Abstract
Opposed rotary piston (ORP) engines can deliver high power density and have few moving parts, being suitable for the power sources of hybrid electric vehicles, range extended electric vehicles, and unmanned aerial vehicles. Hydrogen as a promising alternative fuel is free of carbon emissions during combustion. Hydrogen direct injection avoids the significant power losses of port injection scenarios resulting from the low hydrogen energy density by volume. This paper firstly investigated the ORP engine performance using a 3D numerical simulation method over various hydrogen direct injection strategies (start of hydrogen injection and injection durations). Hydrogen diffusions in combustion chambers, combustion characteristics, engine performance, nitrogen oxides (NOx) emissions, and knock tendency were researched over various hydrogen injection strategies. Hydrogen was unevenly distributed in the combustion chambers for high equivalence ratio scenarios, leading to low combustion efficiency; additionally, the unburned hydrogen was mainly in the cylinder bowls and bottoms. Combustion durations were the shortest within the equivalence ratio range of 0.577–0.865, being approximately 18° crank angle (CA). The maximum in-cylinder pressure was higher than 8.0 MPa over the equivalence ratio of 0.961; and the corresponding heat release rates were higher than 45 J·(°CA)−1. The indicated thermal efficiency was higher than 38%, and it was increased generally by dropping equivalence ratios, with the maximum efficiency being approximately 42.5%. NOx emission factors, approximately 35 g (kW h)−1, reached the maximum value under the equivalence ratio of 0.673 conditions. Knock tendency was decreased continuously by lowering equivalence ratios. This research made a foundation of improving the engine fuel economy and mitigating NOx emissions for hydrogen direct injection ORP engines.
•Hydrogen direct injection applied to ORP engines maintains high power density.•Hydrogen injection strategy affects mixture formation and combustion characteristics.•High ER cases had low combustion efficiency due to uneven hydrogen distributions.•Thermal efficiency was increased by low ER due to less heat loss by cylinder walls.•Cylinder bowls were the man regions having high NOx concentration for all the cases.