[SithLord]

Hey folks,

I am trying to spec out a KERS setup for my 911. I have designed a hydraulic system that looks to have potential but would prefer an electric setup. The goal is 250+hp for around 20sec. with two front wheel motors and ultacapacitor storage.

I have researched a bit and search this forum but have not found my answers. I am drawing a blank at specs and sizing of the storage.

For reference the GT3 R Hybrid using a flywheel stores .2kwh. Output is 150kw for 8sec. Weight of the system is 265lbs.

Please help if you can.

 

[SithLord]

A picture.

 

[Nuts&Volts]

Just some quick math.

250HP is 186kW. 20sec is 0.00555hours.
So that requires 186kw*0.00555 = 1.03kWhrs of energy.

So starting there you need a battery with a C rate of 186C to make that work. Not likely to be able to afford that. So the next best option is to up the battery pack energy to lower the power burden on individual cells. I’ve seen batteries that can do 100C. Check out the cells John Metric uses to drag race, but being conservative you can get 60C cels pretty easy. So 186kW from 60C peak means you need 3.1kWh of battery. At 100Wh/kg that’s 68lbs. That’s being conservative on weight as well.

In terms of motors I’d do maybe two Emrax 228 motors. They are super light and peak above 100kW. So that’s 200kW for 55lbs. Add in two planetary gear sets at say 20lbs each. Then you need two Rinehart PM100controllers. That’s 40lbs.

So 200kW capable. 250lbs (203+cooling/wiring/mounting/axles/etc). Probably a $18-20k budget with brand new parts. Cheaper options available too, but you’ll get heavier and more custom


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[brian_]

250HP is 186kW. 20sec is 0.00555hours.
So that requires 186kw*0.00555 = 1.03kWhrs of energy.

So starting there you need a battery with a C rate of 186C to make that work. Not likely to be able to afford that.

That's probably why the stated plan was:

... ultacapacitor storage.

Capacitors with useful storage capacity exist - and have been used in some hybrids, including some buses - but are expensive. Because they are very expensive per unit energy storage capacity, and most EV builders don't need the very high power capability, they are very rarely used.

 

[Nuts&Volts]

That's probably why the stated plan was:



Capacitors with useful storage capacity exist - and have been used in some hybrids, including some buses - but are expensive. Because they are very expensive per unit energy storage capacity, and most EV builders don't need the very high power capability, they are very rarely used.

A high C rate battery beats out all ultracapacitor systems. Especially for longer than like 3sec pulls. The energy density (volume and weight) and power density (over 20sec) beats out Ultra Caps.

The caps will be heavier, take up more space, give less power (voltage loss as energy is removed), be harder to source and probably cost more. Check out John Metrics Sleeper cells at ampahaulic.com

142wh/kg
100C+ for 10-20secs.
$1.5/Wh

That’s why I ignored the original suggestion of ultra caps. They aren’t needed


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[Karter2]

I assume you realise you will have to completely re-engineer the front end of your 911. ( just for KERS on the front wheels!) using components that will mostly need custom design.
It may be more practical and cost effective to retrofit the front end components from a 100D Tesla. ?
Does anyone know the max regen capacity of either the Tesla front motor, or those Emrax motors.....critical for an effective KERS system.

 

[brian_]

Does anyone know the max regen capacity of either the Tesla front motor...

I have seen the Tesla regen rate quoted at various values, as low as 40 kW and as high as 70 kW, limited apparently by controller logic; the allowed rate depends on multiple factors including battery and/or outside temperature, and presumably battery size. The higher values may only be with dual motors. It looks like allowed regen rate is lower than allowed charging rates, and much lower than drive power levels.

 

[SithLord]

A high C rate battery beats out all ultracapacitor systems. Especially for longer than like 3sec pulls. The energy density (volume and weight) and power density (over 20sec) beats out Ultra Caps.

