An investigation of the impact of injection strategy and biodiesel on engine NOx and particulate matter emissions with a common-rail turbocharged DI diesel engine

Abstract

An investigation of the impact of engine injection strategy on NOx and particulate matter (PM) emissions with biodiesel fueling was conducted with a common-rail turbocharged direct injection diesel engine at moderate speed and different load/torque conditions. The fuels included a baseline ultra low sulfur diesel fuel (ULSD) and a B40 (v/v) blend of a soybean methyl ester (SME)-based biodiesel and ULSD. Single fuel injections at start of injection timings over a range from 9° before to 3° after top dead center with different fuel injection pressures were investigated. It is found that at all load conditions, an increase of fuel injection pressure significantly increases NOx emissions, and that with the same injection strategy as the baseline diesel, biodiesel fueling increases NOx emissions. For particulate matter emissions, it is found that an increase of fuel injection pressure decreases PM emissions for all load conditions. Meanwhile, biodiesel fueling has a more significant effect on PM emissions at low load and a less significant effect at moderate to high load. For an engine running with early SOI (highest brake fuel conversion efficiency in this work), a decrease of fuel injection pressure can counter the biodiesel NOx effect while maintaining a similar or even lower level of PM emissions as ULSD. Both biodiesel fueling and the change of fuel injection pressure did not significantly affect the brake fuel conversion efficiency, but retarding the start of injection appears to decrease it. Apparent heat release analysis shows a faster and higher heat release peak with higher fuel injection pressure. With biodiesel fueling, an earlier of start of combustion can be observed for low load, but no significant difference in combustion phasing can be observed at moderate to high loads. A fuel spray, mixture stoichiometry field and lift-off length model was employed. Linear correlations between the average oxygen equivalence ratio of the fuel–air mixture at the autoignition zone near the lift-off length and brake specific NOx emissions were observed for all load conditions, regardless of fuel type. This confirms that the dominant factor that determines NOx emissions is the ignition event controlled by the oxygen equivalence ratio at the autoignition zone.