Researchers from Colorado State University in Fort Collins have demonstrated that dimethyl ether (DME) could become a viable alternative to diesel fuel for compression-ignition engines. To achieve that, they converted a John Deere research engine to run on DME and discovered through a series of experiments that this fuel can sharply reduce nitrogen oxide (NOₓ) emissions and virtually eliminate soot at the same time, solving two major problems with modern diesel engines.
The biggest challenge with a conventional diesel engine is that NOₓ and soot emissions are closely related. When engineers try to reduce nitrogen oxide emissions, namely, by recirculating some of the exhaust gases back into the cylinders, the engine typically produces more smoke and particulate matter. DME largely breaks this dependency thanks to its chemical properties: its molecule lacks the carbon-carbon bonds that typically form soot, and the fuel itself is approximately 35% oxygen. As a result, a very small amount of particulate matter is formed during combustion.
This was confirmed by direct measurements of exhaust composition. More than 95% of the particulate matter in the exhaust gases of a DME-powered engine was composed of organic residues, i.e., traces of oil and unburned fuel, while the proportion of elemental carbon, or soot, did not exceed 5%. The situation is the opposite for a conventional diesel engine: soot constitutes a significant portion of the particulate emissions.
To gain an understanding of how DME behaves in real-world conditions, the scientists conducted a series of tests at 1,600 rpm and 50% load. First, they varied the fuel injection timing, and then, the level of exhaust gas recirculation, which reduces in-cylinder temperature and thereby suppresses NOₓ formation. They found that DME burns differently than diesel fuel. Unlike diesel, it does not have a sharp peak in heat release at the start of combustion, since the process is smoother and more even. Combustion also completes faster, about 4–5 degrees crankshaft rotation earlier than with diesel, and the maximum cylinder pressure is lower in most operating modes.
Meanwhile, the engine’s thermal efficiency remained virtually unchanged, hovering at 33% in average operating modes. However, fuel consumption by weight was roughly 40–50% higher, since DME contains less energy than diesel.
The most revealing results were obtained with an increased proportion of exhaust gas recirculation. DME maintained stable combustion even with 55% exhaust gas recirculation, a very high value at which a conventional diesel engine becomes unstable and starts smoking heavily. NOₓ emissions dropped to near zero, and particulate matter remained minimal.
Problems only began when NOₓ emissions dropped below approximately 0.1 grams per kilowatt-hour. This would cause carbon monoxide (CO) and unburned hydrocarbon emissions to rise and fuel consumption to increase, with the engine burning the mixture less efficiently. Nevertheless, the researchers found a mode that proved to be the most well-balanced: with exhaust gas recirculation at around 30–40% and a specific injection timing, the engine demonstrated low NOₓ emissions, virtually no soot, stable operation and acceptable efficiency at the same time.
The practical implications of this paper are linked to increasingly stringent environmental requirements for heavy equipment. Modern standards, such as Tier 4 Final in the United States or Europe’s Stage V, require almost complete soot removal and a major reduction in NOₓ emissions. Because of this, manufacturers have to equip diesel engines with complex and expensive exhaust aftertreatment systems, namely, particulate filters and selective catalytic reduction units. DME has the potential to make this much easier.
However, this technology has its limitations. At room temperature, DME becomes a gas, which is why it must be stored under a pressure of about 5–6 atmospheres, similarly to propane. Furthermore, due to its low viscosity and poor lubricating properties, it necessitates fuel injection system modifications. Finally, in order to achieve the same driving range as vehicles with diesel engines, either a larger fuel tank or more frequent refueling will be required. Nevertheless, the potential of producing DME from natural gas, biomass and even coal leads the researchers to consider it one of the most realistic options for a relatively clean fuel for trucks, tractors and other heavy machinery.
