GB1602192A - Variable ratio power transmission - Google Patents

Description

(54) VARIABLE RATIO POWER TRANSMISSION (71) We, THE BRITISH INTERNAL COM- BUSTION ENGINE RESEARCH INSTITUTE LIMITED, of 111-112 Buckingham Avenue, Slough SL1 4PH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a variable ratio transmission, particularly but not exclusively for a vehicle.

Transmission systems have the general disadvantage that the kinetic energy of motion possessed by a vehicle at any speed is completely wasted when the vehicle is deacelerated to a lower speed or brought to rest. Fuel is then consumed at a high rate in order to accelerate the vehicle back to its normal operating speed.

The energy wasted in retardation is dissipated as heat in the vehicle braking system causing general wear and tear.

The power required to maintain a vehicle at steady speed on level ground is a small proportion of the power required to accelerate the vehicle to that speed, or to maintain speed on an uphill gradient. To meet these short term high power requirements it is necessary to equip the vehicle with a high power engine which of necessity spends much of its working life at an uneconomical lower power setting.

Altemative regenerative energy storage systems have been proposed, in particular the flywheel system and the electrical battery storage system. The limitations of the flywheel system are firstly its short storage capacity and secondly its gyroscopic effect in turning corners which is a potential hazard. The battery system, besides adding appreciable weight, does not eliminate in its present form the transmission losses.

Engines operate most economically within a narrow band of speeds and in the region of two thirds to full peak torque capability. With fixed ratio transmissions road conditions normally make the ideal operating conditions unattainable, hence fuel is wasted because of uneconomical engine settings. the use of a manuallyoperated multiple-ratio transmission system requires complexity of manufacture and skill in driving. Automatic vehicle transmission systems are generally based on a fluid flywheel type mechanism which is inefficient because of slippage between the driving and driven members in the low to medium speed range. Hydrostatic variable ratio transmission systems are known in some specialised applications. These are based on an engine driven pump supplying fluid to a hydraulic motor, one or both of these being of variable displacement. These systems suffer from inefficiency due to relatively high fluid flow losses at light loads.

In any commonly used transmission system, fluctiations of load must be met by equal fluctuation of engine power. This type of operation causes high exhaust emissions and high fuel comsumption.

An object of the present invention is to provide a variable-ratio transmission in which the above-mantioned disadvantages are at least reduced.

The invention provides a variable speed ratio power transmission device comprising first and second hydraulically and mechanically interconnected hydrostatic machines, the first machine having a rotatable input member connectable to a source of mechanical power and a rotatable output member and being arranged to operate primarily as a pump, the hydraulic displacement of the first machine being proportional to the difference in speed between the input and output members whereby fluid flow is reduced as said speed difference is reduced until power is transmitted substantially by torque reaction between the input and output members of the first machine, the second machine being arranged to operate primarily as a motor and having a non-rotating casing, a variable displacement mechanism and a rotatable output member which is coupled to the output member of the first machine, the device further comprising an hydraulic fluid reservoir for supplying fluid to the first machine, a first valve having a first and a second position and an hydraulic accumulator, the first and second valves, when in respective first positions, perrritting fluid to flow from the first machine to the second machine and then return to the reservoir, the first valve, when in its second position, preventing flow of fluid from the first machine to the second machine, the second valve, when in its second position, connecting inlet and outlet ports of the second machine to the reservoir, the accumulator being chargeable with hydraulic fluid from the first machine during periods of low power demand and chargeable from the second machine during periods of operation of the second machine as a pump to produce a braking effect and connectable to the second machine during periods of high power demand for supplementing the fluid supplied by the first machine.

As direct drive is approached and reached, fluid flow losses are reduced and then eliminated.

Furthermore at all times while power is being transmitted in the forward direction, part of the output torque of the system is provided by reaction between the input and output shafts of the first machine, thus reducing fluid flow losses and reducing the size requirement of the transmission components. This reaction is hereinafter termed torque reaction.

Preferably, the input member of the first machine is connectable to an engine serving as the source of mechanical power and the first valve has a third position for connecting the first machine to the accumulator such that hydraulic fluid thereby supplied to the first machine will cause the engine to start.

The invention further provides a vehicle in which the engine is coupled to the above-described transmission device.

The advantages described below are related to vehicle operation. similar advantages are obtained in other applications.

During periods of normal vehicle deceleration the second machine acts as a pump which charges an accumulator. This relieves the vehicle braking system of its duty and provides a source of stored energy which is available when required for acceleration. Consequently the engine peak power requirement is reduced and a relatively small engine may be used. This leads to reduced engine friction losses with further reductions in fuel consumption.

