Octane number (octane it means eight carbon atoms )
Octane number is used in petrol engines. It determines how much compression ratio a petrol mixture can withstand. If the limit is crossed the fuel will spontaneously ignite and its called knocking.
Generally octane numbers range from 87 - 90 . If that number is high then the petrol can withstand higher compression ratio. But it comes with more cost.
The number 87 denotes 87 % of octane and rest is heptane ( it can withstand only very lower compression ratio) , that's why as octane number increases the engine can have higher compression ratios.
Octane the name comes because its major composition of petrol.
Cetane number ( 16 carbon atoms )
Its for diesel engine due to composition of diesel has mostly 16 carbon atoms. Its just opposite of octane number.
It determines how fast the fuel will enter combustion stage. It signifies the time gap between time of injection and time of ignition of a diesel. Higher cetane number means this gap is less.
So why you need higher cetane fuel
Fuel is generally injected few degree of crank before piston hits TDC. If cetane number is less the fuel will combust at a time when the pistion would have already left far from TDC. So you dont get enough torque and milegae will be less. So if fuel ignites earlier there will more time for expansion.
Cetane numbers are between 40 - 48
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