Octane is Not What You May Think it Is

Recently there was published a piece that identified octane as the quality of gas that keeps it from self-combusting. It's obvious from the entire piece that the author didn't have a basic understanding of octane.

Octane is simply the point at which spark knock occurs. The higher the octane, the higher the point at which knock occurs. Spark knock is the marble rattling around sound that you hear in a badly mistuned car or a vehicle that is using a gasoline with an octane level that's too low.

The sparkplug is firing the fuel/air mix before top dead center, when it should be fired for maximum compression and an even flame front and it's possible that the piston crowns are being damaged.

There are actually three types of octane that are considered in any measurement. The one we are used to at the gas pump is known as the road octane rating. It is simply the Motor Octane Rating plus the Research Octane Rating divided by two. That's it.

Most cars today are equipped with a piezoelectric sensor that detects knock by actually listening for it. The sensor is a very sensitive microphone and if it heard knock occurring, it adjusts the timing to lean out the fuel/air mix, in most cases. In this way, your 2006 Honda Civic can run on 87 Octane regular for year after year, or until the sensor goes and has to be replaced, not exactly an inexpensive job, but one which may be covered by an extended warranty.

Looking further at octane, you'll find that the Motor Octane Rating is determined by a specially tuned engine and is a ratio of octane versus n-heptane. This rating tends to be very low - around 300 rpm.

The Research Octane Number is run on the same engine, but at higher engine RPM and is usually around 1600 rpm. It is also a ratio of octane versus n-heptane, according various gasoline manufacturers.

To get to the number we need to run out car, we simply use the ratio of MON+RON/2 and that's the Octane Rating for the gasoline. In some cases, you'll find that the octane rating of fuel can be over 100. Such fuels as aviation gasoline or alcohol-based fuels such as ethanol and methanol have higher octane ratings. In other words, they are less prone to spark knock than standard gasolines.

Indeed, ethanol or methanol is added to gasoline today not only to stretch the supplies that we have but also to raise its octane level.

Octane has nothing to do with the specific power output of a car. It does have to do with making sure that your car will run correctly without spitting its pistons out the block or having problems with your input or output valves.

There are cars on the market that require high octane fuels because they run with a higher fuel: air ratio. This is about the only case where you'll find that there's any causal relationship between octane and performance and only insofar as knock is occurring.

One thing to remember in all of this is that we are dealing with a spark-driven engine based loosely on the Otto cycle. The Otto cycle engine is a four-cycle engine with intake, compression, firing and exhaust. The sparkplug fires the fuel: air mix.

There is one type of engine that depends on self-combustion, the diesel engine that operates at far higher pressures and tolerances than a standard engine. Its fuel is also much thicker and quite different that gasoline and if you try using gasoline in a diesel, you'll be opening a can of worms that should best be left shut, unless you have a spare $4,000 or so handy for a new engine.

Just remember that octane is a measure of a gasolines ability to suppress spark knock and no more. It doesn't give you extra power or anything else. It just tells you that the gasoline you are putting in the tank is right.