In many vehicle applications it is common practice to provide between the engine and the load, e.g. the driven wheels of the vehicle, a hydrodynamic/mechanical transmission of which the hydrodynamic unit is constituted by a torque converter or fluid coupling while the mechanical element or unit of the transmission is constituted by a gear transmission shiftable under load and having the various transmission ratios, "gear" or "speeds" selected to appropriate actuation of one or more hydraulically operated clutches.
For the purposes of the present invention and description, such a hydrodynamic/mechanical transmission can be the one described in the aforementioned copending application or in the prior art described below.
In German Pat. DT-PS No. 1 780 067, there is described a hydraulic control system for a hydrodynamic/mechanical transmission of the aforedescribed type in which the pressure in the supply line to the hydrodynamic unit is controlled by a pressure-relief or pressure-limiting valve such that, upon the development of excess pressure in the supply line to the hydrodynamic unit, a port is opened which delivers the surplus fluid to an outlet and thence to the reservoir.
The fluid is usually drawn from the reservoir via a filter by a pump driven by the motive-power source, namely, the engine. It has been found that with such a valve, the inlet pressure is dependent upon the throughflow volume of the valve. In other words, with increasing volume rates of flow of fluid through the valve, the pressure builds up at the inlet side of the hydrodynamic unit in spite of the fact that the valve is intended to limit the pressure at the inlet to the hydrodynamic unit by shunting the excess fluid to the reservoir.
Thus, with relatively large throughflows, the valve spool or member is shifted to its largest opening and thereafter partially blocks the intake so that the pressure in the hydrodynamic unit sharply rises. A greater dimensioning of the flow cross-section of the valve, which might alleviate the problem, is not always possible because of the limited space in which the valve must be located for practical reasons. It has also been found that the use of a pressure-relief valve to shunt the excess fluid to the reservoir has the significant disadvantage that it increases the volume of the hydraulic fluid which must traverse the filter at the intake side of the fluid pressure source, namely, the pump. For a given filter fineness, the requisite of greater volume rates of flow of the fluid, because of these circumstances, means that the filter dimensions must be greatly increased. This has the disadvantage mentioned previously and further increases the cost of the filter.
It is also possible to accommodate the increased throughflows of the fluid by compromising the filter fineness which has the disadvantage that it endangers the functioning of the entire hydraulic network.