Originally posted by GSVT:
That why deisel fuel is like 56 octane (something like that) and deisel engines don't have spark plugs. the fuel ignites because of compresion.



Octane is a specification used for gasoline engines. Cetane is the specification used for diesel engines.

Octane is the anti knock index used in gasoline engines. RON is the research octane number, which is calculated. MON is the motor octane index, which is from a lab test using a gasoline engine to test for knocking. Road Octane what you see at the pump is (RON+MON)/2. The RON, the calculated octane, of diesel is essentially zero. The MON of diesel, well I don't know anyone who would knowingly put diesel into a gasoline engine so there is no MON of diesel.

Cetane is the index used for diesel engines. Cetane measures the ease with which diesel can self ignite under specific engine conditions. The cetane index of diesel sold in the US is 40. The cetane index of gasoline would be very high but like diesel and octane, the cetane of gasoline is meaningless since you would destroy a diesel engine running pure gasoline. The higher the cetane the easier to start the engine and the engine runs smoother.