Nashco
Posts: 537
Joined: Wed May 29, 2013 12:00 am
Location: Portland, OR

Re: Performance Boost!?

Wed Mar 26, 2014 5:57 pm

emotodude wrote:EV power is almost always claimed as "BATTERY POWER". That is the power the battery is delivering. You will lose at least 10% of that in even the most efficient vehicles before it comes out at the contact patch of the tire. The spark motor is probably in the low 90% efficiency range, the controller is probably mid 90%, you probably lost a couple percent in hi-power interconnects (even in the battery itself), and another 5% in mechanical drive-train losses. I would be shocked (pun intended :mrgreen: ) if the Spark EV was getting over 85% Total System Efficiency.

Zero Motorcycles, for whom I used to run R&D, have a very clean drive-train, single stage belt direct from motor to wheel and approach 90% total system efficiency.
Actually, that power curve shown above IS motor output power (labeled mechanical power, not electrical power). The battery output is what is on the instrument cluster, which I've never seen higher than 120 kW but regularly above 105 kW. I posted a link in another thread with lots more information from an SAE presentation that GM participated in.

viewtopic.php?f=2&t=3640&start=10

This presentation shows that the traction system (motor, inverter, drive unit) are 93% efficient at best, and at maximum power obviously less. Giving credit where it's due, GM has made this system incredibly efficient, which is a large part of why the Spark EV has the best MPGe of any vehicle ever classified by the EPA!

Bryce

PS. I'm a big fan of many parts of the Zero bikes (ditto for Brammo). I don't dare test ride one for fear of coming home with it until after I sell my current bike. :)

emotodude
Posts: 30
Joined: Tue Nov 26, 2013 4:38 pm

Re: Performance Boost!?

Wed Mar 26, 2014 6:28 pm

Nashco wrote:
Actually, that power curve shown above IS motor output power (labeled mechanical power, not electrical power). The battery output is what is on the instrument cluster, which I've never seen higher than 120 kW but regularly above 105 kW. I posted a link in another thread with lots more information from an SAE presentation that GM participated in.

viewtopic.php?f=2&t=3640&start=10

This presentation shows that the traction system (motor, inverter, drive unit) are 93% efficient at best, and at maximum power obviously less. Giving credit where it's due, GM has made this system incredibly efficient, which is a large part of why the Spark EV has the best MPGe of any vehicle ever classified by the EPA!

Bryce

PS. I'm a big fan of many parts of the Zero bikes (ditto for Brammo). I don't dare test ride one for fear of coming home with it until after I sell my current bike. :)
That's what I was saying. They are claiming 105kw "mechanical power" though they fail to state if that is at the motor shaft or what. So yeah, if they are getting 105kW at the motor shaft, or even better at the wheels, with 120kW input power that is 87.5% Total System Efficiency. Not too shabby. North of 90% is "the holy grail" for EV engineers. I have heard KERS systems in f1 cars are into the mid 90%'s now!

Skullbearer
Posts: 25
Joined: Sat Mar 15, 2014 6:47 pm
Location: Sacramento, CA

Re: Performance Boost!?

Fri Mar 28, 2014 2:45 pm

On the topic of improving performance, tires are the biggest improvement on a traction limited vehicle such as the Spark EV... but after ~30mph you are already producing maximum power. Power is power, you can only reduce weight or increase power to improve acceleration above 30mph. If you could reflash the battery controller to allow more power you could probably get it, but the motor likely can't safely put out much more. For that matter, the battery probably only has a 15-20% margin before real damage would occur.

So bottom line, tires and weight a faster Spark make.
Development Engineer
EVs, PHEVs, CVTs, ECVTs (Don't know it, look it up!)

High Voltage and Low Voltage;
They both bite, but only one is a funny story. Don't mess with the orange.

nozferatu
Posts: 575
Joined: Sat Feb 01, 2014 4:47 pm

Re: Performance Boost!?

