For the sake of the environment, not only must passenger cars become more environmentally friendly over the coming years, but commercial vehicles must also substantially reduce their CO2 emissions. Especially when it comes to nitrogen oxides (NOx). However, electric propulsion systems are unsuitable for commercial vehicles in long-haul operations. Batteries that are too heavy and charging times that are too long would make these powertrains unprofitable.
Hydrogen is a different matter. Hydrogen-powered trucks are already cruising European roads. But research is also being carried out into synthetic natural gas produced from surplus green energy. As of 2021, the mobility demonstrator from Empa, known as Move, will refuel the first natural gas trucks this way. Researchers are also looking at another alternative to diesel: Dimethyl ether (DME).
Cost-effective infrastructure, clean combustion
Dimethyl ether is already being produced for other purposes on a scale of several tens of thousands of metric tons annually. The chemical is used, for example, as a propellant in spray cans. As well as a component of coolants in refrigeration systems. It also serves as an intermediate product in the chemical industry. Plus, it has one major advantage. It can be produced from methanol at a low cost and with almost no wastage. Methanol, in turn, can be cheaply produced with electricity from solar and wind energy.
In addition, DME has similar properties to liquid gas. Therefore, it can be transported and stored in inexpensive tanks under low pressure in liquid form. In sharp contrast to hydrogen. And the technology for tank systems has also been in use worldwide for years and is reasonably priced. Last but not least, oxygen is chemically bound in dimethyl ether. This means that DME burns in a particularly clean way and produces hardly any soot.
This has also been demonstrated in initial tests with dimethyl ether as a fuel. Volvo Trucks, for example, has been running practical trials with experimental trucks in Sweden and the USA since 2013. In Europe, a research project coordinated by the Ford Research and Innovation Center Aachen has been underway since 2016. The engine has already been installed and tested in a Ford Mondeo.
Trials with a modified truck engine
Empa is now building on the knowledge that has been gained so far. It is doing this together with FPT Motorenforschung AG Arbon, Politecnico di Milano, the lubricant manufacturer Motorex as well as other partners. Since the beginning of July 2020, a pilot engine has been in operation on a test bench in Empa’s Automotive Powertrains Technologies department. It aims to provide sound data on combustion processes, efficiency, and environmental friendliness of DME in the commercial vehicle sector.
“We already know this engine very well,” says project leader Patrik Soltic. “The engine block is derived from a Cursor 11 commercial vehicle engine manufactured by FPT Industrial and has already served us for five years in various research projects. Over the past few months, we have converted it to DME together with our partner FPT.” Nevertheless, this was not easy, as the highly volatile DME has practically no lubricating properties, unlike diesel fuel. This would soon have destroyed the high-pressure pump used in the common rail injection system in particular.
Fuel without additives
In contrast to earlier experiments, the current test engine will run on pure DME. Without the addition of any lubricating additives. To achieve this, the scientists have developed a new, oil-lubricated common rail pump in cooperation with a major European supplier. Moreover, the valves and valve seat inserts had to be converted to material that is compatible with DME. An electrically driven compressor is used to ensure precise exhaust gas re-circulation. Lastly, the combustion chambers and compression ratio of the original diesel engine also had to be adapted, the scientists explain. The new shape of the combustion chambers had been designed using mathematical simulations at the Politecnico di Milano.
“Now we want to get to know the engine with the new fuel,” says Soltic. The researchers are starting with a medium-load driving style which is typical for highway routes, where the engine must deliver 100 kW of power. “Then we modify the timing and pressure of the injection, among other things, and look at exhaust emission values and fuel consumption.” During the test phase, the research team also repeatedly takes samples of the engine oil to examine any chemical changes. The results are used to develop a specially adapted engine oil.
Promising first results
The scientists stress that an initial test phase has already shown that dimethyl ether could indeed be an environmentally friendly alternative to diesel in the future. According to Soltic, the great advantage of using DME “is the opportunity to transfer a very high proportion of exhaust gas to the next charge in the cylinder. This is done through so-called exhaust gas recirculation (EGR).” This technology makes it possible to cut down on a large amount of NOx, which in turn relieves the burden on the exhaust gas purification system behind the engine and makes it possible to safely meet future, stricter emission limits. “With fossil diesel, high exhaust gas recirculation rates lead to an increase in particulate emissions; this is not the case with DME.”
Soltic explains that the company is still in the pre-competitive phase of its research work. Further research is still needed to make the dimethyl ether powertrain suitable for everyday use. This also involves the efficient production of dimethyl ether and special equipment for its production.
The research work is being carried out in collaboration with the University of Zurich and is part of the LightChEC project. The Swiss Federal Office of Energy (SFOE) is co-financing the research project.