Engine braking function for a continuously variable transmission

Abstract

The invention relates to a method for controlling the transmission ratio (105) in a continuously variable transmission that is included in a drivetrain of a vehicle; wherein the transmission ratio (105) is controlled during traction operation such that an absolute value of a torque acting on a shaft of the drivetrain does not overshoot a first limit function (101) which assigns to every transmission ratio (105) of the transmission a limit torque (107) which acts on the shaft in the presence of a maximum torque load on the drivetrain during traction operation; wherein the transmission ratio (105) is controlled during overrun operation such that an absolute value of a torque acting on the shaft does not overshoot a second limit function (103). The second limit function (103) assigns to every transmission ratio of the transmission a limit torque (107) which acts on the shaft in the presence of a maximum torque load, during overrun operation, on a part of the drivetrain which is downstream of an engine of the drivetrain.

Description

[0001] Engine braking function for a continuously variable transmission

[0002] The invention relates to a method according to the preamble of claim 1 and a computer program according to the preamble of claim 6.

[0003] Control systems for automatic transmissions that implement engine braking are known from the prior art. A corresponding control system is disclosed, for example, in JP 6 11 46 645 A2. To achieve engine braking, the transmission is prevented from shifting up.

[0004] The control system is subject to torque limitation. This is intended to protect the drivetrain from overload. The maximum engine torque and the torque capacity of the downstream drivetrain are taken into account when limiting the torque.

[0005] The invention is based on the object of improving the braking effect of a drive train with a continuously variable transmission. This object is achieved by a method according to claim 1 and a computer program according to claim 6. Preferred developments are contained in the subclaims.

[0006] The method is used to control the gear ratio of a continuously variable transmission in a vehicle's drivetrain. A continuously variable transmission, also known as a CVT, is a transmission whose gear ratio between the input and output shafts can be continuously adjusted. For example, it could be a continuously variable power-split transmission used in construction machinery.

[0007] In addition to the transmission, the drivetrain includes all torque-conducting components that are in a torque flow running from the engine to the vehicle's driven wheels. The drivetrain also includes the engine, one or more drive shafts, and one or more axle differentials. The drivetrain can be operated in traction or coasting mode. In traction mode, the engine generates drive torque that provides propulsion. In coasting mode, the drivetrain is subjected to a torque flow in the opposite direction. This torque flow has a braking effect and thus counteracts the vehicle's propulsion.

[0008] During traction operation, the transmission ratio is controlled such that the torque applied to a drivetrain shaft does not exceed a first limit function. Specifically, the transmission ratio is controlled such that the torque applied to the shaft does not exceed a limit torque, which the first limit function assigns to the corresponding transmission ratio. The shaft can be any shaft in the drivetrain whose torque can be determined. Preferably, it is a shaft equipped with a torque sensor.

[0009] The first limit function assigns a limit torque applied to the shaft to each gear ratio. This limit torque occurs when the entire drivetrain is loaded with the maximum possible torque during traction operation. This limit torque is therefore applied to the shaft at the maximum possible torque load on the drivetrain during traction operation.

[0010] During overrun, the transmission ratio is controlled analogously using a second limit function. The ratio is controlled in such a way that the torque applied to the shaft does not exceed a limit torque, which the second limit function assigns to the corresponding transmission ratio.

[0011] According to the invention, the second limit function assigns a limit torque to each transmission ratio, which corresponds to the maximum possible torque load of a part of the drivetrain downstream of the engine during overrun. This limit torque is applied to the shaft at a maximum torque load of the said part of the drivetrain during overrun. According to the invention, the maximum braking effect that the part of the drivetrain downstream of the engine can withstand without damage is thus utilized. This makes it possible to fully exploit the maximum possible braking effect of the engine while taking into account the load capacity of the downstream part of the drivetrain.

[0012] The downstream part of the driveline refers to the part of the driveline located between the engine and the vehicle's driven wheels. This part of the driveline may include the transmission, one or more drive shafts, and / or one or more axle differentials.

[0013] In a preferred embodiment, the first limit function and the second limit function are different for at least one gear ratio of the transmission. Specifically, the limit torque assigned by the first limit function to at least one gear ratio of the transmission differs from the limit torque assigned by the second limit function to the same gear ratio.

[0014] The differences between the first limit function and the second limit function, as described in the advanced training, correspond to different drivetrain load capacities during traction and overrun. The advanced training makes it possible to take these differences into account when controlling the transmission ratio.

