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A system for stratifying recycled exhaust gas (EGR) to substantially increase dilution tolerance has been applied to a multi-cylinder port injected four-valve gasoline engine. This system, dubbed Combustion Control through Vortex Stratification (CCVS), has shown greatly improved fuel consumption at stoichiometric conditions whilst retaining ULEV compatible engine-out NOx and HC emission levels. A production feasible variable air motion system has also been assessed which enables stratification at part load with no loss of performance or refinement at full load.

This paper reports an investigation of the association between flame kernel movement and cyclic variability and assesses the relative importance of this phenomenon, with all other parameters that show a cyclic variability held constant. The flame is assumed to be subjected to a “random walk” by the fluctuating velocity component of the flow field as long as it is of the order of or smaller than the integral scale. However, the mean velocity also imposes prefered convection directions on the flame kernel motion. Two-point LDA (Laser Doppler Anemometry) measurements of mean velocity, turbulence intensity and integral length scale are used as input data to the simulations. A quasi-dimensional computer code with a moving flame center position is used to simulate the influence of these two components on the performance of an S I engine with a tumble-based combustion system.

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.