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points by porphyra 1 day ago | 0 comments

I explained this in my other comment but I'll paste it here:

The thing is that ICE vehicles are so inefficient to begin with, but a towed trailer needs the same amount of energy regardless of what is towing it. Imagine a trailer takes 1 unit of energy to move, and ICE truck takes 5 units to move itself, and EV truck takes 1 unit to move itself.

The ICE truck towing it would go from 5 units to 6 units, reducing the range by 16%.

The EV truck would now go from 1 unit to 2 units, reducing the range by 50%.

So the better your efficiency, the worse you are at towing. Ideally EV trucks would have such a huge battery that even when the range is reduced by 50%, it is still as long as a gas truck's range, but modern battery technology is still too sucky to achieve this.

[–] haberman 1 day ago

This analysis doesn't make sense to me.

An ICE vehicle isn't less efficient because the frame itself takes more energy to move, it's less efficient because the engine burns off a high percentage of its energy as waste heat.

But that will be true regardless of what the load is. If the ICE truck takes 5 units of energy to pull itself compared with 1 unit for the electric truck, that implies that the the truck chassis only actually requires 1 unit to move, but the ICE engine burns off 80% of its energy as waste heat. The engine is only 20% efficient.

Now if you start pulling a trailer, you have (in total) 2 units of energy worth of load. But if the ICE engine is still only 20% efficient, then it will take 10 units to pull both, rather than 6.

I can believe that this effect is real (electric cars suffering more range loss when towing), but it seems to require an explanation beyond this one.

[–] AngryData 1 day ago

It is more complicated with ICE though because the efficiency changes with load amount, and what power output an ICE is most efficient in can be designed with towing in mind. Ive seen 90s trucks that got 18 MPG empty on a good day, but then when loaded with twice or more the vehicle's curb weight on a trailer merely drop to 17 MPG.

Generally this is only true for ICE with an abundance of extra power to start with, but that is basically every truck and most other cars for the last 25 years.

[–] barrkel 1 day ago

It's the effeciency curve. ICEs are more efficient with heavier loads - more fuel goes to useful work.

There's also that electric cars have a marginally better drag coefficients for a few reasons - ICE vehicles have a bot less incentive and they need to use more air for cooling - so the drag from a trailer will be worse, as a percentage extra, on the average electric vehicle.

[–] mahboi 1 day ago

Why would an ICE be more efficient with a heavier load?

[–] bri3d 1 day ago

ICE are most efficient in terms of energy in -> rotation energy out near their peak torque output for a ton of reasons:

* cylinder filling is more efficient at maximum output request (on naturally aspirated engines operating in vacuum, due to less simple pumping loss, on turbocharged or supercharged engines, due to less wasted energy producing charge air and generally more optimized air/exhaust exchange). in short, air gets into the cylinder more efficiently in the first place at high torque requests

* combustion is more complete at high cylinder filling; not only does more air get into the cylinder, but it also burns more completely

* friction losses matter less at higher torque, especially on larger engines

Almost every ICE complexity advancement over the years is to hack around these basic properties: VVL and VVT to optimize pumping, direct injection, stratified injection, and dynamic compression hackery to get around cylinder filling efficiency, and cylinder deactivation / variable displacement to avoid friction and pumping loss from unnecessary engine size. but while ICE engines have made huge strides, they're still more efficient when they're loaded.

And note that this is efficiency, not absolute consumption - of course you'll consume more at higher load still, but the ratio is better. Combine this with the aerodynamics of most ICE pickup trucks and you have a scenario where ICE trucks frequently lose very little fuel economy / actual consumption when towing a trailer.

[–] mahboi 1 day ago

Thanks, this makes sense. Part of my confusion was the RPM for peak torque output isn't the full condition for it, cause you're only getting peak torque at full throttle of course. And a towing truck is at higher throttle than an unburdened one.

[–] apelapan 1 day ago

Efficient in this situation means units of fuel per units of work.

Of course an ICE uses more fuel in absolute terms on a higher load than a lower one. But the proportion of fuel wasted compared to fuel turned into propulsion improves as load is increased (at least up to a point, maybe not all the way to absolute maximum load).

Electric systems also have efficiency curves, but they are less drastic within the primary load range.

[–] mahboi 1 day ago

Er right, I understood it was fuel per units of work but still didn't get it. Makes sense after reading your comment and the other ones.

[–] danielheath 1 day ago

Engine tuning; an engine designed for towing heavy loads is much less efficient when not towing.

[–] icecube123 1 day ago

The range on my diesel truck drops in half when I pull my trailer. Roughly similar to an EV towing. It also happens in winter (not quite as big a drop). Thats what most folks care about. But people like to overstate it as a dramatic downside of EVs.

