Effect of Combustion Chamber Shapes on the Performance of Mahua and Neem Biodiesel Operated Diesel Engines

Shape of combustion chamber plays a major role in controlling combustion process and emission characteristics occurring inside internal combustion engines in general and diesel engines in particular. To optimize a combustion chamber for diesel engine applications, suitable design modifications are required that meet both emission norms as well as acceptable engine performance. In this context, experimental investigations were carried out on a single cylinder four stroke direct injection diesel engine operated in single fuel mode using Mahua oil methyl ester (MhOME) and neem oil methyl ester (NOME). Different combustion chamber shapes were designed and fabricated keeping the compression ratio same for the existing diesel engine. The existing engine was provided with hemispherical combustion chamber (HCC) shape. In order to study the effect of other combustion chamber shapes on the performance of diesel engine, cylindrical (CCC), trapezoidal (TrCC), and toroidal combustion chamber (TCC) shapes were designed and developed. Various engine parameters such as power, torque, fuel consumption, and exhaust temperature, combustion parameters such as heat release rate, ignition delay, combustion duration, and exhaust emissions such as smoke opacity, hydrocarbon, CO, and NOx, were measured. Results revealed that the TCC shape resulted in overall improved performance with reduced emission levels compared to other shapes tested. Total hydrocarbon emission (THC) and carbon monoxide (CO) were also decreased significantly compared to other combustion chambers. Effect of Combustion Chamber Shapes on the Performance of Mahua and Neem Biodiesel Operated Diesel Engines


Introduction
Diesel engines are widely used for transport and power generation applications because of their high thermal efficiency, and their easy adoption for power generation applications as well. However, there is an increased impetus on improved engine performance, lower noise and vibration levels and lower emissions. Increasing energy demand, decrease in fossil fuel reserve in the earth crust and harmful exhaust gases have focused major attention on the use of renewable and alternative fuels. To overcome and meet these demands, use of renewable fuels such as biodiesels for diesel engines has gained greater momentum. To meet the challenge, it is essential in implementing new technologies and methods that improve the efficiency of diesel engine used for both transport and power generation applications. Renewable energy sources can supply the energy for longer periods of time than those of fossil fuels and have many advantages [1]. Liquid biodiesels are more suitable for diesel engine applications as their properties are closer to diesel.
A number of vegetable oils have been used for biodiesel production and their respective biodiesels are used as alternative fuels in diesel engines. Biodiesels derived from jatropha, honge (karanja), honne, palm, rubber seed, rape seed, mahua, and neem seed oils were used in diesel engine applications [2][3][4][5][6][7][8][9][10][11][12][13][14]. Slightly lowered performance with increased emissions and combustion studies was reported for Biodiesels engine operation by several researchers [2,3,8,9,[15][16][17]. Effect of various engine parameters such as compression ratio (CR), injection timing (IT), injection pressure and engine loading on the performance and exhaust emissions of a single cylinder diesel engine operated on biodiesel and their blends with diesel were reported in the literature [18]. Changes in injection timings change the position of the piston and cylinder pressure and temperature at the injection. Retarded injection timings showed significant reduction in diesel NOx and biodiesel NOx [19]. Cylinder pressures and temperatures gradually decreased when injection timings were retarded [20]. Other investigators also have performed experiments on CI engine with different vegetable oils and their esters at different injection pressures. Better performance, higher peak cylinder pressure and temperature were reported at increased injection pressures [20][21][22][23].

