Volatile stuff for heavy trucks

Operating a fleet of trucks is a tough business. Forget trucker romance; strong competition and high price pressure is the name of the game. Increasingly strict environmental regulations aiming at lower CO₂ emissions and strict exhaust gas values, especially with regard to nitrogen oxides (NO_X), will further intensify pressure on the industry in the years ahead. If truck operators do not rely on the latest technology, they will have to fear increased tolls or tax disadvantages in numerous countries.

Many commercial vehicle manufacturers and operators are now considering alternative powertrain systems to improve the environmental performance of their fleets. However, electric drives are hardly suitable for commercial vehicles in long-distance operation: The batteries would be too heavy, the charging times too long and the required charging power too high for competitive use. Hydrogen could solve this problem: As of September 2020, the first Hyundai fuel cell trucks are in commercial test operation in Switzerland. Synthetic natural gas from surplus green electricity is also under investigation: From 2021 the first natural gas trucks will be refuelled at Empa's mobility demonstrator, move. But there is another alternative that would be suitable for long-distance transport and that deserves closer examination: dimethyl ether or DME, in short.

The researchers want to run their test engine with pure DME, without using lubricating additives, as has been done in previous projects. In cooperation with a major European supplier, a new oil-lubricated common-rail pump had therefore been developed. In addition, the valves and valve seat inserts were converted to materials suitable for DME. An electrically driven compressor for precise exhaust gas recirculation is also used. Finally, the combustion chambers and compression ratio of the former diesel engine have also been modified. The new shape of the combustion chambers was developed with the help of mathematical simulations at the Politechnico di Milano. The research project is co-financed by the Swiss Federal Office of Energy (FOEN).

Dimethyl ether (DME), the eco-fuel for compression ignition engines, can be produced from hydrogen and CO₂. If hydrogen is produced with renewable energy and CO₂ is captured from the atmosphere, trucks could be driven with virtually no greenhouse gas emissions.

Empa researcher Andreas Borgschulte and his team are investigating chemical processes that produce DME as efficiently as possible. The method of sorption-assisted catalysis is considered to be very promising: The two gaseous starting materials, hydrogen and CO₂, need to get in contact with active copper particles in order to start the chemical reaction to form methanol or DME. Water is formed as a by-product. If water is removed from the reaction mixture, the chemical equilibrium shifts towards the product. In other words: Only then can the desired large quantities of methanol and DME be produced. To remove water, Empa researchers use zeolite, a water-absorbing mineral.

In lab experiments Borgschulte's team found that, at a certain temperature, CO₂ and hydrogen mainly produce DME, with only a rather small amount of methanol. "DME production using this method is therefore theoretically feasible," says Borgschulte, adding: "Unfortunately, at present the process is not yet very efficient." The next step would thus be to refine the chemical process and develop suitable production facilities. Only then can we assess whether DME production using sorption-assisted catalysis is economically competitive.

The research was carried out in collaboration with the University of Zurich as part of the LightChEC project.

"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 that is common in highway operation, 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." The first results are very promising. The experimental engine runs stable in all engine load ranges, produces practically no soot particles and significantly lower NOx values than a diesel. This leads to a significantly more compact and cost efficient exhaust gas aftertreatment system, even for future extremely strict pollutant emission limits.

The big advantage of DME operation, says Soltic, 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 save a great deal of NOx, which eases the burden on the exhaust gas purification system behind the engine and allows future, stricter limits to be met easily. With fossil diesel, high exhaust gas recirculation rates lead to higher particle emissions, which is not the case with DME.

During the test phase, Empa researchers repeatedly take samples of the engine oil to trace chemical reactions The results are forwarded to project partner Motorex who uses the data to develop a new engine oil specially adapted for DME operation.

"We are currently still in the pre-competitive phase of our research," Soltic explains. The results of the project are partly public and are discussed jointly among competitors in the vehicle industry. The platform for these discussions is the "International DME Association" founded in 2001, which currently has 50 members from science and industry. "But at some future stage everyone will want to keep their results to themselves," the Empa researcher says. "At that point, it is crucial for us to have a good understanding of the technology in order to be able to continue providing valuable input as a research partner for industry".