CHAPTER 8 EFFECTS OF COMBUSTION CHAMBER GEOMETRIES

112 CHAPTER 8 EFFECTS OF COMBUSTION CHAMBER GEOMETRIES 8.1 INTRODUCTION Energy conservation and emissions have become of increasing concern over the past few decades. More stringent emission laws along with the need to conserve the limited resources of petroleum based fuels, engineers concerned are under significant pressure to improve their energy efficiency and reduce exhaust emission levels. In this context, there has been growing interest and extensive research on the processes that take place in internal combustion engines and alternative fuels like biodiesel to provide a suitable diesel oil substitute for internal combustion engines. However the results of the first phase of this research program and most of the studies reviewed on the performance of biodiesel fuelled diesel engine indicate, decrease in engine power and thermal efficiency, increase in specific fuel consumption and increase in emissions particularly NO x, when compared with PBDF operation. The poor performance of the biodiesel operated diesel engine in comparison with PBDF fuelled diesel engine is mainly due to changes in fuel properties, engine design and operating parameters. The performance, emission and combustion characteristics of the DI diesel engine are greatly influenced by the air motion inside the cylinder. The fuel-air mixing and subsequent combustion in a DI diesel engine are controlled by the flow field in the cylinder caused by the combustion chamber geometry. The air motion in a diesel engine, during compression stroke is caused by the combustion

113 chamber. Hence, combustion chamber configurations need more attention to meet the global trends in fuel consumption, performance and emissions. In this phase of experimental work, without altering the compression ratio of the engine, the piston bowl geometry was modified to have Shallow depth Combustion Chamber (SCC), Toroidal Combustion Chamber (TCC), Shallow depth Re-entrant Combustion Chamber (SRCC) and Toroidal Re-entrant Combustion Chamber (TRCC) from the baseline Hemispherical Combustion Chamber (HCC). Engine tests were carried out using these five types of combustion chamber geometries namely HCC, SCC, TCC, SRCC and TRCC to compare the performance, emission and combustion characteristics of 20% blend of POME with PBDF. The shapes and dimensions of five combustion chamber geometries employed in this study are shown in Figure 5.1 and Figure 4.13 shows the photographic view of pistons having the five shapes of combustion chambers. 8.2 AIM AND OBJECTIVES The aim of this experimental phase was to investigate the effects of combustion chamber geometry on the performance, emission and combustion characteristics of POME20 fuelled engine and compare the results with that of baseline PBDF and POME20 operated engine to select the best combustion chamber geometry for biodiesel fuelled engine. To achieve this aim, the following objectives were set: To investigate experimentally the impact of different combustion chamber geometries on the performance, emission and combustion characteristics of POME20 fuelled DI diesel engine at different loads of operation.

114 To compare the above results with the baseline engine, operated with PBDF and POME20 to decide on the best combustion chamber geometry for biodiesel operation. 8.3 EXPERIMENTAL PROCEDURE The performance, emission and combustion tests were conducted using the pistons having five different combustion chamber geometries, following the experimental procedure explained in Chapter-7. Hereafter the term standard engine represents engine with piston having HCC and modified engine means engine with piston having SCC, TCC, SRCC and TRCC. To begin with the performance, emission and combustion tests were carried out using PBDF at various loads for standard engine with fuel injection pressure of 200 bar and fuel injection timing of 23º btdc, which are set by the manufacturer. These performance, combustion and emission values were used as baseline values throughout the experimentation for comparison with the results obtained from modified engine with POME20 and PBDF. Then the performance, emission and combustion tests were conducted for the modified engine having SCC, TCC, SRCC and TRCC with PBDF and POME20 at the same injection parameters i.e. injection pressure of 200 bar and fuel injection timing of 23 btdc. The engine tests were carried out at 0%, 25%, 50%, 75% and 100% load. In order to have a meaningful comparison of emissions and engine performance, the investigation was carried out at same operating conditions i.e. ambient temperature, atmospheric pressure, engine speed and torque were maintained for all combustion chamber geometries. The effects of combustion chamber geometries, on the performance, combustion and emission characteristics of the DI diesel engine operated with POME20 and PBDF at different loads of operation were

