For purposes of description, the PGE and PGEV governors are considered to consist of three major functional sections: a basic governor section, a speed setting section, and a load control section.
Basic Governing Section (Figure 3-1)
This section consists of an oil pump, two accumulators, a speeder spring, a flyweight head assembly, a thrust bearing, a pilot valve plunger, a rotating bushing, a buffer compensation system, and a power cylinder.
The governor drive shaft passes through the governor base and engages the rotating bushing. The pump supplies pressure oil for operation of the basic governor section, the speed setting section, the load control system (except applications using a remote vane servo, or where engine oil is supplied to the control system), and all other auxiliary features or devices.
A spring loaded accumulator and relief valve system maintains governor oil operating pressure. When operating pressure is reached the spring pressure is overcome and the oil is released to sump.
Direction of rotation is normally determined by the engine manufacturer or Woodward Governor Company. Governor rotation is either fixed cw, fixed ccw, or reversible. Figure 3-1 shows the arrangement for reversible rotation with four check valves in the oil pump passages. For fixed rotation the four check valves are removed and two plugs are placed in two of the passages to allow only one direction of flow.
NOTE: Governors for locomotives are built for one direction of rotation.
The governor drive rotates the oil pump and pilot valve bushing. The flyweight head assembly is driven by the rotating pilot valve bushing. A thrust bearing rides on top of the flyweight-head toes permitting the rotational motion between the downward force of the speeder spring and the upward force of the flyweights.
The relative motion between the bushing and plunger minimizes static friction. There are several styles of flyweight head assemblies available. The exact style used depends upon the engine drive train to the governor. A solid head is used where the drive is relatively free of torsional vibrations. "Spring driven" and "spring driven oil damped" head assemblies are used to attenuate objectional levels of torsional vibration which may be imparted to the governor from the engine. These vibrations may originate from a source other than the drive itself but reach the governor through the drive connection. Unless minimized or eliminated, these vibrations are sensed as speed changes and the governor will continually adjust the fuel rack in an attempt to maintain a constant speed.
The greater of two opposing forces moves the pilot valve plunger up or down. Flyweight force tends to lift the plunger while speeder spring force tends to lower the plunger. When the engine is on speed at any speed setting, these forces are balanced and the flyweights assume a vertical position. In this position, the control land on the pilot valve plunger is centered over the regulating port(s) in the rotating bushing. No oil, other than leakage make up, flows to or from the buffer compensation system or power cylinder. A change in either of these two forces will move the plunger from its centered position. The plunger will be lowered (1) when the governor speed setting is unchanged but an additional load slows the engine and governor (thereby decreasing flyweight force), or (2) when engine speed is unchanged but speeder spring force is increased to raise the governor speed setting. Similarly, the pilot valve plunger will be raised (1) when the governor speed setting is unchanged but load is removed from the engine causing an increase in engine and governor speed (and hence, an increase in flyweight force), or (2) where engine speed is unchanged but speeder spring force is reduced to lower the governor speed setting. When the plunger is lowered (an underspeed condition), pressure oil is directed into the buffer compensation system and power cylinder to raise the power piston and increase fuel. When lifted (an overspeed condition), oil is permitted to drain from these areas to sump and the power piston moves downward to decrease fuel.
The buffer piston, springs, and needle valve in the hydraulic circuits between the pilot valve plunger and power cylinder make up the buffer compensation system. This system functions to stabilize the governing action by minimizing over or undershoot following a change in governor speed setting or a change in load on the engine. It establishes a temporary negative feedback signal (temporary droop) in the form of a pressure differential which is applied across the compensation land of the pilot valve plunger. The flow of oil into or out of the buffer system displaces the buffer piston in the direction flow. This movement increases the loading on one spring while decreasing the load on the other and creates a slight difference in the pressures on either side of the piston with the higher pressure on the side opposite the spring being compressed. These pressures are transmitted to opposite sides of the plunger compensation land and produce a net force, upward or downward, which assists in recentering the plunger whenever a fuel correction is made.
Speed Setting Or Load Increase
Increasing the speed setting or increasing load on the engine at a given speed setting have an identical effect. In either case, the flyweights move inward (underspeed) due to the increase in speeder spring force or, to the decrease in centrifugal force caused by the decrease in engine speed as load is added. The movement of the flyweights is translated into a downward movement of the pilot valve plunger. This directs pressure oil into the buffer system, causing the buffer piston to move toward the power cylinder. The oil displaced by the movement of the buffer piston forces the power piston to move upward in the increase fuel direction. The oil pressures on either side of the buffer piston are simultaneously transmitted to the plunger compensation land with the higher pressure on the lower side. The net upward force thus produced is added to flyweight force and assists in restoring the balance of forces and recentering the pilot valve plunger.
In effect, this enables the governor to cut off the additional fuel needed for acceleration by stopping the power piston when it has reached a point corresponding to that amount of fuel required for steady state operation at the new higher speed or load. As the engine continues to accelerate toward the set speed, the compensation force is gradually dissipated to offset the continuing increase in flyweight force. This is done by equalizing the pressures on each side of the compensation land through the needle valve at a rate proportional to the continued rate of acceleration. The rate of dissipation is the same as the rate of increase in flyweight force, the pressure differential is reduced to zero at the instant flyweight force becomes exactly equal to speeder spring force. This minimizes speed overshoot and permits the governor to quickly re-establish stable operation. The needle valve setting determines the rate at which the differential pressure is dissipated and allows the governor to be "matched" to the characteristics of the engine. The compressed buffer spring returns the buffer piston to its centered position as the pressure differential is dissipated.
