GB2191847A - Hydraulically driven engine cooling systems - Google Patents

Abstract

A hydrostatic fan drive for a vehicle engine 3, Fig. 2, comprises a modulating bypass valve 24 which is responsive to coolant temperature selectively to deliver hydraulic fluid to the fan motor 2 or to bypass that motor. Control of the bypass valve 24 is electrohydraulic, with a temperature sensor 25 delivering an electrical control signal to a solenoid- actuated pilot valve stage 42, Fig. 3, of the bypass valve to vary the pilot pressure in a control valve chamber 40 of the bypass valve housing 30. A main valve element is a lift valve member 35 responsive to the pilot pressure to open or close a bypass passage between an inlet 32 and exhaust 33. The bypass valve may also have an overpressure relief valve 47, preferably also controlled from the pilot pressure in the control valve chamber 40. Electrohydraulic control of the kind specified provides many advantages including a more unrestricted choice of operating characteristic, significantly reduced hydraulic ducting and faster and more accurate response to coolant temperature changes. <IMAGE>

Description

SPECIFICATION Hydraulically driven engine cooling systems Technical Field The invention relates to engine cooling systems incorporating a cooling fan driven by a hydrostatic motor, and to modulating bypass valves for such cooling systems.

Background Art Hydrostatic fan drives are well known, for cooling the radiators of liquid-cooled internal combustion engines and for cooling other engine parts such as intercooler air-to-air heat exchangers and engine compartment ventilation systems. Such drives have the advantage that the fan can be at a position remote from the engine, with only hydraulic supply and return lines and no direct mechanical, electrical or viscous drive connecting the engine and the fan. The fan power achieved can be substantially more than an electrically driven fan.

Common applications for hydrostatic fan drives are those where there is a requirement for high fan power and remote fan mounting such as may be specified for example on buses, rail cars, construction vehicles, earthmoving vehicles and other special purpose vehicles. Such applications generally employ high performance pressure-loaded external gear pumps and motors for the hydraulic supply and fan drive. In such drive systems the engine-to- fan speed would be fixed, as determined by the fixed displacements of the pump and motor and the power take-off drive ratio from the engine, if it were not for the provision of a bypass valve.

Bypass valves which have been introduced into the hydraulic system include pressure relief valves designed to trim the fan speed to some value below that corresponding to governed engine speed, temperature responsive modulating bypass valves designed to send only a temperature-responsive proportion of the pumped hydraulic fluid to the motor and to return the remainder to drain; and bypass valves with both these functions.

The temperature-responsive modulating bypass valves that have been used have in general used wax capsules as the temperature sensing elements and have therefore been mounted close to the engine so that the engine coolant temperature could be measured accurately. The modulation may be achieved by direct acting heavy duty wax capsules, but finer control is possible with pilot control using a two valve system in which a pilot valve responsive to engine coolant temperature controls a main switching or modulating bypass valve which is a spool valve in the high pressure hydraulic circuit. Instead of wax capsule temperature sensors, it has been proposed to use an electro-hydraulic control interface in which an electrical temperature-sensing element controls the ON-OFF operation of a solenoid-controlled bypass valve.Modulation can be achieved by varying the ONOFF duty cycle of the bypass valve, for example by pulse width modulation of the solenoid drive current.

The degree of control achieved is however insufficient for modern requirements.

Demands for the required performance of hydrostatic fan drives are ever increasing.

There is a constant search to avoid wasted energy so that more torque is available at the wheels when cooling is not required; a need to reduce expose of passengers to high noise levels; a desire to reduce the duty cycle and lengthen the life; and a desire to avoid sudden changes in fan speed. Also radiator shutters when fitted need to be synchronized with fan operation in order to avoid undesirable combinations of fan and shutter operation. This is greatly simplified if both fan and shutter controls use a common temperature sensor.

Some vehicle manufacturers impose a requirement that there should be a minimum fan speed greater than zero. Others specify a maximum fan speed less than that corresponding to governed engine speed. Often the maximum speed or even the total engine speed/fan speed characteristic has to be a variable element, depending on the intended market for the vehicle and its intended use or the likely ambient temperature. Manufacturers may specify a low hysteresis gap between increasing and decreasing engine coolant temperatures.

