What Is the Difference Between Grades of Gasoline?

One usually sees several types of fuel at a gas station, but what is the difference between the grades of gasoline? Understanding why one type of gasoline is different from another, the price differential between them and what the benefits are for a vehicle can help you during the next trip to a gas station.

Type of Gasoline

The usual type of fuel at gas stations is usually regular, midgrade and premium. Fuel is considered by grade levels and ranked by octane rating. This octane rating is actually an average, noted below:

  • Regular gas: 87 octane rating, average 85 to 88
  • Mid-grade or plus gas: 89 octane rating, average 88 to 90
  • Premium gas: 92 octane rating, average 91 to 94

Octane Meaning and Composition of Gasoline

The two major components of gasoline are the liquids heptane and isooctane (2,2,4-trimethylpentane). The octane rating is the ratio of heptane to isooctane. An octane rating of 87 is 87 percent isooctane and 13 percent heptane.

All gasoline grades are primarily a mixture of hydrocarbons and additives, like ethanol. As their name suggests, hydrocarbons are molecules that contain both carbon and hydrogen.

The exact composition of gasoline varies by climate and environmental regulations, but an approximate is 15 percent C4 - C8 straight alkanes, 40 percent C4 – C10 branched alkanes, 10 percent cycloalkanes, 25 percent aromatics and 10 percent straight-chain and cyclic alkenes.

Gasoline Processing

All gasoline is derived from crude oil, formed from the remains of plants and animals held under great pressure for millions of years, which has a mixture of both long chain and short chain hydrocarbons. How the oil is treated and processed at a petroleum refinery will determine the grade of gasoline fuel.

The process by which these different products are recovered is called fractional distillation. Here, crude oil is pumped into a furnace and heated at high temperatures (over 600 degrees Fahrenheit, 316 degrees Celsius). Most hydrocarbon molecules evaporate and rise into a fractioning column. As the evaporated molecules rise up this 100 (or more) foot-tall column, the heavier molecules will condense at the lower levels, and the lighter hydrocarbons will be at the higher levels.

A 42-gallon barrel of crude oil can be converted to about 20 gallons of motor gasoline, 12 gallons of distillate fuel (diesel), 4 gallons of jet fuel and other products. The extra cost of higher-octane gasoline is due to the expense of adding octane enhancers or additional branched or aromatic hydrocarbon fractions.

Grade Effect on Automobiles

The octane rating is a measure of the fuel’s stability and anti-knocking ability. Lower octane levels can combust easier when compressed, which may make the knocking or pinging sound in an engine. The knocking or pings happen when there is non-uniform combustion causing irregular pressure waves in a cylinder.

Standard-performing vehicles are outfitted to use the correct amount of compression to eliminate internal combustion pings. High-performance cars benefit from premium gasoline because their engines have been built for a higher level of compression for added driving power.

Choosing the Correct Grade

The vehicle’s manufacturer determines the grade of gasoline needed for a particular automobile. Placing the correct type of gasoline will help a vehicle run smoothly and protect the engine from unnecessary engine repair.

However, under normal driving conditions, if the suggested fuel is regular gas, there is little to no benefit to using premium gas.

Historical Notes on Gasoline

Originally, gasoline was discarded in the distillation of kerosene. In the 1890s, with the invention of the automobile, gasoline was then considered a valuable fuel.

In the 1950s, lead was added to gasoline to improve the performance of automobile engines (as an anti-knocking agent). The use of leaded gasoline was phased out due to its health-related issues by the mid-1990s.

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