Pneumatic, or flap valve, governors are less expensive than centrifugal governors and more amenable to repair. The velocity of air moving through the intake manifold is a function of piston speed. The faster the engine runs, the greater the velocity. Pneumatic governors sense this velocity as vacuum developed by a venturi mounted on the air intake.
The venturi restricts the flow of air and, so doing, reduces its pressure. This relationship has a constant of 20, as shown in Fig. 5-30. If we use consistent terms and multiply pressure times velocity at any point in the venturi, the answer is always 20, or something approximating that number. Nor need the venturi be the sort of streamlined restriction shown in the drawing; any impediment accelerates air flow and reduces its pressure.
Pneumatic governors, while by no means the state of the art, are still encountered on marine and industrial engines. The example discussed below is one of the most complex.