Speed Governor - Class 4 Exam

Speed Governor - Class 4 Exam

Speed Governor A speed governor is a device for automatically controlling the speed of an engine by regulating energy medium driving the engine. Majorities of diesel engines are fitted with a speed governor. A marine engine directly coupled to a propeller can be run without a speed governor by fixing up the fuel pump quantity adjustment to any desired value. If the engine speed starts to rise, the propeller torque rises rapidly and so allows only a small change in speed. Definitions Speed droop A progressive drop in speed as load is picked up by the prime mover from no load to full load without manually changing the speed setting of the governor. It may be expressed in percentage and is equal to no load rpm

minus full load rpm divided by full load rpm times 100. Sensitivity The smallest speed change that will induce the governor to alter the fuel flow. A sensitive governor will give a large movement of the control sleeve for only a small change in the radius in which the flyweights are revolving. Stability The ability of the governor to correct a speed disturbance with a minimum of false motions. As the governor becomes more sensitive it becomes less stable. Compensation: A mechanical and/or hydraulic action that prevents over correction of the fuel supply is called compensation. Compensation in a governor produces transient speed droop i.e. a momentary speed droop

during a speed correction. Dead band: A narrow band of speed variation through which the governor makes no correction of fuel supply. Hunting: When the load on an engine changes the governor tends to over-control or under-control. This causes a fluctuation in rpm, which is called hunting. For example, if load is removed from an engine, the speed increases some amount above normal and the governor comes into operation and reduces fuel supply. Due to friction and time lag, the governor causes the fuel reduction to be in excess of that required and the fall in speed is too much. This causes the governor to increase fuel and the engine rpm goes slightly above normal. This swing in speed above and below the mean operating rpm for that load continues until equilibrium is reached and hunting ceases. The more sensitive a governor

is, the greater will be the tendency to hunt. Isochronous: Constant speed i.e. the same average speed regardless of load. Mechanical governor It depends for its operation upon the fact that a force is required to compel a mass to follow a circular path. This force is proportional to square of the speed of rotation and to the first power of the distance of the

mass from the axis of rotation. A pair of spherical flyweight at the end of two arms pivoted near the axis of rotation in such a way that the flyweights can move radially in a plane through the axis. A spring forces the collar down (Hartnell type). Thus when the weights move outwardly, the collar moves up. The centrifugal force acts at right angles to axis of rotation. It exerts a torque about its pivot equal to the product of the force times the vertical distance of the ball from the pivot. The torque of gravity opposes this torque, which is equal to the weight of the ball times the horizontal distance to the pivot plus the spring force if any. The two will be balanced at equilibrium. Thus as speed increases the centrifugal force increases and the ball moves outward, decreasing the centrifugal force torque arm and increasing the gravity torque arm and the

spring force until equilibrium is reached. This results in a unique equilibrium position of flyweights and collar for each speed of rotation. The collar is connected to the fuel control mechanism so as to close it as the flyweights move out ward. It should be noted that the unique relationship between speed and position of flyweight and collar no longer exists if friction is added to the system. For this reason, as the speed increases from an equilibrium condition, the centrifugal force torque must reach a value equal to the gravity torque plus spring force plus that due to friction before movement of collar results. Similarly on decreasing speed the centrifugal torque must go down to a value equal to the gravity torque pus spring force minus that due to friction before movement of collar results. The result is deadband or region in which the speed may change without producing a corrective motion of the collar. To reduce dead-band, the friction should to be reduced, size of weights may be increased and the speed of rotation of the

flyweights may be increased. With help of suitable gear ratio, the speed of rotation of the flyweights is increased. Sketch (a) shows the equilibrium value of the governor at half load of the engine. The flyweights are in vertical position with equilibrium between the spring and the flyweights. Assuming that the engine goes from half load to full load and in doing so slows down. The centrifugal force decreases and the result is as shown in figure (b). The fuel rack increases so that engine can run at the higher load. When a new equilibrium is achieved, the flyweights will not be able to go back to the original vertical position that it had when running at half load. This is because the fuel rack must be increased to

allow greater fuel flow for higher load. Now, assuming the opposite, the engine goes from half load to no load, the new position of the governor can be seen in figure (c). The engine rpm increases, causing the flyweights to move further out and shifting the fuel rack to close the fuel supply to the engine. The engine will be running at a higher rpm at no load condition than at half load condition. Thus the mechanical governor automatically has a speed droop, a decrease in rpm from no load to full load. Mechanical-hydraulic (servo) governors In its simplest form, the flyweight and the pilot valve control a spring-loaded power

piston. The flyweights are driven at a speed proportional to that of the engine, the power piston is connected to the fuel racks and a pump driven by the engine supplies the oil under pressure to move the power piston. The simple combination of flyweight and directly connected pilot valve has only one equilibrium position: the position in which valve is closed, neither admitting oil to nor discharging oil from the power piston. For a given setting of the top end of the speeder spring, the flyweight and the pilot valve will be in this position at only one speed. In other words, such a speed-sensitive device

is inherently isochronous. But such a system is also unstable. The engine speed does not instantly assume a value proportional to the fuel rack position due to inertia of the rotating mass. Therefore, if the engine speed is below the governor speed setting, the pilot valve is positioned to move the power piston to increase the fuel. By the time the speed has increased to the set value so that pilot valve is centred and the power piston stopped, the fuel has already been increased too much and engine runs at a higher speed. This opens the pilot valve the other way and the power piston reduces fuel rack. As before, when the speed of the engine gets to the right value, the fuel control has traveled too far and the engine underspeeds and the whole cycle continues to repeat. Some means of stabilizing the such a system must be added to get a satisfactory governor. Mechanical-hydraulic (servo) governors with speed droop

