What is an automatic gearbox?

  • One of the earliest convenience features in modern cars
  • The latest gearboxes feature 10 speeds
  • Core technology has evolved over decades

An automatic gearbox is a type of transmission that can change gear ratios without any input from the driver. There are several different approaches to creating an automatic gearbox, but the crucial difference is that there is no clutch pedal and in most countries, different licences are available for drivers who pass a test in a manual or automatic vehicle.

Gearboxes and why you need them - for now

Most engines are generally a bit limited in terms of the speeds they can run at - they have to spin fast to generate enough torque to move a car from standstill, but then can only multiply that speed about seven times - so an engine that is geared to do 15mph at its slowest will only do 70mph at its fastest, and that's assuming that it can range from 1.500rpm to 7,000rpm.

That's quite an extreme example, too - a typical diesel will be useful from 1,000rpm to 4,000rpm.

The solution? Ratios - usually referred to as speeds, when people discuss gears.

Most manual gearboxes are like the gears you would find on a bicycle, in that you have two toothed gears of different sizes engaged to achieve a ratio that divides or multiplies the engine's rotation.

To move at a slower speed, you divide the engine's rotation so one turn of the engine is, say, a quarter turn of the wheel. To go faster, you multiply it. Just like swapping to the smallest cog on your bike, where one turn of the crank can be four turns of the wheel.

The crucial thing is that when everything is in motion, the gears are under load - the engine is putting force in at one end, the road wheels are putting force in at the other end, and these are usually working against each other, either pushing forward or dragging back.

When you need to change ratios, you have to release the tension in the gears - and that's achieved by disconnecting the engine from the gearbox with the clutch, allowing both the engine and gears to spin at different speeds, which allows the ratios to be changed.

Citroen e-Dispatch gear selector with drive mode

The gearbox as we know it will, eventually, become invisible as electric motors generate their maximum torque from near-standstill and can spin to 15,000rpm. Even where a gearbox is used, it's part of the motor and invisible to the driver.

Shifting without effort - the automatic gearbox

Back to the tech we have now, though.

As car designers settled a standard layout of three pedals, including one to disconnect the engine from the wheels, and a multi-speed gearbox, the obvious improvement for convenience is to find some way of automating the process of engaging drive and changing gear ratios.

For the user, the gearlever becomes a means of selecting forward or reverse, and the clutch pedal disappears, meaning drivers merely choose between making the car go, or stop, with one pedal for each.

While many solutions exist, the dominant technology remains a fluid-filled torque convertor and multiple automated internal clutches that lock and unlock a system of mechanical gears. Although the gearbox itself is reasonably efficient, the fluid drive element is not; meaning auto gearboxes were usually paired with big, powerful engines.

As such, automatics have had to shake off a reputation for poor economy, and alternative technologies were developed particularly for less powerful, smaller cars.

>> Automatic gearboxes for smaller and more modern cars

How traditional automatic transmissions work

2015 GM Hydra-Matic 8L90

Inside the torque convertor, the input rotation of the engine spins the transmission fluid and impeller, which in turn spin the output to the gearbox.

Ratios in the gearbox are created by engaging combinations of gears in a planetary gearset, so an automatic gearbox's workings are less obvious than a smaller cog turning a bigger cog, but the theory is the same.

This combination of components works to provide low-down torque for pulling away, select new ratios without disconnecting drive, and to allow the fluid to move slowly without overheating when the car is at a standstill; effectively disengaging drive to the gearbox without extra complexity.

The weight and inefficiency of the technology means alternatives such as dual-clutch systems (DSG), or the infinite-ratio CVT, are becoming more popular.

Automatic gearboxes for efficiency

Over time, additional features have been added to enhance efficiency. Torque convertor lock up engages the drive and gearbox with a direct clutch, ensuring fuel economy and refinement are comparable to a high-geared manual car.

TCI Automotive GM400 Clean Neutral Transbrake Valve Body

Gear selection - once determined by road speed, engine speed and hydraulic pressure through an ingenious system of valves (the 'brain' of an automatic gearbox is a surprisingly organic looking object) - is now electronically controlled.

Modern automatic gearboxes in cars can feature up to 10 forward ratios. with fast shifting and intelligent coasting. Rather than being slower and less economical than their manual counterparts, they are often the better option - unless you really want that last bit of control in a performance car.

2017 Chevrolet Camaro ZL1 10 speed auto transmission

Alternatives to the traditional automatic gearbox: CVT or DSG

CVT gearboxes have been available in cars since the 1960s, and though the technology has changed dramatically the principle remains the same. Rather than having fixed ratios, and tuning the engine and gears to find the most efficient compromise for common speeds, the engine is held at the most efficient point and the gearing varies to suit the speed needed.

If a normal gearbox is like having a hi-fi with five settings for volume, and an automatic gearbox switches those five settings based on how loud the recording is, the CVT is a big rotary volume dial that can cope with everything from quiet classical to heavy metal and convert it to the volume you want to hear, always adjusting.

Ironically, the typical pairing of a CVT gearbox and a small, revvy engine means that to keep the engine at peak efficiency for performance, it's loud and constant, rather than building then dropping as the gears change. Most CVT gearboxes now include 'steps' to mimic gears and avoid that constant straining sound.

Automated manual and DSG gearboxes

It's an awful lot of fuss to get rid of a clutch pedal, right? Why not just move the clutch pedal and gear selector? Well, you know how robots still can't match humans for most skilled tasks? The clutch is one of those tasks - and you can't just switch it on and off and expect the gearbox to survive long. So it's taken a while for such systems to become usable and reliable.

SEAT Leon DSG gear selector

Simple robotised manuals do work by automating changes of an almost standard manual gearbox, but a better system was designed where the next required gear would be pre-selected with a second clutch. This means that the car merely disengages one clutch and engages another, rather than disengaging the clutch, moving ratios, then re-engaging the clutch.

It can shift gears faster than a human, and it is as efficient as a manual gearbox because that's essentially what it is in how it works. It has the advantage that the gears work exactly as they would on a manual car, too - so engine braking, performance for overtaking and just the overall feel of the car remains natural.

Does driving an automatic need a special licence?

In the UK and some other countries, the simplified driving technique resulted in a split in licence types; if you take your test in an automatic car (regardless of technology, this is essentially defined as a car without a clutch pedal) you will only be permitted to drive automatics from that point, and will need to take a new test to drive a manual car.

If you have moved to the UK from a country that doesn't differentiate, such as Canada, you can exchange your licence for a UK automatic one, or take a UK test to drive manuals.

Looking for more jargon-busting motoring meanings? Head over to our Parkers Car Glossary page and take a look at our other definitions