Numerical investigations of combustion and emissions characteristics of a novel small scale opposed rotary piston engine fuelled with hydrogen at wide open throttle and stoichiometric conditions

Popularisations of hybrid vehicles and range extender electric vehicles promote the development of high power density and small scale internal combustion engines. Opposed rotary piston (ORP) engines characterised by compact designs, few moving parts and high power density are an ideal power source for the above mentioned vehicles. Due to the short cyclic period of the ORP engine, hydrogen fuel was applied to decrease the combustion duration. This paper investigated the in-cylinder combustion and emissions characteristics of the hydrogen fuelled ORP engine using 3D numerical simulation method at various engine speeds and full load conditions. In cylinder pressure evolutions, heat release rates, nitrogen monoxide (NO) formations, and power density were analysed to evaluate the engine performance. The results indicated that volumetric efficiency of this ORP engine was higher than 88.3% for all the given scenarios, being benefited from large area of intake ports. Peak in cylinder pressure decreased significantly with engine speeds, which was mainly resulted from low fuel mass burn fraction before top dead centre (TDC) for high engine speed conditions. As long as the combustion chambers passed TDC, combustion flame propagated from the bowls into the gaps between end faces of adjacent pistons rapidly. In the exhaust stroke, free discharge process of this ORP engine lasted longer duration than reciprocating engines, which would lead to more energy losses. NO was mainly formed after TDC, with the accumulated NO mass being in the range of 0.07 mg-0.5 mg per cycle per cylinder in the engine speed range of 1000-5000 r/min. Maximum power density and NO emissions factor of this engine fuelled with hydrogen was approximately 69.2 kW.L-1 and 10.60 g.(kW.h)(-1), respectively. Indicated thermal efficiency dropped from 36.2% to 26.5% when the engine speed increased from 1000 to 5000 r/min.