Multiple injection tests were carried out using a 1.2-liter 4-cylinder Ford DIATA (Direct Injection Aluminum Through-bolt Assembly) engine at various operating conditions. The results were simultaneous reductions in NOx and soot emissions. An engineering model based on characteristic times, with consideration of both thermal and N2O formation kinetics, is utilized to gain insight into the reasons for NOx reduction due to multiple injections. Stoichiometric combustion is assumed for NO formation.
In this research, normalization and parametric studies are used to study the effects of injection timing, fuel quantity per injection pulse, and injection rate on NOx emissions. NO formation is reduced by modifications that lower stoichiometric flame temperature at the start of combustion of fuel injection pulses or decrease time spent by a fluid element in the stoichiometric zone. Good agreement is observed between model predictions and experimental results, but the model requires empirical input. Specifically, the cumulative heat release computed from engine test data is used to estimate the fraction of fuel burned in each pulse for the injector used here.
At different EGR rates, multiple injections can show different effects on NOx emissions. For example, multiple injections at the lower EGR rates tested provide increased NOx emissions reduction. Both measurement and model verify that EGR plays a more significant role in NOx reduction than do multiple injections. The model clarifies that the stoichiometric flame temperature decreases due to EGR has an exponential effect through chemical kinetics, while dwell between fuel pulses of multiple injections has only a linear effect on NOx emissions.