Comparative assessment of stoichiometric and lean combustion modes in boosted spark-ignition engine fueled with syngas

Abstract

Synthesis gas (syngas) is considered an intermediate step between conventional carbon-based fuels and future hydrogen-based fuels. Spark-ignition (SI) engines are suitable for converting the chemical energy of syngas for small-scale electrical power generation. However, syngas-fueled SI engines have lower power outputs and thermal efficiencies than SI engines fueled with conventional fuels such as gasoline and natural gas do. The objective of this study was to compare the stoichiometric and lean combustion modes in a single-cylinder SI engine to determine the optimal combustion mode for the development of a syngas engine generator with high power and high efficiency. A high gross indicate power was achieved in the stoichiometric combustion mode by increasing the intake pressure, owing to the increase in the volumetric efficiency and syngas fuel input. The gross indicated thermal efficiency (ITE) improved as the compression ratio was increased from 10:1 to 17.1:1, owing to the high peak heat release rate and short combustion duration. In the lean combustion mode, high gross ITEs were achieved by increasing the excess air ratio to 2.5, but the additional increase led to low combustion efficiencies. However, the gross indicated power decreased with an increase in the excess air ratio. The low gross indicated power was increased through intake boosting. Based on a parametric study, the optimal compression ratio for the stoichiometric combustion mode was selected to be 15:1. Pre-ignition occurred in the stoichiometric combustion mode at a compression ratio of 17.1:1 and an intake pressure of 0.16 MPa. Engine operation with a high compression ratio of 17.1:1 was possible in the lean combustion mode owing to the low combustion temperature. The gross ITE in the lean combustion mode was 18.4% higher than that in the stoichiometric combustion mode, mainly because of a significant reduction in the heat transfer loss. However, the gross indicated power in the lean combustion mode was 25.6% lower than that in the stoichiometric combustion mode.