VARIATIONS
The problem of cyclic variation is more severe in the case of two-
stroke engines. The inherent high level of exhaust dilution, the unsteady
nature of fluid flow, low cylinder peak pressure, and high torque fluctuations
make the problem more complex. Although the literature available on cyclic
115
variation of combustion in a two-stroke engine is limited, the importance of
the problem is well understood. Cyclic variation in combustion affects the
performance and drivability of the two-stroke SI engines (Yamashita 1995).
The light inertia mass and small damping volume of rotating components of
two-stroke engine amplifies cyclic variation and leads to large engine speed
variations (Ishibe 1995). The two-stroke engine running under part load
shows large cyclic variations including misfire, and incomplete combustion,
with high levels of hydrocarbon emissions (Ohira et al 1994). In a crankcase
scavenged two-stroke engine, combustion pressure of previous cycle largely
affects the mass of fresh charge entering in a cylinder, even after two cycles.
The following conclusions are arrived at based on the literature on
cyclic variation in two-stroke engine: (a) speed and torque fluctuations
(b) drivability of the vehicle is affected (c) affects lean operating limit and
(d) excessive UBHC emissions.
7.3 INDICATORS OF CYCLIC VARIATIONS
In-cylinder pressure is an important indicator of the cyclic
variation. The cylinder pressure is measured for individual cycle at each crank
angle interval by a pressure transducer flush mounted in the cylinder. Many
pressure related parameters could be derived from the pressure history, which
indicate the cyclic variations. Some important pressure related parameters are:
In-cylinder peak pressure, Pmax
Crank angle at which the in-cylinder peak pressure occurs,
CAPmax
Maximum rate of pressure rise, (dP/d)max
IMEP of the individual cycles
116
Apart from this, the burn rate and heat release rate related
parameters are also used to indicate the cyclic variations. Some important
combustion related parameters are:
Crank angle occurrence of 5% heat release CAQ5
Crank angle occurrence of 10% heat release CAQ10
Crank angle occurrence of 50% heat release CAQ50
Crank angle occurrence of 90% heat release CAQ90
Pressure related quantities are the easiest to measure and indicate
the direct effect of cyclic variations. However, the cylinder pressure
parameters are affected by volume change, crevice effect and blowby
(Heywood 1989). The heat release related parameters, obtained from the heat
release analysis, indicate the burning history of the trapped charge. The
relation between the variations in combustion rate and variations in cylinder
pressure is complex and care is needed in analyzing the variables.
7.4 AIM AND SCOPE OF PRESENT WORK
The brief review of the literature on cyclic variation presented
above reveals the importance of the problem and the influence of cyclic
variation on engine performance. The cyclic variation imposes constraints
over the lean operation and reduces power and efficiency of the engine. As
the main aim of this work is to develop a lean burn engine, it becomes
necessary to study the effect of cyclic variations. Further, the cyclic variation
affects the drivability of the two-stroke engine under leaner operation (Ishibe
1995). Hence, the present work on cyclic variation is carried out with the
following aims:
117
To investigate the problem of cyclic variation of cylinder
pressures in a lean burn two-stroke engine.
To identify the existence of burn modes among the sample.
To investigate the prior cycle effect on cyclic variation.
To analyze the cyclic variation in heat release angles.
To compare the base, catalytic and magnetic activated engines
on cyclic variations.
7.5 EXPERIMENTAL PROCEDURE
To analyze the cyclic variation, in-cylinder pressure histories are
measured using a pressure transducer flush mounted in the cylinder head. The
experimental setup and the data acquisition system are explained in Chapter 4.
As the aim of the study is to analyze the pressure variation from cycle to
cycle, it becomes necessary to collect large sample of data. To have statistical
consistency and repeatable results, 500 consecutive cycles are obtained.
7.6 METHODOLOGY
The methodology involved in analyzing the cyclic variations is
summarized as follows:
In-cylinder pressures for 500 continuous cycles are measured
at each operating point.
The cyclic variations in cylinder pressures and related
parameters are analyzed.
Cyclic variations in crank angles of heat release are analyzed.
118
Prior cycle effects are identified.
The entire sample is separated in three groups based on their
mode of combustion.
The cyclic variations of base, magnetically activated fuel on
engine and catalytic activated engines are compared.
The cyclic variations are analyzed using the above methodology
and the results are presented in the following sections.
7.7 RESULTS AND DISCUSSIONS
7.7.1 Cyclic Variation in Cylinder Pressures
The variation in the measured cylinder pressures of consecutive
cycles can be seen from the p-v diagram shown in Figure 7.1. The figure
shows the pressure traces of ten consecutive cycles at 3000 rpm. It can be
observed from the figure that there is a wide variation in cylinder pressures.
For certain cycles the peak pressure (Pmax) is higher and for certain cycles it is
lower.
The location of the Pmax (CAPmax) also varies from one cycle to
another. This leads to variation in area under the curve. The IMEP indicates
the work done by the cylinder gas on the piston. Hence the cylinder pressure
variation can be identified either in Pmax, or in rate of pressure rise
(dP/dmax), or in CAPmax, or in IMEP. The engine performance and torque
developed are very much depends upon IMEP and any variation in IMEP
leads to torque fluctuations.
119
0
4
8
12
16
-180 -120 -60 0 60 120 180Crank Angle (degree)
Pres
sure
(bar
)
TDC
Motored
Figure 7.1 Variation of measured cylinder pressure with crank angle
for ten cycles
7.7.1.1 Scatter plot of Pmax, IMEP and crankshaft speed
Figures 7.2 to 7.4 show the scatter plots of Pmax, IMEP and engine
speed of individual cycles. The Pmax is directly obtained from the measured
cylinder pressure trace. The crank angle speed is measured by an optical crank
angle encoder. The mean values of these parameters are also indicated in the
figures. To further enhance the clarity, the spread of these parameters for
100 cycles are shown in Figures 7.5 to 7.7. Here the successive cyclic values
are connected by a continuous line. There seems to be a random variation
from cycle to cycle of these parameters as indicated in the figures.
