Investor Engineering Discussions

[transpondster]

I think petit_bateau's point is that reversing the motors to actively brake, instead of regen, would generate significantly more heat (and much more quickly) than anticipated.

So I think we should look at it this way:

How much power (in KW) does it take to go uphill at a constant speed? That should be the same power (in the form of heat) that you'd need to dissipate going downhill. As long as the motors + cooling system can convert that to heat and dissipate it out into the air, then there's plenty of "braking" capacity to go downhill, even without regen.

Edit: Relying on the friction brakes to "travel" downhill is a no-no. That's riding the brakes and is a sure way to deplete your available braking reserves.

82000 weird weight units are about 37200 kg. Let's assume Semi is traveling 90km/h ~56mph at 6% descent
37200 * 9,8 * 90000 * ( 6 / 100) / 3 600 000 it collects potential energy at ~550 kW rate. From Elon consumption is 1,7kWh/ miles means ~100 kW energy needed for rolling and aero resistance. That means it needs to regen ~450 kW. There is no way in hell it would be able to remove 450 kW as heat

 

[nativewolf]

82000 weird weight units are about 37200 kg. Let's assume Semi is traveling 90km/h ~56mph at 6% descent
37200 * 9,8 * 90000 * ( 6 / 100) / 3 600 000 it collects potential energy at ~550 kW rate. From Elon consumption is 1,7kWh/ miles means ~100 kW energy needed for rolling and aero resistance. That means it needs to regen ~450 kW. There is no way in hell it would be able to remove 450 kW as heat

Brakes are going to brake, in a normal semi we would have an engine brake assisting the regular brakes. I don’t see anything else happening here just a question of how much heat loss you have and we can be sure that the semi have been tested in this critical factor for years. It is the number one issues, they get that wrong and people die. They may have a clever solution that is simple but not patented so it might be savvy to not reveal until someone reveals it. Likely not for months yet. To pass regulation scrutiny it must be simple. It must have a failure mode of stop vehicle (if air brakes fail brakes engage).

I didn’t see how they plan to integrate air brakes into the trailers. Curious about that.

 

[mongo]

Brakes are going to brake, in a normal semi we would have an engine brake assisting the regular brakes. I don’t see anything else happening here just a question of how much heat loss you have and we can be sure that the semi have been tested in this critical factor for years. It is the number one issues, they get that wrong and people die. They may have a clever solution that is simple but not patented so it might be savvy to not reveal until someone reveals it. Likely not for months yet. To pass regulation scrutiny it must be simple. It must have a failure mode of stop vehicle (if air brakes fail brakes engage).

I didn’t see how they plan to integrate air brakes into the trailers. Curious about that.

In a motor failure situation the truck would stop ASAP, so much less kWh to dissipate. I thought engine braking would be the primary deceleration method on a downhill grade, not mechanical brakes.

What are you wondering about in terms of air brakes? Whether accelerator up regen blends them in?

 

[Oil4AsphaultOnly]

82000 weird weight units are about 37200 kg. Let's assume Semi is traveling 90km/h ~56mph at 6% descent
37200 * 9,8 * 90000 * ( 6 / 100) / 3 600 000 it collects potential energy at ~550 kW rate. From Elon consumption is 1,7kWh/ miles means ~100 kW energy needed for rolling and aero resistance. That means it needs to regen ~450 kW. There is no way in hell it would be able to remove 450 kW as heat

Thank you! This is a super simple proof that it would be VERY difficult (impossible?) to go downhill safely with a full load without regen.

 

[mongo]

Thank you! This is a super simple proof that it would be VERY difficult (impossible?) to go downhill safely with a full load without regen.

At least at a reasonable speed...

 

[nativewolf]

In a motor failure situation the truck would stop ASAP, so much less kWh to dissipate. I thought engine braking would be the primary deceleration method on a downhill grade, not mechanical brakes.

What are you wondering about in terms of air brakes? Whether accelerator up regen blends them in?

that And other things, how is the compressor powered? And more. Will be fun to learn.

 

[Oil4AsphaultOnly]

At least at a reasonable speed...

Right. I saw your post about how at 1% grade, rolling resistance and aero drag would limit the speed to 60mph.

Using transpondster's provided formula, this would be a chart of the heat dissipation required for the different steepness (without the help of rolling resistance and aero drag):
6% :: 37200 kg * 9,8 m/s/s * 90000 meters/h * ( 6 / 100) / 3 600 000 = 547kW
5% :: 37200 * 9.8 * 90000 * 5 / 100 / 3 600 000 = 456kW
4% :: 365kW
3% :: 273kW
2% :: 182kW
1% :: 91kW

Do you have a formula for calculating the terminal velocity based on grade? I'm guessing about 85mph for 2% and 100mph for 3%?

 

[nativewolf]

At least at a reasonable speed...

Of course trailers are also braking; in many areas we can no longer engine brake so truck brakes are pretty amazing. It is also helped by huge mechanical gearing In A traditional semi

 

[mongo]

Right. I saw your post about how at 1% grade, rolling resistance and aero drag would limit the speed to 60mph.

Using transpondster's provided formula, this would be a chart of the heat dissipation required for the different steepness (without the help of rolling resistance and aero drag):
6% :: 37200 kg * 9,8 m/s/s * 90000 meters/h * ( 6 / 100) / 3 600 000 = 547kW
5% :: 37200 * 9.8 * 90000 * 5 / 100 / 3 600 000 = 456kW
4% :: 365kW
3% :: 273kW
2% :: 182kW
1% :: 91kW

Do you have a formula for calculating the terminal velocity based on grade? I'm guessing about 85mph for 2% and 100mph for 3%?

