Gasoline, diesel fuel and fuel oil are all petroleum products. Petroleum is a mixture found in the earth, composed of crude oil and natural gas. While the products that are derived from petroleum power the modern world, raw petroleum is of little use until it is refined. It is the refining process that converts crude oil into gasoline, fuel oil, and diesel fuel.
Crude oil is a complex mixture of hydrocarbons that are chained together in different lengths. These chains have different characteristics and properties and form the basis for the individual products created out of raw petroleum. To be able to convert these hydrocarbon chains into gasoline, diesel fuel, fuel oil and other petroleum products, they have to be separated from other chains.
Crude oil is separated into these different chains through distillation. During distillation, crude oil is heated in a distillation column. Collection trays are placed at differing heights in the column. As the crude oil is heated, the vapors formed through the boiling process rise up the column. The lighter chains take longer to condense and collect in the trays towards the top and the heavier chains condense more quickly and collect in the trays towards the middle and bottom of the distillation column. Once they have been separated, they are treated further through various processes to create the different petroleum products.
Gasoline is formed from shorter and lighter chains of hydrocarbons than either diesel or fuel oil. Gasoline is lightweight, extremely volatile and evaporates quickly. These qualities contribute to gasoline powered engines having more horsepower and acceleration than an equivalent diesel engine. However, gasoline is not as efficient a fuel as diesel or fuel oil. Gasoline produces approximately 124,000 British Thermal Units (BTUs) of energy per gallon (3.79 liters) while a diesel engine produces approximately 139,000 BTUs of energy per gallon (3.79 liters).
Diesel is a middle weight product that is heavier than gasoline. It has the appearance of oil and is often referred to as diesel oil. It does not evaporate as quickly nor is it as volatile as gasoline. It takes less refining from crude oil to make diesel fuel, which often makes diesel less expensive than gasoline. The increased mileage and lower cost make diesel a popular choice of fuel in many parts of the world.
Fuel oil used for the heating of homes is slightly heavier than diesel fuel but shares similar properties and is considered a middle distillate as diesel is. Fuel oil for residential use produces approximately the same amount of BTUs as diesel and has a low volatility which makes it ideal for residential heating. Fuel oil that is used in large industrial applications such as electric power generators are considered residual fuel oils and are heavier than gasoline, diesel fuel, and home heating oil.