Whenever large changes in speed setting or load are made, the buffer piston will move far enough to uncover a bypass port in the buffer cylinder. This limits the pressure differential across the buffer piston and permits oil to flow directly to the power cylinder. Thus, the power piston is made to respond quickly to large changes in speed setting or load.
Speed Setting Or Load Decrease
Decreasing the speed setting or load on the engine at a given speed setting are identical in effect and cause a reverse action to that described above. The flyweights move outward (overspeed), lifting the pilot valve plunger and allowing oil to drain from the buffer compensation system. The buffer piston moves away from the power cylinder, permitting oil to drain from the area under the power piston which then moves downward in the decrease fuel direction. The differential pressures acting across the compensation land produce a net downward force, assisting the speeder spring in recentering the pilot valve plunger slightly before the engine has fully decelerated. This stops power piston movement when it has reached a point corresponding to that amount of fuel required for steady state operation at the new lower speed or load. Dissipation of the compensation force in the same manner as previously described and minimizes speed undershoot.
Compensation Cutoff
With large decreases in speed or load, the power piston moves to the "no fuel" position and blocks the compensation oil passage between the power cylinder and needle valve to prevent normal equalization of the compensation pressures. This holds the buffer piston off center and increases the level of the pressure transmitted to the upper side of the plunger compensation land. The increased pressure differential, added to the effect of the speeder spring, temporarily increases the governor speed setting. The governor will thus begin corrective action as soon as engine speed drops below the temporary speed setting and start the power piston upward to restore the fuel supply in sufficient time to prevent a large underspeed transient. The above action is sometimes referred to as "compensation cutoff". When the upward movement of the power piston again uncovers the compensation oil passage, normal compensating action will resume and stabilize engine speed at the actual speed setting of the governor.
NOTE: Due to the location of the compensation cutoff port in the power cylinder wall, the governor/fuel rack linkage must be adjusted so the power piston "gap" does not exceed 1-1/32 inches at idle. no load.
Speed Setting Section (Figure 3-1)
This section consists of a speed setting cylinder, a speed setting pilot valve plunger housed within a rotating bushing, four speed setting solenoids, a triangular plate, and a restoring linkage mechanism.
General
The speed setting section provides a method of changing the compression (force) of the speeder spring which opposes flyweight centrifugal force. It does this by controlling the position of the speed setting piston in the speed setting cylinder. When control oil is admitted to the cylinder, the piston moves downward, compressing the speeder spring and raising the speed setting, When oil is allowed to drain from the cylinder, the piston spring forces the piston upward, reducing speeder spring force and lowering the speed setting. The flow of oil into or out of the speed setting cylinder is regulated by the speed setting pilot valve plunger in the rotating bushing. The plunger is controlled by the solenoids which provide incremental control of speed in equally spaced steps. An integral gear on the governor flyweight head drives the bushing through a splined mating gear on the lower end of the bushing.
The rate of movement of the speed setting piston over its full downward stroke (idle to maximum speed) is usually retarded to occur over some specific time interval to minimize exhaust smoke during accelerations. This is done by admitting governor pressure oil into the rotating bushing through an orifice which registers with the main supply port once in every revolution of the bushing. This retards the rate at which oil is supplied to the control port in the bushing and thus, the rate of oil flow to the speed setting cylinder. The diameter of the orifice determines the specific time interval which may be anywhere within a nominal range of 1 to 50 seconds. Typical engine acceleration periods for switching and suburban service is approximately 5 seconds; for freight or passenger service, approximately 15 to 30 seconds; turbo-supercharged engines the timing may be as much as 50 seconds to permit the supercharger to accelerate with the engine.
On turbo-supercharged units, the rate of movement of the speed setting piston over its full upward stroke (maximum to idle speed) is also retarded to prevent compressor surge during decelerations. This timing may be anywhere within a nominal range of 1 to 15 seconds. In this case, a vertical slot in the drain land of the pilot valve plunger registers with a second orifice in the rotating bushing once each revolution. This retards the rate at which the oil is allowed to drain from the speed setting cylinder.
Speed Setting
Three of the four speed setting solenoids, A, B, and C, actuate the pilot valve plunger by controlling the movement of the triangular plate which rests on top of the floating lever attached to the plunger. The fourth solenoid D controls the position of the rotating bushing with respect to the plunger. Energizing the A, B, and C solenoids, singly or in various combinations, depresses the triangular plate a predetermined distance for each combination. The downward movement of the plate is transmitted through the floating lever to uncenter the plunger. This directs intermittent oil pressure to the speed cylinder, forcing the speed setting piston downward to increase the Governor speed setting. Energizing the D solenoid pushes the rotating bushing downward and opens the control port to drain oil from the speed setting cylinder and thus decrease the speed setting. An identifying letter will be found on the solenoid bracket adjacent to each solenoid.
