As low-temperature diesel combustion (LTC-D) is gaining interest for the reduction of nitrogen oxide (NOx) and particulate matter (PM) emissions, engine manufacturers will need to consider new engine designs to address the unique mixing and combustion challenges presented by these operating strategies. Currently, approaches to optimize engine design for these operating strategies are unclear.
This research examines the role of combustion-chamber design in LTC-D and attempts to offer a more clearly defined direction for the design of these engines. First, a CFD-based optimization methodology is proposed that functions as a design exploration and analysis tool to examine the influence of combustion chamber design in a heavy-duty diesel engine under LTC-D operating conditions. Second, predictions from the optimization study were examined experimentally in a single-cylinder heavy-duty optical diesel engine. Simultaneous planar laser-induced fluorescence (PLIF) measurements of formaldehyde (H2CO), hydroxyl radicals (OH) and polycyclic aromatic hydrocarbons (PAH) and quantitative fuel-tracer (toluene) measurements under non-combusting conditions provided a detailed examination of the spatial mixing, combustion and pollutant-formation processes under LTC-D operation. Finally, CFD models were examined against the experimental PLIF images, providing validation of the models and assisting in the quantification of the experimental results.
The optimization study identified the importance of three design parameters for optimizing NOx and soot emissions and fuel-consumption: swirl ratio, spray targeting and the piston-bowl diameter; establishing these design parameters as the basis for the optical engine measurements. The PLIF measurements showed that fuel-jet interactions with the piston bowl (jet-bowl) and with neighboring jets (jet-jet) can have a significant influence on the combusting jet structure and subsequent pollutant-formation processes, a complexity that was not fully captured in the optimization study. An improved calibration of the CFD models with the PLIF measurements revealed that soot formation and fuel-rich sources of carbon-monoxide (CO) and unburned hydro-carbons (UHC) emissions are significantly affected by the predicted jet-jet interactions and the rebound of these regions away from the piston-bowl wall during late-cycle combustion. Thus, combustion chamber design can tailor these jet-bowl and jet-jet interactions to minimize PM, UHC and CO emissions for LTC-D operating conditions.