Quote:
Originally Posted by Sburn
In general, that's true. But timing needs to be advanced to take advantage of the higher octane and make that extra HP.
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I'm going to quibble a bit here, and I'm not trying to put anyone down - just trying to shed some light on what I think can be a common misconception regarding high-octane fuel.
First, high-octane does not really mean "high power", "better fuel" "cleaner burning", "more efficient" or any of that. It is not "a treat for your engine", as I've heard someone say.
Higher octane refers simply to a fuel's resistance to "detonation" - an explosive burning that may be counterproductive. Back in the bad old days, we had really high compression engines, and these would compress the mixture so much that "standard" grade fuel would actually ignite under diesel compression - before the spark even fired (low octane gas is somewhat easier to ignite). So, if the mixture ignited as the piston was on it's way up, well short of Top Dead Center (TDC), the fuel expansion is working against the piston travel and you get "ping", reduced power, stress on the engine, and higher heat.
High octane kept the fuel from igniting until the spark commanded it, which, DOES occur slightly before TDC, but there is not an appreciable amount of expansion until the piston has passed TDC. And then, the burn is more controlled - instead of being too rapid, it expands at a rate that more nearly matches the piston's movement down the cylinder, giving a "controlled push".
We used to get high octane by adding lead compounds to the fuel, which had the added advantage of supplying some lubrication to the moving parts but the disadvantage of giving everyone lead poisoning.
We got rid of leaded fuels, and most engines had their compressions reduced - way, way down. Then, there were some advances that raised the "octane rating" of gas without adding lead (I think Amoco was a leader in this). So, manufacturers started raising the compression a bit in some engines to take advantage of this. (Higher compression DOES give higher power because there is more total expansion in the power stroke).
Now: Our f150 engines are designed to burn 87 octane gas. The spark is commanded to fire at a point where the burn will be controlled and will give an "even push" on the piston. If you put higher octane gas in it and don't adjust the spark, the burn starts too late! Remember, higher octane has a slower flame front advance rate. You would probably
lose power if you did nothing but add the expensive gas,
exactly as Sburn said!
So, using higher octane, we can advance the spark a bit - about 0.5 degrees for each octane point. The burn starts earlier, and it is expanding most rapidly as the piston is moving most rapidly (in the middle of the power stroke). You DO get more power with this gas now. With a programmer, the amount of fuel delivered (the A/F ratio) can be adjusted for even more power.
But, even though E85 has super high octane (anti detonation) properties, it has much less energy potential compared to ordinary gasoline (it's also highly corrosive). So, we can advance the spark, and even adjust the A/F ratio, but, we're not going to see as much total power output as we had using ordinary gas, so, we're going to see more fuel used for mile driven, simply because there is less energy available per gallon.
This is not to say that you can't design the engine from the ground up to use alcohol instead of gasoline. And if the engine is properly designed, it can outperform a street vehicle. Race cars do this all the time. They also burn a lot of fuel.
Now, I've probably said a few things incorrectly here, and I'm happy to have anyone "clean up" anything I got wrong.
- Jack