GB2466002A - Clutch hydraulic system with a pump controlled as a function of clutch torque - Google Patents

Abstract

A clutch hydraulic system, for a motor vehicle, comprises an accumulator 6, an actuator 7 for operating a clutch 3, a magnetic valve 8, an electric pump 4 for pressurizing hydraulic fluid and a gearbox/pump controller 9 for controlling operation of the pump 4 based on a pressure of hydraulic fluid at an output side of the pump 4 and on torque transmitted by the clutch 3. The pump controller 9 is adapted to switch on the pump 4 if pressure drops below a torque-dependent recharging pressure and to switch off when the pressure reaches a static upper threshold or when it exceeds the recharging pressure. Clutch 3 has a slipping threshold which defines a minimum pressure to prevent clutch slip, wherein the recharging pressure is higher than the slipping threshold by a predetermined amount or percentage. The pump controller 9 may be connected to a torque sensor 12 and to an engine controller 13. A method of controlling pressure in the hydraulic system is also disclosed.

Description

Hydraulic System and Pressure Control Method

Description

The present invention relates to a hydraulic system for a motor vehicle, comprising a pump and one or more devices driven by pressurized hydraulic fluid output from the pump, the devices comprising at least a clutch located between engine and gearbox of the vehicle. Such hydraulic systems are used e.g. in vehicles having an automatic transmission or a dual clutch transmission.

In order to enable a clutch to transmit a desired torque, friction surfaces of the clutch must be pressed against each other with sufficient pressure to prevent slippage between facing friction surfaces. This pressure can be provided by the fluid of the hydraulic system. In other words, the pressure at an output side of the pump of the hydraulic system must be high enough to exercise the desired mechanical pressure on the friction surfaces. Such a pump may be driven by a direct connection to the engine crankshaft, so that it operates at a speed proportional to that of the crankshaft whenever the engine is on. However, such a pump consumes power unnecessarily since it operates also when the pressure demand on the hydraulic fluid is low.

In order to reduce the power consumption of the hydraulic system, systems have been suggested in which the pump is driven electrically. Such a pump can be switched off whenever the pressure at its output side is high enough to ensure proper operation of the clutch, and it is switched on again if the pressure has decreased to a critically low level. This level usually must be set high enough to ensure proper operation of the pump at all times, regardless of the amount of torque which is actually transmitted by the clutch at any given instant.

The objective of the present invention is to provide a hydraulic system for a motor vehicle and a pressure control method which enable to reduce still further the average power which is needed for properly operating the hydraulic system.

According to a first aspect of the invention, in a hydraulic system for a motor vehicle comprising at least one clutch operable by means of a hydraulic fluid, a pump for pressurizing the hydraulic fluid1 and a pump controller for controlling operation of the pump based on the pressure of the hydraulic fluid at an output side of the pump, this object is achieved by adapting the pump controller to operate the pump further based on the amount of torque transmitted by the clutch. Since the transmitted torque is not constant, the pressure required to prevent the clutch from slipping is not constant all the time, either, so that power may be saved by admitting variations of the pressure at the pump output side according to the amount of torque transmitted.

According to a simple and satisfying embodiment, the pump controller is simply adapted to switch on the pump if the pressure of the hydraulic fluid supplied to the clutch drops below a predetermined torque-dependent re-charging pressure threshold.

For any clutch, there can be specified a slipping threshold which specifies, as a function of torque applied to the clutch and, possibly, of the relative speed of friction surfaces of the clutch, a minimum pressure which, if applied to the clutch, enables it to transmit the input torque without slipping. The re-charging pressure threshold should be higher than this slipping threshold.

Preferably, the re-charging pressure threshold is higher than the slipping threshold by a predetermined amount or by a predetermined percentage.

An upper threshold of the pressure at which the controller will switch off the pump may be static. I.e.

it does not depend on variable operation parameters of the hydraulic system and in particular not on the transmitted torque. In this way, the interval between re-charging and upper pressure thresholds can be made wider than in a conventional hydraulic system, so that the frequency of operation of the pump is reduced. Since whenever the pump is switched off the kinetic energy of its moving parts and of the pumped fluid is lost, such losses can be reduced if the frequency of operation of the pump is made smaller.