Fri Mar 28, 2014 3:22 pm

Skullbearer wrote:On the topic of improving performance, tires are the biggest improvement on a traction limited vehicle such as the Spark EV... but after ~30mph you are already producing maximum power. Power is power, you can only reduce weight or increase power to improve acceleration above 30mph. If you could reflash the battery controller to allow more power you could probably get it, but the motor likely can't safely put out much more. For that matter, the battery probably only has a 15-20% margin before real damage would occur.

So bottom line, tires and weight a faster Spark make.
Yea true. If this car was about 2500lbs, it'd be a rocket.

Also I was thinking, there really isn't a really good reason this car couldn't have been made RWD. If anything flattening out the battery pack and moving the motor to the rear would have been ideal. And where the motor is now, all that space under the hood could be for luggage, etc.

Nashco
Posts: 537
Joined: Wed May 29, 2013 12:00 am
Location: Portland, OR

Re: Performance Boost!?

Fri Mar 28, 2014 3:26 pm

Skullbearer wrote: If you could reflash the battery controller to allow more power you could probably get it, but the motor likely can't safely put out much more. For that matter, the battery probably only has a 15-20% margin before real damage would occur.
Sounds like a lot of "probably" and not a lot of actual data...every product with a warranty has a safety margin, nobody knows what those margins are or how to exceed them, at this point. Once they are exceeded, the next step is to figure out how to make those parts more robust and start over again! Hot rodding 101. Obviously, weight reduction and improved traction will make the vehicle quicker, but so will adding more power...just like any other car on the road.

Bryce

Skullbearer
Posts: 25
Joined: Sat Mar 15, 2014 6:47 pm
Location: Sacramento, CA

Re: Performance Boost!?

Mon Mar 31, 2014 8:19 am

Nashco wrote:
Skullbearer wrote: If you could reflash the battery controller to allow more power you could probably get it, but the motor likely can't safely put out much more. For that matter, the battery probably only has a 15-20% margin before real damage would occur.
Sounds like a lot of "probably" and not a lot of actual data...every product with a warranty has a safety margin, nobody knows what those margins are or how to exceed them, at this point. Once they are exceeded, the next step is to figure out how to make those parts more robust and start over again! Hot rodding 101. Obviously, weight reduction and improved traction will make the vehicle quicker, but so will adding more power...just like any other car on the road.

Bryce
The core difference here is that high voltage, and EV equipment in general, is much more difficult to modify in a DIY sense, meaning GM doesn't have to account for people upping power. GM almost certainly runs everything at the most efficient point possible while maintaining driveability. The problem is that the battery pack is an off-the-shelf form factor pack from A123 (thus not being flattened etc), and A123 determines the maximum electric power from the pack that still maintains their warranty. The motor that GM made is almost certainly NOT running any significant safety factor, since motor control is much more precise than engine control, and still very efficient at max torque and power. To get more power from that you'll need to know more about their motor and motor control in general than the team of engineers that made it. Same thing with the battery.

The reason there is a lot of probably involved is because the battery BMS and motor controller are about as 'black box' as anything you'll ever find in a vehicle. Hacking and replacing them is not like an engine controller... engine control doesn't vary much, but fine tuned motor controls is very specific to the motor hardware. You can do about 98% as well with a self-tuning motor controller like a Sevcon or Rinehart, but it simply won't be as good as the GM controller unless GM just sucks at motor tuning, which I really doubt.

Believe me, me and my co-workers work with EV and PHEV powertrains every day. If we wanted to improve the powertrain performance of the Spark EV, we all agree we'd just replace the motor and battery entirely, UNLESS we find that the battery has more power available than the motor is using. I may or may not have a CAN spec for the battery at work to hook a reader up and verify, but I won't share it (covered by NDA). I could only share if the motor is the limiting factor and how much power the battery can put out, which means a motor swap may be an option. I could recommend a motor swap that would work, but you would need to get a transmission (at least a one speed Borg Warner or something) that can take the torque and power, and install a differential. The integration with the GM motor is too tight to put in any of the more powerful motors I know of, and there is no guarantee that more torque could even be run through the reduction and diff. Keep in mind that the motor is a very well known torque that has almost no pulses whatsoever, so the diff may be 'undersized' for the equivalent combustion engine torque (which may see significant pulses above average torque). It's just a whole different ball game with pure EV.

