Effect of compression ratio on engine knock, performance, combustion and emission characteristics of a bi-fuel CNG engine

Highlights: • Optimization of compression ratio for a gasoline and compressed natural gas engine. • Impact of high compression ratio on knock, combustion, performance, and emissions. • Compression ratio up to 16 was achieved with compressed natural gas operation. • Compressed natural gas showed 3% higher thermal efficiency over gasoline engine. • Compressed natural gas showed higher flame speed over gasoline at high CR operation. . Compressed natural gas (CNG) is considered one of the most promising alternative fuel used in the transport sector. CNG has high octane number, wide flammability limit, and high H/C ratio, brings in important aspects regarding its feasibility as a fuel for internal combustion engines. It is necessary to have a dedicated engine rather than retrofitted, bi-fuels or dual-fuel ones to exploit the full potential of CNG properties. Most vehicles use the CNG in bi-fuel mode, where the fuel system is modified to operate either on gasoline or CNG. Therefore, the engine's compression ratio (CR) needs to be determined according to gasoline fuel requirement. However, CNG can be used at a higher CR, and the performance of the engine can be improved. The aim of this study is to investigate the effect of CR on knock intensity, performance, combustion, and emissions of a spark ignition (SI) engine fuelled with gasoline and CNG. Gasoline and CNG engine's performance were compared at different CRs and stoichiometric air-fuel ratio. The results show that the knock intensity was high at CR 12 and 7 bar indicated mean effective pressure (IMEP) for the gasoline-fueled engine. The maximum CR was limited to 12 for the gasoline engine. However, Knock was not observed even at CR 16 for the CNG engine. The engine was successfully operated at CR 16 with CNG. The performance and emission of the engine were also compared and presented. Fuel consumption and thermal efficiency were improved for CNG at higher CR compared to the gasoline engine.