Semi-automatic transmission
A semi-automatic transmission is a multiple-speed transmission where part of its operation is automated, but the driver's input is still required to launch the vehicle from a standstill and to manually change gears. Semi-automatic transmissions were almost exclusively used in motorcycles and are based on conventional manual transmissions or sequential manual transmissions, but use an automatic clutch system. But some semi-automatic transmissions have also been based on standard hydraulic automatic transmissions with torque converters and planetary gearsets.
Names for specific types of semi-automatic transmissions include clutchless manual, auto-manual, auto-clutch manual, and paddle-shift transmissions. Colloquially, these types of transmissions are often called "flappy-paddle gearbox", a phrase coined by Top Gear host Jeremy Clarkson. These systems facilitate gear shifts for the driver by operating the clutch system automatically, usually via switches that trigger an actuator or servo, while still requiring the driver to manually shift gears. This contrasts with a preselector gearbox, in which the driver selects the next gear ratio and operates the pedal, but the gear change within the transmission is performed automatically.
The first usage of semi-automatic transmissions was in automobiles, increasing in popularity in the mid-1930s when they were offered by several American car manufacturers. Less common than traditional hydraulic automatic transmissions, semi-automatic transmissions have nonetheless been made available on various car and motorcycle models and have remained in production throughout the 21st century. Semi-automatic transmissions with paddle shift operation have been used in various racing cars, and were first introduced to control the electro-hydraulic gear shift mechanism of the Ferrari 640 Formula One car in 1989. These systems are currently used on a variety of top-tier racing car classes; including Formula One, IndyCar, and touring car racing. Other applications include motorcycles, trucks, buses, and railway vehicles.
Design and operation
Semi-automatics facilitate easier gear shifts by removing the need to depress a clutch pedal or lever at the same time as changing gears. Most cars that have a semi-automatic transmission are not fitted with a standard clutch pedal since the clutch is remotely controlled. Similarly, most motorcycles with a semi-automatic transmission are not fitted with a conventional clutch lever on the handlebar.Clutchless manual transmissions
Most semi-automatic transmissions are based on conventional manual transmission. They can be partially automated transmission. 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.
Several different forms of automation for clutch actuation have been used over the years, from hydraulic, pneumatic, and electromechanical clutches to vacuum-operated, electromagnetic, and even centrifugal clutches. Fluid couplings 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, 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 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.
Automated manual transmissions
Starting in the late 1990s, automotive manufacturers introduced what is now called an automated manual transmission, 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, 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 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. 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. It has a lower cost than conventional automatic transmissions.
The automated manual transmission is not to be confused with "manumatic" automatic transmission. 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 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 or over-revving. 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.
Sequential manual transmissions
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.Semi-automatic motorcycle transmissions are based on conventional sequential manual transmissions and typically use a centrifugal clutch. 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" is found automatically by equilibrium, where the power is transmitted through the clutch is equal to what the engine can provide. This allows relatively fast full-throttle takeoffs without the engine slowing or being bogged down, as well as more relaxed starts and low-speed maneuvers at lower throttle and RPMs.