In a diesel engine, fuel is injected after the air is compressed and becomes high temperature and pressure. To obtain a high compression pressure even at low engine speeds, a large amount of air must be drawn into the cylinders. Therefore, a throttle valve is not used because of the intake resistance. (Some engines use an intake shutter, which shape is similar to the throttle valve.) In a diesel engine, engine output is controlled by controlling the amount of fuel injected.
Fuel injection volume smallOutput small
Fuel injection volume largeOutput large
REFERENCE:
Gasoline engine output control
The output of a gasoline engine is controlled by opening and closing the throttle valve, thereby controlling the amount of air-fuel mixture taken in.
Air-fuel mixture volume smallOutput small
Air-fuel mixture volume largeOutput Large