120
Speed =3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Mean Pmax =11.05 bar
10.60
10.80
11.00
11.20
11.40
11.60
11.80
0 100 200 300 400 500
Cycle Number
Pmax
(bar
)
Figure 7.2 Scatter plot of Pmax with cycle number
Speed = 3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Mean IMEP = 3.61 bar
3.10
3.25
3.40
3.55
3.70
3.85
0 100 200 300 400 500
Cycle Number
IME
P (b
ar)
Figure 7.3 Scatter plot of IMEP with cycle number
121
Speed = 3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Mean Speed = 2998 rpm
2920
2940
2960
2980
3000
3020
3040
3060
0 100 200 300 400 500
Cycle Number
Spee
d (r
pm)
Figure 7.4 Scatter plot of engine speed with cycle number
Speed = 3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Mean Pmax = 11.05 bar
10.60
10.80
11.00
11.20
11.40
11.60
11.80
0 20 40 60 80 100Cycle Number
Pmax
(bar
)
Figure 7.5 Plot of peak pressure with cycle number
122
Speed = 3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Mean IMEP = 3.61 bar
3.10
3.25
3.40
3.55
3.70
3.85
0 20 40 60 80 100
Cycle Number
IME
P (b
ar)
Figure 7.6 Plot of IMEP with cycle number
Speed = 3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Mean Speed = 2998 rpm
2920
2940
2960
2980
3000
3020
3040
3060
0 20 40 60 80 100
Cycle Number
Spee
d (r
pm)
Figure 7.7 Plot of engine speed with cycle number
123
The Pmax is a measure of rate of pressure rise due to combustion. If
the combustion is faster, higher-pressure rise rate occurs and a higher Pmax
results. The Pmax is shown to depend on both changes in combustion phasing
and burning rate (Heywood 1989). The magnitude of variation depends on
whether the combustion is faster or slower. A faster combustion will produce
a higher Pmax. Also the Pmax will tend to occur closer to TDC whereas a slower
burning cycle will have lower Pmax and that CAPmax will be away from TDC.
The IMEP is a measure of work output from the combustion
products. A faster pressure rise and a quick combustion may result in higher
work output. A higher trapped charge may also lead to increased work output.
The mass of fresh charge trapped in each cycle varies substantially (Galliot
et al 1990). Hence, the IMEP fluctuations may be due to variation in
combustion rate or variation in quantity of energy released.
The speed fluctuation depends upon both combustion rate and the
amount of work done on the piston. A faster combustion will result in
increased rate of pressure rise (dp/d) and hence will accelerate the piston
much faster during its expansion stroke. Whereas, an increased work output
will result in more thrust on piston and hence more speed in crankshaft.
The variation in crankshaft speed indicates the engine roughness
(Heywood 1989). For smooth riding, the engine speed should be constant for
a particular throttle opening.
To find out the relationship between the engine speed, Pmax and
IMEP, the plot of speed versus Pmax and speed versus IMEP are exhibited in
Figures 7.8 and 7.9. The crankshaft speed varies between 3040 rpm to
2957 rpm with a mean value of 2998 rpm. For a two-stroke engine, this
variation is common and the engine is found to be running steadily during the
experiment.
124
Speed = 3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Mean Speed = 2998 rpm,
Mean Pmax = 11.05 bar
2920
2940
2960
2980
3000
3020
3040
3060
10.6 10.8 11.0 11.2 11.4 11.6
Pmax (bar)
Spee
d (r
pm)
Figure 7.8 Plot of engine speed with Pmax
Speed = 3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Mean Speed = 2998 rpm, Mean IMEP = 3.61 bar
2920
2940
2960
2980
3000
3020
3040
3060
3.0 3.2 3.4 3.6 3.8
IMEP (bar)
Spee
d (r
pm)
Figure 7.9 Plot of engine speed with IMEP
125
7.7.1.2 Combustion phasing
The relation between variation in combustion rate and cylinder
pressure is a complex phenomenon (Heywood 1989). The rate of change of'
pressure is affected by both the rate of cylinder volume change and rate of
burning. Matekunas (1983) identified the relationship between Pmax, CAPmax
and IMEP of a SI engine at fixed operation conditions with three different
spark timings. The MBT timing data show a spread in IMEP at a fixed value
of CAPmax. This IMEP data band is relatively flat and is centred around 16°. It
is identified that there are phases of combustion in which both fast burning
cycles and slow burning cycles for similar operating conditions with same
spark timing exists.
The cycles having higher Pmax values with its peak occurring close
to TDC are called fast burn cycles. Whereas the cycles having lower Pmax
values with its peak away from the TDC are designated as slow burn cycles.
Within a limited range, a relatively linear relation exists between Pmax and
CAPmax. The fast burning cycles produce a higher Pmax and the slow burning
cycles a lower value. The ‘hook-back’ of the curve occurs closer to TDC for
the slow burn cycles than for the fast burn cycles.
This happens because, for slow burn cycles the rate of change of
pressure is lower than a pressure change due to piston movement. With this
background, the plot of Pmax and IMEP with CAPmax obtained from the present
work can be examined. Matekunas (1983) obtained similar results with MBT
timing for rich and lean operations. In the present work, the relationship
between Pmax, IMEP and CAPmax at different air-fuel ratio show similar trends.
126
Figures 7.10 to 7.13 show the plot of Pmax and IMEP with CAPmax
at 4000 rpm for the air-fuel ratios of 11.8:1 and 18.1:1. The spread of Pmax and
IMEP at a particular CAPmax indicates the existence of burn phases in both
cases. For rich mixture the Pmax and IMEP correlates well with CAPmax.
A cycle having CAPmax closer to TDC will produce a higher Pmax and IMEP.
There is no misfire or partial burning at the air-fuel ratio of 11.8:1.
Figure 7.10 indicates the linearity of Pmax with CAPmax and a higher value of
Pmax occurs for a cycle having CAPmax closer to TDC.
Speed = 4000 rpm, Power = 2.6 kW, A/F = 11.8:1, Mean CAPmax = 28.2 deg.,
Mean Pmax = 13.97 bar
8.09.0
10.011.012.013.014.015.016.017.0
20 25 30 35 40 45
CAPmax (deg)
Pmax
(bar
)
Figure 7.10 Variation of Pmax with CAPmax at an A/F of 11.8:1
127
Speed = 4000 rpm, Power = 2.6 kW, A/F = 11.8:1, Mean CAPmax = 28.2 deg.,
Mean IMEP = 5.24 bar
2.02.53.03.54.04.55.05.56.06.57.0
18 23 28 33 38 43
CAPmax (deg)
IME
P (b
ar)
Figure 7.11 Variation of IMEP with CAPmax at an A/F of 11.8:1
Figure 7.12 and 7.13 show the relation between Pmax and IMEP
with CAPmax for lean fuel operation. Here the trend is obtained in different
manner. For a two-stroke engine, 18.1:1 is a very lean mixture and hence lot
of misfire can be expected. This is what experienced during the experiment
and is reflected in these figures. A substantial number of cycles undergo
misfire and partial burning which results in lower Pmax and IMEP. Few cycles
have CAPmax well before TDC, indicating the pre-ignition. It is noted that pre-
ignition at the lean operation. One possible explanation for this may be that
the misfire and partial burn leads to increased fuel content in the exhaust gas
that dilutes the fresh charge of next cycle. Hence the next cycle over all
mixture contains more fuel and leads to pre-ignition. This is assisted with the
fact that the combustion chamber temperature is sufficiently high, as the
engine speed is 4000 rpm.
