Your Questions Answered: Fuel and octane ratings

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A nutritious diet is vital for good health, and can help protect us humans from problems and diseases. The same is true for your car, which requires a high-quality diet to keep its various systems and parts healthy and reliable for years to come.

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In addition to the use of appropriate engine oil, coolant, and other fluids, the fuel your car drinks has a role to play in its long-term health — as well as the long-term health of your bank account.

Below, let’s dive into some important facts about gasoline and octane, and answer some popular questions to help you make sure you’re feeding your car the healthiest diet possible, without wasting any money.

To help answer some of the internet’s most popular questions about fuel and octane, I reached out to the American Chemical Society (ACS) to connect with some experts on the topic. Below, we’ll be hearing from three fuel and octane experts to help guide our way through the rest of this deep dive.

But first, we’ll need to understand a few basics.

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How does a gasoline engine work?

A gasoline engine is powered by a continuous series of small and precisely controlled explosions that take place within its combustion chambers. The explosions are created when gasoline is ignited by the spark plug, generating heat and force that the engine uses to drive the car.

“The explosion of gasoline in an engine, like a batter in baseball, has to wait for the right moment to get the best result” says Dr. Mark Jones, an expert on octane additives, and a chemistry consultant with experience in hydrocarbon and petroleum research.

“Swinging early or late will give a poor result, a miss or a foul. Home runs come from swinging at the right time and maximizing the energy transfer.”

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If gasoline ignites within your engine before the spark plug intends it to, extremely harmful engine knock (or detonation) can be the result.

Since your engine runs on a stream of rapid and continuous explosions, precise timing is incredibly critical. In fact, it’s so important that both automakers and fuel manufacturers go to great lengths to make sure that modern engines and modern fuel play nice together, in order to keep that all-important explosion timing in check.

What is engine knock?

“Compressing a gas, like a fuel-air mixture, causes it to heat up. Sometimes, the heating is enough to cause an explosion before the spark plug ignites it. Like a batter swinging too early at a pitch, a premature release of energy isn’t what you want,” explains Dr. Jones.

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Detonation, or knock, is basically an ill-timed explosion within a rapidly-spinning engine that can cause immediate, serious, and even fatal damage.

“To understand this, you have to envision a cylinder in a gasoline engine” adds Dr. Frankie Wood-Black, the Division Chair for Engineering, Physical Science and Process Technology Division at Northern Oklahoma College, and a petroleum and chemical industry expert.

“As part of the stroke, the cylinder chamber is filled with an air/gasoline mixture as the piston is travelling down. As the piston rises it will compress the mixture, and at a pre-set time, the spark plug will ignite the mixture, causing the rapid push on the piston, which is what ultimately propels the car. As the stroke continues, the piston pushes out the exhaust gases, and the cycle repeats.”

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“In order to ensure that the engine doesn’t knock, the air-gasoline mixture needs to have a ‘steady’ push. If it burns too rapidly, you essentially ‘punch’ the piston downward, causing the ‘knock,’ with the timing off.”

What is octane?

“Higher-performance engines operate at higher pressure” says Dr. Jones.

“They are said to be high-compression. Higher compression makes premature explosions more likely, but chemistry can help. The right additives can moderate the tendency of the fuel to pre-ignite. The octane rating is a measure of the fuel’s propensity to pre-ignite and cause knock. The higher the octane, the less likely it is to cause knock.”

This is why filling your car’s gas tank usually involves selecting from one of several gasoline options, each with a different numerical octane rating, and corresponding increase in price.

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In simplified terms, fuel with a higher octane rating is designed for use in certain types of engines, since it offers better resistance to performance-robbing premature explosions. Usually, higher performing engines require higher octane levels to achieve peak performance.

In a nutshell, the difference between fuel grades largely comes down to their octane or Anti-Knock Index (AKI) rating — a standardized measure of that fuel’s ability to resist potentially damaging conditions.

“There is a difference between octane and an octane rating” Dr. Wood-Black says.

“Octane, by itself, is a hydrocarbon molecule with 8 carbons and 18 hydrogens. The Octane Rating, the rating you see on the pump at your local fuelling station, is based on a standard measurement to help the consumer predict how the fuel will respond in a vehicle’s engine. If you go back in time, this was referred to as a “knock” rating. The octane of a fuel is tested as part of the overall quality control of the gasoline”.

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“There are two measurements: research octane (RON), and motor octane (MON). The octane rating that you see on the pump is an average of these two different ratings. The measurements are made on specially designed ‘knock’ engines to determine the ratings.”

How is gasoline made?

Dr. Bill Carroll is an Adjunct Professor in the Department of Chemistry, Indiana University Bloomington, and an expert on octane additives and petrochemical industry processes. According to Dr. Carroll, gasoline comes from separating components out of crude oil.

“A barrel of oil is a mix of chemical compounds, kind of like a bowl of mixed candy. About 10 per cent of the volume of crude oil would qualify as ‘gasoline’ if you just separated the right molecules out.”

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Imagine a bowl of assorted candies, some small, some large, some round, and some flat. This assortment of candies (or molecules) makes up crude oil.

The crude oil is subjected to a process called distillation, which separates those molecules from one another by boiling point. After distillation, each of the molecules are separated by type — not unlike sorting through the bowl of individual candies to arrange each type into their own separate little pile.

“The biggest of these molecules are too big to boil, and they qualify as asphalt if you use them as-is,” Dr. Carroll adds. “But a chemical process called ‘catalytic cracking’ or ‘reforming’ can reduce their size and make them just the right size and shape to be gasoline. That’s what a refinery does. Finally, before it reaches your tank, gasoline can get other additives that help it burn just right in an engine. This is what the researchers invent and the oil companies and automobile companies optimize so you don’t have to.”

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Which octane should I use?

Modern gasoline engines vary widely from one another, even though they all work in more or less the same way. Some high-performance engines require strict use of a high-octane fuel at every fill, while other modern turbocharged engines are designed to deliver optimal power on regular 87-octane gasoline.

Some engines sit somewhere in the middle, delivering top power on high-octane gas, but happy to run with lower power output on regular grade fuel if owners desire.

“An octane rating that is too low will cause the engine to knock and ultimately damage the engine. Similarly, an octane that is too high may also result in engine damage” says Dr. Wood-Black.

“As automobiles have evolved and have become more electronically tuned, it is recommended that you use the octane designated in your owner’s manual”.