PERFORMANCE AND COMBUSTION CHARACTERISTICS OF DIESEL ENGINE: DISCUSSIONS

DISCUSSIONSPollution Levels

Figure.10 shows the variation of smoke levels 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. It is seen that for the same load, the smoke density decreased with induction of alcohol. The combustion of injected fuel in case of pure vegetable oil operation is predominantly one of oxidation of products of destructive decomposition. In this case, there are greater chances of fuel cracking and forming carbon particles. On the other hand, the combustion of alcohol is predominantly a process of hydroxylation and the chances of fuel cracking are negligible. Ethanol does not contain carbon-carbon bonds and therefore cannot form any un-oxidized carbon particles or precursor to soot particles. One of the promising factor for reducing smoke levels with the alcohols is they contained oxygen in their composition which helped to reduce soot density. Soot emissions increased linearly with the increase of carbon to hydrogen atoms (C/H) ratio provided the equivalence ratio is not altered.

This is because higher C/H lead to more concentration of carbon dioxide, which would be further, reduced to carbon. Consequently, induction of alcohol reduced the quantity of carbon particles in the exhaust gases as the magnitudes of C/H for diesel fuel, vegetable oil and ethanol are 0.45, 0.83 and 0.25 respectively. Lower smoke levels are observed in both versions of the engine in dual fuel mode when compared with pure diesel operation on CE. LHR engine with 60% ethanol induction showed lower smoke levels when compared with CE with 35% ethanol induction. Smoke levels decreased with the increase of ethanol induction in both versions of the engine. In dual fuel operation, smoke levels further decreased with the advancing of the injection timing and with increase of injection pressure in both versions of the engine, due to efficient combustion at higher injection pressures, which improved the atomization hence faster rate of combustion and shorter combustion duration at the advanced injection timings caused to reduce the smoke density in both versions of the engine.

Fig10Performance and Combustion-10
Figure 10: Variation of Smoke Levels in Hartridge Smoke Unit 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 NOx levels 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.11. NOx emissions decreased with the increase of percentage of ethanol induction in both versions of the engine, due to lower combustion temperatures. The low value of C/H ratio in ethanol has indirect effect in reducing oxygen availability in the gases, which leads to the reduction of NOx.

However, LHR engine with different percentages of ethanol induction showed higher NOx levels compared with CE with 35% ethanol induction, due to increase of gas temperatures in LHR engine. NOx levels further decreased with the increase of ethanol induction in both versions of the engine.

NOx levels increased marginally in CE while they decreased in LHR engine with the advancing of the injection timing and with the increase of injection pressure. This is due to reduction of gas temperatures in the LHR engine at 31obTDC.

Fig11Performance and Combustion-11
Figure 11: Variation of Nox Levels 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

These aldehydes are responsible for pungent smell of the engine and affect the human beings when inhaled in the large quantities. The volatile aldehydes are eye and respiratory tract irritants. Though Government legislation has not been pronounced regarding the control of aldehyde emissions, when more and more alcohol engines are coming to existence severe measures the controlling of aldehydes emitted out through the exhaust of the alcohol run engines will have to be taken as serious view. Figure.12 (a) shows the variation of formaldehyde concentration while Figure.12 (b) acetaldehyde concentration in CE and LHR engine at recommend injection timing and optimum injection timing at an injection pressure of 190 bar with maximum induction of ethanol. It could be seen that aldehyde emissions are low with pure diesel operation in both CE and LHR engine. Formaldehyde emissions increased drastically with ethanol induction in both CE and LHR engine. With increased induction of ethanol upto 50%, CE registered very high value of formaldehyde emissions in the exhaust, which showed the significant reduction in LHR engine. Hot environment of LHR engine completed combustion reactions and reduced the emissions of intermediate compounds, aldehydes. Hence it is concluded that LHR engine is more suitable for alcohol engines in comparison with pure diesel operation. Advanced injection timing and increase of injection pressure also improved the combustion performance in LHR engine by reducing the intermediate compounds like formaldehyde and acetaldehydes.

Fig12Performance and Combustion-12
Figure 12: (A) Formaldehyde Concentraiton

Fig12aPerformance and Combustion-13
Figure 12: (B) Acetaldehyde Concentraiton
Figure 12: Variation of Aldehyde Concentration in Conventional Engine (CE) and Low Heat Rejection (LHR) Engine at Recommend Injection Timing and Optimized Injection Timings with Maximum Induction of Ethanol.

Combustion Characteristics

Variation of combustion parameters like PP, TOPP and MRPR with maximum induction of ethanol induction in different versions of the engine at recommended injection timing and optimum injection timing and at an injection pressure of 190 bar are represented by Figure. 13(a), 13(b) and 13(c), respectively. From Figure, 13(a), it can be noticed that the magnitude of PP increased with increase of ethanol induction in both versions of the engine. The magnitude of PP increased with advancing of the injection timing in both versions of the engine, with ethanol induction. With the same amount of ethanol induction, LHR engine exhibited higher PP compared with CE with 50% of ethanol induction at 27obTDC and at injection pressure of 190 bar. This is due to increased amount of ethanol with LHR engine. With maximum induction of ethanol, LHR engine at 31obTDC produced higher PP compared with CE at 33obTDC.From the Figure.13 (b), it can be noticed that magnitude of TOPP decreased with the increase of ethanol induction with both versions of the engine.

When the ethanol induction is increased to 50% in LHR engine, the magnitude of TOPP is lower (shifted towards TDC) when compared with CE with 35% ethanol induction. This is once again confirmed by the observation of higher PP and lower TOPP in LHR engine with dual fuel mode, that the performance of LHR engine with 50% alcohol induction is improved over CE with 35% ethanol induction.

The magnitude of TOPP decreased with advancing of the injection timing with both versions of the engine. From the Figure.13©, it can be observed that LHR engine showed higher MRPR when compared with CE at different injection timing. This is due to higher amount of ethanol induction in LHR engine. MRPR increased with the advancing of the injection timing in both versions of the engine. These combustion characteristics improved with increase of injection pressure.

Fig13Performance and Combustion-14
Figure 13: Variation of Combustion Parameters in Conventional Engine (CE) and Low Heat Rejection (LHR) Engine at Recommend Injection Timing and Optimized Injection Timings with Maximum Percentage of Ethanol Induction.

CONCLUSIONS

Maximum induction of alcohol was 35% on mass basis with best possible efficiency at all loads in CE while it is 50% in the LHR engine. LHR engine with 50% alcohol induction showed improved performance when compared to CE with 35% alcohol induction. The maximum induction of alcohol is 35% in CE at 33obTDC, while it is 45% in LHR engine at 31obTDC. Performance, pollution levels (smoke, NOx and aldehyde levels) and combustion characteristics improved in both versions of the engine with maximum induction of alcohol when the injection timings are advanced and with the increase of injection pressure.