or also known as clutchless manual transmission or automated manual
transmission, is a system which uses electronic sensors, processors and
actuators to do gear shifts on the command of the driver. This removes the need
for a clutch pedal which the driver otherwise needs to depress before making a
gear change, since the clutch itself is actuated by
electronic equipment which can synchronise the timing and torque required to
make gear shifts quick and smooth. The system was designed by European
automobile manufacturers to provide a better driving experience, especially in
cities where congestion frequently causes stop-and-go traffic patterns.
In standard mass-production automobiles,
the gear lever appears similar to manual shifts, except that the gear stick only
moves forward and backward to shift into higher and lower gears, instead of the
traditional H-pattern. The Bugatti Veyron uses this approach for its 7-speed
transmission. In Formula One, the system is adapted to fit onto the steering
wheel in the form of two paddles; depressing the right paddle shifts into a
higher gear, while depressing the left paddle shifts into a lower one. Numerous
road cars have inherited the same mechanism.
Hall effect sensors sense the direction of requested shift, and this input,
together with a sensor in the gear box which senses the current speed and gear
selected, feeds into a central processing unit. This unit then determines the
optimal timing and torque required for a smooth clutch engagement, based on
input from these two sensors as well as other factors, such as engine rotation,
the Electronic Stability Program, air conditioner and dashboard instruments.
The central processing unit powers a hydro-mechanical unit to either engage or
disengage the clutch, which is kept in close synchronization with the
gear-shifting action the driver has started. The hydro-mechanical unit contains
a servomotor coupled to a gear arrangement for a linear actuator, which uses
brake fluid from the braking system to impel a hydraulic cylinder to move the
main clutch actuator.
The power of the system lies in the fact that electronic equipment can react
much faster and more precisely than a human, and takes advantage of the
precision of electronic signals to allow a complete clutch operation without the
intervention of the driver.
Historically, the first semi-automatic transmission which was marketed was the
1941 M4/Vacamatic Transmission by Chrysler. It was an early attempt at an
automatic transmission that still
required the use of a clutch, primarily to start and stop. Later, the Volkswagen
Beetle came with an optional "Autostick", which was essentially a clutchless
manual with three forward gears.
CitroŽn is one manufacturer that committed to semi automatic transmission. First
appearing in 1955 on the CitroŽn DS, a hydraulic system was used to select gears
and operate the conventional clutch using hydraulic servos. There was also a
speed controller and idle speed step-up device, all hydraulically operated. This
was a clutchless shifting with a single column mounted selector. The CitroŽn 2CV
gained a device named 'Trafficlutch', a centrifugal clutch that enabled
clutchless changes in the first two ratios only (for town driving). The DS's
semi automatic transmission was nicknamed 'Citro-Matic' in the United States.
Later, the manufacturer introduced optional semi automatic transmissions on
their medium and large saloon and estate models in the 1970s; the CitroŽn GS and
CX models had the option of 3 speed, semiautomatic transmission marketed as 'C
matic'. This was simpler than the previous inasmuch as it used a floor mounted
quadrant lever operating a contact breaker and conventional gear selector rods
in series, a fluid coupling 'torque convertor' and wet plate clutch were cut in
and out of phase by an electro valve controlled by the contact breaker. This
system was simple in that it dispensed with the former use of hydraulics to
operate a clutch AND select the gear ratios. CitroŽn semi automatic transmission
of this era made no use of electronics, the entire gear selecting operation was
carried out by simply moving the gear lever from one ratio to the next.
The German automobile manufacturer, NSU, produced a semi automatic system for
the rotary engined Ro80 saloon car in the 1960s, similar in concept to CitroŽn's
system except that it used an electric switch on the gear shifter which
disengaged the clutch.
The 993cc Diahatsu Charade in 1985 at least, had the option of a 2-speed
semi-automatic transmission which was similar to a conventional auto with torque
converter and planetary gearset but lacked a full valve body for making
decisions regarding shifting. This was left entirely to the driver and as a
result could be accelerated from rest in top gear if desired, depending entirely
on the torque converter action. This was not as bad as might seem; standing
quarter mile time with two 60kg occupants and using low gear appropriately was
21.0 sec while using top gear only was 21.5 sec.
Drag Racers have their own type of clutchless manual transmissions. A Liberty is
basically a manual transmission with no clutch, and is used in Pro Stock. The
Lenco is a transmission also used in drag racers. A Lenco is different from a
Liberty because a Lenco uses planetary gears, like an automatic. Both
transmissions can be manually shifted, or use an air shifter. However, they
require a clutch to use when leaving off the line for
traction. The Lenco uses separate levers to shift while the Liberty uses a
single shifter hooked up to several levers. A variation of the Lenco called a
Lencodrive utilizes a torque converter and no clutch.
Some variants of the system, such as CitroŽn's Sensodrive, allow the driver to
select automatic mode, in which the processor takes responsibility for gear
changes. The car then drives much like a standard automatic, including features
such as kickdown, but with fuel consumption pretty similar to a manual. Gears
can be selected using either stick or paddles, both in manual control and as a
temporary change in automatic mode (for example before starting to overtake
Semi-automatic transmissions have also made its way into the truck and bus
market in the early 2000s. Volvo offers its i-Shift on its heavier trucks and
buses, while ZF Friedrichshafen AG markets its ASTronic system for buses and
coaches. These gearboxes have a place in public transport as they have been
shown to significantly reduce fuel consumption.
In the UK though, semi-automatic transmission has been very popular on buses for
some time, from the 1950's right through to the 1980's, an example being the
well known London Routemaster, although the latter could also be driven as a
fully automatic in the 3 highest gears. Leyland manufactured many buses with
semi-automatic transmission, including it's Leopard and Tiger coaches. Fully
automatic transmission became popular with increasing numbers of continental
buses being bought in the UK, and more and more British manufacturers began
offering automatic options, mostly using imported gearboxes, and semi-automatic
transmission lost favour. These days, very few buses with semi-automatic
transmission remain in service, although many are still on the roads with
private owners. Modern types of semi-automatic transmission though is becoming
more common, mostly replacing manual gearboxes in coaches and small buses.