Semi-automatic transmission

Most semi-automatic transmissions are based on conventional manual transmissions, but are usually operated with an automatic clutch or another kind of partially automated transmission mechanism. Once the clutch becomes automated, the transmission becomes semi-automatic. However, these systems still require manual gear selection by the driver. This type of transmission is called a clutchless manual or an automated manual.

Most semi-automatic transmissions in older passenger cars retain the normal H-pattern shifter of a manual transmission; similarly, semi-automatic transmissions on older motorcycles retain the conventional foot-shift lever, as on a motorcycle with a fully manual transmission. However, semi-automatics systems in newer motorcycles, racing cars, and other types of vehicles often use gear selection methods such as shift paddles near the steering wheel or triggers near the handlebars.[11][12][13][14][15][16][17]

Several different forms of automation for clutch actuation have been used over the years, from hydraulic, pneumatic, and electromechanical clutches to vacuum-operated,[18] electromagnetic, and even centrifugal clutches. Fluid couplings (most commonly and formerly used in early automatic transmissions) have also been used by various manufacturers, usually alongside some form of mechanical friction clutch, to prevent the vehicle from stalling when coming to a standstill or at idle.

A typical semi-automatic transmission design may work by using Hall effect sensors or micro switches to detect the direction of the requested shift when the gear stick is used. These sensors' output, combined with the output from a sensor connected to the gearbox which measures its current speed and gear, is fed into a transmission control unit, electronic control unit, engine control unit, or microprocessor,[19][20] or another type of electronic control system. This control system then determines the optimal timing and torque required for smooth clutch engagement.

The electronic control unit powers an actuator, which engages and disengages the clutch in a smooth manner. In some cases, the clutch is actuated by a servomotor coupled to a gear arrangement for a linear actuator, which, via a hydraulic cylinder filled with hydraulic fluid from the braking system, disengages the clutch. In other cases, the internal clutch actuator may be completely electric, where the main clutch actuator is powered by an electric motor or solenoid, or even pneumatic, where the main clutch actuator is a pneumatic actuator that disengages the clutch.

A clutchless manual system, named the Autostick, was a semi-automatic transmission introduced by Volkswagen for the 1968 model year. Marketed as the Volkswagen Automatic Stickshift, a conventional three-speed manual transmission was connected to a vacuum-operated automatic clutch system. The top of the gear stick was designed to easily depress and activate an electric switch, i.e. when touched by the driver's hand. When pressed, the switch operated a 12-volt solenoid, which in turn operated the vacuum clutch actuator, thus disengaging the clutch and allowing shifting between gears. With the driver's hand removed from the gearshift, the clutch would re-engage automatically. The transmission was also equipped with a torque converter, allowing the car to idle in gear like with an automatic, as well as stop and start from a standstill in any gear.[21][22][23]

Paddle shifter on a car.

Starting in the late 1990s, automotive manufacturers introduced what is now called an automated manual transmission (AMT), which is mechanically similar to, and has its roots in, earlier clutchless manual transmission systems. An AMT functions in the same way as older semi-automatic and clutchless manual transmissions, but with two exceptions; it is able to both operate the clutch and shift automatically, and does not use a torque converter. Shifting is done either automatically from a transmission control unit (TCU), or manually from either the shift knob or shift paddles mounted behind the steering wheel. AMTs combine the fuel efficiency of manual transmissions with the shifting ease of automatic transmissions. Their biggest disadvantage is poor shifting comfort due to the mechanical clutch being disengaged by the TCU, which is easily noticeable as "jolting". Some transmission makers have tried solving this issue by using oversized synchronizer rings and not fully opening the clutch during shifting—which works in theory, but as of 2007, there have not been any series production cars with such functions. In passenger cars, modern AMTs generally have six speeds (though some have seven) and a rather long gearing. In combination with a smart-shifting program, this can significantly reduce fuel consumption. In general, there are two types of AMTs: integrated AMTs and add-on AMTs. Integrated AMTs were designed to be dedicated AMTs, whereas add-on AMTs are conversions of standard manual transmissions into AMTs.

