Semi-automatic transmission

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Transmission types



Continuously variable
Bicycle gearing

Semi-automatic transmission, or clutchless 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 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 shift stick only moves forward and backward to shift into higher and lower gears respectively, 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.

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 transmisson also used in drag racers. A Lenco is different than 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 Citroën'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 temporal change in automatic mode (for example before starting to overtake another car).

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.