The control of NOx emissions is the greatest technical challenge in meeting future emission regulations for diesel engines. In this work, a modal analysis was performed for developing an engine control strategy to take advantage of fuel properties to minimize engine-out NOx emissions. This work focused on the use of EGR to reduce NOx while counteracting anticipated PM increases by using oxygenated fuels.
A DaimlerChrysler OM611 CIDI engine for light-duty vehicles was controlled with a SwRI Rapid Prototyping Electronic Control System. Engine mapping consisted of sweeping parameters of greatest NOx impact, starting with OEM injection timing (including pilot injection) and EGR. The engine control strategy consisted of increased EGR and simultaneous modulation of both main and pilot injection timing to minimize NOx and PM emission indexes with constraints based on the impact of the modulation on BSFC, Smoke, Boost and BSHC. The engine combustion instability region was determined, and the operating parameters were controlled for stable operation.
The engine was operated steady state at four different speed (rpm)/load (kPa, BMEP) conditions as follows: 1500/100, 1500/262, 2000/200, and 2300/420. The test fuel matrix included: an ultra-low sulfur diesel fuel (3 ppm) used in the DOE DECSE Program, an ultra-low sulfur California-type diesel fuel (11 ppm), an ultra-low sulfur fuel (15 ppm) manufactured in a current refinery unit targeted to meet 2007 sulfur level, a blend of ultra-low sulfur California-type diesel fuel with tripropylene glycol monomethyl ether at 7 weight percent oxygen, a blend of ultra-low sulfur with tripropylene glycol monomethyl ether at 7 weight percent oxygen, a blend of ultra-low sulfur with dibutyl maleate at 7 weight percent oxygen, and a neat Fischer-Tropsch fuel. Each operating mode and fuel combination was run in triplicate. Total PM, SOF, NOx, CO, THC, CO2, and O2 were measured.
It was possible to decrease mode-weighted NOx emissions compared to the OEM calibration - generally with corresponding increases in other emissions. These emissions increases could be partially overcome with the use of oxygenated fuels. However, increases in fuel consumption and losses in engine boost proved more significant in limiting practical NOx control than the benefits from fuel compositional changes in the current engine configuration. In subsequent work, emission control devices will be used to control the smoke/particulate and HC emissions, and exhaust measurements will emphasize toxicologically relevant species.