Patent Publication Number: US-2020292067-A1

Title: Hydraulic system and method for controlling pressure in a hydraulic system

Description:
FIELD OF THE INVENTION 
     The invention relates to a hydraulic system for a vehicle transmission with at least a primary friction element and a secondary friction element for coupling and transmitting engine power to wheels of the vehicle by actuation of the friction elements via the hydraulic system. 
     BACKGROUND TO THE INVENTION 
     Generally, a transmission provides controlled application of engine power by conversion of speed and torque from a power source, such as for example an internal combustion engine or an electrical machine. The hydraulic system may provide for actuation of friction elements in the vehicle transmission for coupling the transmission input to the geartrain to transmit engine power to the wheels of the vehicle. 
     Such transmissions comprising friction elements are known, for example a continuous variable transmission (CVT) is a well known transmission incorporating friction elements. The friction elements can be embodied as pulleys between which a flexible element such as a chain or a belt can be clamped by means of friction force. 
     Typically, the hydraulic system is a dual system having a circuit for actuation of the friction elements, the so-called line pressure circuit, and having a circuit for lubrication and/or cooling the so-called lubrication circuit. Typically, the line pressure circuit works on relatively high pressures, whereas the lubrication circuit works on lower pressures. 
     The hydraulic system uses operating elements, preferably operating elements that transfer electrical energy as input to mechanical and/or hydraulically energy as output. Such operating elements typically are embodied as solenoids that control components, such as valves, of various parts of the hydraulic system when receiving an electrical input signal. Typically, there is a line solenoid for controlling the line pressure valve, a primary solenoid for controlling the primary valve of the primary friction element, a secondary solenoid for controlling the secondary valve of the secondary friction element, a clutch solenoid for controlling the clutch valve, torque converter solenoid for controlling the torque converter pressure valve, and a bypass solenoid for controlling a bypass valve. Other solenoids for controlling other valves present in the hydraulic system may be provided. 
     Solenoids are relatively expensive components and require energy for actuation. As the hydraulic system is being pressurized by a pump system which uses energy from a power source for driving the pump system and pressurizing the hydraulic system, there is an ongoing quest for reducing the energy consumption of the hydraulic system. The energy for powering the hydraulic system cannot be used for driving the vehicle. The power source can be e.g. a combustion engine, or a battery, or an electrical machine etc. 
     Further, the so-called boost function may require an acute or rapid change of flow in the line pressure circuit, preferably without a change in pressure level. Thereto, systems can be provided with an additional pump, which is being regulated by an additional operating element or additional solenoid, see e.g. EP1482215 for such a system. A drawback of this system may be that it is relatively expensive in view of the number of components required, in particular if relatively expensive solenoids are used. 
     Alternatively, to reduce the number of solenoids, often a dedicated line solenoid is omitted and only a primary and secondary solenoid is used to control the primary and secondary pressure on the primary and secondary friction element respectively. A drawback of such a control system however may be that the minimum pressure that can be used for clamping of the flexible element between the friction elements, e.g. the belt of a continuous variable transmission (CVT) is the same as the line pressure and thus may remain relatively high. For lower torque levels, this may result in poor efficiency of the transmission. 
     An object of the invention is to provide for a hydraulic system that obviates at least one of the aforementioned drawbacks. In particular, a hydraulic system is aimed for that preferably achieves at least the same performance as the prior art systems preferably in a more cost effective way. 
     SUMMARY OF THE INVENTION 
     Thereto, the invention provides for a hydraulic system according to claim  1 . 
     By providing an operating element having at least two functions, of operating the bypass valve and of operating the secondary valve, less components may be used in the hydraulic system, providing for a more cost effective and/or more reliable hydraulic system. 
