5 ms·

We have many gears for better efficiency with ICE, because every ICE has very narrow areas of RPM×torque that give you acceptable efficiency (typically those wi

We have many gears for better efficiency with ICE, because every ICE has very narrow areas of RPM×torque that give you acceptable efficiency (typically those with high mean effective pressure). You could totally get away with three gears and still get to 150 km/h, it's just that it will be a lot less efficient. Electric motors don't have this particular problem, and they don't need a start gear that is a lot shorter than the other gears (1st to 2nd is usually around a factor two, while from 2nd up to the next higher gear the ratio will change by perhaps 30 % or so) because they can deliver nominal torque at low speeds or from zero speed (so they don't need a clutch, either). An ICE can't do that. Here are the transmission ratios of a random six speed transmission: 1st 2nd 3rd 4th 5th 6th R 4.2 2.5 1.7 1.3 1 0.8 3.8

hwillis 7y ago
> every ICE has very narrow areas of RPM×torque that give you acceptable efficiency (typically those with high mean effective pressure). You could totally get away with three gears and still get to 150 km/h, it's just that it will be a lot less efficient. It isn't that narrow[2]. Engines with fewer gears aren't particularly less efficient, they're just MUCH slower. Look at a map of gears[2]: the top gears are in the efficient range, but the lower gears are only there to allow you to rev the engine high while accelerating. In the ratios you gave, you could cut out 1, 3, 4 and only use gears 2, 5 and 6 and be able to get around fine- you start in second and then shift up as soon as you're rolling. You'd have a much more efficient car but you'd be incredibly slow. [1]: https://qph.fs.quoracdn.net/main-qimg-0c6c45106b28601fc1aef4172c08faf1.webp https://qph.fs.quoracdn.net/main-qimg-0c6c45106b28601fc1aef4... [2]: http://www.posterus.sk/wp-content/uploads/p12975_02_obr02.png http://www.posterus.sk/wp-content/uploads/p12975_02_obr02.pn...
- blattimwind 7y ago
  You could get around fine with the first three gears, it's just that it would be a lot less efficient. You could get around fine with 2, 4, 6, it's just that it would have very slow and less efficient acceleration (accelerating for longer at low RPM and very high torque is overall less efficient than accelerating for a shorter period of time at a higher RPM, given identical start and end speeds). And that's exactly the point I made. You need many gears for an ICE because being able to reach a given speed in a given gear says very little on how efficient that is, therefore you need different gears for accelerating versus holding a given speed to get acceptable efficiency.
  - hwillis 7y ago
    > (accelerating for longer at low RPM and very high torque is overall less efficient than accelerating for a shorter period of time at a higher RPM, given identical start and end speeds). No, that's the whole point of short shifting.

hwillis 7y ago

> every ICE has very narrow areas of RPM×torque that give you acceptable efficiency (typically those with high mean effective pressure). You could totally get away with three gears and still get to 150 km/h, it's just that it will be a lot less efficient. It isn't that narrow[2]. Engines with fewer gears aren't particularly less efficient, they're just MUCH slower. Look at a map of gears[2]: the top gears are in the efficient range, but the lower gears are only there to allow you to rev the engine high while accelerating. In the ratios you gave, you could cut out 1, 3, 4 and only use gears 2, 5 and 6 and be able to get around fine- you start in second and then shift up as soon as you're rolling. You'd have a much more efficient car but you'd be incredibly slow. [1]: https://qph.fs.quoracdn.net/main-qimg-0c6c45106b28601fc1aef4172c08faf1.webp https://qph.fs.quoracdn.net/main-qimg-0c6c45106b28601fc1aef4... [2]: http://www.posterus.sk/wp-content/uploads/p12975_02_obr02.png http://www.posterus.sk/wp-content/uploads/p12975_02_obr02.pn...

blattimwind 7y ago
You could get around fine with the first three gears, it's just that it would be a lot less efficient. You could get around fine with 2, 4, 6, it's just that it would have very slow and less efficient acceleration (accelerating for longer at low RPM and very high torque is overall less efficient than accelerating for a shorter period of time at a higher RPM, given identical start and end speeds). And that's exactly the point I made. You need many gears for an ICE because being able to reach a given speed in a given gear says very little on how efficient that is, therefore you need different gears for accelerating versus holding a given speed to get acceptable efficiency.
- hwillis 7y ago
  > (accelerating for longer at low RPM and very high torque is overall less efficient than accelerating for a shorter period of time at a higher RPM, given identical start and end speeds). No, that's the whole point of short shifting.