moves down. This opens the port to the power piston
and connects the oil supply of oil under pressure. This
oil pressure acts on the power piston, forcing it upward
to increase the fuel.
When the governor speed rises due to a decrease of
engine load, the flyweights move out and the pilot valve
moves up. This opens the port from the power piston to
the drain into the sump. The spring above the power
piston forces the power piston down, thus decreasing
Unfortunately, the simple hydraulic governor has a
serious defect, which prevents its practical use. It is
inherently unstable; that is, it keeps moving continually,
making unnecessary corrective actions. In other words
it hunts. The cause of this hunting is the unavoidable
time lag between the moment the governor acts and the
moment the engine responds. The engine cannot come
back to the speed called for by the governor.
Most hydraulic governors use a speed droop to
obtain stability. Speed droop gives stability because the
engine throttle can take only one position for any speed.
Therefore, when a load change causes a speed change,
the resulting governor action ceases at a particular point
that gives the amount of fuel needed for a new load. In
this way speed droop prevents unnecessary governor
movement and overcorrection (hunting).
The recent introduction of electronically controlled
diesel fuel injection system on several heavy-duty high-
speed truck engines has allowed the speed of the diesel
engine to be controlled electronically, rather than
mechanically. The same type of balance condition in a
mechanical governor occurs in an electronic governor.
The major difference is that in the electronic governor,
electric currents (amperes) and voltages (pressure) are
used together instead of mechanical weight and spring
forces. This is possible through the use of magnetic
pickup sensor (MPS), which is, in effect, a permanent-
magnet single-pole device. This magnetic pickup
concept is being used on all existing electronic systems
and its operation can be considered common to all of
them. MPSs are a vital communications link between
the engine crankshaft speed and the onboard computer
(ECM). The MPS is installed next to a drive shaft gear
made of a material that reacts to a magnetic field. As
each gear tooth passes the MPS, the gear interrupts the
MPSs magnetic field. This, in turn, produces an ac
current signal, which corresponds to the rpm of the
engine. This signal is sent to the ECM to establish the
amount of fuel that should be injected into the
combustion chambers of the engine. Electronic speed
governing systems are set up to provide six basic
1. Idle speed control
2. Maximum speed control
3. Power takeoff speed control
4. Vehicle speed cruise control
5. Engine speed cruise control
6. Road speed limiting
Each of the control modes above is described in
more detail below.
1. The idle speed control provides fixed speed
control over the entire torque capability of the
Also, the idle speed set point is
calculated as a function of the engine
temperature to provide an optional cold idle
speed, which is usually several hundred rpm
higher than normal operating temperature.
2. The engine maximum rpm setting can be
programmed for different settings. This can
improve fuel economy by eliminating engine
overspeed in all gear ranges.
3. The power takeoff speed control setting can
operate at any speed between idle and
maximum. The operator uses rotary control or a
toggle switch in the cab to vary electronically
the engine power to the PTO from idle to the
4. Vehicle and engine cruise control includes set,
resume, and coast features similar to that of a
passenger car, as well as an accelerate (ACCEL)
mode to provide a fixed speed increase each
time the control switch is activated.
5. The road speed limiting function allows the
organization assigned to determine what
maximum vehicle road speed they desire
independent of the maximum governed speed
setting of the engine.
Road speed governing
provides the best method for ensuring ideal fuel
The major advantage of the electronic governor
over the mechanical governor lies in its ability to
modify speed reference easily by various means to
control such things as acceleration and deceleration, as
well as load.