The use of early and late injection diesel Low Temperature
Combustion (LTC) strategies in the low to mid load operating range
are becoming increasingly popular options for production diesel
engines to reduce oxides of nitrogen (NOx) and
particulate matter (PM) emissions. Although opacity-based filter
smoke number (FSN) PM measurements in these operating conditions
have been reduced to near zero for many instruments (which are
standard and very useful in most engine combustion research
laboratories), significant changes can still be seen in the
particle size and number measurements (such as a 2.5 - 4.5 fold
variation in total particle number concentration, depending on
engine operating condition).
The current work presents particle size distribution
measurements from early to late injection timing LTC, varying the
start of injection (SOI) by three crank angle degrees (CAD) per
data point, for two exhaust gas recirculation (EGR) rates, 45% and
50%. For all test points, the end of injection occurred before the
initiation of the high temperature heat release. With both EGR
rates, an increase in the PM FSN measurement could be noticed while
retarding the injection timing from early-LTC, followed by a sharp
decrease in PM upon further retard. Throughout these changes in
injection timing test points, there was a strong correlation
between PM FSN measurement and accumulation mode particles (≺50
nm). Analysis was also made of the total particle number
concentration for each data point.
The earliest injection timing measured was limited by high
in-cylinder pressure rise rate (≻22 bar/CAD) for 45% EGR and high
hydrocarbon (HC) emissions (≻10.5 g/kg_fuel) for 50% EGR, while the
latest injection timing for both EGR rates was limited by decreases
in indicated mean effective pressure (IMEP) greater than 10%. For
early injection timings, a SOI was found with low NOx
and very low PM emissions (≺0.05 FSN) before large increases in HC
and carbon monoxide (CO) could be seen. The latest injection timing
case (also with ≺0.05 FSN), exhibited large number increases in
particles smaller than 100 nm by up to an order of magnitude.
Further emissions and combustion analyses were performed on these
early and late injection timings.