Why does fuel economy get better on the highway, but EV range gets worse?
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You are looking at this from ICE perspective. Let me explain from EV perspective.
EV are very efficient with its electric motor having an efficiency of 90% and above. This doesn’t change whether it is city or highway.
On highway, vehicles are doing, typically, speeds of above 80kph. At this speed, the drag increases and the vehicle has to use more energy to go through this. This drag increases as you go faster and faster. You can watch the classic top gear video of Bugatti Veyron, where Captain slow explains, how much power Veyron needs to hit 300kph and how much it needs to hit 400kph. This is the reason why EV gets lesser efficiency on highway, not due to lack of regen or highway, but the speed. You can go at 40 to 50kph on highway and see incredible range as the aerodynamic drag is very less at those speed.
Now coming to ICE, where fuel economy is higher on highways. The point is that ICE is horrible at energy efficiency, it is less than 35% efficient. And this is even worse when you are at low speed and stop still traffic.
These two points are out of equation on highways, so you see an increase in highway efficiency when compared to city, The drag does affect those car there as well.
Guys you are all forgetting about back EMF. Correct me if I am wrong but As the EV speeds up to highway speeds it’ll start experiencing more and more back emf. It can’t change gears and go into a lower revs. Tesla has a third motor specially geared for highway speeds for this reason.
Explaining Back EMF, as a motor moves faster and faster it starts producing eddy currents which start to oppose the movement of the motor.
Electric motors are approximately equally as energy efficient at all speeds but internal combustion engines have to be tuned to be efficient in a particular band… they can be very efficient at a specific speed but terrible elsewhere or tuned to be OK across a range but not brilliant anywhere…. The issue is that drag from air resistance goes up exponentially so combustion engines are tweaked to give OK acceleration and fuel performance across a range but absolutely they focus on the specific speed that open highway testing is carried out at as it tends to be just before drag really starts to climb…. also around town the EV can use regen braking to recover energy into the battery….
Fuel economy doesn’t really get better, it’s just less bad.
One liter of DIESEL contains an energy of about 10 kWh (sorry to all physics teachers if I use the wrong expression). Even the most advanced and “clean” Diesel cars need about 5 liters for 100km on the highway = 50 kWh. (and then there are many cars that even need 10-15 liters for 100km)
My EV needs exactly 20 kWh for 100km on the highway.
5 liters (=50kWh) of Diesel cost about 7.5€ in my country.
20 kWh charged to EV directly, cost me around 8€.
Find the reason why there are still so many ICE cars on the streets.
(I know the energy contained in Diesel can’t be converted 1 to 1 to kWh, I just want to show that even after 100 years, ICEs are still very inefficient and the last 10 years didn’t bring much progress which suggests we hit at wall at that part of development)
Highway you’re going the same speed for a long period of time generally. Cars are fairly efficient for fuel when cruising about 60mph. Fuel is used up most when accelerating which you don’t need to cruising at highway speeds.
Electric vehicles can generate energy through braking, since you don’t do a lot of braking when cruising, EVs suffer as they cannot generate the energy from the stopping and starting.
Gears!
ICE cars do have gears which allow them to go faster while the engine isn’t working faster (and harder)
EV cars don’t have gears. Some were talking about adding some for highway.
Just imagine doing bicycle without gear to represent an EV car. You may be fine going 10km/h (your sweet spot) but going faster will be harder for you for little more gain.
How have none of the top answers brought up the fact that most EVs don’t have gears?!!
Because combustion engines have a more efficient operating RPM, and the car is geared so that it operates near that ideal RPM at highway speeds.
For EVs, it usually isn’t necessary to have any transmission at all, so they burn more energy to beat the wind.
It’s a bit complicated, but there are three things that primarily affect the efficiency of combustion engines:
1. The inherent issues within combustion engines
2. Rolling resistance, which is more or less constant at all speeds
3. Wind resistance, which increases by the square of the speed. Thus the wind resistance at 50 m/s is 25 times greater than at 10 m/s, 112 mph and 22mph, respectively.
With EVs, the motor is more or less equally efficient at all speeds, so the wind resistance plays a huge role in lowering efficiency at higher speeds, whereas with combustion engines, the engine’s efficiency plays a larger role than wind resistance at highway speeds. In other words, you save more fuel by operating the engine at constant ideal ranges than you lose in fuel by beating the higher wind resistance.
It should be noted here that once you drive the car faster than the ideal operating range, you quickly become significantly less fuel efficient than the EPA rating on the sticker. Those tests are done in such a way that the car is tested at average highway speeds and that efficiency number doesn’t extend beyond that
Many answers focus on RPMs of engines, that’s not really important. Lemme explain
Wind resistance is the number one thing slowing down a car. If you took away that, they would continue roll on a flat road for a very long time. It’s actually such a big factor that you could just say the wind resistance experienced = the power or effort needed from the engine to maintain the speed.
Importantly, wind resistance goes up dramatically with higher speed. At 50mph your wind resistance isn’t twice what it is at 25mph, it’s actually about 4 times as much. (Velocity is squared in the wind resistance equation)
Knowing this, you would figure that the faster you go, the worse fuel economy. This is true with electric cars! Because they are so simple.
More speed
More wind resistance
More power input to the electric motor to overcome the wind resistance
Non EV cars is where it gets complicated…
Now many of these answers focus a lot on the fact that engines like to run at a certain RPM.
That’s not too important to this question…
Whether puttering along at 25mph or cruising at 75mph, modern cars are going to use gears to find a nice happy efficient RPM.
Both gas and electric cars are going to have similar drops in efficiency speeding up from 65mph to 75mph.
The big difference is in city driving. Gas cars are TERRIBLE at city driving. Electric cars are fantastic.
Electric cars aren’t bad on the highway, gas cars are just really bad in the city. So bad that many cars use similar amounts of gas (say like 18mpg versus 20mpg, not a big difference usually) to putter around a city with hardly any wind resistance, versus to scream down the highway with a ton of it.
There are 4 things a car can be doing:
Not moving
Coasting
Slowing down
Speeding up
In a gas car:
Not moving: slow burn of gas
Coasting: slow burn of gas
Slowing down: slow burn of gas
Speeding up: big burn of gas
In an electric car:
Not moving: near zero energy usage
Coasting: near zero energy usage
Slowing down: regenerating energy
Speeding up: using energy
You’ll find when you look at city driving, probably only about 10-20% maximum of your drive are you doing the only thing that uses energy in an EV: speeding up.
And as a bonus, you can recapture some of that energy (with substantial losses) during the portion of time when you’re slowing down.
The gas car burns gas from the moment it turns on to the moment it turns off. And wastes any forward momentum as heat in its brakes.
So while the engine is on, you better hope you’re covering miles at high speeds, because it’s not that different than sitting around in traffic.
EV also perform better due to the regeneration system in stop and go traffic. ICE engines can only burn fuel and lose that energy while braking. EV harvest the energy by engine braking the vehicle thus recharging the batteries as the vehicle slows down.
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