What's So Premium About Premium Gasoline?

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They must call it premium for a reason, right? Let's find out if we're missing out on something big by not shelling out the extra 20 cents.

Knock knock. Who's there?
While you're cruising around town, your car's engine is hard at work, repeating its four-stroke cycle. The piston drops from the top of the cylinder, which fills with a mix of gasoline and air (intake stroke). The piston moves up, compressing the fuel mix (compression stroke), and the spark plug ignites the mix, pushing the piston down (power stroke) again so it can expel what's left of the spent fuel through the exhaust valves and start the process over again (exhaust stroke).

Normally, this cycle keeps repeating without a hitch. But sometimes the mix of gasoline and air loses its patience, and it ignites on its own under compression instead waiting for the spark plug. This is called "pre-ignition," but you may know it as "knocking." The results are actual pinging or knocking sounds, as well as mechanical stress on the engine as the premature ignition pushes downward on the piston before it's completed its stroke. In extreme cases, pre-ignition can burn holes in engine parts.

All in the Numbers

Gasoline is a grab bag of hydrocarbon molecules, and each one behaves differently under pressure. During the auto industry's early days, there was no way to know if a given gasoline would knock in a given engine except for filling up the tank and going for a spin. In 1927, Dr. Graham Edgar of the Ethyl Gasoline Corporation (then a division of General Motors and Standard Oil) suggested using the ratio of two hydrocarbons "“ heptane and iso-octane, which have similar volatility properties and could be produced in sufficient quantity "“ as a reference number for computing a fuel's ability to resist knock.

iStock_000006429094XSmall-gaspumps.jpgA range of test engines and test conditions have popped up over time, but today the most common are the Research Octane Number (RON, which comes from running the fuel in a test engine* and represents typical mild driving), and the Motor Octane Number (MON, which comes from running preheated fuel in a similar test engine at a higher engine speed and with variable ignition timing to represent sustained high speed, high load driving).

What we get from these tests are the numbers you see on the yellow stickers on a gas pump. These numbers, called the octane rating or anti-knock index, are the average of the two different test methods (hence the pump label (RON + MON)/2) and are the measure of the fuels' resistance to knock. If you're pumping "regular," for example, its octane rating of 87 (this varies from state to state; in higher altitude areas, "regular" is sometimes 85) means the gasoline has the same knocking properties as a mixture of 13% heptane and 87% iso-octane.

So what makes premium so great?
What's behind its higher octane rating? It won't make your car faster, give you better gas mileage or make your teeth whiter.

A higher octane rating correlates to higher activation energy, meaning that the fuel needs more energy to start a chemical reaction and is less likely to pre-ignite under compression. That's right. It's simply more resistant to knock.

Modern engines are designed with specific compression ratios "“ the ratio of the combustion chamber's volume, from its largest capacity to its smallest capacity "“ and high-performance engines usually have high compression ratios (higher compression = more power). Premium's anti-knock properties allow it to maintain grace under all that pressure.

Even if you do drive a high-performance car, regular gas isn't going to knock like a woodpecker. Most cars today have knock sensors and engine management systems that use auditory detection to actually "hear" knocking and retard the spark timing to avoid detonation and minimize knocking. If you don't have a high-performance ride, using premium if you don't need it (your manual will tell you which octane rating you should use) just gets rid of more unburned fuel during the engine's exhaust stroke, putting unnecessary stress on the emissions system and sometimes producing a rotten egg smell.

*For the curious, the test engine is a Cooperative Fuels Research (CFR) engine. It's a single-cylinder engine with a variable compression ratio and a special four-bowl carburetor that can adjust individual bowl air-fuel ratios. The only company that makes them is the Waukesha Engine Division of Dresser Industries in Waukesha, Wisconsin, and the complete Octane Rating System package costs $200,000.

If you've got a burning question that you'd like to see answered here, shoot me an email at flossymatt (at) gmail.com. Twitter users can also make nice with me and ask me questions there. Be sure to give me your name and location (and a link, if you want) so I can give you a little shout out.

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February 12, 2009 - 7:09am
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