Quantitative governing In this system of governing, the quality of charge (i.e. air-fuel ratio of the mixture) is kept constant. But the quantity of mixture supplied to the engine cylinder is varied by means of a throttle valve which is regulated by the centrifugal governor through rack and pinion arrangement. The spark ignition engines are governed by Quantitative governing

the part load efficiency of SI engine is poor because air fuel ratio remains constant even if we need low power

Qualitative governing In this system of governing, a control valve is fitted in the fuel delivery pipe, which controls the quantity of fuel to be mixed in the charge. The movement of control valve is regulated by the centrifugal governor through rack and pinion arrangement.

The compression ignition engines are governed by Qualitative governing

part load efficiency of ci engine is good because we can use lean mixture and rich mixtures easily according to our requirement

Detonation: Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug.
Pre-ignition: Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing.