Particle Size Distribution Measurements from Early to Late Injection Timing Low Temperature Combustion in a Heavy Duty Diesel Engine

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 (NO_x) 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 NO_x 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.