A single cylinder, naturally aspirated, four-stroke and camless (Otto) engine was operated in homogeneous charge compression ignition (HCCI) mode with commercial gasoline. The valve timing could be adjusted during engine operation, which made it possible to optimize the HCCI engine operation for different speed and load points in the part-load regime of a 5-cylinder 2.4 liter engine.
Several tests were made with differing combinations of speed and load conditions, while varying the valve timing and the inlet manifold air pressure. Starting with conventional SI combustion, the negative valve overlap was increased until HCCI combustion was obtained. Then the influences of the equivalence ratio and the exhaust valve opening were investigated. With the engine operating on HCCI combustion, unthrottled and without preheating, the exhaust valve opening, the exhaust valve closing and the intake valve closing were optimized next. Finally, with three valve timing events, an HCCI operational window was identified.
Using only three valve timing events, an engine operational window for HCCI combustion was obtained with: a maximum load of about 3.5 bar, a minimum load of about 0.1 bar, a maximum speed about 3000 rpm, and a minimum speed about 1200 rpm. The window for HCCI combustion is restricted by zones of knock (on the high load part) and by cycle to cycle variations in cylinder pressure (on the low load part) at high and low engine speeds. Between HCCI combustion and conventional SI combustion, there is a transient interval during which a spark triggers the auto ignition. A clear benefit in reduced CO emissions, NOx emissions (close to zero), and fuel consumption was observed during HCCI engine operation, compared with conventional SI combustion whereas the level of hydrocarbon emissions was approximately the same. The EVC has the greatest influence on the HCCI auto-ignition delay for the engine operating parameters tested. The influence of EVO is controlled by the gas dynamics and IVC has only a minor effect on engine behavior.