128
Speed = 4000 rpm, Power = 1.0 kW, A/F = 18.1:1, Mean CAPmax = 15.3 deg.,
Mean Pmax = 10.0 bar
8.08.59.09.5
10.010.511.011.512.012.513.0
-30 -10 10 30 50
CAPmax (deg)
Pmax
(bar
)
Figure 7.12 Variation of Pmax with CAPmax at an A/F of 18.1:1
Speed = 4000 rpm, Power = 1.0 kW, A/F = 18.1:1, Mean CAPmax = 15.3 deg.,
Mean IMEP = 2.59 bar
0.0
1.0
2.0
3.0
4.0
5.0
6.0
-30 -10 10 30 50
CAPmax (deg)
IME
P (b
ar)
Figure 7.13 Variation of IMEP with CAPmax at an A/F of 18.1:1
129
Similar results are obtained by Martin et al (1988) for lean mixture
operation, where the cyclic variability in IMEP and Pmax increase with
increased air-fuel ratio. The 'hook-back', described by Matekunas (1983),
occurs at the lean range of air-fuel ratios because the increase in pressure due
to combustion is less than the decrease in pressure due to expansion for some
cycles. In the return region, the Pmax remains low and CAPmax occurs closer to
TDC, as the combustion event is retarded (Whitelaw et al 1995). These slow-
burn cycles in the hook-back and return regions are responsible for
considerable variation in IMEP as seen in Figure 7.13.
7.7.1.3 Burn modes
The relation between Pmax and IMEP for two different air-fuel ratios
at an engine speed of 4000 rpm is shown in Figures 7.14 and 7.15. The mean
values of Pmax and IMEP are also indicated in the figures. It can be observed
that there is a linear relationship at rich mixture operation. The IMEP
increases as the Pmax increases indicating a strong correlation between them at
the rich mixture operation.
However, in the lean mixture operation, certain groups of cycles are
insensitive to Pmax variation. For example, a group of cycles, having a wide
variation of Pmax from 9.8 bar to 12 bar results in near zero IMEP values.
Another group of cycles, having a narrow band of Pmax around 10 bar shows a
wide variation in IMEP from 0.5 bar 4.5 bar. However, a small number of
cycles show a linear relation with IMEP, where the IMEP increases as the
Pmax increases.
130
Speed = 4000 rpm, Power = 2.6 kW, A/F = 11.8:1, Mean Pmax = 13.97 bar,
Mean IMEP = 5.24 bar
3
4
5
6
7
8
9
4 6 8 10 12 14 16 18
Pmax (bar)
IME
P (b
ar)
Figure 7.14 Variation of IMEP with Pmax at an A/F of 11.8:1
Speed = 4000 rpm, Power = 1.0 kW, A/F = 18.1:1, Mean Pmax = 10.0 bar,
Mean IMEP = 2.59 bar
0
1
2
3
4
5
6
8 9 10 11 12 13 14
Pmax (bar)
IME
P (b
ar)
Figure 7.15 Variation of IMEP with Pmax at an A/F of 18.1:1
131
But the mean values indicated in the figure do not represent all
these groups of cycle. There are cycles having both Pmax and IMEP higher
than mean values and cycles having both Pmax and IMEP lower than mean.
Also certain groups of cycles have both Pmax and IMEP close to the mean
values.
Hence, the analysis of cyclic variation and combustion event based
solely on the mean values of entire cycles could be misleading. The present
study concerns mainly with the lean fixtures, where three modes are quite
distinct (Martin et al 1988). Hence, any further analysis must be made on the
individual mode basis. The misfire and fast-burn cycles are to be put in
separate groups.
7.7.1.4 Conditional grouping
The measured cylinder pressure data are grouped into three
different modes. The grouping is done in order to separate dissimilar cycles
for further analysis of cyclic variability in combustion (Blair 1996). By
considering individual cycle pressure data and separating the cycles according
to a specified set of constraint, sub groups can be identified that relate each of
the different combustion modes. These sub-groups are further analyzed by a
heat release analysis code developed for this purpose. The details of the heat
release analysis procedure are presented in Chapter 6.
Although the selection of parameter and limits used in the
conditional grouping of cylinder pressure data are arbitrary, a reasonable
approach is necessary for selecting them. The parameters selected and their
limit should separate the cycles that have the three modes of combustion.
132
An earlier study on cyclic variation indicates that a variation in
IMEP in excess of 10% will produce torque fluctuation and may cause
drivability problem (Heywood 1989). Hence, a 10% variation in IMEP from
the mean value is the logical limit for conditional grouping. Also the present
experiment is carried out with various air-fuel ratios for base, catalytic and
magnetic activated engines, where Pmax and IMEP at each operating point
varies considerably. The 10% limit on IMEP will separate the groups based
on its individual data set of cyclic variation. The following mathematical
formulation is used for grouping the data set:
Upper Mode Cycles (UMC) = Xi > mean of IMEP +
10% of mean of IMEP
Middle Mode Cycles (MMC) = mean of IMEP –
10% of mean of IMEP < Xi < mean of IMEP +
10 % of mean of IMEP
Lower Mode cycles (LMC) = Xi < mean of IMEP –
10% of mean of IMEP
for i = 1,2,... 500
These different modes of cycles can be identified from the
Figure 7.14. The upper mode cycles correspond to the region where the
variation in the flame initiation period alters the phasing of the burn but do
not significantly affect the IMEP. These cycles are influenced by the
variations in initial flame development.
The lower modes cycles correspond to the region where misfire or
near misfire and partial burn occur. These cycles produce lower values of
IMEP and Pmax. The Pmax value occurs near TDC for the LMCs.
The explanation for this may be that, the rate of pressure rise due to
133
combustion is less than or equal to the pressure change due to piston
movement. Also the pressure developed due to combustion is less for these
cycles. For these cycles IMEPs are not much affected by the variation in
CAPmax as seen in the Figure 7.13.
