Gasoline

Concerns over evaporative and tailpipe emissions have promoted new environmentally driven changes in gasoline and greater attention on the development of cleaner burning fuels. These concerns have resulted in compositional changes in fuels, as well as a struggle to balance environmental objectives with performance-based standards.

While regulations set specifications to control the environmental impact of gasoline, there are also specifications and guidelines to control the characteristics of gasoline to ensure that they perform satisfactorily. It is important for today’s automotive technician to understand fuel quality issues, both for diagnostic reasons and for the ability to convey accurate information to the consumer.

Gasoline is not a single substance, it is a complex mixture of components which vary widely in their physical and chemical properties. The properties of gasoline must be able to perform over an extremely wide range of circumstances. Quality standards represent compromises, so that all the numerous performance requirements may be satisfied. Octane and Volatility are two of the most important standards relating to Driveability and Performance in today’s vehicles.

Fuel Octane Quality

Gasoline is most commonly rated based on its Anti-knock Index. The Anti-knock Index is a measure of a fuel’s ability to resist engine knock. It is often stated as a number related to octane quality. Most engines require regular 87 octane while performance engines may require something higher, such as 92 octane. When the octane rating of a fuel is adequate for the engine in which it is burned, optimum performance and fuel economy is achieved. There is generally no advantage in using gasoline of a higher octane than the engine requires for knock-free operation.

There is a widespread perception that the greater the octane the better the performance. This is a myth because, when there is more than enough octane supplied to prevent engine knock, there is little if any performance improvement. Vehicle owners should only use gasoline meeting the vehicle manufacturer’s recommended octane levels.
Another misconception is that using a higher octane fuel will yield improved fuel economy.

Octane is nothing more than a measure of anti-knock quality. Fuel economy is determined by a number of variables including the energy content of the fuel. Two fuels of identical octane could have different energy content due to compositional differences, thus yielding different fuel economy. Diesel fuel has a higher energy content than gasoline, so the fuel economy rating is higher. Alcohol fuels have a lower energy content than gasoline, thus yielding less miles per gallon. The energy content of a fuel is measured in BTU (BRITISH THERMAL UNITS). The higher the BTU, the higher the energy content and the better the fuel economy rating.

Combustion chamber design as well as the compression ratio are the key determinants of an engine’s octane requirement. The higher the compression ratio, the greater the need for higher octane levels. Excessive combustion chamber deposits can increase the octane requirements of an engine due to increased compression ratio and heat retention.

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