     The pump system comprises at least two outlet lines, for example the pump system may be embodied as a single pump having two pump chambers, or may be embodied as a dual pump or as two pumps. In normal operation, both pumps or pump chambers are operating and delivering flow to the two outlet lines. In the normal situation, the flow of the two outlet lines is delivered to the hydraulic system and generates pressure in the hydraulic system, then the bypass circuit is by having the bypass valve closed so there is no flow in the bypass circuit. The pressure in the line pressure circuit, the lubrication circuit and to the various components is controlled by various operating elements. In a determined state, when less torque is required on the friction elements; the bypass operating element can be actuated to open the bypass circuit to reduce the flow supplied to the line pressure circuit and additionally to reduce the pressure on the secondary friction element to a value lower than the pressure on the primary friction element, such that less torque is provided. This can for example be the case in overdrive situations. The pressure on the secondary friction element can then be lower than the pressure on the primary friction element. Then the bypass valve is opened to allow flow through the bypass circuit, such that pressure on one of the two pump system outlet lines is reduced. The flow delivered to the hydraulic system is now reduced partially, as only the flow of one pump is being entered into the circuit. Depending on the configuration of the pump system, the flow is reduced partially, for example when both pumps or pump chambers are equally sized, the flow is reduced by half, but other partiality is possible as well. The second pump can basically be running on idle having about the same inlet and outlet pressure, so less energy is consumed from the engine. The bypass operating element controls the opening of the bypass valve. Additionally, to reduce the pressure on the secondary friction element, the bypass operating element controls the secondary valve to close such that the pressure on the secondary friction element falls lower than the pressure on the primary friction element. As such, a single operating element can perform two functions, so less relatively expensive solenoids can be used resulting in lower costs for the hydraulic system. 
     Also, the maximum pressure level on the primary friction element, regulated by the primary valve and independently controlled by the primary operating element, can be reduced to the minimum system pressure level. This is contrary to master/slave prior art systems. According to the invention, e.g. in overdrive when the pressure on the secondary friction element can be lower than the pressure on the primary friction element, a relatively low secondary pressure can be obtained, resulting in a better performance of the transmission at lower torque. By reducing the pressure on the secondary friction element, the friction force on the secondary friction element is reduced, resulting in less fuel consumption as there is less power required for friction force on the secondary friction element. 
     Advantageously, the line pressure valve is biased towards a predetermined bottom limit line pressure, such that in a fail-safe mode, when no actuation to operating elements is provided, the line pressure is reduced to the bottom limit line pressure resulting in reduced actuation of the friction elements to provide for a slow-down mode of the vehicle. The bias of the line pressure valve typically is provided by the spring force on the line pressure valve. In a fail safe mode, when there is no actuation of the operating elements, e.g. when there is a failure in the electrical circuit or there is an engine failure, or an other failure resulting in loss of the actuation of the operating elements, there is no actuation of the line pressure valve either, then the line pressure valve switches to its biased position, i.e. a predetermined bottom line pressure. As such, there remains sufficient, though limited, pressure in the hydraulic system for providing reduced actuation of the friction elements such that the vehicle can in a controlled mode slow down to allow for a safe stop in case of a failure. 
     More advantageously, in case of a failure, the fail-safe mode provides for a limp-home mode. Thereto, the line pressure valve is biased towards a non-bottom limit line pressure, which is higher than the bottom limit line pressure, such that, in the fail-safe mode, the line pressure is set to a predetermined pressure to provide for sufficient actuation of the friction elements such that the vehicle can safely ‘limp home’. 
     Preferably, to provide for the limp home-mode, the bypass operating element has a third function of regulating the line pressure valve. When actuated, the bypass operating element controls the line pressure valve against the spring action of the bias spring to regulate the line pressure. When there is no actuation of the operating elements, such as in a failure mode, the bias spring of the line pressure valve biases the pressure towards the non-bottom limit pressure to allow for the limp home mode of the vehicle. 
     The invention also relates to a method for controlling pressure. 
     Further advantageous embodiments are represented in the subclaims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention will further be elucidated on the basis of exemplary embodiments which are represented in the drawings. The exemplary embodiments are given by way of non-limitative illustration of the invention. 
       In the drawing: 
         FIG. 1  shows a schematic diagram of an embodiment of a hydraulic system according to the invention; 
         FIG. 2  shows a schematic diagram of the embodiment of  FIG. 1  including the limp home functionality; 
     
    
    
     It is noted that the figures are only schematic representations of embodiments of the invention that are given by way of non-limiting example. In the figures, the same or corresponding parts are designated with the same reference numerals. 