My early guesstimate on max efficiency was 0.8 kWh/ mile rolling and 0.8kWh/mile aero @ 80k and 60 MPH.
Rolling mostly tracks distance, but there are higher order drag factors (oil viscosity) that may kick in. Aero drag is the square of speed, so 120 MPH would be 0.8 kWh/ mile rolling and 3.2 kWh/ mile. Total 4.0 kWh/ mile vs base 1.6 kWh/mile. So a 2.5% grade and 480kW.

Elon called out 1.7kWh/ mile which I expect is more rolling. Maybe 3% grade for 120 MPH?

Handy calculator: Potential Energy Calculator

 

[Stretch2727]

Thank you! This is a super simple proof that it would be VERY difficult (impossible?) to go downhill safely with a full load without regen.

So I guess no mega chargers at the tops of mountains!

Is the concern that there is not enough regen to slow the Semi on a steep grade or that a Semi fully charged or with a cold battery could run away?

 

[mongo]

So I guess no mega chargers at the tops of mountains!

Is the concern that there is not enough regen to slow the Semi on a steep grade or that a Semi fully charged or with a cold battery could run away?

The latter (but it's sort of similar). Downhill speed limits are lower, so less energy needed in regen. Should be able to do so, as long as the pack can absorb it.

 

[BobbyKings]

So I guess no mega chargers at the tops of mountains!

Is the concern that there is not enough regen to slow the Semi on a steep grade or that a Semi fully charged or with a cold battery could run away?

There is a simplified answer to that.

The amount of energy needed for regeneration from the top of the hill to the bottom of the hill is less than the energy needed to reach the top of the hill from the bottom of the hill.
Delta caused by drag and rolling resistance. Plus brakes.
And sure your battery will heat up going uphill.

 

[Oil4AsphaultOnly]

So I guess no mega chargers at the tops of mountains!

Is the concern that there is not enough regen to slow the Semi on a steep grade or that a Semi fully charged or with a cold battery could run away?

The context was a post from petit_bateau (in the investor thread) about a potentially newbie driver leaving near the top of a mountain with a fully loaded (and fully charged) semi truck. Dhanson865 responded that running the motor in reverse (plus other possibilities) would both slow the vehicle as well as consume charge.

After some discussion and seeing transpondster's formula, we can see that running the motor in reverse will NOT work - nor anything that ONLY relies on the powertrain and cooling system.

 

[mongo]

After some discussion and seeing transpondster's formula, we can see that running the motor in reverse will NOT work - nor anything that ONLY relies on the powertrain and cooling system.

Regen is valid.

I-5 Grapevine is 6% grade (5 miles) at 35 MPH. A 316.8 ft drop at 82k is 9.8 kWh/ mile of potential energy. Say 0.8 kWh of rolling and aero and we need to deal with 9 kWh/ mile of energy. At 35 MPH, that's a 315 kW rate. The pack is 900+ kWh, so that is < 1/3C rate. Should not be any problem for system to regen that.
Packs charge at > 1C, and Semi has three inverters at its disposal. Model S Plaid, also with three inverters, reportedly regens 185 kW or more. Dual inverter Model S/X have peak regen over 1/2 C or 50kW on a 100kWh pack.
0.5C * 900 kWh pack = 450 kW which is a 5% grade (no rolling/ aero) from the 60 MPH data set.

 

[Oil4AsphaultOnly]

Regen is valid.

I-5 Grapevine is 6% grade (5 miles) at 35 MPH. A 316.8 ft drop at 82k is 9.8 kWh/ mile of potential energy. Say 0.8 kWh of rolling and aero and we need to deal with 9 kWh/ mile of energy. At 35 MPH, that's a 315 kW rate. The pack is 900+ kWh, so that is < 1/3C rate. Should not be any problem for system to regen that.
Packs charge at > 1C, and Semi has three inverters at its disposal. Model S Plaid, also with three inverters, reportedly regens 185 kW or more. Dual inverter Model S/X have peak regen over 1/2 C or 50kW on a 100kWh pack.
0.5C * 900 kWh pack = 450 kW which is a 5% grade (no rolling/ aero) from the 60 MPH data set.

Ummm, the context of the situation was that no regen was available (because pack is 100% full).

 

[mongo]

Ummm, the context of the situation was that no regen was available (because pack is 100% full).

Yah, but regen is running the motor in reverse and you can rely on a (properly working) powertrain and cooling system. That's where you lost me on the quoted section.

 

[Stretch2727]

The context was a post from petit_bateau (in the investor thread) about a potentially newbie driver leaving near the top of a mountain with a fully loaded (and fully charged) semi truck.

Like I said avoid chargers at the top of a grade. I think a more likely scenario is a cold battery where someone parks overnight at the top and then heads down the grade in the morning.

Does anyone know how much energy the engine brakes actually absorb on a diesel. It is the majority of braking on a downhill or just a small % to save the normal brakes. Visually they seem to just make a lot of noise and not really stop the truck.

 

[mongo]

Like I said avoid chargers at the top of a grade. I think a more likely scenario is a cold battery where someone parks overnight at the top and then heads down the grade in the morning.

Does anyone know how much energy the engine brakes actually absorb on a diesel. It is the majority of braking on a downhill or just a small % to save the normal brakes. Visually they seem to just make a lot of noise and not really stop the truck.

Per Wikipedia, an engine can produce as much (or slightly more) compressive braking HP as motive. RPM dependant though.

 

[nativewolf]

Yeah Diesel engine braking is awesome, I am sure regeneration on such a massive scale as a fully laden truck is one of those things they studied, tweaked and studied and tweaked. Making sure cabling stayed cool etc. I am just very curious to see how the system works.

 

[nativewolf]

winder how much info they give potential buyers?