PERFORMANCE AND COMBUSTION CHARACTERISTICS OF DIESEL ENGINE: RESULTS

RESULTSPerformance Parameters

Investigations are carried out with the objective of determining the factors that would allow maximum use of ethanol in diesel engine with best possible efficiency at all loads.

The variation of brake thermal efficiency (BTE) with brake mean effective pressure (BMEP) with different percentages of ethanol induction in conventional engine (CE) at 27obTDC and at an injection pressure of 190 bar, is shown in Figure.3. Variation of BTE with BMEP with pure diesel operation on CE is also shown for comparison purpose. BTE increased at all loads with 35% ethanol induction and with the increase of ethanol induction beyond 35%, it decreased at all loads in CE when compared with CE with diesel operation (standard diesel).

The reason for improving the efficiency with the 35% ethanol induction is because of improved homogeneity of the mixture with the presence of ethanol, decreased dissociated losses, specific heat losses and cooling losses due to lower combustion temperatures. This is also due to high heat of evaporation of ethanol, which caused the reduction the gas temperatures resulting in a lower ratio of specific heats leading to more efficient conversion of heat into work. Induction of ethanol resulted in more moles of working gas, which caused high pressures in the cylinder. The observed increased in the ignition delay period would allow more time for fuel to vaporize before ignition started. This means higher burning rates resulted more heat release rate at constant volume, which is a more efficient conversion process of heat into work.

Fig3Performance and Combustion-3
Figure 3: Variation of Brake Thermal Efficiency (BTE) with Brake Mean Effective Pressure (BMEP) in Conventional Engine (CE) at Different Percentages of Ethanol Induction

Figure.4 shows the variation of BTE with BMEP with different percentages of ethanol induction in LHR engine at the recommended injection timing and pressure. LHR engine showed an improvement in the performance with the carbureted ethanol at all loads when compared to the standard diesel engine. This is due to recovery of heat from the hot insulated components of LHR engine due to high latent heat of evaporation of the ethanol, which lead to increase in thermal efficiency. The maximum induction of ethanol is 50% in LHR engine, which showed improvement in the performance at all loads when compared to standard diesel engine. However when the ethanol induction is increased more than 50% in LHR engine, BTE is deteriorated at all loads when compared with standard diesel.

Fig4Performance and Combustion-4
Figure 4: Variation of Brake Thermal Efficiency (BTE) with Brake Mean Effective Pressure (BMEP) in Low Heat Rejection

(LHR) Engine at Different Percentages of Ethanol Induction.

The optimum injection timings are at 33obTDC for CE, and at 31obTDC for LHR engine with pure diesel operation [16]. Similar trends are observed on the variation of BTE with BMEP in CE and LHR engine with alcohol-vegetable oil operation when the injection timings are advanced to 31obTDC in LHR engine and 33obTDC in CE as in the case of 27obTDC in both versions of the engine. However, the maximum induction of alcohol is limited to 45% in the LHR engine at 31obTDC against 50% induction at 27obTDC, while maximum induction of alcohol is the same in CE at 33obTDC as in the case of 27obTDC. Ethanol is inducted at these respective injection timings for CE and LHR engine.

The variation of BTE with BMEP in CE and LHR engine with maximum induction of ethanol at recommended and optimum injection timings and at a pressure of 190 bars is shown in Figure.5. LHR engine with 45% ethanol induction at its optimum injection timing showed improved performance at all loads when compared with other versions of the engine. This is due to higher amount of ethanol substitution and improved combustion at advanced injection timing caused better evaporation leading to produce higher BTE.

Fig5Performance and Combustion-5
Figure 5: Variation of BTE with BMEP with Maximum Percentage of Ethanol Induction in CE and LHR Engine at

Recommended and Optimum Injection Timings

There is a limitation to use ethanol due to low cetane number and having higher self-ignition temperature than vegetable oil to use in CE without increasing injection pressure because as percentage of ethanol increases more heat is utilized to evaporate alcohol fuels and less heat is available to evaporate vegetable oil. Therefore a major quantity of alcohol which burns late in the expansion stroke, will not be fully utilized. In order to avert this, injection pressure is increased, which reduces fuel droplet size, increases surface to volume ratio and requires comparatively less heat to evaporate vegetable oil droplet.

