Premixed mixture ignition in the end-gas region (PREMIER) combustion in a natural gas dual-fuel engine: Operating range and exhaust emissions

This paper is concerned with engine experiments and spectroscopic analysis of premixed mixture ignition in the end-gas region (PREMIER) combustion in a pilot fuel ignited, natural gas dual-fuel engine. The results reveal the characteristics and operating parameters that induce and affect this combustion mode. The PREMIER combustion is followed by natural gas flame propagation. Pilot-injected diesel fuel ignites the natural gas/air mixture, and the flame propagates before the natural gas/air mixture is autoignited in the end-gas region. This combustion cycle differs from a knocking cycle in terms of combustion and emission characteristics. The PREMIER combustion can be controlled by pilot fuel injection timing, the equivalence ratio, and the exhaust gas recirculation (EGR) rate, and can be used as an effective method for high load extension on a dual-fuel engine. An analysis of the relationship between the maximum in-cylinder pressure and its crank angle (CA) is used to compare combustion dynamics during conventional, PREMIER, and knocking combustion. In PREMIER combustion, the heat release gradually transforms from the slower first-stage flame rate to the faster second-stage rate. During PREMIER combustion, the maximum indicated mean effective pressure (IMEP) and thermal efficiency increase by about 25 per cent compared with those of conventional combustion, and low carbon monoxide (CO) and total hydrocarbon (HC) emissions can be achieved. However, nitrogen oxide (NO_x) emissions increase. Spectroscopic analysis shows that the intensity of the OH* emissions in the end-gas region increases as the combustion mode transforms from conventional to PREMIER to knocking. In all three modes, emission fluctuations above 650nm can be observed in the end-gas region. These emissions are attributed to the luminosity from soot particles formed during the concurrent diesel fuel combustion.