Temperature sensor response time is also an important factor in some applications, since any sudden surge of heat into the cooling water circuit must be catered for without exceeding safe levels of engine temperature.

Such surges of heat input can be generated; for example, during braking if water-cooled retarders are fitted.

The market trend is thus for progressively higher levels of sophistication in engine cooling management. As the known hydrostatic fan drives and modulating bypass systems have reached their performance limits, it was necessary to look elsewhere for an effective engine cooling system that was capable of meeting the additional demands from engine and vehicle manufacturers without prohibitively increasing the cost and complexity of the cooling system.

Disclosure of the invention The invention not only meets the above needs but it also permits the overall cooling system to be constructed substantially more simply and economically than the prior known systems.

The invention provides an engine cooling system incorporating a cooling fan driven by a hydrostatic motor, and an electro-hydraulic control interface comprising a temperature sensing element delivering an electrical control signal to a modulating bypass valve for the hydrostatic motor, the bypass valve comprising: a valve housing, a pilot valve in the valve housing, actuated by a proportional response solenoid to establish in a control chamber in the housing a pilot pressure that is a linear function of the electrical control signal; and a lift valve responsive to a pressure differential between the pilot pressure in the control chamber and the hydraulic supply pressure in an inlet chamber, selectively to isolate the inlet chamber from an exhaust chamber or progressively to deliver hydraulic fluid from the inlet chamber to the exhaust chamber.

The use of an electro-hydraulic interface between the coolant and the hydraulic circuit enables the use of a semiconductor temperature sensing element such as a negative coefficient thermistor. Thermistor curve matching accuracy can be as low as + 0.250C, which makes for good interchangeability of sensors without the need for recalibration, The semiconductor temperature sensing element may be surface mounted on the engine, the radiator or the coolant ducting therebetween; or it may be immersed in the coolant. It provides for an unrestricted choice of operating characteristic of the bypass valve, and faster and more accurate responses to coolant temperature changes than did the prior used wax capsules.

A control amplifier is preferably interposed between the temperature sensor and control solenoid, to make best use of the relevant thermistor and solenoid characteristics.

The proportional response solenoid is preferably a wet pin proportional solenoid. These are precision manufactured devices with low hysteresis characteristics and reliable operation. As with all proportional response solenoids over a given operating range of pin movement the output force is, for practical purposes, directly proportional to mean solenoid drive current even if the solenoid current is varied by pulse width modulation. The resulting control is superior to the use of pulse width modulation to vary the ON-OFF duty cy cle of a non-proportional solenoid. Preferably the solenoid pin acts directly on the pilot valve in a direction to close the pilot valve.

When the hydraulic pressure in the control chamber in the housing is sufficient to overcome the solenoid force and open the pilot valve, the hydraulic fluid in the control chamber is allowed to bleed to drain, thereby establishing a control pressure differential between the control chamber pressure and the inlet pressure, and actuating the lift valve to deliver hydraulic fluid from the inlet chamber to drain via the exhaust chamber. It will be seen therefore that the extent to which inlet fluid is passed to drain is dependent on the solenoid force and is thus linearly related to the mean solenoid drive current.

The pilot valve is preferably a poppet or lift valve, with the hydraulic pressure in the control chamber acting through a valve seat on a preferably tapered central nose portion of the pilot valve member.

The main lift valve is preferably aligned so that inlet pressure acts on a valve area around the valve seat in a direction to unseat the valve member, with the pilot pressure in the control chamber acting on a larger valve area and in a direction to seat the valve member.

In the situation where the bypass valve is closed and the fan is driven at maximum speed, the pilot pressure in the control chamber is preferably inlet pressure. This condition may be established by delivering fluid from the inlet chamber through a flow restricting orifice to the control chamber. The flow route and flow restricting orifice may be through the lift valve member itself if desired.

