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
7.2.1.2 Combustion Analysis for change in spark plug location
After performance analysis, combustion analysis is carried out to access the effectiveness of spark plug location.
Fig.7.4 Cylinder pressure variation with crank angle for different spark plug location.
The combustion rate due to variable spark plug location can be monitored in terms of combustion chamber pressure variation with respect to crank angle as shown in Fig.7.4. In this figure, compression ratio 10 is considered as representative for discussion. The pressure variation recorded during experiments is smoothened as mentioned in literature (Stone 1999).
From the figure it is clear that the peak cylinder pressure is 25.156 bar at 371 0 CA, 29.160 bar at 370 0 CA, 23.556 bar at 370 0 CA and 18.651 bar at 380 0 CA for SP1, SP2, SP3 and SP4 respectively. Higher the cylinder pressure more will be the homogeneous combustion of fuel air mixture in the combustion chamber.
In this regards, the position SP2 is more optimum than other positions for the complete combustion of charge.
Fig.7.5 Peak cylinder pressure variation with compression ratio for different spark plug location Same fact is logged for all the compression ratios and it clearly reflects from Fig.7.5. At position SP2, the peak cylinder pressure reaches maximum value of 29.1 bar, 26.12 bar and 24.9 bar for CR 10, CR 9 and CR 8 respectively
Fig. 7.6 NHRR variation with crank angle for different spark plug location
The performance improvement for excess protrusion by 2 mm has clearly influenced the combustion process. Thus, for the same ignition timing, combustion duration decreases on account of rise in spark plug location. Lesser time required for the flame to travel in the combustion chamber and thus in turn to raise cylinder pressure is expected for such observation (Taylor, 1977). With the increase in extruded length of spark plug, the crank angle at which peak pressure observed seems to be after TDC.
Locations SP3 and SP4 have protrusion length of 5 mm and 10 mm which might lead to crevice flow around the secondary piston which would delay the combustion (Alger, 2005).
The calculated NHRR [Eq-C-13 (APPENDIX-B)] variation with crank angle is plotted in Fig.7.6. The NHRR for raw biogas combustion at CR10, speed 1440 rpm and ignition timing 450 bTDC shows 18.43% rise when spark plug occupies new position SP2 over original position SP1
Fig.7.7 Variation of MFB with crank angle for different spark plug location
Here the crank angle at which peak heat release takes place also found advanced for SP2 with respect to original SP1 position. Spark plug at position SP3 and SP4 shows 5.85 % decrement and 0.8 % increment respectively in heat release with respect to SP1.The crank angle at which the peak NHRR observed for SP4 is 377 0 CA showing delayed combustion results and thus reduced power output and efficiency.
Mass fraction burned [Eq-C-14 (APPENDIX-C)] with respect to crank angle for variable spark plug locations is presented in Fig.7.7. As the spark timing is fixed at 45 0 bTDC, the 10% mass is found to be burned at 344 0 CA, 342 0 CA, 345 0 CA and 351 0 CA for SP1, SP2, SP3 and SP4 respectively.
Table 7.3. Crank angle during the combustion phase for different CR and VSP
CR
Beginning of combustion (10% MFB), 0 CA
Combustion duration
(90% MFB), 0 CA % MFB at TDC (360 0 CA)
SP1 SP2 SP3 SP4 SP1 SP2 SP3 SP4 SP1 SP2 SP3 SP4
10 344 342 345 351 31 25 29 29 33.65 54.94 32.08 13.78 9 349 349 350 354 27 25 31 28 33.328 44.33 25.92 14.28 8 358 351 353 358 22 21 25 24 14.99 47.22 25.13 15.16 Thus time required for 10 % mass fraction burned is lowest for position SP2 than other spark plug locations. Further, 90 % of mass is seen to be burned at 375 0 CA, 367 0 CA, 374 0 CA and 380 0 CA for SP1, SP2, SP3 and SP4 respectively.
Fig.7.8 Variation of MGT with crank angle for different spark plug location
Thus the combustion duration which is between 10 % to 90 % mass burnt is 31 0 CA, 25 0 CA, 29 0 CA and 29 0 CA for those spark plug locations respectively. Thus it is evident that combustion is faster in case of SP2.
The percentage MFB for different compression ratio with various spark plug position is given in Table 7.2. The MFB for SP2 is 47.22 %, 25.13 % for SP3 whereas 14.99 % for SP1 and 15.16 % for SP4 at CR 8. At CR 9, the percentage MFB is 44.33% for SP2 where as for other spark plug positions the MFB is either found increased or remains steady. At CR10, the percent MFB was 54.94 %, 32.08 %, 33.65 % and 13.78 % for SP2, SP3, SP1 and SP4 respectively.
Variation of MGT calculated [Eq.C-15 (APPENDIX-C)] as a function of crank angle is shown in Fig.7.8. It is known that, MGT depends upon the combustion process. Higher MGT is recorded at SP2 where spark plug will be 2 mm extruded inside the chamber. However the
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.