Octane crucial to engine health
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caption:What you put into your car can help make it run better and more efficiently, and sometimes, that can even mean spending a bit more to save money in the long run.
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Pay attention to the recommendations for your vehicle's fuel, or you'll pay more later
Special to the Star
(1)
May 04, 2009
Regular, mid-grade, premium – there's a lot more to pulling up at the pumps than just figuring out what side the filler neck is on.
Don't look solely at the price per litre. What you put into your car can help make it run better and more efficiently.
Sometimes that can mean spending a bit more to save money in the long run.
First, some background. Engines usually contain anywhere from four to eight cylinders that are fitted with pistons.
These pistons move up and down, turning a central shaft , and the shaft's spinning eventually turns the vehicle's wheels. It's similar to how your legs power a bicycle.
The energy to move the pistons is provided by gasoline, which is ignited by the spark plug. The four-part sequence must be precisely timed if the engine is to run properly: valves open to let fuel in, the pistons compress the mixture, the spark plug ignites the fuel when the piston is at a precise spot in its travel and, after combustion, other valves let the exhaust out.
Peak fuel efficiency is achieved when the engine uses as much power as possible from each spray of gasoline. And in all of that the fuel grade used can play a part.
Gasoline is graded by its octane content.
"Octane is the specific name of a molecule, but the word also refers to an ignition rating for gasoline," says David MacMillan, manager of fuels quality for Suncor Energy Products.
"There are different grades because auto manufacturers make different engines and some have different octane requirements."
The air/fuel mixture is compressed before it's ignited and the more it can be compressed, the more power it can provide.
The problem is that compression makes heat, which can cause the gasoline to ignite before the spark plug fires – and before the piston is where it's supposed to be. This "pre-ignition" wastes power and can harm the engine.
Octane doesn't mind waiting for the spark plug, which is why higher-octane grades are necessary for high-compression engines, such as those in sports cars.
"Octane is not an additive," MacMillan says.
"In a refinery, you produce various streams that have higher- or lower-octane ratings. It's more expensive because when you get into the higher-octane components it's a more complex and, therefore, more costly process. Octane just doesn't come for free."
The numbers on the pump – 87, 89, 91 and sometimes 94 – are obtained by averaging the results of two different quality-control tests.
Because pre-ignition is also called engine knock, due to the sound it makes, the rating is sometimes called an "anti-knock index."
Engine knock is a more serious problem in older cars, which lack computerized sensors; it can cause severe damage.
"Today, the vast majority of cars have knock sensors," says Ken Mitchell, fuels engineer for Shell Canada.
"It's a safety feature to prevent engine damage. You'll get reduced power and fuel efficiency, and if it's too low an octane, it will affect the ability to accelerate. Typically, if the car recommends premium, you're going to get optimal performance on premium grade, both in fuel economy and acceleration."
Gasoline also contains detergents, which clean away engine deposits, and can contain up to 10 per cent ethanol, a plant-based fuel. That's safe for all engines.
But if you should find the rare station selling E85 – 85 per cent ethanol, 15 per cent gasoline – it must only be used in an engine rated for "flexible fuel."
So what should you choose? The experts say to consult your owner's manual. If it recommends 87, you can give your wallet a rest.
"Premium recommended" means it'll run on regular, but will give maximum power and fuel efficiency with a higher grade, while "premium required" means what it says.
Engines can require more octane as they age, so if you notice your performance or mileage slipping, try moving up a grade.
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