Why does fuel economy get better on the highway, but EV range gets worse?
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It helps to break it into two questions:
1. Why does EV range get worse at higher speeds, and does this apply to ICE vehicles too?
2. Why are ICEs so damn terrible for city driving?
Answer to 1:
The faster an object moves, the larger its drag force from air friction. Drag formulas are complex, related to geometry, smoothness, etc. but one thing is for sure: **Drag is proportional to v^2.**
Double the speed = quadruple the drag. This means that as you go faster and faster, your powertrain (EV or ICE) needs to overcome more and more drag. This is why the faster a vehicle goes, the worse the fuel economy/mileage gets. This is true for all vehicles. An ICE vehicle will get better fuel economy driving 100 km/hr than 120 km/hr.
Answer to 2:
ICEs become an exception to the rule “slower is more efficient” in city driving because of idling and frequent stops and starts and varying speeds.
When an ICE is running but the vehicle is not moving, the engine is just keeping itself alive. It’s consuming fuel and not using any of it for moving the vehicle. This hurts fuel efficiency, and it’s the reason that many new ICEs come with auto start stop technology. These engines are made to start very quickly and efficiently, and shut down when you stop at a light to save fuel and emissions.
For comparison, EVs don’t idle. If the wheels aren’t turning, the EV motor doesn’t need to turn at all. The EV still needs to power accessories, but it doesn’t need to keep a whole engine turning.
ICEs also only get their peak efficiency (which is still garbage) at consistent engine speeds and loads. Inconveniently, an ICE needs to change speeds all the time to match the speed of the vehicle. It uses a transmission to try and keep the engine at a load and RPM that’s best suitable for efficiency, but transmissions are imperfect.
City driving results in a lot of speed changes. You’re accelerating on a green light, stopping for a red, slowing for traffic, making turns, etc. All of these activities result in the ICE operating at sub-optimal efficiency.
Electric motors can be made to maintain very high efficiencies at variable loads. Most electric motors are going to be getting 90+% efficiency at all practical RPMs. Many EVs don’t have transmissions in their powertrains, simply because there’s not enough to gain by having them.
EV powertrains maintain their efficiency at varying vehicle and motor speeds, while ICEs do not.
Most importantly at city speeds is braking.
Visualize this scenario with me: You speed up from zero to 50 km/hr on a city street. Most of the fuel you burn is burnt on that acceleration. If you were on a country road with nothing for miles, you could coast for 500m without even touching the gas. But you’re in the city, and as soon as you get up to speed you have to stop at the next light. You brake from 50 km/hr down to zero, and you only got to travel about 100m total. The fuel that would have taken you 500 m took you 100 m instead.
When you accelerate, you’re converting chemical energy from the fuel into mechanical motion energy in the vehicle. When you brake, you’re converting mechanical motion energy into heat (through friction at the pads). That’s why brake rotors get very hot.
Hopefully you see the problem. When you brake, you’re throwing away perfectly good energy, and you can’t reuse it. It’s gone. The best way to get good efficiency with your ICE is to plan ahead and minimize braking. That means noticing when that light ahead has been green for a while and is probably going to turn red before you’re there, and choosing to coast early. It means not following the person in front too closely so that if they slow down you can coast before you have to brake, if you need to at all.
EVs have this same problem, but for EVs it was worth it to engineer a remedy.
EVs have two types of brakes. They have conventional friction disc brakes just like ICEs, but they also have regenerative brakes, often through the motor itself. There are different settings, but typically if you let off the accelerator in an EV, regen braking engages immediately. Essentially, the motor starts running as a generator, and instead of using electricity, it generates it. This resists turning, and slows the powertrain and wheels down, gently braking the car while slightly recharging the battery. This isn’t free energy, it’s just reclaiming *some* of the energy that would have been wasted if you used the friction brakes.
In city driving, braking is very common, so EVs get to reclaim a bit of that wasted energy, while ICEs are wasting it as heat. Braking is less common on the highway, so this disparity doesn’t exist at highway speeds.
ICEs *could* implement regen braking, but there isn’t much value. There’s not a lot of use for the regen’d electricity, and it couldn’t practically replace an alternator, since regen braking only works when braking, which means it wouldn’t be able to charge a battery while idling or highway driving without some serious engineering challenges. Those challenges are just better solved by a belt driven alternator.
So to wrap it all up:
Both ICEs and EVs have worse fuel economy/range at high speeds.
ICEs are just especially terrible for city driving because of energy wasted idling, engine efficiency losses at changing RPM, and most importantly energy lost to braking.
Regen braking is a technology (among others) that EV makers have really had to nail down before going to market. This is because EVs face the massive problem of energy density.
Some of the best Li-Ion batteries have energy densities of 265 Wh/kg, which is nearly 1MJ/kg. That’s nothing to scoff at, but gasoline is about 46MJ/kg. If an ICE were 25% efficient, and an EV was 100% and they weighed the same total mass, a single kilogram of gasoline could take a vehicle 11.5x farther than a kilogram of the best Li-ion batteries.
My compact ICE car has a range of about 600 km on a tank of gas. For an EV of the same size and weight, it would have a range of around 50-60 km. That’s simply not acceptable for consumers. In order to get into even a barely reasonable range of 300+km, the battery needs to weigh at least 5x more. The extra weight also means it’s harder to move the car and its less efficient, so add an extra 50 kg of batteries to make up for it, which is added weight and worse range. You can see the problem: adding weight hurts the range, and the only way to add range is to add even more battery weight. In order to compete with the range of ICEs, EVs need to be far heavier.
This means that EVs have to scrimp and save every bit of energy they can in order to compete on the range front. If you start with a 500 km range with a 1200 kg battery, then every little optimization you squeeze out of tech like regen braking is the option to shave 25kg of battery off without hurting range.
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