On startup when a diesel is cold

why do they knock - is it like pre ignition ?

 

The whole concept of a Diesel engine is based on pre ignition, in a way at least. Could it just be piston slap?

 

bttt

 

let me try : the higher the temperature of the diesel,the easier it is to self-ignite it. when the engine (cilinder/piston) is cold it will take longer for the mixture to reach the 'self-ignition-temperature'. in other words , the piston has to travel further up the cilinder in order to get the mixture to ignite( more compression=higher temperature)
hope this helps..