Homogeneous Charge Compression Ignition (HCCI) is a concept that has for years held potential for increasing gasoline engine efficiency while lowering emissions. HCCI allows efficient part load operation by inducing fast combustion despite high levels of exhaust gas dilution and lean air fuel mixtures, which generally result in slow combustion speeds in spark ignition (SI) engines. By trapping large amounts of exhaust gas, heat from the previous engine cycle is reused in order to help the air-fuel mixture to autoignite in multiple locations throughout the cylinder. Proper control of the mixture conditions is vital for controlling ignition timing and avoiding knock and misfires. HCCI is a viable operating mode only at low to moderate engine loads and speeds. Thus, for practical implementation, an engine must be able to switch in and out of HCCI operation seamlessly. This work studies HCCI operation and mode transition using a production-style valvetrain with 2-step cam profile switching and electric cam phasers mounted on a 4-cylinder engine which has been converted to run as a 1-cylinder engine. Combining this valvetrain with direct injection strategies allows steady state HCCI operation, which displays vastly reduced pumping losses as well as increased load compared to an SI condition run with the same fueling rate. A control strategy has been designed to reliably transition the engine from SI operation into this steady HCCI operation quickly in eight to ten engine cycles without a significant fluctuation in engine output. Cycle-by-cycle analysis of this mode transition shows a distinct hybrid combustion mode which combines traditional spark-induced flame propagation with the fast autoignition present in HCCI. The initial cycles of the transition show a large percentage of SI flame propagation which gives way to a small amount of fast autoignition at the tail end of combustion; on successive cycles during the transition, the onset of autoignition advances, decreasing the percentage of SI flame propagation until full autoignition is present at the end of the transition. The HCCI autoignition process has also been captured via high-speed digital imaging in a single-cylinder optical version of the metal engine. This engine, which uses the same valvetrain as the metal engine, was equipped with a sapphire window in the Bowditch-style piston which allows visible light images of the combustion process to be recorded at 10000 frames per second. The images clearly show multiple ignition points which expand rapidly and result in very fast heat release.
Read
