Design of experiments optimized OME_x-diesel blends on a heavy-duty engine − Part 1: Combustion and emissions analysis with EGR and injection timing variation

Oxymethylene dimethyl ether (OME) as a renewable E-fuel provides huge potential for simultaneous soot and NO_x reduction on heavy-duty engines, but a thorough optimization of combustion and emission characteristics of great interest to achieve that potential. To achieve that goal, diesel-oxymethylene dimethyl ethers (OME_x) blends are investigated on a single-cylinder heavy-duty research engine with different operating strategies using the design of experiments (DOE) method. A representative truck cruising condition with engine speed of 1425 RPM and 30 % load was targeted, because of the relatively high particulate matter (PM) emissions with regular B7 diesel. In general, the required injection duration at a fixed load increases with OME_x addition related to its reduced lower heating value, hence limiting the available energy before the decreased ignition delay, and resulting in a reduced premixed heat release peak. The ignition delay becomes shorter with increasing OME_x content due to its higher reactivity. Because of the higher reactivity of OME_x and higher oxygen content leading to a lower stoichiometric air–fuel ratio, the burn duration is inversely proportional to OME_x addition at this relatively low-load condition, and seemingly independent of injection duration. Subsequently, the combustion phasing (CA50, the crank angle where 50 % of the heat has been released) is advanced with increasing OME_x. In addition to combustion analysis, the particle number concentration is measured using an engine exhaust particle sizer. To obtain good consistency with the well-established AVL smoke meter results an appropriate particle mass density array of EEPS needed to be adopted. Using the DOE approach methodology, a response surface of PM emissions based on the experimental data indicates that the soot emissions strongly correlate to OME_x content in the blends, satisfying EU VI regulations without after treatment when OME_x content surpasses 17 % at this highway cruising condition on the research engine. While OME_x concentration has a slightly negative effect on NO_x, these emissions can be significantly reduced with increasing exhaust gas recirculation (EGR) ratios. On the contrary, CO and unburned hydrocarbon are effectively reduced with OME_x addition. Finally, a comprehensive global emissions map indicates that OME_x has the potential to disrupt the traditional soot-NO_x trade-off relationship compared to diesel. Based on this map at the studied condition, the engine can comply with the emission regulations when OME_x- and EGR- ratio are above 24.1 % and 37.2 % respectively, with an injection timing of 10.5 CAD bTDC.