The Potential of Fuel Metering Control for Optimising Unburned Hydrocarbon Emissions in Diesel Low Temperature Combustion

Low temperature combustion (LTC) in diesel engines offers attractive benefits through simultaneous reduction of nitrogen oxides and soot. However, it is known that the in-cylinder conditions typical of LTC operation tend to produce high emissions of unburned hydrocarbons (UHC) and carbon monoxide (CO), reducing combustion efficiency. The present study develops from the hypothesis that this characteristic poor combustion efficiency is due to in-cylinder mixture preparation strategies that are non-optimally matched to the requirements of the LTC combustion mode. In this work, the effects of three key fuel path parameters - injection fuel quantity ratio, dwell and injection timing - on CO and HC emissions were examined using a Central Composite Design (CCD) Design of Experiments (DOE) method. The experiments were performed on a single-cylinder diesel research engine operating in a high-EGR mixing-controlled LTC mode (EGR ~ 62%, intake O₂ = 8.5%) with a split fuel injection for all conditions.

The experiments identified the potential of fuel metering control for optimizing HC emissions in LTC by showing the effects of fuel control parameters on fuel mixing quality and emission formation mechanisms. The experimental results at this high-EGR operating condition were shown to be highly sensitive to the intake oxygen level. Accordingly, the use of DOE methods was found to be essential to this study. The detailed statistical analysis enabled by the experimental design was able to model and correct for the substantial effects of normal variability in the input oxygen mass fraction noted under these high EGR conditions; thus, permitting a reliable comparison of results.