CHAPTER 7 CYCLIC VARIATIONS

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1 114 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 twostroke 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 twostroke 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

2 115 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, P max Crank angle at which the in-cylinder peak pressure occurs, CAP max Maximum rate of pressure rise, (dp/d) max IMEP of the individual cycles

3 116 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:

4 117 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.

5 118 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 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 (P max ) is higher and for certain cycles it is lower. The location of the P max (CAP max ) 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 P max, or in rate of pressure rise (dp/dmax), or in CAP max, or in IMEP. The engine performance and torque developed are very much depends upon IMEP and any variation in IMEP leads to torque fluctuations.

6 Pressure (bar) Motored TDC Crank Angle (degree) Figure 7.1 Variation of measured cylinder pressure with crank angle for ten cycles Scatter plot of P max, IMEP and crankshaft speed Figures 7.2 to 7.4 show the scatter plots of P max, IMEP and engine speed of individual cycles. The P max 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.

7 120 Speed =3000 rpm, Power = 1.4 kw, A/F = 16.7:1, Mean P max =11.05 bar Pmax (bar) Cycle Number Figure 7.2 Scatter plot of P max with cycle number Speed = 3000 rpm, Power = 1.4 kw, A/F = 16.7:1, Mean IMEP = 3.61 bar IMEP (bar) Cycle Number Figure 7.3 Scatter plot of IMEP with cycle number

8 121 Speed = 3000 rpm, Power = 1.4 kw, A/F = 16.7:1, Mean Speed = 2998 rpm Speed (rpm) Cycle Number Figure 7.4 Scatter plot of engine speed with cycle number Speed = 3000 rpm, Power = 1.4 kw, A/F = 16.7:1, Mean P max = bar Pmax (bar) Cycle Number Figure 7.5 Plot of peak pressure with cycle number

9 122 Speed = 3000 rpm, Power = 1.4 kw, A/F = 16.7:1, Mean IMEP = 3.61 bar IMEP (bar) Cycle Number 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 Speed (rpm) Cycle Number Figure 7.7 Plot of engine speed with cycle number

10 123 The P max is a measure of rate of pressure rise due to combustion. If the combustion is faster, higher-pressure rise rate occurs and a higher P max results. The P max 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 P max. Also the P max will tend to occur closer to TDC whereas a slower burning cycle will have lower P max and that CAP max 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, P max and IMEP, the plot of speed versus P max 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.

11 124 Speed = 3000 rpm, Power = 1.4 kw, A/F = 16.7:1, Mean Speed = 2998 rpm, Mean P max = bar Speed (rpm) Pmax (bar) Figure 7.8 Plot of engine speed with P max Speed = 3000 rpm, Power = 1.4 kw, A/F = 16.7:1, Mean Speed = 2998 rpm, Mean IMEP = 3.61 bar Speed (rpm) IMEP (bar) Figure 7.9 Plot of engine speed with IMEP

12 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 P max, CAP max 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 CAP max. 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 P max values with its peak occurring close to TDC are called fast burn cycles. Whereas the cycles having lower P max values with its peak away from the TDC are designated as slow burn cycles. Within a limited range, a relatively linear relation exists between P max and CAP max. The fast burning cycles produce a higher P max 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 P max and IMEP with CAP max 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 P max, IMEP and CAP max at different air-fuel ratio show similar trends.

13 126 Figures 7.10 to 7.13 show the plot of P max and IMEP with CAP max at 4000 rpm for the air-fuel ratios of 11.8:1 and 18.1:1. The spread of P max and IMEP at a particular CAP max indicates the existence of burn phases in both cases. For rich mixture the P max and IMEP correlates well with CAP max. A cycle having CAP max closer to TDC will produce a higher P max 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 P max with CAP max and a higher value of P max occurs for a cycle having CAP max closer to TDC. Speed = 4000 rpm, Power = 2.6 kw, A/F = 11.8:1, Mean CAP max = 28.2 deg., Mean P max = bar Pmax (bar) CAPmax (deg) Figure 7.10 Variation of P max with CAP max at an A/F of 11.8:1

14 127 Speed = 4000 rpm, Power = 2.6 kw, A/F = 11.8:1, Mean CAP max = 28.2 deg., Mean IMEP = 5.24 bar IMEP (bar) CAPmax (deg) Figure 7.11 Variation of IMEP with CAP max at an A/F of 11.8:1 Figure 7.12 and 7.13 show the relation between P max and IMEP with CAP max 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 P max and IMEP. Few cycles have CAP max well before TDC, indicating the pre-ignition. It is noted that preignition 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.

