Exhaust Gas Recirculation Effects on Diesel Engine Soot Formation, Destruction,
and Emissions. EDWIN HUESTIS (University
of California, Davis, Davis, CA, 95616) MARK P. B. MUSCULUS (Sandia National Laboratory (California), Albuquerque, NM, 87185)
Diesel engine
manufacturers desire insight into the internal processes that
affect emissions in order to design new engines that meet upcoming
US pollutant emissions regulations for particulate matter (soot)
and nitrogen oxides (NOx). Exhaust gas recirculation (EGR) is
one available tool to reduce pollutant emissions of NOx, and
EGR has also been shown to reduce the formation of soot in fundamental
combustion studies. In real engines, however, EGR can increase
soot in the engine exhaust by affecting soot destruction processes
after combustion. In this study, both exhaust-gas soot emissions
and fundamental soot formation/destruction processes inside a
diesel engine are measured to understand how these processes
affect the ultimate soot emissions. The time-evolution of soot
formation and destruction inside an optically accessible diesel
engine is measured by analyzing the radiative emission spectrum
of the combustion-heated soot. The soot radiative emission intensity
is measured within two separate spectral bands, and along with
high-speed luminosity movies, the soot temperature and concentration
inside the engine are determined using the two-color soot thermometry
technique. Gases are also sampled from the exhaust stream and
drawn through a filter to collect the soot particles for exhaust-gas
measurements. The measurements show that soot formation inside
the engine decreased as EGR was added, but soot destruction processes
decreased more rapidly, so that exhaust soot emissions increased
as EGR was added. Only at very high EGR levels did the exhaust
soot emissions decrease, as the soot formation processes became
very slow. Finally, soot temperature measurements show that the
reaction rates for soot formation and destruction depend on EGR,
affecting the soot formation/destruction balance. By combining
measurements inside the engine with exhaust-gas measurements,
the effects of EGR on the soot formation/destruction balance
were quantified in a single experimental facility. The data from
this study shows that EGR decreases soot destruction processes
more than soot formation processes, so that the balance is tipped
toward higher exhaust soot. At very high EGR, however, soot formation
processes are so slow that exhaust soot decreases, even with
reduced rates of soot production. This study also identifies
the tipping points where this balance shifts from increasing
exhaust soot to decreasing exhaust soot.
