A semi-automatic transmission is a very advanced system, which still uses a clutch to perform the gear shift instead of a torque converter. Unlike the manual transmission, the computer does all of the clutch disengaging, gear shifting, and clutch engaging. This not only makes the gear shifting faster than manual transmission, but also prevents the vehicle from stalling when the car is stationary. Like a tiptronic transmission, a semi-automatic transmission can also be switched to manual mode to perform gear shifting at the drivers' wish. The two most common semi-automatic transmissions are direct shift transmission (aka dual-clutch transmission) and electrohydraulic manual transmission (aka sequential transmission).
The direct shift transmission was designed and developed by
the Audi and Volkswagen auto groups. Figure 24 shows the
structure of the direct shift transmission. Like a conventional
manual transmission system, it uses a collar, synchronizer, and gear
set to perform gear shift. The clutch set is like that inside
the automatic planetary gear transmission system, which controls
the gear ratio change. Unlike the conventional manual transmission
system, there are two different gear/collar sets, with each connected
to two different input/output shafts. One set contains odd (1st,
3rd, 5th .
. .) gears, while the other contains even (2nd,
4th, 6th .
. .) gears. It is just like two conventional manual transmission
gear boxes in one. To automatically shift from 1st
gear to 2nd gear, first the computer
detects that the spinning speed of the input shaft is too high,
and engages the 2nd gear's collar to
the 2nd gear. The clutch then disengages
from 1st gear's input shaft, and engages
the 2nd gear's input shaft. Controlled
by computer, the gear shift becomes extremely fast compared with
a conventional manual transmission. Using direct contact of the
clutch instead of fluid coupling also improves power transmission
efficiency. Another advanced technology used for direct shift
transmission allows it to perform "double clutching" by shifting
the gear to neutral first, adjusting the spinning speed of the
input shaft, and then shifting to the next gear. This makes gear
shifting very smooth.
Another famous semi-automatic transmission is an electrohydraulic manual transmission (aka sequential transmission). The gear set is almost the same as the conventional transmission system, except that the shifting of the selector is not an 'H' pattern. Instead, all selector forks are connected to a drum (see Figure 25). The drum has several grooves, and each has a ball sliding in it. Each fork hooks up to a ball and can be moved forward and backward when the drum is turning (see Figure 26). Based on the pattern of the grooves on the drum, by turning the drum, each fork can move forward and backward in turn, which makes gear selection sequential. Therefore, it is impossible for an electrohydraulic manual transmission to perform a gear shift from 1st to 3rd or 4th to 2nd. The shifting must be sequential, like 1st'2nd'3rd'4th, or 4th'3rd'2nd'1st.
Go To Comparison of Transmission Systems
List of Visuals