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Fluid Gears Rotate Without Teeth
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.

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).
1978 film (based on a 1971 musical) set in 1958 with the high school kids repairing a beaten up 1948 model year car.
  • donw
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  • 2 hours ago
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If it’s a hydramatic, be careful to only lubricate it with snake oil.
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.
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.
Tremec makes DCTs that are used in 800hp Mustangs and 1000hp Corvettes
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.
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)
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.
> 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”

  • mjmas
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  • 3 hours ago
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CVT != Automatic transmission (which is generally hydraulic)

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

  • raynr
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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.
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.

  • yason
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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.

The phys.org article and headlines are misleading, the authors did not investigate systems to actually transmit torque. From what I gather, the interesting findings are the parameters for co-rotation and counterrotation of the driving and driven cylinder, depending on the Reynolds number, distance and so on. To illustrate one of the images of their publication: https://i.imgur.com/m8P2iVw.png
Check out what this dude came up with 120 years earlier: https://en.wikipedia.org/wiki/Hermann_F%C3%B6ttinger
Died at age 68 on April 28, 1945 in Berlin, from shrapnel. The Soviets had begun shelling of Berlin on April 20.

This sort of coupling was used to drive the supercharger on the German BF 109, enabling it to maintain manifold pressure from sea level up to 20,000 ft. with a single stage supercharger without throttling. Use of such couplings in automobiles didn't occur until after the war.

https://www.youtube.com/watch?v=NcI67pZe_Ss

As what efficency? The artical doesn't say, but hydraulics and automatic transmissions have been around for a long time and are less efficient than regular gears or electric motors. Cars got a good efficieny boost then the locking torque converter was developed.
Ehm, isn't it the same thing as https://en.wikipedia.org/wiki/Torque_converter ?
I'm not a fan of the bias towards "Gears are old tech, and that makes them bad" but I can see a lot of interesting possibilities with fluid coupling. The variables involved in power transmission for these things would be pretty wild to characterize, and the article video clearly shows inefficiencies in the system with the driven cylinder having counter rotational flow against it.
> I'm not a fan of the bias towards "Gears are old tech, and that makes them bad"

If the gears don't at least require an app with a subscription and regular updates to use, they must be old tech

/sarcasm

  • dvh
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Virtually zero torque right?
I don't see how it could even have any measurable torque.

You could improve it by making the cylinders have sticky-out bits that would scoosh the fluid around better, like little paddle wheels, and if you wanted to get some serious torque transfer you'd push the two paddle wheels so close together that the paddles actually kind of intersect.

Indeed, then this design will truly have come full circle.
That's a slippery slope that leads right back to traditional gears.
you could increase fluid viscosity
I’m super confused how this any more useful than an oil based hydrodynamic torque converter like you’d find in a “slush box” automatic transmission. The video in the article shows such a low rate of transmission it’s crazy, I can’t think of a purpose!
Do we understand fluid mechanics well enough now to just design things like this from scratch, or is it still mostly trial and error? TFA seems to imply the latter, but....
This could have been designed in CFD in an afternoon. Building it in real life is always somewhat cool but the amount of hype they're putting on this is crazy.
  • Y_Y
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There must be some reason PRL chose to publish this, but it's not apparent to me from TFA or the abstract, and I'm not interested enough to login via my institution.

It's not new that you could set up co- or counter-rotation in such a system. This seems like the sort of thing G. I. Taylor had as a bath toy.

Maybe impossibly tiny and unresponsive torques are useful somewhere?

Nice to see that they have discovered the Torque converter https://en.wikipedia.org/wiki/Torque_converter

Used since the 60s lol

How is this different from slushomatics? Sounds exactly like old school fluid couplings.
Or slushboxes …
reminds me of a tesla turbine -- only less efficient