634
you are viewing a single comment's thread
view the rest of the comments
view the rest of the comments
this post was submitted on 06 Jan 2024
634 points (93.0% liked)
Technology
55692 readers
2748 users here now
This is a most excellent place for technology news and articles.
Our Rules
- Follow the lemmy.world rules.
- Only tech related content.
- Be excellent to each another!
- Mod approved content bots can post up to 10 articles per day.
- Threads asking for personal tech support may be deleted.
- Politics threads may be removed.
- No memes allowed as posts, OK to post as comments.
- Only approved bots from the list below, to ask if your bot can be added please contact us.
- Check for duplicates before posting, duplicates may be removed
Approved Bots
founded 1 year ago
MODERATORS
I hate Tesla and especially the Cybertruck as much as the next guy, but this was a highway test and that sounds like a completely normal result.
I own a Bolt EV which is rated for 259 miles of range. On the highway, that's more like ~220. That sounds bad, but the other side of it is that I get ~300 miles of range during my normal work commutes through the city. This is just how EVs are, the estimated range is based on a mixed test. EVs are backwards compared to ICE, you'll get ~20% less range than the EPA estimate driving highway speeds and ~20% more doing purely city driving.
I wrote this in another comment, but Tesla has been known for a long time to game EPA numbers. Here's an article from 2020 talking about it: https://insideevs.com/news/407807/eletric-car-real-world-range-tested/
Several get below their EPA numbers, but several cars also get higher. Tesla models all get significantly below their claimed mileage.
The insideevs article reports 239 miles for a Model 3 Performance while clicking all the way through to the actual source of the testing "Whatcar" reported 324 miles for a M3 LR. What car indeed. I don't believe these low numbers.
I'm sure Tesla has been overly aggressive with the range numbers. Especially people in colder climates must be getting far less than advertised. But these low-effort articles are not the best of sources.
It's worth noting that he recently did the same test, with similar temps, in the EV9 (which is also super inefficient on the highway), and got over the EPA range. IIRC most of his range tests exceed EPA numbers.
Why highways are worse than city streets? Highway doesn't have traffic jams, frequent stops when you just burn fuel\electricity to move a little further. It's just supporting the momentum of a car. With more than one gear it's trivial.
Maybe I don't understand something about e-cars, but from my experience I have wasted like 30% less of fuel just driving on highways from city to city for the same distance I drove in town.
In traffic, the largest reduction of efficiency comes from accelerating and the braking. You use energy to start moving (proportional to m V^2) and then you dump that energy into heat in your brakes to stop. The second comes from idling where you use energy to keep the engine rotating. As others have mentioned, EVs use regenerative braking so a substantial portion of the energy used to slow and stop the car is used to recharge the battery. EVs have no need to keep an engine running so unless you’re running the a/c there are minimal demands on a stopped/idling EV.
On the highway, you have the internal friction in the drivetrain to overcome, the constant deformation of the tires, and - most importantly - wind resistance, which is proportional to cd x rho x V2.
Cd (drag) and rho (air density) are low, but that V (speed) squared means driving at 75mph incurs 25x the energy use as driving at 15 mph. An EV gets no sage harbor here - plowing through a fluid (air) is essentially the same work.
To give you a sense of numbers, my vehicle (F150) gets less than 10mpg the 5 miles to my local pool/gym. The speed limit is 25 mph but there are stop signs every block or two. Lots of braking loss. On back roads with gentle curves and a 45 mph limit I get close to 30 mpg. That’s the sweet spot between overcoming transmission friction and air resistance. On the highway at 60 mph I get 22-23 mpg. At 78-79 mph I get 19 mpg. These are all generally on flat stretches using the 6 min average on my dashboard.
(Sorry for the long post…I’m an engineer and mechanical efficiency and aerodynamics are my happy place)
Ah so actually it's not that ICE vehicles are more efficient at highway speeds, it's that they are just SO MUCH worse in city driving that it only seems that way.
Interesting, I never thought about it that way
It's both. Nearly all ICEs are specifically optimised to cruise at 50mph. Anything more or less will significantly reduce your MPG.
