In the search for clean diesel fuels, dimethyl ether seems like a good choice with good ignition quality and very low emissions, 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,’ said 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,’ said 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 appears to be ideal and pure DME is smokeless when burnt, making no particles.
Boehman, Shirish V. Bhide, graduate student in fuel science, and Dr. Joseph M. Perez, adjunct professor of chemical engineering, investigated and characterised DME and DME diesel blends to determine both the properties of pure DME and DME mixed with diesel. They found that DME would 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,’ said Boehman.
Both viscosity improvers and additives to improve lubrication are available. However, lubrication 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 engine is a Navistar International 7.3 litre, T444E, V8 engine,’ said 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 pressurising 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,’ said 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.