22. How does ethanol behave in the combustion process?In the combustion process ethanol and other oxygenates produce aldehydes. The law provides that in order to minimize the harmful exhaust emissions vehicles must be equipped with catalytic converters that efficiently remove such harmful compounds as aldehydes.
Scientific studies show that aldehydes are hazardous to health. The higher the aldehyde contents, the more health hazards. In vehicles equipped with catalytic converters, however, the low ethanol levels (as 10 % v/v) are assessed not to increase the degree of harmfulness of the exhaust emissions. The catalytic converters are efficient with these relatively low ethanol concentrations keeping the aldehyde emissions minimal when compared to other harmful exhaust components.
If the ethanol concentration in fuel is increased substantially (above 50% v/v) special attention must be paid to the catalytic converters' capacity to remove growing amounts of aldehyde. This is especially important in colder climates where the efficiency of the catalytic converter is not at its best in cold starts.