In an alternative embodiment, the upper threshold is higher than the re-charging threshold by a predetermined amount. In such a case the frequency at which the pump is switched on is not necessarily reduced, but the power consumption is reduced all the same, since the pressure against which the pump operates can at times be substantially lower than in a conventional system having constant re-charging and upper thresholds.

For providing information relative to the torque applied to the clutch, a torque sensor may be connected to the pump controller. Alternatively, the pump controller may be connected to an engine controller so as to receive from said engine controller data representative of an output torque of an engine controlled by said engine controller.

In a preferred embodiment, the clutch is a rotary clutch, in particular for transmitting torque from an engine to a gearbox of a motor vehicle.

Alternatively, the clutch may be of the stationary type, having both mobile and immobile friction surfaces. A clutch of this type is used e.g. for shifting speeds in a planet drive.

The object of the invention is further achieved by a method of controlling the pressure of a hydraulic fluid in a hydraulic system for a motor vehicle, the system comprising at least one clutch operable by means of the hydraulic fluid and a pump for pressurizing the hydraulic fluid, in which the pump is operated based on the torque transmitted by the clutch.

Further features and advantages of the invention will become apparent from the subsequent description of embodiments thereof referring to the appended drawings.

Fig. 1 is a block diagram of a hydraulic system and devices related to it in a motor vehicle; Fig. 2 is an exemplary waveform diagram of transmitted torque, re-charging and upper thresholds according to a first preferred embodiment of the invention; and Fig. 3 is a waveform diagram analogous to that of Fig. 2 according to a second embodiment of the invention.

In the block diagram of Fig. 1, reference numeral 1 denotes a combustion engine, 2 denotes a gearbox, and 3 is a clutch mounted between a crankshaft of combustion engine 1 and an input shaft of gearbox 2.

A hydraulic system comprises an electric pump 4, a low-pressure reservoir 5 from which the pump 4 draws hydraulic oil, an accumulator 6 at a high pressure output of pump 4, a hydraulic actuator 7 for driving the clutch 3, and a magnetic valve 8 having three ports, one of which is connected to the accumulator 6, a second to the actuator 7 and a third to the reservoir 5. Further actuators, not shown, for driving shifting movements within gearbox 2 may be connected to accumulator 6 and reservoir 5 by associated valves. All these valves are under control of an electronic gearbox controller 9, known as such, which monitors engine load related data such as vehicle speed, rotation speed, engine torque and/or throttle position, selects an appropriate gear, and, if the selected gear is different from the one currently set in gearbox 2, controls the valves to open clutch 3, change the gear within gearbox 2 and close clutch 3 again.

Alternatively, gearbox controller 9 may carry out a similar control of the valves for setting in gearbox 2 a gear selected by the driver.

Torque data needed by pump controller 4 may be obtained by measurement, e.g. by means of a torque sensor 12 located at the engine crankshaft. Alternatively, they may be obtained by calculation based on other operation parameters measured at engine 1. The calculation may be carried out by pump controller 4 or by engine controller 13. In the engine controller 13, the engine torque may be known because it is used as a parameter for conventional tasks of the engine controller. In that the only adaptation in the software of the engine controller 13 needed for implementing the invention is to program the engine controller 13 to send current engine torque date to the pump controller whenever required. Alternatively, the pump controller 4 might be connected to a bus system interconnecting the engine controller 13 and various sensors and actuators of the engine, where the pump controller 4 can either request needed data from the appropriate sensors of its own motion, or it may eavesdrop communication between the engine controller 13 and the sensors and actuators associated to it, in order to collect the data it needs.

Hydraulic actuator 7 may be double-acting, i.e. both opening and closing movements of the clutch 3 are driven by oil flowing from accumulator 6 to different chambers of actuator 7, or it may be single-acting, only an opening movement of the clutch or only a closing movement being driven by the oil flow, and the reverse movement being driven e.g. by a spring.

