The cetane rating is very important for the diesel engine, as a measure of the ignitability of the diesel fuel. The very high ignitability of pure cetane is given a value of 100, extremely low ignitable alpha-methyl- naphthalene, has the value 0. As with the octane rating, a test engine is used to compare various fuel mixtures. If the qualities are the same, the fuel is given a cetane count according to its percentage in the mixture.

There is also a single cylinder test-engine. Given a non-varying fuel injection starting point, one can achieve, by changing the amount of air, or the compression ratio, a certain amount of ignition delay. If it is identical with that of the comparative fuel using the same settings, then the cetane rating will also be identical.

Right from the word go, the diesel engine suffered with the problems caused by self-ignition. Either the pressure, and with it the temperature, was too low, or it was too high following the successful ignition. Compared with petrol, which should definitely not self-ignite, this property is really very welcome in the diesel engine because it has no ignition system. One could describe the cetane rating as the 'reciprocal value' of the octane rating. From this, one can recognise, that apart from the lower lubrication capability, just how badly an Otto-engine would run using diesel fuel, or a diesel engine using petrol. The moral of the story is: never mix petrol with diesel fuel!

The higher is the cetane number, the smaller the timeframe between injection and combustion (ignition delay). If you take a look here, you will notice that diesel fuel with a cetane number of 58 exists. 09/08