Method of shifting a transmission

#This disclosure relates to the field of automatic transmissions for motor vehicles. More particularly, the disclosure pertains to a method of changing among speed ratios.

#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.

#When driving conditions change, an automatic transmission changes from one speed ratio to another speed ratio. For example, when a vehicle is cruising using a low speed ratio and a driver demands an increase in wheel torque, the transmission must downshift into a higher speed ratio. For sudden changes in driver demanded wheel torque, the transmission may skip over one or more available gear ratios in a single shift event. Many automatic transmissions have multiple shift elements, such as clutches or brakes, and select particular speed ratios by engaging particular subsets of the shift elements. To perform a shift from one speed ratio to another, one or more previously engaged shift elements are released and one or more previously disengaged shift elements are engaged. Passenger comfort is maximized if the transition is accomplished smoothly. Performance is maximized if the transition is accomplished quickly. These considerations are often in conflict.

#FIG. 1 is a schematic diagram of an exemplary transmission gearing arrangement.

#FIG. 2 is a graph illustrating speed relationships during execution of a downshift.

#FIG. 3 is a graph illustrating torque relationships during execution of a downshift.

#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.

#An example transmission is schematically illustrated in FIG. 1. The transmission utilizes four simple planetary gear sets 20, 30, 40, and 50. Sun gear 26 is fixed to sun gear 36, carrier 22 is fixed to ring gear 58, ring gear 38 is fixed to sun gear 46, ring gear 48 is fixed to sun gear 56, input shaft 60 is fixed to carrier 32, and output shaft 62 is fixed to carrier 52. Ring gear 28 is selectively held against rotation by brake 66 and sun gears 26 and 36 are selectively held against rotation by brake 68. Input shaft 60 is selectively coupled to ring gear 48 and sun gear 56 by clutch 70. Intermediate shaft 64 is selectively coupled to carrier 42 by clutch 72, selectively coupled to carrier 22 and ring gear 58 by clutch 74, and selectively coupled to ring gear 38 and sun gear 46 by clutch 76.

#As shown in Table 1, engaging the clutches and brakes in combinations of four establishes ten forward speed ratios and one reverse speed ratio between input shaft 60 and output shaft 62. An X indicates that the corresponding shift element is engaged to establish the speed ratio.