Gasoline is manufactured as a product of the
petroleum refining process. This process starts with crude oil and separates out the different types of hydrocarbons through what is called fractional distillation. Fractional distillation takes advantage of the fact that different hydrocarbon components, each of which has a different application and different number of carbon atoms in a molecule, are progressively heavier and have progressively higher boiling points. The petroleum is heated to approximately 400 deg C (750 deg F), at which temperature it has become a mixture of liquid and vapor. This mixture, called the feedstock, is introduced into the refining tower (see Figure 1). The most volatile components (those with the lowest boiling points) condense at the top of the column where it is cooler, while the less volatile components condense nearer the bottom. These different fractions, each of which usually consists of a mixture of compounds with similar numbers of carbon atoms, are drawn off separately. The typical fractions collected at refineries are shown with the number of carbon atoms they contain, their boiling points, and their ultimate uses:
- Petroleum gas - used for heating, cooking, making plastics; commonly known by the names methane, ethane, propane, and butane; often liquified under pressure to create LPG (liquified petroleum gas)
- Naphtha or Ligroin - liquid intermediates that are typically further processed to make gasoline, e.g., octane
- Gasoline - liquid used primarily as a motor fuel
- Kerosene - liquid fuel for jet engines and tractors; starting material for making other products
- Gas oil or Diesel distillate - liquid used for diesel fuel and heating oil; starting material for making other products
- Lubricating oil - liquid used for motor oil, grease, other lubricants
- Heavy gas or Fuel oil - liquid used for industrial fuel; starting material for making other products
- Residuals - solids; coke, asphalt, tar, and waxes; starting material for making other products
Refineries then treat the fractions to remove impurities, use chemical processing on some of the fractions to break longer chains into shorter ones, and combine the various fractions into mixtures to make desired products. For example, a refinery could process diesel fuel to turn it into gasoline if demand for gasoline was greater than for diesel. One of the most common examples of combining fractions occurs when refineries combine different mixtures to create gasolines with different
octane ratings. Octane is a hydrocarbon molecule with 8 carbon atoms (see Figure 2) that gives gasoline the ability to resist detonating prematurely in the engine, which causes engine knocking or pinging during combustion.