this post was submitted on 06 Jan 2024
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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 apologise, this was a great post!
Don't be. I'm happy to learn.
So, in other words, the only things EVs are good at are things that public transit would be even better at.
Consider a typical afternoon commute for a parent - you go from work to your kids school to some after school activity (different destination every day) to a few grocery stores then back to the after school activity and then finally home. If you have three kids... now you're going to three afternoon activities. And maybe two schools as well.
Sounds like it'd take about 5 hours with public transit, especially since for some of those you'd literally be getting off the bus stop, then 5 minutes later be back at the bus stop waiting for the next bus. Kids aren't allowed to wait at bus stops these days... whoever is looking after the kid will only release them when the parent comes in to pick them up. Often the parent even has to type their password into a system that checks if they are approved to pick up the kid (most kidnappings crimes are committed by family, often a parent, and staff aren't expected to keep on top of that... so they use databases).
I love public transit. But major cultural changes would be required for it to be my main form of commute.
Not quite. EVs can still do door to door transport, are faster portal to portal, and have a vastly more diverse infrastructure, including the ability to (at least in a limited extent) traverse areas without track or road infrastructure. Public transit is still better, especially for rail, in reducing energy losses due to wheel deformation, reduction of human fatigue and dependence on attentiveness, and in some cases station to station speed and net air resistance per passenger mile. Since this is technology instead of fuckcars, it seems reasonable not to circlejerk too much.
Just to list a few things EVs are good at that public transit isn't:
I don't want a car and I use public transportation, couldn't let that fly though. EVs have their place. Not to mention electric buses are EVs and are even a better riding experience.
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.
Idk if you have an EV, but I imagine the EV owner reading this thread and thinking to themselves 'Holly-molly, my car can even do that?'.
I mean, regenerative breaking isn’t that crazy. An electric motor and generator are literally the same machine, after all.
You can test it out yourself if you have a hobby motor lying around. When you short the two terminals of the motor, it becomes more difficult to rotate the axle by hand.
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.