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Formula E power to weight


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#1 Motie

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Posted 23 November 2014 - 23:49

If you look in the FIA tech regs for Formula E, this is what they say: 28kW-hr is the most energy that you can take out of the "RESS". In other words, the battery. For example: if the race lasts one hour, the battery can put out 28kW for one hour. If the race lasts two hours, the battery can put out 14kW for two hours, since 14kW x 2hrs = 28kW-hr.

 

Problem:  I can't figure out from the regs if the 28kW-hr is for the full race (1 hour) or for half of the race, until the driver comes in and gets a fresh car.  So for now, I will assume that they can consume 28kW-hr in 1/2 hour.  As you will see, if it's for the full hour, things only get worse.  

 

If the race lasts 1/2 hour for one car, the battery can put out 56kW for 1/2 hour, since 56kW x 1/2hr = 28kW-hr.  But 1kW = 1.34HP. So if the battery is putting 56kW into the motors, it's the same as 75HP. Remember that this is the average over the half hour. Let's say you're accelerating for half the time. So that means you can use 2 x 56kW = 112kW during acceleration, which is equal to 2 x 75HP = 150HP. But the specs also say that the drivers have 270HP available to them when they want to use it. So if the driver has a lead foot, he is going to run out of the 28kW-hr very quickly.

 

Also, if you put 150HP of electrical power into the motor, you aren't going to get 150HP of mechanical power out, because the motor isn't 100% efficient.  But we'll ignore that for now, since I don't know the efficiency.

 

Let's compare this with the old Formula Ford series. For Formula E, total weight including driver is 800kg. For Formula Ford, the car is 400kg. Figure 70kg for the driver, and you've got 470kg. Formula Ford is (was?) around 125HP at the club level.

 

Power to weight ratio:                                                                                                                                                                                                                              

Formula Ford: 125HP/470kg = 0.27HP/kg.                                                                                                                                                                                                  

Formula E: 150HP/800kg = 0.19HP/kg average. 

 

Does anybody see a problem with my math?  This says that a club Formula Ford is going to blow the doors off a Formula E over the course of a 1/2 hour race, because the E is so heavy, and because of the 28kW-hr energy limit.  I believe this also means that a Formula E can't use much wing, because it will cause too much aero drag, which will steal precious energy out of the battery.

 

It sounds very slow.  But I may be misreading the rules.  For example, maybe regenerated power is not included in the 28kW-hr energy budget.  For the full rulebook:  http://www.electrica...Déc.2013_0.pdf

 

Thoughts/comments?


Edited by Motie, 23 November 2014 - 23:51.


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#2 gruntguru

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Posted 24 November 2014 - 00:04

You should have calculated Power/Weight using 270 bhp if that is the claimed power.

 

Your energy calculations seem OK. Motor efficiency will be about 95%.

 

28 kW.hr would be battery capacity, so regen would be on top of that. The regen upper limit will normally be similar to motor power ie 270hp but traction will be the limiting factor, particularly if the car is RWD ie no harvesting from the front wheels.


Edited by gruntguru, 24 November 2014 - 00:07.


#3 Motie

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Posted 24 November 2014 - 00:52

I'm pretty sure they can only use the 270HP momentarily.  I've sent an email to the FIA asking them to clarify the rules.

 

Another interesting question:  how often can they replace the battery?  If it's pretty frequent, then the environmental impact is much worse than burning fuel.  I'm waiting for an answer on this also.  I can't find it in the regs.



#4 Greg Locock

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Posted 24 November 2014 - 01:16

Have you been able to find a battery spec?



#5 Motie

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Posted 24 November 2014 - 01:52

Everything that I know is in the link:  http://www.electrica...Déc.2013_0.pdf  

 

The information on the popular racing websites is badly dumbed down.  The problem is that the official regs leave a lot to the imagination.  

 

What if you had a lossless system?  Then you could put 270HP in for x seconds, recover it, and use it again.  I think this means that the 28kW-hr battery usage spec becomes academic.  Your actual HP limit would be 270.  In the real world, what if you could recover enough energy to use 216HP during accel?

 

Power to weight ratio:                                                                                                                                                                                                                              

Formula Ford: 125HP/470kg = 0.27HP/kg.                                                                                                                                                                                                  

Formula E: 216HP/800kg = 0.27HP/kg.

 

Now your E car has performance equivalent to a club Formula Ford (at low speeds where aero drag is not huge).  What am I missing?  This is not impressive.  Maybe they don't care, because they are trying to build an entirely new fanbase that doesn't know about fuel-burning race cars.  But I would like to understand the technical side.



#6 Greg Locock

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Posted 24 November 2014 - 02:06

That's what the capacitors are for, so you can feed the spikes in regen back into the battery.



#7 FullWets

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Posted 16 December 2014 - 21:28

Do any of you know how detrimental would it be to have four engines, one per wheel, in a racing car? Currently available engines reach 225hp+ with a weight of 25 kg, that would mean roughly 1000hp with four wheel traction / regenerative braking. Linked to extremely small cooling needs (power electronics involved could be easily >95% efficient), smaller batteries, and wireless charging while driving (that would be challenging to implement on a race track, for sure!) you could have the most insane and advanced racing in the world and not the current Formula E that is simply not fast or technically advanced to minimally challenge the bigger categories of motosport


Edited by FullWets, 16 December 2014 - 21:34.


