rate of acceleration vs efficiency

[coma24]

Along with swift vs sprint vs smooth, this is the one that has perplexed me for a while. I don't know enough about electric motor efficiency, physics and chemistry to fully understand if or why faster acceleration to a given speed is less efficient than slow acceleration to that same speed.

Yes, the power meter on the dash deflects to a greater extent, but it does so for a shorter period of time. If there a non-linear consumption of energy required to achieve that change in acceleration? If so, then that would answer the question.

I realize that Acceleration is based on Force over Mass (drag notwithstanding), but I suspect there's a bunch of non-linear stuff going on with regards to the production of said Force.

I'm not talking about flooring it every time, but I'm comparing somewhat brisk acceleration vs regular ICE distracted driver acceleration off the line. Does it matter?

My uneducated gut is that commanding a linear increase in acceleration (from a given speed) results in exponentially more power use. In other words, accelerating slowly to 60mph will be the most efficient to get to 60mph. Accelerating more aggressively to 60mph will result in more power being expended compared to the first case, while flooring it to 60 will result in the greatest expenditure of energy. I _hope_ I'm wrong, though!

Thanks for any help from those who are well versed in EV motor and battery tech!

 

[borski]

Along with swift vs sprint vs smooth, this is the one that has perplexed me for a while. I don't know enough about electric motor efficiency, physics and chemistry to fully understand if or why faster acceleration to a given speed is less efficient than slow acceleration to that same speed.

Yes, the power meter on the dash deflects to a greater extent, but it does so for a shorter period of time. If there a non-linear consumption of energy required to achieve that change in acceleration? If so, then that would answer the question.

I realize that Acceleration is based on Force over Mass (drag notwithstanding), but I suspect there's a bunch of non-linear stuff going on with regards to the production of said Force.

I'm not talking about flooring it every time, but I'm comparing somewhat brisk acceleration vs regular ICE distracted driver acceleration off the line. Does it matter?

My uneducated gut is that commanding a linear increase in acceleration (from a given speed) results in exponentially more power use. In other words, accelerating slowly to 60mph will be the most efficient to get to 60mph. Accelerating more aggressively to 60mph will result in more power being expended compared to the first case, while flooring it to 60 will result in the greatest expenditure of energy. I _hope_ I'm wrong, though!

Thanks for any help from those who are well versed in EV motor and battery tech!

You will be driving faster sooner, so there is more air drag sooner and longer. There will be more heat (waste) given the higher power used for acceleration, and then driving longer at a higher speed.

The battery and cabling suffer I²R losses. The motor windings, cables and the inverter all have some resistance (R), which is why motors and inverters need cooling systems; the resistance causes them to get warm when they pass current. The more current they pass (i.e. the more power the motor is delivering) the greater the losses, and because these losses are proportional to the square of the current (I) if you double the current (so double the power) then the losses increase by a factor of four.

The resistive losses are everywhere: within the cells, in the wiring, the inverter and the motor windings… and these make up most of the overall losses in the drive train. There are some eddy current losses at high motor speeds, but generally these tend to be a lot lower than the I²R losses, as motor design has pretty much reduced these losses to a negligible level over the normal motor RPM range.

Regeneration can never recover energy lost as heat, and regen itself has I²R losses that create more heat.

And you possibly overshoot the target speed in your fast acceleration, and then slow down, vs a slower acceleration.

Tires is the big cost here though. Hard acceleration, in a heavy EV, is the best way to kill the tread on your tires.

But: it is still far more efficient than an ICE vehicle traveling at the same speeds and accelerating at the same rate.

 

[MGRMLN]

You will be driving faster sooner, so there is more air drag sooner and longer. There will more heat (waste) given the higher power used for acceleration, and then driving longer at a higher speed.

The battery and cabling suffer I²R losses. The motor windings, cables and the inverter all have some resistance (R), which is why motors and inverters need cooling systems; the resistance causes them to get warm when they pass current. The more current they pass (i.e. the more power the motor is delivering) the greater the losses, and because these losses are proportional to the square of the current (I) if you double the current (so double the power) then the losses increase by a factor of four.