But yeah, slow speeds are really inefficient in an ICE.
Any braking without energy recovery is wildly wasteful. Public transit (busses, trains) are fucking terrible wastes of energy due to their large mass and frequent stops. Hybrid and/or electric busses are, in this respect, potentially far superior to their diesel counterparts. I'm not a train person (engineer...train...haha) but I don't think even the all electric trains use regenerative braking and there are few battery powered trains in service.
I've spent the last year altering my driving habits when I can. I try not to be an asshole when others are around/in traffic, but when I'm not pressed I will coast to a stop as much as possible (esp uphill) and use hills to gain momentum. Over 6000 miles, I've raised my overall mpg around 18%.
For trains and subways, you can build the stations slightly above the rest of the tracks so that a train will naturally break when reaching the station, and accelerate when leaving. Efficiently storing energy in potential gravitational form. I'm not sure how frequently this is done, but at least in my city the subway does this at most stations.
Don't be. I'm happy to learn.
Don't apologise, this was a great post!
Traveling at high speeds just takes a lot of power regardless of fuel, but ICE cars are so inefficient in city driving it makes highways look good in comparison. 25-50mph might be more efficient, but every time you brake that kinetic energy is turned into waste heat, totally negating the benefit of driving slower.
EVs on the other hand have regenerative braking systems. Rather than using friction to slow the car down, they just use the motors by applying resistance to the wheels. The kinetic energy is used to charge the battery while slowing the car down. You get the benefit of slower speeds without much braking loss, so this is where EVs shine.
That sounds really sweet for a street usage.
Frequent lite braking allows the regenerative brakes to do all or almost all the work, meaning you recover a good chunk of the energy you’re using in city/stop-and-go traffic.
Infrequent braking or hard braking (which requires the service brakes) means less energy recovered, so shorter range.
Braking does not increase range. Regenerative braking reduces the losses involved, it doesn't eliminate them. Your last sentence makes it sounds like not braking enough will lower your range - that isn't the case.
Hmm. So it's a sneaky little argument for EVs for an in-city use? I wonder why no company screamed about it through a loudspeaker. If that's so, it's a killer feature for most drivers
It was one of the main selling points of the Prius (which has regenerative braking). Great gas mileage, especially in city driving.
This is correct. I'm a Prius driver. I get 45-55 mpg on the highway, depending on conditions. If it's just city driving, I get 55-65 mpg. I've had as high as 72 mpg, many years ago driving Uber downtown.
And with a plugin model Prius... you won't use the ICE power unit at all on a typical commute (25 mile battery range on the current models).
Man, don't I know it. The 2023 Prime is out with a 39-44 mile range, which would cover about half my total daily commute, plus my employer has free charging. Someday...
I'd happily trade it all for robust public transportation, though.
Not being a car guy, I missed it all.
Thanks, I'd keep that in mind.
If I’m not mistaken, the much more important element is air resistance. The power-efficiency ratio of electric motors is nearly constant, meaning the energy usage per unit distance is nearly the same at all speeds, but there is more air resistance at high speeds.
You are correct. Electric motors are 100% efficient, so a combination of air resistance at speed as well as more regenerative braking being done during city driving are the reasons for better city driving effective mpg.
Umm what? Have you never heard of the laws of thermodynamics? I mean, the overall thrust of your argument is correct, but that statement is just nonsense.
Oh, and using regenerative brakes will always waste more energy than not using brakes at all and simply moving at a constant speed. Regenerative brakes are only efficient compared to conventional brakes, which waste all of their energy as heat. Braking more in the city driving doesn't improve an EV's efficiency there compared to using that same EV on a highway (aside from the fact that you're driving more slowly overall,) it just improves its relative efficiency compared to ICE vehicles with conventional brakes.
That's the same for a fuel-based cars. Some other users told me about the black magick trickery electric cars do in the city. Guess you'd like to check their replies too.
Gasoline powered engines are very inefficient in terms of getting all of the potential energy out of it's fuel source. They do have a sweet spot, usually somewhere between 2,000 and 3,500 rpm and most companies will tune their vehicles to be at highway speeds while in that sweet spot. The rpm range from idle (ICEs needing to idle at stoplights decrease city MPG) up to that sweet spot is less fuel efficient than rolling around in the sweet spot, so a lot of stop and go driving will see a gas motor running out of it's fuel efficient range quit a bit.
