Results of Diesel - Biogas Run Dual Fuel Engine
5.3 Performance Analysis
The performance analyses evaluated are brake thermal efficiency (BTE), brake specific energy consumption (BSEC), exhaust gas temperature (EGT), biogas flow rate (BFR) and
liquid fuel replacement (LFR). The combustion analyses include the cylinder pressure variation, ignition delay (ID), peak cylinder pressure (PCP) and net heat release rate (NHRR).
Finally, emission analysis is performed by measuring carbon dioxide (CO2), carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOX). The theoretical equations, based on which performance and combustion analysis are executed, are included in Appendix-A. In this study, the EGT, BFR, ID, PCP and emission characteristics analysis are carried out on average basis for considering the effect of all loading conditions. The CR variations are made for biogas run dual fuel diesel engine for a constant IT. Therefore, the plots of each performance parameter for constant ITs are clubbed together with diesel mode for comparison. The results are discussed with respect to the two design and performance parameters, namely, CR and IT.
5.3.1 Effect of Compression Ratio
The BTE under DFM is found to be lower in comparison to diesel mode due to a low calorific value of biogas as depicted in Fig. 5.1. At 23° BTDC with 100% load, the BTEs under DFM are found to 20.04%, 18.25%, 17.07% and 16.42% for CRs of 18, 17.5, 17 and 16, respectively as compared to 27.76% for diesel mode. The BTE under DFM improves for high CR. This is due to the fact that the temperature and the pressure rise with the increase in CR. This, in turn, increases the probability of more amount of biogas to undergo complete combustion. Similar trend of rise of BTE with increase of CR was reported earlier (Jindal et al., 2010). The BSEC during diesel mode is found to be lower than dual fuel mode (Fig. 5.2).
This is because diesel is having higher calorific value than biogas. The BSEC drops with the increase of CR. At 100% load, the BSEC reduces by 19.38% as CR increased from 16 to 18.
This is due to better combustion of biogas at high CR. The EGT for all cases of DFM is found to be higher than diesel mode (Fig. 5.3). This is due to late combustion of biogas which shortens the duration of extraction of power. As a result, the combustion products in the form of gases come out at higher temperature. At 23° BTDC, the EGT increases by 13.54%, 20.39%, 22.13% and 26.09% for CRs of 18, 17.5, 17 and 16, respectively as compared to diesel mode. The burning velocity of the biogas air mixture increases with the increase of CR. Therefore, the time required for complete combustion reduces and this produces lower EGT. This means, combustion of biogas starts early in case of high CR. Analogous trend of drop of EGT with the rise of CR was reported earlier (Raheman and Ghadge, 2008). At 23°
BTDC with 100% load, the BFR is found to be 2.73 kg/h, 2.8 kg/h, 2.88 kg/h and 3.02 kg/h
for CRs of 18, 17.5, 17and 16, respectively as indicated in Fig. 5.4. Thus, the BFR decreases at high CR at same loading condition.
Fig. 5.1 Variation of BTE with load, CR and IT
Fig. 5.2 Variation of BSEC with load, CR and IT
On an average, there is a reduction of 15.23% in BFR by increasing the CR from 16 to 18 at 23°BTDC. At 23°BTDC with 100% load, the LFR is found to be 79.46%, 76.1%, 74% and 72% for CRs of 18, 17.5, 17 and 16, respectively as depicted in Fig. 5.4. Thus, the LFR marginally improves with the increase of CR.
Fig. 5.3 Variation of EGT with load, CR and IT
5.3.2 Effect of Injection Timing
For CRs of 18, 17.5, 17 and 16 with 100% load, the BTEs is found to be 22.68%, 20.99%, 18.15 and 17.12% at 26ºBTDC in comparison to 25.44%, 22.99%, 19.89 and 18.35% at 29°
BTDC. However, at 32° BTDC, the BTEs are found to be 23.06%, 21.25%, 18.86% and 17.96% for CRs of 18, 17.5, 17 and 16, respectively. From the above analysis, it is quite evident that the BTE of the biogas run dual fuel diesel engine for this particular study improves with the advancement of IT of pilot fuel up to IT of 29°BTDC. The advancement of IT results in injection of diesel earlier into biogas-air mixture than its standard IT, thereby giving sufficient time for the complete combustion of biogas. This increases the probability of a better combustion of biogas. Similar trend of in BTE with the advancement of IT was observed by Raheman and Ghadge (2008) for a biodiesel run diesel engine. On an average, for CRs of 18, 17.5, 17 and 16, the BSEC drops by 6.6%, 1.61%°C, 3.67% and 2.56% at 26°
BTDC in comparison to 14.7%, 16.7%, 8.42% and 8.2% at 29°BTDC. Further advancement of IT to 32°BTDC lowers the BSEC to 10.88%, 8.52%, 12.24% and 8.07% for CRs of 18, 17.5, 17 and 16, respectively. For CRs of 18, 17.5, 17 and 16, the EGT rises by 10.9%, 16.43%°C, 19.24% and 22.7% at 26° BTDC in comparison to 7.1%, 12.96%, 16.43% and 20.23% at 29°BTDC. Further advancement of IT to 32°BTDC increases the EGT to 4.7%, 10.4%, 12.55% and 16.1% for CRs of 18, 17.5, 17 and 16, respectively. Thus, the EGT decreases with the advancement of pilot fuel IT under DFM. On an average, there is a reduction of 18.51% in BFR is achieved on advancing IT from 23°BTDC to 32°BTDC for CR of 18. For the same range of IT, there is a drop of 12.66%, 10.64% and 7.22% in BFR for CRs of 17.5, 17 and 16, respectively. For CRs of 18, 17.5, 17 and 16 with 100% load, the LFR is found to be 83.17%, 80.67%, 76.06% and 75.22% at 26°BTDC in comparison to 82.67%, 80.70%, 76.06 and 75.22% at 29°BTDC. However, at 32°BTDC, the LFR is found to be 79.32%, 78.4%, 76.13% and 76.1% for CRs of 18, 17.5, 17 and 16, respectively.
Fig. 5.4 Variation of BFR and LFR with load, CR and IT