An Investigation of Multi-Injection Strategies for a Dual-Fuel Pilot Diesel Ignition Engine at Low Load

A multidimensional computational fluid dynamic (CFD) model was developed in order to explore the combined effect of injection timing and fuels quantity ratio of different injection strategies on the combustion performance and emissions characteristics of a dual-fuel indirect injection (IDI) engine with a pilot diesel ignition. The total mass of pilot diesel and premixed natural gas equivalence ratio were kept constant while various injection strategies (single, double, and triple) were investigated at 25% engine load and speed of 800 rpm. Results revealed that the released heat of triple injection pulse during the expansion stroke is the same or higher than that of single and double injection pulses at specified injection timings. It affects positively the engine performance. The highest indicated mean effective pressure (IMEP) can be achieved using single injection pulse at all first injection timings. It is observed that double and triple injection pulses possess comparable indicated thermal efficiency (ITE) and IMEP to those of single injection at specified injection timings. The highest ITE is found 47.5% at first injection timing of −16 deg after top dead center (ATDC) for both single and double injection pulses. Nitrogen oxides (NO_x) mole fraction generally increases when retarding the injection timing. By applying double and triple injection pulses, NO_x emissions decrease, on average, by 9% and 14% compared to that of the single injection pulse. Using double and triple injection pulses, soot emissions increase, on average, by 10% and 32%, respectively, compared to single injection pulse. However, at specified injection timings, the effect of all injection pulses on soot emissions is negligible at relative advanced first injection timing. Carbon monoxide (CO) emissions decrease slightly for all injection strategies when the injection timing varies from −20 deg ATDC to −12 deg ATDC. In this range, dual-fuel operation with triple injection pulse produces the lowest CO emissions. By using triple injection pulse at suitable injection timings, CO emissions decrease by around 7.4% compared to single injection pulse. However, by applying double and triple injection pulses, unburned methane increases, on average, by 16% and 52%, respectively, compared with that of single injection pulse. However, at injection timings of −12 deg ATDC and −8 deg ATDC, triple and double injection pulses produce comparable level of unburned methane to that of single injection pulse.