Electric motors have the same efficiency at any rpm and they don't use energy while the car is sitting still.
I saw those after I posted, and I’m a bit surprised. I always thought the reason EVs lose range on highways was closely related to the reason they don’t have gears. I guess I’ll have to revisit it.
Excuse the meta conversation but why are people downvoting what seems to be an earnest question?
Lighten up and just answer the question.
In addition to what atmur said, EVs don't have the baseline inefficiencies an internal combustion engine requires just to keep itself running. ICEs waste a huge amount of energy just running, which gets lost as heat, vibration, and noise. EVs have the advantage of being able to run just as much as needed, so you don't throw away huge amounts of energy at low speeds.
The efficiency curve of an ICE vehicle generally peaks somewhere around 70-90km/h, due to a combination of wasted energy at low speeds and gearing ratios. EVs peak much lower, generally in the 35-55km/h range. This is due to not having the low speed overhead of an ICE, but still being subject to high speed inefficiencies like rolling resistance and drag.
In fairness there is some baseline inefficiency in that they have to keep the battery pack in an acceptable temperature range. How much of an inefficiency that is depends on outside factors, but it's there.
also, the EPA test cycle for highway has a maximum speed of 60mph, with the average below 50
Yeah this is what it's like with my mach e as well. I have an extended range and I get 300ish miles town/city but on the highway probably 240. So realistically I charge every 190-220 miles on long road trips with the 80% fast charging stations.
I have had my bolt, with new battery, for nearly 3 years. On interstate driving in summer I'm not sure I'd go beyond 180 miles, pretty sure 200 miles I'd be in turtle mode at least. Currently in winter I'm probably limited to about 160 tops.
And EVs and hybrids have regenerative braking so that does some recharging of the batteries. It's not going to be stellar, but in stop and go traffic, it could definitely had some miles to range. There's a lot less stopping on highways.
Regenerative braking isn't magical. It doesn't add range. It reduces range lost by stopping. Conservation of energy is still a thing.
If you were to drive any speed uninterrupted until the vehicle died, then attempted the same drive with stops every mile, the vehicle wouldn't make it to the end.
This is true, but it's neglecting one variable that does complicate the math slightly. There is greater air resistance at highway speeds. IIRC at 60mph 50% of your power is lost due to the air resistance.
So yes, if we lock the speed to a fixed value and compare them, then regenerative of course doesn't increase the range more than not stopping at all. But that's the nuanced gap in the discussion where misunderstanding is going to reside. That's why you two are on different pages. Someone is assuming equal air resistance (speed), and someone is assuming a comparison of average city miles vs highway miles.
Neither is necessarily the ONLY way to look at it. It's all relative.
Important to note though that air resistance is nonlinear
it uses less energy to get from A to B slowly than quickly.
Semantics. Regenerative braking adds miles of range compared to those without.
Yes, but it's unrelated to highway versus city performance in electric/hybrid cars.
Driving under highway speeds is almost always more efficient due to wind resistance. But for ICE cars without regenerative brakes the losses from braking and idling hurt enough to give the illusion of freeway efficiency.
And the reason actual highway speed versus the estimates on the sticker are often so far off with ICE cars is that the test is based on 55mph max highway speeds with an average speed of 48mph. Meanwhile the speed limits on all the freeways near me are between 75 and 85, making actual performance way, way worse.
It's not only not "not stellar", it's the reason hybrids and EVs have higher city miles than highway. Acceleration takes a lot of energy compared to maintaining speed, and regenerative breaking recaptures most of that for use next time you take off.
It's why hybrids can increase fuel mileage so much without being plugged in. It's huge.
If you're ever driving an EV that has the option to see real time flow of watts (with numbers, not just graphics), watch what it does while taking off, coming to a stop, and cruising at speed (both slower city speeds and higher highway speeds). You'll probably see fifty to a hundred plus kW flow either direction while speeding up and slowing down, and under five while cruising