15 128 Speed = 4000 rpm, Power = 1.0 kw, A/F = 18.1:1, Mean CAP max = 15.3 deg., Pmax (bar) Mean P max = 10.0 bar CAPmax (deg) Figure 7.12 Variation of P max with CAP max at an A/F of 18.1:1 Speed = 4000 rpm, Power = 1.0 kw, A/F = 18.1:1, Mean CAP max = 15.3 deg., Mean IMEP = 2.59 bar IMEP (bar) CAPmax (deg) Figure 7.13 Variation of IMEP with CAP max at an A/F of 18.1:1

16 129 Similar results are obtained by Martin et al (1988) for lean mixture operation, where the cyclic variability in IMEP and P max 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 P max remains low and CAP max occurs closer to TDC, as the combustion event is retarded (Whitelaw et al 1995). These slowburn cycles in the hook-back and return regions are responsible for considerable variation in IMEP as seen in Figure Burn modes The relation between P max and IMEP for two different air-fuel ratios at an engine speed of 4000 rpm is shown in Figures 7.14 and The mean values of P max 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 P max 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 P max variation. For example, a group of cycles, having a wide variation of P max from 9.8 bar to 12 bar results in near zero IMEP values. Another group of cycles, having a narrow band of P max 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 P max increases.

17 130 Speed = 4000 rpm, Power = 2.6 kw, A/F = 11.8:1, Mean P max = bar, Mean IMEP = 5.24 bar 9 8 IMEP (bar) Pmax (bar) Figure 7.14 Variation of IMEP with P max at an A/F of 11.8:1 Speed = 4000 rpm, Power = 1.0 kw, A/F = 18.1:1, Mean P max = 10.0 bar, Mean IMEP = 2.59 bar 6 5 IMEP (bar) Pmax (bar) Figure 7.15 Variation of IMEP with P max at an A/F of 18.1:1

18 131 But the mean values indicated in the figure do not represent all these groups of cycle. There are cycles having both P max and IMEP higher than mean values and cycles having both P max and IMEP lower than mean. Also certain groups of cycles have both P max 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 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.

19 132 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 P max 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) = X i > mean of IMEP + 10% of mean of IMEP Middle Mode Cycles (MMC) = mean of IMEP 10% of mean of IMEP < X i < mean of IMEP + 10 % of mean of IMEP Lower Mode cycles (LMC) = X i < mean of IMEP 10% of mean of IMEP for i = 1,2, These different modes of cycles can be identified from the Figure 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 P max. The P max value occurs near TDC for the LMCs. The explanation for this may be that, the rate of pressure rise due to

20 133 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 CAP max as seen in the Figure 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 P max and IMEP. As seen in the figures, the P max 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 P max Microsoft excel sheet is used 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 x i STD = = (1/N (x i ň) 2 ) and COV = STD / Mean = / ň where N = sample size and i = 1, 2,

21 134 Microsoft excel worksheet is used to calculate the above statistical values. Figures shows the scatter plot of each group of data and its corresponding mean, STD and COV for P max 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 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 P max 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 P max 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 P max 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.

22 135 Mean = bar, Stdev = 0.92 bar, COV = 0.07, No. of Cycles = Pmax (bar) CYCLE NUMBER Mean = 3.67 bar, Stdev = 0.13 bar, COV = 0.04, No. of Cycles = 120 IMEP (bar) CYCLE NUMBER Figure 7.16 Scatter plot of P max and IMEP for upper mode cycle operation at 3000 rpm and an A/F of 16.7:1

23 136 Mean = bar, Stdev = 0.61 bar, COV = 0.06, No. of Cycles = 227 Pmax (bar) CYCLE NUMBER Mean = 3.34 bar, Stdev = 0.14 bar, COV = 0.05, No. of Cycles = IMEP (bar) CYCLE NUMBER Figure 7.17 Scatter plot of P max and IMEP for middle mode cycle operation at 3000 rpm and an A/F of 16.7:1

24 137 Mean = 9.95 bar, Stdev = 0.23 bar, COV = 0.03, No. of Cycles = 153 Pmax (bar) CYCLE NUMBER Mean = 3.01 bar, Stdev = 0.27 bar, COV = 0.11, No. of Cycles = 153 IMEP (bar) CYCLE NUMBER Figure 7.18 Scatter plot of P max and IMEP for lower mode cycle operation at 3000 rpm and an A/F of 16.7:1