Figure 3-2 is an additional aid in understanding the various governor components. The oil passages are simplified and color coded for ease in following the oil flow through the system. The lower half of the governor functions to maintain a constant engine speed by controlling fuel flow to the engine cylinders. The upper half of the governor consists of the column and cover and internal related parts for changing governor speed setting, the control valve for the load regulator, and shutdown and protective devices.
Advancing or retarding the throttle control from one step to the next energizes or de-energizes the solenoids in various combinations to increase or decrease engine speeds in approximately equal increments.
Whenever a change in speed setting is made, the movement of the speed setting piston, downward or upward, is transmitted or fed back through the restoring linkage and floating lever to recenter the pilot valve plunger. This stops the flow of oil into or out of the speed setting cylinder at a position corresponding to that speed setting.
Speed Setting Increase
When one or more of the solenoids is energized (or de-energized) by moving the throttle to a higher step, the solenoid plungers move downward and depress the triangular plate and in turn the floating lever. Since the right end of the lever is attached to the lower end of the restoring link, the left end of the lever is forced downward to uncenter (lower) the pilot valve plunger. This directs intermediate pressure oil to the speed setting cylinder which forces the piston downward to further compress the speeder spring and thereby increase the speed setting.
Figure 3-1. Schematic Diagram of Typical PGEV Governor
The downward movement of the piston is transmitted through the restoring linkage to the right end of the floating lever and causes it to move downward a proportional amount. This allows the loading spring under the pilot valve plunger to raise the plunger, with the floating lever pivoting about the triangular plate. This action will continue until the plunger is again recentered, stopping the flow of oil to the speed setting cylinder at the instant the piston reaches the new lower position corresponding to the increased speed setting.
Speed Setting Decrease
Moving the throttle to a lower step de-energizes (or energizes) one of more of the solenoids and causes a reverse action to that of speed setting increase. The triangular plate moves upward, being held in contact with the solenoid plungers by a loading spring. This allows the loading spring under the pilot valve plunger to uncenter (raise) the plunger which allows oil to drain from the speed setting cylinder. The upward movement of the speed setting piston is transmitted through the restoring linkage to recenter the plunger.
Normal Shutdown (See Figure 3-1)
Under normal operating conditions, the engine is shut down by moving the throttle to the STOP position. This energizes the D solenoid pushing the rotating bushing down and opening the control port to drain the oil from the speed setting cylinder. The speed setting piston then moves up lifting the shutdown nuts and shutdown rod in the process. This lifts the governor pilot- valve plunger, draining oil from the buffer compensation system and allowing the power piston to move down to the shutdown (no fuel) position. The upward movement of the speed-setting piston is limited by the stop screw.
The speed-setting-piston stop screw (Figure 3-1) limits piston rod travel. Restarting the engine is easier because less oil volume is required to move the speed setting piston down.
Load Control Section (Figure 3-1)
In most governor applications, the primary function of the governor is to automatically maintain a specific engine speed under varying load conditions by controlling the fuel flow to the engine. With the locomotive governor, a secondary function is included to maintain a constant engine power output at each specific speed setting. Thus, for each throttle setting, there is both a constant engine speed and a predetermined, fixed rate of fuel flow required. To satisfy both conditions, the load on the engine must be adjusted as the locomotive operating conditions (speed and locomotive auxiliaries); vary and it is the function of the load control to do this.
NOTE: Maintaining a constant engine speed does not mean that locomotive road speed will also be constant.
Control of engine load is achieved by regulating engine speed and fuel setting. This is done by adjusting the generator field-excitation current through the use of a vane servo controlled variable resistance in the generator-field circuit. The vane servo is controlled by the load control pilot valve and related linkage in the governor. The load-control linkage is so arranged that for each speed setting there is only one fuel setting (engine power output) at which the load- control pilot-valve-plunger will be centered.
An increase or decrease in either governor speed setting or engine load will change fuel flow. The power piston moving in either the increase or decrease fuel direction will (through the floating lever linkage) move the load-control pilot valve up or down respectively. The vane servo decreases or increases field excitation and in turn engine load.
Figure 3-2. Sectional Diagram PGE Governor
In some applications, the vane servo is a remote unit connected to the governor through tubing and uses oil from the engine lubricating system for its operation. The vane servo may be either a rotary or piston type. In other applications, the vane servo is integral with the governor and uses governor oil for its operation. The integral unit consists of a commutator about which a set of moveable brushes rotate to change the value of the resistance in the generator field excitation circuit. The brushes are driven by the servomotor which, in turn, is controlled by the load-control pilot valve. Remote units, usually provided by the locomotive manufacturer, differ in size and construction from the integral unit, however, the method of control and operation is essentially identical.
The load-control pilot valve plunger is suspended from the load-control floating lever. The lever is connected to the power-piston tailrod at one end and to the speed setting piston rod at the other end. Any movement of either or both pistons causes a corresponding movement of the plunger which is housed within a non-rotating bushing. Pressure oil is supplied to the plunger either externally from the engine lubricating oil system or internally from the governor oil pump. Two lands on the plunger control the flow of oil to or from the vane servo. When internal governor oil is used for operation of the vane servo, a supply (cutoff) valve is provided in the oil supply passage to the load-control valve. The supply valve is closed during starting so that all available oil from the governor oil pump is delivered to the speed setting and power pistons to quickly open the fuel injectors and thus minimize cranking time. After the engine starts, the increase in governor oil pressure opens the supply valve and restores normal load control system operation. This valve also serves a secondary system to control the vane servo response rate (timing).
