How petrol knocking is different from diesel knocking?

they are the same thing. "knocking" in an engine is the fuel igniting from the heat of compression of the fuel and the air together. in a desel engine this is how it is suposed to work because it has no spark plugs so the only way to ingite the fuel is by compressing the fuel and the air by a ratio of about 22 to 1. in a gas engine the fuel is ignitied by a spark plug, if you use low grade fuel the explosion can happen too early (before the spark plug ignites it) and cause a knocking noise in the engine whitch can cause dammage long tearm the typical compression ratio for a gas engine is 10 to 1. the more compression you have the higher grade fuel you need to prevent "knocking" whitch is why higher end cars need higher grade fuel because they have higher compression engines whitch make more horsepower with omperable size engines.