CHAPTER 7 CYCLIC VARIATIONS - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/26382/12/12_chapter7.pdf · max (CAP max) also varies from one cycle to another. This leads to variation

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




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


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.,


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


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



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



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,


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,


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

)


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


10.5010.6010.7010.8010.9011.0011.1011.2011.3011.4011.50

0 100 200 300 400 500CYCLE NUMBER

Pmax

(bar

)


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


137


9.509.609.709.809.90

10.0010.1010.2010.3010.40

0 100 200 300 400 500CYCLE NUMBER

Pmax

(bar

)


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


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


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


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


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


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


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


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


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.


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.,


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



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



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



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

)


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

)


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


12.20

12.40

12.60

12.80

13.00

13.20

0 100 200 300 400 500CYCLE NUMBER

Pmax

(bar

)


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


158


12.40

12.60

12.80

13.00

13.20

13.40

0 100 200 300 400 500CYCLE NUMBER

Pmax

(bar

)


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


159


12.70

12.90

13.10

13.30

13.50

0 100 200 300 400 500CYCLE NUMBER

Pmax

(bar

)


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


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


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


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


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


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


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


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)


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)


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)


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.

Documents

CHAPTER 7 CYCLIC VARIATIONS - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/26382/12/12_chapter7.pdf · max (CAP max) also varies from one cycle to another. This leads to variation