This diagram shows a cross section through the governor head and fuel pump. The by-pass valve beside the governor thrust collar controls both the beginning and the end of injection by allowing pumped oil to return to the tank. It gives the good sharp action necessary to avoid dribble from the nozzle, and meters the injected charge according to the length of time the governor allows it to be closed. This governor is driven through a spring, its heavy head acting as a flywheel to keep it from responding to short-term speed variations as the engine compresses and fires.
Courtesy of John Wilcox