Methanol-fueled diesels may be an attractive means of meeting future, more restrictive diesel particulate standards since methanol combustion forms very little soot. Unfortunately, methanol's autoignition temperature is high, and some means of improving its ignition is required. Therefore, we have investigated the use of dimethyl ether (DME), aspirated with the combustion air, to enhance the ignition of the injected methanol. A small, on-board catalytic reactor could be used to generate DME from the methanol fuel. This system requires minimal modifications to the engine design, and does not require use of an additive or fuel other than methanol.
In this study, we measured maximum cylinder pressure and rate of pressure rise, ignition delay, emissions, and relative efficiencies for a single-cylinder, direct injection, high-speed diesel engine operated on both diesel fuel and methanol-DME. The tests showed DME to be an effective ignition-enhancer over the engine's entire operating range, without excessive maximum cycle pressures, with lower maximum rates of pressure rise, and with efficiencies equivalent to those achieved on diesel fuel. Additionally, ignition timing was controllable (within limits) by adjusting the concentration of DME in the inlet air. At a given operating point, acceptable operation was achieved with a wide range of DME concentrations.
Particulate emissions were greatly reduced during operation on methanol-DME. NOx emissions were also reduced when compared to diesel-fuel operation, but were more a function of ignition timing than fuel. Moderate increases were seen in unburned hydrocarbons. As with diesel-fueled operation, CO emissions were negligible, but were slightly higher during operation on methanol-DME. However, in the air-rich exhaust of a diesel engine, HC and CO are considerably easier to control with catalytic converters than particulates and NOx.