Abstract:
A system for controlling a torque converter of an automatic transmission driven by a power source, the system including a torque converter an impeller, a turbine driveably connected to a transmission input and able to be driven hydrokinetically by the impeller, a stator, an impeller clutch for alternately engaging and disengaging a drive connection between the impeller and the power source, a source of converter charge pressure communicating with the impeller clutch, a source of converter discharge pressure communicating with the impeller clutch, a magnitude of differential force due to charge pressure and discharge pressure across the impeller clutch alternately producing operating multiple operating states of the impeller clutch, and a orifice having a variable fluid flow area for changing a magnitude of converter discharge pressure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a torque converter for an automatic transmission, and, in particular, to control of an impeller clutch among locked released and slipping states. 
     2. Description of the Prior Art 
     A torque converter is a modified form of a hydrodynamic fluid coupling, and like a fluid coupling, is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. A torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. 
     In a torque converter there are at least three rotating elements: the impeller, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator. In practice, however, the stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating the prime mover but allows for forward rotation. 
     Pumping losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch, which physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and therefore virtually no power loss and improved fuel economy. 
     While torque multiplication increases the torque delivered to the turbine output shaft, it also increases the slippage within the converter, raising the temperature of the fluid and reducing overall efficiency. For this reason, the characteristics of a torque converter must be carefully matched to the torque curve of the power source and the intended application. Changing the blade geometry of the stator and/or turbine will change the torque-stall characteristics, as well as the overall efficiency of the unit. Highway vehicles generally use low stall torque converters to limit heat production, and provide a more firm feeling to the vehicle&#39;s characteristics. 
     In a three-pass converter control system, the ability to rapidly and precisely control converter hydraulic system resistance through the converter discharge circuit controls an impeller clutch, which enables neutral idle, variable K-curve and lower load on the transmission oil pump. Without control of the converter discharge circuit it would not be possible to control the impeller clutch, which enables idle disconnect and variable K-curve. 
     In a conventional three-pass torque converter hydraulic control system, converter charge pressure and converter by-pass pressures are controlled by a valve or multiple valves in the main control. Converter charge pressure and by-pass pressure are either based on a percentage of regulated line pressure to prevent cover ballooning of the housing, or in the case of a closed piston, regulated maximum line pressure to optimize converter clutch torque capacity. Solenoids are used as the hydraulic control device. Depending on the precision of control, a pulse width modulated (PWM) or variable force solenoid (VFS) can be used. Converter discharge pressure is generally not separately controlled, but rather in a conventional converter is a dependent variable or a direct function of converter charge pressure and a restriction in the exit passage of the torque converter, stator, main control, case, and cooling circuit. Historically only converter charge pressure and by-pass pressure have been controlled. There has been little practical reason to precisely control and vary converter discharge resistance and pressure. 
     There is a need in the industry to control an impeller clutch in a torque converter during engine idle conditions in order to disconnect the engine and impeller and to vary the torque converter&#39;s K-curve. 
     SUMMARY OF THE INVENTION 
     A system for controlling a torque converter of an automatic transmission driven by a power source, the system including a torque converter an impeller, a turbine driveably connected to a transmission input and able to be driven hydrokinetically by the impeller, a stator, an impeller clutch for alternately engaging and disengaging a drive connection between the impeller and the power source, a source of converter charge pressure communicating with the impeller clutch, a source of converter discharge pressure communicating with the impeller clutch, a magnitude of differential force due to charge pressure and discharge pressure across the impeller clutch alternately producing operating multiple operating states of the impeller clutch, and an orifice having a variable fluid flow area for changing a magnitude of converter discharge pressure. 
     The invention provides precise hydraulic control of the converter discharge circuit allowing the impeller to be decoupled from the engine during idle, producing a variable converter K-curve and reduced oil pump flow demand. 
     Decoupling the impeller from the engine reduces load on the engine caused by the torque converter, thereby reducing fuel consumption during forward drive and reverse drive idle condition. This is accomplished with a single or multiple plate clutch in a torque transmission path between the impeller and a connection to the engine. The impeller clutch is controlled by a pressure differential between converter charge and discharge circuits, which makes precise hydraulic control of converter discharge pressure a requirement for idle disconnect. 
     The K-curve and effective K-curve, determined by slipping the impeller clutch, determine the torque converter load on the engine. Slipping the impeller clutch, thereby raising the effective K-curve, puts a lower load on the engine. For improved off-the-line performance, a raised effective K-curve is desirable. During normal driving at part throttle, a lower K-curve is advantageous for both fuel economy and performance. The ability, through precise hydraulic control of converter charge and discharge circuits, to slip the impeller relative to the engine raises the K-factor when performance is desired. 
