hydraulic governors have more moving parts and are generally more expensive than
mechanical governors, they are used in many applications because they are more sensitive,
have greater power to move the fuel control mechanism of the engine, and can be timed for
identical speed for all loads.
governors (fig. 5-7),
the power which moves the engine throttle does NOT come from the speed-measuring device,
but instead comes from a hydraulic power piston, or servomotor. This is a piston that is
acted upon by fluid pressure, generally oil under the pressure of a pump. By using
appropriate piston size and oil pressure, the power of the governor at its output shaft
(work capacity) can be made sufficient to operate the fuel-changing mechanism of the
speed-measuring device, through its speeder rod, is attached to a small cylindrical valve,
called a pilot valve. The pilot valve slides up and down in a bushing, which contains
ports that control the oil, flow to and from the servomotor. The force needed to slide the
pilot valve is very little; a small ball head is able to control a large amount of power
at the servomotor.
principle of a hydraulic governor (fig. 5-7)
is very simple. When the governor is operating at control speed or state of balance, the
pilot valve closes the port and there is no oil flow.
governor speed falls due to an increase in engine load, the flyweights move in and the
pilot valve 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.
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 the
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.
governors use a speed droop to obtain stability. Speed droop gives stability because the
engine throttle can take only one position for any speed.
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).