Soot and NO _x Reduction by Spatially Separated Pilot Injection

To this day, Diesel engines with direct injection are the most efficient internal combustion engines for passenger cars. The major challenge of these engines with a conventional Diesel combustion process is the high level of particulate matter and nitrogen oxide emissions. Diesel engines in passenger cars normally use a pilot injection strategy for NVH reasons, which influences the engine-out soot emissions negatively. The Diesel fuel of the pilot injection is still burning when the main injection takes place, so, liquid components of the main injection interact with the flame of the pilot injection. The time for mixture formation decreases and the combustion takes place under locally very rich conditions which results in high levels of soot formation. For this reason new emission level restrictions cannot be reached without modern exhaust gas aftertreatment systems, which are quite expensive and can have an impact on the gas exchange. So, a reduction of both soot and nitrogen oxide emissions by in-cylinder methods without any penalty to efficiency is favorable. The present paper describes a new injection strategy for Diesel engines. The pilot injection is spatially separated into the combustion chamber. So, the main injection does not interact with a still burning pilot injection flame. First investigations of this new injection strategy have shown a reduction of soot with persisting nitrogen oxide emissions, so, the trade-off between both emission types is optimized. Additionally a significantly improved tolerance of EGR allows further reduction of nitrogen oxide emissions. Soot formation and oxidation process at the new injection strategy is analyzed by 2-color-measurements.