Effective reduction of NOx emissions from diesel engine using split injections

Good fuel economy and high thermal efficiency of direct injection diesel engines are certainly welcome characteristics from the viewpoints of preserving energy sources and suppressing global warming. Despite the attractive fuel economy, high emissions of oxides of nitrogen (NOx) and particulate matter (PM) with their unresolved trade-off are a major challenge to be addressed by researchers. Downsizing of the engine, dilution using exhaust gases, retardation of injection timing, etc. are widely adopted techniques to lower NOx emissions. Of late, multiple/split injection strategy is garnering much attention from the researchers for its potential to effectively address NOx, soot and piston work trade-offs. The effects of variation of fuel injection timing, dilution using EGR are investigated and compared against the proposed technique of split injections (25/75 and 75/25) on a single cylinder diesel engine using numerical experiments in the present work. For this purpose, a quasi-dimensional model has been developed primarily using the derivations of first law of thermodynamics and ideal gas equation. Models predicting heat release rates, heat transfer losses, ignition delay, chemistry of combustion, NOx and soot formations are coupled to the model using phenomenological considerations. Split injection with a smaller quantity of fuel injected in the first pulse is observed to significantly lower NOx emissions due to the restrain in premixed phase of combustion of second pulse. Split injections are observed to effectively address NO-piston work trade-off compared to increasing dilution rates using EGR and retarding injection timing towards TDC.