Diesel engine governing is accomplished by:

• A. varying the load on the engine
• B. varying the amount of air-fuel mixture drawn into the engine when the load changes
• C. varying the amount of fuel injected on each power stroke
• D. modulating the pressure of the atomizing air.

Answer: (C)

In diesel engines, speed is controlled by how much fuel is injected for each power cycle. The air taken in isn’t throttled to regulate speed; instead, the governor adjusts the amount of fuel delivered with each injection. More fuel per cycle means more energy release and higher torque, which tends to drive the engine faster; less fuel means less torque and slower speed. The governor continually responds to engine speed, increasing fuel when speed drops under load and reducing fuel when it tries to surge.

Why the other ideas don’t fit: you don’t regulate speed by changing the external load or by varying how much air is drawn in, since diesel engines don’t use a throttle to control air and rely on fuel quantity to set power. The atomizing air pressure isn’t what the governor adjusts; the injector’s fuel delivery is what the speed control acts upon.