A closer look at fuel octane

Every week, we take your questions about what is going on under the hood of your vehicle and pose them to a knowledgeable mechanic in the Greater Toronto Area. With a number of readers commenting on our March 6 column, where it was erroneously reported using the wrong octane could clog your filter (it actually causes something called “knock.” More on that in a minute), we decided to take a closer look at what octane is and how it works.

Simply put, octane is an additive that’s put into gasoline said Robert Lirette, a professor of automotive technology at Centennial College in Toronto. When you go to the gas station, there are typically three octane options to choose from. Each type is represented by a number, which correspondence with the rate of octane, made up of chemical compounds, in the fuel. This number measures how much pressure the fuel can withstand before it explodes.

A vehicle’s engine has cylinders that house pistons, a valve that moves up and down. “As the piston in the cylinder moves down, it draws in the fuel and air. As it moves up, it compresses that air fuel mixture and creates heat and pressure in that cylinder,” Lirette said.

If there’s too much pressure in the cylinder, the fuel will pre-ignite as the piston is moving up. This will lead the engine to knock and can cause damage to the vehicle. Knock is very literal: It sounds like a knock on a door.

When this occurs, “It’s going to destroy the piston right away,” said Wilson Almeida, the director of training at Vast-Auto Distribution. He said it can lead to a number of mechanical problems and can cause your engine to fail. “This forced explosion was not supposed to [happen] and it can damage the internal combustion engine,” he said.

The type of gas used in a vehicle depends on the compression ratio of the engine, Lirette added. Every engine has a specific ratio comprised of the volume of the cylinder when the piston is at the bottom versus the volume of the cylinder when the piston is at the top.

Most vehicles on the road have a standard engine and a smaller compression ratio compared to luxury vehicles. If the ratio is higher, the heat is building up in the cylinder faster and you need a higher-octane gas to withstand the pressure.

This brings us back to our reader question from March 6: Could they save some money by using a lower-octane fuel in their luxury vehicle? “If you’re driving a high-end car or a car that’s turbocharged, and you’re putting low octane gas in it, you could potentially have a knock condition. In some cases, it would void your warranty if you caused damage,” Lirette said.

If you drive a luxury vehicle and are tight on money, fill your tank a quarter way with fuel one octane grade lower and listen to how your car reacts, Lirette said. He also warned that if an issue does occur, and you go the dealer to get it fixed, your warranty may not cover the damages if it is discovered you used a lower octane. If you’re confused by what gas to use, Lirette recommended either looking at your owner’s manual or your gas cap or door. It will list the ideal fuel to use in your vehicle.