115 investigated. The results were analyzed and compared with standard engine fuelled with POME20 and PBDF and presented in the next section of this chapter. 8.4 RESULTS AND DISCUSSION The performance, emission and combustion characteristics of the standard engine with HCC and modified engine with TCC, SCC, TRCC and SRCC were measured, analyzed and compared for BSFC, BTE, UBHC, CO, NO x, smoke emissions and combustion parameters such as ignition delay, cylinder peak pressure, exhaust gas temperature and heat release rate. 8.4.1 Combustion Characteristics One of the important parameters in the combustion phenomenon is the ignition delay. The definition of the ignition delay is the time from the start of fuel injection to the start of combustion. Figure 8.1 shows the variation of ignition delay for standard and modified engine. It was observed that the ignition delay period of POME20 was significantly lower than that of PBDF when tested in the standard engine. The fuels with a high cetane number make auto-ignition easily and give short ignition delay period. So, the primary reason for the decrease in the ignition delay was the cetane number of POME20 which was higher than that of PBDF. It was also observed that, for re-entrant combustion chambers the ignition delay periods were lower compared to open type combustion chambers at all loads. For all tested fuels and combustion chamber geometries the reduction in ignition delay increased with the increase in load. This was due to higher combustion chamber wall temperature and reduced exhaust gas dilution at higher loads. The ignition delays for TRCC, SRCC, TCC, HCC and SCC fuelled with POME20 were 2.95 o CA, 2.3 o CA, 2.07 o CA, 1.75 o CA and 1.25 o CA, shorter respectively than that of PBDF operated standard engine having HCC at full load.

116 Figure 8.2 shows the comparison of the heat release rate curves for the standard engine and the modified engine with PBDF and POME20. It can be seen from the Figure 8.2 that, the maximum heat release rate of POME20 blend was lower than that of PBDF in the standard engine. This was due to shorter ignition delay for POME20 compared with that of PBDF. In addition, the poor spray atomization characteristics of biodiesel due to higher viscosity and surface tension may also be responsible for the lower heat release rate. Further it was noticed that the heat release rate during the diffusion combustion phase of POME20 was slightly higher than that of PBDF. However the heat release rate for re-entrant combustion chambers fuelled with POME20 demonstrated similar but slightly better than baseline engine fuelled with PBDF and much improved, compared to open combustion chambers operated with POME20. It was observed that the maximum heat release rate of 87.6 J/ o CA was recorded for TRCC at 9 btdc, while baseline HCC recorded its maximum heat release rate of 67.6 J/ o CA at 7 btdc. This was due to improved air fuel mixing as a result of enhanced swirl and TKE, evaporation and better combustion. Figure 8.3 shows the comparison of the cumulative heat release rate curves for the standard engine and the modified engine with PBDF and POME20. A slight advance in cumulative heat release was observed for diesel fuel as compared to POME20 fuel during pre-combustion phase which can be attributed to longer ignition delay and higher calorific value. Other reasons responsible for this were, better air-fuel mixing and evaporation as a result of lower viscosity and higher volatility. However the cumulative heat release rate for re-entrant combustion chambers fuelled with POME20 demonstrated slightly better trend compared to baseline engine fuelled with PBDF and open combustion chambers operated with POME20. The cumulative heat release rate for TRCC, SRCC, TCC, HCC and SCC fuelled with POME20 were measured as 671 J, 645 J, 641 J, 599 J, and 576 J at 10 o CA atdc respectively at full load.

117 Figure 8.1 Variation of ignition delay for different CC Figure 8.2 Comparison of HRR at full load for different CC

118 The cylinder pressure variation with crank angle for five types of combustion chambers with POME20 and PBDF is shown in Figure 8.4. The pressure variations of all five types of combustion chambers followed the similar pattern of pressure rise as that of PBDF at all load conditions. However when compared to PBDF, the values of pressure data of POME20 with standard engine were lower at all operating conditions. These distinct differences were due to variation of viscosity and heating value of POME20. The cylinder gas pressure trend of TCC with POME20 was found closer to that of the standard engine operated with PBDF and well above SCC and HCC. However the cylinder gas pressures of TRCC fuelled with POME20 was higher than that of standard engine operated with the PBDF. This can be endorsed to better combustion due to enhanced air fuel mixing in TRCC. Other reasons for improved performance may be attributed to POME20 s higher cetane number and Oxygen content. The cylinder pressure trend of SRCC with POME20 lie in between TRCC and TCC. The maximum cylinder gas pressure for TRCC, SRCC, TCC, HCC and SCC fuelled with POME20 were measured at 5 o, 6 o, 7 o, 8 o and 7 o CA atdc respectively at full load operation. The variation of peak pressures with respect to brake power for the modified engine and standard engine with diesel and POME20 is shown in Figure 8.5. It can be seen from the Figure 8.5 that the peak pressure was lower for POME20 (67.24 bar) when compared to that of PBDF (74.35 bar) in the standard engine. This was because of improper mixing and poor combustion of POME20 with air due to higher viscosity and lower calorific value for POME20. However the peak pressure for TRCC operated with POME20 (77.1 bar) was higher than that of HCC fuelled with PBDF (74.35 bar) operation. This can be attributed mainly, to the much improved combustion due to the better in-cylinder air motion as determined in simulation study and mixture preparation.

