This is the third book in the series that included 'The Theory of Nothing' the "The Universe Revealed.' My purpose in this new book is to more fully explain how science is actually done and how science fits into our culture along with religion and p...
Anyone who drives an automobile should be familiar with this term. Most gasoline is sold in three grades: Regular, Mid-Grade and Premium. All grades are unleaded. At one time lead tetraethyl was added to gasoline to prevent knocking. This has been banned because lead is poisonous and fouls newer car engines.
Why the different grades and how are these rated? That's an interesting question that really is not as significant as it once was. At one time engine knocking was a big problem with carbureted engines. Since most engines are fuel injected this is not as big a problem.
Knocking is caused by premature detonation. A gasoline engine runs by injecting vaporized gasoline and air into the cylinders and compressing the mixture before a spark ignites it and makes it burn, causing expansion that drives the piston down. The idea is to have it burn quickly but not detonate. Detonation makes the legendary knock sound when an car is accelerated under load.
The idea of octane rating goes back to the early days of automobiles. The original system was established in what was called a standard carbureted combustion engine using two gasoline components: heptane and 2,2,4-trimethylpentane, better known as iso-octane, which is where the name of the concept comes from. Straight-chain carbon-hydrogen molecules cause serious knocking. Branched hydrocarbons like iso-octane do not.
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ISO OCTANE
The way that the original technicians established the octane rating scale is by comparing the resistance to knocking of pure heptane and calling it 0 and the resistance to knocking of iso-octane and calling it 100. Mixtures of heptane and iso-octane were made and these were rated as to the percentage of iso-octane in the mix. Octane ratings can be below 0. The straight chain n-octane has a rating of -10, while octane ratings above zero can be obtained by using methanol or ethanol. The higher the branching of the hydrocarbon used as gasoline the better the octane rating.
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HEPTANE Modern gasoline is rated by comparing it to this original scheme. The most used rating system is called RON or the Research Octane Number. There are other rating systems, but RON is the most used in the United States. Aviation gasoline has a slightly different rating system, but it's similar to the automobile method.
Most modern automobiles that are not high performance cars can use the lowest grade of gasoline. There is no advantage in using higher, more expensive grades. They don't give one better millage or higher performance. In order to get a higher performance out of an engine using higher grade gasoline one needs a car with a high compression engine. That's because knocking under acceleration is inefficient. That's why dragsters use hydrocarbons like benzene or toluene in their gasoline. Racing engines often use methanol. Dragsters also use nitro methane, which is explosive. The bottom line here is that octane ratings are not as significant as they once were. Gasoline fuels are rated in advertising as to how clean they burn in an engine. Knocking is a thing of the past.
