On the origin of Unburned Hydrocarbon Emissions in a Wall Guided, Low NOx Diesel Combustion System

The formation mechanisms of unburned hydrocarbons (HC) in low NOx, homogeneous type Diesel combustion have been investigated in both standard and optical access single cylinder engines operating under low load (2 and 4 bar IMEP) conditions. In the standard (i.e. non-optical) engine, parameters such as injection timing, intake temperature and global equivalence ratio were varied in order to analyse the role of bulk quenching on HC emissions formation. Laser-induced fluorescence (LIF) imaging of in-cylinder unburned HC within the bulk gases was performed on the optical-access engine. Furthermore, studies were performed in order to ascertain whether the piston top-land crevice volume contributes significantly to engine-out HC emissions. Finally, the role of piston-top fuel films and their impact on HC emissions was studied. This was investigated on the all-metal engine using two fuels of different volatilities. Parallel studies were also performed on the optical-access engine via in-cylinder tracer laser-induced fluorescence (LIF) imaging.

Results obtained in the standard and optical access engines revealed that bulk quenching represents one of the most significant sources of unburned HC for the wall guided combustion chamber geometry. Bulk quenching occurs as a result of incomplete fuel oxidation reactions in regions where the local equivalence ratio is either too fuel-lean or too fuel-rich or alternatively in excessively low temperature zones within the combustion chamber. Experimental data obtained from both the standard and optical access engines also revealed that liquid film formation occurs, and is particularly prevalent for early start of injection (SOI) strategies. Furthermore, liquid films remain present at the end of combustion and are believed to represent a significant source of engine-out HC emissions. In-cylinder imaging of liquid films suggest that the film eventually detaches from the piston surface later during the expansion stroke, resembling a flash boiling phenomenon. The results appear to confirm that unburned fuel arising from piston-top fuel films contribute directly to the engine-out HC emissions.