Auto Engine Performance Vs. Fuel Octane
A typical fueling station sells gasoline in several different grades. While these grades are sometimes branded with catchy names, what really sets them apart from one another is the octane rating, which is represented by a number such as 87, 89, 91 or 93. While some cars will function properly with fuel that has any common level of octane, others require high-octane fuel for maximum performance.
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Definition
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The octane rating of a given type of gasoline is an indication of how resistant that fuel is to self-ignition. The number itself is derived from testing the fuel against a mixture of the chemicals octane and heptane. For example, if a sample of gasoline has the same resistance to ignition as a solution of 91 percent octane (and 9 percent heptane), it received a 91 octane rating (even though its actual percentage of octane may be higher or lower than 91 percent).
Function
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When a car's engine is running, fuel injectors send gasoline to the engine cylinders. There the fuel is blended with air, compressed by the piston and ignited by a spark plug. The combination of high engine temperatures and the force generated by compression can cause fuel with a lower octane rating to ignite early, preventing the engine from running its best.
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Considerations
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While most new vehicle manuals simply specify the need for unleaded fuel, some sports cars and high-performance luxury cars call for high-octane or "premium" fuel. This is because high-performance engines use higher compression ratios, along with different ignition timing and valve timing, to produce more power. This makes the fuel especially susceptible to early ignition, making high-octane fuel necessary.
Requirements
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Traditionally, drivers used high-octane fuel to combat "engine knocking," a noisy symptom of early fuel ignition. However, modern engines feature advanced technology, including computerized engine control units (ECUs) to adjust timing automatically. Still, automakers design certain performance engines to burn high-octane fuel exclusively. Disregarding these requirements will prevent the vehicle from achieving its advertised power rating.
Misconceptions
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A common misconception about the relationship between engine performance and fuel octane is that it extends to cover fuel economy. Octane ratings refer to the amount of energy needed to ignite the fuel, not the power potential of the fuel itself. While drivers who put low-octane fuel in a car meant for high-octane gas may experience a decrease in efficiency, drivers who exceed the octane requirements for a standard engine will not gain any significant increase in miles per gallon.
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References
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- Photo Credit sports car image by Jake Hellbach from Fotolia.com
