Investigation of Transient Emissions and Mixed Mode Combustion for a Light Duty Diesel Engine

The use of low temperature combustion (LTC) modes has demonstrated abilities to lower diesel engine emissions while maintaining good fuel consumption. LTC is assumed to be a viable solution to assist in meeting stringent upcoming diesel engine emissions targets, particularly nitric oxides (NOx) and particulate matter (PM). However, LTC is currently limited to low engine loads and is not a feasible solution at higher loads on production engines. A mixed mode combustion strategy must be implemented to take advantage of the benefits offered from LTC at the low loads and speeds while switching to a conventional diesel combustion strategy at higher loads and speeds and thus allowing full range use of the engine under realistic driving conditions.

Experiments were performed to characterize engine out emissions during transient engine operating conditions involving LTC combustion strategies. Mixed mode transitions between conventional diesel combustion and LTC, or more specifically early premixed charge compression ignition (PCCI) combustion were studied along with non-mode switching load transients within early PCCI combustion mode. The studies were carried out on a 4 cylinder 1.9L EURO 4 emissions certified production diesel engine equipped with common rail direct injection (CRDI), a variable geometry turbocharger and cooled exhaust gas recirculation (EGR).

Results of this study show the effect of load transients on the engine out emissions of NOx and unburned hydrocarbons (UHC) and the effects of fuel command change rate in transients while operating within early PCCI combustion. Emissions from combustion mode switching while operating in a mixed mode strategy are shown to depend on the instantaneous air charge preconditioning at the mode switch location. The resulting NOx and UHC are characterized in terms of critical combustion time constants resulting from the fuel and air delivery systems. It is shown that the charging system is the time critical system that limits the rate of the transitions and leads to higher rates of emissions