The hydrostatic motor displacement, which controls speed and torque ratios, is completely variable and automatically controlled according to accelerator position and road load. This ensures that, except when idling, the engine operates in its economical mode of high torque and moderate speed, which are the conditions for high efficiency and minimum noise.

As the accumulator in the transmission device provides for sudden power requirements, the engine can be operated at comparatively steady conditions which result in low noxious emissions in the exhaust. The clutch and gearbox are eliminated and the use of the conventional braking system is reduced.

An embodiment of the invention will now be described with reference to the accompanying drawing which shows schematically one arrangement of the transmission device in accordance with the invention.

A prime mover 1, normally an internal combustion engine, is directly coupled to one half, shaft or housing, of a hydrostatic pump/motor 2 having ports 2a and 2b. The second half of the pump/motor 2 is coupled to, or integral with, a rotating member 3 of a hydrostatic motor/pump 4 which has a non-rotatable casing and a variable displacement mechanism. The motor/pump 4 has ports 4a and 4b.

The terms pump/motor and motor/pump are descriptive references to the main function of the two components, i.e. pump and motor respectively. For some operating modes the pump/ motor is used as a motor, and for some modes the motor/pump is used as a pump. (For convenience hereafter the pump/motor and motor/ pump are referred to merely as the pump and motor respectively).

Fluid flow through the pump 2 is controlled by a valve 5 which is positioned either by linkage from an accelerator pedal 6, or a first starter solenoid 7. the valve 5 has three positions 5a, 5b, Sc for the idle, drive and start modes respectively.

Fluid flow thorugh the motor 4 is controlled by a valve 8. Three positions are shown namely 8a for idle and direct drive, 8b for acceleration and 8c for retardation. Positions 8b and 8c may be physically the same valve position but the flows in each of the two passages are reversed.

The position of the motor valve 8 is controlled by a variable displacement mechanism 9 of the motor 4 via linkage 10. This causes the valve 8 to move to the idling position 8a, so relieving all pressure loading on the motor, whenever the motor is at zero displacement hence doing no work. The position of the motor displacement mechanism 9 is controlled in thr forward driving mode by the balance of two springs, namely spring 11 which is loaded by depression of the accelerator pedal 6, and spring 12, which is a return spring. The displacement mechanism 9 is also influenced by transmission pressure in a space 13, which is connected to a transmission line 24, and acts on a piston 14, which tends to increase motor displacement as transmission pressure rises. The displacement mechanism is also influenced by a speed sensing device 25 which tends to reduce displacement as output speed rises.

A stop 26 prevents the return spring 12 from moving the displacement mechanism into the reverse displacement position when the loading due to spring 11 and pressure in the space 13 is removed.

The motor valve 8 is moved to position 8c via linkage 15 when the motor displacement mechanism 9 is moved to reverse either by a brake pedal 16 or by a reverse gear selector 17.

Brake pedal 16 varies the displacement control according to the degree of braking required.

Reverse selector 17 sets the displacement mechanism 9 to its maximum position and hence provides the greatest speed ratio for reverse operation.

A cooler 23 is installed in the circuit to control oil temperature. A start/boost valve 18 controls the flow of fluid in and out of an accumulator 19. In position 18a the valve 18 opens automatically to charge the accumulator 19 when transmission pressure exceeds accumulator pressure. In position 1 8b this valve is opened by a second solenoid 20 to provide oil at pressure from the accumulator 19 for starting or acceleration boost.

A pressure limiting valve 21 opens to discharge oil to a reservoir 22 whenever transmission pressure becomes excessive.

A conventional braking system is retained on the vehicle for emergency stops and for parking. The overall braking system is arranged so that braking is performed by the regenerative transmission during the first part of travel of the brake pedal, and the emergency braking becomes operative only when the pedal is more fully depressed.

Reverse gear is selected while the pump and motor are at idling settings, then valve positions 5a and 8a connect all the ports 2a, 2b, 4a, and 4b to the low pressure reservoir. Reverse gear selector 17 is then moved to lock the motor displacement mechanism into full reverse displacement. Depression of the accelerator pedal 6 then moves the pump and motor control valves to drive positions 5b and 8b. The pump then pressurise the transmission line 24 and the transmission pressure is fed to the motor inlet port 4b. Since the motor displacement mechanism 9 is now in reverse the shaft 3 turns in reverse rotation. Torque reaction in the pump tends to tum shaft 3 in the forward direction but the displacement of the motor, being much greater than that of the pump, prevails and the net effort on shaft 3 turns it in reverse.

