Acceleration in an EV: Is it wasteful, as in an ICE?

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F14Scott

F14Scott

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In a gas car, going hammer down from a stop introduces all kinds of inefficiencies:

Less than ideal mixture
Less than ideal timing
Less than optimum RPM range
Operation in less than top gear
Need to grossly cycle throttle to facilitate gear shifts
Probably others I haven't thought of

So, spirited acceleration in an ICE = low MPG.

But, in a one-gear EV with a perfectly flat torque curve, I'm having a hard time picturing why a WOT run to 60 MPH would be appreciably less efficient than an easy-does-it acceleration to the same speed. Maybe the battery has an optimal discharge rate range?

Am I right? Can I launch hard with range impunity? I need data...
 
It's definitely not the most efficient way to drive but spirited driving and regen can warm the battery up giving better efficiency later in your trip so its not all bad
 
I believe conventional wisdom is that slow acceleration is best for maximizing range in EVs.

The range game for EVs is about managing the (more) finite storage of energy over a distance. Since you get maximum torque from ~0RPM, you don't need to apply full power to get moving. Yes, more power will get you moving faster, but at the cost of many more electrons over a much shorter distance. This is why Green mode really dulls the accelerator response.

An analogy: You can chug a can of your favorite beverage in 2 minutes or you can nurse if for 2 hours--either way, you drink the same amount, but the experience you get is very different. The E-Power gauge basically shows you how quickly you're using your energy supply for acceleration, from "sip" to "chug".
 
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Nigel Warnes said:
It's definitely not the most efficient way to drive but spirited driving and regen can warm the battery up giving better efficiency later in your trip so its not all bad
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Concur on the battery efficiency thing with warming. But, to simplify my question, let's assume the car is plugged into the wall with a long extension cord.

Now, does a WOT run to 60 MPH and then finish the drive to a mile use more Watts than a slow acceleration to 60 MPH and then finish the mile? If so, why?
 
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F14Scott said:
Concur on the battery efficiency thing with warming. But, to simplify my question, let's assume the car is plugged into the wall with a long extension cord.

Now, does a WOT run to 60 MPH and then finish the drive to a mile use more Watts than a slow acceleration to 60 MPH and then finish the mile? If so, why?
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Let's remove the complexities of efficiency of the motor controller(s) and drivetrain, etc... and go to the old measure of horsepower... work done over time... so, it stands to reason, that if you are doing the same work, but over less time, you are using more resources... so on that basis, I would say, yes, it uses more watts.
 
MichaelC said:
I believe conventional wisdom is that slow acceleration is best for maximizing range in EVs.

The range game for EVs is about managing the (more) finite storage of energy over a distance. Since you get maximum torque from ~0RPM, you don't need to apply full power to get moving. Yes, more power will get you moving faster, but at the cost of many more electrons over a much shorter distance. This is why Green mode really dulls the accelerator response.

An analogy: You can chug a can of your favorite beverage in 2 minutes or you can nurse if for 2 hours--either way, you drink the same amount, but the experience you get is very different. The E-Power gauge basically shows you how quickly you're using your energy supply for acceleration, from "sip" to "chug".
Click to expand...
On conventional wisdom - This is at the heart of my question... I believe conventional wisdom is based on ICE cars. I'd like the science behind an EV losing efficiency under hard acceleration.

On your chugging a drink analogy - it kind of proves *my* point, I think: Whether you sip or chug, the big gulp is in your belly; the rate makes no difference. The experience is different (fun acceleration vs. boring creep to speed), but the result is the same.

I just look at the torque curve of an EV (it's not a curve, but a straight line) and imagine that the potential energy of the trons delivered correlates directly to kinetic energy in the car, unlike the bent curve of an ICE, where, under the conditions of acceleration, energy in does NOT equal speed out.
 
Toi said:
Send it. We didn't decide on the mini se as the champion of efficiency or range... we wanted to drive!
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No doubt. But, we aviators like our performance charts: Rmax, Max Endurance, BINGO, Min Radius, Max instantaneous turn rate, Max sustained turn rate, Best energy addition, Best climb AOA, Best cruise AOA, Corner airspeed, etc.
 