Mahua and Neem Biodiesel Operation
Biodiesels derived from Mahua and Neem oils have been used as potential alternative fuels to diesel by several investigator. Mahua biodiesel has been used as an alternative fuel for diesel engine application by several investigators [22,24,25]. Effect of injection timing, compression ratio on the performance of Mahua biodiesel has been reported [26]. They reported biodiesel could be blended with diesel fuel up to 20% at any compression ratio and injection timing for getting nearly same performance as compared with diesel. For neem biodiesel a slight drop in efficiency compared to diesel has been reported, while their blends B10, B20 showed performance closer to diesel operation [27,28]. CO emission increased with B100 and increased blends of B60, B80 due to the incomplete combustion. They suggested a change in injection pressure and combustion chamber design for better engine performance. Neem oil methyl ester resulted in lower emissions compared to diesel whereas neem oil gives lower CO 2 emission due to incomplete combustion while smoke opacity increased at part load and decreased at full load [28].
The combustion chamber of an engine plays a major role during the combustion of wide variety of fuels. In this context, many researchers performed both experimental and simulation studies on the use of various combustion chambers [29][30][31]. Improvement in air entrainment with increased swirl and injection pressure were reported [32,33]. Optimum combustion chamber geometry of the engine showed better performance and emission levels. Suitable combustion geometry of bowl shape helps to increase squish area and proper mixing of gaseous fuel with air [29,34]. Designing the combustion chamber with narrow and deep and with a shallow reentrance had a low protuberance on the cylinder axis and the spray oriented towards the bowl entrance reduced the NOx emission levels to the maximum extent [30,31,35]. The behavior of fuel once it is injected in the combustion chamber and its interaction with air is important. It is well known that nozzle geometry and cavitations strongly affect evaporation and atomization processes of fuel. Suitable changes in the in-cylinder flow field resulted in differing combustion.
From the literature survey it follows that very limited work has been done to investigate the effect of combustion chamber shapes on the performance, combustion and emission characteristics of diesel engine fuelled with Mahua and Neem oil methyl esters. In this context, experimental investigations were carried out on a single cylinder four stroke direct injection diesel engine operated on MhOME and NOME with different combustion chamber shapes adopted for this work.

Characterization of Mahua and Neem Oils
In the present study, Diesel, MhOME and NOME were used as injected fuels. Tables 1 and 2 shows the composition of MhOME and NOME, fatty acids contribution, chemical formula, structure and their molecular weight with their chemical structure. The properties of MhOME and NOME were determined experimentally and are summarized in Table 3.

Experimental Setup
Experiments were conducted on a Kirloskar TV1 type, four stroke, single cylinder, water-cooled diesel engine test rig. Figure 1 shows the line diagram of the test rig used. Eddy current dynamometer was used for loading the engine. The fuel flow rate was measured on the volumetric basis using a burette and stopwatch. The engine was operated at a rated constant speed of 1500 rev/min. The emission characteristics were measured by using HARTRIDGE smoke meter and five gas analyzer during the steady state operation. Experiments were conducted by using biodiesels selected for the study with four different combustion chamber shapes (cylindrical (CCC), trapezoidal (TrCC), and toroidal Combustion chamber (TCC) shapes). Figures 2 shows the different combustion chamber shapes. Finally the results obtained with biodiesel operation were compared with Diesel. The specification of the compression ignition (CI) engine is given in Table 4.

Results and Discussions
In the present work, diesel engine was operated on diesel, MhOME and NOME with different configurations of combustion chambers namely cylindrical (CCC), trapezoidal (TrCC), and toroidal combustion chamber (TCC) shapes. The results and discussions on the performance combustion and emission characteristics of diesel engine  Cetane number 42 --------52 52  Figure 3 shows the variation of brake thermal efficiency (BTE) with brake power. It is observed that BTE for diesel fuel mode of operation was higher than both biodiesels of MhOME and NOME operation over the entire load range. This is mainly due to lower calorific value of the biodiesels and lower volatility as well. The study with different combustion chamber shapes showed that for biodiesel operation with TCC resulted in better performance compared to other combustion chambers. It may be due to the fact that, the TCC prevents the flame from spreading over to the squish region resulting in better mixture formation of biodiesel-air combinations, as a result of better air motion and lowers exhaust soot by increasing swirl and tumble. Based on the results, it is observed that the TCC has an ability to direct the flow field inside the sub volume at all engine loads and therefore substantial differences in the mixing process may not be present. Figure 4 shows variation of smoke opacity with brake power. It is observed that the smoke opacity for diesel fuel operation was lower than MhOME and NOME biodiesels over the entire load range. This may be attributed to improper fuel-air mixing due to higher viscosity and higher free fatty acid content of biodiesels considered. However, TCC gives lower smoke emission levels compared to other combustion chambers. It may be due to the fact that, the air-fuel mixing prevailing inside combustion chamber and higher turbulence resulted in better combustion and oxidation of the soot particles which further reduction the smoke emission levels.        Figures 5 and 6 shows the variation of hydrocarbon (HC) and carbon monoxide (CO) emission levels for diesel, MhOME, and NOME at all loads. Both HC and CO emission levels were higher for MhOME, and NOME compared to diesel operation. Incomplete combustion of the MhOME, and NOME biodiesels is responsible for this observed trend. It could be due to their lower calorific value, lower adiabatic flame temperature and higher viscosity and lower mean effective pressures. However, TCC resulted in lower HC and CO emission levels compared to other combustion chamber shapes. It could be due to higher turbulence and comparatively higher temperature prevailing in the combustion chamber that resulted into minimum heat losses and better oxidation of HC and CO and hence reduced both emission levels. However, other combustion chambers may not contribute to the proper mixing fuel combinations.