In the MMC the cycle spread is close to the mean values and
represents the overall mean values of the particular operating condition. They
are optimally phased cycles with medium rate of pressure rise and moderate
combustion duration. The three modes of cycles have distinct property and
hence have different effect on Pmax and IMEP. As seen in the figures, the Pmax
and IMEP values of UMC and LMC are much deviated from the mean
values. The mean values calculated from the overall cycles do not represent
the cycles in UMC and LMC.
The AVL indimeter software is used to calculate the heat release
rate and the crank angle occurrence of 5%, 10%, 50% and 90% heat release
values. For calculating the mean, standard deviation and covariance etc., of
IMEP and Pmax Microsoft excel sheet is used.
7.7.1.5 Statistical calculations
The mean, standard deviation and the covariance of standard
deviation of the cylinder pressure and heat release parameters are determined
for each group of data. They are calculated from the following expressions.
Mean = ň = 1 / N xi
STD = = √ (1/N (xi – ň) 2) and
COV = STD / Mean = / ň
where N = sample size and i = 1, 2,.... 500
134
Microsoft excel worksheet is used to calculate the above statistical
values. Figures 7.16 - 7.18 shows the scatter plot of each group of data and its
corresponding mean, STD and COV for Pmax and IMEP values. The same
procedure is applied to different data set for calculating the mean values and
the results are plotted in Figures 7.19 and 7.20. It can be observed from these
figures, that the overall mean of the data set is very well represented by the
MMC. The UMC and LMC values are well away from the overall values. The
contribution of UMCs in higher Pmax and IMEP is nullified by the LMCs. If
the LMCs are to be eliminated or converted into either MMCs or UMCs by
some means then the engine performance will be improved.
It is noted that the variation in Pmax among these modes is lower at
rich side and higher at lean side. Similarly, the variation in IMEP is less at
rich operation and more at lean operation. Further details are given in
Figure 7.21 and 7.22, where the COV of Pmax and IMEP calculated from the
cycles belonging to different modes are plotted for various air-fuel ratios. The
COV increases with air-fuel ratio and the deviation among the mode
increases. Earlier studies indicate that for a lean mixture the cyclic variation
increases (Yamamoto et al 1987). This trend is reflected in the Figures 7.20
and 7.21, whereas the Figure 7.22 does not show a predictable trend. This
indicates that IMEP is the more appropriate cylinder pressure parameter,
which represents the cyclic variation in a two-stroke engine.
135
Mean = 12.16 bar, Stdev = 0.92 bar, COV = 0.07, No. of Cycles = 120
11.50
11.70
11.90
12.10
12.30
12.50
12.70
12.90
0 100 200 300 400 500CYCLE NUMBER
Pmax
(bar
)
Mean = 3.67 bar, Stdev = 0.13 bar, COV = 0.04, No. of Cycles = 120
3.303.403.503.603.703.803.904.004.104.204.30
0 100 200 300 400 500CYCLE NUMBER
IMEP
(bar
)
Figure 7.16 Scatter plot of Pmax and IMEP for upper mode cycle
operation at 3000 rpm and an A/F of 16.7:1
136
Mean = 11.05 bar, Stdev = 0.61 bar, COV = 0.06, No. of Cycles = 227
10.5010.6010.7010.8010.9011.0011.1011.2011.3011.4011.50
0 100 200 300 400 500CYCLE NUMBER
Pmax
(bar
)
Mean = 3.34 bar, Stdev = 0.14 bar, COV = 0.05, No. of Cycles = 227
2.80
3.00
3.20
3.40
3.60
3.80
4.00
0 100 200 300 400 500CYCLE NUMBER
IMEP
(bar
)
Figure 7.17 Scatter plot of Pmax and IMEP for middle mode cycle
operation at 3000 rpm and an A/F of 16.7:1
137
Mean = 9.95 bar, Stdev = 0.23 bar, COV = 0.03, No. of Cycles = 153
9.509.609.709.809.90
10.0010.1010.2010.3010.40
0 100 200 300 400 500CYCLE NUMBER
Pmax
(bar
)
Mean = 3.01 bar, Stdev = 0.27 bar, COV = 0.11, No. of Cycles = 153
2.602.702.802.903.003.103.203.303.403.50
0 100 200 300 400 500CYCLE NUMBER
IMEP
(ba
r)
Figure 7.18 Scatter plot of Pmax and IMEP for lower mode cycle
operation at 3000 rpm and an A/F of 16.7:1
138
9
10
11
12
13
14
15
16
1.01 1.1 1.25 1.4 1.5 1.6
Equivalence Ratio
Pmax
(bar
)
UMC meanMMC meanLMC meanOVERALL mean
Figure 7.19 Variation of mean Pmax with equivalence ratio
1.52
2.53
3.54
4.55
5.56
1.01 1.1 1.25 1.4 1.5 1.6Equivalence Ratio
IME
P (b
ar)
LMC meanMMC meanUMC meanOVERALL mean
Figure 7.20 Variation of Mean IMEP with equivalence ratio
139
00.010.020.030.040.050.060.070.080.09
0.1
0.9 1 1.2 1.4Equivalence Ratio
CO
V o
f IM
EP
UMC mean
MMC mean
LMC mean
OVERALL mean
Figure 7.21 Variation of COV of IMEP with equivalence ratio
00.005
0.010.015
0.020.025
0.030.035
0.040.045
0.05
0.9 1 1.2 1.4Equivalence Ratio
CO
V o
f Pm
ax
UMC meanMMC meanLMC meanOVERALL mean
Figure 7.22 Variation of COV of Pmax with equivalence ratio
140
7.7.2 Prior Cycle Effect
One of the main causes for cyclic variation is the prior cycle effect
(Daw 1990). The influence of exhaust residuals and the gas dynamic effect of
previous cycle affect the next cycle performance.
As the present engine is a two-stroke cycle, the effect of gas
dynamics and the exhaust residual from the previous cycle will be expected to
pronounce at higher level. To study the previous cycle effect the IMEP of the
Nth cycle versus IMEP from the next cycle (N+1) are usually plotted.
Figures 7.23 and 7.24 show such plots for two different air-fuel ratios. For the
stoichiometric condition, the prior cycle effects are well pronounced as seen
in Figure 7.23 compared to the lean mixture condition, shown in Figure 7.24.
