Novel VCR SI Engine using Gaseous Fuels
7.4 GASEOUS FUEL PERFORMANCE WITH EGR UNDER INFLUENCE OF VCR AND VSPL
As seen from the analysis of raw biogas and LPG fuel with novel VCR combined with VSPL, the performance is found improved. But the exhaust gas emissions are also found raised with rise of CR and adjusting spark location. In order to trace this issue, the technique of exhaust gas recirculation as studied and analyzed in chapter- 6 for conventional VCR SI engine using similar fuels can possibly been implemented. Under such situation, if EGR rate of different magnitudes are utilized in the biogas or LPG run SI engine, then the effect of novel VCR and VSPL on the performance of the engine in presence of EGR becomes utmost importance to select the optimum configuration at each situation.
Fig.7.37 BTE variation with EGR rate for different spark location at CR10
7.4.1 Biogas run SI engine performance with EGR under influence of VCR and VSPL.
Upon the successful operation of the biogas fueled SI engine using novel VCR combined with VSPL mechanism, it is possible to improve the performance of the engine. However the emissions which grows rapidly, needs to be controlled by some technique. To control these emissions, the effective system called as exhaust gas recirculation can be adopted. So the EGR rate 10%, 20% and 25% can be used based on the analysis reported in chapter 6. The
lower heating value fuel raw biogas mixed with EGR supplied to the engine. It is interesting to see the selection of optimum configuration of the engine such as CR and SP location to achieve maximum output with minimum exhaust emission.
7.4.1.1 Performance Analysis
The performance of the engine can be measured in terms of BTE . The BTE variation with EGR rate for different spark location maintaining constant CR10 is as plotted in Fig.7.37. As shown in figure, the BTE keeps on reducing with increase in EGR rate from 0% towards 25%. This is because of the non combustable recirculated exhaust gas when mixed with the fresh charge of fuel and air, makes the charge dilute and hence reduces its heating value. The outcome of this is the reduction in BTE. The corrosponding decrement of 47.56 %, 56.51 %, 68.33 % and 78 % respectively at SP1, SP2, SP3 and SP4 when operated at base CR 10 and speed 1290 rpm. At a particular EGR rate say EGR 0, SP2 shows maximum BTE of 26.4%.
Fig.7.38 BSFC variation with EGR rate for different spark location at CR10.
Other spark locations are having decrement of 5.30 %, 4.17 % and 9.09 % for SP1, SP3 and SP4 with respect to SP2. But this trend doesnot remains same for other EGR rate. At EGR 10, SP3 shows maximum BTE of 18.203 % showing decrement of 1.11 %, 22.37 % and 35.50 % for SP1, SP2 and SP4 respectively with respect to SP3. With the increase of EGR rate, the temperature of recirculated exhaust gas inreases which intern preheat the intake charge of fuel and air. This causes the combustion earlier with lesser compression pressure as compared to previous case of EGR 0. That may be the reason due to which the protrusion of 5mm (SP3) attains homogeneous combustion and increased efficiency. When EGR rate is increased further to 20 % or 25 %, it diluted the mixture to a great extent and hence the optimum location is found to be SP1 followed by SP3, SP2 and SP4. The overall decrement of 47.56 % ,56.15 %, 69.16 % and 77.95 % is recorded for SP1, SP2, SP3 and SP4 respectively with EGR 0 to EGR 25.
The effect of BTE variation is appeaed on the brake specific fuel consumption. Rather, if BTE found decreased with EGR rate, then BSFC gets increased with increse of EGR rate.
This can be seen in Fig.7.38 showing BSFC variation with EGR rate for different SP locations. For EGR 0, the SP2 shows 0.832 kg/ kW-hr BSFC which increases by 78.60 % at EGR 10, 105.52 % at EGR 20 and 135.58 % for EGR 25. Whith increase in EGR rate, brake power reduces which ultimately shows rise in BSFC. For EGR 10 showing optimum location to be SP3, EGR 20 shows SP1 and EGR 25 shows SP1 being maximum BTE or lowest BSFC settings.
