Swiss Research on Dimethyl Ether as an Alternative Fuel for Trucks

The Swiss Federal Laboratories for Materials Science and Technology (Empa) is researching a new drive concept with a special test engine. Commercial vehicles are expected to emit less CO2 in the future while meeting stricter emission limits. Many experts anticipate that fossil diesel could soon face challenges. One possible alternative fuel is dimethyl ether. This easily volatile substance can be produced from renewable energy and burns very cleanly.

(Dübendorf/St. Gallen/Thun) Operating a fleet of trucks is a tough business. There is little romance in long-haul driving, but a lot of strong competition and high price pressure. Increasingly strict environmental laws will further intensify this pressure in the coming years, leading to lower CO2 emissions and strict emission values, especially concerning nitrogen oxides (NOx). If truck operators do not adopt the latest technology, they risk facing higher toll fees or tax disadvantages in many countries.

Alternative Dimethyl Ether

Many commercial vehicle manufacturers and operators are now considering alternative drives to improve the environmental compatibility of their fleets. However, electric drives are hardly suitable for long-distance commercial vehicles: the batteries would be too heavy, the charging times too long, and the required charging capacities too high for competitive use. Hydrogen could solve this problem: Starting in September 2020, the first hydrogen fuel cell trucks from Hyundai will be operating in Switzerland in commercial trials. Synthetic natural gas from surplus green electricity is also being researched: Starting in 2021, the first natural gas trucks will be refueled at the Empa mobility demonstrator move. But there is another alternative that would be suitable for long-distance transport and deserves closer examination: dimethyl ether.

Affordable Infrastructure, Clean Combustion

The chemical dimethyl ether (DME) is produced on a scale of several tens of thousands of tons annually. The substance is used as a propellant in spray cans and is a component of refrigerants in cooling systems. Additionally, DME is widely used as an intermediate product in the chemical industry. Its advantage: it can be produced cost-effectively and almost loss-free from methanol, which in turn can be produced cheaply using electricity from solar and wind energy. DME thus offers the opportunity to enable trucks to operate CO2-neutrally.Another advantage: DME has similar properties to liquefied gas. Unlike hydrogen, it can be transported and stored in liquid form in inexpensive tanks under low pressure; the technology for refueling stations is also cost-effective, globally known, and has been in use for decades. Because oxygen is chemically bound in dimethyl ether, the substance burns particularly cleanly and with low soot formation.

Testing in a Modified Truck Engine

There have been previous attempts to use dimethyl ether as a fuel: Volvo Trucks has been conducting practical trials with experimental trucks powered by DME in Sweden and the USA since 2013. In Germany, a research project has been running since 2016, coordinated by the Ford Research and Innovation Center Aachen. The engine has already been installed and tested in a Ford Mondeo.Now, Empa, together with FPT Motorenforschung AG Arbon, the Politecnico di Milano, lubricant manufacturer Motorex, and other partners, will build on the knowledge gained so far. Since early July 2020, the test engine has been in operation in a test stand at the Empa Department of Vehicle Drive Systems, which aims to provide solid data on combustion processes, efficiency, and environmental friendliness of DME in the commercial vehicle sector. “We know this engine very well,” says project leader Patrik Soltic. “The engine block comes from a Cursor 11 commercial vehicle engine from the manufacturer FPT Industrial and has been used for various research projects for the past five years. In recent months, we have retrofitted it to run on DME together with our partner FPT.” This was not entirely straightforward: the easily volatile DME has practically no lubricating properties compared to diesel fuel, which would have quickly destroyed the high-pressure pump of the common rail injection system.

Operation Without Additives in the Fuel

The researchers want to operate their test engine with pure DME, without the addition of lubricating additives, as was common in previous projects. Together with a large European supplier, a new oil-lubricated common rail pump has been developed. Additionally, the valves and valve seat rings have been retrofitted with DME-compatible materials. An electrically operated compressor for precise exhaust gas recirculation is also used. Finally, the combustion chambers and the compression ratio of the former diesel engine have been adjusted. The new shape of the combustion chambers was calculated using mathematical simulations at the Politecnico di Milano. The research project is co-financed by the Federal Office of Energy (BFE).

Simulated Highway Operation

“Now we want to get to know the machine with the new fuel,” says Soltic. The researchers will start with a typical mid-load range in highway operation, where the engine must deliver 100 kW of power. “Then we will modify the timing and pressure of the injection, look at the emission values and fuel consumption.”The great advantage of DME operation, says Soltic, is the opportunity to take a very high proportion of exhaust gas into the next filling of the cylinder in almost all operating states, using the so-called exhaust gas recirculation (EGR). This technique allows for significant NOx savings, which relieves the exhaust gas treatment behind the engine and enables future, stricter limits to be met safely. In the case of fossil diesel, high exhaust gas recirculation rates lead to increasing particle emissions, which is not the case with DME.During the testing phase, the Empa researchers repeatedly take samples of the engine oil to track chemical changes. The results are sent to the project partner Motorex, which uses the data to develop a new engine oil specifically adapted for DME operation.

Joint Research Among Competitors

“Currently, we are still in the pre-competitive phase of the research work,” explains Soltic. The results of the project are partially public and are discussed jointly among competitors in the vehicle manufacturing sector. The platform for this is the “International DME Association,” founded in 2001, with currently 50 members from industry and research. “But eventually, everyone will want to keep their results to themselves,” says the Empa researcher. “Then it is important that we master the technology to provide valuable input as a research partner for the industry.”

Fuel from the Eco Factory

Dimethyl ether (DME), the eco-fuel for diesel engines, can be produced from hydrogen and CO2. If the hydrogen used is generated with renewable energy and the CO2 is extracted from the atmosphere, trucks could be operated practically without greenhouse gas emissions.Empa researcher Andreas Borgschulte and his team are researching chemical processes to produce DME as efficiently as possible. The method of sorption-assisted catalysis is considered promising: the two gases hydrogen and CO2 must come into contact with active copper particles to bond into methanol or dimethyl ether. Water is produced as a byproduct. If the water is removed from the reaction mixture, the chemical equilibrium shifts towards the product. In other words: only then do the desired large quantities of methanol and dimethyl ether form. To remove the water, the Empa researchers use zeolite, a water-absorbing mineral.In laboratory tests, Borgschulte’s team found that at a certain temperature, dimethyl ether is primarily produced from CO2 and hydrogen, with only a comparatively small amount of methanol. “The production in this way is theoretically possible,” says Borgschulte, adding, “Unfortunately, the process is currently not very productive.” In the next step, the chemical process would need to be refined, and suitable plants developed. Only then can it be assessed whether DME production via sorption-assisted catalysis is economically competitive. The research was conducted in collaboration with the University of Zurich and is part of the project “LightChEC”.Photo/Graphic: © Empawww.empa.ch