Valve Profile Adaptation, Stratification, Boosting and 2-Stroke Strategies for Raising Loads of Gasoline HCCI Engines

Artikel i vetenskaplig tidskrift, 2012

The development of high efficiency powertrains is a key objective for car manufacturers. One approach for improving the efficiency of gasoline engines is based on homogeneous charge compression ignition, HCCI, which provides higher efficiency than conventional strategies. However, HCCI is only currently viable at relatively low loads, primarily because at high loads it involves rapid combustion that generates pressure oscillations in the cylinder (ringing), and partly because it gives rise to relatively high NOX emissions. This paper describes studies aimed at increasing the viability of HCCI combustion at higher loads by using fully flexible valve trains, direct injection with charge stratification (SCCI), and intake air boosting. These approaches were complemented by using EGR to control NOX emissions by stoichiometric operation, which enables the use of a three-way catalyst. Experiments were carried out using a single-cylinder engine of passenger car size running on gasoline and controlled with negative valve overlap. By adapting the valve profiles (lift, duration and phasing) for high loads, a fuel saving of 3% at constant load or a load increase of 6% could be achieved for lean HCCI compared to those obtained using camshafts that were not adapted for high load operation. Further, using intake pressures up to 180 kPa provided almost linear increases in load for lean HCCI, stoichiometric HCCI and stoichiometric SCCI. However, lean SCCI did not profit from boosting because the charge became too lean and stratification lost its effect as a ringing inhibitor. At intake pressures exceeding 140 kPa, stoichiometric HCCI operation becomes redundant since NOX ceases to be a limiting factor. Additionally, promising results were obtained in initial tests of two-stroke operation, which yielded higher maximum loads, lower fuel consumption and lower NOX emissions than the other strategies.