Ethanol and Toxic Emissions
There is a widespread belief that engines burn cleaner on ethanol-containing fuel than on gasoline. This is not really true. The substances present in the exhaust gases change, but the exhaust certainly never becomes just carbon dioxide and water!
All gasoline engines emit quite large amounts of unburnt hydrocarbons – molecules similar to the ones in the original fuel. (Interestingly, diesel exhaust contains almost no unburnt hydrocarbons.) These are the substances which go on to become smog. The catalytic converter was originally devised to oxidize these hydrocarbons, but it only begins to work when it reaches 250oC. In dense urban traffic that may take 5-10km of driving! A large proportion of vehicles on city streets are essentially uncatalysed – not because their catalytic converters are defective but because they have yet to reach operating temperature. Cold catalytic converters are the main reason why our cities continue to suffer from smog.
If a gasoline engine is fuelled with ethanol, hydrocarbon emissions drop dramatically, but emissions of aldehydes soar! These aldehydes are substances like formaldehyde, a powerful irritant and carcinogen, and acetaldehyde, which is similarly irritating and a suspected carcinogen. Depending on who you consult, aldehyde emissions on E-85 are 20-30 times higher than they would be if the same engine was burning gasoline. Again, the catalytic converter burns these off, but only after it has reached operating temperature!
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