Petrol engines will run on ethanol as the properties of ethanol are similar to petrol in many aspects (Table 6.3). Butanol has been included in the table as it is an alternative to ethanol as a petrol replacement with an energy content similar to petrol. The higher heat of vaporization of ethanol means that as the fuel is vaporized in the carburettor, the mixture is cooled to a lower temperature than for petrol. This means that more fuel enters the engine, in part compensating for the lower energy content, but the fuel inlet may need heating. The octane rating is a measure of the resistance of the fuel to pre-ignite when compressed in the cylinder of the engine. A low octane fuel will pre­ignite causing a condition known as ‘pinking’ and this will result in a loss of power. Ethanol has a higher octane number and higher oxygen content than petrol. The heat of combustion (or gross energy) is lower than petrol, which leads to some reduction in performance and a 15-25% increase in fuel consumption.

Ethanol has the disadvantage that it mixes with water and this type of mixture will corrode steel tanks. To avoid separation of an aqueous layer in cold weather the ethanol needs to be anhydrous as ethanol normally contains 4.5% water. At 95.6% ethanol the liquid and vapour have the same concentration, known as an ‘azeotrope’, so no further concentration is possible by simple distillation.