Time for the Peugeot 302?

Nope, splendid though the Peugeot is. Time for something that gives three from two. To be specific, this piece of ironmongery.

Illustration for article titled 3/2!

It’s a Voith T 312 hydraulic transmission for rail use and it gives three speeds from two gears. How so?


Like so.

It has a chain of gears which provide a high and a low ratio. They’re permanently enmeshed with each other and connected to the wheels. They’re also permanently attached to the turbines of a torque converter (low gear) and a pair of fluid couplings (one each for low and high gears). The impellers of the converter and couplings are all driven simultaneously by the engine. This would usually result in a state of hydraulic confusion which is resolved by ensuring that in neutral each of the hydraulic elements is drained of fluid so no power is transmitted anywhere.


In order to proceed fluid is pumped into the converter which might sound a lengthy process but actually takes only a second or two. Power is then applied. The converter is designed to slip considerably so engine revs rise to something approaching their governed maximum and torque is applied to the wheels via low gear. Because everything’s linked the turbines and impellers of the two couplings rotate as well but without fluid between them they have no effect on progress. We’re off then and accelerate until the wheels catch up with the engine which is maintaining a steady speed throughout so it’s like having a CVT. Now we need more speed. The converter is drained and simultaneously one of the couplings is filled. This also drives low gear. Sounds pointless? No. The coupling has much less slip than the converter so engine revs fall and we can accelerate again after experiencing what feels exactly like a conventional mechanical gearchange. This is second speed while still using low gear but without the CVT effect so revs now rise with speed.

When even more go is called for one coupling is drained and the second filled. This drives high gear so this is the third speed.


So there we are. Three speeds, two gears and no involvement of friction materials (as in a conventional automatic) or mechanical elements (as in a manual) so little to wear out other than a hydraulic pump.

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