shorter the ignition delay

"In order to reduce diesel knock, it's essential to reduce the ignition delay since it's during this time that the fuel and air become premixed. The shorter the ignition delay, the less premixed fuel and air there is available to combust when the combustion process is initiated."

With reference to the above statement, is the problem causes by the fact that there is already premixed fuel and air prior to the beginning of combustion? So does it mean that mixing of air and fuel should be happening during the combustion itself to eliminate knocking? Do I understand it correctly?

Basically the diesel knock is caused by the rapid pressure rise in the cylinder when a large quantity of fuel and air combusts. So we want the delay between the start of the fuel injection and the beginning of combustion to be as short as possible so we don't see this large spike in pressure. You're correct that we want the mixing of fuel and air to occur during combustion. This is the idea behind pilot injection where a small quantity of fuel is injected to initiate combustion and prevent a large pressure spike prior to the introduction of the main injection event.