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F14Scott said:
No doubt. But, we aviators like our performance charts: Rmax, Max Endurance, BINGO, Min Radius, Max instantaneous turn rate, Max sustained turn rate, Best energy addition, Best climb AOA, Best cruise AOA, Corner airspeed, etc.
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Aye... sadly, automotive manuals (POH) feel lacking to me in regards to performance data and power curves :D
 
Toi said:
Let's remove the complexities of efficiency of the motor controller(s) and drivetrain, etc... and go to the old measure of horsepower... work done over time... so, it stands to reason, that if you are doing the same work, but over less time, you are using more resources... so on that basis, I would say, yes, it uses more watts.
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Hmm. I like that. Gotta think about why it might be wrong, but nothing's coming off the top of my head.
 
F14Scott said:
Hmm. I like that. Gotta think about why it might be wrong, but nothing's coming off the top of my head.
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OK, how about this: you have just stipulated that the work done is the same. Only the time changed. So, the horsepower is different, which seems right, as a 189 HP electric motor will get you to speed faster than a 100 HP motor, but, at the end of the day, one has moved a 3000 lb. car one mile. That's where I'm hung up. Does faster matter, in terms of Watts spent?
 
F14Scott said:
OK, how about this: you have just stipulated that the work done is the same. Only the time changed. So, the horsepower is different, which seems right, as a 189 HP electric motor will get you to speed faster than a 100 HP motor, but, at the end of the day, one has moved a 3000 lb. car one mile. That's where I'm hung up. Does faster matter, in terms of Watts spent?
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Well, watts too, are a statement of work done over time... I probably should have used that term instead of horsepower... so then the question is distilled, does watts spent physically equate to watts consumed electrically? Which brings the question of efficiencies of drivetrain back in...

that being said, whilst WOT vs gentle would use more watts electrically to satisfy the demand of watts physically, you are still leagues more efficient than an ICE would be doing the same activities.
 
F14Scott said:
Concur on the battery efficiency thing with warming. But, to simplify my question, let's assume the car is plugged into the wall with a long extension cord.

Now, does a WOT run to 60 MPH and then finish the drive to a mile use more Watts than a slow acceleration to 60 MPH and then finish the mile? If so, why?
Click to expand...

From what I can gather, there are two things here, but neither would seem to be huge.
1) In your scenario above, your average speed over the mile is close to 60MPH in the fast case and as low as 30MPH in the slow case. Higher speed for a longer period of time = more losses to aero drag.
2) Batteries have "internal resistance", which causes energy lose (which essentially leads to warming the battery). Power loss through a resistor is proportional to current squared, whereas the power output of the battery is proportional to the current. In other words, double the power out of the battery, and you quadruple the energy losses within. Now that's only for half the time, but you are now back down to double the total energy lost due to internal resistance.

I can't think of other factors off hand, but I'm sure there are some.

I think the issue is more likely to be cumulative. Someone who is driving spiritedly is not going to just goose it to 60MPH and then hold a flat speed for the rest of the drive. More likely, they are going to accelerate more quickly more often and those incremental losses add up.
 
GvilleGuy said:
Andy Miles attempted to address this very question in 2018. He thinks the rapid acceleration causes losses in the system (air resistance, rolling resistance, frictional losses in the drive train).

https://cleantechnica.com/2018/04/16/how-ev-range-is-affected-by-quick-acceleration/
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Ah, ha! This was a great article. Based on it, and the excellent EE lesson by @GetOffYourGas, I am seeing these factors as important:

Air resistance, once up to speed, is greater
Rolling resistance, once up to speed, is greater
Reduction gearbox and other transmission gears experience more torque and, therefore, more loss
Increased loss from a battery delivering more current

What doesn't seem to be important, is the acceleration, itself. For grins, I plotted a time/distance graph for two cars accelerating to 60 and then completing a quarter mile. The areas underneath each graph (which I assume represent energy) are identical.

Quarter mile run.webp
 
My underdeveloped knowledge of physics leads me to concur that the motor efficiency should be pretty consistent, going "faster" is just spinning things more. So yeah, the various resistances and friction are going to be greater. On the other hand, don't wind turbines "make" more electricity by spinning faster, and isn't an EV motor the opposite so it would cost more electricity to go faster?

What we have seen is running the SE on the track for a period of time causes the batteries to heat up to a point of losing efficiency, but I doubt a quick acceleration from an intersection is going to heat them up that much.

What we really need is some performance tests of an EV in a vacuum...
 
A real-world consideration would seem to be that if you're always giving it full-current, you're also probably going to need to use friction-braking more often.
 
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