       FIG. 1  shows a schematic diagram of a hydraulic system  1  for a vehicle transmission with at least a primary friction element  2  and a secondary friction element  3 . An example of a transmission using friction elements is a continuously variable transmission (CVT). 
     The primary and secondary friction elements  2 ,  3  provide for coupling and transmitting engine power to wheels of the vehicle by actuation of the friction elements  2 ,  3  via the hydraulic system  1 . 
     The hydraulic system  1  comprises a pressure pump system  4  having at least two outlet lines  5 ,  6 . The pressure pump system  4  can be embodied as a pump having two pump chambers, or as a two pumps each having a pump chamber, etc. Many variants are possible. Here, the pressure pump system  4  is schematically represented by two pumps  4   a ,  4   b  having outlet line  5  and  6  respectively. 
     The pump system  4  is being powered by the engine and pressurizes the hydraulic fluid of the hydraulic system  1 . The pressurized fluid is supplied to the hydraulic system  1 . The hydraulic system  1  is typically a dual system comprising a line pressure circuit  7  in which the fluid has a relatively high pressure (approximately 5-80 bar, preferably 7-70 bar) and a lubrication circuit  8  with a lower pressure (approximately 5-10 bar, preferably 6-9 bar). The lubrication circuit  8  is mainly for cooling and lubrication of components of the transmission and will not be elaborated further. It is noted that the high pressure range and the low pressure range are overlapping, but it is also noted that the high pressure is at any time higher than the low pressure, so there is no overlap of the pressure during use. 
     In the line pressure circuit  7 , operating elements or solenoids operate valves in the line pressure circuit for controlling the pressure on components of the transmission, such as the primary friction element, or the secondary friction element, or a clutch, or the pressure in the line pressure circuit itself, etc. 
     The pressure pump system  4  has two outlet lines  5 ,  6  which are coupled to the line pressure circuit  7 . The pressure pump system  4  is further provided with a bypass circuit  9  that is controlled by a bypass valve  10 . When the bypass circuit  9  is open, with bypass valve  10  open, there is flow through the bypass circuit and the output flow of the pressure system  4  is reduced and the pressure in one of the outlet lines, here outlet line  5 , becomes reduced. When the bypass circuit  9  is closed, the bypass valve  10  is closed, and the output flow of one of the outlet lines, here outlet line  5 , is supplied to the line pressure circuit  7 . This is also referred to as the “boost” function, as the output flow of the pump system  4  then rapidly, almost immediately, increases. The bypass valve  10  is controlled by a bypass operating element  11 . By providing the bypass circuit  9 , the pressure and output flow of the second pump or pump chamber  4   a  is always available, but is not always supplied to the line pressure circuit  7 . As such, when the additional pump flow is not required (no boosting), less energy is consumed from the engine. 
     In the line pressure circuit  7  various valves are provided for controlling pressure on components of the transmission and/or hydraulic system. There is the solenoid feed valve, not shown here, that controls the pressure on the operating elements or solenoids. A line pressure valve  13  is provided that controls the pressure in the line pressure circuit  7 . An output of the line pressure valve  13  is supplied to the lubrication circuit  8  with a lubrication valve. An output of the lubrication valve typically is fed back to the pump system  4 . 
     There is also provided a primary valve  15  for controlling the pressure on the primary friction element  2 . Via the primary valve  15  further fluid flow can be provided to a selection valve, not shown here, that controls a forward clutch and a reverse clutch. Further, a secondary valve  19  is provided in the line pressure circuit  7  that controls the secondary friction element  3 . The line pressure circuit  7  also comprises a clutch valve and torque control valve, not shown here. 
     The various valves  13 ,  15 ,  19  are operated by operating elements or solenoids. The line pressure valve  13  is operated by the line operating element  20 . The primary valve  15  is operated by the primary operating element  21 . Further, in an embodiment of the hydraulic system, a clutch valve and/or a torque control valve can be provided, that operate the clutch valve and torque control valve respectively. 
