3 ms·

>regenerative braking on the new trains is generating and sending back to the electric grid approximately 23% of the energy consumed by the system does this ma

>regenerative braking on the new trains is generating and sending back to the electric grid approximately 23% of the energy consumed by the system does this make sense? let's round that 23 up to 25%, and say brake energy regeneration is 100% efficient (it's not): one quarter of the energy put into the system comes back out? city driving in a car, you might have your foot on the brake 25% of the time (yes, that's not how it works, just think conceptually) but a train spends a lot of time running, and comparatively a small amount of time braking, and if you were to detach the engine from the train, one doesn't get a sense the train would roll very far unpowered, so while there's momentum to scavenge, it doesn't seem like 25% of the total I'm just thinking, how do these numbers make sense?

mikepurvis 2y ago
Firstly, I believe the trains are EMUs, so every bogey is powered rather than having the whole thing hailed by an engine; this is actually a huge benefit of electric as the trains can accelerate to speed much faster when they’re all wheel drive. But to address your main point, the rolling resistance of a train in motion is tiny; basically all the energy cost is at acceleration time— it probably cruises at like 5-10% power.
Groxx 2y ago
"rolling very far unpowered" is like the entire point of rail systems
- RWSen 2y ago
  Exactly. There's a train route here in The Netherlands, from Den Bosch to Utrecht, where the train can coast the last 20 km. That's out of a 48 km trip, so almost half!
- fsckboy 2y ago
  I would say that's not precisely true. the point of rail systems started out as "big power plant pulls lots of load". Once that was in place, yes, rolling very far unpowered becomes a compelling improvement. a good comparison would be the invention of the travois https://en.wikipedia.org/wiki/Travois https://en.wikipedia.org/wiki/Travois which benefitted from the addition of wheels.
  - ianburrell 2y ago
    Rails were invented for horse drawn carts. The low rolling resistance was the point. First steam locomotives were pretty weak and needed the efficiency. They were also used on existing tramways that had to be upgraded as trains got bigger.
justinc8687 2y ago
Trains have little friction on the tracks. That's one of the ways they are much more efficient than a car (with rubber tires). If the moving friction is very low, it makes sense that not that much energy is used to move the train once it's at speed. Slowing it down would return some of the energy back (with the rest dissipated as heat).
- lutorm 2y ago
  Even with a car's higher rolling resistance, most of the drag at cruising speed is aerodynamic, though. But a commuter train, by definition, starts and stops a lot so it makes sense that a large fraction of the energy used would be for acceleration.
  - renhanxue 2y ago
    Aerodynamic drag is proportional to cross sectional area. A train only has maybe 2-3x the drag of a car at the same speed, but it has on the order of 100 to 1000x the mass and momentum. It takes forever for drag to do anything to a train at speed.
renhanxue 2y ago
Try out the video game Derail Valley and you'll get a decently realistic idea of just how much trains tend to just coast along without any power input. On flat ground you just need a little nudge every once in a while to keep going.
Symbiote 2y ago
> one doesn't get a sense the train would roll very far unpowered Even a model/toy train wagon can easily coast the length of a large room when given a push. See how little metal is in contact, about 1cm²: https://www.reddit.com/r/Damnthatsinteresting/comments/17ysdb8/this_is_the_size_of_the_contact_patch_between/ https://www.reddit.com/r/Damnthatsinteresting/comments/17ysd... There's very little friction.

mikepurvis 2y ago

Firstly, I believe the trains are EMUs, so every bogey is powered rather than having the whole thing hailed by an engine; this is actually a huge benefit of electric as the trains can accelerate to speed much faster when they’re all wheel drive. But to address your main point, the rolling resistance of a train in motion is tiny; basically all the energy cost is at acceleration time— it probably cruises at like 5-10% power.

Groxx 2y ago

"rolling very far unpowered" is like the entire point of rail systems

RWSen 2y ago
Exactly. There's a train route here in The Netherlands, from Den Bosch to Utrecht, where the train can coast the last 20 km. That's out of a 48 km trip, so almost half!
fsckboy 2y ago
I would say that's not precisely true. the point of rail systems started out as "big power plant pulls lots of load". Once that was in place, yes, rolling very far unpowered becomes a compelling improvement. a good comparison would be the invention of the travois https://en.wikipedia.org/wiki/Travois https://en.wikipedia.org/wiki/Travois which benefitted from the addition of wheels.
- ianburrell 2y ago
  Rails were invented for horse drawn carts. The low rolling resistance was the point. First steam locomotives were pretty weak and needed the efficiency. They were also used on existing tramways that had to be upgraded as trains got bigger.

justinc8687 2y ago

Trains have little friction on the tracks. That's one of the ways they are much more efficient than a car (with rubber tires). If the moving friction is very low, it makes sense that not that much energy is used to move the train once it's at speed. Slowing it down would return some of the energy back (with the rest dissipated as heat).

lutorm 2y ago
Even with a car's higher rolling resistance, most of the drag at cruising speed is aerodynamic, though. But a commuter train, by definition, starts and stops a lot so it makes sense that a large fraction of the energy used would be for acceleration.
- renhanxue 2y ago
  Aerodynamic drag is proportional to cross sectional area. A train only has maybe 2-3x the drag of a car at the same speed, but it has on the order of 100 to 1000x the mass and momentum. It takes forever for drag to do anything to a train at speed.

renhanxue 2y ago

Try out the video game Derail Valley and you'll get a decently realistic idea of just how much trains tend to just coast along without any power input. On flat ground you just need a little nudge every once in a while to keep going.

Symbiote 2y ago

> one doesn't get a sense the train would roll very far unpowered Even a model/toy train wagon can easily coast the length of a large room when given a push. See how little metal is in contact, about 1cm²: https://www.reddit.com/r/Damnthatsinteresting/comments/17ysdb8/this_is_the_size_of_the_contact_patch_between/ https://www.reddit.com/r/Damnthatsinteresting/comments/17ysd... There's very little friction.