Device for controlling a continuously variable automatic transmission

This invention relates to a device to control a continuously variable automatic transmission (CVT) which is particularly adequate for installation in a motor vehicle.

A CVT is a continuous transmission with a first bevel pulley pair upon an input shaft and a second bevel pulley pair upon an output shaft, each bevel pulley pair consisting of a first pulley stationary in the axial direction and a second bevel pulley movable in the axial direction (the primary pulley and secondary pulley, respectively). Between the bevel pulley pairs extends a belt drive organ such as a pushing linked band. The primary pulley and secondary pulley are adjusted by a pressure medium with the aid of an electronic control unit which controls the pressure level of the primary and secondary pulleys via electromagnetic actuators and hydraulic valves.

DE-OS 42 34 103 disclosed a device to control the pressure of the primary pulley. The device comprises an electromagnetically controlled primary valve and a hydraulic secondary valve. The primary valve is controlled by an electronic control unit. If the line from the electronic control unit of the primary valve is interrupted, the device changes over to emergency operation. The pressure level of the primary pulley is then determined by the secondary valve. The position of the secondary valve, in turn, is determined by the primary pulley via a mechanical clutch.

The hydraulic shifting circuit for such a CVT usually has one pump which, from a tank serving as a lubricant sump, conveys the pressure medium to the lines provided with pressure-reducing and pressure-regulating valves through which lines, among others, the adjusting cylinders for the primary pulley and secondary pulley are filled with pressure medium. The leakage flows which appear during adjustment of the primary and secondary pulleys pressurelessly flow back to the tank, the same as the leakage flows appearing in the pressure regulators, and are again returned by the pump, via a filter, to the hydraulic circuit. In the prior art, the known pressure regulators work in the range from 0 to 6 bar at an operating pressure of 8 bar.

The problem solved by the invention is based on increasing the efficiency and reducing the construction expenses of a hydraulic circuit for a CVT, in particular for a belt with bevel pulley transmission.

According to the invention, it is proposed that the hydraulic circuit be a closed hydraulic circuit with a constantly regulated pressure so that leakage flow currents from the primary pulley and secondary pulley, as well as the pressure regulators, no longer be delivered to the tank, but e.g. directly to the secondary circuit, or to a constant pressure system that supplies the hydrodynamic torque converter and the lubrication.

The invention is explained in detail with reference to the drawings which show:

FIG. 1 is a simplified diagram of a conventional pressureless hydraulic circuit; and

FIG. 2 is an analogous diagram of a closed hydraulic circuit designed according to the invention.

Both representations for the hydraulic control of a CVT transmission in the design of a belt 12 with a bevel pulley transmission for a motor vehicle is shown only diagrammatically and very simplified, to make understanding the invention easier. FIG. 1 shows a conventional pressureless hydraulic circuit where both adjusting cylinders 1 and 2 , for the primary pulley pair 3 and secondary pulley pair 4 are loaded via corresponding lines and pressure-reducing valves 8 . The hydraulic circuit is connected on one side with a tank 7 serving as a lubricant sump to which pressurelessly return the leakage flows of the adjusting cylinders 1 and 2 , respectively, of the primary pulley pair 3 and secondary pulley pair 4 , as well as the leakage flows of the pressure-regulating valves DBV. In its other section, the hydrodynamic circuit is connected with a hydraulic secondary circuit 9 which supplies a hydrodynamic torque converter 13 and the lubrication points ( 14 ) of the CVT. In this hydraulic circuit 9 , the pressure regulators DBV usually work in the range from 0 to 6 bar at an operating pressure of 8 bar.

The primary pulley 3 comprises a pair of beveled sheaves on an input shaft 10 . One of the sheaves, e.g. the primary sheave, is movable in an axial direction along the input shaft 10 while the other one of the sheaves, e.g., the secondary sheave, is fixed to the input shaft 10 .

The secondary pulley 4 comprises a pair of beveled sheave on an output shaft 10 . One of the sheaves, e.g. the secondary sheave, is movable in an axial direction along the output shaft 11 while the other one of the sheaves, e.g. the primary sheave, is fixed to the output shaft 11 .

FIG. 2, like FIG. 1, shows diagrammatically, and only in a simplified representation, a closed hydraulic circuit in which the leakage flows from the adjusting cylinders 1 and 2 , respectively, for the primary pulley pair 3 and secondary pulley pair 4 , as well as the individual pressure-regulating valves DBV, are no longer returned to the lubricant sump 7 , but are directly supplied to the secondary circuit 9 , via a designated line L, so that said leakage flows for the torque converter 13 and lubrication points 14 of the transmission can be used directly or, arbitrarily, in a constant-pressure system. The former pressureless, vented circuit, therefore, is converted to a closed system with a constantly regulated pressure of about 5 bar. The working range of the pressure regulators DBV is preferably approximately 5 to 11 bar, the minimum contact pressure at said values amounting to about 5 bar. The leakage flows of both the secondary pulley pair 4 and primary pulley pair 3 , together with the pressure regulators DBV (which as a function of pressure and temperature can amount to 1.5 liters per pressure regulator) are delivered directly to the secondary circuit 9 .