119 Figure 8.3 Comparison of CHRR at full load for different CC Figure 8.4 Comparison of cylinder pressure at full load for diff CC

120 Figure 8.6 shows the variation of EGT for standard engine and modified engine with PBDF and POME20. EGT increased with engine load for all test fuels and all combustion chambers employed. The EGT increased Figure 8.5 Variation of peak pressures for different CC Figure 8.6 Comparison of EGT for different CC

121 with the load because more fuel was burnt at higher loads to meet the power requirement. It was observed that the EGT of the POME20 blend was higher than that of PBDF. The reason can be due to the presence of excess Oxygen in POME20 which improves combustion. In addition this can also be reasoned to shorter ignition delay, which resulted in reduced premixed combustion, which in-turn increased the diffusion combustion. Longer afterburning resulted in higher EGT. It was also observed that, EGT for re-entrant combustion chambers was higher than the open combustion chambers. This was due to more complete combustion as a result of better air fuel mixing and the presence of Oxygen in the POME. The EGT for TRCC, SRCC, TCC, HCC and SCC fuelled with POME20 were measured as 548 o C, 532 o C, 504 o C, 481 o C and 468 o C, respectively at full load operation. 8.4.2 Performance Characteristics The BSFC variations for standard engine and modified engine operated with POME20 and PBDF are shown in Figure 8.7. The BSFC for TRCC (0.252 kg/ kw-hr) was lower than any other type of combustion chamber with POME20 at full load operation. This may be attributed to better combustion of POME20 due to better air fuel mixing, as a result of improved swirl velocity and turbulent kinetic energy (TKE) as determined in simulation study. Further the BSFC for SCC (0.308 kg/ kw-hr) was higher than HCC (0.293 kg/ kw-hr), TCC (0.280 kg/ kw-hr) and SRCC (0.272 kg/ kw-hr). It can be seen from the Figure 8.7 that re-entrant combustion chambers demonstrated better results compared to open type combustion chambers. Compared to HCC, the BSFC for the modified re-entrant combustion chambers viz. SRCC and TRCC was lower by 7% and 13.9% respectively at full load operation with POME20. However BSFC for POME20 (0.252 kg/ kw-hr) was slightly higher than that of PBDF (0.242 kg/ kw-hr) in the

122 modified engine having TRCC at full load because of lower calorific value of POME20 than that of PBDF. Figure 8.8 shows the comparison of BTE of standard engine and modified engine fuelled with PBDF and POME20. It shows that the BTE increased with the increase in brake power for all fuel and all types of combustion chambers. BTE of POME20 was lower (28.48%) compared to that of PBDF (31.35%) with the standard engine having HCC. Since the engine was operated under constant injection timing and POME had a smaller ignition delay, combustion was initiated much before TDC was reached. This increased the compression work and more heat loss and thus reduced the BTE of the engine. On the other hand, BTE for TRCC (33.09% at full load) was higher, when compared to the other types of combustion chambers at all loads with POME20. This was due to better mixture formation of POME20 and air, as a result of better air motion in TRCC as determined in simulation study, which led to better combustion of the POME20. This increased the BTE. At all loads the BTE for SRCC was lower than that of TRCC and higher than that of TCC and SCC when fuelled with POME20. However BTE for TRCC operated with POME20 was slightly lower, when compared with PBDF (33.75% at full load) at the same operating condition. BSEC is the energy consumed per unit power. Figure 8.9 shows the variation of BSEC for standard engine and modified engine fuelled with PBDF and POME20. It can be seen from the Figure 8.9 that the BSEC decreased with the increase in brake power for all fuel and all types of combustion chambers. Best BSEC was observed for TRCC (10.879 MJ/ kwhr) when the engine was operated with POME20 fuel. However the BSEC for POME20 operated engine having TRCC, SRCC, TCC, HCC and SCC was