The caps will be heavier, take up more space, give less power (voltage loss as energy is removed), be harder to source and probably cost more. Check out John Metrics Sleeper cells at ampahaulic.com

142wh/kg
100C+ for 10-20secs.
$1.5/Wh

That’s why I ignored the original suggestion of ultra caps. They aren’t needed


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Thanks for the replies! This kinda answers my next question but not fully. So the 100C+ batteries will support the rapid charging from regen braking like caps or the flywheel? This setup will be constantly boosting power coming out of turns and overtaking. My want for the caps, of course, was about the quick charge and output.

Thanks again!

 

[SithLord]

I assume you realise you will have to completely re-engineer the front end of your 911. ( just for KERS on the front wheels!) using components that will mostly need custom design.
It may be more practical and cost effective to retrofit the front end components from a 100D Tesla. ?
Does anyone know the max regen capacity of either the Tesla front motor, or those Emrax motors.....critical for an effective KERS system.

Fabbing up a front subframe to mount these motors etc. will not be a problem. Not sure I would go the Tesla motor direction because I like the idea of possibly having torque vectoring. I will still check pricing on the parts from a recycler.

Thanks

 

[SithLord]

Looks like the front drive of the Tesla will hit the scales pretty hard without all the supporting components.

https://hsrmotors.com/hsr/products/front_small_du

 

[Karter2]

Thanks for the replies! This kinda answers my next question but not fully. So the 100C+ batteries will support the rapid charging from regen braking like caps or the flywheel? This setup will be constantly boosting power coming out of turns and overtaking. My want for the caps, of course, was about the quick charge and output.

Thanks again!

100C is the discharge rating on that lipo.
The charge rate will not be that high, and im not sure there is much detailed information regarding maximum charge rate or its effects (heat etc)
15-20C is the highest charge rate i have seen quoted.
How much regen time (braking) do you expect to have available between those discharge periods ?

 

[lastcyrol]

100C is the discharge rating on that lipo.
The charge rate will not be that high, and im not sure there is much detailed information regarding maximum charge rate or its effects (heat etc)
15-20C is the highest charge rate i have seen quoted.
How much regen time (braking) do you expect to have available between those discharge periods ?

Good point.
The maximum charge rate of the 'BRAND NEW 200C "Sleeper Cells"' is 2C according to the table on top of the battery page. The recommended charge rate is actually 0.5C.

 

[Nuts&Volts]

100C is the discharge rating on that lipo.
The charge rate will not be that high, and im not sure there is much detailed information regarding maximum charge rate or its effects (heat etc)
15-20C is the highest charge rate i have seen quoted.
How much regen time (braking) do you expect to have available between those discharge periods ?

Ah yes very good point. I missed the charge current. No idea what the practical limit is, but 15-20C is about what I’ve heard as limits as well. The ultracaps would kick ass in that regard at least.

The Emrax motors should be able to provide the same regen power/torque as it does in motoring mode. So the full 100kW is possible. The battery would be your limiter. In my Volt it only allows like 55kW max or 3C on the battery.

Doing some quick ultra cap research just to add data here.
Sleeper cells are 12kW/kg peak (2sec) and caps are close to 18kW/kg (8.5kW/kg typical usable)
Sleeper cells are 140Wh/kg and caps are 7.7Wh/kg

That’s the Maxwell K2 2.85V/3400F Cap


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[Nuts&Volts]

Lithium Titanate may be a good option.
https://www.ebay.com/itm/Lithium-Titanate-LTO-Kokam-Altairnano-11ah-Battery-cell-24-pcs-case-/152947660172
https://www.ebay.com/i/162718692045?chn=ps&dispItem=1

50-70C charge and discharge peaks and energy density around 40-70 Wh/kg

 

[SithLord]

Wow, this is really challenging and getting heavy. So I would need 66kg of those Caps?
Maybe hydraulic is the way to go... but I am dreading leaks on a road course.

