Performance and emission characteristics of a hydrogen-enriched compressed-natural-gas direct-injection spark ignition engine diluted with exhaust gas recirculation

Compressed natural gas (CNG) has been widely used in city buses or taxis because of its relatively low price and great abundance. Most of the CNG engines externally mix the natural gas with the intake air and utilize a lean-burn strategy to achieve low nitrogen oxide (NOx) emissions and high fuel economy. However, the well-examined external mixing of gas fuel with the intake air will result in a volumetric efficiency loss at high engine loads and may cause backfire when the natural gas is enriched with hydrogen. Also, the lean-burn technique may not sufficiently satisfy increasingly restricted future legislation. In this paper, the use of the stoichiometric equivalence ratio with exhaust gas recirculation (EGR) was investigated experimentally on a direct-injection spark ignition engine fuelled with hydrogen-natural-gas blends. Attention is focused on the advantages of combining EGR with hydrogen enrichment. The results show that hydrogen enrichment increases the brake mean effective pressure, which dramatically deteriorates under high-level EGR. Optimal efficiencies are achieved with an EGR rate of 5-10 per cent; the NOx emissions can be decreased by about 80 per cent with 25 per cent EGR dilution, but the combustion variation increases simultaneously. With hydrogen enrichment, the combustion variation can be controlled under 5 per cent, while the NOx, hydrocarbon, and carbon monoxide emissions are kept at a low level. Worse EGR tolerance was exhibited for the late-injection-timing case.