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What Kind Of Fuel Should You Put In Your Car?

What Kind Of Fuel Should You Put In Your Car? - General Mechanical Advice

We’ve all been there. Pulling up to the pumps, you pretty much know what grade of petrol or diesel you should be putting in your car’s fuel tank. Yet, there’s always the temptation to be rebellious. You may have wanted to fill your tank with premium fuel in the hopes of getting better performance from your Honda Civic; or fill your BMW up with regular fuel to save a bit of money. Is this a good idea?

The short answer? No. Use the fuel recommended by your owner’s manual. In order to explain why, let’s take a look at what octane actually does.

An octane molecule. The light grey atoms are hydrogen, and the dark grey atoms are carbon.
An octane molecule. The light grey atoms are hydrogen, and the dark grey atoms are carbon.

Octane is what’s known as a hydrocarbon, a molecule made up of carbon and hydrogen atoms. When combined with oxygen molecules and a source of ignition, hydrocarbons create a special chemical reaction called combustion. In other words, octane is the chemical in petrol that makes it explosive.

The number that you see on the gas pump is the octane rating. The octane rating is a measurement of the fuel’s ability to withstand premature detonation due to compression inside the combustion chamber. Contrary to popular belief, it is not a measure of the amount of octane in the fuel. Rather, the octane rating is calibrated according to the knocking resistance of isooctane (pure octane) and heptane. Heptane has an octane rating of 0, while pure octane has a rating of 100. The octane ratings of commonly available petrol vary from region to region. Where I live, 87-octane gasoline is considered “regular”, and most “premium” gasoline is 91-octane.

What Kind Of Fuel Should You Put In Your Car? - General Mechanical Advice

It’s important to know the recommended octane rating for your vehicle. This is because engines are designed and engineered differently, and these differences may require the vehicle to run on a higher-octane fuel. Most commonly, especially in European vehicles, engines with high compression ratios require petrol with a higher octane rating. With higher compression ratios, the risk of engine knocking increases. Knocking occurs when the fuel-air mixture ignites when it isn’t supposed to. In a four-stroke engine, this happens when the mixture ignites (due to compression) before the power stroke. This can result in a distinct pinging or knocking sound coming from the engine, and can potentially cause serious damage to the engine over time.

Engine knock can occur for many different reasons, but compression issues are one of the most common causes. Fuel quality can affect the timing of the combustion cycle, especially when too low-octane fuel is being burned. This results in a loss of power, a loss of efficiency, and can result in engine knocking.

What Kind Of Fuel Should You Put In Your Car? - General Mechanical Advice

Back to the original question: what happens when you use the wrong grade of petrol in your car? The answer to that depends on the age of your vehicle. Newer engines, especially ones with variable valve timing, have knock sensors able to adjust to achieve the optimal performance characteristics given the type of fuel that is being burned. Older engines without such technology simply won’t run properly, and are more likely to experience knock.

Generally speaking, if your car was designed to run on premium fuel, put premium fuel in the tank. This is especially true of older engines that are more susceptible to knocking. Some newer engines will come with an octane rating that says something to the effect of “Premium fuel recommended for optimum performance”. This could mean that your engine may still be able to run on regular fuel without risking engine damage, although the power output and overall engine performance will decrease. However, it’s still best to use premium fuel.

But what happens when you use premium in a car that’s designed to run on regular petrol? The answer: probably not much. Since premium fuel only reduces the likelihood of engine knock, running it in an engine that’s already optimized for regular fuel shouldn’t provide any real improvement. Having said that, it won’t hurt your engine, but it will hurt your wallet.

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