Octane is a rating of how slow the gasoline burns. Simply put, gasoline is a chain of hydrocarbons. Hydrocarbon chains all burns at the same speed, but higher octane fuels contain longer hydrocarbon chains, and therefore take longer to burn completely, so it's said 92 octane burns "slower" than 87. For high octane fuel, this means more stored energy per fuel particle, because it's burning at the same rate, but for a longer period of time. By advancing your spark timing (on the focus, this is done with a chip) you can make the spark happen earlier than stock, to take advantage of the longer burn time of the fuel.
In a healthy engine using the right octane, the spark happens right before the piston reaches the top of it's stroke. a wave of flame makes it's way from the spark toward the piston. It takes a few milliseconds to happen, but in a well tuned car, just as the piston is starting to move downward, the force of the fuel igniting sends the piston rocketing downward.
Wrong octane is bad!
Let's look at what happens in a high performance engine that's supposed to use high octane fuel:
On a car with high compression or turbo, the fuel burns relatively faster because of the higher temperature and pressure in the combustion chamber. If you use a faster burning low-octane fuel, the spark happens too early. The wave of flames first hits the piston while it's still trying to move upward, or just as it hits top dead center. There's nowhere for the flame and pressure to expand to. For a few milliseconds, the flame is causing extreme pressure on the piston, the head gasket, and the cylinder head. The spark plug gets extremely hot. With continued use, you may char or crack the insulator of the spark plug, or worse yet, burn or break an electrode off. This condition is known as PRE-IGNITION, often called PING, KNOCK. Needless to say, this is bad news.
On a side note, if you have carbon deposits on the piston face, the carbon can remain hot enough to ignite the fuel mixture without a spark. Sometimes, as the piston approaches top dead center on the compression stroke, the fuel will ignite before the spark. The source of the flames is from the deposits on the piston head instead of the spark plug (can also happen from deposits on the cylinder head), but the effects are the same. High temps and high pressure in the combustion chamber causing carnage on the upper internals. This is DETONATION, but it's also commonly called PING or KNOCK.
Now that that's out of the way...
In an econobox engine like a stock Zetec from a Ford Focus, they tuned the PCM to run on 87 octane, relatively fast burning fuel. Not as much energy in each fuel injected squirt, but it gets the job done quite nicely.
If you run high octane on an engine meant for low octane, the fuel doesn't get a chance to burn completely. This means you have parts of "spent" hydrocarbon chains (basically, unburned fuel), that can damage your catalytic convertor, can cause carbon deposits and build-up on your piston face, Cyl head, valves, and anywhere else the exhaust flows. Eventually, these carbon deposits can cause a mild form of detonation that becomes more evident whenever you go back to using the correct octane (faster burning) fuel, and therefore creating what some people call "engine dependency of high octane".
I hope this helps a little. If I find a better explanation, I'll link to it. I was just having this discussion w/ my co worker yesterday.
In a healthy engine using the right octane, the spark happens right before the piston reaches the top of it's stroke. a wave of flame makes it's way from the spark toward the piston. It takes a few milliseconds to happen, but in a well tuned car, just as the piston is starting to move downward, the force of the fuel igniting sends the piston rocketing downward.
Wrong octane is bad!
Let's look at what happens in a high performance engine that's supposed to use high octane fuel:
On a car with high compression or turbo, the fuel burns relatively faster because of the higher temperature and pressure in the combustion chamber. If you use a faster burning low-octane fuel, the spark happens too early. The wave of flames first hits the piston while it's still trying to move upward, or just as it hits top dead center. There's nowhere for the flame and pressure to expand to. For a few milliseconds, the flame is causing extreme pressure on the piston, the head gasket, and the cylinder head. The spark plug gets extremely hot. With continued use, you may char or crack the insulator of the spark plug, or worse yet, burn or break an electrode off. This condition is known as PRE-IGNITION, often called PING, KNOCK. Needless to say, this is bad news.
On a side note, if you have carbon deposits on the piston face, the carbon can remain hot enough to ignite the fuel mixture without a spark. Sometimes, as the piston approaches top dead center on the compression stroke, the fuel will ignite before the spark. The source of the flames is from the deposits on the piston head instead of the spark plug (can also happen from deposits on the cylinder head), but the effects are the same. High temps and high pressure in the combustion chamber causing carnage on the upper internals. This is DETONATION, but it's also commonly called PING or KNOCK.
Now that that's out of the way...
In an econobox engine like a stock Zetec from a Ford Focus, they tuned the PCM to run on 87 octane, relatively fast burning fuel. Not as much energy in each fuel injected squirt, but it gets the job done quite nicely.
If you run high octane on an engine meant for low octane, the fuel doesn't get a chance to burn completely. This means you have parts of "spent" hydrocarbon chains (basically, unburned fuel), that can damage your catalytic convertor, can cause carbon deposits and build-up on your piston face, Cyl head, valves, and anywhere else the exhaust flows. Eventually, these carbon deposits can cause a mild form of detonation that becomes more evident whenever you go back to using the correct octane (faster burning) fuel, and therefore creating what some people call "engine dependency of high octane".
I hope this helps a little. If I find a better explanation, I'll link to it. I was just having this discussion w/ my co worker yesterday.
