Most of you know that your particular car’s engine is designed to operate with a specified minimum Octane Rating (it’s in the Owner’s manual!), but wonder why. The answer is all about engine knock.
Combustion Knock
The source of power in a gasoline engine is burning of a mixture of gasoline and air. Some might assume that this “burning” is an explosion, but it isn’t. It is an orderly and rapid burning along a flame-front. Imagine a round piece of paper that you start burning in the center with a match, there will be ring of flame that forms around the center that eventually consumes the entire circle. That process, greatly sped-up, is similar to what occurs in normal combustion. In knocking combustion, instead of the entire piece of paper being consumed by the traveling flame, some portion of the paper around its outer edge explodes, like if it was coated with a layer of gunpowder.
This happens in a real engine when the last bit of fuel to be consumed doesn’t wait for the flame and instead nearly spontaneously combusts. The reason why this happens near the end of the normal combustion process is complicated, but in simple terms it is because the temperature and pressure in the combustion chamber gets higher and higher during the combustion process and the last bit of fuel tends to approach its “auto-ignition” temperature. If auto-ignition of some of the fuel occurs, you hear it as a knocking noise because of how fast the auto-ignition explosion raises the pressure in the combustion chamber. This high pressure-rise can also damage the engine, because it wasn’t designed for it.
Knock and Octane
So what does knock have to do with Octane? The Octane rating of gasoline is proportional to how well the fuel resists auto-ignition. A high Octane fuel resists auto-ignition better than low-Octane, but still gets ignited just as easily by the spark plug, and burns just as fast and completely. So if you use a fuel with a lower Octane than your engine was designed for, you can expect to experience knock, and if you run higher Octane than the engine was designed and calibrated for, the engine will run just fine. The only downside is spending more money on gasoline than you really have to. Whether you actually have knock in daily driving depends on a number of factors besides Octane:
- Engine load, higher is worse
- Engine speed, lower is worse
- Engine temperature, higher is worse
- Air temperature, higher is worse
- Air humidity, lower is worse
- Barometric pressure (altitude), higher pressure is worse
- Combustion chamber deposits, more is worse
Modern Engines with Knock Detection
If you have a modern engine (last 10 – 15 years or so), your engine is equipped with a system to detect and prevent knock, no matter what fuel you use. If the system detects borderline knock, it immediately retards the ignition timing (from optimal), which is very effective at eliminating knock. There is a big downside to this: retarding ignition timing greatly reduces the efficiency of the engine and raises the exhaust temperature. At the very least, your fuel economy gets worse, The worst case is that the higher exhaust temps can cause premature failure of the catalytic converter, if you are running at high power levels. In some engines, running much less Octane than you need can also result in rough running and misfiring at high load as a result of overly-retarded ignition timing. If you must run some too-low Octane fuel in your car, you can protect the engine and catalyst by staying away from high load.
Gasoline Choice
So the main difference between the grades of gasoline at the pump is just related to the ability to resist knock at high engine load. Unfortunately, most of the gasoline companies also blend their best detergent packages in the higher-Octane fuels. These detergents are important for keeping fuel injectors clean and spraying properly. This isn’t a significant issue for most engines, especially if you use a major brand of gasoline (even regular grade). Regular low-Octane grade (from a major brand) should have sufficient detergents to keep the injectors in good shape for a decade, but the higher grades have more.
