I had a few questions about the different grades of gasoline, (or petrol, as you chaps across the pond call it Please Register or Log in to view the hidden image! ) and some assumptions people make about them. This is how I understand it to work: (please tell me if I have it right) When crude oil is refined, it is sent through a tower. The higher it goes in the tower, the lighter the final product is. From what I understand, oil form chains of hydro carbons, and the number of these chains determine what kind of refined fuel is produced. Methane has 1 chain, and is the lightest. heptane has 7, octane has 8. FWIU, Gasoline is made of a combination of heptane and octane. When you see the 87, 89, or 93 octane rating, it tells you what the total percentage of octane to heptane is. The higher percentage of octane, the more compressible the fuel is before it self-combusts....and that's the only difference. In high-compression, performance engines, when the piston raises and compresses the air/fuel mixture, if it compresses it too much, the mix will ignite on it's own, causing knocking in the engine. Higher octane gas prevents this. To the best of my knowledge, premium gas is no more explosive, or powerful, than regular gas...it's just more compressible. Is this true? Many people claim premium gives them better gas mileage, more power, fresher breath, and generally think it's better for their engine than regular. If my idea is correct...then all these assumption are false, and are just marketing hype. I was hoping someone with a little more knowledge of chemistry or the oil business could either confirm or deny my assumptions. Thanks, Mac
But if it ignites prematurely the energy yield is less than normal. And if it doesn't 'burn clean' when prematurely igniting, what they say would be true.
I would agree with that. But, I'm talking about people who use premium in a regular compression engine that doesn't knock when running on regular gas. Would it still be true? Thanks.
Hmm I really don't know much about it. I was just trying to approach this logically Please Register or Log in to view the hidden image! You will have to wait for someone else to give you the answer Please Register or Log in to view the hidden image! But, whatever you do, ignore OIM Please Register or Log in to view the hidden image!
The octane rating is a measure of the autoignition resistance of gasoline and other fuels used in spark-ignition internal combustion engines. It is a measure of anti-detonation of a gasoline or fuel. Octane number is the number which gives the percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptane, that would have the same anti-knocking capacity as the fuel which is under consideration. For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90. [1] The octane rating of a spark ignition engine fuel is the knock resistance (anti-knock rating) compared to a mixture of iso-octane (2,2,4-trimethylpentane, an isomer of octane) and n-heptane. By definition, iso-octane is assigned an octane rating of 100 and heptane is assigned an octane rating of zero. An 87-octane gasoline, for example, possesses the same anti-knock rating of a mixture of 87% (by volume) iso-octane and 13% (by volume) n-heptane. This does not mean, however, that the gasoline actually contains these hydrocarbons in these proportions. It simply means that it has the same autoignition resistance as the described mixture. A high tendency to autoignite, or low octane rating, is undesirable in a spark ignition engine but desirable in a diesel engine. The standard for the combustion quality of diesel fuel is the cetane number. A diesel fuel with a high cetane number has a high tendency to autoignite, as is preferred. It should be noted that octane rating does not relate to the energy content of the fuel (see heating value), nor the speed at which the flame initiated by the spark plug propagates across the cylinder. It is only a measure of the fuel's resistance to autoignition. It is for this reason that one highly branched form, or isomer, of octane (2,2,4-trimethylpentane) has (by definition) an octane rating of 100, whereas n-octane (see octane), which has a linear arrangement of the 8 carbon atoms, has an octane rating of -10, even though the two fuels have exactly the same chemical formula and virtually identical heating values and flame speeds. http://en.wikipedia.org/wiki/Octane_rating
Thank you for the response, Cosmic. So, basically what you are saying is that 87 octane gas has the same energy potential as 93 octane? So if I had two identical cars, with low enough compression ratios as to not cause knocking with regular fuel...and I filled one with 87 and one with 93 octane gas...and then put both cars on a "dyno"...both would have the same horsepower output?
The horsepower doesn't come from the gas alone. The engine also has to have been built to produse that type of horsepower as well. Once you've done the engine increase you should also be using a higher octane of fuel as well.
Higher octane gasoline actually has a tiny bit lower BTU rating than lower octane. So there is actually less energy in it (but this difference is very small). An engine designed for 87 fuel isn't going to work any better with 89 or 93. It would be a complete waste of money. But an engine that is designed for higher octane fuel will have more power if run on the octane it is rated for than it will if a lower rated fuel is used. In a modern EFI engine, the ECU will detect the detonation, and will retard the spark advance. This saves the engine, but reduces output and efficiency. If the fuel filler door has a sticker recommending 89 or 93 octane fuel (we only go to 92 here in California), you are best off using what it is rated for. An older car that was designed for 87 octane fuel may benefit from going to the next grade up. As the engine ages, deposits in the combustion chamber actually increase compression, and act as hot spots that can trigger detonation. This is known as octane creep.
Isn't it funny that some products other than gas are advertised as "high octane" to mean "high power"...i.e. "high octane energy drink"
