Emissions of Unburned Hydrocarbons (UHC) from diesel engines are a particular concern during the starting process, when after-treatment devices are typically below optimal operating temperatures. Drivability in the subsequent warm-up phase is also impaired by large cyclic fluctuations in mean effective pressure (MEP). This paper discusses in-cylinder wall temperature influence on unburned hydrocarbon emissions and combustion stability during the starting and warm-up process in an optical engine. A laser-induced phosphorescence technique is used for quantitative measurements of in-cylinder wall temperatures just prior to start of injection (SOI), which are correlated to engine out UHC emission mole fractions and combustion phasing during starting sequences over a range of charge densities, at a fixed fueling rate.
Squish zone cylinder wall temperature shows significant influence on engine out UHC emissions during the warm-up process. Higher surface temperatures correlate with lower levels of engine-out UHC. Moreover, the UHC emissions are more sensitive to wall temperatures at higher charge densities. Engine-out UHC is less sensitive to bowl surface temperatures, which are observed to quickly attain values nearly 100°C greater than those measured in the squish zone. Increasing charge density reduces the time required to attain steady temperatures, but has little effect on the final surface steady state temperature. Misfire and incomplete combustion occurs more often at low bowl temperature. However, high bowl temperatures are observed to assist residual fuel evaporation, advance combustion phasing and reduce the probability of misfires.