Performance and emission characteristics
The NOx emission levels were found to be higher for diesel fuel operation compared to biodiesel over the entire load range (Figure 7). This is because of higher heat release rate during premixed combustion phase observed with diesel compared to biodiesel operation. Slightly higher NOx resulted with TCC compared to other combustion chambers tested. This could be due to slightly better combustion occurring due to more homogeneous mixing and larger part of combustion occurs just before top dead center. Presence of oxygen in the biodiesels is also responsible for this trend. Therefore it is resulted in higher peak cycle temperature.

Combustion characteristics
In this section combustion characteristics of diesel engine fuelled with MhOME, NOME biodiesel has been presented.   Ignition delay: The variation of ignition delay with brake power for different combustion chamber shapes were shown in Figure 8. The ignition delay is calculated based on the static injection timing.
It is observed that ignition delay decreased with an increase in brake power for almost all combustion chamber shapes. With an increase in brake power, the amount of fuel being burnt inside the cylinder gets increased and subsequently the temperature of in-cylinder gases gets increased. This leads to reduced ignition delay with all combustion chamber shapes. However, the ignition delay for diesel was lower compared to biodiesel operation with all combustion chamber shapes. However, lower ignition delays were observed for biodiesel operation with TCC compared to the operation with HCC, CCC and TrCC. It could be attributed to better air-fuel mixing and increased combustion temperature.

Combustion duration:
The combustion duration shown in Figure  8 was calculated based on the duration between the start of combustion and 90% cumulative heat release. The combustion duration increases with increase in the power output with all combustion chamber shapes. This is due to the amount of fuel being burnt inside the cylinder gets increased. Combustion chamber being same, higher combustion duration was observed with biodiesel compared to diesel operation. It could be due to higher viscosity of biodiesels leading to improper air-fuel mixing, and needs longer time for mixing and hence resulting in incomplete combustion with longer diffusion combustion phase. However with combustion duration was reduced with TCC compared to other combustion chambers tested. This could be attributed to improvement in mixing of fuel combination due to better squish. Significantly higher combustion rates with biodiesel operation leads to higher exhaust temperatures and lower thermal efficiency. However, biodiesel operation with TCC showed improvement in heat release rate compared to other combustion chamber shapes.       Figure 9 shows the effect of combustion chamber shapes on the in-cylinder pressure operated on different fuel combinations. The peak pressure depends on the combustion rate and amount of fuel consumed during rapid combustion period. Mixture preparation and slow burning nature of biodiesel during the ignition delay period were responsible for peak pressure and maximum rate of pressure rise.
Results showed that biodiesel with TCC resulted in higher peak pressure as shown in Figure 9. The pressure for MhOME biodiesel operation with TCC was higher compared to NOME biodiesel tested. It could be due to the combined effect of longer ignition delay, lower adiabatic flame temperature and slow burning nature of the biodiesel operation. This could be attributed to incomplete combustion due to improper mixing of fuel combinations, reduction of air entrainment, and higher viscosity of biodiesel. The sharp increase in combustion acceleration showed increased cylinder pressure during the piston's descent and that the combustion energy as efficiently converted into work.
Heat release rate Figure 10 shows rate of heat release versus crank angle for different two biodiesels with TCC combustion chamber shapes. Biodiesel operation for TCC resulted into higher heat release rate compared to the operation with other combustion chambers. Combustion chamber being common NOME has higher second peak in the diffusion combustion phase compared to the MhOME operation with TCC operation.

Conclusions
From the exhaustive experimentation on the use of biodiesel in diesel engines with different combustion chamber shapes the following conclusions were made for the present study.
• Both biodiesels of Mahua and Neem resulted into inferior engine performance with increased emissions compared to diesel operation. The performance was improved with TCC combustion chamber provision.
• Improved air motion, better mixture formation with TCC compared to other combustion chambers was observed.
• Higher brake thermal efficiency with lower emission levels obtained with TCC.
• Combustion chamber optimization coupled with optimized injector position further improves the diesel engine performance.  Figure 9: In-cylinder pressure versus crank angle at varying compression ratio for NOME and MhOME biodiesels at 80% load.
Heat release rate, J / crank angle Crank angle, degree CA NOME (TCC) Diesel MhOME (TCC) Figure 10: Rate of heat release versus crank angle at varying compression ratio for NOME and MhOME biodiesels at 80% load.