Effect of varying compression ratio on combustion, performance, and emissions of a hydrogen enriched compressed natural gas fuelled engine

Compressed natural gas (CNG) and hydrogen are potential alternate fuels for internal combustion (IC) engines, which are capable of sustaining large part of global transportation system in future. However these fuels suffer from some combustion related issues. A blend of CNG and hydrogen cancels their negative individual properties, mutually. As a consequence, hydrogen enriched compressed natural gas (HCNG) is emerging as an acceptable alternative fuel for spark ignition (SI) engines. HCNG resists knocking hence it delivers superior engine performance. Therefore it is usable at higher compression ratios (CRs). Effect of CR on HCNG fuelled engine's performance, combustion and emission characteristics is experimentally evaluated in this study. Experiments were performed at three CRs (10, 11 and 12) under identical conditions, in order to assess their effect on two different HCNG compositions (20HCNG and 30 HCNG) for comparison. Brake specific energy consumption (BSEC) was lower at higher CRs which decreased with increasing engine load/ brake mean effective pressure (BMEP). Brake thermal efficiency (BTE) was higher at higher CRs for both HCNG mixtures. 30HCNG showed relatively higher BTE and lower BSEC compared to 20HCNG. Higher peak cylinder pressures were obtained for higher CRs. Combustion parameters such as rate of pressure rise (RoPR), heat release rate (HRR) and cumulative heat release (CHR) improved at higher CRs for both HCNG mixtures however 30HCNG performed slightly better amongst the two HCNG compositions. Regulated emissions such as HC, CO and NOx were relatively higher at higher CRs, but CO2 emission was relatively lower.