Knocking is sudden rise of pressure and temperature . It releases of large quantity of heat due to abnormal burning of the fuel. This knocking adversely decreases the thermal efficiency and life of the engine. The process of knocking in diesel engines is similar to detonation in petrol engines. Knocking/detonation is highly undesirable. Thus rate the fuels by their antiknock property.


Decrease in knocking property is antiknock property. Express it in the following ways:

1. By Octane number in Petrol Engines or Spark Ignition Engines

2. By Cetane number in Diesel Engines or Compression Ignition Engines


It gives an idea about the performance of the fuel. If the Octane number of the fuel is 63. Thus it has 37 % knocking tendency. Higher is the Octane number, lesser will be the knocking. Octane number is applicable only to petrol in spark ignition engines. Petrol is C8H15.

In petrol engines, desirable Octane number is between 80 and 95. Still higher Octane number is desirable in racing petrol cars and aero-planes. Improve the octane number with little addition of Tetraethyl lead (TEL). Add only a few milliliters of TEL to several liters of fuel. However exhaust of petrol engine becomes poisonous. Assigne 100 octane number to100 % by volume of ISO-OCTANE (C8H18). It represents zero knocking or best antiknock. HEPTANE (C7H16) has 0 Octane number and has 100 % knocking. Fuel having 60 Octane number has 60 % Iso-octane and 40 % Heptane by volume.


It represents the performance of the diesel fuel. It indicate ignition property of diesel. Cetane number of 100 means burning of diesel is perfect. There is zero  delay period.

α Methyl Naphthalene (C11H10) has a Zero Cetane number is 100 % knocking and has 100 % delay period.

Cetane number is % by volume of Cetane in a mixture of Cetane (C16H34) and α Methyl Naphthalene (C11H10). Cetane number is a measurement of the ignition properties of a fuel.

The range of Cetane number of diesel is 40 to 55. Therefore, it is not good enough as antiknock fuel.  Ignition Delay is the time interval between start of fuel injection and the start of Combustion. Larger the ignition delay, larger will be the knocking. Diesel engine with short ignition delay is highly desirable. Further add different types of additives in diesel to increase its antiknock property.

Discuss various types of Diesels.

Light diesel (C123H222) has a Cetane number of 40 to 55.

Heavy diesel (C146H248) has a Cetane number of 35 to 50.

Therefore, light diesel with short ignition delay and additives will be the final choice to have minimum knocking.