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 greenhouse gas emissions and increase the fuel efficiency
of vehicles, a sector which is a major contributor to society's
global CO₂ 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 CO₂
production and higher efficiencies than other potential powertrain
solutions.
Diesel engines, however, typically exhibit higher nitrogen
oxides (NOx) and soot engine-out emissions than their
gasoline counterparts. NOx 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 NOx - soot tradeoff
where a reduction in NOx 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
NOx 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 NOx 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.