Underrun is a simple term to describe the ability of the governor to prevent engine speed from dropping below a set idle, particularly when the throttle has been moved rapidly to a decreased fuel setting from maximum full-load position.

Deadband is the change in speed required before the governor will make a corrective movement of the throttle.

State of balance is used to describe the speed at which the centrifugal force of the rotating flyweights of the governor matches and balances the spring force of the governor.

Types of Governors

The type of governor used on diesel engines is dependent upon the application required. The six basic types of governors are as follows:

1. Mechanical centrifugal flyweight style that relies on a set of rotating flyweights and a control spring; used since the inception of the diesel engine to control its speed.

2. Power-assisted servomechanical style that operates similar to the mechanical centrifugal flyweight but uses engine oil under pressure to move the operating linkage.

3. Hydraulic governor that relies on the movement of a pilot valve plunger to control pressurized oil flow to a power piston, which, in turn, moves the fuel control mechanism.

4. Pneumatic governor that is responsive to the air flow (vacuum) in the intake manifold of an engine. A diaphragm within the governor housing is connected to the fuel control linkage that changes its setting with increases or decreases in the vacuum.

5. Electromechanical governor uses a magnetic speed pickup sensor on an engine-driven component to monitor the rpm of the engine. The sensor sends a voltage signal to an electronic control unit that controls the current flow to a mechanical actuator connected to the fuel linkage.

6. Electronic governor uses magnetic speed sensor to monitor the rpm of the engine. The sensor continuously feeds information back to the ECM (electronic control module). The ECM then computes all the information sent from all other engine sensors, such as the throttle position sensor, turbocharger-boost sensor, engine oil pressure and temperature sensor, engine coolant sensor, and fuel temperature to limit engine speed.

The governors, used on heavy-duty truck applications and construction equipment, fall into one of two basic categories:

1. Limiting-speed governors, sometimes referred to as minimum/maximum models since they are intended to control the idle and maximum speed settings of the engine. Normally there is no governor control in the intermediate range, being regulated by the position of the throttle linkage.

2. Variable-speed or all range governors that are designed to control the speed of the engine regardless of the throttle setting.

Other types of governors used on diesel engines are as follows:

1. Constant-speed, intended to maintain the engine at a single speed from no load to full load.

2. Load limiting, to limit the load applied to the engine at any given speed. Prevents overloading the engine at whatever speed it may be running.

3. Load-control, used for adjusting to the amount of load applied at the engine to suit the speed at which it is set to run.

4. Pressure regulating, used on an engine driving a pump to maintain a constant inlet or outlet pressure on the pump.

At this time on heavy-duty truck and construction equipment applications, straight mechanically designed units dominate the governor used on nonelectronic fuel injection systems.

Mechanical Governors

In most governors installed on diesel engines used by the Navy, the centrifugal force of rotating weights (flyballs) and the tensions of a helical coil spring (or springs) are used in governor operation. On this basis, most of the governors used on diesel engines are generally called mechanical centrifugal flyweight governors.