Why do Electric Vehicles have higher fuel efficiency at sub-highway speeds?

Question

For consumer vehicles, it seems that most people understand that while conventional gasoline motors operate most efficiently at highway speeds, electric motors provide better mileage for "city" driving than "highway" driving, at lower speeds.

I ask because when this arises in conversation, with electric vehicle owners and others alike, the discussion either asserts or implies that Regenerative Braking accounts for this phenomenon. But this answer does not sit right with the physicist in me (and I apologize if this belongs on Physics.SE). If indeed Regenerative Braking provides the better efficiency, wouldn't the most efficient driving involve lots of quick changes in speed (i.e. driving constantly fluctuating between 60-80 km/h would use less energy than the same drive at 70 km/h)?

So, why do electric vehicles get better mileage at "city" speeds than they do at "highway" speeds? If the answer is Regenerative Braking, I can accept that, but I would like some explanation, and I prefer Physics over Statistics.

Answer 1

Combustion engines have greatly varying degrees of efficiency, depending on the load and rpm. This is usually shown in a specific fuel consumption diagram like this: Generally, lower speed and higher load results in less fuel required for the same amount of power. This applies to electric motors too, but is far less pronounced there - to the point where it can be disregarded in practice.

For every ICE, this creates a fuel consumption sweet spot. Higher speeds at the same fuel consumption per hour would result in better mileage, but drag forces due to the wind also add more load to the engine at those speeds. Therefore, mileage improves up to around 50 mph, and drops off after that again - the exact optimum is slightly different for every vehicle.

Since electric cars don't experience improved fuel efficiency at the higher loads generated by higher speeds, they are most energy efficient at lower speeds.

Regenerative breaking only helps when you want to vehicle to decelerate anyway, and thus does not play a role when considering constant speeds. It still results in energy losses in the "generator", and should be avoided if possible - gliding is always more efficient, whether it's a combustion engine or an electric motor, it's just that the electric motor can still regain some of the energy otherwise lost as heat when you have to break.

Answer 2

It is unclear to me what you are comparing with what

• electric vehicles in the city vs on the highway

• city usage of combustion vs electric.

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For both types, the most efficient energy use will be at a steady speed.

For both types, the resistance to motion is significantly worse at a higher speeds than when travelling slowly.

So for an electric motor, the most efficient travel will be at the slowest steady speed possible.

That would be true for combustion engines too, were it not for the fact that they need gears, and I determined (empirically) in my previous diesel car that its fuel consumption was best at the slowest speed it will go in top gear, which was about 40 mph. Any faster, and the fuel consumption increased. Any slower, and the gear shifted down, again increasing the fuel consumption.

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With electric propulsion, for normal braking, the energy is reclaimed by reversing the flow of electricity so instead of the battery powering the motion, the motion powers the battery: "regenerative braking" (but you don't get back all of the energy that was used to accelerate).

For a combustion powered vehicle, that energy is lost, wasted, dissipated as heat through the brakes.

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So in slower speed urban driving, the electric vehicle has it for efficiency: it is more efficient both under power and when braking.

But on the open highway, there is less of a difference between the two types of engine, from the speed factor, and less braking too.

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As for a hybrid vehicle: combustion and electric power. For short urban journeys under battery power, they do much better than purely combustion engines, for the reasons stated.

However, on the highway, they can't go far on the battery alone, so they have to burn fuel. Now, it is better to power the wheels directly, than to convert the energy to electricity, and drive the wheels with that, because no energy conversion process is 100% efficient.

So if your journeys are mainly on fast highways at a steady speed, there is little point having the more expensive hybrid technology.

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Edit:

The simple answer as to why an electric vehicle is more efficient in urban use than on the highway, is because there is considerably less resistance to motion at lower speeds. Regenerative braking doesn't affect that at steady speeds, but it reduces the energy wastage in stop-go running. The regenerative system can never recover 100% of the energy though, so I presume there must be a break-even point somewhere: repeated rapid acceleration to the urban speed limit alternated with sharp braking might reduce the efficiency so it's less than steady speed running on the highway.

It's the other way round with ICE vehicles, because in town they are working below their most efficient speed, as well as the unrecoverable losses due to stop-go running, and idling when stationary.

Answer 3

From a layman's point of view, augmented by my owning of an electric vehicle, I offer a broader viewpoint.

ICE (infernal combustion engine) vehicles in stop-and-go traffic continue to use energy when slowing and when stopped. With either an automatic transmission or a manual transmission, the engine is operating for a portion of time at an inefficient rpm, using/wasting yet more energy.

EVs have a much broader range of rpm for efficiency, although I've learned that I can max mine at about 45-50 mph. Even though that's a peak performance (economy) figure, the motor is quite efficient at most speeds.

As you've noted, regenerative braking is a factor as well. I've been able to return an estimated fifteen to twenty percent although the upper figure is rare. Still, a plus is good.

Fifteen percent isn't efficient enough to justify "pulsing" one's speed. It's great on the downhills, but it never returns the energy used to climb the hill.

One would also note that the aerodynamic drag of the vehicle is greater at high speeds, although this impacts both the ICE and EV type of transport.

I would place a higher value on the increased efficiency at low, stop and go speeds to reduced aero drag and no ICE type idling losses.