Gasoline engines are probably the most common type of internal combustion engine you see in everyday life. In the United States, most passenger vehicles use gasoline as their main fuel.
Gasoline engines are called spark-ignition engines because the combustion step begins when a spark plug ignites the compressed air-fuel mixture inside the engine. This small explosion pushes the piston downward and produces power.
Gasoline is made of short-chain hydrocarbons refined from petroleum. It has a relatively low boiling point, around 85°C (185°F), which is lower than the boiling point of water. Gasoline is also highly volatile, which means it evaporates easily. You may notice this when fueling a car or filling a lawn mower on a hot day. The smell you notice comes from gasoline vapors entering the air as some of the hydrocarbons evaporate.
Gasoline is sold based on its octane rating. In the United States, gasoline is usually sold as regular, midgrade, and premium. These values can vary somewhat by state, but regular gasoline is usually 87 octane, midgrade is around 89 octane, and premium is typically 91 to 93 octane.
Octane ratings are based on two reference fuels:
Iso-octane (C8H18), which is assigned a value of 100.
Text description of the Iso-octane image.
Ball-and-stick model of an iso-octane molecule showing a branched hydrocarbon structure composed of eight carbon atoms and eighteen hydrogen atoms. The central carbon chain has three methyl branches, creating a compact, irregular shape. Carbon atoms are typically shown as dark gray or black spheres connected by rods, with smaller white hydrogen atoms attached around the outside.
Credit: Moleview
Note: You can use your mouse to rotate and zoom in on the Iso-octane molecule.Normal heptane (C7H16), which is assigned a value of 0.
Text description of the Hectane image.
Ball-and-stick model of a heptane molecule showing a straight-chain hydrocarbon structure composed of seven carbon atoms and sixteen hydrogen atoms. The carbon atoms form a zigzag chain, with smaller hydrogen atoms attached around the outside. Carbon atoms are typically shown as dark gray or black spheres connected by rods, while hydrogen atoms are shown as smaller white spheres.
Credit: Moleview
Note: You can use your mouse to rotate and zoom in on the Heptane molecule.
Fuels with more branched hydrocarbon chains tend to have higher octane ratings, while fuels with straight-chain hydrocarbons tend to have lower octane ratings. Some fuels can even have octane values above 100. For example, ethanol (C2H5OH) has an octane rating of about 113 and is often blended into gasoline. The fuel you buy is a mixture of many different types of compounds. The octane rating (87, 89, 93) is the average mix of all of the compounds found in that fuel.