Mechanical or Pneumatic Governors

Governors on in-line pumps are usually mechanical or pneumatic. The primary job of the governor is to maintain a set engine speed regardless of load. Maintaining the speed is accomplished by balancing governor spring tension with fly weight position. For example, increased governor spring tension increases fuel delivery and engine speed; increased engine speed causes the fly weights to move outward from centrifugal force, reducing fuel delivery and engine speed.

A mechanical governor uses rotating fly weights to control movement of the fuel control rack against a spring. Removing the load from the engine lets its speed rise. Centrifugal force pushes out the weights, which push a sleeve against the spring. The force from the spring tries to push the rack to the maximum fuel position. The force on the sleeve from the fly weights acts against the spring to try to push the rack to the minimum fuel position Figure 54-23.

FIGURE 54-23
A typical mechanical governor.

For any governor position, the fuel control rack determines the volume of fuel delivered, and therefore engine speed. During idling, the governor prevents the engine from stalling. It also stops it from overspeeding. Thus, mechanical governors in automotive use are called idling and maximum speed governors, because idling speed and maximum speed are all they control. They can also be called limiting speed governors. For other throttle positions, the operator determines the rack position by moving the position of the floating link, which increases or decreases governor spring tension.

A pneumatic governor takes advantage of a principle called the venturi effect to control fuel delivery Figure 54-24. When air flows through a constricted section of pipe, there is a reduction in pressure. To be clear, when we talk about a pressure lower than atmospheric pressure, we are talking about vacuum. Therefore, vacuum increases as airflow increases through the constriction.

The pneumatic governor has a manifold-mounted venturi unit, linked by tubing to a sealed diaphragm assembly on the in-line injection pump housing. This venturi unit has a main venturi and an auxiliary one. A throttle butterfly in the intake manifold controls airflow through the venturi and into the engine.

FIGURE 54-24
The pneumatic governor takes advantage of the venturi effect to control fuel delivery.

Here is how it works: When the engine is not running, the diaphragm spring pushes the diaphragm and fuel control rack toward the full-fuel position. With the engine running at idle, the throttle butterfly is almost closing the intake, and air flows through the auxiliary venturi at high velocity. The high airflow produces low pressure, which is transferred through the connecting hose to the sealed chamber on the spring side of the diaphragm. Atmospheric pressure on the pump side now forces the diaphragm and rack toward the no-fuel position. This reduces the effective pump stroke and the amount of fuel injected.

Depressing the accelerator allows more air to enter the engine but decreases the air velocity through the auxiliary venturi. Pressure in the sealed chamber rises and allows the spring to move the diaphragm, and control rack, against atmospheric pressure to increase the fuel delivered. The diaphragm position at any given time is determined by the air velocity through the auxiliary venturi, in accordance with engine speed and load. This provides a rack setting that allows the correct quantity of fuel to be injected to match the operating condition.