A simple mechanical governor must overcome friction in the linkages and exert a control force. These forces act in different simple servo is however unstable because the pilot valve allows full oil to the servo or it drains the servo thus giving full fuel on or off conditions.
Stability could be provided by making the value larger than the servo supply ports. This would however reintroduce a dead band. The solution is to provide feed back from the fuel linkage and this comes about with speed droop i.e. graph of % speed against engine load directions depending upon whether the load is increasing or decreasing. This in fact produces a dead band in which the governor will not exert any fuel control. By having the rotating weights only move a pilot valve which directs oil to or from a servo. The control force and friction forces are eliminated and hence the dead band is removed. The Speed control is carried out by varying the main spring tension acting on the flywheels.
Feed back is provided by means of a lever connecting the servo cylinder pilot valve and rotating weights. Consider a load increase, the engine slows down and the rotating weights move inwards lowering that end of the floating lever. This causes the pilot valve to open allowing oil to the servo. This rises increasing the fuel and lifting that end of the floating lever. This raises the pilot valve until it closes the port to the servo cylinder. The engine will now runs with increased fuel on increased load but at a reduced speed. This provides stability but the engine cannot run at the same speed for all loads i.e. it is isochronous. It is possible to introduce a mechanism which will provide a further slight fuel change to restore the engine speed to normal. This is known as compensation.
The compensating governor allows the engine to operate at a constant speed no matter what the load. Adjustment can be considered in two stages. Compensation and speed droop. In practice these stages operate together and a number of times during each load change. Consider a load increase, the engine speed slows , rotating weights move inwards and the pilot valve is opened. Oil flows to the servo which rises increasing the fuel. At the same time the transmitting piston is moved downwards forcing oil to the receiving piston which rises compressing the top centering spring . this also lifts the pilot valve shutting off further oil to the servo. The engine now operates with increased load and increased fuel but at a reduced speed.
Compensation takes place to provide a further slight fuel change to return the speed to normal. The centering spring forces the receiving piston downwards and oil escapes through the adjustable valve.. This lowers that end of the floating lever until both centering springs are equally loaded and that end of the floating lever is in its original position. The pilot valve is open slightly allowing oil to the servo which gives a further slight increase in fuel. The engine speeds up, the rotating weights move out and the pilot valve is lifted until it is closed. The engine now operates with increased load, increased fuel but at the same original speed. Engines connected together by gearing or electrically are required to operate at the same speed. The loads they take will depend upon the values of their speed droops. Ideally both governors should have the same speed droop but they will never have identical values of droop. Slight variations will exist due to mechanical considerations. This is not a problem for most operating situations
When setting engine systems it is usual to have one engine governor as the master and the others as the slave. The actual practicalities will vary depending upon the installation.
Load sharing
Engines share load increase in the inverse ratios of their speed droop. i.e. the lower the value of droop the greater the share of the load increase taken
Electric governors have become in favour due to their compact size, rapid response and high reliability allied to low maintenance costs.
The main part of the governor is the controller and signal amplifier. This receives a D.C. signal proportional to the engine speed and compares it to a speed set signal. The difference between the measured value (engine speed) and the set value is the offset; this offset value is passed to the output circuit which produces an appropriate output signal. In this case, a signal which raises or lowers the fuel rack by an amount dependent on the degree of offset. This system is inherently stable due to the feedback layout.
For this system the engine speed is measured using an alternator driven off the camshaft- this is a common arrangement. The speed set signal is typically supplied by the bridge control arrangement via the engine management system.
An arrangement for a generator set might replace the camshaft driven alternator with a tapping off the alternator output. The frequency of the alternator output is now the measured value. In addition a load sensing element can be introduced detecting changes in current flow. For increased current, that is an increased electrical load, the governor can act to supply increased fuel before the engine has began to slow.
Stability could be provided by making the value larger than the servo supply ports. This would however reintroduce a dead band. The solution is to provide feed back from the fuel linkage and this comes about with speed droop i.e. graph of % speed against engine load directions depending upon whether the load is increasing or decreasing. This in fact produces a dead band in which the governor will not exert any fuel control. By having the rotating weights only move a pilot valve which directs oil to or from a servo. The control force and friction forces are eliminated and hence the dead band is removed. The Speed control is carried out by varying the main spring tension acting on the flywheels.
Feed back is provided by means of a lever connecting the servo cylinder pilot valve and rotating weights. Consider a load increase, the engine slows down and the rotating weights move inwards lowering that end of the floating lever. This causes the pilot valve to open allowing oil to the servo. This rises increasing the fuel and lifting that end of the floating lever. This raises the pilot valve until it closes the port to the servo cylinder. The engine will now runs with increased fuel on increased load but at a reduced speed. This provides stability but the engine cannot run at the same speed for all loads i.e. it is isochronous. It is possible to introduce a mechanism which will provide a further slight fuel change to restore the engine speed to normal. This is known as compensation.
The compensating governor allows the engine to operate at a constant speed no matter what the load. Adjustment can be considered in two stages. Compensation and speed droop. In practice these stages operate together and a number of times during each load change. Consider a load increase, the engine speed slows , rotating weights move inwards and the pilot valve is opened. Oil flows to the servo which rises increasing the fuel. At the same time the transmitting piston is moved downwards forcing oil to the receiving piston which rises compressing the top centering spring . this also lifts the pilot valve shutting off further oil to the servo. The engine now operates with increased load and increased fuel but at a reduced speed.
Compensation takes place to provide a further slight fuel change to return the speed to normal. The centering spring forces the receiving piston downwards and oil escapes through the adjustable valve.. This lowers that end of the floating lever until both centering springs are equally loaded and that end of the floating lever is in its original position. The pilot valve is open slightly allowing oil to the servo which gives a further slight increase in fuel. The engine speeds up, the rotating weights move out and the pilot valve is lifted until it is closed. The engine now operates with increased load, increased fuel but at the same original speed. Engines connected together by gearing or electrically are required to operate at the same speed. The loads they take will depend upon the values of their speed droops. Ideally both governors should have the same speed droop but they will never have identical values of droop. Slight variations will exist due to mechanical considerations. This is not a problem for most operating situations
When setting engine systems it is usual to have one engine governor as the master and the others as the slave. The actual practicalities will vary depending upon the installation.
Load sharing
Engines share load increase in the inverse ratios of their speed droop. i.e. the lower the value of droop the greater the share of the load increase taken
Electric Governors:
Electric governors have become in favour due to their compact size, rapid response and high reliability allied to low maintenance costs.
The main part of the governor is the controller and signal amplifier. This receives a D.C. signal proportional to the engine speed and compares it to a speed set signal. The difference between the measured value (engine speed) and the set value is the offset; this offset value is passed to the output circuit which produces an appropriate output signal. In this case, a signal which raises or lowers the fuel rack by an amount dependent on the degree of offset. This system is inherently stable due to the feedback layout.
For this system the engine speed is measured using an alternator driven off the camshaft- this is a common arrangement. The speed set signal is typically supplied by the bridge control arrangement via the engine management system.
An arrangement for a generator set might replace the camshaft driven alternator with a tapping off the alternator output. The frequency of the alternator output is now the measured value. In addition a load sensing element can be introduced detecting changes in current flow. For increased current, that is an increased electrical load, the governor can act to supply increased fuel before the engine has began to slow.
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