The main source of liquid fuels is petroleum which is obtained from drilled wells under the Earth’s crust. Petroleum has originated probably from organic matter such as fish and plants either by bacterial action or distillation under pressure and heat. It consists of a mixture of gases, liquids, and solid hydrocarbons with small amounts of nitrogen and sulfur compounds.
Liquid fuels are classified as follows:
- Natural or crude oils: These oils are further distilled to obtain petrol, benzene, diesel, and other light oils such as kerosene, alcohol, and so on.
- Distilled artificial oils: These oils include coal–tar, tar–oil, shale–oil, and natural gas oil.
Various oils obtained from natural or crude oils are briefed below:
- Petrol: It is the lightest and the most volatile liquid fuel. It is mainly used for spark-ignition petrol engines. It is obtained by fractional distillation of petroleum at 65°C to 220°C. It is also known as
- Kerosene: It is obtained by fractional distillation of crude oil at 345°C to 365°C. It is also known as paraffin oil. It is heavier and less volatile than petrol. It is used for heating and lighting purposes.
- Diesel: It is obtained by fractional distillation of crude oil at 345°C to 470°C. It is mainly used in diesel engines and oil-fired furnaces and boilers. It is also known as heavy fuel oil.
- Benzene: It is obtained by the redistillation of tar, which is the byproduct collected during the production of coal gas. It is used as an alternate fuel for internal combustion (IC) engines. It is less prone to detonation than standard petrol.
- Alcohol: It is formed by the fermentation of vegetable matter. The calorific value of alcohol is lower than that of petrol and the cost is higher. It is mainly used for industrial purposes.