The best place to start on an EV is not to put in an AEM intake and a turbo, its to decrease vehicle weight and improve traction. Weight loss options you might not have thought of:

- AC-DC Charging Module (3.3kW OBC)
- DC-DC 350VDC to 12VDC unit
- Emergency Brake system

Would need to provide a stand in CAN unit to keep the vehicle happy, and it may still throw some faults if the DC-DC isn't there, but you can charge your 12V battery after track runs and wire up so the OBC can become an Off-Board Charger. Need to be dedicated to weight loss though!


On the rear wheel vs. front wheel question, front wheels can apply more regen torque (just like front brakes can apply more braking torque), so I'll bet GM chose it to get more range and MPGe on the EPA drivecycle. Rear wheel would have been better for performance, no doubt (plus EV torque is perfect for drifting!).
Development Engineer
EVs, PHEVs, CVTs, ECVTs (Don't know it, look it up!)

High Voltage and Low Voltage;
They both bite, but only one is a funny story. Don't mess with the orange.

Nashco
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Joined: Wed May 29, 2013 12:00 am
Location: Portland, OR

Re: Performance Boost!?

Mon Mar 31, 2014 12:52 pm

Skullbearer...I'm very familiar with how high voltage power electronics and controls work in a modern OEM application. The point of my comment was to say these things are all technically possible, even if you think they might be difficult, and that you don't know the limitations of the system so you're hypothesizing based on assumptions but not real data. If you've got actual experience with this particular system, I'd love to discuss it and compare notes, whether it's on the phone, in person, or via the web.

There are many ways to trick the system into doing something different than what it was originally intended to do. I'm not going to argue the variety of cat skinning methods as it's irrelevant until I'm actually trying them and reporting out what works and what does not work.

I'd love to see people documenting what they have tried and if they had success or not in this thread to provide data rather than theory.

Bryce

Oberon
Posts: 75
Joined: Sun Nov 24, 2013 9:47 pm
Location: Oakland, California

Re: Performance Boost!?

Tue Apr 01, 2014 12:17 am

As long as we are speculating, in other applications A123 cells are known for their high power density, that is one of their good points. The others being safety and long life. The downside is lower energy density.

The Spark pack is made of 336 20Ah 3.3V A123 flat pouch cells arranged as 112s3p. That is, 60Ah at 369V. The A123 spec sheet for 20Ah flat pouches claims a 20C discharge rate which for the Spark pack would be 1200A. At 369V = 443kW. Even allowing for sag etc the pack should be capable of 400kW. I don't think the battery will be a limitation. Indeed, a

The motor is rated for 105kW. PM motors like this don't really have strict power limits, the only way to really damage one is to get is so hot that the insulating varnish burns, or the magnets lose their strength. The rating is basically a measure of the cooling capacity of the motor, ie how much power can you feed in without it continuing to get hotter. For short bursts I'd be surprised if you could not at least quadruple the rated power input.

Power output is a different thing. If you get the extra power by increasing the current (given a fixed pack voltage that is the option) you get increased I^2 losses, ie the motor is less efficient. For drag racing or autocrossing this can be ignored. As can the heating. There is also magnetic field saturation where increasing the current past a point does not increase torque, just heating. This is probably the real limit on the Spark motor. Since it is very efficient, it can't be operating too close to saturation, so there is some headroom.

The above may be part of the reason that the motor current is limited below 30ish mph in the spark in the sense that it is mainly to keep the motor efficiency high for heavy footed drivers by limiting I^2 losses at low speeds. There will also be a limit on the current capacity of the controller power drivers.

Speculating based on the above I expect the motor could handle 50% more current and still be reasonably efficient and with no risk of damage if it was done only for limited duration. IF the voltage could be increased then the only real limit is rpm and cooling.