To further explore the effect of prior cycle, the plot of IMEP from
the cycles belonging to three modes is plotted in Figures 7.25 and 7.26 for
two different air-fuel ratios. The dotted line indicates the order of successive
cycles belonging to the original sample. For clarity, only 50 cycles are chosen.
The plots exhibit some interesting phenomena:
There is a distinct relation between the UMCs and LMCs.
The MMCs do not have any predictable relation with either
LMCs or UMCs.
Most of the UMCs occur immediately after LMCs and
vice versa.
This distinct relation is common for rich and lean mixtures.
141
Speed = 3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Mean IMEP = 3.61 bar
0
1
2
3
4
5
6
0 1 2 3 4 5
IMEP of Nth Cycle
IME
P of
N+1
Cyc
le
Figure 7.23 Plot of IMEP of N+1 cycle with Nth cycle at an A/F of 16.7:1
Speed = 3000 rpm, Power = 0.6 kW, A/F = 18.1:1, Mean IMEP = 2.59 bar
0
1
2
3
4
5
6
0 1 2 3 4 5
IMEP of Nth Cycle
IMEP
of
N+1
Cyc
le
Figure 7.24 Plot of IMEP of N+1 cycle with Nth cycle at an A/F of 18.1:1
142
Speed = 3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Overall Mean = 4.3 bar,
UMC Mean = 5.3 bar, MMC Mean = 4.8 bar, LMC Mean = 4.2 bar
3
3.5
4
4.5
5
5.5
6
250 260 270 280 290 300Cycle Number
IME
P (b
ar)
UMC LMC MMC
Figure 7.25 Plot of IMEP with cycle number
Speed = 3000 rpm, Power = 0.6 kW, A/F = 18.1:1, Overall Mean = 2.0 bar,
UMC Mean = 3.0 bar, MMC Mean = 2.0 bar, LMC Mean = 1.0 bar
0
0.5
1
1.5
2
2.5
3
3.5
4
250 260 270 280 290 300Cycle Number
IME
P (b
ar)
UMC LMC MMC
Figure 7.26 Plot of IMEP with cycle number
143
As connected by the dotted line, for most of the cycles, the
occurrence of high IMEP is preceded by a LMC. This confirms the prior cycle
effect and the deterministic behavior proposed by certain researchers (Daily
1987). The possible explanation for this kind of behavior may be as follows:
The cycles having low IMEP experience either misfire or partial
burn. This leaves more unburned fuel in the exhaust residue and hence the
total energy content of the next cycles trapped charge increases. The heat
release from these cycles is higher resulting in higher IMEP for the cycles.
This may be the reason for higher IMEP of cycles which occur immediately
after the low IMEP cycle.
Similarly, most of the cycles following the UMCs belong to LMCs.
One possible reason for this may be due to the gas dynamic effect. As the
UMC cycle has more IMEP, the thrust on the piston will be more and this
leads to increased acceleration and speed of the piston. This high piston speed
may have an adverse effect on the engine gas exchange process: i.e. it reduces
the real time available for the exhaust-intake gas exchange process, and hence
the next cycle will have less fresh charge and more exhaust residue. This
leads to less energy content of trapped charge and hence a lower IMEP
results.
Figures 7.27 and 7.28 show the crankshaft speed plotted against the
cycle number for the corresponding data set shown in Figures 7.25 and 7.26
supports the above explanation. The higher thrust on piston obtained during
an UMC accelerates the piston at a faster speed. The speed of the piston
increases after 90° from TDC in the expansion stroke. The crankshaft speed of
a particular cycle starts at BDC and ends at BDC i.e. 180° on either side of
TDC.
144
Hence, the effect of piston speed increase is felt only in the next
cycle, where the piston continues to have high speed up to TDC. This results
in a higher crankshaft speed for LMC and lower speed for UMC. This is
what is exactly reflected in Figures 7.27 and 7.28, where the UMC's have
lower speed and the LMC's have higher speed.
The above findings substantiate the explanation for the cause and
effect of prior cycle on cyclic variation. The oscillations between UMCs and
LMCs some time return to MMC mode. However, there are other factors
such as air movement, combustion and flame propagation, mixture non-
homogeneity etc, which may force the cycles to deviate from the MMC
mode and start the oscillations between UMC and LMC.
Speed = 3000 rpm, Power = 1.4 kW, A/F = 16.7:1, Overall Mean = 2998 rpm,
UMC Mean = 2992.2 rpm, MMC Mean = 2998 rpm, LMC Mean=2999.6 rpm
2950
2975
3000
3025
3050
3075
3100
250 260 270 280 290 300
Cycle Number
Spee
d (r
pm)
UMC LMC MMC
Figure 7.27 Plot of engine speed with cycle number
145
Speed = 3000 rpm, Power = 0.6 kW, A/F = 18.1:1, Overall Mean = 2966.2 rpm,
UMC Mean = 2870.2 rpm, MMC Mean = 2946.2 rpm, LMC Mean=2956.4 rpm
2850
2950
3050
250 260 270 280 290 300
Cycle Number
Spee
d (r
pm)
UMC LMC MMC
Figure 7.28 Plot of engine speed with cycle number
7.7.3 Cyclic Variation of Combustion Parameters
From the measured cylinder pressure trace, the heat release rate
is calculated by the procedure described in Chapter 6. From the heat release
rate, the crank angle positions of 5%, 10%, 50% and 90% heat release values
are calculated. The crank angle position of heat release values indicates the
combustion history (Nakagawa et al 1982). The variation in the early phase
of combustion can be identified from the 5% heat release angle. The 90%
heat release angle is the measure of combustion duration. Any variation in
these parameters will affect the cylinder pressure.
The following sections describe the cyclic variations of these
crank angle positions of heat release values and their effects on IMEP.
146
7.7.3.1 Heat release rates
The instantaneous heat release rates calculated from the cylinder
pressures belonging to the three modes are shown in Figure 7.29. The
UMC has a higher maximum heat release rate, indicating a faster
combustion. The cycle belonging to LMC has a lower maximum heat
release rate, indicating slow burning. The cycles belonging to MMC mode
show intermediate trend. The corresponding mass fraction burned curves
are shown in Figure 7.30. The UMC completes its combustion well in
advance and the mass fraction burned is close to unity.
The LMCs continue to burn even during the latter part of expansion
stroke and have lower IMEP. These figures indicate that the UMCs have a
faster combustion with higher heat energy release and the LMCs have a
slower combustion with lower values of heat release, and hence produce less
work. The MMC falls in-between these two.
