The homogeneous charge compression ignition engine, HCCI, has the potential to combine the best of the spark ignition and compression ignition engines with high octane number fuel. The engine operates with high compression ratio and lean mixtures giving CI engine equivalent fuel consumption or better. Due to premixed charge without rich or stoichiometric zones the production of soot and NOx can be avoided but HC and CO emission are high due to low temperature in combustion zones. HCCI combustion is achieved by controlling the temperature, pressure and composition of the air/fuel mixture so that it auto ignites near top dead centre (TDC) as it is compressed by the piston. Expanding the controlled operation of an HCCI engine over a wide range of speeds and loads is probably the most difficult hurdle facing HCCI engines. Out of this follows that timing/phasing of the combustion is one of the main difficulties with HCCI combustion concepts. This paper deals with control of HCCI by aid of variable valve timing with specialization in usage of a non-linear quasi-static compensation to lower HC and CO emission in CNG HCCI engine.

Keyworks: Valve tining, Non-linear Quasi-static compensation system.