Why Does a Car Require Premium Gas?
New automobiles come with a long list of manufacturer specifications and recommendations. These include tips for maintenance, driving style and what type of fuel to use. Some cars, especially sports and luxury cars, require premium grades of gasoline to function properly.
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Octane Ratings
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Gasoline is sold in a number of grades, each of which is distinguished by an octane rating. Gas with a higher octane rating, such as 91 or 93, is more resistant to combustion. Lower octane ratings, like 87 or 88, describe fuels that will ignite more quickly or at a lower temperature. Octane ratings are determined by comparing a sample of fuel to a mixture of the isomer iso-octane and the alkaline n-Heptane. The percentage of iso-octane needed to mimic the combustibility of the gasoline is used to apply the octane rating to the fuel.
Engine Knocking
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Until the 1980s, most car engines used carburetors to blend fuel and air before they were ignited in the engine. Over time, carburetors would often allow for a higher percentage of fuel to be used, and this high-fuel mixture would ignite more quickly, resulting in a condition known as engine knocking. To correct engine knocking and restore an engine to its most efficient state, drivers could begin using fuel with a higher octane rating, which would delay the combustion.
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Fuel Injection
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In the 1980s, most automobile manufacturers began building engines that used fuel injectors instead of carburetors. Fuel injected engines use a computerized system to determine how much fuel should be injected into the engine by monitoring the timing of the actual ignition sequence and adding or subtracting fuel continuously. Today, most cars are sold with a minimum octane recommendation of 87, meaning that any gasoline with an octane rating of 87 or higher will resist combustion long enough for the fuel injection system to provide an appropriate amount of fuel for continuous, efficient operation.
Modern Cars
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Today, some cars are still sold that require premium fuel, usually with a minimum of 91 or 93 octane. While some of these cars do, in fact, use fuel injectors designed to inject high octane gasoline, they will be able to self-adjust to any fuel grade above 87 octane. Other cars, such as high-performance sports cars or luxury cars, may have engines with a high compression ratio. In these cases, high octane fuel may be needed to delay combustion since more compression means more combustible fuel in the cylinder at one time.
Need Versus Want
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For the vast majority of cars, premium gasoline is unnecessary. Contrary to common beliefs, all grades of gasoline are legally required to contain cleaning agents that help keep fuel injectors and engines running efficiently and producing as few harmful emissions as possible. Using premium gas in a car that only needs 87 octane may result in wasted fuel that does not burn and enters the exhaust system, often collecting in the catalytic converter where it can cause harm over time. If engine knocking doesn't occur when using a lower grade than specified by the manufacturer, a car is proven to work well with the lower octane.
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