Investigation of Mixing and Temperature Effects on HC/CO Emissions for Highly Dilute Low Temperature Combustion in a Light Duty Diesel Engine 2007-01-0193

There is a significant global effort to study low temperature combustion (LTC) as a tool to achieve stringent emission standards with future light duty diesel engines. LTC utilizes high levels of dilution (i.e., EGR > 60% with <10%O₂ in the intake charge) to reduce overall combustion temperatures and to lengthen ignition delay, This increased ignition delay provides time for fuel evaporation and reduces in-homogeneities in the reactant mixture, thus reducing NO_x formation from local temperature spikes and soot formation from locally rich mixtures. However, as dilution is increased to the limits, HC and CO can significantly increase.

Recent research suggests that CO emissions during LTC result from the incomplete combustion of under-mixed fuel and charge gas occurring after the premixed burn period [1, 2]¹. The objective of the present work was to increase understanding of the HC/CO emission mechanisms in LTC at part-load. To do this, fluid mechanics and chemical kinetics were decoupled by selectively varying in-cylinder mixing and charge temperature to influence not only the formation of CO and HC but also their oxidation during the latter stages of combustion and expansion.

Controlled experiments in a single-cylinder light duty diesel engine were combined with three computational tools, namely heat release analysis of measured cylinder pressure data, analysis of spray trajectory and in-cylinder thermodynamics with a phenomenological model, and 3-D in-cylinder CFD computations with a version of the KIVA-3V Chemkin code recently tested at LTC conditions. The effect of such variables as rail pressure, swirl number and inlet temperature were explored using statistical experimental designs to first correlate with the kinetic behavior of HC and CO and second to help identify and understand the mechanisms of HC/CO formation and oxidation. The results were analyzed for major effects using heat release computations. Both the phenomenological and CFD models were used to understand the key phenomena driving the increased HC/CO.

A unique behavior was found in the HC/CO emissions while performing injection sweeps in these highly dilute environments. This behavior, termed a CO “sweet spot”, originally seen by Kook et al. [1, 2], was again observed, but in this case at higher loads and speeds. Use of a phenomenological engine spray model showed that increasing amounts of liquid fuel that miss the bowl (i.e., spray trajectory too wide to enter bowl) for timings advanced from the sweet spot coincide with increasing CO. This model also showed that less than 100% of the fuel was vaporized prior to start of combustion (SOC) at retarded timings, where again CO increased from the observed minimum “sweet spot”. With the use of detailed CFD results, this behavior was found to be the result of mixing-related phenomena. While reduced injection pressure increased the engine out HC/CO it also advanced the location of the CO minimum by reducing the amount of fuel that misses the bowl at a given timing. Varying swirl was found to dramatically change the behavior of the “sweet spot” due to changes in available O₂ within the cylinder. Intake temperature was found to have only a small affect on the emissions.