Publications Details

Two-Stage ignition and unburned fuel emissions for heavy- duty diesel low-temperature combustion of neat n-Heptane

Musculus, Mark P.B.; Lachaux, Thierry

New low-temperature combustion (LTC) strategies can reduce both NOx and soot emissions from compression-ignition engines, but unburned hydrocarbon (UHC) emissions typically increase. Incylinder UHC evolution can be marked by formaldehyde, an intermediate species in the combustion process. Formaldehyde is formed during the first stage of ignition of diesel-like fuels, and it persists along with UHC in regions that do not achieve complete combustion. During the second stage of ignition, fuel and formaldehyde are largely consumed as OH radicals become prominent. The appearance of OH therefore indicates second-stage ignition and relatively complete combustion of fuel. Simultaneous planar laser-induced fluorescence (PLIF) images of formaldehyde and OH are acquired for two LTC conditions with different ignition delays, using neat n-heptane fuel. For both cases, formaldehyde PLIF is initially observed throughout the jet. Later, OH PLIF first appears downstream in the jet, where formaldehyde and UHC are locally consumed. For the shorter ignition-delay condition, OH PLIF quickly appears upstream locally where formaldehyde PLIF decreases, marking second-stage ignition and consumption of formaldehyde and UHC. For the longer ignition-delay condition, however, OH PLIF does not appear upstream, even late in combustion. Rather, formaldehyde PLIF, and therefore UHCs, persist near the injector late in combustion, indicating that regions near the injector do not achieve complete combustion, and may contribute to UHC emissions for the longer ignition delay condition.