Characterizing the Influence of EGR and Fuel Pressure on the Emissions in Low Temperature Diesel Combustion
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In the wake of global focus shifting towards the health and conservation of the planet, greater importance is placed upon the hazardous emissions of our fossil fuels, as well as their finite supply. These two areas remain intense topics of research in order to reduce green house gas emissions and increase the fuel efficiency of vehicles, a sector which is a major contributor to society's global CO 2 emissions and consumer of fossil-fuel resources. A particular solution to this problem is the diesel engine, with its inherently fuel-lean combustion, which gives rise to low CO 2 production and higher efficiencies than other potential powertrain solutions. Diesel engines, however, typically exhibit higher nitrogen oxides (NO x) and soot engine-out emissions than their gasoline counterparts. NO x is an ingredient to ground level ozone production and smoke is a possible carcinogen, both of which are facing stricter emissions regulations. The typical diesel engine exhibits a NO x - soot tradeoff where a reduction in NO x results in an increase in soot, and vice versa. There exists the possibility to simultaneously reduce both emissions with the application of low temperature diesel combustion, or LTC. LTC allows for low flame temperatures within the combustion, in order to prohibit both soot and NO x formation, while at the same time allowing for premixed combustion to eliminate fuel rich combustion zones which further reduces soot formation. While exhibiting great characteristics in simultaneous reductions in nitrogen oxides and soot, LTC faces challenges with carbon monoxide (CO) emissions, hydrocarbon (HC) emissions, penalties in fuel efficiency, and difficulty in attainment during high loads. The following study examines the characteristics of LTC which contribute to the differences in emissions and efficiency compared to typical conventional diesel combustion. More specifically, key engine parameters which are used to enable LTC, such as EGR and fuel pressure are swept through a full range to determine their effects on each combustion regime. Analysis will focus on comparing both combustion regimes to determine how EGR and fuel pressure relate to lowering NO x and smoke concentrations, and how these relate to penalties in CO and HC concentrations. This study identifies that with the application of LTC on a conventional combustion diesel engine, a 99% reduction in NO emissions and a 15% simultaneous reduction in smoke can be realized. The typical soot - NO tradeoff is reduced with application of EGR, relative to conventional combustion operation. Further, increasing fuel pressure shows typical increases in NO and decreases in smoke for both LTC and conventional combustion, thus suggesting that LTC may not necessarily defeat the soot-NO tradeoff, but shift its behavior to lower NO / soot concentration regimes. Copyright 2011 SAE International.
