Semi-Automatic is a type of transmission fitted to many cars nowdays (although most manufacturers have their own name for it such as "Sequential Automatic", "Steptronic", "Tiptronic" etc. It comes in two major different versions.
Conventional Clutch
This is the ideal form of Semi-Automatic transmission, in terms of having a minimum of power loss. This system has a conventional friction plate clutch, but instead of having a clutch pedal, a computer controls it being activated and de-activated. The user simply selects the gear (s)he wants to use, at whch point the computer disengages the clutch, changes gear and re-engages the clutch. This is the type of system used on Formula 1 racing cars, and can change gears in less than half a second.
Fluid Clutch
The alternative method is to use a Fluid Clutch, also known as a Torque Converter. This is the system which is often fitted to road cars with automatic transmission, that can be switched into Semi-Automatic mode. In this case, the system - when changing gear - is no different to when running in automatic mode, except that it changes gear when you say, rather than when it wants to. The fluid clutch absorbs the engine / wheel speed difference as normal.
Ways of changing gear
There are typically three different ways to select the gear on cars with Semi-Automatic options.
- Paddles behind the steering wheel - This is how things are done on Formula 1 cars, and also cars like the Honda Jazz. There are two paddles behind the steering wheel, one on each side. Pulling them briefly towards you activates a switch to change up or down a gear (usually down on the left and up on the right).
- Buttons on the steering wheel - This is like the paddles option, but instead of paddles, there are buttons on the steering wheel itself. These can either be one on each side, like with paddles, or (such as on the Lexus GS300, two on the front and two on the back. The two on the front go down and the two on the back go up.
- On the gear shift - Some cars have the selection on the gear shift itself. In all cars which are automatic or Semi-Automatic, you select the mode on the gear shift, often by moving it to the side. On these cars, where the gear shift is used to select the gear, you then usually push it forwards or backwards to change up or down a gear. This is the method used on the Toyota Avensis.
It's important to realise that in all cases, there's no direct link from the selection buttons/paddles/lever to the gearbox. Everything is just a microswitch that sends an electronic message to the management system, which actually changes the gear.
Summary
There's no doubt that the Semi-Automatic boxes with real clutches are amazing devices - hence their use in Formula 1 cars - and that they can change gear faster than any person can. On the other hand, the ones that are just adaptations of conventional automatic boxes are fun to have, and do give you slightly more control of your car, but on the whole, modern automatic systems are good enough for 99% of driving.