Experimental Study of an Automotive Diesel Engine Running with Stoichiometric Combustion 2012-01-0699

Stoichiometric Diesel combustion (SDC, also called stoichiometric compression ignition) is a concept which tries to combine high efficiency of Diesel engine with the use of a relatively inexpensive three-way catalyst (TWC) for NOx post-treatment. A preliminary literature survey shows that relatively few studies have been performed in this regard. They show the major role of the injection system and the piston shape and confirm that a TWC effectively removes NOx, CO and HC on such an engine. The aim of this paper is to present an experimental study carried out on a modern turbocharged, common-rail automotive Diesel engine running under stoichiometric conditions. Most engine parameters are modified: EGR rate, inlet air temperature and pressure, injection strategy (single injection and split injection, start(s) of injection(s), rail pressure). A particular emphasis is put on intake strategies: the influence of boost pressure and EGR rate is studied; and two levels of swirl are tested. A sensitivity study to lambda is also performed near stoichiometric conditions, with a particular focus on TWC efficiency. Finally, various speed and loads covering a wide range of the engine map are tested, in order to check whether or not the same analysis can be performed in all operating conditions. For all tested conditions, the focus of analysis is on combustion (combustion efficiency, heat release), emissions (raw emission of PM, TWC efficiency), as well as engine efficiency. The best results are obtained with large amounts of cooled EGR allowed by a high boost pressure, an injection composed of a pilot injection targeting the squish and a main injection targeting the bowl, a relatively high injection pressure (but not too high to prevent wall wetting) and a moderate swirl level. In these conditions, the gross indicated thermal efficiency (GITE) under SDC is maximal, close to GITE obtained with traditional lean combustion, while PM emissions are still manageable with a DPF.