What Is Octane?

A primer on octane’s role in fuel and its impact on vehicles.
January 15, 2019

Each day, more than 40 million drivers pull up to a fuel dispenser to refill their vehicle, typically choosing from three types of fuel: regular, midgrade or premium.

With each fuel is a number that measures the level of octane in the gasoline. In most areas of the United States, regular unleaded gasoline is 87 octane, midgrade is 89 and premium is 91 to 93. These numbers are determined through engine testing, resulting in two measurements: Research Octane Number (RON) and Motor Octane Number (MON).

In the United States, gasoline is marketed with an octane value calculated as an average of RON (octane tested under normal driving conditions) and MON (octane tested under more rigorous conditions) and designated on fuel pumps as (R+M)/2. This value, known as the Anti-Knock Index (AKI), is the primary indicator of octane in gasoline that consumers see on fuel dispensers.

Octane-101-image.jpgMany vehicle owners may not know how an internal combustion engine that runs on gasoline works, or why octane matters. Some may even think that selling regular to premium gas, because of their lower to higher price points, is a method of selling “OK gas” to “fancy gas.” In fact, the different grades are related to the types of vehicle engines that require a different level of octane in gasoline.

In May 2016, the Fuels Institute asked consumers, “What is an octane grade as it relates to gasoline?” Most respondents did not know the answer, and only 2% accurately identified that octane referred to the anti-knock properties of gasoline. Consumer awareness of octane, however, can be the difference between a well-maintained vehicle engine or costly engine damage.

Octane and Engines

Depending on a vehicle's engine design, octane plays a key role in engine performance and measures the ability to resist auto-ignition, which is commonly referred to as knock.

As engine compression increases, so does the amount of power and efficiency generated by a given amount of fuel. The higher the octane, the greater the fuel can withstand compression and resist knock.

A gasoline internal combustion engine compresses an air-fuel mixture in its cylinders, thereby raising the mixtures’ temperature and pressure. The air-fuel mixture is ignited with a spark during compression, and the resulting combustion releases heat energy that ultimately powers the vehicle. Knock can occur at a sufficiently high temperature (a consequence of compression) in the engine cylinders. Long-term, knocking reduces a vehicle’s fuel economy, robs the engine of power and causes engine damage.

History of Octane

In the United States, refineries have used a variety of methods to increase octane levels in gasoline.

Prior to 1974, the primary and least expensive source of incremental octane was the additive tetra-ethyl lead (TEL, or lead). Between 1960-1980, regular- and premium-grade gasoline had high octane ratings. However, air- and soil-borne lead caused by vehicle emissions presented a significant risk to public health and the environment. Consequently, unleaded fuel was introduced in 1975, and all new vehicles as of that model year were required to operate on unleaded fuel only. The removal of lead resulted in a slight decline in the average octane rating of the U.S. gasoline pool.

After the removal of lead, refiners began adding methyl tertiary butyl ether (MTBE) to gasoline, but as concerns about the environmental implications of MTBE increased, refiners removed MTBE and switched to blending fuel with ethanol to boost octane levels.

Today, nearly all gasoline sold in the U.S. is blended with 10% ethanol (E10), and more retailers are selling E15 (15% ethanol, 85% gasoline), a higher octane fuel that is available in 30 states. E15 was approved for use in model year 2001 and newer cars, light-duty trucks, medium-duty passenger vehicles (SUVs), and all flex-fuel vehicles by the U.S. Environmental Protection Agency in 2012. These vehicles make up more than 80% of the cars, trucks and SUVs on the road today.

Bringing higher octane fuels to market is a complex and timely process that has garnered increasing attention from a diverse group of policymakers, stakeholders and industries, including the U.S. Congress.

On April 13, 2018, the U.S. House Energy & Commerce Subcommittee on the Environment held a hearing, “High Octane Fuels and High Efficiency Vehicles: Challenges and Opportunities,” to examine the potential for high octane fuels. During the hearing, General Motors communicated its support for making 95 RON the minimum standard for gasoline in the United States to increase fuel economy and reduce greenhouse gas emissions.

The U.S. Department of Energy, through its Co-Optima initiative, is exploring opportunities to maximize performance and energy efficiency for both transportation fuels and vehicles, which includes research on development related to higher-octane fuels.