Most have heard the word octane used in regards to the hydrocarbon fuel gasoline. Octane is actually the generic name for molecules having eight carbon atoms and the chemical formula C8H18. More commonly, octane is used in reference to grades of gasoline. In this case, the numbers seen at the pump, 87, 89, and 93, refer to the fuel's octane number. So what vehicle owners are actually interested in knowing is "What is octane number?" Before getting into what octane number is and what it means to gasoline consumers, it is useful to have a basic understanding of how an engine works.
Vehicles that use gasoline have a four-stroke spark ignition engine. The first stroke is the induction stroke. The piston travels down the cylinder. A valve is opened allowing a mixture of fuel and air to enter into the cylinder. Next is the compression stroke. In this stage of the cycle, the valves are closed, and the piston travels back up the cylinder causing the air and fuel mixture to compress. When the piston has traveled to the top of the cylinder, the spark plug fires, causing the air-fuel mixture to ignite. The flame propagates through the mixture causing the temperature and pressure inside the cylinder to increase. The mixture expands forcing the piston down in the power stroke. Finally, the exhaust valve is opened and the piston travels back up the cylinder expelling any remaining gases in the exhaust stroke.
Under the high temperature and pressure conditions of the compression stroke, it is possible for the fuel-air mixture to ignite without the spark plug. This phenomenon is known as engine knock. Engine knocking is bad for a vehicle. It reduces a car's gas mileage and acceleration, creates wear and tear on parts, and in severe cases can lead to engine failure. Octane number is a rating that refers to a fuel's resistance to auto-ignition under specific engine conditions. Specifically, the octane number is the percentage of "octane" (2,2,4-trimethylpentane) blended with another chemical called pentane, that is required to achieve the same knocking characteristics as the fuel being tested. Since octane is very resistant to knocking and pentane knocks very easily, the higher the fuel's octane number the less likely it will cause engine knocking.
So why are there three different octane numbers? The different grades of gasoline are needed to match the different types of engines available. The most important engine characteristic to consider is the compression ratio: the ratio of the volume of the cylinder at the piston's lowest point and the volume at the piston's highest point. A car with a high compression ratio will perform better in terms of acceleration and power but will also subject the fuel to more severe temperature and pressure conditions, and will therefore require gasoline with a higher octane number. For example a Porsche 911 has a compression ratio of 11.3:1 and requires 93 octane number gasoline, while a Mercury Tracer has a ratio of 8:1 and requires 87. Consult your vehicle's owner's manual to determine the best grade of gasoline for your vehicle. You may be spending extra money on premium gasoline when you don't really need to.
