Sources and Tradeoffs for Transient NO and UHC Emissions with Low Temperature Diesel Combustion

High bandwidth transient data from a multi-cylinder diesel engine operating in a low temperature combustion regime was analyzed to identify and characterize the transient response behaviors primarily responsible for transient emissions of NO and UHC. Numerous different speed and load transients as well as different combustion modes and control strategies were studied to determine how these parameters affect transient performance. Limitations in the transient response of the air system were found to be the largest contributor to transient emissions, although the mechanism by which these limitations affect performance can vary greatly depending on conditions.

Analysis of the data shows that transient emissions for low temperature combustion strategies are highly dependent on cycle-to-cycle changes in intake charge conditions. No fundamental difference was observed between the transient processes controlling speed and load changes. In addition, the fundamental transient response of the engine system is not a function of the combustion mode or control strategy used, although these factors do have large effects on the magnitude of emissions and other performance parameters. Cyclic build-up or depletion of oxygen caused by the feedback effect of EGR often influences transient behavior even when EGR fraction does not change or after charge flow rates have stabilized. This severely limits the effectiveness of EGR control strategies using mass flow rate of fresh air or EGR for control feedback.