In order to reduce the fuel consumption and the output of pollutant emissions of vehicles built with combustion engines and familiar from general practice, it has become common to switch off the combustion engines of vehicles under appropriate operating conditions of the vehicle with the help of various vehicle concepts. Such functions are known among other things as engine stop-start functions, which are activated or deactivated depending on the operating conditions of the most varied vehicle components, and trigger the switching off of the combustion engine even when the vehicle is briefly at rest.
So that conventional driving operation is not compromised by an engine start-stop function, if a driver wishes to drive on so that the vehicle's combustion engine has to be started, particularly when entering a congested street with a right of way, it is necessary to have a quick engine starting process and an immediate build-up of the force flow in a transmission device of the vehicle. In conventionally made automatic transmissions or automated shift transmissions constructed with shift elements made as wet-running disk clutches for engagement and disengagement of various transmission ratio steps of the transmission device, the shift elements are essentially supplied with the required control pressure from a transmission pump only when the combustion engine is running. In this, the control pressure or an actuating pressure of a shift element is in each case adjusted by way of at least one electric actor.
For the build-up of the force flow in the transmission, an air gap of a shift element to be actuated first has to be closed and then the shift element to be actuated is fully engaged by increasing the actuating pressure in accordance with predetermined engagement characteristics. Closing the air cap in a shift element and then engaging it in the force flow of a transmission are both carried out by passing a certain hydraulic fluid volume flow into a piston space of the hydraulically controlled clutch to be engaged, and this has to be supplied by the transmission pump driven by the started combustion engine.
If, before the vehicle is re-started, several shift elements of a transmission are open because the combustion engine is switched off and these have to be closed for the vehicle to re-start, the time between the beginning of the combustion engine's starting process and the time when the force flow in the transmission has been fully established is prolonged sometimes to such an extent that a vehicle made with an engine start-stop function cannot be operated as effectively as desired.
For vehicles with conventional transmissions having an implemented engine start-stop function to be able, despite this, to be operated in the desired manner, in some vehicles known as such, besides the main transmission pump a further hydraulic pump driven by an electric motor is provided, whose delivery volume is independent of the speed of the combustion engine and which, if insufficient pressure is supplied by the main transmission pump of the transmission unit, produces a pressure in the hydraulic system, in collaboration with the electric actors of the transmission unit, for the shift elements to be actuated at least sufficient to close the respective air gaps in the shift elements to be engaged in order to establish the force flow in the transmission unit.
Unfortunately, compared with transmission devices having no additional pump, the above-described design of a transmission with a main transmission pump driven by the Combustion engine and an additional pump driven by an electric motor reduces the efficiency of the transmission and increase its manufacturing costs.
Measures which improve the transmission efficiency only slightly and increase manufacturing costs only by a small amount, in order to be able to operate a vehicle that works with an engine start-stop function with the desired spontaneity, are known from DE 10 2006 014 756.1. For the sufficient supply of a transmission device when a combustion engine is to be switched on, a device for storing hydraulic fluid of a hydraulic system of a transmission device and a method for the operation of such a device are proposed, the device or hydraulic storage unit being made as a spring-loaded oil volume store with an electro-mechanical detent system.
While the combustion engine is switched on, the hydraulic storage device is filled by a hydraulic pressure produced by a main transmission pump so that as the hydraulic storage device fills up a piston, of the hydraulic storage device, is moved to a position in which it can be arrested and held by a holding device When the combustion engine is switched off so that the drive power to the main transmission pump is zero, the hydraulic pressure in the hydraulic system of the transmission device falls essentially to zero. If an engine start-stop function then calls for the combustion engine to be switched on, then the production of the force flow in the transmission unit is supported in a simple manner by expelling the volume of hydraulic fluid stored in the hydraulic storage device during a starting process of the combustion engine.
Furthermore, when the unpressurized condition in the hydraulic system of the transmission device brought about by switching off the combustion engine has been reached, the electric actors are energized to a so-termed standby level in which only so-termed diagnosis functions can be carried out by the actors in the transmission device and in which the electric power uptake of the transmission is minimized, so that the vehicle's electric power supply system is relieved to a not inconsiderable extent. Moreover, when the engine start-stop function calls for the combustion engine to be switched on or when it is recognized that the combustion engine has been started up from the standby level to an operating level in which the actors are respectively energized with a current corresponding to a required operating condition such as the engagement of a gear, a neutral operating condition, or a parking operation, the energizing current of the actors is changed. By changing the energizing current of the actors from the standby level to the operating level, the shift elements to be engaged, in order to produce the desired operating condition of the transmission unit, are at the same time acted upon and filled with hydraulic fluid from the hydraulic system of the transmission unit.
The sequence in which the respective shift elements reach their transmission capacity required for producing the operating condition of the transmission device called for, depends essentially on the filling flow resistance in the lines of the hydraulic system and on the clutch volumes to be filled. During a starting process of a combustion engine, these dependencies sometimes result in undesired operating condition variations within the transmission unit, in which rotating masses inside the transmission are first accelerated and then synchronized and undesirably braked by the shift element when it eventually reaches its intended transmission capacity. This results in undefined rotation speed conditions in the transmission, with the consequence of non-reproducible behavior and which sometimes causes a jerk in the drive train that reduces driving comfort.
Accordingly, the purpose of the present invention is to provide a method for operating a transmission device, which avoids compromising the driving comfort of a vehicle.