DRA WINGS Figure 1 is a schematic partly exploded assembly diagram of a known hydrostatic fan drive engine cooling system; Figure 2 is a schematic assembly diagram; similar to that of Figure 1; of a hydrostatic fan drive engine cooling system according to the invention; Figure 3 is an axial section through the modulating bypass valve of Figure 2; Figure 4 is a side elevation of the bypass valve of Figure 3 mounted on a hydrostatic motor; Figure 5 is a rear elevation of the motor and bypass valve assembly of Figure 4; Figure 6 is a rear elevation, similar to that of Figure 5; of the modulating bypass valve of Figure 3 mounted on a hydrostatic pump; and Figure 7 is a graph showing the force/distance operating characteristic of a proportional output solenoid as opposed to an ON/OFF driven solenoid.

Referring first to Figure 1, there is shown a conventional internal combustion engine cooling system utilizing a cooling fan 1 driven by a hydrostatic motor 2. The engine 3 is liquid cooled, and coolant is passed to a radiator 4 through a conduit 5 and returned through a conduit 6.

Mounted on and driven by the engine 3 is a hydrostatic pump 7 which draws hydraulic fluid from a tank 8 through a conduit 9, and delivers it to the motor 2 via a conduit 10, a filter 11, a further conduit 10a, a bypass valve 12 and a conduit 13. A return conduit 14 is provided from an exhaust port of the motor 2 to the tank 8. The bypass valve 12 is a spool valve actuable in response to pilot pressure delivered through flow and return conduits 15 and 16 from a thermostatic pilot. valve 17.

The thermostatic pilot valve 17 incorporates a wax capsule temperature sensing element 18 which in use is immersed in the flow of liquid coolant passing from the engine 3 into the conduit 5.

When the coolant is cold, the thermostatic pilot valve 17 causes the bypass valve 12 to open, so that hydraulic fluid delivered to the bypass valve from the pump 7 is diverted to the tank 8 through a fluid dumping conduit 19. Thus none of the pumped hydraulic fluid passes through and actuates the motor 2.

When the coolant is hot, the pilot valve 17 causes the bypass valve 12 to close, so that all the hydraulic fluid delivered by the pump 7 passes through and drives the motor 2. Between these two extremes is a range of coolant temperatures which cause a partial bypassing of the motor 2, so that motor speed is controlled progressively in response to the engine coolant temperature as sensed by the wax capsule 18.

Disadvantages of the above cooling system include the relative insensitivity of wax capsules as temperature sensing elements, requiring the wax capsule 18 to be physically immersed in the liquid coolant flow; the sensitivity of the spool valve 12 to contaminants in the hydraulic fluid, which makes it necessary to provide the filter 11 immediately before the bypass valve 12; and the excessive amount of pipework 9,10, 10a, 13 to 16 and 19 which is needed.

Description of Preferred Embodiments of the Invetion Figure 2 illustrates a cooling system according to this invention. The fan 1, engine 3 and radiator 4 are referenced as in Figure 1, as are the hydrostatic pump 7 and motor 2.

The pump 7 draws hydraulic fluid from a tank 20 through a conduit 21. The tank 20 is provided with its own internal filter, and no additional filter is required as will be explained below. Fluid is delivered via flow and return conduits 22 and 23 to a bypass valve 24 mounted on the motor casing. Only three conduits 21 to 23 are required, as opposed to the eight conduits 9,10, 10a, 13,14,15,16 and 19 used in the prior art cooling system of Figure 1.

Associated with the engine coolant circulating system is a temperature sensor 25 which is a surface-mounted thermistor device which delivers an electrical signal over a wire 26 to a semiconductor control unit 27. The control unit 27 preferably has an amplification function and delivers a pulse width modulated output over an electrical wire 28 to a solenoid 29 of the bypass valve 24. Pulse width modulation is preferred because it provides high stability in electrically noisy environments, at relatively low cost. However linear amplification of the signal from the sensor 25 is an alternative that may be considered. Whatever form the amplification and signal control takes, the out put over the wire 28 produces in the solenoid 29 a force directly related to the sensed temperature.

The internal construction of the bypass valve is illustrated somewhat schematically in Figure 3. A valve body 30 is made in two parts, 30a and 30b, which are clamped together in sealing relationship. A stepped bore 31 in the body part 30b communicates with an inlet port 32 and an exhaust port 33. An annular valve seat 34 is positioned at a shoulder of the stepped bore between the inlet port 32 and the exhaust port 33, and a valve poppet 35 slidable in the larger diameter portion of the stepped bore 31 has a tapered nose 36 which is engageable with the valve seat 34 to isolate the exhaust port 33 from the inlet port 32.

