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Gasoline—a petroleum product

Gasoline is a fuel made from crude oil and other petroleum liquids. Gasoline is mainly used as an engine fuel in vehicles. Refineries and companies that produce the finished motor gasoline sold in retail gasoline fueling stations may add various liquids so that the gasoline burns cleaner and meets air pollution control standards and requirements.

Finished motor gasoline meets the basic requirements for fuel that is suitable for use in spark ignition engines. Some finished motor gasoline may require additional blending with ethanol (a renewable fuel and oxygenate), detergents and other additives, and higher octane gasoline before it is delivered to retail outlets for sale to end users. Most of the gasoline that oil refineries produce is actually unfinished gasoline (or gasoline blendstocks). Gasoline blendstocks require additional blending and usually require ethanol before delivery to retail outlets as finished gasoline. Refineries produce some finished gasoline, but most of the finished gasoline sold in the United States is actually produced at blending terminals where finished gasoline, gasoline blendstocks, and fuel ethanol are processed into finished gasoline ready for consumer use. Blending terminals are more numerous and widely dispersed than refineries, and they have equipment for loading trucks that transport finished gasoline to retail outlets.

Most of the motor gasoline now sold in the United States contains about 10% fuel ethanol by volume. Ethanol is added to gasoline mainly to meet the requirements of the Renewable Fuels Standard, which is intended to reduce greenhouse gas emissions and the amount of oil that the United States imports from other countries.

A gasoline pump showing different grades of gasoline
A gasoline pump showing different grades of gasoline

Source: Stock photography (copyrighted)

Gasoline varies by grade

Three main grades of gasoline are sold at retail gasoline refueling stations:

  • Regular
  • Midgrade
  • Premium

Some companies have different names for these grades of gasoline, such as unleaded, super, or super premium, but they all indicate the octane rating, which reflects the anti-knock properties of gasoline. Higher octane ratings result in higher prices.

Before 1996, lead was added to gasoline as a lubricant to reduce wear on engine valves. Leaded gasoline was completely phased out of the U.S. fuel system by 1996. Manufacturers recommend the grade of gasoline for use in each model of a vehicle.

Gasoline also varies by formulation

Did you know?

Gasoline changes with the seasons.

The primary difference between winter–and summer–grade gasoline is vapor pressure. Gasoline vapor pressure is important for an automobile engine to work properly. During winter months, vapor pressure must be high enough for the engine to start easily. In the summer, a lower vapor pressure is required in many areas to reduce air pollution.

Gasoline evaporates more easily in warm weather, releasing more volatile organic compounds that contribute to health problems and to the formation of ground-level ozone and smog. To cut down on pollution, the U.S. Environmental Protection Agency requires petroleum refiners to reduce the vapor pressure of gasoline during summer months.

In addition to the different grades of motor gasoline, the formulation of gasoline may differ depending on the location where it is sold or the season. Federal and state air pollution control programs that aim to reduce carbon monoxide, smog, and air toxins require oxygenated, reformulated, and low-volatility gasoline. Some areas of the country are required to use specially formulated gasoline to reduce certain emissions, and the formulation may change during winter and summer months. These area-specific requirements mean that gasoline is not a homogenous product nationwide. Gasoline produced for sale in one area of the United States may not be authorized for sale in another area.

The characteristics of the gasoline depend on the type of crude oil that is used and the setup of the refinery where the gasoline is produced. Gasoline characteristics are also affected by other ingredients that may be included in the blend, such as ethanol. Most motor gasoline sold in the United States contains some fuel ethanol.

Last updated: November 6, 2017

What is Octane?

A gasoline pump showing different grades of gasoline and octane ratings on the yellow labels
A gasoline pump showing different grades of gasoline

Source: Stock photography (copyrighted)

Did you know?

The large number on the yellow gasoline pump octane label is the minimum octane rating. (R+M)/2 Method on the label refers to the octane testing method used, where R is Research Octane Number and M is Motor Octane Number.

Normal combustion in a gasoline engine cylinder
Image of engine cylinder when normal combustion occurs.

