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Comment by mcherm | Hacker News Reader

Ever driven a vehicle with an automatic transmission rather than a manual gearshift with a clutch? Then you almost certainly used a fluid coupling: basically two fans in a can with oil so turning one turns the other.

The article is so full of hype it doesn't bother to explain how this is different from the "fluid gears" invented in 1905.

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As immortalised in the 1978 song "Greased Ligthnin'" from the film Grease:

    Well, this car is automatic
    It's systematic
    It's hydromatic
    Why it's greased lightnin' (greased lightnin')

I am pretty sure that "hydromatic" there is actually "Hydramatic" (https://en.wikipedia.org/wiki/Hydramatic).

If it’s a hydramatic, be careful to only lubricate it with snake oil.

1978 film (based on a 1971 musical) set in 1958 with the high school kids repairing a beaten up 1948 model year car.

Many automatics these days are manual transmissions with a computer controlling the clutch. They have nothing in common with the slushboxes of old, the oil is just for lubrication.

Even slushboxes tend to aggressively lock up the torque converter. It's usually only in a "fluid dynamics" mode for brief moments. (Except maybe on a gentle hill start)

It's worth mentioning hard acceleration causes a torque multiplication to occur in torque converters vs a slow acceleration does not (check me on that). Once locked up, multiplication no longer occurs. Thank you hot rod magazines of the 90's.

ZF 8HP is still a traditional torque converter transmission. Most high performance or high torque applications use that design. Dual clutch automatics or automated manuals can't take it.

Virtually every truck has an automated manual transmission, every EV 2-speed is a DCT, Porsche uses PDKs etc.

Every truck? Like which ones?

As an American (land of the pickup truck) I can't think of any

I think GP meant eighteen-wheelers and the like.

> American (land of the pickup truck)

More like SUVs with beds

But yes, sibling is correct, I'm talking about commercial vehicles.

Citation needed. By truck if you mean commercial truck (lorry, artic, etc.) then no, Allison still makes hydraulic automatics which are very common in vocational work the world over.

Tremec makes DCTs that are used in 800hp Mustangs and 1000hp Corvettes

I mean, certainly the VAG group likes to use their dual clutch automatics, but "true" torque converters are still very common. ZF makes them for like a million different cars, and AISIN makes them for the Volvo and Geely group.

Maybe in sports cars but the majority of vehicles still come with hydraulic autos or CVTs.

> Ever driven a vehicle with an automatic transmission rather than a manual gearshift with a clutch? Then you almost certainly used a fluid coupling

Are you sure?

https://en.wikipedia.org/wiki/Continuously_variable_transmis...:

“The most common type of CVT uses a V-belt which runs between two variable-diameter pulleys.

[…]

A belt-driven design offers approximately 88% efficiency, which, while lower than that of a manual transmission, can be offset by enabling the engine to run at its most efficient speed regardless of the vehicle's speed.

[…]

Disadvantages of a hydrostatic CVT include:

Reduced efficiency. Gears are one of the most efficient methods of mechanical power transmission, with efficiencies as high as 90 percent in many cases. In contrast, few hydrostatic transmission systems achieve more than about 65 percent efficiency”

CVT != Automatic transmission (which is generally hydraulic)

https://en.wikipedia.org/wiki/Automatic_transmission#Hydraul...

Not sure why you are making this statement.

> "A continuously variable transmission (CVT) is an automatic transmission ..."

https://en.wikipedia.org/wiki/Continuously_variable_transmis...

CVT is not the same as "automatic transmission", but it is a subtype of automatic transmission, i.e. CVT is a kind of automatic transmission, but there are also other kinds of automatic transmissions, which are more frequently used.

"Automatic transmission" just means that you do not change gears manually, which is also true for CVT.

There are also DCTs which IIRC don't use a torque converter. But chances are pretty good that an automatic transmission car uses a torque converter.

A hydrostatic CVT is not a torque-converter transmission (which is hydrodynamic). A hydrostatic CVT is basically a hydraulic pump, control valves plus hydraulic motor. So what you would typically find on construction or forest equipment.

Except a fluid clutch actually works, and a torque converter works even better and has three fans inside it ;-)

I can see the "passive" cylinder getting dragged around a little by viscosity but I don't see how this could transfer even the tiniest amount of power.

Yeah, and like it's name a torque converter can actually not only transmit torque as well but also convert rpms to torque. Running the engine at high rpm at standstill converts revolutions of the input shaft to torque on the output shaft, thus allowing the car to start accelerating at slow speed but with high torque, operating without a clutch.

That's as close to fluid gearing as you can imagine.

Yup. You can hold the RPM steady and watch the road speed climb as the inner blades "catch up" with the outer ones.

Not as funny as with CVTs though where you can have the road speed increasing as the engine RPM decreases. Or there was a guy about 20 years ago who had a Volvo 340 automatic fitted with a Volvo 760GLT 2.3 litre turbocharged engine, which he used to compete with at drag races at the local raceway. It did not too bad against similar vehicles in its class, but it sounded pretty funny because it would just race up to about 4500RPM and stay there for the whole run, as you had a rising howl from the drive belts at the back.

Of course my car has a torque converter and two viscous couplings - the TC between the engine and gearbox, one viscous coupling that makes the centre diff act like a very stiff LSD, and of course one that works the other way (eases off as it slips) to let the cooling fan stay at a steady speed.

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