Cetane Value of Fuel

This is a measure of the fuel, when used in compression-ignition (diesel) engines, with regards to its ability to react with oxygen under explosion conditions and hence enable the engine to produce shaft power.

A variable-compression ratio engine is used (the Richardo E6 engine being commonly used) where the cylinder block can be moved relative to the crank shaft to vary the volume of the combustion chamber-when the piston is at t.d.c. Such an engine enables the highest useful compression ratio. (H.V.C.R.) to be measured since:-

NORMINAL COMPRESSION RATIO =

Swept Volume + Clearance Volume =
Clearance Volume

Vs + Vc =
Vc

Vs + 1
Vc

When the H.V.C.R. is found, for a test fuel, the engine is then run at the same running conditions but supplied with a mixture of Cetane (a very good burning fuel oil) and Alpha Methyl Naphthalene (a very bad burning fuel oil) which are measured by separate fuel flow meters. The particular blend f these two oils which enables the engine to produce the same power as the test fuel oil, has the percentage of Cetane allocated as the Cetane value of the test fuel. A typical value, for diesel engine road vehicles (D.E.R.V.) is 55 CETANE VALUE (i.e. it is equivalent to the ability of a mixture of 55% CETANE and 45% Naphthalene.