Ethanol, with the chemical formula C2H5OH is a colourless liquid with a boiling point of 78°C and has been used to large extent as a chemical compound in the medical and food industries. Ethanol is highly flammable and has a flame which is difficult to be seen. It is soluble in water and forms an azeotrope, so it is difficult to achieve 100% pure ethanol by distillation. Ethanol can be used as a pure fuel or blended with gasoline or diesel in a transport system. Ethanol has lower energy density (about 34% lower) and lower vapour pressure than gasoline which makes starts in cold weather difficult. Ethanol is less toxic than gasoline, diesel or methanol regarding safety and environmental issues. Ethanol can be broken down by bacteria to carbon dioxide and water and it can be produced from ethene obtained from fossil sources in oil refining and also from biomass as bioethanol.

The most important characteristic of ethanol which makes it suitable as a fuel for Otto engines is its high octane number. The octane number is a numeric representation of the anti-knock properties of a motor fuel. By definition, the octane number is zero for n-heptane and 100 for iso-octane (2,2,4-trimethyl pentane) and for other fuels the octane number is decided by comparison with a mixture of these two compounds. Liquid fuels with a high octane number have better properties during engine combustion. For ethanol, with a high octane number (129), it is possible to push more fuel-air mixture into the engine’s cylinders (higher compression ratio gives higher efficiency and less fuel consumption) without any risk of uncontrolled self-ignition which may cause ‘knocking’ and serious damage to the engine as a consequence.

One disadvantage of ethanol is its low cetane number (8) and it can be used in diesel engines only if some ignition improver (e. g. di-tert-butyl-peroxides) is added to it. These kinds of additives are often costly, but there are commercially feasible alternatives in the market. The cetane number is a numeric representation of a fuel’s ignition properties. By definition the cetane number is 15 for hepta — methyl-nonane and 100 for n-hexadecane. For other fuels, it is decided by comparison with a mixture of those two compounds. Too low a cetane number causes slow ignition and poor engine performance.

It is technically possible to add at least 10% bioethanol to gasoline without any need for changes in the engine of cars and this can reduce gasoline consumption and the net concentration of fossil CO2 in the atmosphere worldwide. One obstacle to mixing a higher percentage of ethanol in gasoline (petrol) is that car manufactures, in many cases, do not guarantee, for ethanol blends more than 5-10%, cars with ordinary gasoline engines. Several modifications are needed to minimise the risk of any damage to some parts of the engine if higher blends are used.

It is possible to use neat ethanol (99% pure, water free) or blended with petrol or diesel in Otto engines and diesel engines, respectively. There are two types of vehicles: one is the flexible fuel vehicle (FFV) in which it is possible to use up to 85% ethanol in petrol, the second group is the vehicles that use pure (neat) ethanol.

When it comes to blends, bioethanol with diesel in private cars or heavy vehicles (buses, trucks, etc), and the addition of emulsifying agents is essential to achieve a homogenous emulsion of ethanol and diesel (aliphatic hydrocarbons). In pure ethanol fuel for a diesel engine, addition of an ignition improver is necessary. The ignition improver will increase the production cost of the bioethanol as fuel in transport sector.

All bioethanol may not be used as transport fuel. In fact, ethanol is used in the production of other industrial chemical compounds such as ethylene, ethyl acetate, acetic acid and acetaldehyde by various chemical reactions, e. g. oxidation, esterification. Therefore, as the production of bioethanol increases, it will replace fossil sources for ethanol production in many aspects.