> This is simply not going to be an issue with EV trucks. A fire is possible but much less likely in general, and MUCH less likely in that specific scenario.
I'm actually not so sure. If the gradient is so steep that the vehicle is struggling to move at all, the current through the motor windings will be very high, causing the windings to overheat, potentially fail and potentially short circuit. There's a high risk of damaging the MOSFETs in the motor controller, which very much could lead to a fire risk depending on the failure mode.
There's not really many ways to solve this problem - in a normal 3-phase winding, all you can do is remove the current until it cools down and try again, but that will force the motor to stop and then try to restart, so creating an even larger load. Possibly if you have 6 more more phases and more magnets such that each of the normal 3-phases has multiple windings and magnets, you can cycle through the different ones and still keep applying some torque, but obviously this would still not really solve the fundamental problem.
Essentially the problem is the same for ICE vs EV - if the gradient is so steep and load so heavy that the engine / motor can't provide enough force, then it will be overloaded. Whether that's through pressure / shearing / excess heat in an ICE or through excess current / excess heat in EV, the outcome is failure to continue forward at best.
The only real solution is to massively over engineer the engine for normal situations, but human nature being what it is, there will always push things way beyond the designed limits and safety margins until it fails.
It’s a lot easier to engineer an EV truck to handle steep gradients without overloading. We see from real examples that EV trucks are by default much more capable of driving fast uphill.
One part of this is the batteries. When you have the amount of batteries needed to drive a truck for a reasonable distance, you automatically get a high amount of power output as well. The power is distributed over many cells, so no overload there.
EV motors are significantly smaller than their ICE counterparts, they’re relatively cheap, don’t require significant maintenance and they generate much less waste heat for a given power output. Adding more motors+inverters to handle the required power is not over engineering in the case of an EV truck, it’s just good engineering. I suppose it’s even necessary to some degree, to deal with the lack of a multi speed gear box
As mentioned in the other comment, the problem is often overheating in brakes. This is also less of an issue with EVs. You can distribute the energy dissipation to the motors/batteries and the brake pads, so the heat load is less concentrated. Energy sent to the batteries is absorbed as energy stored, with very little waste heat.
I'm specifically talking about the windings in the motors themselves. There's only so much current they had take before they start heating up, and that's when they can start to fail - just as melting or burning the insulation, creating a short across some of the winding, making them even less effective and even more prone to short circuiting which can cause even higher currents.
In normal use, only one phase is active an a time, so the duty cycle is 1/3. When the motor slows almost to a stop, the duty cycle on that winding is 100% meaning that the effect of that current on heating the wire is much worse than normal.
The catastrophic failure is when the MOSFET fails in a way that it doesn't protect the winding or battery from a short circuit which could lead to runaway heating in the battery as well as the motor. But even before then, unless the controller is actively limiting current to safe levels, the motor will get destroyed.
The only happy day scenario is if the motor control is actively limiting the current to safe levels well below the expected failure point, and then the EV will just fail to move at all under that load, other than rolling backwards.
As I said, the limit for this will be based on what the manufacturer expects the maximum load will be, but people have a knack for trying to carry more weight than their vehicle was designed for, or taking it places that are unsuitable. That's just humans being humans.
It's possible to design an EV that could withstand significantly steep hills with heavy loads, e.g. by putting many more sets of individually wired windings in parallel, but it'd be expensive and unnecessary for the typical situations that they'd be used in, and so unlikely to be commercially viable.
>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.
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.
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.
> 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.
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.
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?
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.)
In a gasoline engine, engine speed is controlled by a throttle valve which meters the air entering the engine. A carburetor or fuel injection system then provides an appropriate amount of fuel for the incoming air. If you release the accelerator the throttle valve shuts and the engine effectively becomes a vacuum pump sucking air through small openings that only flow enough air to idle, which causes it to actively slow down.
