The potential of dimethyl ether (DME) to meet current and future emissions standards in heavy-duty compression-ignition engines

Abstract

Dimethyl Ether (DME, H3C-O-CH3) is regarded as a meaningful alternative fuel as it has a comparably high energy density, can easily be stored on vehicles, can be produced via different renewable pathways, has a high Cetane number and is therefore suitable for use in efficient and durable compression ignition engines. Moreover, the DME molecule lacks carbon bonds and due to the inbuilt oxygen atom enables low-sooting diffusion combustion. This study reports on the performance and emission characteristics of a state-of-the-art heavy-duty compression ignition engine with 11 L displacement which was optimized specifically for DME combustion. This includes the adaptation of the fuel supply and injection system as well as the combustion chamber geometry. In addition, the engine is equipped with an electrically driven volumetric pump in the exhaust gas recirculation (EGR) path which gives the freedom to set any desired EGR rate, independently from the pressure ratios across the turbocharger. The experimental results show that DME operation retains the performance on the base Diesel engine (338 kW peak power) while significantly reducing the engine out emissions. The NOx-soot tradeoff, typical of Diesel applications, basically does not exist anymore so that a desired NOx level can basically be set without the restrictions of sooting combustion. In addition, the combustion is very complete and very low levels of regulated and non-regulated pollutant emissions can be detected (e.g. for carbon monoxide, DME, methane, benzene, toluene, acetaldehyde, formaldehyde, formic acid). Transient operation is unproblematic and very low emissions can be achieved using the same exhaust gas treatment system as for diesel. Actually, total hydrocarbon, carbon monoxide and particle emission levels of the actual EuroVI and possibly even for the proposed EuroVII on-road legislation could be met without any exhaust gas purification. In terms of tailpipe CO2 emissions, a reduction of around 11% is possible compared with a diesel-fueled engine.