A variety of “alternative” fuels are being considered as potential future replacements for petroleum-based fuels. One of the leading contenders for replacing diesel fuel in some heavy duty applications is methanol, since methanol can be made from abundantly- available materials such as natural gas, coal, and biomass, and because neat methanol can result in reduced NOx and particulate exhaust emissions relative to diesel fuel. Because of various fuel property differences between methanol and diesel fuel, engine modifications must be made to the conventional diesel engine to allow it to utilize methanol fuel.A patented (1) ignition-assist combustion system initially applied to a 2 valve, 4 stroke Caterpillar 3306 DIT engine (121 mm bore) demonstrated methanol combustion feasibility in a tractor application (2). This ignition-assist technology has since been transferred to a 4 valve, 4 stroke Caterpillar 3406 DITA engine (137 mm bore) and further evaluated in line- haul truck application in the Canadian Methanol- In-Large-Engines (MILE) Program.Modifications were made to several 3406 DITA diesel engine components. These components included fuel injection pump, fuel nozzles, cylinder head and pistons. Design calculations indicated that these modifications would allow methanol engine performance and major component durability to be approximately the same as those of the diesel.Laboratory performance and preliminary durability tests verified the design calculations. Transient emissions (without exhaust after-treatment) showed the expected NOx and particulate emissions reduction relative to the diesel.Two 3406 DITA methanol engines were installed in Kenworth W900 trucks and operated in line-haul service across the Rocky Mountains between Coquitlam, B.C. and Canmore, Alberta, Canada, accumulating 327,027 km (203,122 miles)/5310 hours and 272,171 km (168,983 miles)/4404 hours, respectively. Despite the fact that the methanol engines were still in a very early stage of development, availability and fuel energy consumption closely approached that of the two diesel control trucks employed in this program. Post-test inspection showed major component life to be equal to or better than the diesel. Life of components such as glow plugs, fuel injection nozzles, and valves require additional development before the methanol engine is commercially viable.This long duration 3406 methanol engine demonstration in a grueling, high utilization application has provided further evidence of heavy-duty methanol engine feasibility. With additional development, the heavy-duty methanol engine is capable of becoming a commercially viable product should future economic and social conditions dictate the use of methanol as a heavy-duty engine fuel.