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>I'm actually not so sure. If the gradient is so steep that the vehicle is struggling to move at all The problem isn't overloading the engine when you go up, i

>I'm actually not so sure. If the gradient is so steep that the vehicle is struggling to move at all The problem isn't overloading the engine when you go up, it's overheating the brakes when you go down. The reasoning here is probably that EV semis will use regeneration for some of the braking thus avoiding the overheating to some extent.

xienze 1y ago
To be fair, the parent didn’t specify which direction they were talking about. And is brake overheating a cause for an entire truck to catch on fire? That sounds more like an engine failure kind of thing.
- diggan 1y ago
  If I were to guess, it's about using "motor breaking" (not sure that's the correct English term for it) that you do if you travel downwards for a long time, in order to avoid over-heating the brakes so they are ineffective. I'm guessing doing motor breaking for too long, in a hot environment, might overheat the engine as well?
  - Symbiote 1y ago
    In Britain we call it "engine braking", I think Americans say the same. A quick search says it heats the engine less than accelerating, so shouldn't cause overheating.
  - gmueckl 1y ago
    I don't see how that could overheat the engine. You're backdriving the engine from the wheels and fuel injection should be complete shit down so there is no combustion.
  - mikestew 1y ago
    Engine braking doesn’t work on diesels, that’s why they have “Jake brakes”, that loud braaaaaaaaap you hear when they go downhill (and why you see signs at the edge of town saying “compression brake use illegal”). I highly doubt a diesel engine would overheat going downhill. (Not a diesel mechanic, but FWIW used to be an auto mechanic decades ago.) https://en.wikipedia.org/wiki/Compression_release_engine_brake#Function https://en.wikipedia.org/wiki/Compression_release_engine_bra...
    - diggan 1y ago
      > Engine braking doesn’t work on diesels Are you sure? I'm pretty sure I can engine break with my diesel car and do that pretty often in fact. Or I misunderstand what engine breaking is.
      - mikestew 1y ago
        Am I sure? No, but I have been told this numerous times by actual diesel mechanics, and otherwise why was it necessary for Jacobs to invent the compression brake? Your car also has rolling resistance and aerodynamic drag to contend with. Might work well at slowing a passenger car, not so much with a 50K lb. vehicle, hence the Jake brake.
        diggan 1y ago
        > Your car also has rolling resistance and aerodynamic drag to contend with I like to believe I'd recognize the difference, especially since it's a manual car and I can tell the difference between letting it roll in neutral and shifting down a gear which slows down the car :) Maybe it's not that it doesn't work on diesel cars but the effect is just less than with petrol?
        mikestew 1y ago
        Manual tranny, why didn't you say so? :-) Yeah, I guess you would notice the difference. But I think you've hit on the difference, as I vastly simplified what is going on. Not that I expected you to read the link I gave, but it does give some explanation as to what's going on. And what's going is that the pistons are still going up and down because air is continually drawn into the engine. Air is compressed, and even though there is no fuel to make it go bang, that air still needs to uncompress and so returns a lot of the energy back to the crankshaft. Ergo, very little engine braking. As a personal example, our diesel Sprinter van (automatic tranny, FWIW) had some degree of engine braking, but so little that I wouldn't rely on it for much more than coasting to a stoplight or other low-stakes slowing. If I'd like to stop sometime in the next day or two, I hit the brake pedal.
- audunw 1y ago
  Yes, last time I read about a fire in that tunnel I believe it was the brakes. I don’t know how. Even if it’s an engine issue, I don’t see how an EV would be more likely to catastrophically overheat. An EV will generate a lot less heat for a given amount of power. There’s also less potential for oil and fuels leaks which exacerbates the issue.
  - AnthonyMouse 1y ago
    > Yes, last time I read about a fire in that tunnel I believe it was the brakes. I don’t know how. Friction brakes convert momentum into heat. If you ride the brakes going down a mountain you generate more heat than the brakes can dissipate into the air and the brake temperature keeps going up until they're hot enough to start a fire.
    - xienze 1y ago
      Presumably truck drivers are skilled enough to know not to ride the brakes, aren’t they?
      - AnthonyMouse 1y ago
        The vast majority of truck drivers, yes. The driver of the truck whose brakes caught on fire, nope.
  - giantg2 1y ago
    The risk would be from power cell failure and how difficult it is to put out those fires for most of the chemistries used, how they're packed, etc. I would guess the rate of occurrence will be pretty similar. I don't think we'll fully know until we have a bunch of older EV trucks to know how the risk compares to older diesel ones.