Speed droop in the above governor can be provided by mechanical interconnection between the power piston and governor speed setting such that as fuel is increased, the speed setting is decreased. The engine 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 not isochronous. Such a device may consist of a lever of suitable ratio

between power piston and speeder spring. Since the application of feedback reduces output, it is considered negative. Negative feed back is a method employed to reduce sensitivity to the governor. As compared to an arrangement without mechanical feedback, the rack movement is arrested earlier in this case. This introduces stability but brings in a permanent speed droop. Speed droop occurs since there is a reduction in speeder spring tension following load increase. The equilibrium relationship between speed setting and power piston position for such a system may be represented by a line sloping or drooping downward to indicate decreased speed setting with movement of the power piston in the increase fuel direction. Mechanical-hydraulic (servo) governors with isochronous speed It is sometimes desirable to have an isolated diesel engine run isochronously (speed constant

regardless of load within the capacity of the engine). Mechanical feed back introduces stability but also brings in a speed droop. To eliminate this droop without sacrificing on stability, compensation is applied. Compensation introduces a transient speed droop and helps in achieving isochronous operation. A temporary readjustment of speed setting with power piston to produce the stabilizing speed droop characteristic followed by a relatively slow return of the speed to its original value. Woodward PG Type Governor In this direct application of pressure to the pilot valve plunger, adding or subtracting from the speeder spring force

in order to effect a change in speed setting. The pressure oil actuating power piston is required to deflect a buffer piston against a centring spring load which produces a pressure differential across a receiving piston attached to the pilot valve. A needle valve permits equalizing of pressure across the pilot valve receiving piston to restore the initial speed setting. In operation as oil flows to the power piston, the buffer piston is moved against the force of the centring spring, resulting in higher pressure on the lower side of the receiving piston of the pilot valve which produces an upward force on the pilot valve. This in effect decreases the force, which the flyweights must balance, resulting in centring of the pilot valve at a lower speed, thus

providing speed droop. As the displaced oil is permitted to leak through the needle valve, the buffer piston returns to its equilibrium position, the differential pressure disappears and the speed setting reverts to its original value. Woodward UG Type Governor The mechanism for giving compensation action consists of two hydraulic cylinders and pistons, one being considerably larger than the other. The bottoms of the cylinders are connected with each other. An adjustable needle valve has an opening has an opening into the

connection between the cylinders and allows oil to bleed into or out of the cylinders from or to the oil sump formed by governor casing. Consider a load increase occurring. The rpm will drop, the flyweights will move in, the floating lever, with the right side as fulcrum point will pivot and push the control valve down. This will allow pressure oil to flow to the power piston, thereby move the rack towards higher fuel. In the second stage, the rack moves up toward increase in fuel, the transmitting piston comes down, the receiving piston goes up. When the receiving piston goes up, the compensating spring gets compressed and also the floating lever will pivot about the left end and lift the control valve and close the flow of pressure oil to the power piston.

In the third stage, due to increase in fuel to the engine, the rpm increases and the flyweights regain their previous position. But this happens at the same at which the compensating spring recovers its original position. The floating lever just pivots about the point where the control valve plunger is connected The piston in larger cylinder is referred to as transmitting piston while that in the smaller cylinder is called the receiver piston. The receiver piston is spring loaded in such a manner that it acts in opposition to the governor sleeve spring. The compensation mechanism is connected to the governor in the following manner: the larger piston is connected to the power piston through a lever. The lever is pivoted in the middle and has a fixed point and the larger piston and power piston are connected to the opposite ends of the lever. The smaller piston, control valve and governor sleeve are connected through a three-point floating lever.

The control valve is connected to the mid point of the lever. The springs on the governor sleeve and the smaller piston, which act in opposition to one another, always hunt back to the same equilibrium position at any constant load and the engine speed remains exactly the same for any load. Except for the transient speed reduction, which must occur when load changes, the governor is isochronous. The compensating lever The compensating lever adjusts the fulcrum point of the compensating link. When this lever is at the minimum, the fulcrum

point is towards the transmitting piston. In this case only a small fraction of the power piston movement is fed to the receiving piston. Compensation is minimum and a large correction of fuel rack will occur following a given rpm change. The compensating needle The needle valve controls the rate of oil flow in or out of the compensating system. With more opening of the needle valve, the compensation spring will release itself to normal state quickly. The needle valve is adjusted in such a way that rate at which the flyweights return to normal matches the rate of release of the compensating ring. Isochronous operation Isochronous operation is confirmed by the fact that finally at stability, the arrangement of floating lever resets to the position before the disturbance occurred.

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