Whenever oil is withdrawn from the accumulator 6, the pressure therein decreases and is detected by a pressure sensor 10 connected to a pump controller 11. The pump controller 11 compares the pressure detected by sensor 10 to a re-charging threshold Prc* When the pressure in accumulator 6 drops below the re-charging threshold Pre, controller 11 switches on pump 4, replenishing accumulator 6, until an upper pressure Pniax is reached, at which the pump 4 is switched off again.

In a preferred embodiment, the re-charging threshold Pre is a function of the torque transmitted by clutch 3 from combustion engine 1 to gear box 2 at any given time, i.e. it varies with time just as the transmitted torque does. In practice, a re-charging threshold is given by p,cM+d (1) or by p-(c+d)M (2) wherein M denotes the torque from engine 1, c is inversely proportional to an angular friction coefficient of clutch 3, and d, d are positive offset values. The factor c is defined so that cM gives the minimum pressure which must be applied to the clutch 3 in order to prevent it from slipping under torque M. The pressure Pmax at which pump controller 11 switches off pump 4 is constant in this first embodiment.

In a second embodiment it can be equal to Prc plus a predetermined offset.

Fig. 2 illustrates an exemplary waveform of Prc as a function of time t. A limit pressure Piim can be defined, which corresponds to the highest torque Mum the clutch 3 will be able to transmit without slipping: Piim CMim In a conventional hydraulic system, the pump would have to be switched on whenever the accumulator pressure drops to Puim, in order to ensure that said limit torque Mum can be transmitted across the clutch at any time. The accumulator pressure is represented in Fig. 2 by a dash-dot curve Pacc. It is seen that in the conventional approach a pump would have to be switched on at time t1, in order to bring back the accumulator pressure to Pmax* If it is assumed that the pressure in the accumulator increases linearly with the oil volume pumped, it can be shown that in that case the average work done by the pump per volume unit of pumped oil is proportional to (p+ Pmax)/2. According to the invention, Prc is considerably lower than Plim most of the time. Therefore, the criteria for switching on the pump 4 is met only at a much later time t2, and at that time the pressure in the accumulator is increased from p(t2) to Pmax It is immediately seen that the smaller p(t2) is, the less is the average work is W -(p(12) + Pmax)'2 done by pump 4.

In practice, the pressure increase in the accumulator is much stronger than linear, so that the energy economy is still higher.

The work done by the pump may be decreased even more if a threshold Pmax' for switching off the pump is not constant but higher than the variable switching-on threshold Prc by a predetermined offset D. This case is illustrated in Fig. 3. It is noted that the pump 4 is operated more frequently than in the embodiment of Fig. 2, at times t1', t2', t31, but if the pressure increase D in each pumping event is the same as (PmaxPiim) the amount of work done by the pump 4 in each of these events is much smaller than in the conventional case, since the average pressure in accumulator 6 is much lower.

The invention has been described above referring to a simple power train design comprising a single combustion engine 1 and a single clutch 3 between the combustion engine 1 and the gearbox 2. It will be readily apparent to the skilled person that the invention is also applicable to more complex clutch systems, e.g. to a double clutch made up of two single clutches, each of which drives one of two subassemblies of the gearbox.

Such a double clutch allows interruption-free shifting between gears associated to different ones of the two subassemblies. Another possible application is a hybrid drive, where clutches driven as described above enable selective coupling of a combustion engine and an electric engine to the same power train.

Another interesting application is a so-called stationary clutch, i.e. a clutch having an immobile friction surface. Such a clutch is used for shifting speeds in a planet drive by selectively braking one or another of its rotatable components.

List of reference signs 1 combustion engine 2 gearbox 3 clutch 4 pump reservoir 6 accumulator 7 hydraulic actuator 8 valve 9 gearbox controller pressure sensor 11 pump controller 12 torque sensor 13 engine controller