End-of-Injection Over-Mixing and Unburned Hydrocarbon Emissions in Low-Temperature-Combustion Diesel Engines

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Abstract
Although low-temperature combustion (LTC) strategies for compression-ignition engines can achieve very low emissions of nitrogen oxides (NOx) and particulate matter (PM) at high efficiency, they typically have increased emissions of other pollutants, including unburned hydrocarbons (UHC). In the current study, the equivalence ratio of mixtures near the injector are quantified under non-combusting conditions by planar laser-Rayleigh scattering (PLRS) in a constant-volume combustion chamber and by planar laser-induced fluorescence (PLIF) of a fuel tracer (toluene) in a single-cylinder direct-injection heavy-duty diesel engine at typical LTC conditions. The optical diagnostic images show that the transient ramp-down at the end of fuel injection produces a low-momentum, fuel-lean mixture in the upstream region of the jet, which persists late in the cycle. At LTC conditions with long ignition delay, this mixture likely becomes too lean to achieve complete combustion, thereby contributing to UHC emissions. The potential contribution of this phenomenon to the total UHC emissions is comparable to the observed engine-out UHC emissions under long ignition-delay conditions. Therefore, stagnant, fuel-lean regions that form during the ignition dwell after the end of injection are likely a significant source of UHC emissions for EGR-diluted LTC diesel engines.

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