Automatic transmissions explained

Choosing a transmission type isn't quite as simple as just manual or automatic.

For the overworked driver there are several types of self-shifting transmissions available, each offering their own particular characteristics and attributes, so there is a genuine choice between automatic transmission types too.

For internal combustion vehicles there are three main automatic transmission types available. Let's have a closer look at how they work and what they offer to the driver.

Over the last decade we've heard a lot about dual-clutch transmissions (DCT), especially in reference to performance-oriented cars. Although at least as complex as other automatics the DCT is the transmission most closely related to a traditional manual transmission, at least from the perspective of the mechanical workings.

As the name suggests there are two clutches which connect the gearbox to the engine. Each clutch acts on a separate gear set. The odd numbered gears are on one shaft, with the even gears on the other. As the driver pulls away in first gear, second gear is already pre-selected on the second gear shaft in anticipation of the upward shift. When the computer (or driver, if using the common manual shift option) decides it's time to shift up the first clutch disengages and the second clutch engages. The shift happens extremely quickly and smoothly and the pre-selection of the next gear occurs whether accelerating or decelerating, as the control electronics anticipate the next shift by analysing the driver inputs.

Dual-clutch transmissions are renowned for their shifting speed and consequently are the go-to choice for many sports cars and performance variants of mainstream models. The gearshift feels most like a manual shift compared to other automatic transmissions and this, in combination with a manual shifting option, appeals to enthusiast drivers. Fuel economy is typically better than conventional automatics and even manual transmissions due to low power losses in the transmission and the benefits of increased number of gears (seven-speed in many applications).

Dual-clutch transmissions do have some downsides though. They are complex, and therefore consequently can be expensive to fix if (when?) things go wrong. Also, some can be tricky to drive smoothly at low speeds due to the delay between throttle application and clutch engagement, which can sometimes result in a rather jerky drive.

In theory Continuously Variable Transmissions (CVT) should be the ideal transmission for any scenario. With a nearly infinite choice of ratios within the transmission's range there is potential to achieve the best possible performance and efficiency from this transmission type.

There are no gears in a CVT; instead there are two variable-diameter pulleys (input and output). Each pulley is made up of two parts that are shaped like opposing cones and are movable. The pulleys are connected by a steel belt which transmits the power from one to the other. The gap between the two parts of the pulleys is adjusted hydraulically by the transmission's electronic control unit depending on the demand of the driver. As a result the belt rides lower or higher on the walls of each pulley resulting in continuously variable changes to the gearing of the car.

With no defined gear ratios the CVT has the advantage of keeping the engine at optimum RPM (revolutions per minute, or 'revs') for either performance (power output) or fuel economy (best efficiency) while the transmission adjusts to the changing road speed. The engine RPM is therefore not dependent on vehicle speed and this can sound and feel a little strange compared to more conventional transmissions and tends to make CVTs a less popular choice for driving enthusiasts. Manufacturers often overcome this by programming a 'sport' mode with predefined ratios, which the driver can shift between via a manual mode for the shifter or paddles.

Relying on friction to transmit power between the pulleys means that the CVT has quite high power loss in the transmission, but that is in many ways negated by the benefits of the variable ratios. CVTs therefore tend to be good for fuel economy and of course comfort is maximised with no shifting between gears.

The most conventional and traditional automatic transmission is commonly referred to as a torque-converter automatic. The torque converter is actually the device that acts in place of a clutch to create the connection for power to be transmitted from the engine to the gearbox. Torque converters don't actually provide a solid connection between engine and gearbox in the way that a mechanical clutch does.

Instead, the power is transmitted via a fluid which is pumped around a sealed casing by an impeller. That in turn drives a turbine connected to the transmission. When the engine is at idle the blades of the impeller move through the fluid at a slow speed, and very little torque is transmitted to the wheels. As RPM increases the fluid is pushed to the outer edges of the casing driving the turbine, transmitting more torque and accelerating the vehicle.

Inside the gearbox the ratios are defined by planetary gearsets. The gear ratios are determined by a series of bands and clutches which can hold certain elements of the gearsets stationary, let them turn freely or engage them to transmit power. The bands and clutches are hydraulically controlled and in modern cars the hydraulic pressure is regulated by electronic solenoids, allowing sophisticated control strategies depending on variables such as throttle position, engine revs and vehicle speed.

With modern electronic control and increasing number of gears (eight and nine-speed automatics are becoming increasingly common, with six-speed typical) the performance of the 'conventional' automatic transmission has continued to make impressive improvements during the last decade. Performance and efficiency has improved substantially as the newer transmissions offer more available ratios and a consequent reduction in power losses in the torque converter. With more gears the shift gaps between ratios are reduced giving quicker and smoother gearshifts to the point where the overall performance is getting close to the Dual-clutch transmissions but with slightly better low speed comfort that a torque converter provides when pulling away from a standstill.

History is littered with different types of automatic transmission technology, but two relatively recent types are the so-called robotised manual and the Mercedes-AMG Speedshift transmission. The Speedshift transmission is essentially an epicyclic transmission with a planetary gearset, but with a multiplate wet-clutch pack in place of the torque converter.

Several European brands have marketed the robotised manual transmission – basically a single-clutch manual with automated clutch actuation and shifting. This type of transmission is rapidly falling out of favour with the rise of the DCT. Drivers have found the single-clutch auto to be difficult to drive smoothly unless gear changes are made manually – which negates the whole point of offering it as an automatic option. Alfa Romeo's version is branded 'Selespeed', Fiat's is 'Dualogic' and Citroen's is 'SensoDrive'. Maserati had its own version, as did Ferrari and Aston Martin. BMW's first 'SMG' transmission in the E36 generation of M3 was another example, but the German brand quickly moved on to DCTs.

Pictured in order:
Porsche PDK,
Subaru Lineartronic cutaway (courtesy of Qurren/Wikimedia Commons),
ZF 8HP70 cutaway (courtesy of Stefan Krause/Wikimedia Commons),
Mercedes-AMG Speedshift