Effect of Exhaust Catalysts on Regulated and Unregulated Emissions from Low Temperature Diesel Combustion with High Rates of Cooled EGR

Unregulated emissions from a DI diesel engine with ultrahigh EGR low temperature combustion were analyzed using Fourier transform infrared (FTIR) spectroscopy and the reduction characteristics of both regulated and unregulated emissions by two exhaust catalysts were investigated. With ultra-high EGR suppressing the incylinder soot and NOx formation as well as with the exhaust catalysts removing the engine-out THC and CO emissions, clean diesel operation in terms of ultra-low regulated emissions (NOx, PM, THC, and CO) is established in an operating range up to 50% load. To realize smokeless low temperature combustion at higher loads, EGR has to be increased to a rate with the overall (average) excess air ratio less than the stoichiometric ratio. As a result, the thermal efficiency severely deteriorates, the THC and CO emissions drastically increase, and although approximately 50% reductions in THC and CO are obtained with the exhaust catalysts, these emissions are still high in the ultra-high EGR smokeless low temperature combustion regime. The unregulated emissions increase drastically with intake oxygen content below 14% due to EGR. These trends correlate well with the drastic increases in THC emissions. The exhaust catalysts are effective to reduce some unregulated toxic emissions including aldehydes and some unsaturated hydrocarbons. However, aromatics and methane generated from ultra-high EGR low temperature smokeless combustion are hardly reduced with the catalysts, particularly under overall rich conditions. Catalysts of higher oxidation ability can help to further reduce methane and aromatics to some degree but these emissions are still at high levels due to the low temperature combustion operation with ultra-high EGR.