Doctor Diesel

Torque converter automatic transmission

GM Hydramatic transmission_cropped1We’re taking a look at what we’ll call traditional automatic transmissions, as they have existed for many decades, which are one of the more complex car components.

They have a number of key items, primarily the torque converter, a form of fluid coupling that transmits drive from the engine, in place of the clutch of a manual transmission car, and sets of planetary gears on a single shaft that provide a number of gear ratios, but in a totally different way from a two-shaft manual gearbox. Then there’s an engine vane pump, to provide high pressure transmission fluid for the system, mainly to facilitate the selection of gears.

This works through a complex valve body that’s essentially the heart of the transmission, distributing high pressure fluid through tiny passages to activate gear selection, using hydraulically or electric servo operated clutch packs and brake bands.

Finally, there’s an inevitable computer, either a section of the car’s central ECU, or a separate TCU (transmission control unit) directing the operations of the transmission system, operating in close conjunction with the valve body.

The complexity of these systems demands that we must focus on the key items, most distinctive of which is the torque converter, which provides cushioned drive transfer, in place of the clutch of a manual transmission. In this, an engine-driven impeller creates a rotational flow of the transmission fluid (in which the whole system is immersed) that, modified by the blades of a stator, drives a turbine on the main transmission shaft.

The torque converter’s flexible drive function can hold the car on mild gradients at low engine speeds, but transmits more and more torque as the engine speed rises and the fluid flow increases, as power demands increase. There’s generally significant energy loss (in the form of heat) within the torque converter, due to a speed differential between impeller and turbine, although this is eliminated by a process of lock-up in suitable conditions. The fluid-driven turbine transmits power to the final drive unit through a single shaft epicyclic gear system, which inquisitive readers may wish to look at in Wikipedia’s “Epicyclic Gearing” for a detailed description.

Essentially all gearwheels are constantly in mesh, with no possibility of crunching of engaging gear teeth, and gear changes are achieved by holding stationary or releasing different planet gears, carriers, or sun gears, enabling the drive ratios to be seamlessly changed over a wide range. Today, as many as eight and nine ratios are provided, and ten and twelve ratio transmissions are coming soon!

The base principles of such epicyclic transmissions are shown in the technical section of for the 1908 Ford Model T, which has a basic two-speed and reverse planetary gear transmission, controlled by the driver with manually applied transmission bands. Much has changed since, but the Model T was the undoubted pioneer of today’s automatics.

As electronics have become more sophisticated, the efficiency and function of such automatic transmissions has improved greatly. Lock up of gears reduces the heat generated and wasted in the torque converter and more gear ratios mean that ideal gearing is available to allow the engine to operate close to its maximum efficiency, at varying loads and power demands. Automatic transmission fluid has become more advanced and many transmissions are now filled for life and virtually maintenance-free, as the stresses imposed by heat-generating components such as brake bands have been reduced.

Computerisation means that a massive amount of engine and ambient sensor data is continuously factored into gear selection. The availability of manual override in Tiptronic-type systems in many ways now offers the best of both worlds compared with manual transmissions. The controllability of driver gear selection is blended with fast ratio changes and computer-directed override gives protection of engine and transmission when necessary.

But automatic transmission components are expensive to produce and the complexity of them means that manufacturers are using automated, computer-controlled variants of conventional manual transmissions to achieve somewhat similar results, whilst we also have twin-clutch DSG and CVT transmissions competing for the same business, all of which we’ll be looking at soon.



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