     When the drive cycle demands a high percentage of available engine torque, either during vehicle launch or while towing a load, a raised or effective K-curve achieves better performance. The objective of a properly calibrated converter control system is to allow a controlled amount of impeller clutch slip at stall and low effective speed ratios. As efficiency becomes more important, controlled slip across the impeller clutch must be reduced to zero, such that the effective speed ratio equals speed ratio. The amount of controlled impeller slip must be characterized for partial to wide open throttle engine torque curves. The key is to characterize the effective K-curve to align peak available engine torque for a range of throttle positions from part-throttle to WOT with maximum converter torque multiplication, thus improving the attribute of wide open throttle performance and part-throttle performance feel. The ability to selectively raise the effective K-factor through slipping of the impeller clutch allows the K-factor defined by the internal torque converter hardware to be lower. Compared with a conventional torque converter, the ability to raise or lower the K-curve leads to a much more flexible system to balance vehicle performance with system efficiency. 
     Control of the converter discharge circuit resistance in certain driving modes and converter states lowers the load on the oil pump. Automatic transmissions are equipped with either a fixed or variable displacement oil pump to generate flow that is regulated, controlled and directed to friction elements such as clutches and bands, torque converter, lubrication and cooling circuit. The transmission oil pump is a power-take-off from the engine. If flow demand on the pump is decreased through reducing converter flow requirements, then the oil pump will operate more efficiently. 
     Whenever converter discharge resistance is raised above the downstream system resistance of the cooling circuit, then converter system flow demand is decreased, thus lowering the load on the pump by either redirecting unused flow back to the pump, in the case of a fixed displacement pump, or decreasing pump displacement, in the case of a variable displacement pump. 
     There are many instances where converter discharge resistance can be increased to lower the demand on the pump and increase efficiency. These include forward drive, reverse drive, neutral idle, vehicle deceleration, and when the converter clutch is locked. In a three-pass system in which converter discharge flow is directed to the cooling and lube circuit, the key calibration parameters that need to be characterized in order to increase hydraulic resistance are sump temperature, flow through the torque converter to maintain ATF cooling (lower flow requirement during idle and when the converter clutch is locked) and lube flow requirements. The flow requirements will need to be characterized for each converter system and where applicable the converter discharge resistance can be increased when the flow out of the converter is not required to maintain vital transmission function. However an enabler to lower the hydraulic demand on the oil pump is control of the converter discharge resistance. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a cross section through a torque converter having a bypass clutch and impeller clutch; 
         FIG. 2  is schematic diagram of orifice, solenoid and controller; and 
         FIG. 3  is a chart showing the relative magnitudes of converter bypass pressure, converter discharge pressure and converter charge pressure for various operating modes. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in  FIG. 1  a torque converter  10 , which is arranged about a central axis  12  and includes a bladed impeller  14 , a bladed turbine  16 , and a bladed stator  18 . The impeller, stator and turbine define a toroidal fluid flow circuit, whereby the impeller is hydrokinetically connected to the turbine. 
     The stator  18  is secured to, and supported for rotation on a stationary stator sleeve shaft  20 . An overrunning brake  22  anchors the stator to shaft  20  to prevent rotation of the stator in a direction opposite the direction of rotation of the impeller, although free-wheeling motion in the direction of rotation of the impeller is permitted. The turbine  16  is secured to a rotating transmission input shaft  24 , which transmits torque to the transmission gear box (not shown). A torque converter housing  26 , surrounding the turbine, impeller and stator, is driveably connected to the crankshaft of an internal combustion engine (not shown) or another power source, such as an electric motor. 
     Located within a torque converter housing  26  is an impeller clutch  28  for alternately driveably connecting and releasing the impeller  14  and engine. Impeller clutch  28  includes a disc  30 , supported for rotation on a turbine hub  32  by a bearing  34 ; a ring  36  secured to a shroud  38 , which is attached to the periphery of each blade of the impeller  14 ; and friction material  40 , located between ring  36  and disc  30 . A ring  42 , secured to disc  30 , is connected also to a torsion damper  44 , which resiliently connects the engine shaft  45  through the cover  26  to disc  30 . The engine shaft  45  is secured to cover  26 . 
     Also located within a torque converter housing  26  is a lockup clutch  46  for alternately driveably connecting and releasing the turbine  16  and engine through cover  26 . Clutch  46  includes a first set of friction discs  48 , splined at their outer circumference to a surface of ring  42 , and a second set of friction discs  50 , each splined at their inner circumference to piston  52 , interleaved between consecutive first discs and secured to the turbine  16 . Lockup clutch  46  is actuated by a piston  52 , which is supported on turbine hub  32  and disc  30  and secured to hub  32  permitting axial displacement rightward and leftward along axis  12 . A disc  54 , secured by a spline  56  to turbine hub  32 , is separated from piston  52  by a volume  58 , which, when pressurized, moves piston  52  rightward forcing discs  50 ,  48  into mutual frictional contact and engaging clutch  46 . When lockup clutch  46  is engaged, the engine shaft  45  and turbine  16  are mechanically interconnected and driveably connected to the transmission input shaft  24 . When lockup clutch  46  is disengaged, the turbine  16  and engine shaft  45  are mechanically disconnected, and the turbine may be hydrokinetically driven by the impeller  14 , provided impeller clutch  28  is fully engaged or slipping. 
