Transient NOx Emission Reduction Using Exhaust Oxygen Concentration Based Control for a Diesel Engine

Meeting EPA Tier 2 emission standards presents a great challenge to engine manufacturers. In addition to having an actively controlled aftertreatment system, engine-out NOx emission needs to be reduced significantly to achieve regulatory compliance. Using advanced combustion methods, such as low temperature combustion and/or HCCI, has been shown to reduce engine-out NOx emissions. However, all this new combustion technologies are yet to permeate down into any production system. In current practice, large amount of exhaust gas recirculation (EGR) into the cylinders is widely used to reduce emissions. However, NOx emission from transient engine operation still constitutes a very large percentage of the total NOx output during a Federal Test Procedure (FTP) cycle and has yet to be adequately addressed.

Currently, the EGR flow is controlled using the intake mass airflow (MAF) measurement. In this study, an alternative control method was developed based on the measured air-to-fuel ratio (A/F) ratio in the exhaust manifold, which is proportional to the oxygen concentration of the intake manifold, which in turn is one of the main factors determining the formation of NOx and particulate matter. The proposed control scheme was implemented in dSPACE and tested on a 4.5 liter V6 prototype engine. We show that the new strategy achieved significant reduction in transient NOx spikes and contributed up to 15% improvement in engine-out NOx emissions during FTP 74 transient test cycles with an increase in particulate matter (PM) emissions of around 6.5% and little difference in fuel consumption. It was also found that there is slight performance degradation during some very hard acceleration maneuvers. However, this is negligible during real world driving.