Operation With Load Increase
Assuming that the train is in motion and that the electrical load is balanced with the desired engine fuel (power output) at the existing governor-speed setting, the load control system will be stationary with the pilot valve plunger centered. When a compressor turns on (or any situation occurs that increases load) electrical load on the generator is increased and transmitted to the engine. Engine speed decreases and the governor increases fuel flow to bring the engine back to the preset speed while still carrying the added load.
The power piston moves upward simultaneously raising the right end of the load control floating lever which, in turn, lifts the pilot valve plunger above center. This directs pressure oil through the upper control port in the bushing to the decrease-excitation side of the vane servo while opening the lower port in the bushing to drain. With a reduction in load, the engine will overspeed and the governor will then act to reduce fuel. The reduction in field excitation current and engine fuel will continue until the power piston and floating lever have returned to their original position. This recenters the pilot valve plunger and stops the servomotor. Consequently, the electrical load is reduced sufficiently to again balance the required engine power output (fuel flow). At this point, the engine will have also returned to an on-speed condition.
Operation With Load Decrease
Under the same conditions as stated above, a decrease in electrical load will reduce engine load and cause the engine to decrease fuel and, in the process, lower the right end of the floating lever. This moves the pilot-valve plunger below center and directs pressure oil through the lower control port in the bushing to the increase excitation side of the vane servo. With an increase in load, the engine will underspeed and the governor will act to increase fuel. This increase in field excitation current and engine fuel will continue until the power piston and floating lever have returned to their original positions. This recenters the pilot-valve plunger and stops the servomotor. Consequently the electrical load is increased sufficiently to again balance engine-power output with the engine on-speed.
Operation With Speed Setting Increase
Advancing the throttle to a higher step causes the piston to move downward. This lowers the left end of the load-control floating lever which displaces the load control pilot valve plunger below center. Pressure oil is directed to the increase-excitation side of the vane servo. The governor acts to increase fuel to compensate for both the increase in speed setting and the simultaneous increase in electrical load. As the power piston moves upward, it raises the right end of the floating lever to return the pilot-valve plunger to its centered position. This stops the servomotor as the power piston reaches its new higher position corresponding to the increased speed setting. At this point, the electrical load has been sufficiently increased to balance the increase in engine power output.
Operation With Speed Setting Decrease
Moving the throttle to a lower speed setting causes the speed setting piston to move upward. This raises the left end of the load-control floating lever and lifts the pilot valve plunger above center. Pressure oil is directed to the decrease excitation side of the vane servo. The governor acts to decrease fuel to compensate both for the decrease in speed setting and the simultaneous decrease in electrical load. As the power piston moves downward, it lowers the right end of the floating lever to return the pilot valve plunger to its centered position. This stops the servomotor as the power piston reaches its new lower position corresponding to the decreased speed setting. At this point, the electrical load has been sufficiently decreased to balance the decrease in engine power output.
Load Control Balancing
The rate of vane servo movement (timing) must be controlled to effect a controlled rate of load application and to provide stability of the overall system. Several methods are commonly used to provide a balanced action and are identical in that they restrict the flow of oil to and from the vane servo and thus determine its rate of movement.
In some governors, the oil flow is restricted by the number, size and position of a group of orifice holes (restricted porting) in the load control bushing, which are opened and closed by the movement of the pilot valve plunger. With this arrangement, a progressively increasing (or decreasing) rate of movement occurs depending on the degree of movement of the plunger. These rates will not necessarily be the same in both directions.
Governors used with remote servos may have a separate timing valve assembly consisting of two adjustable ball check valves in series within a common housing. The assembly may be externally mounted on the governor or remotely located and connected into one of the lines between the governor and servo. The valves are individually adjustable to provide the desired maximum rate of movement over the full travel of the servomotor in either the increase or decrease excitation direction.
Governors with an integral-vane servo may use a similar arrangement to the timing valve assembly, except that the ball valves are individually housed and internally installed in the top of the governor column
Minimum Or Maximum Field Start Adjustment
The load control system in the governor may be set up for either "Minimum or Maximum" field start.
MINIMUM FIELD START - builds up engine load slowly, providing a smooth take-up of slack in the train. The load control pilot valve is mechanically set above center with the throttle in IDLE position. Field excitation is retarded due to the retarded position of the pilot valve plunger. The vane servo rheostat remains in the minimum excitation position until the throttle is moved in the increase speed direction. This lowers the load control pilot valve to the recenter position and beyond to increase excitation.
MAXIMUM FIELD START - enables the engine load to build up immediately, for rapid accelerations. The load control pilot valve is mechanically set below center with the throttle in IDLE position. Field excitation is advanced due to the advanced position of the pilot valve plunger. The vane servo rheostat remains in the maximum excitation position until the throttle is moved in the increase speed direction to raise the load control pilot valve.
