DIFFERENCE BETWEEN KNOCKING AND DETONATION EXPLAINED !!

Both knocking and detonation means sudden increase in pressure inside the cylinder of engine.But,there is a small difference in the way at which they take place.

In a spark ignition engine combustion which is initiated between spark plug electrodes spreads across the combustible mixture. A definite flame front which seperates the fresh charge from the products of combustion travels from spark plug to the other end of combustion chamber. In order to effect pressure equilisation the burned part of the mixture expand, and compresses the unburned mixture adiabatically increasing its pressure and tempetature, The process continues as the flame front advances through the mixture and the tempereture and pressure of the unburned mixture are increased furthur.

If the temperature of unburnt mixture exceeds the self Ignition temperature of the fuel and remains at or above this temperature during the period of pre-flame reactions auto Ignition occurs.This auto-ignition results in sudden rise in pressure.THIS PHENOMENON IS CALLED KNOCKING.This is also called auto-ignition because it is initiated automatically with very less control. 

In diesel engines,during the cold starting conditions a situation might arise in which the vaporization of fuel is limited due to cold combustion chamber. Low vaporization result in misfiring or no combustion at all in cylinders. This results in accumulation of fuel inside combustion chamber. This fuel after certain cycle then burns rapidly, producing tremendous power. This might occur while the engine is in start of compression stroke or exhaust stroke. Hence producing huge stresses within the engine. This phenomenon is called detonation

So in short, knocking occurs in spark ignition (petrol engine) due to auto-ignition of fuel resulting in sudden rise in pressure whereas detonation occurs in compression ignition engine (diesel engine) due to sudden burning of accumulated fuel particles resulting in sudden increase in pressure.