5 ms·

Trains do it because they need to apply a lot of torque at zero speed to get moving. The only way to do that with a mechanical connection to a combustion engine

Trains do it because they need to apply a lot of torque at zero speed to get moving. The only way to do that with a mechanical connection to a combustion engine is with a clutch, which would burn up from friction before the train got going. In theory a hydraulic pump would work, but I'm sure that electric motors are better for that usage, since that is what they use.

jasonwatkinspdx 4y ago
There's also torque converters, which are a hydraulic mechanism. This is the basis of most (non sporting) automatic transmissions. Higher performance torque converters have a lock up mechanism that functions as a clutch making it rigid at high speeds. So you can kinda get both.
- masklinn 4y ago
  > There's also torque converters, which are a hydraulic mechanism. This is the basis of most (non sporting) automatic transmissions. By virtue of being the dominant automatic transmission in the US, where automatic transmission is the overwhelming majority. In Europe, torque converters have never been much of a thing, historically because of the small engines (at the low end, and demand for control and responsiveness at the high end), and more recently for efficiency reasons. AT has been shooting up in popularity (in parts because gearings have been getting shorter which makes manual transmissions really annoying), but mostly on the back of DCTs, even at the low end e.g. these days it's pretty common to find a 6-speed DCT on a B-segment car, and C-segments getting 7 or 8-speed DCTs (AMTs sadly still survive at the lower end of AT, but I feel CVTs are eating their bacon, especially with progressive hybridation).
  - SigmundA 4y ago
    Modern lock-up torque converter transmissions have pretty much eliminated any efficiency gains from other designs, I believe everyone is moving to them even in Europe. I know here in the US many care from various manufacturer are coming with German designed ZF transmissions including the 9 speed in my Honda Pilot. https://www.autocar.co.uk/car-news/technology-news/under-skin-why-torque-converters-are-coming-back-dead https://www.autocar.co.uk/car-news/technology-news/under-ski... https://www.caranddriver.com/features/a23367341/automatic-transmission-best-zf-eight-speed/ https://www.caranddriver.com/features/a23367341/automatic-tr...
  - CRConrad 4y ago
    > In Europe, torque converters have never been much of a thing, historically because of the small engines [...] and more recently for efficiency reasons. The small-engine problem was basically just another aspect of the efficiency problem: Torque converters used to be so inefficient that if you had very little power to begin with, close to none at all got through to the drive wheels.
Toutouxc 4y ago
> electric motors are better for that usage, since that is what they use Except those that actually use hydraulic couplings or torque converters, which is the cheaper and lighter solution compared to electric, albeit less efficient.
frosted-flakes 4y ago
They also do it for simplicity (reliability and less maintenance). A that scale, a mechanical drivetrain to all those wheels would be much more complicated than electric motors and cables.
SigmundA 4y ago
There are hydraulic trains, they have poor running efficiency worse then electric. My understanding is in order to get a large train moving with steel on steel requires very precise traction control that lends it self to electric, while doing a mechanical coupling for cruise speed would be very complicated and expensive so they eat the efficiency loss there. They are also saving brake wear using the traction brakes which just run heat strips on the top of the train.

jasonwatkinspdx 4y ago

There's also torque converters, which are a hydraulic mechanism. This is the basis of most (non sporting) automatic transmissions. Higher performance torque converters have a lock up mechanism that functions as a clutch making it rigid at high speeds. So you can kinda get both.

masklinn 4y ago
> There's also torque converters, which are a hydraulic mechanism. This is the basis of most (non sporting) automatic transmissions. By virtue of being the dominant automatic transmission in the US, where automatic transmission is the overwhelming majority. In Europe, torque converters have never been much of a thing, historically because of the small engines (at the low end, and demand for control and responsiveness at the high end), and more recently for efficiency reasons. AT has been shooting up in popularity (in parts because gearings have been getting shorter which makes manual transmissions really annoying), but mostly on the back of DCTs, even at the low end e.g. these days it's pretty common to find a 6-speed DCT on a B-segment car, and C-segments getting 7 or 8-speed DCTs (AMTs sadly still survive at the lower end of AT, but I feel CVTs are eating their bacon, especially with progressive hybridation).
- SigmundA 4y ago
  Modern lock-up torque converter transmissions have pretty much eliminated any efficiency gains from other designs, I believe everyone is moving to them even in Europe. I know here in the US many care from various manufacturer are coming with German designed ZF transmissions including the 9 speed in my Honda Pilot. https://www.autocar.co.uk/car-news/technology-news/under-skin-why-torque-converters-are-coming-back-dead https://www.autocar.co.uk/car-news/technology-news/under-ski... https://www.caranddriver.com/features/a23367341/automatic-transmission-best-zf-eight-speed/ https://www.caranddriver.com/features/a23367341/automatic-tr...
- CRConrad 4y ago
  > In Europe, torque converters have never been much of a thing, historically because of the small engines [...] and more recently for efficiency reasons. The small-engine problem was basically just another aspect of the efficiency problem: Torque converters used to be so inefficient that if you had very little power to begin with, close to none at all got through to the drive wheels.

Toutouxc 4y ago

> electric motors are better for that usage, since that is what they use Except those that actually use hydraulic couplings or torque converters, which is the cheaper and lighter solution compared to electric, albeit less efficient.

frosted-flakes 4y ago

They also do it for simplicity (reliability and less maintenance). A that scale, a mechanical drivetrain to all those wheels would be much more complicated than electric motors and cables.

SigmundA 4y ago

There are hydraulic trains, they have poor running efficiency worse then electric. My understanding is in order to get a large train moving with steel on steel requires very precise traction control that lends it self to electric, while doing a mechanical coupling for cruise speed would be very complicated and expensive so they eat the efficiency loss there. They are also saving brake wear using the traction brakes which just run heat strips on the top of the train.