Results of Emulsified Rice Bran Biodiesel - Biogas Run Dual Fuel Engine
8.3 Performance Analysis
The performance analyses include 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 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 used for performance and combustion analysis are given in Appendix-A. In this investigation, 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 WIRBB-biogas run dual fuel diesel engine for a constant IT. Therefore, the plots of each performance parameter for constant ITs are group together with diesel mode for comparison. The results are discussed with respect to the two design and performance parameters, namely, CR and IT.
8.3.1 Effect of compression ratio
The BTE under DFM is found to be lower in comparison to diesel mode due to lower calorific values of both biogas and WIRBB as depicted in Fig. 8.1. In addition, the other factors like biogas residual, combusted residual gases, low combustion temperature and low flame propagation speed, higher total fuel flow rate during combustion and increased negative compression work caused by induction of large biogas air mixture are contributing to the low thermal efficiency. With 100% load, at 23°BTDC, the BTEs under DFM are found to be 19.84%, 18.07% and 17.4% for CRs of 18, 17.5 and 17, 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 resulted in occurrence of fast microexplosion, which initiates the earlier ignition of pilot fuel by appropriate mixing of air and pilot fuel, thereby giving sufficient time for the combustion of biogas. This increases the probability of a better combustion of biogas. The BSEC during diesel mode is found to be lower than DFM (Fig. 8.2). On an average, the BSEC reduces by 11.01% as CR increased from 17 to 18. The EGT for all cases of DFM is found to be higher than diesel mode (Fig. 8.3). This is owing to late combustion of biogas which reduces the duration of extraction of power. As a result, the combustion products in the form of gases come out at a higher temperature. Further, it seems that the cooling effect of microexplosion produced by the emulsified pilot fuel has a marginal effect on the rise of EGT in comparison to late
combustion of raw biogas as engine mainly run on biogas under DFM. At 23°BTDC, there is an increase of 7.23%, 12.26% and 14.78% of EGT at CRs of 18, 17.5 and 17, respectively as compared to diesel mode. The burning velocity of the biogas air mixture increases with the increase of CR.
Fig. 8.1 Variation of BTE with load, CR and IT
Fig. 8.2 Variation of BSEC with load, CR and IT
Fig. 8.3 Variation of EGT with load, CR and IT
Therefore, the time required for the complete combustion shortens and this produces lower EGT. This means, combustion of biogas starts early in case of high CR. With 100% load, at 23°BTDC, the BFR is found to be 2.67 kg/h, 2.95 kg/h and 3.06 kg/h for CRs of 18, 17.5 and 17, respectively as indicated in Fig. 8.4. Thus, the BFR decreases at high CR at same loading condition. At 23°BTDC, there is a reduction of 15.84% in BFR by increasing the CR from 17 to 18 at 23° BTDC. With 100% load, at 23° BTDC, the liquid fuel replacement (LFR) is found to be 79.25%, 77.77% and 76.79% for CRs of 18, 17.5 and 17, respectively as depicted in Fig. 8.4. Thus, the LFR increases marginally for high CR. Analogous trend of rise of BTE and drop of EGT with increase of CR for different test fuels was reported earlier (Raheman and Ghadge, 2008; Jindal et al., 2010).
8.3.2 Effect of Injection Timing
For CRs of 18, 17.5 and 17, with 100% load, the BTEs are found to be 21.36%, 19.29% and 18.35% at 26° BTDC in comparison to 23.62%, 20.97% and 19.89 % at 29° BTDC.
However, at 32°BTDC, the BTEs are found to be 22.36%, 19.91% and 18.27% for CRs of 18, 17.5 and 17, respectively. From the above investigation, it is quite evident that the BTE of the biogas run dual fuel diesel engine for this particular study improves with the advancement of pilot fuel IT upto IT of 29°BTDC. The advancement of IT results in injection of WIRBB
earlier into the combustion chamber than its standard IT, thereby giving ample time for the WIRBB to form a homogeneous mixture with air and biogas. This results in more efficient burning of biogas air mixture. For CRs of 18, 17.5 and 17, the BSEC drops by 10.55%, 4.57%°C, and 5.77%, respectively at 26°BTDC in comparison to 19.66%, 9.56% and 10.9%
at 29°BTDC. Further advancement of IT to 32°BTDC lowers the BSEC to 9.05%, 4.21%
and 3.42% for CRs of 18, 17.5 and 17, respectively. On an average, for CRs of 18, 17.5 and 17, the EGT rises by 5.97%, 10.37% and 12.89% at 26° BTDC in comparison to 4.08%, 7.86% and 10.69% at 29°BTDC. However, at 32°BTDC, the EGT is increases by 2.83%, 5.97% and 9.11% for CRs of 18, 17.5 and 17 respectively. Thus, the EGT under DFM drops with the advancement of IT. Researcher (Raheman and Ghadge, 2008, Debnath et al., 2014b) observed similar trend of EGT with the advancement of IT for different test fuels.
Fig. 8.4 Variation of BFR and LFR with load, CR and IT
On advancing IT from 23°to 32°BTDC for CR of 18, there is a reduction of 11.21% in BFR.
`For the same range of IT, there is a drop of 8% and 11.7%, respectively in BFR at CRs of 17.5 and 17. For CRs of 18, 17.5 and 17 with 100% load, the LFR is found to be 79.47%, 78.27% and 78.21% at 26° BTDC in comparison to 82.22%, 80.24% and 79.25% at 29°
BTDC. However, at 32°BTDC, the LFR is found to be 80.24%, 79.75% and 75.46% for CRs of 18, 17.5 and 17, respectively.