Quote:
Originally Posted by superchow
Jay,
I agree.
I respectfully disagree.
It is true that one is trying to keep the air/fuel mix to self-ignite to avoid knocking. Wikipedia (I know, I know, don't trust anything you read on the internet...) states that Octane rating:
As far as I know it does not affect the actual speed at which the gasoline will burn when it does ignite. (Please correct me if I'm wrong - THAT would be fascinating!)
Hm... how can I re-phrase what I am thinking. What about Diesel engines?
They operate without spark plugs based on the principle that the air/fuel mix self-ignites because of the high pressures within the cylinder. Compression ratios are higher (around 16:1 if I recall). Because temperatures are higher, and also because diesel contains more energy per unit of volume, the explosions are more efficient. Same amount of fuel + bigger bang = more efficiency.
By extension a higher compression ratio in a gasoline engine should yield higher efficiency. The only way I know of how to achieve this is to have a air/fuel mix that can be compressed more without self-igniting. And since the engine has a fixed displacement this can only be achieved by allowing more air without allowing more fuel to be injected. Therefore the valve timing adjusts to allow maximum air within the cylinder. This increases the air temperature as the air is compressed more. Now you have a lot of really hot air that needs a fuel that does not self-ignite - ergo - a higher octane rated fuel.
And since all cars I know have excess cooling capacity the excess heat is negligible in the life-expectancy-of-components scheme of things.
But please, (no sarcasm) correct me where I am wrong. I am here for the same reason as all of us: to save fuel and coin and improve efficiency.  |
I agree. In an engine designed with higher compression, lower octane fuel will have a tendency to pre-detonate because it will be under more pressure by the time the plug sparks. If the fuel does detonate early, it will not be at the correct "time" and performance will suffer. The computer will attempt to correct by adjusting the air fuel ratio. IF your engine is designed for it, the higher octane fuel will produce more power for the same engine displacement. More pressure means more potential energy. This is why race engines do not run on pump gas. The higher the compression ratio the higher the octane requirement to run with proper timing.
If your engine is designed with standard piston compression, the fuel will not pre-detonate, as long as you use regular or better. In this case, using higher octane fuel will offer no benefit.
Refer to the manufacturers recommendation for what octane you should use and don't wast your money on anything higher. As you engine ages (cylinder rings wear) the engines compression ration will gradually decrease to the point that it no longer runs at it's best on the recommended octane. A luxury car with a ton of miles often run great on regular even if it is supposed to require premium.