Retired VIP wrote:
The problem with that is it is a textbook explanation that is outdated and completely ignores reality. In order to maximize fuel efficiency and performance many modern engines are programmed to run right up to the point where detonation occurs. Ideally you would have a small amount of detonation every firing of a cylinder. Detonation is not something that is black and white, it is not on or off it is dynamic and varies in degree. Modern engines are designed to hold the tune so that detonation is at a point where it maximizes performance. The knock sensor is not the only thing to respond to more than the minimum amount of detonation. The air/fuel trim can also respond due to disruption of complete combustion when detonation exceeds that point of maximum efficiency. That means slight changes in octane do make a difference. Couple that with the fact that octane is a whole lot more complicated than a single number on a pump. Add to that that the number on the pump is not as reliable as you might expect and the reality is that no one can predict the actual mileage effect of going from one brand to the next or one octane level to the next within one brand. You can read a book and have it tell you it won't make a difference but you are only fooling yourself. People who test this in a controlled manner almost always find some increase in mileage as octane increases. Often that increase is too small to pay for the extra cost. It isn't just the fuel that is an unreliable input to your comfortable formulas it is the engine itself that is unknown quantity. Lots of things in the engine also affect detonation and thus how much the engine retunes to accommodate. Even slight changes in engine temperature affect detonation. So does slight changes in the intake air temp as well as atmospheric pressure. When a low pressure system is passing thru everyone's engine is less likely to detonate. And of course Air/fuel mixture and EGR mix both raise or lower the tendency to detonate at any particular moment. And then there are slight defects like vacuum leaks and dirty injectors and dirty or poorly gapped spark plugs that all affect the engines tendency to knock. Carbon accumulation in the combustion chamber can also cause detonation. And what happens then? Well the engine detunes and that may cause more carbon accumulation. A tank of premium at that point could break what might become a slow death spiral. So you tell me you can read a book and read the mfg's specs and tell me exactly how my engine will react to tank of premium. I don't think so.
No it is a rating indicating how much the fuel theoretically has a tendency to detonate. As a counter example to your claim, hydrogen catches fire much easier than any gasoline component yet it has an octane rating much higher than any gasoline component. It is theoretical limit because there is no way to predict with certainty how any particular blend of fuels will work in any particular engine. Testing in an engine is still the method for determining octane even though today they can determine the exact molecular composition of the fuel. If someone really want to know how their particular vehicle responds to any particular gasoline you have to try it and see. It's true the vast majority of vehicles will see only diminishing returns for octane levels above what the manufacture recommends. But how much your engine varies from the norm can only be determined by doing controlled tests.
Even when no ethanol is present on average higher octane fuel contains less energy than low octane. It is just in the nature of the gasoline fractions that the ones that contribute the most to detonation (for example heptane) also contain the most energy. But the energy content is for all practical purposes irrelevant. If energy content was all that was important for fuel economy then you would burn diesel in your gas engine.
-jim
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