Parametric Tests Demonstrate SI-CI Combustion Milder than PCI Combustion and More Efficient: To control the start of combustion and heat release rate in order to expand PCI combustion operating range, researchers at Kyoto University and Nissan continue to investigate the characteristics of SI-CI combustion by observing conditions in tests that would produce SI-CI combustion as intended to produce smooth and efficient combustion. A rapid compression expansion machine (RCEM) was used to produce the SI-CI combustion for only 1 cycle. The RCEM was operated on primary reference fuel. Hydrogen was supplemented to secure reliable ignition of lean mixture. The equivalence ratio of PRF, RON of PRF, hydrogen supplement rate, and compression ratio were varied as the test parameters. This chapter reports experimental results of the parametric tests systematically conducted on the RCEM.