-10
0
10
20
30
40
50
60
-30 -10 10 30 50 70 90
Crank Angle (degree)
Inst
anta
neou
s Hea
t Rel
ease
Rat
e (k
J/m
3 de
g)
MMC
LMC
UMC
Figure 7.29 Variation of instantaneous heat release rate with crank
angle
147
0
500
1000
1500
2000
2500
-60 -10 40 90Crank Angle (degree)
Cum
ulat
ive
Hea
t Rel
ease
Val
ue
(kJ/
m3)
UMC
MMC
LMC
Figure 7.30 Variation of cumulative heat release value with crank angle
7.7.3.2 Scatter plot of the heat release angles
Figure 7.31 shows the scatter plot of IMEP with different heat
release angles for a lean air-fuel ratio of 18.1:1. The mean values of the heat
release angles are also indicated. It can be observed that for a small
variation in 5% heat release angle, the 90% heat release angle varies
considerably.
The CAQ5 and CAQ10 angles have a narrow band of variation.
The CAQ50 and CAQ90 scatter wide for the cycles having high IMEP.
The lower IMEP cycles have a less variation and hence the heat release
angles occur early.
148
Speed = 3000 rpm, Power = 0.6 kW, A/F = 18.1 : 1 Mean IMEP = 2.59 bar
Mean CAQ5 = 3.24 deg., Mean CAQ10 = 9.70 deg., Mean CAQ50 = 36.86 deg.,
Mean CAQ90 = 55.62 deg.
1
1.5
2
2.5
3
3.5
4
4.5
0 20 40 60 80Crank Angle (degree)
IME
P (b
ar)
CAQ5 CAQ10 CAQ50 CAQ90
Figure 7.31 Plot of IMEP with heat release angles
7.7.3.3 Modes of heat release angles
To further investigate the problem, the cycles belonging to
different modes are separated and their scatter plot is presented in
Figure 7.32. As expected, the UMCs occupy the upper portion and the
LMCs occupy the lower portion. For clarity, the MMCs are not plotted but
their absence can be seen in the figure.
The UMCs heat release angles occur early and have higher
IMEP. This confirms the earlier hypothesis that they have a faster
combustion. The UMCs heat release angles also occur early but result in
lower IMEP. The LMCs also have the CAQ90 similar to UMCs, but
149
produce less IMEP. This indicates that the slow burning LMCs undergo
either misfire or partial burn.
Speed = 3000 rpm, Power = 0.6 kW, A/F = 18.1:1, Mean IMEP = 2.59 bar,
Mean CAQ5 = 3.24 deg., Mean CAQ10 = 9.70 deg., Mean CAQ50 = 36.86 deg.
Mean CAQ90 = 55.62 deg.
1
1.5
2
2.5
3
3.5
4
4.5
0 20 40 60 80Crank Angle (degree)
IME
P (b
ar)
CAQ5 CAQ10 CAQ50 CAQ90
UMC
LMC
Figure 7.32 Plot of IMEP with heat release angles for UMC and LMC
7.7.3.4 Effect of initial burning on cyclic variation
Figure 7.33 shows the dependence of CAQ90 with CAQ5. Earlier
investigations (Sztenderowicz 1990) on cyclic variation indicate that the very
early period of combustion is stable and the flame development period is
erratic and causes cyclic variations in the latter stage of combustion. The
flame development period can be taken as indicated by CAQ5, and its
variation is expected to affect the CAQ90. The figure shows that the cycles
belonging to the three modes exhibit different relations between CAQ5 and
CAQ90.
150
Speed = 3000 rpm, Power = 0.6 kW, A/F = 18.1:1, Mean CAQ5 = 3.24 deg.,
Mean CAQ90 = 55.62 deg.
40455055606570758085
0 1 2 3 4 5 6CAQ5 (deg)
CA
Q90
(deg
)UMC
MMC
LMC
Figure 7.33 Plot of CAQ90 with CAQ5
The UMCs CAQ5 scatters around 5° aTDC and their corresponding
CAQ90 varies between 45° to 75°. The LMCs CAQ5 and CAQ90 show a
linear trend, where a longer CAQ5 produces a higher CAQ90. The MMCs do
not show any specific trend. This explains the earlier findings of wide scatter
in UMCs CAQ9Q, where the scattering in CAQ5 is amplified in CAQ90 only
for UMCs.
7.7.3.5 COV of heat release angles
The COVs calculated from the different heat release angles for the
three modes are shown in Figures 7.34 to 7.37. The COVs calculated from
the entire sample are shown in dotted line along the figures. These figures
indicate that the COVs of heat release angles are lower at stoichiometric air-
fuel ratio and increases with both lean and rich mixtures. Higher COVs are
obtained at the leaner side which indicates higher cyclic variations.
151
0
0.3
0.6
0.9
1.2
1.5
11.8 13.9 15.3 16.2 16.7 18.1Air-Fuel Ratio
CO
V-C
AQ
5
UMC mean
MMC mean
LMC mean
OVERALL mean
Figure 7.34 Variation of COV of 5% heat release angle with air-fuel
ratio
0
0.1
0.2
0.3
0.4
11.8 13.9 15.3 16.2 16.7 18.1Air-Fuel Ratio
CO
V-C
AQ
10
UMC meanMMC meanLMC meanOVERALL mean
Figure 7.35 Variation of COV of 10% heat release angle with air-fuel
ratio
152
0
0.04
0.08
0.12
0.16
0.2
0.24
0.28
11.8 13.9 15.3 16.2 16.7 18.1Air-Fuel Ratio
CO
V-C
AQ
50
UMC meanMMC meanLMC meanOVERALL mean
Figure 7.36 Variation of COV of 50% heat release angle with air-fuel
ratio
0
0.04
0.08
0.12
0.16
0.2
11.8 13.9 15.3 16.2 16.7 18.1Air-Fuel Ratio
CO
V-C
AQ
90
UMC meanMMC meanLMC meanOVERALL mean
Figure 7.37 Variation of COV of 90% heat release angle with air-fuel
ratio
153
An interesting feature that can be observed from the figures is that
the COVs of overall sample is close to the LMCs COVs. Among the three
modes, the LMCs have higher COVs compared to the MMCs and UMCs. The
conclusion that can be arrived from this is that the cyclic variation of the
LMCs mode cycles affects the overall samples of cyclic variation. The MMCs
show minimum cyclic variation at all the air-fuel ratios.