Servo systems may be used to assist the effort exerted by the operation of accelerator pedal 6, the brake pedal 16, reverse selector 17 and the speed sensing mechanism 25.

Known means are used for the initial charging of the accumulator 19 with hydraulic fluid.

The operation of the system in various driving modes is as follows: Starting is achieved with the motor control valve 8 at position 8a with both motor ports connected to low pressure so as to exert no torque, and the output drive shaft 3 is locked by the vehicle parking brake. Operation of the solenoid 7 moves the pump control valve to position Sc and operation of the solenoid 20 opens the start valve 18 to position 18b. Oil then flows from the accumulator 19 via valves 18b and Sc to the input port 2a of the pump/ motor 2 which then operates as a motor and cranks the engine 1 until it fires. The Pump/ motor outlet port 2b is connected via the valve Sc to the low pressure reservoir 22. For idling, the pump valve 5 is at position 5a and the motor valve 8 is at position 8a at which inlet and outlet ports of both components are at low pressure so no work is done. Springs 11 and 12 controlling the motor displacement mechanism 9 are set to cause the motor displacement mechanism 9 to be at zero.

Depression of the accelerator 6 moves the pump control valve 5 to position 5b which connects the pump inlet port 2a to the low pressure reservoir 22, and connects the pump outlet port 2b to the transmission pressure line 24.

Depression of the accelerator 6 also compresses spring 11 to move the motor diaplcaement mechanism 9 to forward displacement and the motor valve 8 to position 8b at which the motor inlet port 4a is connected to the transmission pressure line 24, and the motor outlet port 4b is connected to the low pressure reservoir 22 via the cooler 23. Movement of the motor displacement mechanism 9 and the motor control valve 8 is partly opposed by the return spring 12.

Before the motor 4 turns there can be no fluid flow, hence the output of the pump 1 causes a rise of pressure in the transmission line 24 and space 13. The resultant increase of load on piston 14 assists the spring 11 to icnrease the motor displacement, thus creating high speed and torque ratios for acceleration from rest. As the motor speed increases, its oil demand causes a lowering of pressure in the transmission line 24 hence a reduction of force on piston 14 due to the falling transmission pressure in space 13. This enables the spring 12 to reduce the motor displacement so reducing the speed and torque ratios. The reduced displacement, by reducing oil demand, offsets the original drop in transmission pressure. As the transmission output shaft 3 increases in speed a speed sensitive device 25 exerts a force on the return spring 12 thus urging the motor displacement mechanism 9 to reduce the motor displacement. The effect of the speed sensitive device 25 is that the transmission pressure at which direct drive can be maintained rises with the speed of the output shaft 3. This enables the transmission to provide a high speed ratio at ralatively low transmission pressure for starting, and thereafter direct drive at higher transmission pressure for normal road speed conditions.

When the speed of the output shaft 3 is low and the speed of the engine 1 is high, the pump output is greater than that which the motor can accept. The excess output of the pump 2 causes a rise of transmission-pressure in line 24 which opens the valve 1 8a and charges the accumulatorl9. At normal road speeds full depression of the accelerator pedal 6 overcomes the force due to the return spring 12 and the speed sensitive device 25. The resultant displacement of the motor 4 is then greater than that of the pump 2, at its effective speed which is the difference of the speeds of the engine 1 and the shaft 3.

As the accelerator pedal 6 approaches full dis displacement the solenoid 20 is actuated to open the start/boost valve 18 to position 18b.

Oil from the accumulator 19 then supplements the pump output to provide short term high power in excess of that which the engine alone can produce. Continued operation with the accelerator pedal 6 fully depressed results in equilibrium conditions in which the pump output due to the difference in speeds of engine 1 and shaft 3 balances the motor displacement due to the speed of the shaft 3 at some intermediate position of the displacement mechanism 9.

Transmission pressure in this phase cannot fall below the level due to the engine torque applied to pump 2, so the accumulator cannot become fully discharged.

