Syngas composition for improving thermal efficiency in boosted homogeneous charge compression ignition engines

Abstract

Various syngas compositions were experimentally investigated in a homogeneous charge compression ignition (HCCI) engine to suggest an optimal composition that can maximize thermal efficiency. Intake boosting was also implemented to extend the high-load operating range over a wide range of equivalence ratios. The engine speed was selected as 1,800 rpm and the intake pressure was increased from 0.19 MPa to 0.26 MPa. The thermal efficiency improved by increasing the intake pressure, and incrementally increasing the equivalence ratio until the maximum pressure rise rate (MPRR) limit was reached. The increase in intake pressure also extended the engine load by increasing the syngas fuel input. The syngas composition was systematically varied to clarify the effects of H2 and CO contents on the combustion and emissions of the HCCI engine. H2 was increased from 30% to 50%, whereas the CO amount was reduced from 25% to 8% for maintaining a constant lower heating value (LHV). Although the intake temperature decreased with increasing the H2 content, the thermal efficiency improved. The increase in H2 content was more effective in enhancing combustion performance than the intake temperature reduction. The engine load also slightly increased owing to the improvement in the thermal efficiency. Furthermore, the CO emission also decreased. The NOX emissions remained low despite the enhanced combustion performance. The CO-absent fuel compositions were also evaluated to improve the thermal efficiency. The highest gross indicated thermal efficiency of 43.4% was recorded with a CO-absent fuel composition (50% H2 and 50% CO2). However, the achievable engine load reduced because the LHV was lower than that of CO-present syngas. In the case of CO-absent fuel compositions, it was difficult to operate the HCCI engine with an H2 content higher than 50% owing to its excessive MPRR. Therefore, a syngas composition with a high H2 content and a low CO content was favorable for high-load extension in the HCCI engine.