Quote:
Originally Posted by FourBinLabs
How can diesel fuel have a lower octane rating then regular gasoline then?
I hope I'm not pestering anyone, I really am trying to wrap my head around this.
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The KEY thing to remember is that a Diesel engine operates differently than gas. In a Diesel, the only substance being compressed is air. As the air is compressed, there is nothing to auto-ignite. Ignition curves and timing are meaningless to Diesels as there is no spark.
Cetane Number rates the ignition delay, and the higher the number the less delay there is. You WANT the fuel in a Diesel engine to auto-ignite, so that's why the scale is set up so high numbers indicate a fuel that is better at autoignition.
Octane Number rates the resistance to autoignition. In an Otto cycle engine (aka gasoline, or spark-ignition), you DON'T WANT the fuel to autoignite since the fuel and air are mixed before compression starts. For this reason, a higher number indicates better resistance to autoignition.
Any fuel that has a high Cetane Number MUST have a low Octane Number.
Any fuel that has a low Cetane Number MUST have a high Octane Number.