An automated manual transmission may include a fully automatic mode where the driver does not need to change gears at all.[24] These transmissions can be described as a standard manual transmission with an automated clutch and automated gear shift control, allowing them to operate in the same manner as traditional automatic transmissions. The TCU automatically shifts gears if, for example, the engine is redlined. The AMT can be switched to a clutchless manual mode wherein one can upshift or downshift using a console-mounted shift selector or paddle shifters.[25] It has a lower cost than conventional automatic transmissions.[26]

The automated manual transmission (trade names include SMG-III) is not to be confused with "manumatic" automatic transmission (marketed under trade names such as Tiptronic, Steptronic, Sportmatic, and Geartronic). While these systems seem superficially similar, a manumatic uses a torque converter like an automatic transmission, instead of the clutch used in the automated manual transmission. An an automated manual can give the driver full control of the gear selection, whereas a manumatic will deny a gear change request that would result in the engine stalling (from too few RPM) or over-revving. [24] The automatic mode of an automated manual transmission at low or frequent stop start speeds is less smooth than that of manumatics and other automatic transmissions. [27]

Several semi-automatic transmissions used by motorcycles and racing cars are actually mechanically based on sequential manual transmissions. Semi-automatic motorcycle transmissions generally omit the clutch lever, but retain the conventional heel-and-toe foot shift lever.[28][29][30][31][32][33]

Semi-automatic motorcycle transmissions are based on conventional sequential manual transmissions and typically use a centrifugal clutch.[34] At idle speed, the engine is disconnected from the gearbox input shaft, allowing both it and the bike to freewheel—unlike with torque converter automatics, there is no idle creep with a properly adjusted centrifugal clutch. As the engine speed rises, counterweights within the clutch assembly gradually pivot further outwards until they start to make contact with the inside of the outer housing and transmit an increasing amount of engine power and torque. The effective "bite point" or "biting point"[35] is found automatically by equilibrium, where the power is transmitted through the (still-slipping) clutch is equal to what the engine can provide. This allows relatively fast full-throttle takeoffs (with the clutch adjusted so the engine is at peak torque) without the engine slowing or being bogged down, as well as more relaxed starts and low-speed maneuvers at lower throttle and RPMs.

Bollée Type F Torpedo with gear shift ring located inside the steering wheel

In 1901, Amédée Bollée developed a method of shifting gears that did not require the use of a clutch and was activated by a ring mounted within the steering wheel.[36] One car using this system was the 1912 Bollée Type F Torpedo.

Prior to the arrival of the first mass-produced hydraulic automatic transmission (the General Motors Hydra-Matic) in 1940, several American manufacturers offered various devices to reduce the amount of clutch or shifting input required.[37] These devices were intended to reduce the difficulty of operating the unsynchronised manual transmissions, or "crash gearboxes", that were commonly used, especially in stop-start driving.

An early step towards automated transmissions was the 1933–1935 REO Self-Shifter,[38][39][40][41] which automatically shifted between two forward gears in the "forward" mode (or between two shorter gear ratios in the "emergency low" mode). Standing starts required the driver to use the clutch pedal. The Self-Shifter first appeared in May 1933 and was offered as standard on the Royale and as an option on the Flying Cloud S-4.[42]

In 1937, the four-speed Oldsmobile Automatic Safety Transmission was introduced on the Oldsmobile Six and Oldsmobile Eight models.[38] It used a planetary gearset with a clutch pedal for starting from a standstill and switching between the "low" and "high" ranges.[43][44][45] The Automatic Safety Transmission was replaced by the fully-automatic Hydra-Matic for the 1940 model year.[46][47]

The 1938–1939 Buick Special was available with another Self-Shifter 4-speed semi-automatic transmission,[48][49][50] which used a manual clutch for starting from standstill and an automated clutch for gear changes.

The 1941 Chrysler M4 Vacamatic transmission was a two-speed manual transmission with an integral underdrive unit, a traditional manual clutch, and a fluid coupling between the engine and the clutch.[51][52][53] The two-speed transmission had "high" and "low" ranges, and the clutch was used when the driver wanted to switch between ranges. For normal driving, the driver would press the clutch, select the High range, and then release the clutch. Once the accelerator was pressed, the fluid coupling would engage and the car would begin moving forward, with the underdrive unit engaged to provide a lower gear ratio. At between 15–20 mph (24–32 km/h), the driver would lift off the accelerator and the underdrive unit would disengage. The Vacamatic was replaced by a similar M6 Presto-Matic transmission for the 1946 model year.

