DIMETHYL ETHER AS A COMPRESSION IGNITION ENGINE FUEL FOR SIMULTANEOUS NOX AND PM REDUCTION

Abstract

Dimethyl ether (DME) is an alternative compression ignition (CI) fuel capable of achieving low nitrogen oxide (NOx) and particulate matter (PM) emissions simultaneously while maintaining diesel equivalent power. However, its behavior under varied injection timing, exhaust gas recirculation (EGR) rates, and multi-condition operation remains insufficiently characterized for practical calibration and commercialization. This research investigates the combustion, emissions, and performance characteristics of DME relative to diesel using a fully instrumented John Deere 6068CI550 single cylinder research engine (SCRE) modified for high pressure common rail DME operation. Baseline diesel and DME tests were conducted at three ISO 8178 C1 steady-state modes, matching location of 50% mass burned (CA50), indicated mean effective pressure (IMEP), and EGR to isolate fuel property effects. Subsequently, an injection timing sweep and a full EGR sweep at 1600 rpm and 50% load were performed to evaluate phasing sensitivity, emissions tradeoffs, and combustion stability limits.Baseline comparisons show that DME produces substantially lower PM compared to diesel which is composed of more than 95% organic carbon (OC) with less than 5% elemental carbon (EC). DME also reduces NOₓ by 10%–35% across baseline conditions, while maintaining diesel equivalent thermal efficiency. Combustion analysis confirms that DME exhibits a single-stage premixed heat release structure with significantly lower peak apparent heat release rates (AHRR) and 4–5 deg shorter 10 to 90% mass fraction burned duration (CA10–90) compared to diesel. These trends reflect rapid fuel vaporization, uniform mixture preparation, and elimination of diffusion limited burning. Injection timing sweeps demonstrate that retarding CA50 progressively decreases NOₓ, with all cases beyond 15 deg ATDC falling below the diesel baseline, but at the cost of increasing brake specific fuel consumption (BSFC). Phasing also becomes more consistent at retarded timings, as indicated by reduced coefficient of variation (COV) of CA50. EGR sweeps reveal that DME sustains stable combustion up to 55% EGR, beyond which COV of IMEP and CA50 start to increase. Higher EGR extends start of injection to 5% mass burned (SOI–CA5) and CA10–90 and reduces peak AHRR, indicating slower reaction rates under heavy dilution. A combustion incompleteness threshold emerges below approximately 0.1 g kWh⁻¹ NOₓ, where carbon monoxide (CO), total unburned hydrocarbons (THC), and BSFC rapidly rise. Overall, this study fulfills its objectives by identifying optimal operating conditions for low NOₓ and low PM DME combustion while maintaining diesel baseline power. The results demonstrate that DME can simultaneously achieve low NOₓ and low PM without the traditional diesel PM–NOₓ tradeoff, with optimal performance occurring near CA50 of 16 deg ATDC and EGR levels of 30%–40%. These findings provide a foundation for future DME engine development and support its viability as a low emission fuel.