7.7.4 Cyclic Variation of Magnetically Activated Fuel on Catalytic
Coated Engines
The analytical procedure developed in the above sections is applied
to the magnetically activated fuel on catalytic coated engines. Selection and
shape of magnet material is the prime factor (Paul Leangpanich 2004).
Circular shape of magnets is uniformly arranged in a steel cylinder. This is
shielded so as to provide single polarity with more magnetic lines of flux
(Christioph Tschegg 2002). The cyclic variation of base, catalytic coated and
magnetically activated fuel engines are discussed in the following sections.
7.7.4.1 Cyclic variations in cylinder pressures
Figures 7.38 and 7.39 depict the variation of STD and COV of
IMEP for base, catalytic and magnetically activated fuel engines. It can be
observed that both STD and COV of IMEP of catalytic coated and
magnetically activated fuel engines are lower than the base engine. Earlier
studies suggest that when COV increases beyond 0.10 then the drivability of
the vehicle will be affected. The low value of COV is experienced with
ZIRMGE engine.
154
0.20
0.30
0.40
0.50
0.60
0.70
0.80
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
STD
of
IME
P (b
ar)
BASEBASEMG1BASEMG2BASEMGECOPPMGEZIRMGE
Figure 7.38 Variation of STD of IMEP with air-fuel ratio
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
CO
V o
f IM
EP
(bar
)
BASEBASEMG1BASEMG2BASEMGECOPPMGEZIRMGE
Figure 7.39 Variation of COV of IMEP with air-fuel ratio
155
Experimental results show that the magnet with more than 9000
gauss magnetic flux will have good effect on fuel (Masaru 1988). The high
gauss magnetically activated fuel on catalytic coated engine shows lower
cyclic variations compared to the base engine.
7.7.4.2 Effect on cycle variation
The widely used parameter to analyze the combustion variation in
SI engines is peak pressure (Pmax), measured inside the cylinder during
combustion. As combustion rate increases due to magnetically activated fuel,
gas force developed by combustion of the charge inside activated fuel
combustion is found more, compared to that developed at the base
combustion (Christioph Tschegg 2002). This increased gas force leads to
higher peak pressure for the same supply of air-fuel mixture in magnetically
activated fuel engine. Also, cyclic variations of peak pressures are controlled
because combustion rate depends on diffusion rate of the fuel, which further
varies with crank angle position. So, maximum pressure is developed more or
less at a constant crank position in a cycle. So the peak pressure at different
cycles is improved.
Figures 7.40 - 7.43 show the scatter plots of Pmax and IMEP of
individual cycles for both base and magnetically activated fuel engine at an
optimal air-fuel ratio of 16.7:1. The Pmax is directly obtained from the
measured cylinder pressure trace. The crank angle speed is measured by an
optical crank angle encoder. The mean values of these parameters are also
indicated in the figures.
Improvement of cyclic variations in the BASEMGE engine is
14.1%, COPPMGE engine is 19.2% and ZIRMGE engine is 25.1% compared
with base engine running at an A/F of 16.7:1.
156
Mean = 11.05 bar, Stdev = 0.90 bar, COV = 0.081
10.60
10.80
11.00
11.20
11.40
11.60
11.80
0 100 200 300 400 500CYCLE NUMBER
Pmax
(bar
)
Mean = 3.61 bar, Stdev = 0.15 bar, COV = 0.041
3.20
3.40
3.60
3.80
4.00
4.20
0 100 200 300 400 500CYCLE NUMBER
IMEP
(bar
)
Figure 7.40 Scatter plot of peak pressure and IMEP for BASE engine at
3000 rpm and an A/F ratio of 16.7:1
157
Mean = 12.55 bar, Stdev = 0.822 bar, COV = 0.065
12.20
12.40
12.60
12.80
13.00
13.20
0 100 200 300 400 500CYCLE NUMBER
Pmax
(bar
)
Mean = 3.74 bar, Stdev = 0.22 bar, COV = 0.059
3.40
3.60
3.80
4.00
4.20
4.40
0 100 200 300 400 500CYCLE NUMBER
IMEP
(bar
)
Figure 7.41 Scatter plot of Pmax and IMEP for BASEMGE engine at
3000 rpm and an A/F ratio of 16.7:1
158
Mean = 12.78 bar, Stdev = 0.820 bar, COV = 0.064
12.40
12.60
12.80
13.00
13.20
13.40
0 100 200 300 400 500CYCLE NUMBER
Pmax
(bar
)
Mean = 3.83 bar, Stdev = 0.29 bar, COV = 0.075
3.40
3.60
3.80
4.00
4.20
4.40
4.60
0 100 200 300 400 500CYCLE NUMBER
IMEP
(bar
)
Figure 7.42 Scatter plot of Pmax and IMEP for COPPMGE engine at
3000 rpm and an A/F ratio of 16.7:1
159
Mean = 13.01 bar, Stdev = 0.791 bar, COV = 0.060
12.70
12.90
13.10
13.30
13.50
0 100 200 300 400 500CYCLE NUMBER
Pmax
(bar
)
Mean = 4.05 bar, Stdev = 0.36 bar, COV = 0.089
3.60
3.80
4.00
4.20
4.40
4.60
0 100 200 300 400 500CYCLE NUMBER
IMEP
(bar
)
Figure 7.43 Scatter plot of Pmax and IMEP for ZIRMGE engine at
3000 rpm and an A/F ratio of 16.7:1
160
Among the various combinations at a leaner side, ZIRMGE has higher IMEP of 4.05 bar and lower cyclic variation of 0.791 bar. The
variations of Pmax for continuous cycles of magnetically activated catalytic coated engines are less than that of the base engine.
The coefficient of variation of Pmax and IMEP are calculated from
the cycles belonging to different modes are plotted. The COV of Pmax decreases from base engine to catalytic coated engine whereas COV of IMEP is increased.
7.7.4.3 Cyclic variations in crank angle of heat release values The variation of STD and COV of CAQ5 and CAQ90 are
presented in Figures 7.44 to 7.47. CAQ5 and CAQ90 are related to the start and end of combustion. At rich mixtures, the COV of CAQ5 and CAQ90 are less compared to the lean mixtures. The variation in the crank angle of 5%
heat release indicates the time variation in igniting the mixture. If the mixture near the vicinity of spark plug is in flammable state, combustion starts and
CAQ5 will occur at a consistent crank angle. On the other hand, if the mixture near the spark plug is too lean or
too rich, then the start of combustion will be delayed. This will be reflected in
the variation of CAQ5. It was illustrated in the earlier studies that the variation in the early flame development leads to cyclic variation of combustion (Ho 1987). In the present work similar trend is observed, where higher cyclic variation in the CAQ5 leads to higher COV in CAQ90 as seen in
the Figures 7.45 and 7.47.