It may also be possible to exceed the current ratings on the controller too if you can figure out how the limiting is done. On the cheap electric bike controllers its as simple as soldering the shunt used to sense current. The Spark may be more sophisticated ;-). Again, heat is the main limitation, and given that a transistor junction has less thermal inertia than a big copper winding, it's probably easier to damage the controller. Still, the Spark is designed to last years and hopefully not have a lot of them hanging around the dealer shop having the inverter replaced, so there is probably some head room here too.

Finally, according to "reputable internet sources" (I'll see if I can find the link) the differential in the Spark EV is the same as the one in the Sonic, a somewhat larger car. I'm tempted to believe this since the Spark is clearly a parts bin car as much as possible.

Tl;dr, I'd be surprised if 150 kW was not possible fairly safely if you can get at the programming. Depending on your definition of "safely".
If simplicity worked, the world would be overrun with insects
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2016 White Spark EV with DCFC (used to have 2014 Spark EV)

Skullbearer
Posts: 25
Joined: Sat Mar 15, 2014 6:47 pm
Location: Sacramento, CA

Re: Performance Boost!?

Tue Apr 01, 2014 8:07 am

We have to be careful with speculation on the A123 cells, they don't publish all their options and have both 'energy' and 'power' cells. The cells that are commonly in this form factor pack are 'energy' cells and have a spec 10s peak discharge rate of only 10C, and a 30s of 5C.

Saturation will definitely be a hard limit on the motor, this is a cored PMAC, though having a true sinusoidal back emf means that it will be able to push a little closer to saturation without as extreme of losses (saturation will only occur for a brief period, in a trapezoidal back emf it occurs for roughly 25% of full load). Increasing voltage means exponentially increasing insulation breakdown, unless GM paid extra for higher rated insulation, but certainly possible to run higher voltage for a while. Could couple a Rinehart 480-750VDC inverter or a Sevcon Gen4 Size10 (beta stage Inverter) with a buck boost bi-directional DC-DC unit. Higher losses due to the DC-DC, but could nearly double the motor top speed, and have full torque out to twice the corner speed... seems like corner speed is around 15 mph in the Spark, so full accel up to ~30mph then. Running more current isn't just an issue with saturation though, copper loss increases with the square of current. Very efficient motors will still run pretty close to their shutdown temperature under maximum continuous load.

For example:
Motor A (based on real vehicle traction motor)
Rotor temp at max continuous load: 96C
Stator temp (liquid cooled): 74C
Shutdown temp for Rotor: 110C
Delamination temp for Rotor: 140C
Shutdown temp for Stator: 90C
Delamination temp for Stator: 140C
Efficiency: >96% (Motor and controller)

I've seen a BLDC motor with only a 20% increase over 'maximum peak current' go from its continuous heat plateau to delamination in under 30s. Safety margin is generally about 5-10% for 10s bursts, but we have no specs on the GM motor, so who knows. Someone can put a data logging clamp on one of the 3 phase cables and record the Amps rms under full throttle pulses at various speeds, we can see where it's hitting currently. My clamp meter doesn't have any sort of logging output or a remote display so I haven't done it.
Development Engineer
EVs, PHEVs, CVTs, ECVTs (Don't know it, look it up!)

High Voltage and Low Voltage;
They both bite, but only one is a funny story. Don't mess with the orange.

Oberon
Posts: 75
Joined: Sun Nov 24, 2013 9:47 pm
Location: Oakland, California

Re: Performance Boost!?

Tue Apr 01, 2014 3:40 pm

Skullbearer wrote:We have to be careful with speculation on the A123 cells, they don't publish all their options and have both 'energy' and 'power' cells. The cells that are commonly in this form factor pack are 'energy' cells and have a spec 10s peak discharge rate of only 10C, and a 30s of 5C.
Given that the 105kW / 369 volts is 285 A, with a 60 Ah pack that would be 4.75 C. I doubt that the 105kW is intended to be only a 30s rating, so it seems likely these are not the 5C cells.
If simplicity worked, the world would be overrun with insects
-----
2016 White Spark EV with DCFC (used to have 2014 Spark EV)

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