Load Control Override (Optional)
Under certain conditions of locomotive operation (transition, maximum-field start and wheel slip), it is sometimes desirable or necessary to override the normal action of the governor load control mechanism to cause a reduction in generator excitation current when it would normally respond by increasing excitation current.
The load control override mechanism in the governor consists of an overriding solenoid (ORS), a two-position overriding control valve, and an overriding piston within a cylinder which surrounds the upper end of the load control pilot-valve plunger. See Figure 3-1.
Energizing the ORS pushes the overriding valve plunger down, closing the drain to Sump and allowing pressure oil to flow into the overriding cylinder. The overriding piston moves upward, contacting the spring collar on the stem of the pilot-valve plunger and lifting the plunger above its centered position. The slot in the link connecting the pilot-valve plunger to the floating lever permits the plunger to rise independently of the lever. This directs pressure oil to the decrease-excitation side of the vane servo, thus reducing generator output. When the ORS is de-energized, the overriding-valve plunger moves upward, closing the pressure port and allowing the oil to drain from the overriding cylinder. This restores normal load-control system operation.
TRANSITION - A condition where the electrical circuits between the generator and traction motors are automatically changed, as road speed changes, to provide more efficient transmission of electrical power. Overriding is used in this circumstance to protect the switchgear from arcing which would occur during transition if high current existed in the traction motor circuits.
MODIFIED MAXIMUM FIELD START - A variation used in some applications where the load control mechanism is arranged for maximum field start but it is desirable to hold the vane servo rheostat in the minimum excitation position at idle speed. Normal operation is restored when the throttle is advanced to the first step, allowing the vane servo to increase excitation toward maximum as the train starts.
WHEEL SLIP - When rail and load conditions cause drive wheel slip, an immediate decrease in load occurs at the traction motors and generator. The resulting increase in engine speed would normally cause the load-control system to respond by increasing generator output at a time when there is no demand. Overriding is used in this circumstance in conjunction with wheel-slip relays, if the locomotive is so equipped, to cause a reduction in generator output until wheel slippage ceases.
Operation of the ORS is done through automatic switching devices.
FAST UNLOADING - may be used in conjunction with the load control override mechanism. It provides a quick unloading of the integral-vane servo. Fast unloading cannot be used for remote-servo applications using external timing valves. Two methods are available. (1) The lower end of the load-control pilot-valve plunger has an additional land. An additional port in the bushing bypasses the restricted port in the bushing. When the ORS energizes, the plunger uncenters upward and oil is released through the extra port for fast unloading of the vane servo, (2) The plunger has an additional land on its lower end. Instead of a bypass port, the bushing has small orifice holes in one side of the bushing. A radial slot is located in the bushing and midway between the orifice holes. As the ORS energizes, the plunger uncenters upward releasing oil through the orifice holes first and if a large displacement of the pilot valve takes place, oil is released from the servo through both the orifice holes and the radial slot. Both methods allow the vane servo to move rapidly in the direction of minimum excitation.
Integral Vane Servo Assembly (Optional)
The integral-vane servo is used with low wattage pilot or amplifier type excitation systems. It functions in conjunction with the load control mechanism in the governor to automatically regulate generator output and thereby maintain a constant engine-power output at each throttle setting.
Figure 3-3a. Integral Vane Servo (with Resistor Pack Assembly
Vane servos use either a resistor pack or a ceramic resistor. Figure 3-3a shows a vane servo with a resistor pack. A vane servo with a ceramic resistor appears in Figure 3-3b.
Figure 3-3b. Integral Vane Servo (with Ceramic Resistor)
Both servos have a vane-type rotary servomotor. Drain oil flows through the covers of both units to cool the resistor pack or the ceramic resistor. The commutator and resistor pack or ceramic resistor are electrically insulated from the vane servo unit.
The vane servo shaft output shaft has external serrations with one missing tooth. The slot formed by the missing tooth mates with a ridge on the brush drive shaft to make sure that the two shafts assemble correctly.
NOTE: Contact Woodward Governor Company for details and actual limits.
Whenever the load-control pilot valve in the governor column is uncentered, pressure oil is directed to one or the other side is the vane servo while the opposite side is opened to drain. This causes the vane to rotate which, in turn, rotates the contact-brush assembly about the commutator. The position of the brushes on the commutator segments determines the circuit resistance and thereby the generator field-excitation current.
Lube Oil Pressure Shutdown At Alarm (See Figure 3-4)
Figure 3-4. Lube Oil Pressure Shutdown and Alarm
Engine oil pressure is directed to the oil-pressure diaphragm. The shutdown-valve plunger is connected to the diaphragm which has three forces acting on it; load-spring and engine-oil pressures act to move it to the right, governor speed-setting-servo oil acts to move it to the left. Normally, load-spring and engine-oil pressures hold the diaphragm and shutdown-valve plunger to the right, permitting oil to the left of the shutdown piston to drain to sump. When engine lube-oil pressure drops below a safe level, speed-setting-servo oil pressure (which is dependent on the speed setting and on the rate of the speed-setting servo spring) overcomes the load spring and engine-oil pressure forces and moves the diaphragm and shutdown-valve plunger to the left. Governor pressure oil is directed around the shutdown-valve plunger to the shutdown piston and moves it to the right. The shutdown piston moves the inner spring and shutdown plunger to the right. The differential piston allows a high engine-lube oil-pressure trip point without a corresponding increase in the speed-setting-servo oil pressure. The engine-lube oil pressure required to initiate shutdown is increased. When the shutdown plunger moves sufficiently, it trips the alarm switch.