123 Figure 8.7 Variation of BSFC for different CC Figure 8.8 Comparison of BTE for different CC

124 higher than that of PBDF operated TRCC by about 2%, 10.11%, 13.15%, 18.54% and 24.45%, respectively. Because of the lower calorific value of biodiesel fuel, the BSEC was higher than that of PBDF fuel. However because of improved swirl velocity, TKE and air fuel mixture preparation, the BSEC of POME20 fuelled engine having TRCC was very marginally higher than that of PBDF fuelled engine (10.663 MJ/ kw-hr). 8.4.3 Emission Characteristics The comparison of UBHC emissions for all shapes of combustion chambers operated with POME20 and PBDF is shown in Figure 8.10. UBHC emissions were reduced over the entire range of loads for all types of combustion chambers fuelled with POME20 when compared to PBDF operation. It was noticed that re-entrant combustion chambers emit less level of UBHC compared to open type combustion chambers. This was due to better combustion of POME20 as a result of improved swirl and squish motion of air in re-entrant combustion chambers and the presence of Oxygen in POME20. The UBHC emission for TRCC (46 ppm) was lower than any other type of combustion chamber with biodiesel operation at full load. This was because of better combustion of POME20 due to better air fuel mixing and the presence of Oxygen in the fuel. Better combustion of POME20 increased temperatures both in the gases and at the combustion chamber walls of the engine, which assist in the oxidation reactions to proceed close to completion. Another reason for reduction in UBHC emission from TRCC may be due to decreased quenching distance due to improved air motion. There was a reduction of 20.7% UBHC emissions for the TRCC when tests were carried out with POME20 and 30% reduction with PBDF compared to the standard engine at full load operation.

125 Figure 8.9 Variation of BSEC for different CC Figure 8.10 Variation of UBHC emissions for different CC

126 Figure 8.11 shows the comparison of CO emissions with brake power for five types of combustion chambers. CO emissions from all types of combustion chambers fuelled with biodiesel blend decreased significantly when compared with those of standard PBDF at all loads. This shows that CO emissions were greatly reduced with the addition of POME to PBDF. It decreased more with TRCC than with the other four types of combustion chambers. Higher swirl velocity and turbulent kinetic energy (TKE) as foundin simulation study for TRCC and the presence of Oxygen in POME, led to better combustion of fuel resulting in the decrease in CO emissions. Secondly increase in the proportion of Oxygen in POME promotes further oxidation of CO during the engine exhaust process. There was a reduction of 44.5% CO emissions for the TRCC compared to standard engine when tests were carried out with POME20 and 59% CO emissions for the TRCC compared to standard engine when tests were carried out with PBDF. It was observed that, a reduction of 26% and 12.5% of CO emissions for the SRCC and TCC respectively and an increase of 12% for the SCC compared to standard engine having HCC when tests were carried out with POME20 at full load operation. Figure 8.12 compares the CO 2 emissions of various combustion chambers employed using PBDF and POME20 with respect to brake power. The CO 2 emissions increased with the addition of POME in PBDF and reached a maximum value for TRCC than the other combustion chambers. The more amount of CO 2 in the exhaust emission was an indication of the complete combustion of fuel. So higher CO 2 emission from the engine having TRCC indicates effective combustion due to the Oxygen content in POME20. This was also attributable to better air motion as ascertained in simulation study and air fuel mixture preparation. It was noticed that CO 2 varied from2.5% at low load to 7% at full load for standard engine fuelled with POME20 and from 1.9% at low load to 6.8% at full load for standard engine

