In-cylinder unburned hydrocarbon visualization during low-temperature compression-ignition engine combustion using formaldehyde PLIF

Formaldehyde (H2CO) is visualized by planar laser-induced fluorescence (PLIF) in a heavy-duty direct-injection diesel engine to better understand the sources of unburned hydrocarbon (UHC) emissions. H2CO is used as a tracer for UHC based on chemical kinetic simulations, which show that after the first-stage of ignition, the evolution of H2CO is very similar to that of UHC in the bulk gases. Modern low-temperature, low-nitrogen oxides (NOx) combustion conditions are achieved by diluting the intake stream with nitrogen to 12.7%, oxygen by volume, simulating exhaust gas recirculation. A range of ignition delays (IDs) is produced by adjusting the intake temperature and/or the fuel injection timing. Spectral measurements of the laser-incduced emission after 355-nm excitation show that broadband emission from sources other than H2CO can be significant, so the PLIF images must be interpreted with care. A frequency-domain covariance analysis is offered as a way to quantify the contribution of H2CO fluorescence in the images. Longer ID conditions are found to have both higher-UHC emissions and stronger H2CO fluorescence persisting late in the cycle, especially near the injector. Based on kinetic model predictions of longer H2CO lifetimes in leaner regions, it is concluded that for long ID conditions, mixtures near the injector after the end of injection are too lean to achieve complete combustion, thus contributing to UHC emissions. Published by Elsevier Inc. on behalf of The Combustion Institute.