Most efficient way to drive an electric car and why?

[arusse02]

Hello,

I wanted to get a physics analysis of the best way to drive an electric car for maximum range. For example, would it be more efficient to drive on side streets at a slower speed of say 30 mph, or would it be better to get on the freeway and drive 60. Should I be accelerating as slowly as possible? Should I try to coast or rely on regenerative breaking? What factors are the most significant? Things like this and any other recommendations are welcome. I'm also looking for explanations too, so thank you in advance.

 

[sophiecentaur]

There is no definitive answer to this, except to say the slower the better.

 

[anorlunda]

@sophiecentaur was right, there is no simple answer, but here are two guiding principles.

Air drag and friction tend to increase as the square of speed.

Every time you touch the brakes, you discard energy (even if the brakes are regenerative.)

You'll never break the record of Elon Musk's Tesla. It is traveling very fast right now while consuming no energy.

 

[jrmichler]

The range record for an electric car is 670 miles: https://www.thedrive.com/sheetmetal/13221/new-670-mile-tesla-range-record-set. That's for a Tesla P100D, which has a rated range of 270 miles.

The article discusses some of the driving techniques, and how driving an electric car for maximum range is different from driving a car with an internal combustion engine.

One general rule is that coasting is better than regenerative braking, and regenerative braking is far better than regenerative breaking. The physics analysis is that you get back only part of what you put into a battery, so it is better to use your kinetic energy directly towards rolling friction and air drag.

Air drag goes up as the square of speed, while electric motor efficiency stays about the same, so slow is good. Too slow is bad because certain loads (lights, heater, AC, electronics) are constant regardless of speed. The result is an optimum speed for maximum range. The maximum range speed for electric cars is about 20 to 30 MPH.

 

[cjl]

You'll never break the record of Elon Musk's Tesla. It is traveling very fast right now while consuming no energy.

It did use over 140,000 gallons of kerosene to get to that state though (and around twice that much liquid oxygen), so I'd be a bit hesitant to declare it the economy king at this point...

As for the original question, I've seen some data to indicate peak efficiency around ~40-45mph for most cars (electric or otherwise), and you want to minimize stops and changes in speed as much as possible. With regenerative braking, they won't harm your efficiency as much as they would for a normal gas car, but your best bet is still to minimize speed changes.

 

[sophiecentaur]

It did use over 140,000 gallons of kerosene to get to that state though (and around twice that much liquid oxygen), so I'd be a bit hesitant to declare it the economy king at this point

True but its mpg is going up every orbit.

 

[Anand Sivaram]

I guess ambient temperature also plays a role on Electric car mileage. Too cold or too hot weather could reduce the mileage, Also headlight, AC etc. could drain battery. I think gasoline/petrol cars may not show those much of variance based on weather, AC etc as the efficiency is too bad already.

 

[wfpjr]

What about pulsed acceleration? Could this capitalize on the advantages of momentum and intermittent application of torque, reducing expenditure of electrical power, like a boy striking a rolling hoop intermittently with a stick to keep it turning, instead of applying impetus continuously?

 

[CWatters]

The range record for an electric car is 670 miles: https://www.thedrive.com/sheetmetal/13221/new-670-mile-tesla-range-record-set. That's for a Tesla P100D, which has a rated range of 270 miles.

That's for a production car. Others have gone further.. 1600km...

https://www.electricvehiclesresearc...cord-for-longest-range-of-an-electric-vehicle

 

[CWatters]

What about pulsed acceleration? Could this capitalize on the advantages of momentum and intermittent application of torque, reducing expenditure of electrical power, like a boy striking a rolling hoop intermittently with a stick to keep it turning, instead of applying impetus continuously?

Drag forces aren't linear. So for example increasing speed by 10mph (eg from 50 to 60mph) needs more power than you would save by reducing speed by 10mph (eg from 50 to 40mph). This suggests that you should probably try and drive at a constant speed and avoid accelerating and decelerating.

I'm not sure where most of the losses occur in the electrical system of a modern ecar but... Power is IV and resistive losses are I^2R. That also suggests you should try and run at a near constant current/constant power.

Battery charging/discharging is inefficient so you should minimise it, again that suggests driving at a constant speed is better than driving fast and then using regenerative braking.

 

[cjl]

Pulsed acceleration improves efficiency for most cars because internal combustion engines tend to be most efficient at 70-80% throttle. Adding kinetic energy to the car with the engine at this load level then allowing the engine to idle while the car coasts back down is therefore more efficient than running the engine steadily at 20-30% load. Electric motors don't suffer from this issue - in fact, they often become less efficient at very high load, so for an electric car, you should (theoretically, though I haven't looked at the data) be better off just steadily cruising at a moderate speed (I'd guess ~45mph).