A diesel engine does not have a throttle valve. Engine speed is controlled purely by the amount of fuel being injected. This is why diesel engines can "run away" if an uncontrolled fuel source such as oil leaking from a turbocharger enters the intake, and why older pre-computerization diesels can be smoky under hard acceleration (or any time if poorly tuned). When you release the accelerator the engine stops receiving fuel and thus stops producing power, but aside from friction there's nothing working to slow it down. Air is sucked in more or less unrestricted and some energy is spent compressing it as the pistons rise in the cylinders, but much of that energy comes back out as the pistons come back down so the only energy loss in the system is what's converted to heat. You would get the same effect in a gasoline engine if you shut off the fuel pump but kept the throttle wide open.
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.
> 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?
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.
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.
> If the gradient is so steep that the vehicle is struggling to move at all
But such a gradient would be completely unusable for any ICE! EVs can deliver an absolutely insane amount of power at even the slowest speeds. If an EV has the power to sustain highway speeds, it'll also have the power to go up a steep hill at a snail's pace. On the other hand, an ICE will struggle significantly with steep hills - even if they technically have the horsepower to do it.
I highly doubt it'll result in a fire, though. Measuring motor current isn't exactly rocket science, so it'll just go into an overload mode. Heck, I wouldn't be surprised if EV motors were explicitly designed to survive short-term stall currents. Measure the wheel speed and it's pretty obvious to figure out when you're stalling and should shut down to avoid damage.
> If an EV has the power to sustain highway speeds, it'll also have the power to go up a steep hill at a snail's pace.
Most EVs are direct drive from the motor without a gearbox. There absolutely is a minimum speed depending on how the motor is wound. If you imagine, say 12 magnets around the motor driven by 3 phases, the idea is to have one winding attracting the magnets, and this phase is the one the motor is "trying to get to". In this situation, this winding is active until the motor is in the correct position (actually slightly before) when it is depowered and the next phase is powered. The longer the motor stays in that position, the hotter the winding gets for the same current. At the point where the motor stops being able to rotate at all, that winding is receiving 100% duty cycle.
There are strategies that you could try - the simplest is cutting power and letting it cool down, but then the motor will stop where it is, and starting the motor from stationary requires even more power than keeping an already moving motor going. You might apply a reverse polarity to the previous winding for a bit to repel the magnets and then swap back. If the motor has passed the mid point this will be less effective, but still better than nothing.
If you ignore feedback from the axle and just cycle through the windings at the desired speed, then there definitely is a minimum speed. Because when the load is too great and the motor doesn't move and you cycle to the next phase, then next phase won't exert much magnetic field on your magnets and the next phase after that, you're working against the direction you want to go.
It's true that if you gear an EV motor, the torque should be able to be geared down to any speed. But typically pure EVs are direct drive, because usually they have enough torque for the low speeds and adding a gearbox just adds inefficiencies. But for a heavy load, maybe gearboxes are required. As a disclaimer, I don't know if EV trucks have gearboxes or not, but certainly EV cars tend not to have except for hybrids.
I'm actually not so sure. If the gradient is so steep that the vehicle is struggling to move at all, the current through the motor windings will be very high, causing the windings to overheat, potentially fail and potentially short circuit. There's a high risk of damaging the MOSFETs in the motor controller, which very much could lead to a fire risk depending on the failure mode.
There's not really many ways to solve this problem - in a normal 3-phase winding, all you can do is remove the current until it cools down and try again, but that will force the motor to stop and then try to restart, so creating an even larger load. Possibly if you have 6 more more phases and more magnets such that each of the normal 3-phases has multiple windings and magnets, you can cycle through the different ones and still keep applying some torque, but obviously this would still not really solve the fundamental problem.
Essentially the problem is the same for ICE vs EV - if the gradient is so steep and load so heavy that the engine / motor can't provide enough force, then it will be overloaded. Whether that's through pressure / shearing / excess heat in an ICE or through excess current / excess heat in EV, the outcome is failure to continue forward at best.
The only real solution is to massively over engineer the engine for normal situations, but human nature being what it is, there will always push things way beyond the designed limits and safety margins until it fails.