Semi-automatic transmission

Most semi-automatic transmissions are actually based on conventional manual transmissions, but usually are operated with an automated clutch or another kind of partially automatic transmission mechanism, thus partially automating the manual transmission, to a certain degree. Once the clutch becomes automated, then the transmission becomes semi-automatic. However, they still require full control of the manual gear selection, which is controlled 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 in newer motorcycles, racing cars, and other types of vehicles often use gear selection methods such as paddle-shifters 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 electro-mechanical-operated clutches, while other manufactures have used alternate methods of actuation, like vacuum-operated clutches, electromagnetic clutches, or even centrifugal clutches.

A typical semi-automatic transmission design may work by using hall-effect sensors to detect the direction of the requested shift, and with this input, together with a sensor connected to the gearbox, which senses the current speed and gear selected, feeds into either a transmission control unit, electronic control module, microprocessor,[18][19] central processing unit, or another type of electronic control system. This control unit will then determine the optimal timing and torque required for smooth clutch engagement, based on input from these sensors.

When the driver touches the gearshift to change gear, this activates an electric micro-switch. The electronic control 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. In some cases, the hydro-mechanical unit contains a servomotor coupled to a gear arrangement for a linear actuator, which uses hydraulic fluid from the braking system to impel a hydraulic cylinder which moves the main clutch actuator, therefore disengaging the clutch. In other cases, the internal clutch actuator may be completely electric (sometimes through the use of a solenoid); where the main clutch actuator is powered by an electric motor, or even pneumatic; where the main clutch actuator is a pneumatic actuator, and uses compressed air to impel the pneumatic cylinder that disengages the clutch, allowing the driver to make a smooth gear change. The actuators and sensors which control the clutch are all connected and controlled through a dedicated electronic control unit or module.

One now-famous application of this clutchless manual system was the Autostick 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 shift was designed to easily depress and activate an electric switch, i.e., when touched/engaged by the driver's hand. When pressed, the switch operated a 12-volt solenoid, in turn, operating the vacuum clutch servo, 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 an automatic. The torque converter was operated by transmission fluid. This would allow the car to stop in any gear and start from a standing stop in any gear.[20][21][22]

Starting in the late-1990s, automotive manufacturers introduced what is now called an Automated Manual Transmission (AMT), which are mechanically similar, and have their roots in earlier clutchless manual transmission systems – These function in the same way as older semi-automatic and clutchless manual transmissions, but with two exceptions; they can do everything fully-automatically, and they do not have or require a hydrodynamic torque converter. The shifting signal comes either automatically, from the Transmission Control Unit (TCU), or via manual control from the shift knob, or shift paddles mounted behind the steering wheel. These AMTs combine the fuel efficiency of manual transmissions with the shifting ease of an automatic transmission. Their biggest disadvantage is the poor shifting comfort because shifting requires a disruption in power and torque transmission from the engine to the wheels, due to the mechanical clutch being disengaged by the TCU (transmission computer), which is easily noticeable as "jolting". Some transmission makers have tried solving this issue by using oversize synchronizer rings and not fully opening the clutch during shifting – now in theory, this works, but as of 2007, there have not been any series production cars with such functions. In passenger cars, modern AMTs generally have at least six gears (though some have seven-speeds) and a rather large spreading. 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 AMTs, whereas an add-on AMT is a conversion of a regular manual gearbox into an AMT.

Modern automated manual transmissions are a type of automatic transmission which have a fully-automatic mode, where the driver does not need to change gears at all.[23] These transmissions can be described as a standard manual transmission with an automated clutch and automated gear shift control. Therefore, the transmission can operate in the same manner as a traditional automatic transmission, by allowing the transmission's computer to automatically shift gear if, for example, the driver were redlining the engine. The AMT can be engaged in a manual mode wherein one can up-shift or down-shift using the console-mounted shifter selector or the paddle shifters just behind the steering wheel, without the need of a clutch pedal.[24] It has a lower cost than any other types of automatic transmissions.[25]

Several semi-automatic transmissions used by motorcycles and racing cars are actually mechanically based on a sequential manual transmission. (Semi-auto motorcycle transmissions generally omit the clutch lever, but retain the conventional heel-and-toe foot-shifter lever.)[26][27][28][29][30][31]

Semi-automatic motorcycle transmissions are based on conventional sequential manual transmissions, and typically use a centrifugal clutch, which operates in a different fashion, by using centrifugal force. At idle, the engine is disconnected from the gearbox input shaft, allowing both it and the bike to freewheel (unlike torque converter automatics, there is no "idle creep" with a properly adjusted centrifugal clutch). As the throttle is opened and engine speed rises, counterweights attached to movable inner friction surfaces (connected to the engine shaft) within the clutch assembly are thrown gradually further outwards, until they start to make contact with the inside of the outer housing (connected to the gearbox shaft) and transmit an increasing amount of engine power and torque. The effective "bite point" is found automatically by equilibrium where the power being 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 will be turning near its maximum-torque rpm) without the engine slowing or bogging down, as well as more relaxed starts and low-speed maneuvers at lower throttle settings and RPMs.