25 138 Pmax (bar) UMC mean MMC mean LMC mean OVERALL mean Equivalence Ratio Figure 7.19 Variation of mean P max with equivalence ratio IMEP (bar) LMC mean MMC mean UMC mean OVERALL mean Equivalence Ratio Figure 7.20 Variation of Mean IMEP with equivalence ratio

26 139 COV of IMEP UMC mean MMC mean LMC mean OVERALL mean Equivalence Ratio Figure 7.21 Variation of COV of IMEP with equivalence ratio COV of Pmax UMC mean MMC mean LMC mean OVERALL mean Equivalence Ratio Figure 7.22 Variation of COV of P max with equivalence ratio

27 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 N th 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 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.

28 141 Speed = 3000 rpm, Power = 1.4 kw, A/F = 16.7:1, Mean IMEP = 3.61 bar 6 IMEP of N+1 Cycle IMEP of Nth Cycle 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 6 IMEP of N+1 Cycle IMEP of Nth Cycle Figure 7.24 Plot of IMEP of N+1 cycle with Nth cycle at an A/F of 18.1:1

29 142 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 UMC LMC MMC IMEP (bar) Cycle Number 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 UMC LMC MMC 3 IMEP (bar) Cycle Number Figure 7.26 Plot of IMEP with cycle number

30 143 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.

31 144 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 nonhomogeneity 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 = rpm, MMC Mean = 2998 rpm, LMC Mean= rpm UMC LMC MMC Speed (rpm) Cycle Number Figure 7.27 Plot of engine speed with cycle number

32 145 Speed = 3000 rpm, Power = 0.6 kw, A/F = 18.1:1, Overall Mean = rpm, UMC Mean = rpm, MMC Mean = rpm, LMC Mean= rpm 3050 UMC LMC MMC Speed (rpm) Cycle Number Figure 7.28 Plot of engine speed with cycle number 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.

33 Heat release rates The instantaneous heat release rates calculated from the cylinder pressures belonging to the three modes are shown in Figure 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 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. Instantaneous Heat Release Rate (kj/m3 deg) MMC LMC UMC Crank Angle (degree) Figure 7.29 Variation of instantaneous heat release rate with crank angle

34 147 Cumulative Heat Release Value (kj/m3) UMC MMC LMC Crank Angle (degree) Figure 7.30 Variation of cumulative heat release value with crank angle 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.

35 148 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 = deg., Mean CAQ90 = deg CAQ5 CAQ10 CAQ50 CAQ90 IMEP (bar) Crank Angle (degree) Figure 7.31 Plot of IMEP with heat release angles 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 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

36 149 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 = deg. Mean CAQ90 = deg CAQ5 CAQ10 CAQ50 CAQ90 IMEP (bar) UMC LMC Crank Angle (degree) Figure 7.32 Plot of IMEP with heat release angles for UMC and LMC 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.

37 150 Speed = 3000 rpm, Power = 0.6 kw, A/F = 18.1:1, Mean CAQ5 = 3.24 deg., Mean CAQ90 = deg. CAQ90 (deg) UMC MMC LMC CAQ5 (deg) 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 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 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 airfuel ratio and increases with both lean and rich mixtures. Higher COVs are obtained at the leaner side which indicates higher cyclic variations.

38 151 COV-CAQ UMC mean MMC mean LMC mean OVERALL mean Air-Fuel Ratio Figure 7.34 Variation of COV of 5% heat release angle with air-fuel ratio COV-CAQ UMC mean MMC mean LMC mean OVERALL mean Air-Fuel Ratio Figure 7.35 Variation of COV of 10% heat release angle with air-fuel ratio

39 152 COV-CAQ UMC mean MMC mean LMC mean OVERALL mean Air-Fuel Ratio Figure 7.36 Variation of COV of 50% heat release angle with air-fuel ratio COV-CAQ UMC mean MMC mean LMC mean OVERALL mean Air-Fuel Ratio Figure 7.37 Variation of COV of 90% heat release angle with air-fuel ratio

40 153 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 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 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.

