Governor basics

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Identify the following components:-
• Flyweights
• Droop fulcrum lever

• Compensation range fulcrum lever
• Compensation rate needle valve
• Servo/power piston
• Synchronizer clutch

Explain the purpose/function of the following:-
• Load limit
• Droop fulcrum lever
• Compensation range fulcrum lever
• Remote synchronizing/speed setting
• Pressure oil accumulators
• Gear pump N/R valves

Droop

·               the difference between stable full load speed and stable no load speed.

Isochronous

·               constant, stable speed regardless of load.

Governor Effort

·               the resultant force due to imbalance between centrifugal and spring force which is available to move the fuel control.

Dead band

·               the change in speed required, increase or decrease, before the governor takes action.

Stability

·               ability to achieve equilibrium (stable speed) for different loads.

Sensitivity

·               measure of the deviation in speed required to achieve a given output. A sensitive or fine governor requires only a small change in speed to give a large output movement. Similarly a coarse governor requires a large speed change for a small output movement.

Diesel engine governors are essentially devices which are required to control either the engine speed or the engine load. They must be capable of detecting change in speed or load and then apply the corrective action required with sufficient force to overcome fuel linkage resistance.

They may be designed to control either with a fall in speed with an increase in load (droop), or with a constant speed regardless of load (isochronous).

We can start by looking at a basic mechanical governor and from this develop the more sophisticated mechanical - hydraulic and electronic governor.

Basic mechanical governor

A basic mechanical, or centrifugal, governor consists of a pair of flyweights connected to a pair of pivoted bell cranks and driven from the engine. The bell crank arms act on a spring loaded collar connected to the fuel pump linkage. It operates in such a way that an increase in engine speed (e.g.due to a load reduction) results in the flyweights moving out, due to the increased centrifugal force, which causes the pivoted bell cranks to act to move the collar and fuel pump linkage in the direction of reduced fuel. This also acts to compress the spring and so increases the spring force. The increased spring force and centrifugal forces will reach an equilibrium point at some new, higher speed.

If we take the practical case into consideration then we must consider the effect of friction within the governor and linkage. The engine speed must now rise by an amount that will generate an increase in the centrifugal force sufficient to overcome the friction in the mechanism before any governor action occurs. Similarly if we consider a fall in engine speed then the decrease in centrifugal force will have to be equal to the frictional force present before any movement of the fuel linkage can occur. This range of speed without response is termed the dead band and is inherent in this type of governor.

Before equilibrium at the new speed is reached there will be a certain amount of speed fluctuation, or hunting, about the steady speed. The period and magnitude of these fluctuations will depend on the sensitivity of the governor which in turn depends on the masses of the flyweights. The greater the mass the coarser the regulation and this results in a short period but a large temporary deviation. Conversely small masses give a long period with small temporary deviations. It is also possible to increase the governor effort by using large masses and high speeds (step up gearing) but there is a limit to this.

It is obvious that this type of governor would be of little use where close control of speed is necessary, (e.g. A C generators), or where a high governor effort is required, (e.g. movement of main engine fuel rack). These deficiencies can be overcome by the introduction of hydraulic servo governors.

Inadequacies of mechanical governors

·               spring type

·               limited governor effort

·               high friction potential

·               unstable

·               unique speed with each loadGovernor adjustments

Compensation range

This adjustment is effected by altering the position of the fulcrum on the lever connection between the servo out put linkage and the compensation transmitter piston.

Compensation rate

This adjustment is effected by setting the compensation needle valve once the range has been set. In practice the needle valve is opened out until the control just becomes unstable and then the valve is closed back in 1/4 of a turn.

In addition to setting the compensation rate at initial installation the needle valve is also used to assist cleaning of the governor when flushing through and also to  purge system of air in the event that air becomes entrained in the hydraulic circuit. Adjustment necessary if response is sluggish (possibly air in system), engine speed hunts or if engine over-speeds at start up.

Local speed setting (Synchonising control)

Primarily there in case remote system fails but can be used for fine tuning and for testing of overspeed.

Speed setting indicator (Synchonising indicator)

Essentially a counter to indicate the number of turns that have been made by the speed setting control. Usually has a minimum and maximum fuel setting limit facility incorporated.

Load limiting

Limits the working maximum fuel and so maximum load within the absolute maximum limit set for the governor. This facility is usually required following overhaul when a limit on the engine load is required during running in. Can also be used for local shutdown in some cases depending on the minimum load setting set above.

Speed droop

This setting is effected by altering the position of the droop lever fulcrum point. The setting is adjusted to give stable operation of the governor. Also needs to be used to select isochronous or droop operation when generator load sharing is required.