In addition, oil trapped above the governor speed-setting-servo piston flows around the smaller diameter on the left end of the shutdown plunger and drains to sump. This action allows the speed-setting-servo spring to raise the speed setting servo piston. When the piston moves up sufficiently, the piston rod lifts the shutdown nuts and rod. The shutdown rod lifts the governor pilot-valve plunger. When it is lifted above its centered position, oil trapped below the power piston drains to sump and the power piston moves to the fuel off position
NOTE: The shutdown plunger must be pushed back in to restart the engine except on modulating governors.
Adjustment of the spring seat in the field is not recommended. This adjustment biases the lube-oil-pressure required for shutdown. Adjust the spring seat on a test stand during testing after an overhaul. No further adjustment should be necessary.
Water Pressure Shutdown At Alarm
A water box monitors engine water pressure to shut down the engine when water pressure is too low. This device operates like the Lube Oil Shutdown device described above except that low water pressure initiates shutdown of the engine.
Bypass Valve
Governor pressure oil is supplied to the shutdown piston in one of two ways, depending on the speed setting. At rated speed settings, the bypass valve is moved down off its seat by the speed-changing mechanism. Governor pressure oil passes directly to the shutdown piston and immediately initiates engine shutdown in the event of lube-oil failure.
When starting and at idle speeds, the bypass valve is closed and governor pressure oil travels through an intermittent-flow orifice in the rotating speed- setting-pilot-valve bushing. With each rotation of the bushing, a slot in the bushing registers with an oil-supply passage in the governor column and a hole in the adjustment sleeve. Thus, intermittent pressure oil is passed to the shutdown-valve plunger. The adjustment sleeve may be turned (by readjusting the time-delay pointer) so the cross-sectional area of the oil passage is increased or decreased. Thus, the volume of oil supplied with each rotation of the bushing is increased or decreased. Turning the pointer cw increases volume and decreases the time required to pass sufficient oil to initiate shutdown.
Fuel Limiter
General
The fuel limiter is an auxiliary system designed primarily for use on Woodward PG load control governors installed on turbo-supercharged locomotive engines. It is used with manifold air pressure as a reference. This governor is equipped with a load-control-overriding solenoid and provisions for fast unloading.
The function of the load control is independent of the fuel limiter. They are related only through an optional common reference to manifold air pressure. Figure 3-6a illustrates the basic fuel limiter, the load-control override and bias linkages installed on a locomotive governor equipped with load control, an overriding solenoid, and solenoid speed setting.
During acceleration, on turbo-supercharged engines, it is possible to supply more fuel to the engine than can be burned with the available air. This results from the normal lag of supercharger speed, and consequently manifold air pressure decreases with respect to engine speed.
The fuel limiter restricts the movement of the governor power piston toward the increase-fuel direction, limiting engine fuel during acceleration as a function of manifold air pressure (an approximation of the weight of air available at any instant). Fuel limiting improves the fuel-to-air ratio and, during acceleration, allows complete combustion. This improves acceleration and reduces smoke. Fuel limiting also protects the engine if the turbo-supercharger fails or reductions in engine air supply occur.
Figure 3-5. Typical Limited Acceleration Fuel Schedule Curve
Figure 3-5 illustrates the unlimited, limited, and steady-state fuel schedules for a typical engine together with a typical acceleration transient from one steady-state condition to another.
Description
The fuel limiter (Figure 3-6a) is essentially a floating lever, a bellcrank, a pressure sensor and cam, and a hydraulic amplifier together with a feedback lever and a fuel-limit lever. The right end of the floating lever is connected to the tailrod of the governor power piston and pivots about one leg of the bellcrank. The left end of the floating level rests on the right end of the hydraulic-amplifier feedback lever. The position of the bellcrank, and therefore the position of the floating-lever pivot point, is determined by the position of the fuel-limit cam. Raising the floating-lever pivot as manifold air pressure increases, allows the governor power piston to move upward a proportionally greater distance before fuel limiting occurs.
The pressure sensor is a force-balance device consisting of an inlet check valve, an orifice-pack restriction, a piston-and-cam assembly, a restoring spring, a bleed valve, and either a gauge-pressure or an absolute-pressure bellows arrangement. The sensor establishes a corresponding piston (and cam) position for each different manifold air pressure. The relationship between manifold air pressure and governor power-piston position (fuel flow) where limiting occurs is determined by the profile and angular tilt of the cam. Cam profiles are either linear or non-linear depending on engine and turbo-supercharger characteristics. The hydraulic amplifier is a pilot-operated, single-acting hydraulic cylinder. The amplifier provides the force necessary to overcome the resistance of the speeder spring, lift the shutdown rod and recenter the governor pilot-valve plunger when the fuel limit is reached for a given manifold air pressure.
