Pump gas explained 87 89 92… What does it all mean?
To fully understand the octane ratings, it is important to first understand what is really going on inside your motorcycle’s engine.
The common misperception is that the fuel air mixture “explodes” driving the piston down and the motorcycle forward. Not true. The more accurate and familiar description of the process of combustion is: “The fuel/air mixture is ignited causing rapid expansion of the gasses forcing the piston down.” This description is closer but still doesn’t really describe what is going on in there very well.
That description still implies that the burning of the fuel (gasoline, diesel, alcohol, etc), is responsible for forcing the piston down, thereby creating power. This is also not true. The gasoline you feed your engine has only one job to do and it does not involve moving the piston. The gasoline is burned to heat the air.
It’s time for the science overload.
In gases (think oxygen not gasoline), there is a direct relationship between volume and temperature. Charles Law states: "For a fixed mass of gas, at a constant pressure, the volume (V) is directly proportional to the absolute temperature (T)." In other words if you change one (temperature) the other (pressure) is also changed (or vise versa).
If you think about a running engine where the piston has just reached top dead center, there is a fixed mass of gas (air) trapped in that space. By burning fuel inside the engine that trapped air is heated. When the air is heated its volume increases, and the pressure inside that space rises (dramatically.) This increase in pressure due to the expansion of the heated air is what forces the piston down.
You may be wondering what that has to do with gasoline and octane ratings; a little more science and a definition is necessary yet, but hang in there its coming.
Because of the temperature / volume / pressure relationship when the air is compressed (its volume reduced and pressure raised) during the compression stroke the temperature rises significantly. The high temperatures created by the compression stroke can cause the fuel / air mixture to ignite (as in a diesel engine) before the spark-plug fires. This ignition happens at the point or points of highest temperature and the flame front that is created by this premature ignition races across the cylinder and collides with the flame front created by the spark plug firing (or those created by other hot spots). This collision is what causes the noise commonly called pinging or knocking. This collision produces shock waves that can, over time, cause significant damage to your motorcycle’s engine, melting spark plugs and pitting cylinder heads and piston crowns.
A fuels ability to resist detonation is called its octane rating. The higher the octane rating the more resistant a fuel is to igniting from the temperature increase caused be the compression stroke.
If you have stuck with the science lesson you will most likely realize that a motorcycle with a high compression engine is more likely to suffer from detonation than a motorcycle with a low compression engine. However there are also numerous other factors that can contribute to detonation including high ambient temperatures, heavy engine loads and lean mixtures.
Back on the gas
When crude oil is refined hydrocarbon chains are produced. These chains are named after the number of carbon atoms contained in the chain. The two that are important to our discussion are heptane (seven carbon atoms) and octane (eight carbon atoms). Heptane is easily ignited by compression, octane on the other hand is very resistant to compression ignition.
A fuels octane rating is its ability to resist detonation relative to a mixture of octane and heptane. For example an 87 octane fuel performs like an 87/13 mixture of octane and heptane. Keep in mind that gasoline is not actually made of these chemicals in this ratio merely that it should act like it is when it is compressed.
To produce a batch of gasoline that has the desired octane rating there are several things that can be added to gasoline to increase its resistance to detonation. One of those potential additives is ethanol. Ethanol has a research octane rating (more on this in the side bar) of 129. By mixing ethanol with gasoline it is possible to raise the octane rating. The down side to this approach is that ethanol is not as energy dense as gasoline so you need to burn more to get the same amount of heat from it.
By adding ethanol the octane rating is increased but it will take more fuel to produce the same amount of power because burning ethanol does not produce as much heat as the same amount of gasoline. More fuel will have to be introduced to get the same temperature increase inside the engine. In other words gasoline with ethanol added will reduce your fuel economy.
Ethanol can be harmful to rubber seals and is also very hydroscopic the former of which is of particular concern in a two stroke engine where the fuel-air mixture passes through the crankcase and comes in contact with the crank seals, and the later can also be a concern if you store your fuel for extended periods as it will attract moisture.
Not so Super
There is a common, and intentionally created misconception that higher octane fuels are higher quality fuels.
If premium gasoline simply has a higher octane rating than “regular” gasoline why is it called “Premium?” Marketing.
It is a mistake to associate a higher octane rating with higher quality (rememebr we are talking pump gas here and not race fuels.) A higher octane rating does not indicate that the fuel is any higher in quality only that is resists detonation better. And depending on how those higher octane ratings are achieved you may actually get worse gas mileage by using it.
What about all those additives with the cool names? Also just marketing.
According to the Federal Trade Commission: “As a rule, high octane gasoline does not outperform regular octane in preventing engine deposits from forming, in removing them, or in cleaning your car's engine. In fact, the U.S. Environmental Protection Agency requires that all octane grades of all brands of gasoline contain engine cleaning detergent additives to protect against the build-up of harmful levels of engine deposits during the expected life of your engine.”
