A team from Empa - Swiss Federal Laboratories for Materials Science and Technology and FPT Motorenforschung reports in an open-access paper in the journal Fuel on the performance and emission characteristics of an 11-liter heavy-duty compression ignition engine which was optimized specifically for DME (dimethyl ether) combustion. The DME engine was based on an FPT Cursor 11 serial-production-engine.

Brake thermal efficiency of the DME engine. The BTE map looks as one would expect for a compression-ignition engine: best efficiency levels at cruising speed and medium-to-high load and a reduction in efficiency at lower load conditions. Above around 6–7 bar bmep, the engine’s efficiency is above 40% and the maximum efficiency is slightly above 45%. However, the test engine did not have the most recent friction reduction package. With the most recent design, an increase of the efficiency level of around 1–2 percent-points can be expected. Soltic et al.

Modifications included a DME-adapted high-pressure common rail pump, adapted injectors as well as a low-pressure fuel-supply system able to deliver DME at 10 bar and return the injectors’—as well as the common rail pump—backflows accordingly. The common rail pump was externally lubricated by engine oil so that it can handle the low fuel lubricity.

The engine was also 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.

EGR system consisting of an EGR cooler, an electrically driven EGR pump and respective piping. Soltic et al.

The researchers also optimized the combustion chamber geometry and compression ratio (to 20.5 from 16.5) for DME.

The experimental results showed that DME operation retains the performance on the base diesel engine (338 kW peak power) while significantly reducing the engine-out emissions.

Configuration of the engine, mounted on the test bench. Soltic et al.

The NO_x-soot tradeoff, typical of Diesel applications, basically does not exist anymore so that a desired NO_x 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. Total hydrocarbon, carbon monoxide and particle emission levels of the actual Euro VI and possibly even for the proposed Euro VII on-road legislation could be met without any exhaust gas purification.

Schematic of the experimental setup (not shown: ammonia measurement post-ATS). Soltic et al.

In terms of tailpipe CO₂ emissions, a reduction of around 11% is possible compared with a diesel-fueled engine.

The fair energy density of DME in combination with the numerous pathways for sustainable production, the diesel-like high conversion efficiency together with the possibility to meet extremely strict emission regulations with a comparably low technical effort for exhaust gas purification render DME a very attractive fuel for a number of applications in the future.

The authors therefore recommend that DME should be considered seriously as a sustainable future fuel for the heavy-duty on– and off-road sector. DME has the technical, economic and ecological potential to ideally complement other sustainable fuels such as hydrogen, methanol, methane or ammonia.

During the experiments described in this paper, no issues were found with the externally lubricated DME pump. However, a thorough assessment of the durability of such a pump over the desired lifetime has to be made before this technology can be industrialized.

—Soltic et al.

Resources

• Patrik Soltic, Thomas Hilfiker, Yuri Wright, Gilles Hardy, Benjamin Fröhlich, Daniel Klein (2023) “The potential of dimethyl ether (DME) to meet current and future emissions standards in heavy-duty compression-ignition engines,” Fuel, Volume 355, doi: 10.1016/j.fuel.2023.129357