A blind bore 37 formed in the valve poppet from the end remote from the tapered nose 36 communicates with a transverse bore 38 which is positioned to be in fluid communication with the inlet port 32. A flow restrictor 39 is inserted in the blind bore 37 to establish a restricted flow path for hydraulic fluid from the inlet port 32 to a control pressure chamber 40 formed between the valve poppet 35 and the part 30a of the valve body 30. A spring 41 is in compression between the part 30a of the valve body 30 and an internal shoulder of the valve poppet 35, biasing the valve nose 36 against its seat 34. In use, the forces constraining the movement of the valve poppet 35 are the inlet pressure acting on an area immediately around the valve seat 34; the pressure in the control pressure chamber 40; and the force of the spring 41.

The pressure in the control pressure chamber 40 is a pilot pressure controlled by a pilot poppet or lift valve 42. The valve 42 comprises a valve poppet 43 having a tapered nose engageable with a valve seat 44 to block fluid flow from the control pressure chamber 40 into passages 45 and 46 formed in the valve body 30. A variable bias closing the pilot poppet valve 42 is imposed by the actuating pin 46 of the solenoid 29 (see figure 2) which acts directly on a rear face of the valve poppet 43.

When the bias of the solenoid 29 is sufficient to resist the control pressure acting on the valve poppet 43 through the valve seat 44, the control pressure in the chamber 40 rises to inlet pressure, at a rate dictated by the size of the aperture in the flow restrictor 39.

When the bias of the solenoid 29 is insufficient to maintain the valve poppet 43 seated (for example when the engine coolant temperature is low), hydraulic fluid is bled from the control pressure chamber 40, past the valve poppet 43, and through passages 45 and 46.

It then passes past an overpressure relief valve 47 and through a passage 48 to the exhaust port 33. The overpressure relief valve 47 does not interfere in any way with this fluid flow, and the result is that the control pressure in chamber 40 falls sufficiently to upset the balance of forces acting on the main valve poppet 35 so that the inlet pressure acting on the front face of the poppet around the valve seat 34 lifts the valve poppet from its seat and permits hydraulic fluid at the inlet port to discharge directly to the exhaust port and to drain via the return conduit 23 of Figure 2. The resulting fall in inlet pressure is of course immediately accompanied by a reseating of the valve poppet 35, so that the net result is a proportional or progressive amount of bypassing, directly dependent on the force of the solenoid 29 on the pilot valve poppet 43.

The bypass valve 24 has a rapid response to changes in the force exerted by the solenoid 29, as the valve poppet 35 is a lift valve member exposing a substantial fluid path when lifted from its seat 34. Because the valve poppets 35 and 43 are both lift valve members, the bypass valve 24 is not susceptible to malfunction due to dirt particles in the hydraulic fluid, and the valve surfaces are to a large extent self-cleaning under the conditions of fluid flow. Also the pressure differential across the valve poppet 35 is low, with only slight falls in the control pressure below inlet pressure being sufficient to cause bypass valve actuation. The valve poppet 35 is not therefore as likely to stick, due to dirt particles being forced between the valve poppet and its bore, as would be the case for example with a valve member exposed to high pressure differentials.

The passages 46 and 48 in the valve body 30 communicate with one another as indicated above without any element of control by the overpressure relief valve 47 with which they communicate. A third passage 49 also communicates with the overpressure relief valve 47 and leads to the control pressure chamber 40. When the inlet pressure tries to exceed a given threshold and the valve poppet 43 is seated, the control pressure and the pressure in the passage 49 rise until the overpressure relief valve 47 opens and discharges the control pressure fluid to drain via the passage 48 and the exhaust port 33. The result is the maintenance of the inlet pressure at that threshold value, with the imposition of a preset maximum speed on the motor 2 and fan 1.

Figures 4 and 5 show how the bypass valve 24 can conveniently be attached directly to the hydrostatic motor 2 of Figure 2, without adding excessively to the length of the motor.

The inlet port of the motor is connected directly to the inlet port 32 of the bypass valve 24, and the outlet port of the motor is connected to an exhaust port 33 of the bypass valve by means of an integrally connected port connector 50.

