Torque Converter

[James02]

I have never gotten an answer to this:

1.  What is a torque converter?  I have assumed it is an automatic clutch.  What is its function and purpose?

2.  A friend told me that modern (probably common today) torque converters can stay "unlocked" on a long, steep grade, which is not good for them.  What does this mean?

Thanks.

[Tim]

It's a fluid coupling which transfers rotating motion from the prime mover and can multiply the torque like a reduction gear. It's an automatic transmission.

http://en.wikipedia.org/wiki/Torque_converter

tim

[James02]

I read the article.  The lock-up clutch is used for cruising down the highway.  So the engine is directly connected to the transmission when this happens.  I imagine that when it unlocks, you can generate a lot of heat.

[Tim]

Got to get equal the energy; heat+entropy.

tim

[SoCalLocal]

The simplest way to describe how it works is the classic example of two fans pointing at each other. Turn one on, it starts blowing, and it causes the other fan to start turning. Torque converters get a lot more complicated than that, with one-way sprags and the stator throwing fluid back for a second go-around, which almost doubles the torque.

The torque multiplication is why automatics don't have a low first gear like a manual transmission. They don't need it. It's not perfect, since it's in the real world, but at zero RPM, torque will be doubled and speed will be halved. As the RPM goes up, it gets closer and closer to same speed, same torque. That particular RPM is the stall speed, which you may see listed in hotrod catalogs. Try to remember that the old Chevy Turboglide was only a two-speed automatic, and it was pretty bombproof. They let the torque converter handle the spread.

Lockup torque converters came along around 1979, for sure by 1980. It adds another layer to the internals. It actually is a clutch, like the one in a manual transmission car, except it's wet. When it gets applied, there is no slippage between engine and transmission. This is programmed into a transmission computer or the main engine computer. In the 1980s, most cars had to be above 35 mph, in D or OD, with moderate throttle. Step on the brakes, it unlocks. Floor it, it unlocks. Slow down, it unlocks. Some Volvos and BMWs locked up whenever the RPM hit 2000. Now, with faster, more powerful computers, there are a lot more variables.

If it unlocks on a long grade, it's because it's downshifting and senses that you need more torque. Yes, it generates more heat. But them huge old 1960s American cars managed just fine without lockups or overdrive. It's not that big a deal.

Clear as mud?

[James02]

Got it. thanks.