An Investigation Into the Impact of Post-Injection on Combustion and Emission Characteristics in Diesel and DME Fueled Compression Ignition Engines

Abstract

Increasingly stringent emissions regulations have intensified the demand for advanced combustion strategies in compression ignition (CI) engines to reduce nitrogen oxides (NOx) and particulate matter (PM) without compromising performance. Lean NOx trap (LNT) systems require periodic generation of reductants such as carbon monoxide (CO), total hydrocarbons (THC), and hydrogen (H2) to enable effective NOx conversion. Post injection, where a small fuel quantity is introduced late in the cycle, offers a potential solution for delivering these reductants directly within the cylinder. This study investigates the impact of the intake oxygen concentration and post injection timing on the combustion characteristics and exhaust emissions of both diesel and dimethyl ether (DME) in a single-cylinder CI engine. Rate of injection (ROI) analysis was used to characterize injection dynamics and determine conditions for separating main and post injection events. Engine tests were performed under low to moderate load conditions with fixed main injection parameters, while systematically varying exhaust gas recirculation (EGR) and post injection timing. The results showed that reducing the intake oxygen concentration significantly promoted CO and THC formation. Notably, DME exhibited a superior capacity for reductant generation compared to diesel; under extreme dilution (9.3% oxygen), DME yielded a CO peak of approximately 11700 ppm while maintaining negligible soot emission across all test conditions. These findings highlight that DME’s intrinsic soot-free nature allows it to decouple the conventional NOx-soot trade-off, providing a broader operational window for high-yield, in-cylinder reductant generation to support LNT regeneration.