15 Combustion Characteristics of an Improved Design of a Stratified Charge Spark Ignition Engine

The characteristics of the combustion process in an improved design of a novel spark ignition engine studied by means of Computational Fluid Dynamics are presented. The engine is designed to work at low average combustion temperatures to achieve very low NOx emissions.

The engine is a two-stroke, two piston in-line engine. The main combustion occurs in four combustion pre-chambers that have an annular shape with a nozzle on the side facing the cylinder. Fuel is directly injected into the pre-chambers by using high-pressure fuel injectors. A progressive burning process is expected to keep the flame inside the pre-chambers while the fast ejection of combustion products should produce effective mixing with the cold air in the cylinder. This fast dilution should guarantee a temperature drop of the combustion products thus reducing the formation of NOx via a thermal path.

The improved design of the combustion chamber showed a positive effect in keeping the flame confined inside the pre-chambers. Changing the ignition timing by calibrating the spark advance also contributed to the confinement of the flame. With respect to the preliminary design, higher turbulent burning velocity and shorter combustion duration were found. Due to the quick and effective dilution of the burned gases with the colder air, the average temperature was found reasonably low for yielding low NOx production.