[–] panopticon 1 day ago

But the range on the Cybertruck is starting much lower, so that halving is a lot more painful.

I frequently tow between Washington and Montana, and I only have to fill up the F-350 once or twice when loaded. You would need to recharge a loaded Cybertruck 5+ times on that same route. My 11 hour drive just turned into a 15 hour drive in ideal conditions. And I don't think there are the latest superchargers available on most of the route, so the time spent charging is likely much higher.

Don't get me wrong, I like EVs and have a Model 3 as my daily driver. But I would never get rid of the gasser given today's battery tech with the number of roadtrips we take.

[–] mschuster91 1 day ago

> I frequently tow between Washington and Montana

But... why? Like, at that point it should make more sense shipping whatever freight you want to ship by train?!

[–] donkers 1 day ago

Regular people aren’t towing freight. More likely a camper or a boat or other recreational toy. Shipping those via train whenever you wanted to go on a trip would be absurd.

[–] HeyLaughingBoy 1 day ago

The answer from someone who owns a truck and a trailer would be "why would it make sense to ship by train when I have a truck and a trailer?"

[–] defrost 1 day ago

Tonnages, distance, and cost per calculations.

Locally farmers built a rail to port system as a grain carriage has more tonnage than a road train.

Two axle trailer is it? What's that max out at, two tonne?

[–] HeyLaughingBoy 1 day ago

Probably more like 7-10k lbs, so 3-5 tons, depending on the specifics of the trailer. More if it's a fifth-wheel type.

Yes, clearly there are cases when it makes sense to ship by train, otherwise they wouldn't exist. I can't tell if I'm not getting my point across, or you just want to be pedantic, but the point is that most people who own the capability of transporting something, will transport it themselves.

[–] dboreham 1 day ago

The problem is charging stations are few and far between in rural areas already.

[–] rootusrootus 1 day ago

> This analysis doesn't make sense to me.

Because the numbers are a bit off. I've towed a lot, gas trucks do not lose 15% when towing. My old F150 went from 18mpg to 10mpg. My super duty went from 16mpg to 8mpg.

The advantage is that gas is so energy dense we just routinely equip trucks with 35+ gallon fuel tanks. There's no battery option equivalent yet.

[–] numpad0 1 day ago

I think what it actually says is that EV has short ranges.

If a car held gas worth of 750mi of driving and towing would reduce it to 500mi, and your actual driving distance was significantly less than both, the difference would be unnoticed.

If a car held gas worth of just 500mi with extremely well optimized aero and driveline loss minimization and highly efficient regen, and a towed trailer would absolutely ruin that, and your driving distance was too close to the diminished range, you will notice that.

If your EV held like 12500kWh of charge for like 3000 miles of range, and it was mostly for Costco runs, towing range penalties will not be among your top concerns.

[–] brianwawok 1 day ago

It’s also the part that if you go from 400 miles to 200 miles between 5 minute gas fill ups, eh. But doubling 20-30 minute charges? Super annoying.

But boy does my model Y tow tiny trailers well.

[–] rootusrootus 1 day ago

The EV trucks usually do more like 30-40 minute charges due to the large battery. But also, gas vehicles don't do 5 minute fillups either, AAA says average is about 15 min.

[–] mh- 1 day ago

> But also, gas vehicles don't do 5 minute fillups either, AAA says average is about 15 min.

What? It takes about 3 minutes to add 350 miles of range to my gas vehicle.

[–] rootusrootus 15 hours ago

When people stop to refuel on a road trip they typically stop for ~15 minutes. According to AAA, who has no vested interest in making an argument on refueling times of gas vs electric, unlike commenters in discussions like this.

[–] testing22321 1 day ago

3 minutes on a gas pump? Slower than diesel.

In my little commuter car with a 40l tank the fastest I’ve ever filled up was 6 minutes.

[–] tempestn 1 day ago

That's part of the answer, but the other part is that towing is going to reduce range significantly regardless of your fuel. With a gas or diesel vehicle that's not a big deal, because you can fill back up in 5 minutes or less. With an EV, it takes 20+, and the fast charging locations are less plentiful than gas stations. If you're not towing, that doesn't matter too much, but when you have to charge 2-3 times as often due to towing, it becomes a real issue.

[–] InvertedRhodium 1 day ago

Fast charger stations also rarely make allowances for cars with trailers. You often have to detach the trailer in order to get into the charging bay or park across multiple.

[–] kraig911 1 day ago

Not to mention you can't really charge while having a trailer.

