Alcohol as a fuel 4878Guess you don't know what a strawman is, because I am not making someone else's arguement into an easily knocked down extreme. No, that wasn't what I was doing. I explained it to scott...
Detonation is different from pre-ignition, which is the early ignition of fuel-air mixtures. it can be caused by the timing being early, or by hot carbon spot in the cylinder. There is no "octane" in gasoline. The octane rating is a comparative rating: A special test engine, having variable compression, is run on a sample gasoline and the compression increased until detonation occurs. The engine is then switched over to a straight octane fuel, with the compression still set at the gasoline's detonation setting, and run on the octane while reducing the octane percentage by mixing increasing amounts of heptane into it. When it begins to detonate, the percentage of octane remaining becomes the sample gasoline's octane rating. There are gasolines with higher ratings than 100. They are arrived at mathematically and chemically. We couldn't have an octane test fuel with 110% octane in it, of course. Detonation is different from the desireable normal combustion. In normal burn, the flame front travels across the cylinder at about 100 feet per second. During detonation, the front travels at 5000 feet per second or more; it explodes rather than burns. It can destroy an engine real quick. Pistons and rings break and heads can crack. Cylinder temperatures rise beyond limits. Detonation happens when too much time is allowed for the fuel to burn; as the burn begins, the pressure and temperature ahead of the flame front rise rapidly, and the complex gasoline molecules, which tend to burn in a controlled manner, break into simpler structures and autoignite almost all at once. Lean mixtures burn more slowly, giving time for detonation to occur. Large cylinders take more time. Slow piston travel, such as at low RPM and high power, gives more time and pressure for detonation to happen. Tetraethyl lead used to be used to control the burn. Now the refinery has to break the petroleum down to basic molecules and rebuttemble them to get the fuel they want with anti-detonation properties, which is one reason gasoline costs more than it used to.
Alcohol as a fuel 4879Brent P Maybe its not what you intended, but from my chair that's exactly what you did. I pointed out that high(er) compression combustion is a more thermally efficient process by...
Dan
