r i What is the difference between octane numbers and cetane numbers and what is their importance?
Page 87
If you've noticed an error in this article please click here to report it so we can fix it.
AThe textbook answer is that an octane number is applicable to a fuel used in a spark-ignition engine and is a measure of its anti-knock property. To facilitate comparison the designation is derived from the use of a reference fuel comprising a mixture of isooctane, a non-detonating fuel, and heptane, which has a low anti-knock value. A petrol with a given octane number is equivalent to a reference fuel tested in a laboratory research engine, with a percentage of iso-octane equal to this number. Thus a fuel with an octane number of 80 has an anti-knock rating equal to that of a test mixture containing 80 per cent of iso-octane.
A cetane number is a measure of the ignitiondelay period in a diesel engine provided by the fuel—the higher the cetane number the shorter the delay period. Cetane is the shortdelay reference fuel and is mixed with Alphamethyl-naphthalene which gives a long delay. Therefore a fuel with a cetane number of 80 gives a delay-period equal to that of an engine running on a mixture of the two fuels with 80 per cent cetane. The longer the delay the more prone is the engine to develop a diesel knock.
Because the detonation tendency of a fuel limits the compression ratio that can be used in a petrol engine and the thermal efficiency is increased by raising the compression ratio, output and specific fuel consumption are favoured by high compression ratio. But, in practice, a ratio of more than 10 or 11 to 1 is unrewarding as the efficiency curve flattens out at higher ratios. Difficulties are also created with regard to combustion-chamber shape if the volume of the chamber is reduced below a certain proportion of the swept volume. For example. although a hemispherical chamber normally promotes efficient burning, a very high compression ratio gives an orange-peelshaped chamber that is inefficient.
The cetane number of diesel fuels is less important to a user of a diesel engine than octane is to the petrol user. Other things being equal, variations in cetane number have no effect on specific fuel consumption of a diesel engine or on its output. The only disadvantage of using a fuel with a low octane number is that it normally produces heavier diesel knock at low speeds. A diesel engine with a high compression ratio does not necessarily produce a heavier knock than an engine of lower compression ratio. It is pertinent that in smalland medium-sized diesels, a ratio of. say, more than 15 to 1 is generally employed as the only practical means of providing easy starting. Above a ratio of about 12 to 1, depending on the size of the unit. the gain in thermal efficiency afforded by a ratio increase may be more than offset by higher pumping and friction losses.