1. Field of the Invention
This invention relates generally to a system for cooling a starting clutch of a continuously variable transmission, and more particularly to a control system for clutch cooling which allows greater coordination between the amount of clutch cooling and the amount of clutch slippage, thereby enabling greater flexibility in regulating and modifying clutch slippage.
2. Description of the Prior Art
Continuously variable transmissions (CVT's) provide continuously variable drive ratios by using a belt between two variable-diameter pulleys. A detailed discussion of how such systems operate is set forth in U.S. Pat. Nos. 4,433,594, 4,522,086 and 4,982,822, each of which is incorporated herein by reference. In general, a belt intercouples two variable-diameter pulleys, an input or primary pulley and an output or secondary pulley. Each pulley includes a fixed sheave and a movable sheave. A hydraulic control system is employed to enable axial movement of both the primary and secondary movable sheaves, thereby changing the effective diameters of the pulleys. When the transmission is in operation the effective diameter of the primary pulley is varied to provide a continuously variable drive ratio, while the effective diameter of the secondary pulley is varied in order to maintain belt tension and avoid belt slippage.
As with conventional transmissions, a automobile having a CVT system also requires a start clutch to permit engagement and disengagement of coupled shafts which transmit torque. The start clutch for a CVT system can be located either upstream or downstream of the belt and sheave system of the CVT. Various types of start clutches can be employed with a CVT, including wet clutches or electromagnetic clutches. With wet start clutches, however, removal of heat generated during engine stall is required. Furthermore, clutches that are designed to provide slippage in order to reduce coupling shock during the engagement period, or when the torque exceeds its maximum rating, produce heat during such slippage which must be dissipated.
One type of start clutch typically used in combination with a CVT is a fluid actuated and fluid cooled start clutch. Such start clutch assemblies commonly include a cup-shaped cover plate, a pressure plate, a reaction plate, a clutch disc assembly, a Belleville spring, and connecting elements. The clutch disc assembly includes a clutch disc having an energy-absorbing layer and a friction material layer mounted on each of its faces. This type of start clutch operates as follows. The pressure plate is biased by the Belleville spring to a disengaged condition. The clutch is engaged by fluid actuating the pressure plate. This is accomplished by increasing fluid pressure in a clutch fluid pressure chamber that is defined by the cover plate and the pressure plate. When the fluid pressure is increased in the fluid pressure chamber, the force of the Belleville spring is overpowered. The pressure plate is thereby thrust into contact with the clutch disc assembly, and into driving communication with the reaction plate through the friction faces of the clutch disc.
The start clutch also includes a cooling fluid cavity chamber. Cooling fluid is circulated through the cooling chamber in order to dissipate heat generated by the friction between the reaction plate and the friction faces during clutch engagement, i.e. during clutch slippage. When minimal or no cooling flow is required (e.g., when the clutch is fully engaged or fully released, and at low relative speed creep) the clutch cooling flow port is closed in order to reduce drag and provide energy savings. In accordance with prior art cooling system designs, however, there was limited coordination between the need for cooling and the circulation of cooling fluid.
Systems have been proposed for cooling a starting clutch in a CVT during the clutch slippage period. Smith et al. U.S. Pat. No. 4,458,318 discloses a starting clutch cooling system which includes an FEMA valve, a spool valve and a solenoid valve. The spool valve is responsive to pressure changes in two diametrically opposed chambers contained therein. A spool contained within the valve is movably located between these two chambers. The pressure in one chamber remains fixed, whereas the pressure in the second chamber is varied by the FEMA valve. An electrical signal controls FEMA valve. The electrical signal causes a plunger in the FEMA valve to restrict a bore, thereby restricting fluid flow between the second chamber in the spool valve and the FEMA valve. This causes an increase in the pressure in the spool valve's second chamber. The resulting pressure differential between the two diametrically opposed chambers causes the spool to laterally move permitting fluid to flow to the solenoid valve. The solenoid valve is vacuum operated. When open, the solenoid valve allows fluid flow to a clutch cooling fluid chamber.
Other systems have been employed as well, including non-electronic systems which offer limited control of cooling fluid flow. The present invention, however, provides a better system design which incorporates electronic control means to enable exceptional control of the clutch cooling, while also enabling coordination between the need for clutch cooling and the coolant flow rate.