Source: General Chemistry: Principles, Patterns,
and Applications
, 2011 (copyrighted)

Spontaneous combustion in a gasoline engine cylinder causing engine knock
Image of engine cylinder when spontaneous combustion occurs, causing engine knock.

Source: General Chemistry: Principles, Patterns,
and Applications
, 2011 (copyrighted)

Preignition in an engine cylinder
Image of engine cylinder when preignition occurs.

Source: General Chemistry: Principles, Patterns,
and Applications
, 2011 (copyrighted)

In recent years, car manufacturers have been requiring or recommending premium gasoline (a high-octane grade of fuel) for use in more of their vehicle models. There has also been an increase in the difference between prices for premium and lower octane grades. As a result, more people are curious about what octane is and what those octane rating numbers on gas pumps mean.

Octane ratings are measures of fuel stability. These ratings are based on the pressure at which a fuel will spontaneously combust (auto-ignite) in a testing engine. The octane number is actually the simple average of two different octane rating methods—Motor octane rating and Research octane rating—that differ primarily in the specifics of the operating conditions. The higher an octane number, the more stable the fuel. Retail gasoline stations in the United States sell three main grades of gasoline based on the octane level:

  • Regular (the lowest octane fuel–generally 87)
  • Midgrade (the middle range octane fuel–generally 89–90)
  • Premium (the highest octane fuel–generally 91–94)

Some companies have different names for these grades of gasoline, such as unleaded, super, or super premium, but they all refer to the octane rating.

Of the 18 isomers of normal octane (C8H18), octane gets its name from the 2,2,4-Trimethylpentane compound, which is highly resistant to auto-ignition. This iso-octane has been assigned the reference value of 100 for testing purposes. The extremely unstable normal heptane (C7H16) molecule is the 0 octane reference fuel.

How does the octane level affect my vehicle?

Engines are designed to burn fuel in a controlled combustion. A flame starts at the spark plug and burns throughout the cylinder until all of the fuel in the cylinder is burned. In comparison, spontaneous combustion, also called auto-ignition, detonation or knock, happens when rising temperature and pressure from the primary combustion causes unburned fuel to ignite. This uncontrolled secondary combustion causes pressure in the cylinder to spike and causes the knock to occur.

The competition between the intended (controlled) and unintended (spontaneous) combustion causes the energy from the burning fuel to disperse unevenly, which can cause damage and place high pressure on the engine's piston before it enters the power stroke (the part of the cycle when the piston's motion is generating power).

Before electric computerized ignition was widely used, this knocking commonly occurred and could cause significant engine damage. Most modern engines have sensors to detect knocking. When detected, the computer delays the initial spark, which causes the controlled combustion to take place at a point when compression is not at its highest point. Although this eliminates the knock, it can cause the engine to run less efficiently.

A similar undesirable condition is called pre-ignition, when the fuel ignites on its own before the spark ignites it. Modern engine computers minimize this condition by controlling the timing of valves and fuel injection, however this control mechanism can also come with a fuel-efficiency or emissions penalty.

How is octane measured?

The standard means of testing octane is with an octane testing engine. This test is similar to the way the mass of an object can be determined by comparing it to objects (references) of known mass on a balance scale. Primary Reference Fuels (PRF) of precisely known octane are formed by combining iso-octane, heptane, and other well-known standards such as toluene. These PRFs are used to bracket a given fuel sample to determine the pressure at which similar knock intensities are observed. This measurement is taken by adjusting the octane engine's cylinder height, which changes the compression ratio/pressure in the engine until the knocking reaches a specific intensity level.

The (R+M)/2 you see on the label refers to the average of the Research Octane Number (RON) and the Motor Octane Number (MON) ratings. To determine the RON, the fuel is tested under engine idle conditions with a low air temperature and slow engine speed. To determine the MON the fuel is tested under the more stressful conditions of higher air temperature and engine speed.

Historically, RON and MON were determined on separate testing machines specifically configured for each test. Current designs (see image below) allow the same engine to perform both tests. Despite this flexibility, many testers still prefer to use more than one machine, each specifically set up and calibrated to perform either RON or MON tests.

An octane testing engine
Image of an octane testing engine

Last updated: November 6, 2017