How an automatic gearbox works
THE PICTURE above shows a generic cutaway view of a ZF eight-speed automatic gearbox. It’s not necessarily the exact layout in the eight-speed Range Rover and Discovery models, but it illustrates the basic internal components that also apply to the six- and four-speed ZF, and other Land Rover auto boxes.
It’s obviously a complex piece of engineering, but we can simplify it down a bit. To better understand the illustration, the torque converter, the epicyclic gears and the mainshaft running though the centre, are all as we would see them with the casing cut away. For clarity though, the multiple clutch discs are cut away in the image, so you have to imagine that these clutch discs are actually fully circular discs running concentrically around the main shaft. We’ll deal with each section in turn, following the power flow through the gearbox.
Torque converter
That plate at the right hand end of the illustration is attached to the engine, and it transmits the drive to the torque converter (TC). The torque converter is in two sections: one connected to the drive from the engine, the other connected to the gearbox. Inside the TC are vanes and blades in the form of a turbine, and fluid within the TC transmits the drive between the two sections. Because the medium is a fluid (gearbox oil) the drive through the torque convertor can be varied and the incoming engine torque can be increased (hence the name). A torque converter will perfectly apply the drive when pulling away with a heavy load with no chance of stalling. It’s why autos are so competent off-road.
The torque converter has an internal lock-up clutch which causes its two sections to work together. This eliminates slip from the fluid drive, producing a direct drive from the engine via the torque converter to the gearbox. Lock-up improves the drive efficiency, reducing heat, fuel consumption and thus emissions. The lock-up clutch is automatically applied hydraulically by an electronically-controlled solenoid valve according to the driving conditions.
Epicyclic gears
Auto boxes use epicyclic gear sets. In the illustration you’ll see gear sets just to the left of the torque converter and a further set towards the rear. These provide all eight ratios for this gearbox. Epicyclic gear sets are made up of a central ‘sun’ gear, around which three planet gears (rigidly spaced on a carrier) revolve. Around the outside of the planet gears, an annulus gear revolves, and its teeth are on the inside so they mesh with the planet gears, therefore all gears are interlinked. The trick with epicyclic gear sets is that if one gear, such as the sun, the three planets, or the annulus is slowed or stopped, the final output speed will change, in other words, the gear ratio varies. So, by restraining the annulus or the planet gears, we can change the gear ratio. Drive is transferred via the gear sets from the input shaft to the output shaft.
Wet clutches and brakes
As we said above, it’s the wet clutch system that defines the type of gearboxes in our Land Rovers. The clutches are formed of packs of circular metal discs with a friction coating. They are operated hydraulically, and they work in oil, hence wet clutch. It is these clutches that operate on the epicyclic gears to brake, or alter, their rotational speed to provide the different gear ratios. Their application is progressive, allowing smooth gear changes, and they are the heart of automatic transmission, allowing the gear ratios to be changed without disconnecting the engine drive.
Solenoid pack
The clutches are controlled by solenoido-perated valves that send hydraulic fluid under pressure to apply and release the clutches and brakes according to the gear changes required.
Electronic control
The whole system is orchestrated electronically. In the eight-speed example shown here (and in six-speeds) a Mechatronic unit inside the gearbox casing comprises the solenoids, valves and electronic control unit. The control unit receives information relating to accelerator position, engine speed and load, engine temperature, road speed, and in later Land Rovers, driving style, road gradient, traction and braking systems. It controls torque converter lock-up, gear selection and the characteristics of the gear change.
As internal components wear, the time taken for a gear change can increase. This unwanted effect is countered by the electronic system which calculates the new pressures required to restore the shift speed and stores this data for subsequent gear changes – a process known as adaptation. Adaptation also modifies the shifting characteristics to suit the current driving style.
Transmission fluid
Automatic transmission fluid operates throughout the gearbox. It is inside the torque convertor, it provides bearing lubrication, and it transmits the hydraulic power to operate the multi-plate clutch packs. The fluid is pressurised by a pump driven from the mainshaft and passes repeatedly through a filter in the fluid pan (sump) underneath the gearbox. Much of the auto box’s reliability, smoothness and longevity depends on the condition of the fluid.
To control its temperature, the transmission fluid is circulated through a cooler, usually integral with the engine coolant radiator.
Manual and Sport options
These are simply an override arrangement in which the driver can signal the control unit which will activate the appropriate solenoids to operate the necessary clutch packs for the required gear selection and, in the case of Sport mode, of altering the points at which the gearbox changes up and down and perhaps hold a gear for longer before changing up. Manual mode does not give the driver full control because the system will override the driver if a low gear is held at too high an engine speed, or if a high gear is causing the engine to labour.