[0015] In a further preferred development, the second limit function is greater than the first limit function for at least one transmission ratio. Specifically, a limit torque assigned by the second limit function to at least one transmission ratio is greater than the limit torque assigned by the first limit function to the same transmission ratio. This means that, according to the development, a greater torque can be transmitted in overrun mode than in traction mode.

[0016] In a further preferred development, the engine is adapted so that it can apply an increased braking torque. This can be achieved, for example, by means of variable valve control. During overrun, the piston movements can then be specifically impeded by closing the valves. Accordingly, the development provides that the torque applied by the engine to the downstream part of the drive train is greater in magnitude for at least one speed during overrun than during traction.

[0017] If it turns out that the torque applied to the shaft during overrun exceeds the second limit function or a limit torque that the second limit function assigns to the respective transmission ratio, a brake of the vehicle is automatically applied in a further preferred embodiment. This supports the engine braking according to the invention.

[0018] A computer program according to the invention implements the method according to the invention or a preferred development. The computer program is thus designed to execute the method according to the invention or a preferred development of the method. This means that the computer program comprises instructions for executing the method according to the invention or a preferred development. The computer program causes a transmission control unit to execute the method according to the invention or a preferred development when the computer program is executed by the transmission control unit.

[0019] The computer program can be contained in the transmission control unit, stored on a storage medium, or encoded in one or more transmittable or transmitted signals. In particular, the computer program can be present as a computer program product, i.e., as a tradable unit or a unit that serves the purpose of transferring ownership of the computer program. The computer program can exist not only as software, but also in the form of hardware-based circuit logic.

[0020] A preferred embodiment of the invention is shown in Fig. 1. In detail:

[0021] Fig. 1 shows two limit functions. A first limit function 101 and a second limit function 103 are shown in Fig. 1. The two limit functions 101, 103 each assign a maximum output torque 107 to each gear ratio or reciprocal gear ratio 105 of a continuously variable transmission.

[0022] The first limit function 101 corresponds to the maximum torque load capacity of the entire drivetrain during traction operation. The drivetrain also includes an engine. This limits the maximum load capacity of the drivetrain at higher reciprocal ratios 105.

[0023] The second limit function 103 corresponds to the maximum torque load capacity of a part of the drivetrain downstream of the engine during coasting. Accordingly, a limitation imposed by the engine is not included in the second limit function 103. Therefore, the second limit function 103 allows for greater torque loads at larger reciprocal ratios.

[0024] A denotes an operating state in overrun mode. The output torque 107 present in state A exceeds the permissible limit torque according to the first limit function 101 for the corresponding reciprocal ratio 105. If the first limit function were used to control the ratio in overrun mode, the reciprocal ratio 105 would have to be increased accordingly. However, this would result in a reduced braking effect.

[0025] To maximize the drivetrain's maximum load capacity, the second limit function 103 is used instead. This allows for a higher output torque 107 at the reciprocal ratio 105 of operating state A. This increases the achievable braking torque. Reference symbol Limit function Limit function Reciprocal ratio Output torque

Claims

Patent claims 1 . Method for controlling the gear ratio (105) of a continuously variable transmission included in a drive train of a vehicle; wherein the gear ratio (105) is controlled in traction mode such that the magnitude of a torque applied to a shaft of the drive train does not exceed a first limit function (101) which assigns a limit torque (107) to each gear ratio (105) of the transmission, which limit torque is applied to the shaft at a maximum torque load of the drive train during traction mode; and wherein the gear ratio (105) is controlled in overrun mode such that the magnitude of a torque applied to the shaft does not exceed a second limit function (103); characterized in that the second limit function (103) assigns a limit torque (107) to each gear ratio of the transmission, which limit torque is applied to the shaft at a maximum torque load of a part of the drive train downstream of an engine of the drive train during overrun mode.

2. Method according to claim 1; characterized in that the first limit function (101) and the second limit function (103) are different for at least one gear ratio (105) of the transmission.

3. Method according to the preceding claim; characterized in that the second limit function (103) for at least one gear ratio of the transmission is greater than the first limit function (101).

4. Method according to the preceding claim; characterized in that a torque with which the engine applies to the downstream part of the drive train is greater in magnitude for at least one speed in overrun mode than in traction mode.

5. Method according to one of the preceding claims; characterized in that a brake of the vehicle is automatically applied when, in overrun mode, the torque (107) applied to the shaft exceeds the second limit function (103).

6. Computer program; characterized by Instructions which cause a transmission control unit of a vehicle to regulate the gear ratio (105) of a continuously variable transmission included in a drive train of the vehicle according to a method according to one of the preceding claims.

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