Originally Posted by THD
... how does the injection system sense the engine load?
Marin pretty much described it.
A diesel doesn't have a throttle that works the way a gas engine does, it doesn't vary the amount of air admitted to the cylinder as a means to control the weight of fuel in the mixture. A diesel always (though that is a highly nuanced "always" for reasons we needn't go into here) admits the same amount of air regardless of the speed or load. It is the amount of fuel that determines how fast the engine turns or how much load it can handle. The governor controls how much fuel is injected.
Moving the throttle on a mechanically injected engine does not open or close a fuel valve or throttle plate, it changes the pressure on a spring in the governor. That spring resists the force created by a spinning weight that is hinged so that increasing speed and resultant centrifual force trys to fling the weight outwards. As it moves outwards against the spring it moves a lever that, in the short version, opens or closes a valve that regulates how much fuel is above the injector pump plunger and sent to the injector itself.
The force on the flyballs is the same at a given rpm regardless of the load on the engine so we use the throttle lever (speed lever) to change the force on the spring (depending on the governor, it may increase the force or decrease it) so that the position of the valve that controls the fuel matches the amount of fuel required to oppose the spring and keep the engine turning at the rpm we command.
At no load, it requires very little fuel to reach max rpm, at high load, it takes a lot. With a properly matched fixed pitch propeller the maximum load possible matches the rated output of the engine at the "speed stop" rpm set by those screws on the side of the governor that you are not supposed to touch. The governor is set so that the range of fuel flow matches the range of power and all we have to do is move the throttle to obtain the speed we want.
If we had a controllable pitch propeller, we could "fool" the governor and overload the engine very easily. If the engine control system did not have a "combinator" or a more sophisticated governor to match pitch with rpm automatically, we could set a low rpm and a high pitch and quickly overheat and destroy the engine by overloading. Sort of like driving around in 4th gear all the time. In an overload condition more fuel is delivered than the engine can burn and all that excess fuel turns to heat and black smoke.
That is the super simple description. If you want to really get a firm grounding on governors, take a look at the Woodward governor website. they have probably built more governors than anyone else on the planet and they have some excellent training materials online. If you are the least bit of a geek, the subject is fascinating.