Simulation on the effect of compression ratios on the performance of a hydrogen fueled opposed rotary piston engine

Hydrogen internal combustion engines are attracting increasing attention because of no carbon dioxide (CO₂) emission and high thermal efficiency for hydrogen combustion. Opposed rotary piston (ORP) engines have simple structures, small size and mass, contributing to the performance improvement of hybrid electric vehicles (HEVs) and range extended electric vehicles (RE-EVs). Compression ratios as an important factor affecting engine performance should be considered in the process of engine designs and optimizations. In this paper, in-cylinder combustion characteristics of a small-scale ORP engine were investigated using a numerical simulation method over different compression ratios (8.90, 9.66, and 10.55). Compression ratio adjustment of the ORP engine may be achieved by the movement patterns of the two shafts. The results indicated that maximum in-cylinder pressure was increased from approximately 3.5 MPa–5.0 MPa when the compression ratios were increased from 8.90 to 10.55. The crank angle (CA) of the maximum in-cylinder pressure was slightly retarded by increasing compression ratios. The hydrogen combustion rates were almost the same before top dead centre (TDC) for the three cases. The combustion durations were dropped from approximately 38.7 °CA to 28.3 °CA when the compression ratios were increased from 8.90 to 10.55; however, the combustion phase of the compression ratio of 9.66 was the earliest among the three cases. The proportions of energy loss by cylinder walls were almost the same under different compression ratios, being approximately 10%; additionally, the indicated thermal efficiency was increased from approximately 34%–39% by changing compression ratios from 8.90 to 10.55. The nitrogen oxides (NO_x) emission factors of the ORP engine were almost linearly increased by increasing compression ratios, with the values being higher than 17.2 g/kWh for all the three cases. NO_x distributions in combustion chambers around 50 °CA after TDC agreed well with those of in-cylinder temperature and hydrogen residuals.