41 154 STD of IMEP (bar) BASE BASEMG1 BASEMG2 BASEMGE COPPMGE ZIRMGE Air-Fuel Ratio Figure 7.38 Variation of STD of IMEP with air-fuel ratio COV of IMEP (bar) BASE BASEMG1 BASEMG2 BASEMGE COPPMGE ZIRMGE Air-Fuel Ratio Figure 7.39 Variation of COV of IMEP with air-fuel ratio

42 155 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 Effect on cycle variation The widely used parameter to analyze the combustion variation in SI engines is peak pressure (P max ), 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 show the scatter plots of P max and IMEP of individual cycles for both base and magnetically activated fuel engine at an optimal air-fuel ratio of 16.7:1. The P max 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.

43 156 Mean = bar, Stdev = 0.90 bar, COV = Pmax (bar) CYCLE NUMBER Mean = 3.61 bar, Stdev = 0.15 bar, COV = IMEP (bar) CYCLE NUMBER 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

44 Mean = bar, Stdev = bar, COV = Pmax (bar) CYCLE NUMBER Mean = 3.74 bar, Stdev = 0.22 bar, COV = IMEP (bar) CYCLE NUMBER Figure 7.41 Scatter plot of P max and IMEP for BASEMGE engine at 3000 rpm and an A/F ratio of 16.7:1

45 Mean = bar, Stdev = bar, COV = Pmax (bar) CYCLE NUMBER Mean = 3.83 bar, Stdev = 0.29 bar, COV = IMEP (bar) CYCLE NUMBER Figure 7.42 Scatter plot of P max and IMEP for COPPMGE engine at 3000 rpm and an A/F ratio of 16.7:1

46 159 Mean = bar, Stdev = bar, COV = Pmax (bar) CYCLE NUMBER Mean = 4.05 bar, Stdev = 0.36 bar, COV = IMEP (bar) CYCLE NUMBER Figure 7.43 Scatter plot of P max and IMEP for ZIRMGE engine at 3000 rpm and an A/F ratio of 16.7:1

47 160 Among the various combinations at a leaner side, ZIRMGE has higher IMEP of 4.05 bar and lower cyclic variation of bar. The variations of P max for continuous cycles of magnetically activated catalytic coated engines are less than that of the base engine. The coefficient of variation of P max and IMEP are calculated from the cycles belonging to different modes are plotted. The COV of P max decreases from base engine to catalytic coated engine whereas COV of IMEP is increased 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 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 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.

48 161 STD of CAQ BASE BASEMG2 COPPMGE BASEMG1 BASEMGE ZIRMGE Air-Fuel Ratio Figure 7.44 Variation of STD of CAQ5 with air-fuel ratio COV of CAQ BASE BASEMG2 COPPMGE BASEMG1 BASEMGE ZIRMGE Air-Fuel Ratio Figure 7.45 Variation of COV of CAQ5 with air-fuel ratio

49 162 STD of CAQ BASE BASEMG1 BASEMG2 BASEMGE COPPMGE ZIRMGE Air-Fuel Ratio Figure 7.46 Variation of STD of CAQ90 with air-fuel ratio 0.30 BASE BASEMG1 COV of CAQ BASEMG2 COPPMGE BASEMGE ZIRMGE Air-Fuel Ratio Figure 7.47 Variation of COV of CAQ90 with air-fuel ratio

50 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 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 MMC BASE BASEMG1 BASEMG2 BASEMGE COPPMGE ZIRMGE Air-Fuel Ratio Figure 7.48 Variation of number of middle mode cycles with air-fuel ratio

51 164 UMC BASE BASEMG1 BASEMG2 BASEMGE COPPMGE ZIRMGE Air-Fuel Ratio Figure 7.49 Variation of number of upper mode cycles with air-fuel ratio LMC BASE BASEMG1 BASEMG2 BASEMGE COPPMGE ZIRMGE Air-Fuel Ratio Figure 7.50 Variation of number of lower mode cycles with air-fuel ratio

52 165 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 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. IMEP of MMC (bar) BASE BASEMG1 BASEMG2 BASEMGE COPPMGE ZIRMGE Air-Fuel Ratio Figure 7.51 Variation of IMEP of MMC with air-fuel ratio

53 166 IMEP of UMC (bar) BASE BASEMG1 BASEMG2 BASEMGE COPPMGE ZIRMGE Air-Fuel Ratio Figure 7.52 Variation of IMEP of UMC with air-fuel ratio 3.5 IMEP of LMC (bar) BASE BASEMG1 BASEMG2 BASEMGE COPPMGE ZIRMGE Air-Fuel Ratio Figure 7.53 Variation of IMEP of LMC with air-fuel ratio

54 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.

55 168 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.

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