Compared to the base engine, the high gauss magnetically activated fuel on catalytic coated engines show lower cyclic variation in the
lean mixture ranges. In the rich mixture range, all the configurations show reduced cyclic variations.
161
0.00.20.40.60.81.01.21.41.61.82.0
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
STD
of
CA
Q5
BASE BASEMG1BASEMG2 BASEMGECOPPMGE ZIRMGE
Figure 7.44 Variation of STD of CAQ5 with air-fuel ratio
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
CO
V o
f C
AQ
5
BASE BASEMG1BASEMG2 BASEMGECOPPMGE ZIRMGE
Figure 7.45 Variation of COV of CAQ5 with air-fuel ratio
162
0.0
2.0
4.0
6.0
8.0
10.0
12.0
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
STD
of
CA
Q90
BASEBASEMG1BASEMG2BASEMGECOPPMGEZIRMGE
Figure 7.46 Variation of STD of CAQ90 with air-fuel ratio
0.05
0.10
0.15
0.20
0.25
0.30
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
CO
V o
f C
AQ
90
BASE BASEMG1BASEMG2 BASEMGECOPPMGE ZIRMGE
Figure 7.47 Variation of COV of CAQ90 with air-fuel ratio
163
7.7.4.4 Modes of cyclic variation
Figures 7.48 to 7.50 illustrate the different modes of cycles
belonging to different catalysts, gauss values of magnet and base engine for
various air-fuel ratios. The numbers of cycles belonging to MMC are plotted
for various air-fuel ratios in Figure 7.48. In the rich range, almost all the
500 cycles are belonging to MMC and only few cycles are in the LMC group.
For lower cyclic variation the number of cycles belonging to MMC should be
more.
In addition, when the number of cycles belonging to either UMC or
LMC group increase, the cyclic variations increase. This can be observed
from Figures 7.40 and 7.48, where COV of IMEP increases as the number of
cycles belonging to the MMC decrease in the lean range.
100
150
200
250
300
350
400
450
500
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
MM
C
BASEBASEMG1BASEMG2BASEMGECOPPMGEZIRMGE
Figure 7.48 Variation of number of middle mode cycles with air-fuel
ratio
164
0
117
233
350
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
UM
C
BASEBASEMG1BASEMG2BASEMGECOPPMGEZIRMGE
Figure 7.49 Variation of number of upper mode cycles with air-fuel
ratio
0
50
100
150
200
250
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
LMC
BASEBASEMG1BASEMG2BASEMGECOPPMGEZIRMGE
Figure 7.50 Variation of number of lower mode cycles with air-fuel ratio
165
This trend is observed for all the categories of the engines. As the
number of cycles in MMC group decrease, the corresponding cycles in the
UMC and LMC increase. Compared to the base engine, the high gauss
magnetically activated fuel on catalytic coated engines has more cycles in the
MMC group and hence less cyclic variations in the lean range.
The respective IMEP of the different groups are plotted along with
air-fuel ratios in Figures 7.51 to 7.53. It can be observed that the cycles
belonging to UMCs have more IMEP and their contribution is nullified by the
lower IMEP produced by the LMCs. This can be observed from Figures 7.49
and 7.50, where the number of cycles belonging to UMCs and LMCs are
almost equal. Whereas, the IMEPs plotted in Figures 7.52 and 7.53 show a
higher value for UMCs and a lower value for LMCs. Among the different
gauss values of magnets and catalysts, the ZIRMGE has higher IMEP and
lower cyclic variation compared to the base engine.
1.0
1.5
2.0
2.5
3.0
3.5
4.0
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
IME
P of
MM
C (b
ar)
BASEBASEMG1BASEMG2BASEMGECOPPMGEZIRMGE
Figure 7.51 Variation of IMEP of MMC with air-fuel ratio
166
1.0
1.5
2.0
2.5
3.0
3.5
4.0
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
IME
P of
UM
C (b
ar)
BASEBASEMG1BASEMG2BASEMGECOPPMGEZIRMGE
Figure 7.52 Variation of IMEP of UMC with air-fuel ratio
1.0
1.5
2.0
2.5
3.0
3.5
10.0 12.0 14.0 16.0 18.0Air-Fuel Ratio
IME
P of
LM
C (b
ar)
BASEBASEMG1BASEMG2BASEMGECOPPMGEZIRMGE
Figure 7.53 Variation of IMEP of LMC with air-fuel ratio
167
7.8 SUMMARY
The following points are arrived at based on the above work:
The cyclic variation increases as the mixture becomes leaner.
The COV of IMEP at 11.8 air-fuel ratio is 0.05 and at 18.1
air-fuel ratio is 0.35 for an engine speed of 3000 rpm.
Three separate modes of cycle can be identified with different
combustion phasing i.e. upper mode, middle mode and lower
mode cycles.
The cyclic variation in UMC is more compared to MMC and
LMC. This increases with air-fuel ratio. The COV of IMEP at
11.8 air-fuel ratio is 0.02 and at 18.1 is 0.03 for the UMC
whereas the corresponding COVs are 0.03 and 0.05 for
MMCs.
The mean value of cylinder pressure parameters of the entire
sample are represented very well by MMCs.
The prior cycle effect shows a distinct relation between UMCs
and LMCs. Most of the UMCs occur immediately after the
LMCs and vice versa.
The cyclic variation is affected by both gas dynamic effects
caused by engine speed and the variation in the amount of fuel
trapped in each cycle.
The UMC completes its combustion well in advance and
contribute higher IMEP.
The LMCs have lower values of mass fraction burned and
contain both misfire and partial burn cycles.
168
The cyclic variation in the early burn period affects the latter
stages of combustion. This effect is more pronounced in the
case of UMCs.
The COVs were calculated from heat release angles indicate
that minimum cyclic variation occurs near the stoichiometric
air-fuel ratio. The cyclic variation increases for lean mixtures
for base, magnetically activated and catalytic coated engines.
Improvement of cyclic variations in the BASEMGE engine is
14.1%, COPPMGE engine is 19.2% and ZIRMGE engine is
25.1% compared with base engine running at an air fuel ratio
of 16.7:1.
Among the varieties of magnets and catalysts, the ZIRMGE
has higher IMEP of 4.05 bar and lower cyclic variation of
0.79 bar compared to the base engine.