XME: Alternative fuels for the diesel combustion process

What is the potential of oxygenated methyl ether-based synthetic fuels for replacing fossil-based diesel in an compression-ignition combustion process? In the “XME Diesel” research project supported by FVV, funded as part of the “Neue Fahrzeug- und Systemtechnologien” [New Vehicle and System Technologies] specialist programme of the German Federal Ministry for Economic Affairs and Energy (BMWi), research institutes at RWTH Aachen University and the Technical University of Munich investigated the suitability of DME (dimethyl ether) and OME-1 (mono-oxymethylene ether) in collaboration with Denso, Ford and IAV. The main goal of the project was to adapt the combustion process to the physical and chemical properties of the new fuels in such a way that raw pollutant emissions are reduced significantly while the efficiency of the engine improves.

Motivation

The literature quotes numerous individual investigations into low-emission or even soot-free combustion of methyl ether-based fuels. When used in a vehicle, however, carburation and combustion must be adapted to the properties of new fuels. If this is successful, synthetically produced fuels can not only contribute towards air pollution control, but also make road traffic climate-friendly.

Methodology

The first task was to develop an injection system capable of handling the physical properties of DME and OME-1, which are very different to those of fossil diesel. The subsequent tests on a single-cylinder engine were primarily aimed at designing a suitable combustion process. Alongside these basic investigations, test series were conducted on fully assembled engines – these confirmed the benefits of the fuels in terms of soot emissions. Following this, two demonstrator vehicles were modified to run on DME and were subjected to an emission test as per the WLTP.

Result

Even the tests on the single-cylinder engine illustrated that soot-free combustion is possible with the two fuels; furthermore, it was possible to match the power density of the combustion engine run on fossil fuels using DME. Tests on the fully assembled car engine confirmed the predicted emissions behaviour of the single-cylinder engine while producing the same CO2 emissions as the corresponding diesel engine. A further reduction in CO2 (up to approximately 10%) can be achieved when the combustion process and the air and exhaust gas path are adapted for operation with DME. A commercial vehicle engine was built for the test with OME-1; using this, significant emission benefits were also demonstrated for application-specific load points. Finally, two cars were equipped with the optimised engine concept developed on the test bench and a liquid gas tank system derived from a conventional LPG system. In the WLTC test cycle, these demonstrator vehicles (with unlimited driveability) achieved completely soot-free operation while reducing nitrogen oxides by a third at the same time. Despite the air path and combustion process not being adapted in the vehicle, CO2 emissions remained almost constant. Final tests on the single-cylinder engine showed that the engine efficiency can still be raised considerably – and that emergency operation with conventional diesel fuel is possible with a minimised worsening of emissions.

Documentation

XME Diesel | (Bio) methyl ethers as alternative fuels in bivalent diesel combustion processes | Project No. 1005 | Collaborative research | Funding No. 19U15007 A-D

We would like to thank Oberon Fuels and Prins Westport for providing dimethyl ether and DME tank systems.

Themis