Numerical Investigations on Oxides of Nitrogen Mitigation Strategies in a Homogeneous Charge with Direct Injection Engine

Homogeneous charge with direct injection (HCDI) is a single-fuel low-temperature combustion (LTC) strategy that injects diesel into the intake port and inside the engine cylinder. The present study aims to numerically evaluate various oxides of nitrogen (NO_x) mitigation methods such as split injection, exhaust gas recirculation (EGR), and water vapor induction in a single-cylinder diesel engine operated in HCDI mode. Numerical investigations are carried out using a commercial computational fluid dynamics (CFD) code CONVERGE. Experimental data are generated in a light-duty diesel engine operated in HCDI mode at 2.4 bar indicated mean effective pressure (imep) (low load) and 4.6 bar imep (high load) conditions to validate the CONVERGE predictions. The production engine is modified to run in HCDI mode through suitable modifications in the intake system, cylinder head, and fuel injection systems. The predicted combustion and emission parameters compared well with the measured data. The parametric investigations conducted with CONVERGE show that increasing the main injected fuel quantity in the split injection reduces NO_x emissions. An increase in EGR fraction reduces NO_x emissions; however, EGR reduces the indicated thermal efficiency (ITE) significantly even at a moderate proportion. Among the NO_x mitigation methods chosen for investigation, water vapor induction greatly benefits NO_x reduction. The efficacy of the three methods is compared at fixed combustion phasing at 2.4 bar and 4.6 bar loads in HCDI. It is observed that water vapor induction reduces NO_x emissions by up to 71.3%, with a slight penalty of carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions by 8.5% and 28.4%, respectively, at a 4.6 bar load.