Environmental Engineering Reference
In-Depth Information
The total gear shift ratio coverage is split into five (or six) steps with tighter
spacing in the higher, more frequently used, gears. The A750E, for example, has a gear
shift ratio coverage of 4.92:1, and the presence of five steps permits wider ratio cov-
erage so higher overall gearing is available for improved vehicle launch, yet sufficient
overdrive remains for highway cruise performance. Figure 4.3 illustrates the differ-
ence in total driveline ratio for the various transmissions under consideration.
6HP26
A750E
55-50SN
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Overall driveline ratio
Figure 4.3 Illustration of gear shift ratio coverage mapped to total driveline ratio
In Figure 4.3 the final drive for the transmissions considered are selected based
on the typical vehicle application - for example, the 55-50SN is used in the Volvo
S80 sedan. The final drive ratios used in this comparison are FD = 2.93 for 55-50SN,
FD = 3.97 for A750E and FD = 3.53 for the 6HP26.
The following two points from Figure 4.3 should be considered in addition to the
wider ratio spread of the 6-speed transmission: First, a 6-speed transmission has
higher ratio in first gear for improved launch and goes deeper into overdrive in sixth
gear for better fuel efficiency in cruise; and second, for all stepped automatics, the
lower gears have tall ratios with the gear steps decreasing for higher gears. Small
gear steps in higher gears offer smoother shifting feel and improved economy by
mapping the road load into the higher efficiency fuel islands of the engine. The
following three subsections elaborate further on the architectures of each transmis-
sion type listed in Table 4.4. These architectures are the Simpson type, or basic
3-speed design, the Wilson type and the Lepelletier with Ravigneaux type gear.
4.1.3.1 Simpson type
In the Simpson architecture a double planet epicycle gear set receives its input
torque from the torque converter turbine at the inner planet and outputs its torque to
the Simpson base transmission on the counter shaft via the second planet set. The
transmission schematic, including torque converter with integral M/G rotor, is
shown as Figure 4.4. Notice that input torque enters via a clutch to either the double
planet sun or inner carrier and exits via the outer carrier. Control is imposed over
the transmission by brakes on the sun and ring gears.
