DIESEL ENGINE SPEED,
DOE-HDBK-1018/1-93
Diesel Engine Fundamentals
FUEL CONTROLS, AND PROTECTION
Governor
Diesel engine speed is controlled solely by the amount of fuel injected into the engine by the
injectors. Because a diesel engine is not self-speed-limiting, it requires not only a means of
changing engine speed (throttle control) but also a means of maintaining the desired speed. The
governor provides the engine with the feedback mechanism to change speed as needed and to
maintain a speed once reached.
A governor is essentially a speed-sensitive device, designed to maintain a constant engine speed
regardless of load variation. Since all governors used on diesel engines control engine speed
through the regulation of the quantity of fuel delivered to the cylinders, these governors may be
classified as speed-regulating governors. As with the engines themselves there are many types
and variations of governors. In this module, only the common mechanical-hydraulic type
governor will be reviewed.
The major function of the governor is determined by the application of the engine. In an engine
that is required to come up and run at only a single speed regardless of load, the governor is
called a constant-speed type governor. If the engine is manually controlled, or controlled by an
outside device with engine speed being controlled over a range, the governor is called a variable-
speed type governor. If the engine governor is designed to keep the engine speed above a
minimum and below a maximum, then the governor is a speed-limiting type. The last category
of governor is the load limiting type. This type of governor limits fuel to ensure that the engine
is not loaded above a specified limit. Note that many governors act to perform several of these
functions simultaneously.
Operation of a Governor
The following is an explanation of the operation of a constant speed, hydraulically compensated
governor using the Woodward brand governor as an example. The principles involved are
common in any mechanical and hydraulic governor.
The Woodward speed governor operates the diesel engine fuel racks to ensure a constant engine
speed is maintained at any load. The governor is a mechanical-hydraulic type governor and
receives its supply of oil from the engine lubricating system. This means that a loss of lube oil
pressure will cut off the supply of oil to the governor and cause the governor to shut down the
engine. This provides the engine with a built-in shutdown device to protect the engine in the
event of loss of lubricating oil pressure.
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