     The secondary valve  19  is in prior art system controlled by a secondary operating element. Here, according to the invention, the secondary valve  19  is also controlled by the bypass solenoid  11 . As such, the bypass solenoid  11  has two functions of operating the bypass valve  10  and of operating the secondary valve  19 . This is advantageous, as, according to the invention, a dedicated solenoid can be obviated and thus energy, for controlling the solenoid, and costs, of providing the solenoid, can be saved by omitting one solenoid. 
     Here, the bypass solenoid  11  is arranged such that, after actuation of the bypass valve  10 , the pressure on the secondary friction element  3  falls lower than the pressure on the primary friction element  2 . This is particularly advantageous when the transmission is in overdrive. Then, the pressure on the secondary friction element  3  is advantageously lower than the pressure on the primary friction element  2 , so that less energy is required for providing friction force on the secondary friction element during overdrive. Thereby, the energy consumption is reduced and more engine power is available for driving the vehicle. Thus, energy and costs can be saved. 
     The line pressure circuit  7  has a predetermined bottom limit line pressure such that, in a fail-safe mode, when no actuation to the operating elements is provided, e.g. in case of electrical failure, or engine failure, the line pressure is reduced to the predetermined bottom line pressure, e.g. 5 bar or 7 bar, depending on the specific hydraulic circuit. By providing this predetermined bottom line pressure, a limited actuation of the primary and secondary friction elements  2 ,  3  can be provided so the vehicle can run in slow-down mode, such that the vehicle can e.g slow down until a hard shoulder along the road or an other safe place can be reached. The predetermined limit bottom line pressure is provided by the spring bias on the line pressure valve  13 . The spring bias is provided by spring  24  on the line pressure valve  13 . 
     According to the invention, the line pressure valve  13  can additionally and/or alternatively be biased towards a non-bottom limit line pressure, e.g. 17 bar or 15 bar. Then, there remains sufficient pressure available in the line pressure circuit  7  for the primary and secondary friction elements  2 ,  3  to provide for a limp home function of the vehicle. Thereto, the bypass solenoid  11  is provided with a third function to bias the spring force  24  to the non-bottom limit line pressure. This third function of the bypass operating element  11  is optionally provided and illustrated in  FIG. 2 . In this embodiment, the function is provided via the connection  25  between the bypass solenoid  11  and the line pressure valve  13 . In other embodiments, this connection  25  may be absent. As such, by providing this additional functionality, safety may be increased as the vehicle can safely continue its journey in case of e.g. an electric failure, until a safe heaven is reached. 
     In an embodiment, the secondary valve  19  is embodied as a pressure differential valve. Alternatively, the secondary valve  19  can be embodied as a pressure reduction valve. In an embodiment, the primary valve  15  is embodied as a pressure reduction valve and the line pressure valve  13  is can be embodied as a pressure relief valve. 
     According to the invention, there is a pump system  4 , having the two pumps or pump chambers  4   a ,  4   b  with the outlet lines  5 ,  6  respectively. The pressure system  4  has the bypass circuit  9  with the bypass valve  10 . Further, the line pressure circuit  7  is partially represented, showing the line pressure valve  13 , the primary valve  15  and the secondary valve  19 . Here, the primary valve  15  and the secondary valve  19  are both embodied as a pressure reduction valve. The valves  13 ,  15 ,  19 ,  10  are operated by operating elements  20 ,  21 ,  11 ,  11  respectively. The operating elements are typically embodied as solenoids which are operated by electrical currents passing through the solenoid. According to the invention, the bypass operating element  11  is arranged for operating the bypass valve  10  and additionally is arranged for operating the secondary valve  19 . As such, a solenoid can be omitted saving costs and energy, thus fuel consumption. Also, by providing the additional operation for the secondary valve  19 , in overdrive, the pressure on the secondary friction element can become lower than the pressure on the primary friction element. Thus less energy is required for actuating the secondary pulley, resulting in reduced required power, which may result in less fuel consumption or more energy available for driving the vehicle. So, a semi-independent pulley pressure control is provided that saves costs, one solenoid less, and provides for efficient performance of the hydraulic system by allowing the secondary pressure to be lower than the primary pressure in overdrive. 
     For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. 
     Many variants will be apparent to the person skilled in the art. All variants are understood to be comprised within the scope of the invention defined in the following claims. 
     In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to an advantage.