7.4.1.2 Combustion Analysis
The combustion analysis is the only tool by which one can justify the engine performance at a particular compression ratio, spark location and EGR rate. To understand further the BTE (Fig.7.37) and BSFC (Fig.7.38) variations for the particular EGR rate, variable spark location for varying CR , the analysis has been carried out for cylinder pressure, NHRR, MFB and MGT.
(a) (b)
(c)
Fig.7.39 Cylinder pressure variation with CA for different spark location at CR10 with (a) EGR 10; (b) EGR 20 and (c) EGR 25.
The combustion parameter such as cylinder pressure is as plotted in Fig.7.39(a-c) with CA for different SP locations and EGR rate. For EGR0 the cylinder pressure variation shows that SP2 was the optimum location. For EGR 0, peak cylinder pressure is 25.156 bar at 371 0 CA
increases to 29.160 bar at 371 CA for SP2. Further to SP3 and SP4, it is 23.556 bar and 18.642 bar occurred at 370 0 CA and 378 0 CA respectively. If EGR rate increased to 10%, the peak pressure is at SP1 followed by SP3 showing a decrement of 9.40 %.
(a) (b)
(c)
Fig.7.40 NHRR variation with CA for different spark location at CR10 with (a) EGR 10; (b) EGR 20 and (c) EGR 25.
However at EGR 20, the peak pressure located at SP3 having magnitude 19.033 bar with decrement of 6.81 %, 12.42 % and 31.53 % respectively for SP1,SP2 and SP4 with respect to SP3. Highest EGR rate carried by biogas is EGR 25 in our case showing maximum pressure at SP1 location. As biogas is a poor heating value, it needs more time for combustion to occur and hence the peak pressure observed is quite far ahead of TDC.
The NHRR is the parameter directly depending upon the fuel being used and the working condition of the engine such as load, speed, ignition advance, compression ratio and spark location. The NHRR for biogas fueled SI engine varying with CA is as plotted in Fig. 7.40 (a- c) for different EGR rate with SP location at CR10 and speed 1290 rpm. From the diagram it is seen that the peak NHRR doesnot remain fixed at one CR and signifies the combustion behavior in engine with varying EGR rate. Peak NHRR at EGR 0 is 15.649 J/ 0 CA for SP1 raises further when engine is operated to SP2 having magnitude 19.187 J/ 0 CA. At other SP location the decrement of 22.74 %, 21.66 % is recorded respectively for SP3 and SP4 with respect to SP2. If the EGR rate is increased to 10 %, then SP3 is comparable with SP1
showing maximum NHRR in all locations considered. In terms of CA, SP3 reaches peak at 366 0 CA where as SP1 reaches at 372 0 CA and shows delayed combustion.
(a) (b)
(c)
Fig.7.41 Percent MFB variation with CA for different spark location at CR10 with (a)EGR 10; (b)EGR 20 and (c)EGR 25.
This shows that the protrusion of 5 mm inside combustion chamber can be optimally used in case of 10 % EGR rate.
Table 7.5 Percent MFB variation with CA for different spark location at CR10 with (a) EGR 10; (b) EGR 20 and (c) EGR 25.
EGR RAT
E
SP1 SP2 SP3 SP4
10%
MF B
90%
MF B
MFB
@ TDC
10%
MF B
90%
MF B
MFB
@ TDC
10%
MF B
90%
MF B
MFB
@ TDC
10%
MF B
90%
MF B
MFB
@ TDC 0 344 375 54.45 342 367 75.24 345 374 53.81 351 380 30.04 10 349 375 48.36 343 379 23.71 354 377 29.00 360 384 11.08 20 344 377 35.57 354 383 20.57 355 379 23.92 367 384 0 25 351 377 33.14 342 379 23.71 364 381 18.30 374 384 0 If the EGR rate increases further then it shows SP3 be the optimum location attaining peak NHRR of 14.162 J/ 0 CA at 371 0 CA. The heavy EGR rate 25 % is carried optimally by