Adding oxygenates to gasoline has the effect of making the engine run leaner: since some oxygen is already present in the fuel molecules, the total amount of oxygen (from the air and from the oxygenates) relative to carbon and hydrogen in the fuel is greater. Running the engine leaner reduces the emissions of CO substantially; it also reduces the emissions of unburned hydrocarbons to some extent. Since the largest source of CO emissions is the automobile exhaust, use of oxygenated fuels substantially reduces the emissions of this pollutant. It is for this reason that, since January 1995, the oil companies have to offer this ‘reformulated’ gasoline in.
