1. As the air enters the combustion chamber, the
shape of the intake port (fig. 5-5) introduces a
swirling effect to it.
2. During the compression stroke, the swirling
motion of the air continues as the temperature in
the chamber increases (fig. 5-5).
3. As the fuel is injected, approximately 95 percent
of it is deposited on the head of the piston and
the remainder mixes with the air in the spherical
combustion chamber (fig. 5-5).
4. As combustion begins, the main portion of the
fuel is swept off the piston head by the high-
velocity swirl that was created by the intake and
the compression strokes. As the fuel is swept off
of the head, it burns through the power stroke,
maintaining even combustion and eliminating
detonation (fig. 5-5).
A governor is required on a diesel engine to control
the idling and maximum speeds of the engine, with
some governors being designed to control the speed
within the overall operating range of the engine. It is
possible for the operator to control the engine speed
between idle and maximum through the operation of the
throttle. Idle and maximum speeds must be controlled
to prevent the engine from stalling during low-speed
idle and to keep the speed from exceeding the maximum
desired limits desired by the manufacturer. The main
reason that a diesel requires a governor is that a diesel
engine operates with excess air under all loads and
Even though it is not part of the fuel system, a
governor is directly related to this system since it
functions to regulate speed by the control of fuel or of
the air-fuel mixture, depending on the type of engine. In
diesel engines governors are connected in the linkage
between the throttle and the fuel injectors. The
governor acts through the fuel injection equipment to
regulate the amount of fuel delivered to the cylinders.
As a result the governor holds engine speed reasonably
constant during fluctuations in load.
Before discussing governor types and operations,
governor terms should be addressed and understood
since they are commonly used when discussing engine
To understand why different types of governors are
needed for different kinds of job, you will need to know
the meaning of several terms that are used in describing
the characteristics of action of the governor.
Maximum no-load speed or high idle is used to
describe the highest engine rpm obtainable when
the throttle linkage is moved to its maximum
position with no load applied to the engine.
Maximum full-load speed or rated speed is
used to indicate the engine rpm at which a
particular engine will produce its maximum
designed horsepower setting as stated by the
Idle or low-idle speed is used to indicate the
normal speed at which the engine will rotate with
the throttle linkage in the released or closed
Work capacity is used to describe the amount of
available work energy that can be produced to
the output shaft of the governor.
Stability refers to the ability of the governor to
maintain speed with either constant or varying
loads without hunting.
Speed droop is used to express the difference in
the change in the governor rotating speed which
causes the output shaft of the governor to move
from its full-open throttle position to its full-
closed position or vice versa.
Hunting is a repeated and sometimes rhythmic
variation of speed due to overcontrol by the
governor. Also called speed drift.
Sensitivity is an expression of how quickly the
governor responds to a speed change.
Response time is normally the time taken in
seconds for the fuel linkage to be moved from a
no-load to a full-load position.
Isochronous is used to indicate zero-droop
capability. In others words, the full-load and no-
load speeds are the same.
Overrun is used to express the action of the
governor when the engine exceeds its maximum