Operation
Pressured oil enters the fuel limiter through the inlet check valve. Oil is directed to the upper side of the sensor piston and through the orifice-pack restriction to the under side of the sensor piston. The inlet check valve prevents siphoning of the oil from the limiter housing during shutdown periods and omits the time lag to refill the orifice pack and piston cylinder. This prevents the sensor piston from going to maximum-fuel position during start-up. The bleed valve regulates the rate of oil flow from the area under the sensor piston to sump as a function of manifold air pressure. When the bleed valve bypasses a greater flow of oil from this area than is admitted through the orifice pack, the sensor piston moves downward. Conversely, reducing the bypass-oil flow to less than that admitted causes the sensor piston to rise. When the inflow and outflow of oil are equal, the piston remains stationary.
The sensing element of the absolute-pressure-type fuel limiter consists of two opposed, flexible, metallic bellows of equal effective area. The upper bellows is evacuated, and the lower bellows senses manifold air pressure. A spacer joins the bellows at the center while the outer end of each bellows is restrained to prevent movement. Manifold air pressure acting internally on the sensing bellows produces a force causing the spacer to move toward the evacuated bellows. The evacuated bellows provides an absolute reference, therefore, the sensing-bellows force is directly proportional to the absolute manifold-air pressure. Movement of the bellows spacer is transmitted through an output strap and a bleed-valve pin to the bleed-valve diaphragm.
The sensing element of the gauge-pressure-type fuel limiter consists of a single, flexible, metallic bellows. Movement of the gauge-pressure bellows is transmitted directly to the bleed-valve pin. The bellows force tends to open the bleed valve while the restoring-spring force tends to close the valve.
When these opposing forces balance, the bleed-valve diaphragm floats just off of its seat bypassing oil to sump. This rate of oil flow maintains a constant volume of oil in the area under the sensor piston.
Assume that the governor speed setting is advanced to a higher speed setting and a higher manifold-air pressure. The governor power piston moves upward supplying the additional fuel required for engine acceleration. Since manifold air pressure lags engine acceleration, the fuel-limiter cam and bellcrank initially remain stationary until manifold air pressure rises.
As the governor power piston moves upward increasing fuel, the fuel-limit floating lever pivots about the upper leg of the bellcrank and depresses the right end of the feedback lever on the hydraulic amplifier. This pushes the amplifier pilot-valve plunger below center, allowing pressured oil to flow into the area under the amplifier piston, causing the piston to rise. As the piston rises, it simultaneously lifts the left ends of both the fuel-limiter lever and the feedback lever.
Figure 3-6a. Schematic Diagram, Fuel Limiter and Linkage
When the fuel limit lever contacts the fuel-limit nut on the shutdown bushing, it begins lifting the shutdown rod to recenter the governor pilot-valve plunger. The upward movements of the fuel-limit and feedback levers continue until the left end of the feedback lever raises far enough to recenter the amplifier-pilot-valve plunger and stop the flow of oil to the amplifier piston. At this point, the fuel-limit lever recenters the governor pilot-valve plunger, stopping the upward movement of the governor power piston. This limits the amount of fuel to provide a proper fuel/air ratio for efficient burning. Although the governor flyweights are in an underspeed condition at this time, the power piston remains stationary until manifold air pressure rises.
As engine speed and load increase, manifold air pressure rises after a short time lag. The increase in manifold air pressure produces a proportionate increase in the sensing-bellows force. The bellows force, now greater than the restoring-spring force, causes the bleed-valve diaphragm to move further off its seat. This allows a greater flow of oil to sump than is admitted through the orifice pack. Governor oil pressure acting on the upper side of the sensor piston forces the piston (and cam) downward and, in the process, further compresses the restoring spring. The piston continues its downward movement until the net increase in restoring-spring force equals the net increase in bellows force. This restores the bellows and bleed-valve diaphragm to their original positions. At this point, the outflow of oil is again equal to the inflow, and the piston stops moving.
As the sensor piston and cam move downward in response to a rise in manifold air pressure, the bellcrank rotates in a cw direction. This allows the floating-lever pivot point, the left end of the lever, and in turn the hydraulic-amplifier pilot-valve plunger to rise.
The loading spring under the pilot-valve plunger maintains a positive contact between the plunger, levers, bellcrank, and cam. When the pilot-valve plunger rises above center, the oil under the amplifier piston bleeds to sump through a drilled passage in the center of the plunger. The passage in the plunger restricts the rate of oil flow to sump and decreases the rate of movement of the amplifier piston to minimize hunting. As the amplifier piston moves downward, the left end of the fuel-limit lever also moves downward. This lowers the shutdown rod which in turn lowers the governor pilot-valve plunger and increases engine fuel.
The above events occur in continuous and rapid sequence. Normal governor operation is overridden during an acceleration transient and engine fuel is scheduled as a function of manifold air pressure, regardless of governor speed setting. To prevent interference with normal governing action during steady-state operation, the sensor piston and cam continue their downward movement until sufficiently below the effective limiting point.
