Effects of combustion phasing, combustion duration, and their cyclic variations on spark-ignition (SI) engine efficiency

A zero-dimensional two-zone model was employed to investigate the influence of the combustion process of the spark-ignition (SI) engine on its thermal efficiency. Attention was mainly paid to the effects of combustion phasing, combustion duration, and their cyclic variations. These combustion parameters were varied by changing spark timing and the hydrogen/natural gas blending ratio (hydrogen and natural gas mixtures were used as the fuel). The results show that there always exists an optimized combustion phasing for best engine thermal efficiency. Combustion phasing that deviates from this optimized value would decrease thermal efficiency. The fact that heat transfer loss increases with the advance of combustion phasing was thought to be the reason why best efficiency thermal efficiency does not occur under the condition when the combustion rate peaks at TDC. It is also found unexpectedly that although reduction in combustion duration can increase the degree of constant volume combustion, it does not have obviously positive effects on the final thermal efficiency. Finally, with regard to the cyclic variations, it is concluded that cyclic variations in the combustion process exert a heavy effect on thermal efficiency. The higher the variations, the larger the negative effect.