A process for enhancement of combustion control in rotary and reciprocating IC engines for improving fuel efficiency by enabling leaner combustion at higher compression ratios. Embodiments supporting this process employ a fluid of higher heat of vaporization and higher volatility but lower ignitability than the fuel to increase the compression ratio required for self ignition. These have secondary chambers in a cylinder periphery for radical ignition (“RI”) species generation in an earlier cycle for use in a later cycle. These chambers communicate with a main chamber via conduits. Measures regulate the RI species generated and provided to the main chamber. These species then alter the dominant chain-initiation reactions of the main combustion ignition mechanism by lowering the heat and the fuel ratios required for combustion. This improves combustion in radical ignition engines and radical augmented spark and compression ignition engines.