Conversely, a drop in manifold air pressure rotates the bellcrank ccw. This lowers the fuel-limit lever, depressing the pilot-valve plunger, and releases pressured oil to the underside of the amplifier piston. The shutdown rod and governor pilot-valve plunger are raised, releasing oil from the power-piston cylinder to sump, and decreasing fuel to the engine. The left end of the fuel-limit floating lever pivots upwards releasing the hydraulic amplifier pilot-valve plunger upward. As the control land of the pilot-valve plunger opens the port from the piston cylinder, oil is bled to sump through a hole in the pilot-valve-plunger shaft. The shutdown rod is lowered, allowing the governor pilot-valve plunger to recenter.
Load Control Override Linkage
The load-control-override linkage (Figure 3-6a) consists of an overriding lever which connects the left end of the fuel-limit lever to the load-control-overriding solenoid through a pin-and--yield spring combination. The overriding solenoid adjustment set screw must be adjusted to fully depress the overriding-solenoid plunger completely, at a point just before the fuel-limit lever contacts the fuel limit nut. Pressured oil is released to the underside of the overriding piston, lifting the load-control pilot-valve plunger in the decrease-load direction. During acceleration transients, when fuel limiting occurs, the integral-vane servomotor begins to unload prior to an acceleration lag, reducing overload and poor acceleration. Depending on engine and turbo supercharger characteristics, premature unloading can permit the engine to accelerate quickly and raise the manifold air pressure rapidly enough to prevent any fuel limiting from taking place.
NOTE: On this governor application, load on the engine is adjusted through a servomotor-operated rheostat in the field excitation circuit of a generator. The servomotor, in turn, is controlled through the governor's load-control system.
As engine speed nears the new setting, and manifold air pressure rises, a downward movement of the fuel-limit lever permits the overriding-solenoid plunger to rise. Oil is released from under the load-control-overriding piston to sump, lowering the load-control pilot-valve plunger. The load-control pilot-valve plunger moves down, releasing pressured oil to the vane servomotor, and increases excitation. This increases load in proportion to the increase in engine speed.
LVDT Load Control System
The secondary purpose of the governor is to maintain a definite horsepower output of the engine for a specific speed setting of the governor. To achieve this objective, the LVDT (Linear Variable Differential Transformer), adjusts (through external circuitry) the generator field excitation current to keep the traction motor load at a set point.
The LVDT provides a linear output voltage over a displacement measuring range of 1.000 inch. It consists of a primary coil and two separate secondary windings of enameled copper wire wound on a common cylindrical, resin bonded, glass fibre core. A ferromagnetic stainless-steel case houses the coil assembly and provides full electromagnetic and electrostatic shielding. All internal voids are filled with epoxy resin. The spring-loaded captive core is manufactured from nickel iron alloy and moves freely in its guides. The device requires an excitation of 6 Vac (RMS) at 2.5 KHz. The device has a resolution of 25 mV per 0.001 inch displacement.
The excitation voltage is applied to the primary winding. The two secondary windings are wound in opposite directions to each other. When the movable core is centered, the secondary voltages are equal in amplitude, but opposite in phase. At other core positions the secondary voltages will still be opposite in phase but no longer equal in amplitude. The amount of amplitude difference is proportional to the distance of the core from center.
A rectifier assembly connected to the secondary windings of the LVDT converts the secondaries' ac voltages to a dc voltage. The amplitude of the dc voltage is proportional to core distance from center and the polarity of the dc indicates the core direction from center. The rectified dc voltage has a resolution of 10 mV per inch.
The LVDT core center position depends on the governor speed setting. The core is positioned by a plunger which senses engine load and governor speed setting. A change in engine load (horsepower) moves the core from center position. When the engine load increases, the LVDT core moves to cause a decrease in generator excitation voltage to decrease load. Since the load is reduced, the governor decreases fuel and, at the same time, the LVDT core position. This continues until the engine speed is that called for by the governor speed setting and the LVDT core is once again centered.
The horsepower is now at the designed value for the present speed setting. The governor has responded to an increase in load without a long term change in speed. When engine load decreases the response is similar, but in opposite directions.
Figure 3-6b. Start Fuel Limiter Linkage
Start Fuel Limiter
The Starting Fuel Limiter minimizes the tendency of engines to flood when starting and minimizes excessive smoking during engine cranking. The starting fuel-limiter linkage consists of a fuel limit lever, an adjustable limit screw, and a lever spring. Figure 3-6b shows the limiter linkage arrangement.
The limit lever extends over the floating lever between the speed-setting servo piston and the power piston tailrod. The tailrod is positioned as a function of the fuel setting. The speed-setting servo piston is positioned as a function of speed setting. When the tailrod moves up (as fuel increases) sufficiently far, the floating lever lifts the free end of the fuel limit lever. The lever spring continually urges the limit liver down in the direction to contact the floating lever. The adjustable limit screw attaches to a lug in the fuel-limit lever. The head of the limit screw fits under the shutdown nuts. The limit screw is adjusted so that the shutdown nuts (and shutdown rod) are lifted as the tailrod reaches the point corresponding to the desired maximum starting fuel. Lifting the shutdown nuts and shutdown rod prevents the governor from increasing fuel further.
When the governor speed setting is increased, the speed setting servo position moves down. This action moves the floating lever down away from the fuel limit lever so that the limit level no longer restricts fuel.