Extending Exhaust Gas Recirculation Limits in Diesel Engines

This paper addresses the application of high exhaust gas recirculation (EGR) for reduced nitrogen oxide emissions from diesel engines. The research objective is to develop fundamental information about the relationship between EGR parameters and diesel combustion instability and particulate formation so that options can be explored for maximizing the practical EGR limit, thereby further reducing nitrogen oxide emissions while minimizing particulate formation. A wide range of instrumentation was used to acquire time-averaged emissions and particulate data as well as time-resolved combustion, emissions, and particulate data. The results of this investigation give insight into the effect of EGR level on the development of gaseous emissions as well as mechanisms responsible for increased particle density and size in the exhaust. A sharp increase in hydrocarbon emissions and particle size and density was observed at higher EGR conditions while only slight changes were observed in conventional combustion parameters such as heat release and work. Analysis of the time-resolved data is ongoing. INTRODUCTION Exhaust gas recirculation (EGR) has been used in recent years to reduce NOx emissions in light duty diesel engines. EGR involves diverting a fraction of the exhaust gas into the intake manifold where the recirculated exhaust gas mixes with the incoming air before being inducted into the combustion chamber. EGR reduces NOx because it dilutes the intake charge and lowers the combustion temperature. A practical problem in fully exploiting EGR is that, at very high levels, EGR suppresses flame speed sufficiently that combustion becomes incomplete and unacceptable levels of particulate matter (PM) and hydrocarbons (HC) are released in the exhaust. This transition to incomplete combustion is characteristically very abrupt due to the highly nonlinear effect of EGR on flame speed. In a transient operating environment, it is particularly difficult to reliably approach this instability limit without occasionally producing undesirable bursts of HC and PM emissions. The result is that diesel engines must be typically operated significantly below their maximum EGR potential, thus penalizing NOx performance. The objective of this work is to characterize the effect of EGR on the development of combustion instability and particulate formation so that options can be explored for maximizing the practical EGR limit. We are specifically interested in the dynamic details of the combustion transition with EGR and how the transition might be altered by appropriate high-speed adjustments to the engine. In the long run, we conjecture that it may be possible to alter the effective EGR limit (and thus NOx performance) by using advanced engine control strategies. All experiments described here were performed on a modern turbo-charged, direct-injection automotive diesel engine. This engine was selected on the basis that it is likely to reflect the