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In the twenty-first century, exhaust emission control will remain a major technical challenge especially as additional pressures for fuel and energy conservation mount. To address these needs, a wide variety of engine and powertrain options must be considered. For many reasons, the piston engine will remain the predominant engine choice in the twenty-first century, especially for conventional and/or parallel hybrid drive trains. Emissions constraints favor the conventional port fuel-injected gasoline engine with 3-way exhaust catalyst, while energy conservation favors direct-injection gasoline and diesel engines. As a result of recent technological progress from a competitive European market, diesels, and most recently, direct-injection (DI) diesels now offer driveability and performance characteristics competitive with those of gasoline engines. In addition, DI diesels offer the highest fuel efficiency.

Exhaust emission and performance characteristics of a single-cylinder engine fueled with methanol are compared to those obtained either with gasoline or a methanol-water blend. Our measurements of engine efficiency and power, and CO and NOx emissions agree with trends established in the literature. Consequently, the emphasis is placed on organic emissions (unburned fuel including hydrocarbons, and aldehydes), an area in which there is no consensus in the literature. In all cases with methanol fueling, the unburned fuel (UBF) emissions were virtually all methanol as opposed to hydrocarbon compounds. Without special measures to overcome methanol's large heat of vaporization, UBF emissions were four times greater with methanol than those with gasoline. Similarly, aldehyde emissions were an order of magnitude greater with methanol.

The dioiefin content of unstable gasolines has been implicated in the restriction of multi-port fuel injectors by deposits. Vehicle tests of the effect of adding a diolefin mixture to a stable (olefinic) gasoline indicated that a deposit-producing gasoline resulted. Laboratory oxidation tests showed that addition of a “dienophile” to an unstable gasoline significantly reduced deposit formation, increased oxidation stability, and reduced gum formation. Additional vehicle tests with a dienophile-treated fuel showed that some injector deposits were reduced, but new deposits occurred in other areas of the fuel system. Apparently, diolefins can be a factor in producing plugged fuel injectors, but other fuel factors are also likely to play important roles whether diolefins are present or not.