For normal level road operation the accelerator pedal 6 is partly retracted when the desired vehicle speed is reached. The reduced load on the spring 11 allows the combined effects of the return spring 12 and speed sensitive device 25 to move the displacement mechanism 9 to the zero displacement position. The linkage 15 between the displacement mechanism 9 and the motor valve 8 causes the valve 8 to move to position 8a at which both ports 4a, 4b are connected to the low pressure return to the reservoir 22. With zero displacement and both ports at low pressure the motor 4 neither does, nor absorbs, any work. The pump valve 5 is at position 5b at which the output port 2b is connected to the transmission pressure line 24. The engine torque, applied to the first half of the pump 2 to maintain transmission pressure, causes a reaction on the second half which maintains an equal driving torque on the shaft 3. As long as valve 18 remains closed there is no pump output and therefore no relative motion between the two halves of the pump, hence direct drive is obtained.

'The transmission provides for a free-running mode in which the engine idles and provides no power. In this mode the accelerator pedal 6 is retracted to the idle position. The motor 4 operates at zero dieplacement and no applied pressure, as in the direct drive mode, and neither does, nor absorbs, any work. The pump valve 5 moves to position 5a at which the pump ports 2a and 2b are both connected to the low pressure reservoir. The output shaft 3 runs faster than the engine 1, so fluid flow through the pump 2 is reversed. The fluid pressure is low, hence very little power is absorbed. Energy is saved since it is not necessary to maintain an engine speed equivalent to the road speed of the vehicle. The stop 26 fitted to the return spring 12 of the motor displacement mechanism 9 prevents the spring pushing the displacement mechanism 9 to the reverse mode during free running.

For vehicle retardation the brake pedal 16 moves the motor displacement mechanism 9 to the reverse mode and the motor valve 8 to position 8c. The motor 4 then acts as a pump, drawing oil from the reservoir 22 via cooler 23 and valve 8c to port 4a and discharging it via port 4b and valve 8c to the transmission line 24. As the transmission pressure rises the valve 18a opens to charge the accumulator 19. The rate of charge of the accumulator hence the degree of braking depends on the setting of the displacement mechanism 9 which is controlled by the position of the brake pedal 16. During braking the pump 2 and pump valve 5 are at idling conditions. Prolonged periods of retardation, as on a long downhill gradient, cause the accumulator 19 to become fully charged, then the pressure limiting valve 21 opens to discharge excess motor/pump output to the reservoir 22.

The braking energy is then dissipated as heat in the oil and this is reheated via the cooler as oil is recirculated. The transmission system then operates as a device generally known as a retarder.

WHAT WE CLAIM IS: 1. A variable speed ratio power transmission device comprising first and second hydraulically and mechanically interconnected hydrostatic machines, the first machine having a rotatable input member connectable to a source of mechanical power and a rotatable output member and being arranged to operate primarily as a pump, the hydraulic displacement of the first machine being proportional to the difference in speed between the input and output members whereby fluid flow is reduced as said speed difference is reduced until power is transmitted substantially by torque reaction between the input and output members of the first machine, the second machine being arranged to operate primarily as a motor and having a non-rotating casing, a variable displacement mechanism and a rotatable output member which is coupled to the output member of the first machine, the device further comprising an hydraulic fluid reservoir for supplying fluid to the first machine, a first valve having a first and a second position, a second valve having a first and a second position and an hydraulic accumulator, the first and second valves, when in reapective first positions, permitting fluid to flow from the first machine to the second machine and then return to the reservoir, the first valve, when in its second position, preventing flow of fluid from the first machine to the second machine, the second valve, when in its second position, connecting inlet and outlet ports of the second machine to the reservoir, the accumulator being chargeable with hydraulic fluid from the first machine during periods of low power demand an chargeable from the second machine during periods of operation of the second machine as a pump to produce a braking effect and connectable to the second machine during periods of high power demand for supplementing the fluid supplied by the first machine.

2. A transmission device as claimed in Claim 1, wherein the device further comprises a pressure release valve permitting fluid to flow from the second machine to the fluid reservoir when

