Under the Hood: What is Octane?

Craig Cole
by Craig Cole

Anyone that’s ever topped off their tank or filled up a jerry can knows gasoline comes in different grades, from regular unleaded to pricey premium. Each “flavor” has a number that corresponds to its octane level, but what does this mean? What is octane and what role does it play? AutoGuide reached out to experts in the fuels field for answers.

To understand the function of octane, which is basically a measure of the chemical resistance of gasoline to auto ignition – the higher the number the less likely it is to explode under pressure – you first have to know the basics of how a gasoline internal-combustion engine functions. If you’re already familiar with pistons, valves and spark plugs feel free to skip the next three paragraphs, if not, please continue reading for a quick overview.

The engines found in almost all vehicles today function on something known as a four-stroke cycle. Within each engine are pistons that move up and down inside cylinders. The pistons are attached to connecting rods, which spin on a crankshaft. With the help of a transmission, the force applied to these parts is what ultimately drives your Camry’s wheels and gets you to work in the morning.

What are the four strokes of this all-important process? Well, in order of operation they’re intake, compression, power and exhaust.

The cycle starts as a piston moves downward in a cylinder. This is called the intake stroke. As it drops, a precisely controlled mixture of air and fuel vapor is drawn into the cylinder through open intake valves. Next, the intake valves shut and the piston rises, beginning the compression stroke where the air-fuel mixture is squeezed. Just as the piston reaches the top of its travel the spark plug fires, which ignites the air and fuel resulting in a potent bang. The power stroke has begun. This pushes the piston down again and provides the driving force that keeps the engine running and the vehicle moving. No sooner does the piston reach the bottom of its travel than it’s time to go up again. This begins the exhaust stroke, where all of the hot, leftover gasses from the combustion process get pushed out of the now-open exhaust valves. Once the piston reaches the top the intake valves open and the whole process starts again. Imagine how fast all of this happens when you’re merging on the highway!

Hopefully that makes sense; if you’re a gearhead welcome back. Now for the main event and the reason you’re here in the first place – the role octane plays in an engine.

Stephen Russ, Technical Leader with Ford Motor Company’s engine engineering department is a fuels expert. According to him, “octane is just a measure of the chemical resistance to auto ignition.” He also said “high-octane fuel is more resistant to auto ignition.”

So what does that have to do with anything? Well remember, during the compression stroke a piston is squeezing a combination of air and fuel. If this mixture gets put under too much pressure it will spontaneously ignite, which is a serious problem should it happen before the spark plug fires.

Auto-ignition, more commonly referred to as “spark knock,” can result in an audible pinging noise. This potentially destructive clatter sounds a bit like pennies rattling in an empty pop can.

According to Bill Studzinski, Fuels Technical Specialist at General Motors, auto ignition can lead to “pressure waves that run into each other that cause the ringing or the knocking that you hear.”

Beyond noise, spark knock can wreak havoc on internal engine components. Surprisingly it can melt holes in pistons and even bend connecting rods. Thankfully Russ said “it’s really, really rare nowadays” because of advanced engine-control computers.

“We have knock sensors on our vehicles,” Studzinski said. These are small electronic transducers attached to an engine’s block that listen for specific sound frequencies associated with spark knock. If it’s detected the powertrain control module can do a number of things to get combustion under control again. It can lower boost levels on forced-induction engines, pull spark timing or enrich the air-fuel mixture to prevent internal damage.

High compression ratios allow engines to create more power while burning less fuel. A compression ratio is basically how tightly the air and fuel mixture inside a cylinder gets squeezed. Modern engines typically have ratios right around 10-to-1, but they can exceed 12-to-1 or more if they have direct fuel injection. Also, forced-induction engines will be a little bit lower.

Automakers have to walk a fine line though; they can’t go overboard. This is where octane fits into the equation. Engines with higher compression ratios – often found in performance cars – almost always need higher octane fuel because it is less likely to automatically ignite. Russ said higher octane gasoline “does enable us to put more spark advance in and get more efficiency.”

To recap, higher cylinder pressures require higher octane, which prevents engine-damaging auto-ignition. But mistakes can happen, what if you put the wrong grade of fuel in your vehicle?

“If you’ve got a premium-required car and you put in 87 octane and you start hearing audible knock, you need to baby that car until you can get that fuel changed” Studzinski said, adding “even if you don’t hear the audible knock the vehicle is reacting.” It’s reducing performance, lowering fuel economy and sending more heat into the exhaust catalyst, which reduces its durability.

So, how is an octane rating calculated? According to Studzinski, in the United States the average of two separate tests is used to determine the number you see on a gas pump.

You’ve probably noticed the formula (R+M)/2 before. A lot of times it’s printed right on fuel pumps. The “R” stands for “research octane” and the “M” for “motor octane.” Each of these is one of the tests just mentioned. The average is designed to give an acceptable rating for all drivers in all different conditions. The fuel needs of someone towing a trailer up a mountain pass in the desert are quite different from someone driving a small passenger car at sea level.

According to Studzinski the research-octane test favors larger, older vehicles – cars and trucks typically powered by big-bore, naturally aspirated engines. It features higher in-cylinder temperatures than the motor-octane test, which is geared more toward small, efficient, turbocharged engines.

In America the average of two tests determines octane, but Studzinski said “we don’t have a harmonized global octane standard,” and “the rest of the world only uses research octane numbers.”

If you’ve ever been to Europe for instance you may have noticed something like “95 RON” on their fuel pumps. RON, of course, stands for “research octane number.”

Interestingly, Studzinski said Europe’s regular-grade gasoline, which is rated at 95 RON equates to about 90 in the (R+M)/2 test. That means European vehicles have to be recalibrated to run on America’s regular-grade fuel, which is only 87 octane.

One thing Studzinski asked was “is (R+M)/2 an appropriate rating for modern engines?” a question many fuels experts and oil refiners are no doubt asking themselves. For the average driver though, it’s best to follow what’s in their vehicle’s owner’s manual by running the recommended grade of gasoline for the best performance and highest efficiency.

Craig Cole
Craig Cole

Born and raised in metro Detroit, Craig was steeped in mechanics from childhood. He feels as much at home with a wrench or welding gun in his hand as he does behind the wheel or in front of a camera. Putting his Bachelor's Degree in Journalism to good use, he's always pumping out videos, reviews, and features for AutoGuide.com. When the workday is over, he can be found out driving his fully restored 1936 Ford V8 sedan. Craig has covered the automotive industry full time for more than 10 years and is a member of the Automotive Press Association (APA) and Midwest Automotive Media Association (MAMA).

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