On the Cyclic Variability and Sources of Unburned Hydrocarbon Emissions in Low Temperature Diesel Combustion Systems

The cycle-to-cycle variability and potential sources of unburned hydrocarbon (UHC) emissions are examined in a single-cylinder, light-duty diesel test engine operating in low-temperature combustion regimes. A fast flame ionization detector (FID) was employed to examine both cycle-to-cycle variations in UHC emissions and intra-cycle emissions behavior. A standard suite of emissions measurements, including CO, CO₂, NO_x, and soot, was also obtained. Measurements were made spanning a broad range of intake O₂ concentrations-to examine the UHC behavior of dilution-controlled combustion regimes-and spanning a broad range of injection timings-to clarify the behavior of increased UHC emissions in late-injection combustion regimes. Both low- and moderate-loads were investigated.

The cycle-resolved UHC data showed that the coefficient of variation of single-cycle UHC did not increase with increases in UHC emissions as either O₂ concentration or injection timing was varied. Increases in UHC emissions thus result from deterioration in the performance of the mean (or typical) cycle, not from increasing single-cycle variation. Additionally, the crank-angle-resolved UHC measurements were coupled with a 1-D engine model to examine the variation of exhaust mass flow rate of UHC within each cycle. The results showed that most of the UHC mass exited the cylinder during the latter part of the exhaust process, and that UHCs originating from cylinder wall and piston top quench layers are likely of greater importance than UHCs emitted from quench layers along the head.