Abstract
Homogeneous-charge compression-ignition (HCCI) combustion has been expected to exhibit higher thermal efficiency and lower NOx emission. HCCI operations are, however, restricted at a narrow range of engine output due to knock at higher loads and misfire at lower loads. In recent, the spark assisted compression-ignition (SI-CI) combustion was introduced to confirm the ignition of lean mixture and to control the heat-release rate, in which compression-ignition (CI) combustion of the end gas may moderately follow spark-ignition (SI) combustion. In this study, in order to clarify the possibility of SI-CI combustion, experiments were made by using the rapid compression/expansion machine (RCEM) which provides advantages compared to conventional engine for its completely homogeneous-charge preparation and its simplicity of the setting of engine condition. Histories of incylinder pressure and heat-release rate were measured at different equivalence ratios and compression ratios to investigate the combustion processes of the lean mixture of primary reference fuels (PRFs) with hydrogen addition. From the results of systematic tests for various octane numbers of PRF (RON) and hydrogen ratios rH, the criteria of SI-CI combustion was exhibited at different compression ratios ε and equivalence ratios φ. For ε =13 and φ= 0.45, at lower octane number (RON) fuel, HCCI combustion was observed at lower hydrogen ratios and SI-CI combustion was realized with higher hydrogen ratios. In the case of further hydrogen addition, combustion was completed with a simple SI form. For higher ε and φ, the operation range for SI-CI combustion extended to higher RON and rH. Also, based on the pressure-rise rate and engine output, the feasibilities of SI-CI combustion were demonstrated.
Keywords: HCCI, Spark ignition, Lean mixture, Octane number, Primary reference fuels, Hydrogen