In order to control the heat-release rate at homogeneous charge compression ignition (HCCI) operation, the feasibility of spark-assisted compression ignition (SI-CI) combustion was demonstrated with lean mixture of primary reference fuel (PRF) and hydrogen. In this study, the criteria at octane number and hydrogen ratio were investigated using a rapid compression/expansion machine for different equivalence ratios, compression ratios and spark timings. Experimental results of in-cylinder pressure and heat-release rate indicated possible advantageous characteristics of SI-CI operation with lower pressure-rise rate and higher engine output. Also, flame images were visualized to observe various combustion forms including HCCI, SI-CI and SI combustions.

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