127 Figure 8.11 Variation of CO emissions for different CC Figure 8.12 Variation of CO 2 emissions for different CC

128 fuelled with PBDF. The CO 2 emissions from SRCC, TCC, SCC and HCC in comparison with that of TRCC decreased by 6%, 9.5%, 20.2% and 16.6% respectively at full load. Figure 8.13 shows the variation of NO x emissions for standard engine and modified engine with POME20 and PBDF. The NO x emissions were higher for TRCC than the baseline engine having HCC. The reason for the increase in NO x was due to higher combustion temperatures arising from improved combustion due to better mixture formation in TRCC and the availability of Oxygen in POME. Another reason for increased NO x emissions from TRCC can be reasoned to that, a larger part of the combustion was completed before top dead centre for POME20 compared to PBDF due to their lower ignition delay. At full load, for the TRCC using POME20, the level of NO x emission was 784 ppm compared to 712 ppm for standard engine having HCC. There was an increase of about 9.2% of NO x emissions for TRCC compared to the baseline engine when fuelled with POME20 and 20.98% with standard PBDF. The smoke intensity comparison of five combustion chamber geometries with POME20 and PBDF is shown in Figure 8.14. At all loads and for all types of combustion chambers, smoke emissions for the blend decreased significantly when compared with that of PBDF. The reduction in smoke emission was due to the presence of Oxygen in biodiesel blend. It was also observed that the smoke emissions were lower for TRCC than with other chambers. Among all combustion chambers, TRCC gave 28.2% reduction of smoke opacity when compared with standard engine fuelled with PBDF. Further, it was noticed that smoke emissions from the re-entrant combustion chambers were lower than open combustion chambers. This was due to more complete combustion as a result of better air motion and air fuel mixing.

129 Figure 8.13 Comparison of NO x emissions for different CC Figure 8.14 Comparison of smoke emissions for different CC

130 8.5 SUMMARY In this experimental phase the effects of combustion chamber geometry on the performance, combustion and emission characteristics of POME20 fuelled DI diesel engine were investigated, at standard injection timing and injection pressure of 23º btdc and 200 bar respectively. The results were compared with that of PBDF operated standard engine to select the best combustion chamber geometry for biodiesel fuelled engine and PBDF and POME20 operated modified engine for validation. Even though properties of POME20 were comparable with PBDF the viscosity of POME20 was found to be about 20.34% higher and calorific value was 2.3% lower, when compared to PBDF. These properties strongly affect atomization, evaporation and air-fuel mixing. The mixing quality of biodiesel spray with air can be generally improved by selecting a better design of the combustion chamber. To evaluate the effects of combustion chamber geometry on the performance of POME20 operated engine, different combustion chamber geometries like HCC, SCC, TCC, SRCC and TRCC were employed. The experimental results obtained using the different combustion chambers investigated in the present work show that, 1. Due to the lower calorific value and higher viscosity of POME20 the BSFC and BSEC increased and the BTE decreased in the standard engine. However better results were obtained from the engine having re-entrant combustion chambers particularly from TRCC mainly due to better swirl velocity and turbulent kinetic energy as determined in simulation study and charge mixing. 2. As determined in simulation study, the modified engine having re-entrant combustion chambers particularly for TRCC, the CO, UBHC and smoke emission levels were

131 reduced significantly due to improved mixture formation as a result of enhanced swirl velocity and complete combustion compared to standard engine having HCC. 3. The higher Oxygen content in the POME resulted in better combustion and increased the combustion chamber temperature. As a result the NO x emission was increased for POME20 compared to PBDF in standard engine. The increased swirl and squish motion in modified engine having TRCC, improved charge mixing which resulted in better combustion and increased further the combustion chamber temperature. As a result NO x emission from modified engine was further increased. 4. The ignition delay of POME20 blend was found to be lesser and exhaust gas temperature was higher compared to that of PBDF. The improved air motion and better mixing in reentrant combustion chambers as determined in simulation study further decreased ignition delay and increased exhaust gas temperature when compared to open combustion chambers. 5. The engine developed maximum peak pressure and maximum heat release rate for PBDF, when compared to POME20 in the standard engine. However TRCC with POME20 showed maximum peak pressure and maximum heat release rate compared to baseline engine operated with PBDF due to better air-fuel mixing and enhanced combustion. The present analysis reveals that the performance and emission characteristics of biodiesel fuelled engine can be improved by suitably designing the combustion chamber. In general, the TRCC was found to be

132 superior in terms of performance, combustion and exhaust emissions improvements over the other configurations of combustion chambers. Compared to standard engine operated with PBDF, the modified engine, particularly the engine with TRCC delivered a performance enhancement of 5.5%, 3.8% and 5.2% in terms of BTE, BSFC and BSEC respectively and an emission level improvement of 44.54%, 30.3% and 28.1% in terms of CO, UBHC and smoke opacity. However the NO x emission level was deteriorated by about 20.9% compared to standard engine. Based on the above results, the engine having TRCC was selected for further studies, to evaluate the effect of varying the injection timing and injection pressure to further enhance the existing performance and emissions characteristics of a biodiesel fuelled DI diesel engine.