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. displacement the solenoid 20 is actuated to open the start/boost valve 18 to position 18b. Oil from the accumulator 19 then supplements the pump output to provide short term high power in excess of that which the engine alone can produce. Continued operation with the accelerator pedal 6 fully depressed results in equilibrium conditions in which the pump output due to the difference in speeds of engine 1 and shaft 3 balances the motor displacement due to the speed of the shaft 3 at some intermediate position of the displacement mechanism 9. Transmission pressure in this phase cannot fall below the level due to the engine torque applied to pump 2, so the accumulator cannot become fully discharged. For normal level road operation the accelerator pedal 6 is partly retracted when the desired vehicle speed is reached. The reduced load on the spring 11 allows the combined effects of the return spring 12 and speed sensitive device 25 to move the displacement mechanism 9 to the zero displacement position. The linkage 15 between the displacement mechanism 9 and the motor valve 8 causes the valve 8 to move to position 8a at which both ports 4a, 4b are connected to the low pressure return to the reservoir 22. With zero displacement and both ports at low pressure the motor 4 neither does, nor absorbs, any work. The pump valve 5 is at position 5b at which the output port 2b is connected to the transmission pressure line 24. The engine torque, applied to the first half of the pump 2 to maintain transmission pressure, causes a reaction on the second half which maintains an equal driving torque on the shaft 3. As long as valve 18 remains closed there is no pump output and therefore no relative motion between the two halves of the pump, hence direct drive is obtained. 'The transmission provides for a free-running mode in which the engine idles and provides no power. In this mode the accelerator pedal 6 is retracted to the idle position. The motor 4 operates at zero dieplacement and no applied pressure, as in the direct drive mode, and neither does, nor absorbs, any work. The pump valve 5 moves to position 5a at which the pump ports 2a and 2b are both connected to the low pressure reservoir. The output shaft 3 runs faster than the engine 1, so fluid flow through the pump 2 is reversed. The fluid pressure is low, hence very little power is absorbed. Energy is saved since it is not necessary to maintain an engine speed equivalent to the road speed of the vehicle. The stop 26 fitted to the return spring 12 of the motor displacement mechanism 9 prevents the spring pushing the displacement mechanism 9 to the reverse mode during free running. For vehicle retardation the brake pedal 16 moves the motor displacement mechanism 9 to the reverse mode and the motor valve 8 to position 8c. The motor 4 then acts as a pump, drawing oil from the reservoir 22 via cooler 23 and valve 8c to port 4a and discharging it via port 4b and valve 8c to the transmission line 24. As the transmission pressure rises the valve 18a opens to charge the accumulator 19. The rate of charge of the accumulator hence the degree of braking depends on the setting of the displacement mechanism 9 which is controlled by the position of the brake pedal 16. During braking the pump 2 and pump valve 5 are at idling conditions. Prolonged periods of retardation, as on a long downhill gradient, cause the accumulator 19 to become fully charged, then the pressure limiting valve 21 opens to discharge excess motor/pump output to the reservoir 22. The braking energy is then dissipated as heat in the oil and this is reheated via the cooler as oil is recirculated. The transmission system then operates as a device generally known as a retarder. WHAT WE CLAIM IS:

1. A variable speed ratio power transmission device comprising first and second hydraulically and mechanically interconnected hydrostatic machines, the first machine having a rotatable input member connectable to a source of mechanical power and a rotatable output member and being arranged to operate primarily as a pump, the hydraulic displacement of the first machine being proportional to the difference in speed between the input and output members whereby fluid flow is reduced as said speed difference is reduced until power is transmitted substantially by torque reaction between the input and output members of the first machine, the second machine being arranged to operate primarily as a motor and having a non-rotating casing, a variable displacement mechanism and a rotatable output member which is coupled to the output member of the first machine, the device further comprising an hydraulic fluid reservoir for supplying fluid to the first machine, a first valve having a first and a second position, a second valve having a first and a second position and an hydraulic accumulator, the first and second valves, when in reapective first positions, permitting fluid to flow from the first machine to the second machine and then return to the reservoir, the first valve, when in its second position, preventing flow of fluid from the first machine to the second machine, the second valve, when in its second position, connecting inlet and outlet ports of the second machine to the reservoir, the accumulator being chargeable with hydraulic fluid from the first machine during periods of low power demand an chargeable from the second machine during periods of operation of the second machine as a pump to produce a braking effect and connectable to the second machine during periods of high power demand for supplementing the fluid supplied by the first machine.

2. A transmission device as claimed in Claim 1, wherein the device further comprises a pressure release valve permitting fluid to flow from the second machine to the fluid reservoir when

the output pressure of the second machine operating as a pump to produce a braking effect reaches a pre-selected value, and a cooler for removing from the fluid heat generated during such periods of operation.

3. A transmission device as claimed in Claim 1 or Claim 2, wherein the input member of the first machine is connectable to an engine serving as the source of mechanical power and wherein the first valve has a third position for connecting the first machine to the accumulator such that hydraulic fluid thereby supplied to the first machine will cause the engine to start.

4. A variable speed ratio transmission device substantially as herein described with reference to and as shown in the accompanying drawing.

5. A vehicle in which the engine is coupled to a transmission device as claimed in any one of the preceding claims for transmitting power to propulsion means of the vehicle.