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The speed governor senses speed by the use of flyweights hich are mounted on a ballhead. The ball head s driven mechanically by the engine. Therefore, the entrifugal force of the flyweights is a function of core ngine speed. This centrifugal force is applied as an pward force on the governor pilot valve plunger. The pward force is opposed by the force of the restoring pring which is set by the power lever cam, full throttle cam, and power trim adjustment. Since the upward force is a function of engine speed and the downward force is set by the power lever, full throttle cam and power trim adjustment, the position of the pilot valve plunger is determined by comparing engll1e speed with the desired speed.
When engine speed and desired speed are identical, the flyweight force equals the spring force and the pilot valve plunger is on null. This is called "on speed" condition.
When engine is greater than desired speed, the pilot valve plunger is moved up by the force of the flyweights, porting governor intermediate pressure to Pb. This is called "overspeed" condition and it lowers the governor servo pressure through the buffer piston. Lower servo pressure allows Pcr on the right side of the governor servo piston to move the piston to the left which turns the fuel valve rotor counterclockwise, decreasing fuel flow to the engine. Decreasing fuel flow results in decreasing engine speed, which decreases the force of the flyweights.The restoring spring then retu rns the pilot valve plunger to null.
When engine speed is less than desired speed, the flyweight force is less than the restoring spring force and the pilot valve plunger moves down, porting governor intermediate pressure to Pc. This is called "underspeed" condition and it increases the governor servo pressure through the buffer piston. Higher servo pressure overcomes Pcr on the right side of the governor servo piston to move the servo piston to the right which turns the fuel valve rotor clockwise, increasing fuel flow to the engine.Increasing fuel flow results in increasing engine speed, which increases the force of the flyweights. The force of the flyweights overcomes the force of the restoring spring, pushing the pilot valve plunger back to null.
The governor buffer system compensates for the time delay between changes in fuel flow to the engine and the change in engine speed. This action is necessary to provide stability to the governor system. The buffer piston provides a feedback force to the governor pilot valve plunger that is proportional to fuel flow to the governor servo, as supplied by the governor pilot valve.
This feedback force is used to null the governor pilot valve in advance of the engine reaching the desired speed. Hydraulic lines for sensing the pressure drop across the buffer piston are routed to each side of the compensating land.
The buffer spring is an extension-compression spring that is attached to the buffer piston sleeve on one end and the buffer piston on the other end. Consequently,the buffer piston is free to slide in the sleeve but is restrained by the buffer spring.
The buffer piston sleeve contains bypass ports that provide a parallel flow path around the buffer piston orifice when the buffer piston moves a predetermined distance.
The amount of stroke to bypass from the free length of the buffer piston spring is commonly referred to as anticipation and is usually expressed in terms of percent speed error.
When there is no flow into or out of the buffer system, the buffer piston remains at a zero displacement position established by the free length of the buffer spring. When small, steady amounts of fuel are allowed to enter or leave the buffer system, the buffer piston orifice causes a slight restriction to the flow and produces a pressure drop across the buffer piston orifice. Since the pressure drop is also applied across the buffer piston, which is restrained by the buffer spring, the buffer piston will be displaced proportional to the pressure drop. This condition. occurs when small transients in speed occur while operating on the governor.
If a large amount of fuel is suddently ported into or out of the buffer system, a small portion of the flow will pass through the buffer piston orifice but the rest of the flow will produce a large displacement of both the buffer piston and the fluid volume downstream of the buffer piston. | The buffer piston continues to move until the flow driving the system stops, or until it moves far enough to bypass the excess flow through the bypass ports in the buffer piston sleeve. Under the latter condition, the buffer piston tends to regulate on bypass until the excessive flow condition is reduced. This condition exists when large transients in speed occur while operating on the governor. |
The buffer piston produces a pressure drop between governor intermediate pressure and governor servo pressure whenever there is flow through the buffer system.
Since the pressure drop across the buffer piston is applied to the compensating land, any displacement of the governor pilot valve plunger results in flow through the buffer system that produces a feedback force to the governor plunger. This adds to the flyweight force and the governor pilot valve plunger will reach null before the commanded speed is reached, thus preventing an overshoot.
| The jump and rate limiter permits the governor to make rapid changes to the fuel flow for about 15 percent of the full fuel flow range. Beyond this limit, the jump and rate piston meters on the accel or decel ports, depending on whether fuel flow is increasing or decreasing, and flow of the governor servo supply is restricted by the orifice in the jump and rate piston |
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发表于 2012-4-12 15:46:46
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