"Diesel engines expect fuels that have a certain lubricating quality and a certain viscosity," says Dr. Andre L. 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 V. Bhide, graduate student in fuel science, and Dr. Joseph M. 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 today (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.
In the laboratory, the researchers' results on pure DME, obtained on a refurbished high pressure viscometer originally constructed at Penn State in the 1950s, closely match other recent characterizations of DME by the Danish Technical University. Numerical calculations produced by AVL Power Train Technology of Plymouth, Mich., are also very consistent with the Penn State results.
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," says Boehman. "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."
A test bed engine is already fitted with the extra pressurizing equipment needed to use blended DME diesel. The engine easily coped with the increased pressure in the fuel reservoir.
"The engine responds to its state, so it is difficult to determine exactly what the changes are that are being made to compensate for the different fuel," says the Penn State researcher.
To find out what is going on in the engine, Boehman's team will soon install a video system to record the inside of the engine. They are currently using a computer program that can both listen to and talk to the engine's electronics through the Controller Area Network ‘CAN' communications bus on the engine.
Funding for the DME studies have come from the U.S. Department of Energy, Air Products and the Pennsylvania Department of Environmental Protection. Expanded fuel property experiments will be funded by the DOE's National Energy Technology Laboratory, including the testing of additives for improving lubricity and increasing the viscosity of the fuel mixtures. The researchers will also begin compressibility tests on the fuel mixtures.