Diesel knock
The knock phenomenon of C.I. engine depends upon delay period. If delay period is small then less amount of fuel is admitted into cylinder during ignition delay period. As it burns smoothly knocking do not occur. If the delay period is long, then more amount of fuel is accumulated in the combustion chamber during ignition delay period. When it actually burns, sudden pressure rise will cause the engine to vibrate, resulting in noise and this is said to be knocking.
Methods of controlling Diesel knocking:
1) The delay period can also be reduced by reducing the degree of turbulence as it will reduce heat loss. However, it will increase the combustion period and thus reduce thermal efficiency. 2) The delay period can be reduced by adding chemical dopes, called ignition accelerators (ethylnitrate and amyl – nitrate). 3) Delay period can be reduced by multiple injections so that only small amount of fuel is injected at first (pilot injection).