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What is the difference between diesel and petrol?

Petrol is a petroleum-derived liquid mixture consisting mostly of aliphatic hydrocarbons and enhanced with aromatic hydrocarbons toluene, benzene or iso-octane to increase octane ratings, primarily used as fuel in internal combustion engines. Diesel is a specific fractional distillate of petroleum fuel oil or a washed form of oil that is used as fuel in a diesel engine invented by German engineer Rudolf Diesel.

Petroleum is refined to produce petrol and diesel. Fractional Distillation process is used on petroleum and at various temperatures; different by products is formed from it. Petrol and diesel both are derived at varying temperatures during refining process. Petrol is produced at temperature between 35 degrees to 200 degrees while diesel is produced at a boiling point of 250-350 degrees. After distillation, in order to use these byproducts as commercially acceptable petrol and diesel, some blending with other elements has to be done. Petrol is produced first in this process as it is produced at a lower temperature than diesel.

Diesel is composed of about 75% saturated hydrocarbons (primarily paraffins including n, iso, and cycloparaffins), and 25% aromatic hydrocarbons (including naphthalenes and alkylbenzenes). The average chemical formula for common diesel fuel is C12H23, ranging from approx. C10H20 to C15H28. Petrol consists of hydrocarbons with between 5 and 12 carbon atoms per molecule but then it is blended for various uses. Overall a typical petrol sample is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes), aromatics and olefins (alkenes). The ratios vary based on a variety of factors.

Petrol is more volatile than diesel, not only because of the base constituents, but because of the additives that are put into it.

Gasoline contains about 34.6 mega joules per litre (MJ/l) while diesel contains about 38.6 mega joules per litre. This gives a higher power to diesel.

The U.S. used about 510 billion litres (138 billion gallons) of petrol (called "gas") in 2006, of which 5.6% was mid-grade and 9.5% was premium grade. The U.S. annual consumption of diesel in 2006 was about 190 billion litres (42 billion imperial gallons or 50 billion US gallons).

Diesel is used to run diesel engines that are used in cars, trucks, motorbikes etc. Some varieties of diesel are also used in running heating systems in houses. Bad quality (high sulfur) diesel fuel has been used as a palladium extraction agent for the liquid-liquid extraction of this metal from nitric acid mixtures. Petrol is primarily used to run petrol engines in cars, motorbikes etc.

Paraffin, another byproduct of crude oil produced at 190-250 degrees is used as airline fuel.

In countries like India, diesel is subsidized because it is heavily used for transportation. There is almost an Rs.30 difference between petrol and diesel prices in India. In the international markets both are almost equally priced. In the US, petrol was priced at $3/gallon in December 2007; diesel was priced at $3.39/gallon.

When discussed in terms of motor fuel, diesel is said to be more fuel efficient, giving almost 1.5 times the fuel efficiency of petrol.