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:
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.
Power-assisted servomechanical style that
operates similar to the mechanical centrifugal
flyweight but uses engine oil under pressure to
move the operating linkage.
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.
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.
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
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
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
Constant-speed, intended to maintain the engine
at a single speed from no load to full load.
Load limiting, to limit the load applied to the
engine at any given speed. Prevents
overloading the engine at whatever speed it may
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.
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
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