Hacker News

points by haberman 1 day ago | 0 comments

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.

[–] defrost 1 day ago

> depending on the specifics of the trailer.

I did specifically ask about your trailer - not a generic set of trailers, but, okay.

What I'm getting is you don't have a prime mover actual truck with one, two, or three trailers to tow single or in a road train.

> the point is that most people who own the capability of transporting something, will transport it themselves.

to some degree but by no means always, sometimes it makes sense to have others do transport while getting on and doing others things in the same time.

> or you just want to be pedantic

Just familiar with transport, a great deal across a remote state 3x size of Texas, and a fair amount to other parts of the world, by air and sea.

[–] 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.