 

[brian_]

Looks like the front drive of the Tesla will hit the scales pretty hard without all the supporting components.

https://hsrmotors.com/hsr/products/front_small_du

Yes, this is the complete powertrain (other than battery) for a large sedan weighing over two tonnes.

Also, from that page, an idea of how much more limited regeneration is compared to driving:

• Input power (peak) 220 kW (294 HP)*
• Input current (HV, peak) 650A DC
• Torque (peak output) 330 Nm (~243 ft/lb)
• -
• Output power (regenerative braking, peak) 45 kW
• Output current (regenerative braking, peak) 140A
• Torque (regenerative braking, peak) 115 Nm

 

[Ada Diamonds]

The goal is 250+hp for around 20sec.

To play the devil's advocate, why not go with a 700rwhp LS7 and a Renegade Hybrid kit? Would be significantly lighter, cheaper, faster, and easier to drive at the limit...

https://www.mastmotorsports.com/collections/650-749hp/products/ls7-427-black-label-harrop-crate-engine-700hp

http://www.renegadehybrids.com/

I'm all for pushing the boundaries and am here researching my own performance swap project; however, getting a proper KERS system dialed in for serious track days seems like a daunting project.

 

[Nuts&Volts]

Wow, this is really challenging and getting heavy. So I would need 66kg of those Caps?
Maybe hydraulic is the way to go... but I am dreading leaks on a road course.

Maybe even more than 66kg. I’d also hate to have a hydraulic system, but maybe it’s smaller/lighter solution.

One question that popped in my head was why do you need that power for 20sec? 250HP for 20sec would have most cars at over 140mph I would guess. Is your duty cycle that long? Are you really at full throttle for 20sec straight? Maybe you’re full throttle for 7secs then braking for 5sec. If so will that 7sec need the boost for the whole time?

What I’m getting at is you could downsize the battery or supercap overall capacity to save weight if you don’t really need that long of boost.


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[SithLord]

Maybe even more than 66kg. I’d also hate to have a hydraulic system, but maybe it’s smaller/lighter solution.

One question that popped in my head was why do you need that power for 20sec? 250HP for 20sec would have most cars at over 140mph I would guess. Is your duty cycle that long? Are you really at full throttle for 20sec straight? Maybe you’re full throttle for 7secs then braking for 5sec. If so will that 7sec need the boost for the whole time?

What I’m getting at is you could downsize the battery or supercap overall capacity to save weight if you don’t really need that long of boost.


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So no, I do not NEED 20secs. of boost. The number was wishful thinking and for if I used it for some standing start racing(1/2, 1 mile). For 7secs what do you think the weight will be?

 

[Nuts&Volts]

So no, I do not NEED 20secs. of boost. The number was wishful thinking and for if I used it for some standing start racing(1/2, 1 mile). For 7secs what do you think the weight will be?

I’ll go into a little more detail/more specific. Assume a 90% motor controller efficiency at full power

7sec would be 0.001944 hours.
250HP(186kW)/0.90 = 207kW from the battery
One pulse drains 402 Wh from the battery. At max you want to use say 10% to 95% of the battery. Buffer on each end for imbalance, error and to improve battery. But you at racing so gave you down to 10%. So 402Wh/0.85 operating window is a 473Wh battery. The titanate cells are 70Wh/kg. So that’s 6.7kg minimum based on energy needed.

Let’s look at power now. 207kW/473Wh would be a 437C battery. Not possible. If the titanate battery can truly do 50C peaks then you a battery that is 437/50 = 8.75x bigger. So that’s a 4.14kWh battery at 59kg.

So honestly that’s not much lighter and that would give you enough energy to make a 20sec+ boost. The key is in what the try C rate is of the cells.

The 2.9Ah Toshiba Titanate cells have a 65C rate at 50% SOC for 10sec. So let’s say at higher SOC and 7sec you can do 75C. You’d need 2.55kWh to make the power. But at 46Wh/kg you’d be at 55kg or battery.

So I guess I can figure out what 55kg of supercaps would get you in terms of total power for how long.





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