Rick's Free Auto Repair Advice

What is Octane?

Octane and gasoline

Octane eliminates detonation.

What is detonation?

Let’s review the combustion process in an engine. Air and fuel are combined during the intake stroke. Then the air/fuel mixture is compressed during the compression stroke. That creates heat, not quite enough heat to start the mixture on fire though. As the piston reaches the top of the compression stroke, the air/fuel mixture is ignited by the spark plug. As the air/fuel mixture burns, a flame front expands away from the ignition point near the spark plug, expanding pressure in the combustion chamber increases dramatically.

The air/fuel mixture was already compressed during the compression stroke, so the rapid pressure rise caused by combustion raises the temperature of the uncombusted air/fuel mixture to the point where it self-ignites. causing multiple flame fronts in the combustion chamber. When those multiple flame fronts collide, they create audible shock waves known as detonation or “knock”.

Detonation robs the engine of power, and if allowed to continue, can destroy the engine.

Octane prevents the high-pressure unburned air/fuel mixture from self-igniting. So it prevents detonation and knock.

What is an octane rating

A fuel’s octane rating is a numerical index of the fuel’s ability to resists detonation. It is not an indication of power. A higher octane fuel doesn’t provide any more power or cleaning capabilities to a fuel. A higher octane rating is only an indication of its ability to resist detonation.

Fun combustion facts about detonation

When an engine fires the spark plug, the combustion begins, the flame front moves smoothly across the combustion chamber, traveling at 45 to 90-MPH. Late-model engines with high-swirl combustion chamber designs may have faster airflow and thus faster flame spread. However, when you use regular-grade (87-octane) gas in a high compression engine that was designed for 91 or 93-octane, detonation occurs. The combustion gasses in that scenario can reach a speed of over 700-MPH!

That kind of rapid expansion increases combustion temperatures and can damage pistons, gaskets, valves, and cylinder heads. The metallic knock you hear is the force of the combustion slamming against the piston while the piston is still moving upwards on its compression stroke.

What is pre-ignition?

preignition is caused by hot carbon deposits in the engine that ignite the air/fuel mixture before the spark plug fires. The result is the same as detonation–it causes multiple flame fronts; one from the hot carbon and one from the spark plug.

Will a higher octane gas provide better MPG?

If your engine is in good condition, with no carbon deposits or knocking, using a higher octane gas than the carmaker recommends with NOT get you better MPG or more power.

If your car requires premium and you fill with premium, you’ll get the performance the car was rated for.

High octane or “premium gasoline” is one of the most misunderstood fuels. So I’ll explain the myths and legends surrounding it.

What is premium gas?

Premium is usually listed on the pump as 91 or 93-octane. Whoever coined the phrase “premium” was a marketing genius because it makes you think you’re buying a higher quality product—a better gasoline, or gas that’s more powerful. It’s neither. It just has a higher octane rating. Period.

Understanding the AntiKnock Index (AKI)

You get optimum engine performance by filling your vehicle with gasoline that ignites only from the ignition spark and spreads in a predictable way. You want to avoid compression-induced and combustion-chamber deposit-induced ignition and out of control flame spread that result in knock.

Refiners follow two testing methods to determine a fuel’s anti-knock rating. These are referred to as the Research Octane Number (RON) and the Motor Octane Number (MON). Refiners determine the RON and MON results, add them together, and divide by two to get the AKI. The RON number affects knock that occurs at low to medium speeds, and engine run-on after shutting off the engine. MON affects knock at higher speeds and part throttle conditions. If the MON rating is too low, you’ll experience knock during power acceleration during passing or climbing hills.

When a carmaker specifies a particular “octane rating” they’re talking about RON+MON÷2. So a car equipped with a low compression engine and a high swirl combustion chamber design may specify a “regular” fuel rating of 87, while a different high compression engine may require a fuel rating of 93 to prevent knock.

But the octane rating has NOTHING to do with the fuel’s overall energy content or its performance. In other words, using a higher octane gas in an engine designed to run on regular gas will NOT get you more performance. You may however, see some slight MPG differences between two different gasoline brands with the same octane rating. That’s because each refiner’s recipe differs slightly to attain their gasoline’s octane rating, and some of those chemicals may result in a slightly higher btu content. The difference in btu content from one refiner’s 87-octane gasoline to another 87-octane gasoline is small and may account for an MPG increase of 1-MPG at max.

gas pump signage telling the octane rating of each fuel



Premium gas MYTHS

• Premium gas doesn’t provide more power than regular. The available energy may vary slightly from brand to brand due to the brand’s proprietary gasoline formula. But within the brand, regular and premium have the same energy.

• Premium gas doesn’t have better additives or better detergents. Adding premium will NOT clean out your engine better than regular.

• Premium gas ISN’T always ethanol-free unless you buy it from a station that specializes in selling non-oxygenated gas. Over 95% of all gas (regular and premium) sold in the U.S. contains 10% ethanol.

•Premium gas burns slower—nope.

• Premium makes it harder to start when used in an engine designed for regular—nope. A lower compression engine doesn’t produce enough heat during the compression stroke to ignite premium gas. But the spark always provides enough energy to ignite both regular and premium gas.

What is gasoline

Gasoline is not a single substance. It’s made of over 150 flammable hydrocarbon liquids including pentane, hexane, heptane—all refined from crude oil. In addition, all gasoline contains anti-knock additives like Benzene, xylene, and toluene and alcohols: ethanol, methanol, tertiary butyl alcohol (TBA). In winter blends the refiner may also add propane and butane. However, they reduce those two additives in summer because they contribute to hot-weather driveability problems.

Gasoline also contains other additives like;

Detergents to eliminate or remove system deposits

Anti-icers to prevent fuel line freeze up

Fluidizer oils used along with other deposit control additives to control intake valve deposits

Corrosion inhibitors to minimize fuel system corrosion

Anti-oxidants to reduce gum formation in stored gasoline

Metal deactivators to reduce the effects of metal-based components that might occur in gasoline

Lead replacement additives to reduce valve seat wear

Additives amount to a very small percentage of the total product. In fact, refiners add as little as 100-lbs of deposit control additive to treat 20,000 gallons of gas.

Can any car get better MPG with premium?

No. In fact, the EPA mandates that engines certified to run on regular CANNOT show a fuel economy increase of more than 3% by switching to premium. That ruling is designed to prevent car makers from setting factory spark timing too late so that adding premium results in advanced spark timing. In car makers were allowed to alter their software this way, it would create an unnecessary high demand for premium gas.


gas pump sticker showing 93 octane

©, 2015 Rick Muscoplat




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