Bolt EV Acceleration Comparison

[Zoomit]

I was playing around with some acceleration models and thought this forum might be interested in the results. I created the models based on Car&Driver test data:

2017 Bolt EV - http://media.caranddriver.com/files...-review-car-and-driverchevroletboltev2017.pdf
2017 Volt - http://media.caranddriver.com/files...st-review-car-and-driverchevroletvolt2017.pdf
2014 Spark EV - http://media.caranddriver.com/files/2014-chevrolet-spark-ev2014-chevrolet-spark-ev-mar-2014.pdf
2014 Model S 60 - http://media.caranddriver.com/files...eview-car-and-driver2014-tesla-model-s-60.pdf
2014 i3 - http://media.caranddriver.com/files/2014-bmw-i32014-bmw-i3-comparo.pdf
2011 Volt - http://media.caranddriver.com/files...-volt-complete-specs-and-performance-data.pdf

The results don't perfectly match the C&D data, but it's pretty close and well within the normal variation due to test methods and conditions.
Note 1: The 2015-16 Spark EV's have a 3.87 final drive ratio, vs 3.17 in the 2014 Spark EV's, which would produce different results.
Note 2: The S 60 is the original version with the smaller ~60 kWh battery.
Note 3: The i3 is the BEV with the lighter 60-Ah (22kWh) battery.

There are a few nuances to pull from the charts, but my biggest takeaways are:
1) The Bolt EV will accelerate decently right up to 93 mph. Hitting that speed limiter will be noticeable as the acceleration falls from 0.13g to zero. This jerk is probably atypical, as many cars aren't accelerating as much when they hit their rev-limiter or they are drag limited.
2) The Bolt EV acceleration does appear to be optimized for better highway acceleration, as indicated in interviews with GM personnel.
3) At highway speeds, the Bolt EV will respond almost as well as the old S 60.
4) I'm bummed that C&D never tested the 15-16 Spark EVs, because that's what I drive.

 

[SeanNelson]

The Bolt EV will accelerate decently right up to 93 mph. Hitting that speed limiter will be noticeable as the acceleration falls from 0.13g to zero. This jerk is probably atypical, as many cars aren't accelerating as much when they hit their rev-limiter or they are drag limited.

I can't think of any reason to assume that the accelleration cut-off caused by the speed limiter will be any more abrubt than that caused by the cruise control. None of the cars I've ever driven stops accelerating abruptly when reaching the set cruise speed, and I can't imagine the Bolt being any different. And since it's all done by engine control system software, why wouldn't the speed limiter work the same?

 

[Zoomit]

I was trying to be careful about my word choice and said hitting the speed limit would be noticeable, not abrupt or jarring. There's really no need for GM to purposefully reduce acceleration when approaching that limit. I know of no cars that close the loop on acceleration when approaching a rev-limit.

My reference to jerk was in the physics sense, as it's the most appropriate term. It's the rate of change of acceleration or derivative of acceleration with respect to time.

 

[oilerlord]

4) I'm bummed that C&D never tested the 15-16 Spark EVs, because that's what I drive.

I had a deal pending on a 2014 Spark EV. Great car, but wife veto'd it only because it was too small for us. After a few months with my EV, I can tell that 3.3kW charging would have been a problem too (not sure why they didn't go with at least 6.6kW), but I digress. The car would have been even quicker if it was able to put all those 400 lb-ft down without it's traction control putting a stop to the shenanigans. Perhaps a couple of wider Michelin PS/2's in front may have helped, but no doubt would have added torque steer.

Thanks for putting those number down for us.

 

[WetEV]

EVs have constant torque up to speed where they are at the power limit.

http://cdn.shopify.com/s/files/1/0196/5170/files/04-911_and_Model_S_torque_curve-new_copy_large.png?1891

So the acceleration should be roughly constant at lower speeds, rather than falling rapidly with increasing speed.

 

[Zoomit]

EVs have constant torque up to speed where they are at the power limit.

http://cdn.shopify.com/s/files/1/0196/5170/files/04-911_and_Model_S_torque_curve-new_copy_large.png?1891

So the acceleration should be roughly constant at lower speeds, rather than falling rapidly with increasing speed.

The curves are based solely on empirical test data and not the result of physics-based equations using power, torque, mass, traction, etc.

 

[Anonymous]

4) I'm bummed that C&D never tested the 15-16 Spark EVs, because that's what I drive.

