What are the different types of automatic gearbox?

Most cars have a gearbox, which is a device that transfers power from a car’s engine to the wheels. Broadly speaking, there are two types of gearbox – manual and automatic. Manual gearboxes are basically all the same but there are several types of automatic gearboxes, all working in different ways with particular pros and cons.

If you’re interested in getting an automatic car, or already have one, knowing more about its gearbox could help you understand more about how it feels to drive, what’s good about it and what might not be so good about it.

In most non-electric cars, the power needed for movement is generated by an engine, fuelled by petrol or diesel. The engine spins a crankshaft connected to a gearbox, which is in turn connected to the wheels.

The crankshaft on its own can’t spin with a large enough range of speed and force to drive the wheels efficiently, so the gearbox – literally a metal box of different-sized cogs – is used to adjust the force that comes from the engine. Low gears transmit a higher force to the wheels in order to get the car moving, while higher gears transmit less force, but at a higher speed, when the car is moving faster.

Gearboxes are also known as transmissions, because they transmit the power from the engine to the wheels. Transmission is probably a better term, because not all transmissions actually have gears, but in the UK, gearbox is the usual catch-all term.

Put simply, you need to change gear manually when driving a car with a manual gearbox while an automatic gearbox changes gear, well, automatically as needed.

A car with a manual gearbox has a clutch pedal on the left that has to be pressed down, disconnecting the engine and gearbox so you can move the gear lever and select a different gear. An automatic car doesn’t have a clutch pedal, just the gear stick that you put into Drive or Reverse as needed or into Park when you want to be stationary or Neutral for when you don’t want any gears selected (if, for example, your car needs to be towed).

If your driving licence is only valid for an automatic car, you aren’t allowed to drive any car that has a clutch pedal. If you have a manual driving licence, you can drive both manual and automatic cars.

Now that we’ve described what an automatic gearbox is and what it does, let’s look at the main types.

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Torque converters are one of the most common types of automatic gearbox. They use hydraulics to change gear, which gives you a lovely smooth shift. They’re not the most fuel-efficient of automatics though they’re much better than they used to be, in part because carmakers have added extra gears to improve efficiency.

Torque converter gearboxes typically have six to 10 gears, depending on the car. They tend to be fitted to more luxurious and powerful cars because of their smoothness and physical strength. Many carmakers give theirs brand names – Audi calls it Tiptronic, BMW uses Steptronic and Mercedes-Benz goes with G-Tronic.

Torque, by the way, is rotational force, and is different from power, which in the car world is usually referred to as horsepower. To give a very basic illustration of torque versus power, torque is how hard you can push a pedal on a bicycle, while power is how fast you can pedal.

CVT stands for ‘continuously variable transmission’. Most other types of gearbox use cogs for the gears, but CVTs have a series of belts and cones. The belts move up and down the cones as speed rises and falls, continuously finding the most efficient gear for the situation. CVTs don’t have individual gears, though some carmakers have designed their systems to have simulated gears in a bid to make the experience feel more traditional.

Why? Well, cars with a CVT gearbox can feel a bit strange to drive because the engine noise doesn’t rise and fall with a change in gear. Instead, the noise just keeps rising as speed increases. But CVTs are very smooth and can be extremely efficient – all Toyota and Lexus hybrids have them. Brand names for CVT gearboxes include Direct Shift (Toyota), Xtronic (Nissan) and Lineartronic (Subaru).

Mechanically, these are the same as conventional manual gearboxes, except that electric motors activate the clutch and change gear when needed. There’s no clutch pedal and the only choice of gear is Drive or Reverse.

Automated manuals cost less to produce than other types of automatic gearbox and are usually found in smaller, less-expensive cars. They’re more fuel-efficient as well, but gear changes can feel a bit jerky. Brand names include ASG (Seat), AGS (Suzuki) and Dualogic (Fiat).

Like an automated manual gearbox, a dual-clutch is, in essence, a manual gearbox with electric motors that change gear for you. As the name suggests, it has two clutches, whereas an automated manual just has one.

Even with electric motors doing the work in an automated manual gearbox, changing gear takes a relatively long time, leaving a noticeable gap in the engine’s power when you accelerate. In a dual-clutch gearbox, one clutch is engaged with the current gear and the other is primed to change into the next. That makes changes quicker and smoother, and improves fuel efficiency. Clever software can anticipate which gear you’re likely to shift into next, and line it up accordingly.

Brand names include DSG (Volkswagen), S tronic (Audi) and PowerShift (Ford). In many cases, it’s simply abbreviated to DCT (dual-clutch transmission).

Unlike a petrol or diesel engine, the power and torque produced by electric motors are constant, regardless of how fast the motors are spinning. Electric motors are also much smaller than an engine and can be mounted closer to the wheels. As such, most electric cars don’t actually need a gearbox (though some really powerful ones do, which helps them reach very high speeds). Electric cars still have a gear lever to set the direction of travel to forward or reverse, and they don’t have a clutch pedal, so they’re classed as automatics.

It’s worth noting that some electric cars have a separate motor for reversing, while others simply spin the main motor backwards.

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