Studies with Alternative SI Engine Fuels
B. Effect of EGR
Exhaust gas recirculation effect on torque variation can be seen from Fig. 6.12 (a, b and c) when engine running at MBT timing and constant air-fuel ratio.
Fig.6.13(a) Brake Torque variation with speed for different EGR rate at CR10
Fig.6.13(b) Brake Torque variation with speed for different EGR rate at CR9
Fig.6.13(c) Brake Torque variation with speed for different EGR rate at CR8
Fig.6.14 (a) Brake Power variation with speed for different EGR rate at CR10
Fig.6.14 (b) Brake Power variation with speed for different EGR rate at CR9
Fig.6.14 (c) Brake Power variation with speed for different EGR rate at CR8
Fig.6.15(a) BTE variation with speed for different EGR rate at CR10
Fig.6.15 (b) BTE variation with speed for different EGR rate at CR9
Fig.6.15(c) BTE variation with speed for different EGR rate at CR8
Fig.6.16(a) BSFC variation with speed for different EGR rate at CR10
Fig.6.16(b) BSFC variation with speed for different EGR rate at CR9
Fig.6.16(c) BSFC variation with speed for different EGR rate at CR8
The torque decrement of 28-30 % is observed with increase in speed from 1293 rpm to 1700 rpm for all EGR 0 to EGR 40 (Fig.6.12-a). The maximum decrement is observed at EGR40.
At a particular speed 1293 rpm and CR10, the torque decrement with EGR10, EGR 20, EGR 30 and EGR 40 is 13.2%, 14.9%, 21.37% and 33.08% respectively with respect to EGR 0 (Fig. 6.12-a). With decrease in compression ratio to 9, the torque decrement is 28.45%, 33.23%, 30.85%, 31.34% and 26.26% at EGR0, EGR10, EGR20, EGR30 and EGR40 respectively over a speed range of 1253-1700 rpm (Fig. 6.12-b). Similarly for a constant speed of 1293 rpm, the torque percentage decreases to 17.8, 27.01, 29.85 and 39.54 percent at CR9 with EGR10, EGR20, EGR30 and EGR40 (Fig. 6.12-b). At CR 8 the torque decrement is 30.75%, 35.65%, 40.40%, 46.59% and 22.45% over speed range of 1253-1700 rpm at EGR0, EGR10, EGR20, EGR30 and EGR40 respectively (Fig. 6.12-c). At a constant speed of 1293 rpm the decrement of torque is 14.04, 27.89, 37.76, 48.25 percent at 1293 rpm at EGR10, EGR20, EGR30 and EGR40 respectively (Fig. 6.12-c). This decrement in torque due to rise in EGR at 90% WOT shows that the fuel air mixture strength deteriorates with rise in EGR percentage which results in poor performance in terms of torque over fixed speed range 1200-1700 rpm (Haywood,1988). EGR effect is seen for LPG fuel from lower EGR10 to very heavy EGR40 over brake power as shown in Fig. 6.13 (a,b and c). It is acknowledged that the trend followed by magnitude of power at maximum load is of decreasing order with speed. The brake power shows decrement with increase in EGR rate. At a particular speed of 1293 rpm and CR10, EGR10, EGR20, EGR30 and EGR40 in intake charge, shows power decrement of 14, 16, 22, 33% with respect to EGR0 (Fig. 6.13-a). Similarly at lower CR9 and same speed 1293 rpm, the power decrement was 17, 26, 30, 40% (Fig. 6.13-b) whereas at CR8 the power decrement was 13, 25, 33, 47% with respect to EGR0 (Fig. 6.13-c). Intake
charge mixture dilution rate in terms of EGR introduction leads to decrease in flame propagation speed and amount of air and fuel into the engine cylinder, this consequently results in decrease in brake power with increase of EGR rate (Hu et al.,2009). The VCR and EGR effect on the brake thermal efficiency can be explored from Fig. 6.14 (a,b and c) where the BTE variation with speed for different percentage EGR is shown for CR10, CR9 and CR8. The BTE is decreasing with increasing speed for EGR0 to EGR40. The decrement in BTE is more if EGR percentage is more in the intake charge. At a particular speed of 1293 rpm and CR10 the BTE decrement is 6, 16, 22 and 30% at EGR 10, EGR 20, EGR 30 and EGR 40 dilution of exhaust gas with respect to EGR0 (Fig. 6.14-a). At similar speed and CR9, the BTE shows 11, 23, 29 and 33% decrement at EGR 10 EGR 20, EGR 30 and EGR 40 respectively with respect to EGR0 (Fig. 6.14-b) whereas at CR8 the decrement in BTE is 8, 23, 22 and 39% at speed 1293 rpm with respect to EGR0 (Fig.6.14-c). This decrement in BTE is found higher among all EGR tests from medium to very heavy EGR (EGR 30 and EGR 40). As the EGR increases from 0% to 40%, there will be more dilution of untreated exhaust gas with the fresh air-fuel charge, so the heat supplied by this charge gets reduced due to decrease of lower calorific value of the charge inducted in the engine cylinder. So less heat was released and the power produced gets lower. The ultimate effect of this is the reduction in brake thermal efficiency.
The BSFC variation with speed due to variation in EGR for CR10, 9 and 8 can be studied from Fig. 6.15 (a,b and c). Here, BSFC increases with the increase in speed for the complete speed range at a particular CR combined with EGR 0 or EGR 10 or EGR 20 even at EGR 30 and EGR 40. At 1293 rpm and CR 10, the increment in BSFC from EGR 10, EGR 20, EGR 30 and EGR 40 is 6, 18, 27 and 42 % respectively with respect to EGR0 (Fig. 6.15-a). At similar speed and CR 9 this increment is 13, 30, 42 and 49% with respect to EGR0 (Fig. 6.15- b). Whereas at CR8 this increment at lowest speed 1253 rpm is 8, 30, 48 and 63% with respect to EGR0 (Fig.6.15-c).The higher EGR rate means lower flame development and longer
Fig.6.17 (a) Cylinder pressure variation with crank angle for different untreated EGR rate using LPG fuel for 90% WOT
Fig.6.17 (b) Cylinder pressure variation with crank angle for different untreated EGR rate using LPG fuel for 50%throttle
Fig.6.17 (c) NHRR variation with crank angle for different untreated EGR rate using LPG fuel for 90%
WOT
Fig.6.17 (d) Cylinder pressure variation with crank angle for different untreated EGR rate using LPG fuel for 50% WOT
Fig.6.17 (e) MGT variation with crank angle for different untreated EGR rate using LPG fuel for 90%
WOT
Fig.6.17 (f) MBT variation with crank angle for different untreated EGR rate using LPG fuel for 50%
WOT
combustion duration results in slow combustion and thus lower power output. The decrement in power produced for the constant fuel consumption results ultimately in higher BSFC (Fontana and Galloni,2010)
6.3.3.3. Combustion Analysis in presence of EGR
The combustion analysis is also presented herein where the effect of EGR is clearly seen. The combustion parameters include the cylinder pressure, net heat release rate (NHRR),percent mass fraction burned (MFB) and mean gas temperature (MGT) are of primary concerned.
Along with this the combustion duration and fuel conversion efficiency are also important for selection of right combination of CR and EGR rate. Based on the evaluation of spark advance in section 6.2.3, the optimum spark advance is used for the analysis.