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Comment by alexanderstears | Hacker News Reader

I hope to be proven wrong, but a battery-only truck seems incomplete.

Trains use diesel engines to power the electric motors that spin the wheels. Electric motors rock compared to internal engines, but the batteries have terrible energy density compared to fossil fuels.

It'd be nice if they offered a truck with a smaller battery and a turbine that generated electricity. It'd be super efficient compared to diesel trucks, and it could go battery only in urban areas and avoid pollution while retaining the electrification advantages of acceleration and torque control. But it would give the trucks flexibility to cross Nebraska and Iowa without charging a few times.

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I agree trucks like that are useful (and appear to be coming to market now~ish from other brands) but I think Tesla should stick to battery only.

There's enough short haul routes through stop-go traffic in pollution hotspots to keep Tesla busy for a while, and as they gain experience and reduce prices the range can expand.

The best thing about Tesla is that they're starting with battery only vehicles and throwing intelligence towards solving within that constraint.

Slow moving convoys of autonomous trucks that travel by night is one interesting possibility I'd like to see them explore.

You're right that nearly anything could improve upon the status quo, but batteries are frankly a crappy technology and this seems to use batteries intensively for not a lot of benefit.

If you can make your peace with short range, why not use a hydrogen fuel cell? Sure, hydrogen often comes from natural gas but electricity often comes from coal/natural gas (in the U.S) and it seems like they make better catalysts for cracking hydrogen from water all the time - increased hydrogen gas demand could motivate people to commercialize the technology.

Hydrogen doesn't have an appreciably higher energy density but it's much less cobalt intensive.

Battery packs could be swapped out and charged out of the vehicle. This would make a lot of sense.

It would make a lot of sense, but Tesla already floated the idea and decided not to pursue it [0]. That seems like a lot of work just to make a bad technology 'good enough'. Swapping bad batteries for more charged bad batteries doesn't change the fact that batteries are a bad technology.

[0]: https://www.teslarati.com/tesla-shuts-down-battery-swap-prog...

Personally, I would put most of the batteries on the flatbed. Have beds at either end that are charging while they are loaded/unloaded, and one truck autonomously juggling a 3rd bed between both locations.

I don't quite understand your point about reducing battery size. What would that accomplish?

It would free up weight / space for a turbine, it would mean Tesla needs fewer raw materials (and less cobalt) to make a truck.

It's the difference between a hybrid and a battery electric vehicle - the hybrid needs a much smaller battery.

Most trains don't use a turbine engine, they're usually diesel electric.

Are there any trains that use turbines? I was under the impression they're all diesel.

It's 1000 miles from Denver to Chicago

Book range of a Tesla 3 is 310 miles (maybe round down to 250). I don't think it's inconceivable they could fit enough battery into the footprint of a semi to reach that distance

A Tesla model 3 weighs 3,800 lbs. A fully loaded semi weighs 80,000 lbs. Let's just pretend that the energy requirements scale with weight -

the semi needs 20x the battery just to go the same distance, if it needs to go ~3x as far, the semi will need ~60x the capacity of a Tesla model 3.

The model 3 long range has a ~75 kwh battery pack. The Semi truck would need a battery pack that's 60*75 kwh = 4,500 kwh.

The only figure I could find on a Tesla battery is that their 85 kwh pack (which had an old architecture and chemistry) weighs 1,200 lbs.

A 4,500 kwh pack would weigh around 63,000 lbs.

A semi truck's max legal weigh is 80,000 lbs. You'd have 17,000 lbs to make the rest of the truck and load it with cargo.

Batteries have a terrible power to weight ratio.

An average truck gets 6.5 mpg, a semi truck only needs about 153 gallons of diesel to go 1000 miles - that only weighs about 1,400 lbs.

My point is this - rather than make the battery / cargo trade-offs, burn fossil fuels in the most efficient way possible and carry more cargo. It's a net win because we'll burn much less fuel near people (and burn less fuel than conventional trucks) and the trucks will be much faster - they'll interrupt traffic less.

> Let's just pretend that the energy requirements scale with weight

That would make the truck accelerate just as fast as the car at the traffic light - there is no need for that. And once it is at speed, energy requirements scales with drag - the truck may only need 2 or 3 times as much, not 20.

You're conflating energy and power.

Maybe the truck doesn't burn energy 20x as fast going down the road, but if we compare the fuel efficiency of cars to semi trucks, it's apparent that trucks burn at least 5x as much fuel as passenger cars. But the truck does need to burn 20x the energy to go up hills so the system efficiency is going to reflect the weight difference. We can decompose the friction into two categories - air friction and mechanical friction. The mechanical friction should scale linearly with the weight, air friction is the larger term at freeway speeds so maybe trucks aren't that bad but they're not going to make an electric truck that can carry a profitable amount of cargo with the current technology.

Scale the numbers accordingly, but there's a max weight of 80k lbs. Every ounce of battery cuts into the payload that pays for the truck. Battery technology just isn't there for heavy transportation - it'll get there and I hope it gets there soon but in the mean time, we should have a conversation about using fossil fuels better.

In that case I wonder if Tesla is going to try to stretch the definition of 'semi'

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