A modifcation that may be useful when space is at a premium near the radiator is for the bypass valve 24 to be mounted directly on the pump 7 as shown in Figure 6. Alternatively the bypass valve 24 may be mounted elsewhere on the vehicle at any location at which the supply and return conduits 22 and 23 are, or can be brought into; close proximity. The running of an electric wire 28 from the control unit 27 to the solenoid 29 presents no problems whatever the location of the solenoid on the vehicle, and the installation is always much simpler, more flexible, more efficient and more reliable than the allhydraulic control of Figure 1.

Figure 7 shows the operating characteristic of a proportional response solenoid such as the solenoid 29 (shown in solid line as 50) in comparison with that for a conventional nonproportional solenoid (shown in broken line as 52). The armature of the proportional response solenoid is specially shaped, in relation to the solenoid windings, so that over a given operating range R of stroke positions the output force on the armature is substantially constant for a given solenoid drive current, and is substantially proportional to that drive current.

In comparison, a conventional nonproportional solenoid has an output characteristic 52, for a given solenoid drive current, which varies significantly with the stroke of the solenoid armature.

Such conventional solenoids are suitable for simple ON/OFF control of a hydraulic valve but are not capable of establishing the linear relationship between the solenoid drive current and the pilot pressure which is an essential element of the cooling system of the invention.

It is also a characteristic feature of proportional response solenoid that it has low hysteresis, and this is an additional important advantage in cooling systems of the invention.

Claims (12)

1. An engine cooling system incorporating a cooling fan driven by a hydrostatic motor, and an electro-hydraulic control interface comprising a temperature sensing element delivering an electrical control signal to a modulating bypass valve for the hydrostatic motor, the bypass valve comprising; a valve housing, a pilot valve in the valve housing, actuated by a proportional response solenoid to establish in a control chamber in the housing a pilot pressure that is a linear function of the electrical control signal; and a lift valve responsive to a pressure differential between the pilot pressure in the control chamber and the hydraulic supply pressure in an inlet chamber selectively to isolate the inlet chamber from an exhaust chamber or progressively to deliver hydraulic fluid from the inlet chamber to the exhaust chamber.

2. An engine cooling system according to claim 1, wherein the temperature sensing ele ment is a negative coefficient thermistor.

3. An engine cooling system according to claim 1 or claim 2, wherein interposed between the temperature sensing element and the solenoid-actuated pilot valve is a control amplifier with a pulse width modulated output.

4. An engine cooling system according to any preceding claim wherein the solenoid of the solenoid-actuated pilot valve is a wet pin proportional solenoid.

5. An engine cooling system according to any preceding claim, wherein the solenoid of the solenoid-actuated pilot valve acts directly on a valve poppet of the pilot valve in opposition to the pilot pressure in the control chamber which acts on the valve poppet through a valve seat.

6. An engine cooling system according to any preceding claim, wherein the lift valve is aligned so that inlet pressure acts on a valve area of a main valve poppet around a valve seat in a direction to unseat the main valve poppet, and the pilot pressure in the control chamber acts on a larger area of the main valve poppet in a direction to seat the main valve poppet.

7. An engine cooling system according to claim 6, wherein a spring acts on the main valve poppet in a direction to augment the force of pilot pressure thereon.

8. An engine cooling system according to any preceding claim, wherein pilot pressure in the control chamber is established as a balance between hydraulic fluid at inlet pressure flowing into the control chamber through a flow restricting orifice and hydraulic fluid bled from the control chamber to drain past the pilot valve.

9. An engine cooling system according to any preceding claim, wherein the valve housing of the modulating bypass valve is mounted on a housing for the hydrostatic motor.

10. An engine cooling system according to claim 9, wherein the hydrostatic motor receives its high pressure hydraulic motive fluid directly from the inlet chamber of the modulating bypass valve and delivers its low pressure discharge fluid to the exhaust chamber of the modulating bypass valve through a port connector conduit.

11. An engine cooling system according to any of claims 1 to 8, wherein the valve housing of the modulating bypass valve is mounted on a housing of a hydrostatic pump supplying the hydrostatic motor.

12. An engine cooling system according to claim 1, substantially as described herein with reference to the drawings.