Patent Application
20150307101
This disclosure relates to the field of automatic transmission controls. More particularly, the disclosure pertains to a method of employing a braking system to reduce vehicle speed before engaging an opposite direction gear ratio.
Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. Transmission speed ratio is the ratio of input shaft speed to output shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Generally, transmissions include at least one negative speed ratio which is engaged when the driver selects reverse.
Many automatic transmissions implement a discrete number of different transmission ratios in which each ratio is establish by engaging a particular subset of clutches. Clutches may include devices that couple two rotating elements to one another and devices which couple a rotating element to a stationary element. To shift from one speed ratio to another speed ratio, one clutch, called the off-going clutch, is released and another clutch, called the oncoming clutch, is engaged. To maintain power transfer during an upshift, the oncoming clutch must absorb energy. Designing the clutch to absorb and then dissipate this energy may involve increasing the friction area or fluid flow rate above what would be required simply to have adequate torque capacity. Increasing friction area and fluid flow rate increase the parasitic drag when the clutch is open reducing fuel efficiency. Some types of clutches, such as dog clutches, have no capability to absorb energy. When the oncoming clutch is a dog clutch, the elements to be coupled by the dog clutch must be at the same speed before engagement.
When the vehicle is stationary, the gearbox input is also stationary even for very high speed ratios. Since an internal combustion engine cannot generate torque at zero crankshaft speed, a launch device is necessary to permit the engine to rotate and transmit torque to the gearbox input. Many automatic transmission utilize a torque converter having an impeller driven by the engine crankshaft and a turbine driving the gearbox input shaft. Torque is transferred from the impeller to the turbine whenever the impeller rotates faster than the turbine. Torque is transferred in the opposite direction when the turbine rotates faster than the impeller.
When a driver shifts from drive to reverse while the vehicle is moving forward or from reverse to drive while the vehicle is moving backwards, the gearbox input shaft and turbine rotate backwards. The load on the engine is higher when the turbine rotates backwards than when the turbine is stationary. If the backwards speed of the turbine is too high, the load on the engine may cause the engine to stall. Since the backwards speed of the turbine is proportional to the vehicle speed, a controller may inhibit the shift until the vehicle speed is below a threshold, placing the transmission in neutral in the meantime. However, the deceleration rate of the vehicle in neutral is low, so this approach may delay engagement of the desired gear ratio may for an excessive amount of time. An alternative approach is to apply the oncoming clutch gradually to avoid exerting excessive load on the engine. However, this approach may force the oncoming clutch to absorb and dissipate more energy than it is capable of absorbing and dissipating.
A vehicle includes a transmission, a braking system, and a controller. The braking system may include friction brakes located at each wheel and arranged to apply a braking torque in response to a command from the controller. The controller commands the braking system to slow the vehicle in response to a command from the driver, such as by depressing a brake pedal. The controller is also programmed to command the braking system to slow the vehicle in response to the driver moving a shift lever while the vehicle is moving. For example, the controller commands the braking system to slow the vehicle if the driver moves the shift lever from a Drive position to a Reverse position while the vehicle is moving forward at a speed above a threshold. Similarly, the controller commands the braking system to slow the vehicle if the driver moves the shift lever from a Reverse position to a Drive position while the vehicle is moving backward at a speed above a threshold.
A method of controlling a vehicle includes responding to a change of position of a shift lever from a position corresponding to one direction of motion to a position corresponding to the opposite direction of motion by applying a first friction element to slow the vehicle to a speed less than a threshold before engaging a second friction element to establish the commanded power flow path. The change in shift lever position may be from Drive to Reverse or from Reverse to Drive. The first friction element may be a wheel brake.
A controller includes input communication channels, output communications channels, and control logic. The input communications channels receive a signal from a shift lever and may also receive a signal from a brake pedal. The output communications channels send command signals to a transmission and may also send command signals to a set of wheel brakes. The control logic is programmed to respond to movement of the shift lever indicating a change in intended direction of motion while the vehicle is moving by commanding a braking system to reduce vehicle speed and then, when vehicle speed is less than a threshold, by commanding the transmission to establish a power flow path corresponding to the opposite direction of vehicle movement.
Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.
A front wheel drive (FWD) vehicle powertrain 10 is illustrated schematically in
The engine and gearbox respond to commands from controller 30. The controller sends signals to gearbox 16 to apply particular clutches. The controller sends signals to engine 12 indicating what amount of torque to produce. Controller 30 receives signals from a shift lever 32, an accelerator pedal 34, and a brake pedal 36. The driver moves shift lever 32 among several positions to indicate the desired direction of travel. A D position indicates a desire to move forward. An R position indicates a desire to move backwards. An N position indicates a desire for neutral. A P position indicates a desire to engage park. The term shift lever is used here to represent any user interface element intended to indicate these choices including, for example, a console mounted lever, a steering wheel mounted lever, or a touch screen. Controller 30 may be implemented, for example, as a single micro-processor or as multiple communicating micro-processors.
Each wheel is associated with a friction brake 38, 40, 42, and 44 which applies torque to slow the wheel in response to a command from controller 30. Typically, controller 30 would issue such a command in response to the driver depressing brake pedal 36. However, the brake system command is not necessarily proportional to the brake pedal depression. The controller may limit the brake torque to avoid wheel slip or, in a hybrid electric vehicle, may coordinate the friction brake torque with regenerative braking supplied by a motor.
An exemplary arrangement of gearbox 16 is illustrated in
A method of controlling a vehicle during a direction change from reverse to forward is illustrated in
A method of controlling a vehicle during a direction change from forward to reverse is illustrated in
While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.
