Studies with Alternative SI Engine Fuels
6.2 GASEOUS FUEL OPTIMUM SPARK TIMING ANALYSIS .1 Performance analysis
6.2.2 Combustion Analysis .1 Effect of ignition timing
Combustion analysis is important for attaining the optimum spark timing for the LPG and raw biogas fuels. These outcomes will closely justify the performance parameters reveled in the last section 6.2.
According to the performance analysis, 26 0 bTDC and 45 0 bTDC are the optimum spark advance for 100% LPG and raw biogas fuels. To backup those outcomes, the study of cylinder combustion pressure variation with crank angle for different spark advance at CR10 and speed 1430 rpm is as plotted in Fig. 6.3(a) for LPG fuel and in Fig. 6.4(a) for raw biogas.
As shown in figure, the peak cylinder pressure is 29.61bar at 15 0 aTDC, 34.16 bar at 7 0 aTDC, 36.83 bar at 5 0 aTDC, 38.39 bar at 1 0 aTDC, 37.07 bar at 2 0 bTDC for 24 0,26 0,28 0, 43 0 and 46 0 bTDC spark advance respectively when engine running at CR10 with speed 1430 rpm. It is seen that with advancing the spark timing, the peak pressure increases (Heywood,1988, Stone,1999). At the same time the crank angle at which peak pressure occurred also gets close to TDC. As the MBT timing for LPG fuel is 26 0 bTDC, the peak pressure 34.16 bar is the optimum pressure at 7 0 aTDC. The optimum timing of spark ultimately helps to generate more heat from the fuel which can be understood clearly from heat release analysis.
The Net Heat Release Rate (NHRR) which is the heat gets converted to work out of total heat or gross heat produced at the end of spark with crank angle. According to 1st law of thermodynamics, the NHRR analysis has been carried out as detailed in APPENDIX-C.
NHRR for LPG fuel at CR10 and speed 1430 rpm is as shown in Fig. 6.3(b).The peak NHRR is observed for 26 0 bTDC and its amount is 32.681 J/0CA at crank angle 20bTDC as compared to other spark timings. With increase in spark timing bTDC, the peak NHRR keeps on shifting towards the left of TDC (bTDC) (Heywood, 1988, Stone, 1999). In case of 46 0 bTDC, the peak NHRR is 25.47 J/0 CA occurred at 21 0 bTDC. This causes the combustion pressure reaches at its peak close to TDC as shown in Fig. 6.2(a). The overall effect is the
loss of work during compression and reduced the conversion of heat energy in to useful work during expansion stroke. In regards to the maximum brake power, from the literature it was noticed that peak NHRR should be before TDC (Heywood, 1988; Taylor, 1999). The same is observed at 26 0 bTDC where the brake power is also found more as shown in Fig. 6.1(b).
Fig.6.3(a) Cylinder Pressure variation with crank angle for spark timing using LPG fuel
Fig.6.3(b) NHRR variation with crank angle for spark timing using LPG fuel
Fig.6.3(c) Percent MFB variation with crank angle for spark timing using LPG fuel
Fig.6.3(d) Mean Gas Temperature (MGT) variation with crank angle for spark timing using LPG fuel
Percentage mass fraction burned (MFB) is a ratio of actual mass burned to the theoretical mass burned which can be calculated by approach as discussed in APPENDIX-C (Stone,1999). Percent MFB as function of crank angle for CR10 and speed 1430 rpm is as shown in Fig. 6.2(c). Percent MFB at TDC is 30.37%, 51.90%, 84.50%, 93.68% and 100%
for 24 0, 26 0, 28 0, 43 0 and 46 0 bTDC respectively. As the ignition timing advances to 460 bTDC, the combustion initiation gets advanced and that may be the reason, the MFB is 100%
at TDC. However at optimum spark location 260 bTDC the MFB obtained was 62% at TDC.
This shows that with advancing the ignition timing, the magnitude of percentage MFB represent higher consumption of charge. But the spark timings such as 46 0 bTDC could not able to attain the maximum brake torque (Fig. 6.1-a). This means that the necessary compression stroke which generates the compression pressure is less for 46 0 bTDC spark advance. This ultimately reduced the brake power. However for 26 0 bTDC, the percent MFB is 51.90% at TDC which signifies that there should be maximum compression pressure required for combustion of fuel air charge to cause complete combustion. So the crank angle
of 26 0 bTDC is the optimum ignition timing for the LPG fuel in this experimental test set up.
