Novel VCR SI Engine using Gaseous Fuels
7.2 NOVEL VARIABLE SPARK PLUG LOCATION RESULTS
7.2.1 Biogas performance under variable spark location concept
7.2.1.3 Emission analysis for spark plug location
MGT is equally high at SP1 with delayed combustion than that of SP2 from TDC. Therefore MGT variation also suggests that the SP2 location is most appropriate for the engine. This observation is in phase with the conclusion made from Fig.7.6 since MGT depends upon the combustion process in-side the combustion chamber.
The coefficient of variance (COV) [Eq-C-16 (APPENDIX-C)] with respect to spark plug location for different compression ratio using raw biogas is as shown in Fig 7.9. The COV can be found to be reduced with change in spark plug location from SP1 to SP2 for all the compression ratios. This decrement in cycle-by-cycle variations shows that the combustion of raw biogas is smoother when spark plug is at SP2 (Han et al., 2000).
Fig.7.9 Percent COV IMEP variation with spark plug location for different compression ratio
But thereafter at SP3 and SP4 the variations is found more and COV increases with positive slope for all CRs. At position SP2, the cycle-by-cycle variations are 0.215 %, 0.64 % and 0.52 % while these variations at SP4 are 1.53 %, 1.78 %, 1.135 % for CR 8, CR 9 and CR 10 respectively.
Thus the combustion analysis from combustion chamber pressure, NHHR, MFB, MGT and COV clearly indicate that the combustion in the combustion chamber gets largely influenced with axial location of the spark plug. Further SP2 location is seen to be effective in making combustion faster with maximum mass burned, high energy release rate and high mean gas temperature. It has also been noticed that the cycle-by- cycle variation is also less for this spark plug location.
has been demonstrated in terms of carbon monoxide (CO) emission as shown in Fig. 7.10.
CO emission decreases with increase in compression ratio for any spark plug location.
Further, it is found that the CO emission decreases by 18 %, 31 % and 25 % for change in spark plug location from SP1 to SP2 for compression ratios CR 8, CR 9 and CR 10 respectively. But thereafter it increases for further change in position to SP3 and SP4.
Fig.7.10 CO emission for different spark plug locations compression ratios.
The increment reported is 38 %, 56 % and 47 % for CR 8, CR 9 and CR 10 respectively.
Mean gas temperature remains high for SP2 location due to higher heat release rate associated with it.
Fig.7.11 NOx emission for different spark plug locations compression ratios.
In view of these facts emission of CO remains lower for SP2 location as it decreases with compression ratio. Nitrogen oxide (NOx) emission from the biogas fuelled engine is plotted for variable spark location along with variation in compression ratio. It has been found that the NOx emissions are increased with change in spark plug location from SP1 to SP2.
Fig.7.12 HC emission variation with spark plug location for variable compression ratio. This increment was 63 %, 76 % and 74 % respectively for CR 8, CR 9 and CR 10 with respect to SP1(Fig.7.11). The rise in NOx is due to more complete combustion which causes the increase in combustion temperature. However further change in spark plug location from SP2 to SP3 and further SP4 is shown to decrease the NOx emission. Thus, increase in NOx
emission with change in spark plug location has same reason for its increment with compression ratio for same spark plug location. Higher combustion temperature is leading to high NOx emission for either cases.
The effect of variable spark location on unburnt hydrocarbon (HC) emissions is explained in Fig.7.12 for different compression ratios. As shown in figure, the HC emission found minimum at SP2 as compared to that of SP1, SP3 and SP4 for all CR. The compression ratio effect is clearly visible from the figure where at higher CR10; the HC is lower as compared to other CR9 and 8. This is a outcome of maximum fuel combustion in presence of higher compression pressure. So there is lower the chance that fuel remains unburnt in the crevices of piston ring or around the secondary piston. This is called flame quenching which depends upon cylinder piston geometry (Heywood,1998). But with lowering the CR these effect influence the fuel combustion and leaves the fuel unburnt which ultimately rises HC in exhaust. At SP2, the HC is found to be 130, 170 and 200 ppm for CR10, 9 and 8 respectively.
The percentage increment of 44 to 50 % for SP1, 22 to 40 % for SP3 and 22 to 45 % for SP4 with respect to SP2 for all CR10, 9 and 8.
The green house gas CO2 emission with spark plug location is as explained in Fig.7.13 for different crank angles. As shown in figure, the lowest CO2 percentage in exhaust is found at
SP2 location for all CR under consideration. However, higher CR10 shows 3.63% CO2 at SP2. The CO emission (Fig.7.10) is lowest for CR10 at SP2.
Fig.7.13 CO2 emission variation with spark plug location for variable compression ratio.
This reflects the oxidation of hydrocarbon to be maximum. So CO gets converted into CO2
emissions at CR10 and SP2 location. Whereas at CR 9 and 8, CO2 emissions are lower 2.21
% and 1.18 %. This clearly reflects in rising of CO emission at SP2 location. In terms of magnitude, the percentage CO2 at SP2 is 3.63% which is 9.21 %, 8.08 % and 9.64 % at SP1, SP3 and SP4 respectively for biogas at CR10.