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I posted this one the old TCCoA forum this morning, and I thought that it should have been a new thread. But now I saw this new forum, and had a chance to be the first to post in a section, so why not.
The octane rating of a fuel is actually it's resistance to burn. A higher octane fuel has to reach a higher temperature before it begins to burn.
Gotta include one law: when you compress air, it gets hotter. Don't believe me? Start up an air compressor, let it run a little and then feel how hot the compressor gets.
As the piston travels up the cylinder, it compresses the air/fuel mixture. Close to the top of piston travel (TDC) the computer fires the spark plug, this is called advanced timing. This ignites the air/fuel mixture and forces the piston downward. This is all fine and dandy when the engine is new and clean inside. When the combustion chamber is gets some carbon deposits on the inside, that's when pinging and knocking start to become a problem. If a piece of carbon is deposited on the top of the piston, for example, this chunk can and will be pre-heated by the previous combustion stroke. This chunk can get so hot that as the piston moves up it becomes hot enough to ignite the air/fuel mixture before the spark plug has a chance to fire. This is called pre-ignition, that's bad. A pre-ignition source does not have to be on the piston, it can be an overheated exhaust valve or spark plug, or a deposit somewhere in the combustion chamber.
To fix the pre-ignition, also called pinging, one can either clean out the combustion chamber of all possible pre-ignition sources, or run a fuel that is less affected by those pre-ignition sources. This fuel is a gasoline with a higher octane rating. This fuel will have extra additives to help prevent it from pre-igniting. This should help to eliminate the pinging.
Factors that will increase an engines octane requirement:
1. Ignition timing is advanced
2. Air density rises due to supercharging or a larger throttle opening or higher barometric pressure.
3. Humidity or moisture content of the air decreases
4. Inlet air temperature is increased
5. Coolant temperature is raised
6. Antifreeze (glycol) engine coolant is used
7. Engine load is increased
excerpt from Internal Combustion Engine Fundamentals by John B. Heywood, 1988.
Another factor that would require an increase in octane requirement is increase of compression ratio. The mose you compress air, the hotter it gets. Pre-Ignition can also take place when air/fuel mixture gets too hot just from the compression of the gases. You may notice that all cars that require the higher octane rating are either artificially aspirated (supercharged or turbocharged) or high compression ratio (10.5:1 or so).
Diesel engines don't even have spark plugs. They rely on the heating of the air by compression to ignite the oil mist that is injected into the combustion chamber. Due to the fact that the fuel is not injected until the piston is almost at TDC, Diesel engine designers can utilize higher compression ratios than our gasoline engines because they don't have to worry about pre-ignition and knock.
As for running a fuel that has higher rating that is required for your car: unless your computer is re-programmed to take advantage of the higer octane by advancing the timing and other adjustments, you really shouldn't see any difference. Now there might be a different mixture of additives and detergents between different octane grades that might help to clean the injectors and combustion chamber. This would help,
by cleaning out the engine and make it more effiecient.
I don't want to say that money is wasted by purchasing premium when regular will do, but it might make no difference at all.
Sorry to make this soooooo long, but I tried to explain it as best as I could. This book was my textbook from my ME333 Internal Combustion Engines class that I took as an elective when I was in school getting my Mechanical Engineering degree.
The design of engines is very complex, to say the least.
Scott
1995 T-Bird LX 4.6L
The octane rating of a fuel is actually it's resistance to burn. A higher octane fuel has to reach a higher temperature before it begins to burn.
Gotta include one law: when you compress air, it gets hotter. Don't believe me? Start up an air compressor, let it run a little and then feel how hot the compressor gets.
As the piston travels up the cylinder, it compresses the air/fuel mixture. Close to the top of piston travel (TDC) the computer fires the spark plug, this is called advanced timing. This ignites the air/fuel mixture and forces the piston downward. This is all fine and dandy when the engine is new and clean inside. When the combustion chamber is gets some carbon deposits on the inside, that's when pinging and knocking start to become a problem. If a piece of carbon is deposited on the top of the piston, for example, this chunk can and will be pre-heated by the previous combustion stroke. This chunk can get so hot that as the piston moves up it becomes hot enough to ignite the air/fuel mixture before the spark plug has a chance to fire. This is called pre-ignition, that's bad. A pre-ignition source does not have to be on the piston, it can be an overheated exhaust valve or spark plug, or a deposit somewhere in the combustion chamber.
To fix the pre-ignition, also called pinging, one can either clean out the combustion chamber of all possible pre-ignition sources, or run a fuel that is less affected by those pre-ignition sources. This fuel is a gasoline with a higher octane rating. This fuel will have extra additives to help prevent it from pre-igniting. This should help to eliminate the pinging.
Factors that will increase an engines octane requirement:
1. Ignition timing is advanced
2. Air density rises due to supercharging or a larger throttle opening or higher barometric pressure.
3. Humidity or moisture content of the air decreases
4. Inlet air temperature is increased
5. Coolant temperature is raised
6. Antifreeze (glycol) engine coolant is used
7. Engine load is increased
excerpt from Internal Combustion Engine Fundamentals by John B. Heywood, 1988.
Another factor that would require an increase in octane requirement is increase of compression ratio. The mose you compress air, the hotter it gets. Pre-Ignition can also take place when air/fuel mixture gets too hot just from the compression of the gases. You may notice that all cars that require the higher octane rating are either artificially aspirated (supercharged or turbocharged) or high compression ratio (10.5:1 or so).
Diesel engines don't even have spark plugs. They rely on the heating of the air by compression to ignite the oil mist that is injected into the combustion chamber. Due to the fact that the fuel is not injected until the piston is almost at TDC, Diesel engine designers can utilize higher compression ratios than our gasoline engines because they don't have to worry about pre-ignition and knock.
As for running a fuel that has higher rating that is required for your car: unless your computer is re-programmed to take advantage of the higer octane by advancing the timing and other adjustments, you really shouldn't see any difference. Now there might be a different mixture of additives and detergents between different octane grades that might help to clean the injectors and combustion chamber. This would help,
by cleaning out the engine and make it more effiecient.
I don't want to say that money is wasted by purchasing premium when regular will do, but it might make no difference at all.
Sorry to make this soooooo long, but I tried to explain it as best as I could. This book was my textbook from my ME333 Internal Combustion Engines class that I took as an elective when I was in school getting my Mechanical Engineering degree.
The design of engines is very complex, to say the least.
Scott
1995 T-Bird LX 4.6L