In the search for clean diesel fuels, dimethyl ether seems like a good choice. But mixing DME with diesel fuel to run these engines is more complicated than simply combining the two fuels, according to Penn State researchers.

"Diesel engines expect fuels that have a certain lubricating quality and a certain viscosity," says Andre Boehman, associate professor of fuel science. "DME has no lubricity and very low viscosity."
DME is normally produced by dehydration of methanol, but can also be produced from natural gas and from coal derived syngas. A gas, DME in liquid form is stored under pressure similarly to the way liquefied natural gas is stored.
"There is pressure to produce fuel combinations that do not pollute the environment," says Boehman. "We are looking for ways to make fuels burn so they produce fewer particulates. The key approach is to reformulate fuel by adding compounds that contain oxygen."
DME fits the bill and pure DME burns smokeless, making no particles.
Boehman, Shirish Bhide, graduate student in fuel science, and Joseph Perez, adjunct professor of chemical engineering, investigated and characterized DME and DME diesel blends to determine both the properties of pure DME and DME mixed with diesel. They
found that DME will mix completely with diesel fuel. However, a 25 percent DME by weight mixture of fuel had a viscosity rating well below the acceptable range for diesel fuel. When DME is mixed with diesel, viscosity drops off rapidly.
"While DME does not have good lubricating properties, it appears that viscosity may be the more important property in developing these fuels," Boehman told attendees Aug. 27 at the American Chemical Society Meeting in Chicago.
Both viscosity improvers and lubricity additives are available. However, lubricity additives have been used in lubricants, not in fuels, and may cause problems in the fuel injection system of diesel engines.
Although the normal range of viscosity for diesel engines is above that of the 25 percent DME, Boehman plans to test the fuel in both a test bed engine and on a University Shuttle Bus. The bus, purchased partly with research funds, has been run on straight diesel so that the engine properties could be well characterized. The bus is now being converted to run blended DME diesel fuel.
The bus engine is a Navistar International 7.3 liter, T444E, V8 engine. Normally fuel is delivered at 70 to 80 pounds per square inch, but to accommodate the DME, the pressure will be between 120 and 150 pounds per square inch.
Funding for the DME studies have come from the U.S. Department of Energy, Air Products and the Pennsylvania Department of Environmental Protection.