[–] mahboi 1 day ago

This would only make sense if the EV efficiency gain were due to lightness of the vehicle or better aerodynamics. It's definitely not the first, and don't imagine the second is a big factor but maybe I'm wrong. I'd even expect EV to help here because a heavy trailer takes more energy to get moving that can be partially recouped when regen-braking.

Is it just that these EV trucks don't have multiple gears to tune themselves for towing? Like how many EV cars are geared for city speeds but suffer on the highway. I know dual-motor ones have different ratios, but that's still 2 speeds vs 6-10. Edit: Could also be about engine tuning.

[–] porphyra 1 day ago

The second is in fact a big factor. The vast majority of a car's drag is aerodynamic drag since it grows quadratically with speed; rolling resistance and stuff are much smaller at highway speeds. In order to compensate for the tiny capacity of batteries, EVs try really hard to squeeze out better efficiency in aerodynamics and other sources. That's why EVs have flush door handles and stuff.

[–] mahboi 1 day ago

That would explain it if it's large. But if this matters so much, why don't the diesel trucks make all the same resistance/aerodynamics improvements? The only EV-specific advantage there is not needing air intake... again I have to say, intake drag doesn't sound huge but maybe I'm wrong.

[–] WALoeIII 1 day ago

The large fleet operators do care about this and do go out of their way to improve aero. Look at a UPS 18-wheeler and you'll see trailer skirts, no bug deflectors, window buffeters, etc.

Ironically, the private owner/operators are most often the ones installing accessories to make their trucks look cool and sacrificing immensely for it.

[–] pge 1 day ago

Vehicles have two primary components of resistance, air drag and rolling resistance. Rolling resistance is a function of weight, and air drag is a function of the frontal surface area (and to a lesser extent the total body surface area). For a small passenger car, rolling resistance is much lower than for a large truck, so they get much greater efficiency gains from reducing air drag. For a large truck, the rolling resistance is so much greater that the gains from reducing air drag are not large. That's why big tractor trailers don't have the same optimized aerodynamics that a tesla or a prius has.

[–] Mawr 1 day ago

1. No real market pressure — if you cared about fuel burn rate you wouldn't be looking into buying a truck in the first place. EVs on the other hand live or die by their battery range.

2. When optimizing, you want to start with the biggest problem and work your way down. ICEs are so laughably inefficient that improving aerodynamics just won't have that much of an impact.

[–] nicoburns 1 day ago

A large amount of the energy of an ICE is wasted as heat and noise. Some of that is a constant factor that isn't sensitive to load. So when increase load, it doesn't make as much of a difference to energy used.

By contract in EVs, much more of the energy is going towards moving the vehicle and therefore both weight and air resistance greatly affect the amount of energy use.

[–] mahboi 1 day ago

The heat and noise do scale with how much gasoline you burn. That's part of the whole ICE being less efficient than battery in terms of input chemical energy vs output mechanical energy. It cancels out when you're calculating regular vs towing range. The engine is the same efficiency either way.

[–] mahboi 1 day ago

Oh, it's not the same efficiency either way though, got some good explanations elsewhere

[–] thecompilr 1 day ago

You can’t recoup the braking energy when the trailers have their own brakes.

[–] mahboi 1 day ago

True that

[–] dguest 1 day ago

I'm confused by where efficiency comes into this.

The efficiency is usually defined as the energy the car can use to move itself over the energy going in (electrical, or chemical for gas). If an ICE is 20% efficient then using 5 units means you get one unit out. Towing a trailer that doubles the load means you need 2 units out, which means you need 10 units in. So you get half the range either way.

The engine doesn't get more efficient just because you hook a trailer to it: it's still 20% efficient regardless.

[–] danielheath 1 day ago

ICE engine efficiency is not a flat figure, it varies by RPM and load.

“20% efficient” is like an EVs “270km range” - how far you can actually drive an EV depends on temperature, speed, etc.

[–] kraig911 1 day ago

I think you're missing the cost of ownership in your efficiency calculation. EV's without incentives cost a lot compared to ICE so much so that the savings don't equate usually until I hear year 4 of ownership. Now that's getting better I mean look at the new Bolt or Ford Fathom 30k Retail usually (which I think is where there's a lot of room in small midsize trucks and EV's)

[–] rootusrootus 1 day ago

> EV's without incentives cost a lot compared to ICE

This is highly variable. My Lightning was cheaper than the ICE version I had been shopping for. It has been better for my wallet from day 1. And it has depreciated less. The credits did distort the market for a while, though, that is for certain.

[–] oddIdentities 23 hours ago

>So the better your efficiency, the worse you are at towing

The fuck?

