Diesel engines are generally

In the IC engine, there are two terms we often use, first is quantity governing and another is quality governing.

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 part-load efficiency of SI engine is poor because the air-fuel ratio remains constant even if we need low power.
• Thus, the petrol engines use Quantity governing.

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 the control valve is regulated by the centrifugal governor through rack and pinion arrangement.
• The part-load efficiency of CI engine is good because we can use a lean mixture and rich mixtures easily according to our requirement.
• The Diesel engines use Quality governing.