what is the purpose of a lock up torque-converter?

Answer 1

*A torque converter is a type of hydrodynamic drive whose function is very similar to that of a fluid coupling. The principal difference is that whereas a fluid coupling is a two element drive that is incapable of multiplying torque, a torque converter has at least one extra element—the stator—which alters the drive's characteristics during periods of high slippage, producing an increase in output torque. It is suggested to the reader that he or she become familiar with the principles of hydrodynamic drives before continuing by reading the fluid coupling article.

In a torque converter there are at least three rotating elements: the pump, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the pump and turbine so that it can alter oil flow returning from the turbine to the pump. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator. In practice, however, the stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating the prime mover but allows for forward rotation.

*The key to the torque converter's ability to multiply torque lies in the stator. In the classic fluid coupling design, periods of high slippage cause the fluid flow returning from the turbine to the pump to oppose the direction of pump rotation, leading to a significant loss of efficiency and the generation of considerable waste heat. Under the same condition in a torque converter, the returning fluid will be redirected by the stator so that it aids the rotation of the pump, instead of impeding it. The result is that much of the energy in the returning fluid is recovered and added to the energy being applied by the pump itself. This action causes a substantial increase in the mass of fluid being directed to the turbine, producing an increase in output torque. Since the returning fluid is initially traveling in a direction opposite to pump rotation, the stator will likewise attempt to counter-rotate as it forces the fluid to change direction, an effect that is resisted by the one-way stator clutch.

*A torque converter cannot achieve 100 percent coupling efficiency. The classic three element torque converter has an asymptotical efficiency curve that resembles an inverted U: zero efficiency at stall, generally increasing efficiency during the acceleration phase and poor efficiency in the coupling phase. The loss of efficiency as the converter enters the coupling phase is a result of the turbulence and fluid flow interference generated by the stator, and as previously mentioned, is commonly overcome by mounting the stator on a one-way clutch.

*Lock-up Torque Converters:
As described above, pumping losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch that physically links the pump and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and therefore virtually no power loss.

The first automotive application of the lock-up principle was Packard's Ultramatic transmission, introduced in 1949, which locked up the converter at cruising speeds, unlocking when the throttle was floored for quick acceleration or as the vehicle slowed down. This feature was also present in some Borg-Warner automatics produced during the 1950's. It fell out of favor in subsequent years due the extra complexity and cost it added to the transmission. However, in the late 1970's lock-up clutches started to reappear in response to demands for improved fuel economy. They are now nearly universal in automotive applications.

Answer 2

Lock Up Torque Converter

Answer 3

Lockup Torque Converter

Answer 4

I assume you mean in reference to a car/transmission.

It allows the engine to turn faster(produce more power), and convert this to a lower rotation speed but have more torque. This is needed when you are taking off at low speeds.

However, there is an efficiency cost to this. So, at highway, cruising speeds, the torque-converter will lock up; meaning that there is a direct mechanical connection between the engine and the wheels.

I assume you already know that a torque-converter is basically two-disks in a fluid.

Answer 5

There are two prominent methods of torque conversion; hydrostatic and mechanical variable ratio. Both types have continuously variable drive to driven ratios from n:1 to 1:1. A Lock-Up converter is designed to "lock" at a predetermine ration under high load conditions, such as in a drag racing automatic transmission (hydrostatic).

Answer 6

it just changes the torque converter from a fluid coupling to a purely mechanical one, helps to improve fuel efficiency

Answer 7

Torque converters are not 100% efficient. They pump fluid between the two parts to couple them together and there is heat generated and slippage.

Lock-up ones are designed so that they start out pumping fluid but at a target speed they firmly engage and have mechanical contact rather than fluid contact. That is 100% efficient.

Answer 8

To conserve fuel.