Internal Combustion Engines
The two reference fuels used for cetane rating are

Cetane and alpha-methyl naphthalene
Cetane and iso-octane
Cetane and tetra ethyl lead
Cetane and normal heptane

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Internal Combustion Engines
The delay period in compression ignition engines depends upon

All of these
Nature of the fuel mixture strength
Temperature and pressure in the cylinder at the time of injection
Relative velocity between the fuel injection and air turbulence pressure of residual gases

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