Nitrogen Oxide Reduction Potentials Using Dimethyl Ether and Oxymethylene Ether in a Heavy-Duty Diesel Engine

The synthetic fuels dimethyl ether (DME) and polyoxymethylene dimethylether (POMDME or OME) are promising oxygenated fuels to meet the rising challenges of air pollution control, CO₂-neutrality, and sustainability. The sootless combustion and high ignitability of DME and OME represent ideal properties for an application in diesel engines. However, recent investigations of oxygenates reported an increase of nanoparticles, which are known to have fatal effects on human’s health. Besides nanoparticles, ongoing discussions about future emission legislation focus on a drastic reduction of NO_x.

For this reason, the present work investigates different measures to reduce NO_x emissions using DME/OME and a paraffinic diesel fuel (PDF) as reference. Different rail pressures, exhaust gas recirculation (EGR) rates, and injection timings are evaluated, considering the effectivity on NO_x reduction and the impact on other emissions, especially on nanoparticles. Besides these calibration parameters, a variation of the injector nozzle design for DME/OME is carried out to find an optimum between efficiency and NO_x. Finally, the influence of water injection into the intake port is investigated and compared against the NO_x reduction using direct injection of a mixture of fuel with water.

It is found that the number of nanoparticles using DME/OME shows no dependency of the abovementioned NO_x-reducing measures and significantly decreases in comparison to PDF. Furthermore, a nozzle design taking account of both the lower energy density of the fuels and decreased rail pressure proves to be the optimum for DME/OME, leading to a benefit in NO_x emission levels and thermal efficiency in comparison to PDF. Moreover, a notable reduction in NO_x is observed by injecting small amounts of water into the intake port. Though, increased water mass does not lead to a proportional NO_x reduction effect. Direct injection of fuel/water mixtures shows a comparable effect on NO_x emission levels when applying an equal amount of water. However, the direct injection of a fuel/water mixture shows higher impact on NO_x reduction at increased load.