The cetane number is a rating of a diesel fuel that expresses ignition quality and defines how easily the fuel will ignite when it is injected into the cylinder. The lower a fuel's cetane number, the longer it takes to reach ignition point. Using a fuel with a cetane number that is too low will increase the amount of diesel knock in an engine. A higher cetane number means the fuel is more prone to self-ignition. Better self-igniting properties of the fuel result in a shorter ignition delay period, which produces better control of the fuel combustion process. A shorter delay period results in easier starting, quieter engine operation, and lower emissions due to complete combustion of the fuel mixture in the cylinder.
When diesel fuel is injected into the cylinder, it does not ignite instantly. It takes time for the heat of the compressed air in the cylinder to heat the fuel sufficiently for it to ignite. This period of time from the start of injection to the start of combustion is called the ignition delay period. During this delay period, fuel continues to be injected into the cylinder.
When the fuel is heated sufficiently, it erupts into flame and burns. Combustion occurs. The sudden pressure rise sends a shock wave through the combustion chamber that can be heard outside the engine. This sound is called diesel knock.
Diesel knock can also be caused by poor atomization of the fuel, which can take too long to reach combustion temperature. However, the knock referred to here is the result of uncontrolled combustion. This type of combustion means the fuel ignites when it wants to, not at the proper time for complete controlled combustion and heat energy transfer, as designed by the engine manufacturer. Uncontrolled combustion is damaging to the engine.