I had a deal pending on a 2014 Spark EV. Great car, but wife veto'd it only because it was too small for us. After a few months with my EV, I can tell that 3.3kW charging would have been a problem too (not sure why they didn't go with at least 6.6kW), but I digress. The car would have been even quicker if it was able to put all those 400 lb-ft down without it's traction control putting a stop to the shenanigans. Perhaps a couple of wider Michelin PS/2's in front may have helped, but no doubt would have added torque steer.

Thanks for putting those number down for us.

It should be noted that torque and gearing were changed from 2014 to 2015 for chevy spark ev... Battery got a tad smaller and efficiency went up - with no net drop in range.

jeff

 

[Zoomit]

It should be noted that torque and gearing were changed from 2014 to 2015 for chevy spark ev... Battery got a tad smaller and efficiency went up - with no net drop in range.

Indeed they are different.

Note 1: The 2015-16 Spark EV's have a 3.87 final drive ratio, vs 3.17 in the 2014 Spark EV's, which would produce different results.

 

[Michael1]

EVs have constant torque up to speed where they are at the power limit.

http://cdn.shopify.com/s/files/1/0196/5170/files/04-911_and_Model_S_torque_curve-new_copy_large.png?1891

So the acceleration should be roughly constant at lower speeds, rather than falling rapidly with increasing speed.

This is a mistake many non-engineers make. They see one graph, and assume all EV's operate this way. They also assume that what they see is a native characteristic of all EVs.

 

[WetEV]

This is a mistake many non-engineers make.

Really.

Do explain, and do show an example of an car electric motor torque curve that isn't closely resembling constant torque until it reaches peak power, and constant power less switching losses until it hits maximum RPMs.

Do show the physics of how it can be significantly different.

Oh, and I'm an engineer.

 

[Michael1]

This is a mistake many non-engineers make.

Really.

Do explain, and do show an example of an car electric motor torque curve that isn't closely resembling constant torque until it reaches peak power, and constant power less switching losses until it hits maximum RPMs.

Do show the physics of how it can be significantly different.

Oh, and I'm an engineer.

Really,

What kind of engineer are you?

Google is your friend. "electric motor torque curve".

 

[WetEV]

Google is your friend. "electric motor torque curve".

Sure. And here is one curve that you might think proves your point.

And it would, if you are directly connected the battery to a DC permanent magnet motor, like a toy car I owned when I was a kid. Not much in the way of speed control. Not practical.

Electrical Engineer.

 

[Michael1]

There you go. The torque curve for an electric car is set by the engineers, and is not a characteristic of just motor itself other than the upper limit. The torque peak, or plateau does not have to match the horsepower peak. They very easily could set a declining torque curve with the horsepower continuing to rise. That's typical for an internal combustion gasoline engine.

 

[Zoomit]

EVs have constant torque up to speed where they are at the power limit.

http://cdn.shopify.com/s/files/1/0196/5170/files/04-911_and_Model_S_torque_curve-new_copy_large.png?1891

So the acceleration should be roughly constant at lower speeds, rather than falling rapidly with increasing speed.

The curves are based solely on empirical test data and not the result of physics-based equations using power, torque, mass, traction, etc.

To add more detail to my previous statement...WetEV is correct in that EV motors have a constant torque when they are current limited. The Bolt EV motor has constant torque below ~30mph, per this paper: http://papers.sae.org/2016-01-1153/

My acceleration curves were based on C&D acceleration timings to 30, 40, 50, 60 etc, but I have no incremental acceleration data below 30 mph. With timings from 0-10 and 0-20, we should theoretically be able to extract and show the constant torque portion. This should show up as a flat acceleration vs speed curve below 30 mph.

Incidentally, the paper also shows the Bolt EV power peak is at ~43 mph. That peak is ~130 kW axle mechanical power.

 

[Zoomit]

They very easily could set a declining torque curve with the horsepower continuing to rise.

This appears to be what the GM engineers did with the Bolt EV. The torque curve is flat to ~30 mph, where it starts linearly decreasing slightly until speed reaches ~43 mph. Faster than 43 mph, the torque falls off along a more classic power-limited curve.

 

[michael]

This is typical practice for EVs. It allows the drivetrain to be designed for some limited torque, rather than for the very high torque that would otherwise be seen at lower speeds. Also improves control at low speeds (avoids tire spin, etc)

 

[Michael1]

This is typical practice for EVs. It allows the drivetrain to be designed for some limited torque, rather than for the very high torque that would otherwise be seen at lower speeds. Also improves control at low speeds (avoids tire spin, etc)

You'll often see this with turbocharged ICE's, too.