Similar designs were used for the 1941–1950 Hudson Drive-Master[54][55] and the ill-fated 1942 Lincoln Liquimatic.[56][57] Both of these combined a 3-speed manual transmission with automated shifting between the 2nd and 3rd gears, instead of the Vacamatic's "underdrive" unit.

The Packard Electro-Matic, introduced in the 1941 Packard Clipper and Packard 180, was an early clutchless manual transmission that used a traditional friction clutch with automatic vacuum operation, which was controlled by the position of the accelerator.

The automotive products manumatic system, available on the 1953 Ford Anglia 100E, was a vacuum-powered automatic clutch system that was actuated by a switch that was triggered whenever the gear stick was moved. The system could control the throttle cable (to keep the engine at the required RPM for the gear change) and vary the rate of clutch engagement.[58] The successive Newtondrive system, available on the 1957–1958 Ford Anglia, also had a provision for choke control. A similar product was the German Saxomat automatic clutch system, which was introduced in the mid-1950s and available on various European cars.[59]

The Citroën DS, introduced in 1955, used a hydraulic system with a hydraulically-operated speed controller and idle speed step-up device to select gears and operate the otherwise conventional clutch. This allowed clutchless shifting with a single column-mounted selector, while the driver simultaneously lifted off the accelerator to change gear. This system was nicknamed "Citro-Matic" in the U.S.

For the 1962 model year, American Motors introduced the E-Stick, which eliminated the clutch pedal in the Rambler American with standard three-speed manual transmissions.[60] This automatic clutch used engine oil pressure as a hydraulic source and was available for less than $60.[61] Compared to fully automatic transmissions of the time, the E-Stick offered the fuel economy of a stick-shift, with vacuum and electric switches controlling the clutch. The E-Stick three-speed transmission was offered on the larger Rambler Classic models, along with an overdrive unit.[62] The system was only available with 6-cylinder engines, and the lack of a clutch proved unpopular, so it was discontinued after 1964.[63]

The 1967 Volkswagen WSK (Wandlerschaltkupplungsgetriebe; English: Torque converter shift/clutch gearbox), used in the Beetle, Type 3 and Karmann Ghia, was one of the first gearboxes of its kind, with an automatic mechanical clutch and a torque converter. It was also known as the Autostick. Shifting was done manually by the driver. The automatic mechanical clutch allowed the car to accelerate from a stop, whereas the torque converter enabled it to do so in any gear. Dampening engine vibrations and providing torque multiplication, it functioned as a sort of "reduction gearbox", so the actual mechanical gearbox only needed three forward gears (this is why conventional automatic transmissions with torque converters normally have fewer gears than manual transmissions). The WSK had no "first" gear; instead, the first gear was converted into reverse gear, and the second gear was labeled first (with the third and fourth gears respectively being labeled second and third).[64]

The Chevrolet Torque-Drive transmission, introduced on the 1968 Chevrolet Nova and Camaro, is one of a few examples where a semi-automatic transmission was based on a conventional hydraulic automatic transmission (rather than a standard manual transmission). The Torque-Drive was essentially a 2-speed Powerglide automatic transmission without the vacuum modulator, requiring the driver to manually shift gears between "Low" and "High". The quadrant indicator on Torque-Drive cars was "Park-R-N-Hi-1st". The driver would start the car in "1st," then move the lever to "Hi" when desired. The Torque-Drive was discontinued at the end of 1971 and replaced by a traditional hydraulic automatic transmission. Other examples of semi-automatic transmissions based on hydraulic automatics were the Ford 3-speed Semi-Automatic Transmission used in the 1970–1971 Ford Maverick, early versions of Honda's 1976–1988 Hondamatic 2-speed and 3-speed transmissions, and the Daihatsu Diamatic 2-speed transmission used in the 1985–1991 Daihatsu Charade.