Fully-automatic transmissions require no driver input when changing gears; semi-automatic transmissions will only require partial driver involvement, usually in the form of manually selecting the next gear, with the clutch operation being completely automated. Transmissions that offer the driver the option of manually selecting a gear (such as with paddle shifters or "+" and "-" controls on the gear selector) are not considered true semi-automatic transmissions, because the transmission is capable of operating in a fully-automatic mode. This type of transmission is called a manumatic.

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

In 1901, Amédée Bollée developed a method of shifting gears which did not require the use of a clutch and was activated by a ring mounted within the steering wheel.[32] One car using this system was the 1912 Bollée Type F Torpedo.

Prior to the arrival of the first mass-produced hydraulic automatic transmission in 1940 (the General Motors Hydra-Matic), several American manufacturers offered various devices to reduce the amount of clutch or gear shifter usage required.[33] These devices were intended to reduce the difficulty of operating the unsynchronised manual transmissions ("crash gearboxes") that were commonly used, especially in stop-start driving.

An early step towards automated transmissions was the 1933-1935 REO Self-Shifter,[34][35][36][37] 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 Reo Flying Cloud S-4.[38]

In 1937, the Oldsmobile 4-speed Automatic Safety Transmission was introduced on the Oldsmobile Six and Oldsmobile Eight models.[34] It used a planetary gearset with a clutch pedal for starting from standstill and switching between the "Low" and "High" ranges.[39][40][41] The Automatic Safety Transmission was replaced by the fully-automatic Hydra-Matic for the 1940 model year.

The 1938–1939 Buick Special was available with the Self-shifter 4-speed semi-automatic transmission.[42][43][44] transmission. This transmission uses a manual clutch for starting from standstill, and an automated clutch for gear changes.

The 1941 Chrysler M4 Vacamatic transmission used a two-speed manual transmission with an integral underdrive unit, a traditional manual clutch, and a fluid coupling between the engine and the clutch.[45][46][47] The two-speed transmission was used as "High" and "Low" ranges, and the clutch was needed 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 Vacmatic 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[48][49] and the ill-fated 1942 Lincoln Liquimatic.[50][51] Both of these used a 3-speed manual transmission with automated shifting between 2nd and 3rd gears, instead of the Vacamatic's "underdrive" unit.

The Packard Electro-Matic — introduced in the 1941 Packard Clipper and Packard 180 — is an early clutchless manual transmission that uses a traditional friction clutch with automatic operation. In the case of the Electro-Matic, the clutch was vacuum-operated and 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 lever was moved. The system could control the throttle cable (to keep the engine at the required RPM for the gear change) and could vary the rate of clutch engagement.[52] 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.[53]

The Citroën DS, introduced in 1955, used a hydraulic system 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 allowed clutchless shifting with a single column selector mounted behind the steering wheel, 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" that eliminated the clutch pedal on the compact Rambler American with standard three-speed manual transmissions.[54] This automatic clutch used engine oil pressure as a hydraulic source and was available for less than $60.[55] Compared to the fully-automatic transmissions of the time, the E-Stck 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 and also together with an overdrive unit.[56] The system was only available with 6-cylinder engines and the clutch-less option proved be unpopular so it was dropped after 1964.[57]

The 1967 VW WSK (Wandlerschaltkupplungsgetriebe; English: "hydrodynamic torque converter shift/clutch gearbox"), used in the Beetle and Karmann Ghia, was one of the first gearboxes of this kind, which had an automatic mechanical clutch and a hydrodynamic torque converter; shifting gears was done manually by the driver. The automatic mechanical clutch enabled the "taking off" function, whereas the hydrodynamic torque converter enabled "taking off in any gear", it dampened the engine vibrations, and it provided torque multiplication, functioning as some sort of "reduction gearbox", so the actual mechanical gearbox only needed three forward gears (maybe you've wondered why "traditional" automatic transmissions with hydrodynamic torque converters normally have fewer gears than manual gearboxes – this is why). In the WSK, there was no "first" gear, instead, the first gear was converted into reverse gear, the second gear was labeled first, the third second, and the fourth third.[58]

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 are the Ford Semi-Automatic Transmission 3-speed transmission used in the 1970–1971 Ford Maverick (Americas), early versions of Honda's 1976-1988 Hondamatic 2-speed and 3-speed transmissions, and the Diahatsu Diamatic 2-speed transmission used in the 1985-1991 Daihatsu Charade.