#8 gruntguru

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Posted 17 December 2014 - 00:07

The only detriment is the complexity and unsprung weight (if hub mounted). Apart from that the performance balance is all postive - AWD traction, differential action, torque vectoring. Formula SAE is currently undergoing a revolution because this technology - implemented well - is dominant.



#9 desmo

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Posted 17 December 2014 - 01:07

Is it worth mounting the motors inboard on the sprung mass and using drive shafts?  My guess is that wheel motors and little suspension travel will be the easiest.



#10 Greg Locock

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Posted 17 December 2014 - 01:53

Lotus did a study on the effect of unsprung mass on ride and handling, and found that perhaps it wasn't as clearcut as the base assumption for many years, that is more=bad. On rough roads I'd guess less unsprung mass has clear advantages for traction, as the wheelhop resonance caused by the almost undamped tire radial rate is the defining feature in the frequency domain.

 

Inboard motors will add a deal of complexity and weight and be more difficult to package, but has the advantage in real life that (a) there is more room for the brakes and (b) the relatively fragile motors are no longer as vulnerable to shock loads and vibration.



#11 imaginesix

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Posted 17 December 2014 - 02:53

Lotus did a study on the effect of unsprung mass on ride and handling, and found that perhaps it wasn't as clearcut as the base assumption for many years, that is more=bad. On rough roads I'd guess less unsprung mass has clear advantages for traction, as the wheelhop resonance caused by the almost undamped tire radial rate is the defining feature in the frequency domain.
 
Inboard motors will add a deal of complexity and weight and be more difficult to package, but has the advantage in real life that (a) there is more room for the brakes and (b) the relatively fragile motors are no longer as vulnerable to shock loads and vibration.

1000 hp on a lightweight car should be sufficient to give you reasonable braking without friction brakes, no?

#12 Greg Locock

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Posted 17 December 2014 - 04:37

Yes, it should. What do you do if you have to brake and your batteries are fully charged? That's not an issue for an EV, but it is for a PHEV.



#13 gruntguru

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Posted 17 December 2014 - 05:18

Yes, it should. What do you do if you have to brake and your batteries are fully charged? That's not an issue for an EV, but it is for a PHEV.

Put some heat somewhere - motor windings?

Better solution is to limit battery charge to 98% or whatever coincides with a stop from Vmax.

 

Lotus did a study on the effect of unsprung mass on ride and handling, and found that perhaps it wasn't as clearcut as the base assumption for many years, that is more=bad.

Interesting. Did they publish anything?



#14 FullWets

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Posted 17 December 2014 - 07:20

Yes, it should. What do you do if you have to brake and your batteries are fully charged? That's not an issue for an EV, but it is for a PHEV.

I guess if you start with 100% loaded batteries and have to brake at a certain point, the kinetic energy you are going to recover has already come out the battery pack, therefore you shouldn't have a problem putting it back there



#15 Greg Locock

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Posted 17 December 2014 - 07:30

@FW, except that a PHEV can fully charge its battery once it is at high speed. This is a real issue. That's why i compared EV to PHEV

 

@gg- http://www2.eng.cam....11_Hurdwell.pdf

 

http://www.proteanel...-muchthe-truth/

 

http://www.proteanel...n-Services3.pdf


Edited by Greg Locock, 17 December 2014 - 07:37.


#16 gruntguru

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Posted 17 December 2014 - 08:04

Thanks Greg.



#17 GVera

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Posted 17 December 2014 - 11:18

I'm pretty sure they can only use the 270HP momentarily.  I've sent an email to the FIA asking them to clarify the rules.

 

 

From the  Formula E page http://www.fiaformul....aspx?page=1334

 

Power

  • Maximum power: 200kw, equivalent to 270bhp
  • Race mode (power-saving): 150kw, equivalent to 202.5bhp
  • FanBoost (race-only): Temporarily increases max power from 150kw to 180kw. (Increase of 30kw / 40.5bhp)

The 270HP are only available during qualy and it seems the cars can't do two consecutive laps in that configuration.

I was at the Punta del Este race last saturday and qualy was one hot lap/one VERY slow cooling down lap for everybody, without a single exception.

Acceleration is fast, but top speed is lacking, they reached only 195km/h in PdelE ( the Uruguayan Mercedes CLA250 class reached 205km/h in the same circuit).


Edited by GVera, 17 December 2014 - 11:21.


#18 FullWets

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Posted 17 December 2014 - 18:58

@FW, except that a PHEV can fully charge its battery once it is at high speed. This is a real issue. That's why i compared EV to PHEV

 

@gg- http://www2.eng.cam....11_Hurdwell.pdf

 

http://www.proteanel...-muchthe-truth/

 

http://www.proteanel...n-Services3.pdf

Great info, thanks! :up:



#19 Fat Boy

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Posted 18 December 2014 - 23:22

So it's basically like a heavy F3 car with really good drivers. Still not feeling it.



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#20 Ross Stonefeld

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Posted 19 December 2014 - 17:14

Would it have that much downforce? It has, however, really really **** tires. They're nearly street specification.