Mechanical Governors

Many of the governors installed on today’s diesel engines use the centrifugal force of rotating weights (flyballs) and the tensions of a helical coil spring (or springs). On this basis, most of the governors used on diesel engines are generally called mechanical centrifugal flyweight governors.

In mechanical centrifugal flyweight governors (fig.5-6), two forces oppose each other. One of these forces is tension spring (or springs) which may be varied either by an adjusting device or by movement of the manual throttle. The engine produces the other force. Weights, attached to the governor drive shaft, are rotated, and a centrifugal force is created when the engine drives the shaft. The centrifugal force varies with the speed of the engine.

Transmitted to the fuel system through a connecting linkage, the tension of the spring (or springs) tends to increase the amount of fuel delivered to the cylinders.

On the other hand, the centrifugal force of the rotating weights, through connecting linkage, tends to reduce the quantity of fuel injected. When the two opposing forces are equal, or balanced, the speed of the engine remains constant

To show how the governor works when the load increases and decreases, let us assume you are driving a truck in hilly terrain. When a truck approaches a hill at a steady engine speed, the vehicle is moving from a set state of balance in the governor assembly (weights and springs are equal) with a fixed throttle setting to an unstable condition. As the vehicle starts to move up the hill at a fixed speed, the increased load demands result in a reduction in engine speed. This upsets the state of balance that had existed in the governor. The reduced rotational speed at the engine results in a reduction in speed, and, therefore, the centrifugal force of the governor weights. When the state of balance is upset, the high-speed governor spring is allowed to expand, giving up some of its stored energy, which moves the connecting fuel linkage to an increased delivery position. This additional fuel delivered to the combustion chambers would result in an increase in horsepower, but not necessarily an increase in engine speed.

When the truck moves into a downhill situation, the operator is forced to back off the throttle to reduce the speed of the vehicle; otherwise, the brakes or engine/transmission retarder has to be applied. The operator can also downshift the transmission to obtain additional braking power. However, when the operator does not reduce the throttle position or brake the vehicle mass in some way, an increase in road speed results. This is due to the reduction in engine load because of the additional reduction in vehicle resistance achieved through the mass weight of the vehicle and its load pushing the truck downhill. This action causes the governor weights to increase in speed, and they attempt to compress the high-speed spring, thereby reducing the fuel delivery to the engine. Engine overspeed can result if the road wheels of the vehicle are allowed to rotate fast enough that they, in effect, become the driving member.

The governor assembly would continue to reduce fuel supply to the engine due to increased speed of the engine. If overspeed does occur, the valves can end up floating (valve springs are unable to pull and keep the valves closed) and striking the piston crown. Therefore, it is necessary in a downhill run for the operator to ensure that the engine speed does not exceed maximum governed rpm by application of the vehicle, engine, or transmission forces.

Favorable, as well as unfavorable, characteristics are to be found in mechanical governors. Advantages are as follows:

• They are inexpensive.
• They are satisfactory when it is not necessary to maintain exactly the same speed, regardless of load.
• They are extremely simple with few parts.
• Disadvantages are as follows: They have large deadbands, since the speed-measuring device must also furnish the force to move the engine fuel control.
• Their power is relatively small unless they are excessively large.
• They have an unavoidable speed droop, and therefore cannot truly provide constant speed when this is needed.