     Fluid that causes lockup clutch  46  alternately to engage or apply and to disengage or release is supplied from a converter apply pressure circuit of the hydraulic system, whose magnitude is varied and regulated by the hydraulic control and actuation system of the transmission. Converter apply pressure C APY  is transmitted from the converter apply pressure circuit of the hydraulic system to volume  58  through an axial fluid passage  60 , radial passage  62  formed in input shaft  24 , axial passage  64 , and radial passage  66  formed in turbine hub  32 , i.e., the converter bypass pressure source. 
     A source of converter charge pressure of the hydraulic system includes axial passage  68 , which communicates through radial fluid passage  70  to the toroidal volume of the torque converter  10 . Converter charge pressure C CL  supplied from the converter charge pressure circuit of the hydraulic system through lines  68 ,  70  to the torque converter  10  develops a pressure force against the inner surface of impeller clutch disc  30  that is directed radially outward. 
     A converter discharge hydraulic circuit of the hydraulic system includes axial passage  72  and communicates with passages  74 ,  75  and  76 . Converter discharge pressure C ouT  in the converter discharge pressure circuit of the hydraulic system fills a volume  78  between impeller shroud  38  and cover  26  and develops a pressure force against the surface of ring  36  that is directed radially inward. The engaged, disengaged and slipping state of impeller clutch  28  is determined by the magnitude of the pressure differential across the impeller clutch  28 , i.e., (Δ C CL  C OUT ). 
     The impeller clutch  28  enables the decoupling of the impeller  14  from the engine shaft  45  during engine idle conditions. Decoupling of the impeller reduces load on the engine caused by the torque converter and fuel consumption in forward drive, reverse drive and neutral idle operation. 
     Referring to  FIG. 2 , the cross sectional area of an orifice  80 , located in the converter discharge circuit  72 , is changed by a variable force solenoid (VFS)  82 , which responds to command signals from an electronic controller  84  in a transmission control unit. Solenoid  82  is supplied with a variable electric current such that the size of orifice  80  varies in response to the magnitude of the current. The controller  84  repetitively executes control algorithms, which control the lockup clutch  46 , impeller clutch  28 , pressure regulator valves, friction clutches and brakes in the transmission, etc, in response to signals representing operator control of the engine throttle position, wheel brakes, vehicle speed, temperatures, engine parameters and inferred road conditions. Controller  84  issues command signals in response to the results of executing the algorithms. 
     The control algorithms ensure proper operation of the transmission, compatible with engine operation and driver demands. For example, while operating in engine idle impeller disconnect mode, the cross sectional area of orifice  80  is such that a uniform pressure is produced across impeller clutch  28  allowing the impeller clutch  28  to disengage. During idle disconnect mode, the impeller clutch  28  is in the open state, i.e., C OUT  pressure is equal to or greater than than C CL  pressure.  FIG. 3  shows for the neutral idle mode, the relative magnitudes of converter apply pressure C APY , converter discharge pressure C OUT , and converter clutch pressure C CL . 
     The disengagement of the impeller clutch  28  decouples the engine  45  from the impeller  14 . Since the impeller  14  is disconnected, there is little if any relative motion between the impeller and turbine reducing vortex flow and parasitic losses. 
     When the vehicle operator transitions from engine idle to drive away mode by releasing the brake pedal and depressing the throttle pedal, the impeller clutch  28  must immediately engage, i.e., either hard-lock the impeller  14  to the engine  45  or slip the impeller relative to the engine, thereby raising the effective K-curve of the torque converter. When the engine shaft  45  is coupled to the impeller  14 , the torque converter  10  increases torque transmitted to output shaft  24  from the engine shaft  45 . This torque amplification is accomplished by maintaining high converter charge pressure C CL  while concurrently decreasing converter discharge pressure C OUT  thereby increasing the pressure differential across impeller clutch  28 . 
     The speed ratio of a torque converter equals turbine speed divided by impeller speed (Speed Ratio=N turbine/N impeller). The effective speed ratio of a torque converter equals turbine speed divided by engine speed (Speed Ratio=N turbine/N engine). If the impeller clutch  28  is hard-locked such that impeller speed equals engine speed, then the torque converter speed ratio equals the effective speed ratio. If the impeller clutch  28  is slipping such that impeller speed is less than engine speed (N impeller&lt;N engine), the converter speed ratio is greater then the effective speed ratio. 
     The torque converter constant is equal to N engine/(T impeller) 1/2  and the effective torque converter constant is equal to N engine/(T engine) 1/2  When impeller clutch  28  is slipping, the effective K-curve is raised relative to the normal K-curve as defined by the torque converter geometry 
     In order to avoid an undesirable state or a driver perceived hesitation, the hydraulic control system must be capable of generating a pressure delta sufficient to hold the combination of engine torque, inertia torque and a safety factor before engine torque reaches the impeller clutch. This is only possible with accurate and speedy control of the converter discharge circuit resistance with either a VFS or PWM. 
     In summary in a three-pass converter control system, the ability to rapidly and precisely control converter hydraulic system resistance through the converter discharge circuit enables neutral idle, variable K-curve and lower load on the transmission oil pump. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.