xienze 1y ago

To be fair, the parent didn’t specify which direction they were talking about. And is brake overheating a cause for an entire truck to catch on fire? That sounds more like an engine failure kind of thing.

diggan 1y ago
If I were to guess, it's about using "motor breaking" (not sure that's the correct English term for it) that you do if you travel downwards for a long time, in order to avoid over-heating the brakes so they are ineffective. I'm guessing doing motor breaking for too long, in a hot environment, might overheat the engine as well?
- Symbiote 1y ago
  In Britain we call it "engine braking", I think Americans say the same. A quick search says it heats the engine less than accelerating, so shouldn't cause overheating.
- gmueckl 1y ago
  I don't see how that could overheat the engine. You're backdriving the engine from the wheels and fuel injection should be complete shit down so there is no combustion.
- mikestew 1y ago
  Engine braking doesn’t work on diesels, that’s why they have “Jake brakes”, that loud braaaaaaaaap you hear when they go downhill (and why you see signs at the edge of town saying “compression brake use illegal”). I highly doubt a diesel engine would overheat going downhill. (Not a diesel mechanic, but FWIW used to be an auto mechanic decades ago.) https://en.wikipedia.org/wiki/Compression_release_engine_brake#Function https://en.wikipedia.org/wiki/Compression_release_engine_bra...
  - diggan 1y ago
    > Engine braking doesn’t work on diesels Are you sure? I'm pretty sure I can engine break with my diesel car and do that pretty often in fact. Or I misunderstand what engine breaking is.
    - mikestew 1y ago
      Am I sure? No, but I have been told this numerous times by actual diesel mechanics, and otherwise why was it necessary for Jacobs to invent the compression brake? Your car also has rolling resistance and aerodynamic drag to contend with. Might work well at slowing a passenger car, not so much with a 50K lb. vehicle, hence the Jake brake.
      - diggan 1y ago
        > Your car also has rolling resistance and aerodynamic drag to contend with I like to believe I'd recognize the difference, especially since it's a manual car and I can tell the difference between letting it roll in neutral and shifting down a gear which slows down the car :) Maybe it's not that it doesn't work on diesel cars but the effect is just less than with petrol?
        mikestew 1y ago
        Manual tranny, why didn't you say so? :-) Yeah, I guess you would notice the difference. But I think you've hit on the difference, as I vastly simplified what is going on. Not that I expected you to read the link I gave, but it does give some explanation as to what's going on. And what's going is that the pistons are still going up and down because air is continually drawn into the engine. Air is compressed, and even though there is no fuel to make it go bang, that air still needs to uncompress and so returns a lot of the energy back to the crankshaft. Ergo, very little engine braking. As a personal example, our diesel Sprinter van (automatic tranny, FWIW) had some degree of engine braking, but so little that I wouldn't rely on it for much more than coasting to a stoplight or other low-stakes slowing. If I'd like to stop sometime in the next day or two, I hit the brake pedal.
audunw 1y ago
Yes, last time I read about a fire in that tunnel I believe it was the brakes. I don’t know how. Even if it’s an engine issue, I don’t see how an EV would be more likely to catastrophically overheat. An EV will generate a lot less heat for a given amount of power. There’s also less potential for oil and fuels leaks which exacerbates the issue.
- AnthonyMouse 1y ago
  > Yes, last time I read about a fire in that tunnel I believe it was the brakes. I don’t know how. Friction brakes convert momentum into heat. If you ride the brakes going down a mountain you generate more heat than the brakes can dissipate into the air and the brake temperature keeps going up until they're hot enough to start a fire.
  - xienze 1y ago
    Presumably truck drivers are skilled enough to know not to ride the brakes, aren’t they?
    - AnthonyMouse 1y ago
      The vast majority of truck drivers, yes. The driver of the truck whose brakes caught on fire, nope.
- giantg2 1y ago
  The risk would be from power cell failure and how difficult it is to put out those fires for most of the chemistries used, how they're packed, etc. I would guess the rate of occurrence will be pretty similar. I don't think we'll fully know until we have a bunch of older EV trucks to know how the risk compares to older diesel ones.