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).

GOVERNORS

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 speeds.

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 speed regulation.

Terms

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 manufacturer.

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 position,

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 governed speed. 5-8