The trend exhibited by both versions of the engine with dual fuel operation at higher injection pressure of 270 bars is similar to the corresponding to the injection pressure of 190 bars. However, the maximum induction of alcohol is 40% in CE at an injection pressure of 270 bars against 35% at 190 bars, while maximum alcohol induction remained same with LHR engine at 270 bars as in the case of 190 bars.

Figure.6 shows bar charts which represents the variation of brake specific energy consumption (BSEC) at peak load operation with different versions of the engine at maximum induction of ethanol at recommended and optimum injection timings. BSEC decreased with the increase of ethanol induction, as higher amount of alcohol substitution caused better evaporation and produced lower BSEC in both versions of the engine. BSEC is lower in LHR engine at its optimum injection timing, which shows the suitability of the engine for alternate fuels. It also decreased with the increase of injection pressures in both versions of the engine. This is due to early initiation of combustion with improved fuel spray characteristics.

Fig6Performance and Combustion-6
Figure 6: Bar Chart Showing the Variation of Brake Specific Energy Consumption (BSEC) at Peak Load Operation with Induction of Ethanol in CE and LHR Engine at Recommended and Optimum Injection Timings.

Variation of exhaust gas temperature (EGT) with BMEP in CE and LHR engine with maximum induction of ethanol at recommended and optimum injection timings and at an injection pressure of 190 bars is shown in Figure.7. The magnitude of EGT decreased with the increase of percentage of ethanol induction in both versions of the engine. At the recommended injection timing, the magnitude of EGT is lower in CE with 35% induction of ethanol induction at all loads when compared with standard diesel engine. Lower exhaust gas temperatures are observed in the LHR engine with 50% ethanol induction when compared with CE with 35% ethanol induction. This showed that the performance of the LHR engine is improved with 50% ethanol induction over CE with 35% ethanol induction. EGT further decreased, when the injection timings are advanced in both versions of the engine. This is due to increase of thermal efficiency, reduction of coolant load and decrease of gas temperatures.

Fig7Performance and Combustion-7
Figure 7: Variation of Exhaust Gas Temperature (EGT) with Brake Mean Effective Pressure (BMEP) in Conventional Engine (CE) and Low Heat Rejection (LHR) Engine at Recommend Injection Timing and Optimized Injection Timings with Maximum Induction of Ethanol.

Variation of coolant load (CL) with BMEP in CE and LHR engine with maximum induction of ethanol at recommended and optimum injection timings and at an injection pressure of 190 bars is shown in Figure.8. Coolant load is less in both versions of the engine at different percentages of ethanol induction at all loads when compared with pure diesel operation on CE. This is due to the reduction of gas temperatures with ethanol induction. Cooling load is less in the LHR engine with 50% ethanol induction when compared with CE with 35% ethanol induction at all loads. This is due to the insulation provided in LHR engine. Cooling load increased in CE and decreased in the LHR engine with the advancing of injection timing and increase of injection pressure. This is due to increase of gas temperatures in CE and decrease of the same in LHR engine, when the injection timing is advanced.

Fig8Performance and Combustion-8
Figure 8: Variation of Coolant Load (CL) with Brake Mean Effective Pressure (BMEP) in Conventional Engine (CE) and Low Heat Rejection (LHR) Engine at Recommend Injection Timing and Optimized Injection

Timings with Maximum Induction of Ethanol.

Variation of volumetric efficiency (VE) with BMEP in CE and LHR engine with maximum induction of ethanol at recommended and optimum injection timings and at an injection pressure of 190 bars is shown in Figure.9. VE decreased marginally in both versions of the engine with the dual fuel operation when compared with pure diesel operation on CE, as percentage of alcohol induction increased, the amount of air admitted into the cylinder of the engine reduced. However, CE with different percentage of ethanol induction showed higher volumetric efficiency when compared with LHR engine. This is because of increase of temperatures of insulated components in LHR engine, which heat the incoming charge to high temperatures and consequently the mass of air inducted in each cycle is lower. VE increased marginally with the increase of injection pressure in both versions of the engine. This is due to improvement of air utilization and combustion with the increase of injection pressure. However, these variations were very small.

Fig9Performance and Combustion-9
Figure 9: Variation of Volumetric Efficiency (VE) with Brake Mean Effective Pressure (BMEP) in Conventional Engine (CE) and Low Heat Rejection (LHR) Engine at Recommend Injection Timing and Optimized Injection Timings with Maximum Induction of Ethanol.