Abgas
Exhaust gas is very hot if retained or re-inducted from the previous combustion cycle or cool if recirculated through the intake as in conventional EGR systems. The exhaust has dual effects on HCCI combustion. It dilutes the fresh charge, delaying ignition and reducing the chemical energy and engine output. Hot combustion products conversely increase gas temperature in the cylinder and advance ignition. Control of combustion timing HCCI engines using EGR has been shown experimentally.