Experimental investigation of the combustion behaviour and NOx emissions of LPG-diesel, dual-fuel engine

Abstract

In this research, the effects of diesel and dual-fuel combustion, by introducing liquified petroleum gas (LPG) fuel (in the vapour state) in the intake manifold, were investigated using a compression ignition engine. The engine performance combustion characteristics were analysed for both configurations, i.e., in diesel-only operation and dual-fuel operation. Before investigating the performance and combustion behavior, it was found that since the engine was previously turbocharged and then converted to naturally aspirated, at the optimum injection timing, the rate of heat released occurred after top dead center (TDC) not before when compared to a turbocharged diesel engine. This means that the combustion initiation occurred after the piston reached TDC not occurring while the piston is moving towards the TDC, where a continuous buildup of the in-cylinder pressure and temperature occurs. Following the determination of the optimum injection timings, the results obtained were inputted into the injection timing table of the Reata electronic control unit (ECU). It was shown that for both engine operations, the coefficient of variation (COV) of indicated mean effective pressure (IMEP) did not exceed 10%. The mass fraction burned (MFB) and cyclic variability of the MFB were then investigated at MFB10%, MFB50%, and MFB90% for both operations and it was shown that the maximum COV was 2.5%, which is well below 10%. Regarding the brake thermal efficiency (BTE), during dual-fuel (DF) operation it was found that as the diesel pilot injection (DPI) was increased, the BTE was slightly increased, however, for the majority of the operations, as the LPG quantity was increased, the BTE decreased and in cases the BTE was reduced more than diesel-only (DO) operation. When comparing the performance for both operations, it was found that initially when the DPI was relatively small, the performance of the DO operation was superior to that of the DF operation. However, as the diesel pilot quantity was increased, it was found that the engine was operating slightly better than the DO operation. Regarding the emissions, the nitrogen oxide (NOx) emissions at DO operation, increased at low to mid load conditions and then reduced again at full load conditions. At DF operation, the NOx values increased with the increase in LPG mass fraction. The oxygen quantity during DO and DF operation was observed to decrease with the increase in diesel injection and LPG quantity. The combustion behavior was then investigated in terms of in-cylinder pressure, heat release rate (HRR) and MFB for both operations and compared. At DF operation, further investigation was done and three pilot sweeps were investigated. It was observed that as the LPG quantity was increased, the combustion ignition was shown to be delayed. However, as the pilot injection was relatively increased, the ignition delay was drastically reduced.