The resistive losses are everywhere: within the cells, in the wiring, the inverter and the motor windings… and these make up most of the overall losses in the drive train. There are some eddy current losses at high motor speeds, but generally these tend to be a lot lower than the I²R losses, as motor design has pretty much reduced these losses to a negligible level over the normal motor RPM range.

Regeneration can never recover energy lost as heat, and regen itself has I²R losses that create more heat.

And you possibly overshoot the target speed in your fast acceleration, and then slow down, vs a slower acceleration.

Tires is the big cost here though. Hard acceleration, in a heavy EV, is the best way to kill the tread on your tires.

But: it is still far more efficient than an ICE vehicle traveling at the same speeds and accelerating at the same rate.

Easy for you to say Mr. Engineer...

So...accelerating gently and keeping speeds to 75 mph or lower beats getting up to speed faster and overshooting your target speed.

 

[coma24]

That was very helpful, thank you, Borski! I'm not at all concerned for daily driving as I don't have any sort of long range needs on the daily. However, when it come to road trips, I wanted to dive a little deeper, beyond the known quantity of exponential increases in drag with speed, something that becomes very obvious when operating small airplanes.

I'll plan on moderate acceleration, then, gentle overtakes, and keeping the speed as close to the limit as I can stand on long trips if max range is the goal.

 

[borski]

Easy for you to say Mr. Engineer...

So...accelerating gently and keeping speeds to 75 mph or lower beats getting up to speed faster and overshooting your target speed.

Yup. I still drive with a lead foot anyway, but that’s because I like driving and enjoying it, and I’m not aiming for maximizing efficiency.

On a road trip, you’re right.

 

[MGRMLN]

Yup. I still drive with a lead foot anyway, but that’s because I like driving and enjoying it, and I’m not aiming for maximizing efficiency.

On a road trip, you’re right.

Oh yeah. I have yet to take my GT on a long trip since it's my daily commute vehicle, but this great info.
I have never gotten more than 3.5 miles/kWh because the car is just too fun to accelerate.

 

[coma24]

Oh yeah. I have yet to take my GT on a long trip since it's my daily commute vehicle, but this great info.
I have never gotten more than 3.5 miles/kWh because the car is just too fun to accelerate.

I'm at 2.9, lol.

 

[borski]

I'm at 2.9, lol.

This is the thing that I laugh out loud about whenever I read one of the efficiency threads. These numbers matter to me like once or twice a year. Everyone is out there hitting their 4.6’s, and I’m over here in Sprint at like 1.7 enjoying myself haha

 

[MGRMLN]

This is the thing that I laugh out loud about whenever I read one of the efficiency threads. These numbers matter to me like once or twice a year. Everyone is out there hitting there 4.6’s, and I’m over here in Sprint at like 1.7 enjoying myself haha

My normal is right around 3.0 but occasionally I like to make it into a game to try for higher.

 

[tonybot]

My Air Pure RWD is my first EV and I am still learning. I took mine for the first road trip around 560 miles round trip from North Dallas to Houston. Only charged at Houston - EA for free. Averaged 4.1 miles per KWh, with AC, phone charging, massage on and driving 2-5 above speed limit - a long stretch is 75 speed limit and drove at 80 on cruise for longer stretches of that. With occasional 85 and 90s for passing. Drove through a huge storm with wipers and windshield heaters on. I am very impressed!

 

[RM-S8]

Along with swift vs sprint vs smooth, this is the one that has perplexed me for a while. I don't know enough about electric motor efficiency, physics and chemistry to fully understand if or why faster acceleration to a given speed is less efficient than slow acceleration to that same speed.

Yes, the power meter on the dash deflects to a greater extent, but it does so for a shorter period of time. If there a non-linear consumption of energy required to achieve that change in acceleration? If so, then that would answer the question.

I realize that Acceleration is based on Force over Mass (drag notwithstanding), but I suspect there's a bunch of non-linear stuff going on with regards to the production of said Force.

I'm not talking about flooring it every time, but I'm comparing somewhat brisk acceleration vs regular ICE distracted driver acceleration off the line. Does it matter?

My uneducated gut is that commanding a linear increase in acceleration (from a given speed) results in exponentially more power use. In other words, accelerating slowly to 60mph will be the most efficient to get to 60mph. Accelerating more aggressively to 60mph will result in more power being expended compared to the first case, while flooring it to 60 will result in the greatest expenditure of energy. I _hope_ I'm wrong, though!