[–] tonymet 1 day ago

best explanation of towing I've heard great job.

Once I discovered that the entire EV battery holds only 2 gallons-of-gas-worth of energy (roughly about 65kwh) , all of the range issues made a lot more sense.

[–] dotancohen 1 day ago

Half the range for 1/20 the energy is a win no matter how hard you spin it.

[–] tonymet 1 day ago

Not when you are towing

[–] mahboi 1 day ago

Drivers care about range and cost per mile. The EV is better for one and worse for the other, not strictly better.

[–] reassess_blind 1 day ago

I don’t think this is true at all. If towing the trailer doubles the energy required at the wheels (and both engines maintained the same efficiency under the additional load) then both ranges would be halved.

In practice, diesel engines can become more efficient under additional load, and it obviously varies between models but it’s not a 16% vs 50% difference like your example.

The reason EVs don’t tow well is they don’t have a high enough range to begin with. Diesels have nearly double the starting range of an equivalent EV, and storing additional fuel on board is an option.

[–] reassess_blind 1 day ago

Getting some downvotes on the above. I’d appreciate if those who disagree explain why they disagree.

[–] rootusrootus 1 day ago

The anti-EV crowd is dominated by people with very strong political ideology, I wouldn't take the downvotes personally.

[–] dotancohen 1 day ago

So the EV still beats the ICE vehicle threefold (2 units vs 6 units), it just _feels_ worse (half as efficient as without a trailer) so you declare it _is_ worse.

With that type of reasoning, let me guess, the competitor you work for is General Motors?

[–] nextaccountic 1 day ago

EV beats ICE in isolation but without just as much charging stations as petrol stations the advantage is theoretical. Also you need faster charging too (I heard BYD latest tech can do this, can Tesla just license?)

A federal law could easily fix this though, but you would need an administration that cares about climate change or at least about EV

[–] kraig911 1 day ago

I think the whole thing about climate is kind of disingenuous but that's another topic. Mining the minerals for EV's is utter strip mining and terrible. China is selling it's future away. Look at the story of Mountain Pass Mine in California and do 20-50x that in China at less efficiency.

[–] hansvm 1 day ago

Efficiency was brought up to explain the real problem, range. I didn't fill up between Reno and SF in my F350 towing a massive trailer and still had plenty of fuel to spare. A cybertruck would've barely made it past Sacramento with a much lighter load and would've taken much longer to fill up.

[–] mikeryan 1 day ago

Actually from Tahoe to Sac is probably one of the better stretches to pull that off, it’s all downhill.

That doesn’t change the gist of your comment however - which is correct. (I’m a happy Bay Area Rivian owner who’s made that trip often)

[–] hansvm 1 day ago

Tahoe to Sac for sure isn't bad. The uphill section before that adds 2-3 gallons by itself when I'm towing though (compared to a flat route the same distance and speeds). IIRC net fuel economy winds up being similar to a flat stretch (even the downhill stretch burns a lot of energy in your brakes and has enough flat and uphill segments -- slightly ameliorated with regenerative braking, but I don't think it moves the needle enough).

How is your Rivian by the way? Is it in any way comparable to a cybertruck or is it just far and away better?

[–] kraig911 1 day ago

IMO it is worse. And I thought I said where i worked in my original comment. You just can't get the range to work on EV's and towing (my personal opinion) large pickups have big gas tanks and it's quick and easy to fill up. It's all about ergonomics. Yes an EV with a full battery vs a ICE with a full tank EV will always be more 'efficient' but day to day it doesn't add up.

Ergonomics and ease of use are why people get cars. Ergnomics right now to me are reason why EV's in America don't sell well. Infrastructure. If every apartment complex and shopping mall had chargers or we had a novel way to charge a battery like an ICE engine then yeah it will take off. Culturally though I think we just love gas stations, emissions and pollusion.

[–] rootusrootus 1 day ago

> Ergonomics and ease of use are why people get cars.

It's why I got my EVs, for certain. It would be hard to convince me to go back to the inconvenience of a gas vehicle at this point. I'm hooked.

[–] cwel 1 day ago

They put the competitor they work for in their comment, it was not edited, it was there when you posted this reply.

Excellent sleuthing, either way.

[–] dotancohen 1 day ago

Yes, it was a dig at GM.

[–] mahboi 1 day ago

The topic is a now-discontinued EV truck, or two of them if you're counting the Lightning, and a real measured range drop when towing. And GM makes EV trucks too. So can we please make this honest and not a fanboy fight?

[–] kanbankaren 1 day ago

Dude, your comment does not compute.

Care to clarify with actual math? We are a technical audience here.