The effect of rise in pressure of the engine at CR10 and speed 1430 rpm by different spark advance affects the mean gas temperature (MGT) of the combustion. MGT is the temperature after combustion of fuel air charge inside combustion chamber at the prevailing pressure and volume of chamber. Using the universal gas equation, MGT can be evaluated as explained in APPENDIX-C. This is found to be 1062.420C at 370 0 CA when spark advance is 26 0 bTDC.
During combustion the optimum spark timing is of utmost importance for the flame development and propagation as well as length of flame travel across the combustion chamber and finally the flame termination after reaching the wall (Heywood,1988).The optimum spark location of 26 0 bTDC is again justified in terms of MGT.
Fig.6.4 (a) Cylinder Pressure variation with crank angle for different spark timing using raw biogas fuel for different CR
Fig.6.4 (b) NHRR variation with crank angle for different spark timing using raw biogas fuel for different CR
Fig.6.4 (c) Percent Mass Fraction Burned (MFB) variation with crank angle for different spark timing using raw biogas fuel for different CR
Fig.6.4 (d) Mean Gas Temperature(MGT) variation with crank angle for different spark timing using raw biogas fuel for different CR
6.2.2.2 Effect of compression ratio
With the very poor heating value fuel raw biogas, the optimum spark timing is 45 0 bTDC.
The cylinder pressure in case of raw biogas is 25.536 bar observed at4 0 aTDC as represented in Fig. 6.4(a) while engine working at CR10 and speed 1430 rpm with 90% WOT. At the same time the peak cylinder pressure found to be reduced and keep on shifting away from TDC on right side (aTDC) with other spark locations such as 37 0 bTDC, 39 0 bTDC, 41 0
bTDC , 47 0 bTDC. As the auto ignition temperature of 100% raw biogas is higher, it takes 49 0 CA (angle from spark initiation to the peak pressure) to attain the peak pressure.
However it is seen here that with respect to LPG fuel (Fig. 6.3-a), the peak pressure is lowered. There are numerous reasons for this but the major reason from combustion point of view is the reduction in effective compression stroke causing reduced pressure of charge before spark. This is seen clearly in Fig. 6.4(b) where NHRR variation with crank angle is shown for the spark timings considered using raw biogas fuel. Here the optimum spark timing is 45 0 bTDC as NHRR is found to be 13.382 J/0 CA which is maximum and occurred at 7 0 bTDC. An increment of 0.5%, 0.1%, 0.9% and 28% is observed with respect to 47 0 bTDC, 41 0 bTDC, 39 0 bTDC, 37 0 bTDC respectively while combustion takes place at optimum spark advance. The contents of raw biogas such as H2S and water lower the heating value of fuel. In addition, whatever heat is produced is being absorbed by such ingredients lowering the NHRR further (Razbani, 2011).
The mass fraction burned for raw biogas fuel combustion is as plotted in Fig. 6.4(c) for different crank angles. The significance of MFB is the understanding about the speed of combustion inside combustion chamber. Accordingly, at TDC, the percent MFB is 64% at MBT spark timing and shows maximum charge gets burned. However it is seen that before TDC the speed of combustion is faster as compared to that during expansion stroke (aTDC).
This is the reason that is why combustion duration is increased to 28 0 CA. At other spark timings, the percent MFB is 23% to 40% and combustion extended till 19 0 aTDC. At 45 0 bTDC, the flame gets enough time to spread across the chamber so that homogeneous combustion may be happened. That is the reason NHRR shows peak value at 45 0 bTDC (Fig.
6.4-b) as well.
Table 6.1 The experimental matrix for multifuels VCR engine
Throttle EGR0 EGR10 EGR20 EGR30 EGR40
Conventional VCR Mechanism Engine
LPG WOT
CR8,CR9 and CR10 +Untreated
EGR
CR8,CR9 and CR10 +Untreated
EGR
CR8,CR9 and CR10 +Untreated
EGR
CR8,CR9 and CR10 +Untreated
EGR
CR8,CR9 and CR10 +Untreated
EGR
LPG WOT(IT) ALL CR (+5,+10,+15,+18,0,-2,-4)
Conventional VCR Mechanism Engine Biogas
WOT CR8,CR9, CR10+Untre
ated EGR
CR8,CR9, CR10+Untreated
EGR
CR8,CR9, CR10+Untreat
ed EGR
CR8,CR9, CR10+Untr eated EGR POT
Biogas WOT(IT) ALL CR
(+2,0,-2,-4,-6,-8,-10)
The mean gas temperature for the 100% raw biogas fuelled engine is as illustrated in Fig.
6.4(d). The MGT is 823 0 C, 833 0 C, 786 0 C, 811 0 C and 669 0 C respectively at 47 0 , 45 0 , 41 0, 39 0 and 37 0 bTDC. At MBT timing, the MGT found maximum as compared to other spark advances