Thanks for any help from those who are well versed in EV motor and battery tech!

Very interesting post.

 

[Steveinarizona]

Yup. I still drive with a lead foot anyway, but that’s because I like driving and enjoying it, and I’m not aiming for maximizing efficiency.

On a road trip, you’re right.

Me too. I would put it down to "why buy a Lucid and drive it like a Buick". Just like people, the Lucid enjoys and needs regular exercise.

I'm at 2.9, lol.

Exactly what I am at on my Genesis GV60 Performance after a year and a half. 20K miles and 2.9 average. Sometimes I pass people just because it is so much fun.

 

[Kibana]

Along with swift vs sprint vs smooth, this is the one that has perplexed me for a while. I don't know enough about electric motor efficiency, physics and chemistry to fully understand if or why faster acceleration to a given speed is less efficient than slow acceleration to that same speed.

Yes, the power meter on the dash deflects to a greater extent, but it does so for a shorter period of time. If there a non-linear consumption of energy required to achieve that change in acceleration? If so, then that would answer the question.

I realize that Acceleration is based on Force over Mass (drag notwithstanding), but I suspect there's a bunch of non-linear stuff going on with regards to the production of said Force.

I'm not talking about flooring it every time, but I'm comparing somewhat brisk acceleration vs regular ICE distracted driver acceleration off the line. Does it matter?

My uneducated gut is that commanding a linear increase in acceleration (from a given speed) results in exponentially more power use. In other words, accelerating slowly to 60mph will be the most efficient to get to 60mph. Accelerating more aggressively to 60mph will result in more power being expended compared to the first case, while flooring it to 60 will result in the greatest expenditure of energy. I _hope_ I'm wrong, though!

Thanks for any help from those who are well versed in EV motor and battery tech!

Your qualitative description is right and so is what is given by others. I thought, I will provide some quantitative answer to provide some more insight. When one wants to accelerate an object of certain mass, static friction between road and tire needs to be taken into account. If we consider Lucid GT with a weight of 5400 lb (2454 kG), the force due to static friction with typical friction coefficient value of ~0.4 comes out to be 0.4*2454=981N. Please remember this number for a moment. Now in order to accelerate GT from 0 to 60mph in 3 seconds, one would need to apply 2545kg*9.8m/s^2=24941N. One needs to add 981N force to overcome friction as well and so total comes out to be 24941+981=25922N. I will skip other match to calculate the distance car will move in 3 seconds with above force but the energy required to move that distance (~8.4 meter) comes out to be 7.2wh. If I further convert it into kWh per mile, it turn out to be 1.4kWh. Sure we are not accelerating for one mile but I am trying to compare this energy of acceleration with constant highway speed energy at typical efficiency of 3.5 mile/kWh to be 1/3.5mile/kWh=0.285 kWh per mile. So the energy to accelerate is almost 5 times more than that spent by care magically driving at 60mph for a mile without acceleration !! As energy cost of acceleration depends on time^2, accelerating from 0 to 60 in 5 seconds will reduce energy consumption by (5/3)^2=2.75 times!!!

This also explains why highway driving is much more effective than stop and go. If one hypothetically accelerates once to 60 mph in 3 seconds and then continues driving at that speed (of course we could have calculated energy to accelerate to 70 or 70mph) for 100 miles at 3.5 miles/kWh efficiency, it will cost total energy of ~28kWh.(100/3.5). Compare this to 0.0072kWh to accelerate once and it is nothing compared to total energy spent over 100 miles

For bumper to bumper traffic, regenerative braking won't bring any advantage and one would keep accelerating small distances and spend lot of energy mainly due to heavy weight of the car. But if one starts increasing the distance between car and assume that one drives longer distances and can use regenerative braking then things start to turn around. Lower speed means drag is almost nothing compared to weight related force/energy needed and one can anticipate when to let go of accelerator and starts saving kinetic energy back to battery and that's why efficiency at lower speed is ridiculously high. I have created an Excel calculator to see how efficiency and range change with speed and it explains things so well even without considering second order effects.

I hope this helps!

Cheers!