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Discussion Starter · #1 · (Edited)
Hi all,

having read the "I want to build an EV! Where do I start?" sticky thread - here I am :)

The idea would be for the following

very light (500 kg) - like a Caterham (weight and torque-wise) but a completely different (highly aerodynamic) shape. Welded steel space frame, with aluminium panels.

high peak torque (500 Nm) - EMRAX 268 synchronous PM motor (20 kg)

"direct drive" (ie. no transmission (gear box). Just a rear LSD (limited slip differential).

medium range (16 kWh battery) - (approx 100 kg) 200 km ?

high performance (t 0-100 kmh = 5 s, Vmax = 160 kmh) - OK, 160 kmh isn't really high speed for a "high performance" vehicle, but in practice that's my comfortable cruising speed on an Autobahn, and if you want to get any sort of range at all, you have to go pretty slow (100 kmh) anyway.


My (back of envelope) calculations so far:

The EMRAX 268 MV has a max peak torque (T.pk.mot) of 500 Nm and max continuous rotation speed (w.max.mot) of 4500 rpm. If LSD is 3:1 that gives peak torque (T.pk.whl) of (500 x 3) 1500 Nm and max speed (w.max.whl) of (4500 / 3) 1500 rpm at the wheel.

"Normal" wheels (225/55 R16 ... 235/35 R19) have radius (r.whl) 0.32 m and circumference (C.whl) 2.01 m

Fmax = T.pk.whl / r.whl = 1500 Nm / 0.32 m = 4688 N

Vmax = w.max.whl / 60 * C.whl = 1500 rpm / 60 s/min * 2.01 m = 50.25 m/s (= 180.9 kmh)

acc.max = F.max / m = 4688 N / 600 kg = 7.8 m/s2 (=> 28 m/s / 7.8 m/s2 = t.0-100 = 3.6 s)

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Power is required to overcome drag (wind resistance) and rolling resistance (tyres).

F.drag = 1/2 * rho * v^2 * [ c.d * A]
: 0.5 * 1.22 * 0.25 * v^2
: 0.15 v^2

where rho is air density = 1.22 kg/m^3
c.d is drag coefficient
A is frontal area.
A very aerodynamic car might have a [c.d * A] of 0.25 m^2.

F.rr = c.rr * F.n = 0.01 * 750 kg * 9.8 = 74 N

where c.rr (coefficient of rolling resistance) is ~ 0.01
F.n is normal (downward) force which is 750 kg * 9.8 m/s^2

F.drag is proportional to v squared. F.rr is more or less constant.

Energy is force x distance. Power is Energy per second = force * distance per second = force * speed.

P = (0.15 * v^2 + 74) * v.

[ 160 kmh = 160,000 / 3600 = 44.4 m/s (/2.01 * 60 = 1320 rpm (wheel) = 3960 rpm (mot) ]

P.160 = ( 0.15 * 44.4^2 + 74) * 44.4 = 370 N * 44.4 m/s = 16.5 kW

"Free running losses" in the motor are given as ~ 1.9 kW at 4000 rpm.

P.tot.160 = 16.5 + 1.9 = 18.4 kW

[ 70 mph = 112 kmh = 31.3 m/s = 934 rpm.w = 2800 rpm.m]

P.112 = ( 0.15 * 31.3^2 + 74) * 31.3 = 221 * 31.3 = 6.9 kW
P.frl = ~ 1 kW
P.tot.112 = 7.9 kW

16 kWh battery would give 2 hrs at 70 mph. ie 200 km range.

The EMRAX 268 MV "medium" voltage requires up to 680 V and can cope with up to 400 A (for max torque, when accelerating for a few seconds).

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I haven't really looked into the battery yet. Using NCR18650B cells as a nominal unit (3.6 V, 3.35 Ah [= 12 Wh] 2C = 6.7 A, 48.5 g, 5 EUR):

16 kWh = 1333 cells. = 65 kg.
400 A (peak, only) requires 60 cells in parallel (at 2C, continuous)
180 cells in series = 650 V.

Clearly I don't want 180 x 60 = 10,800 cells = 130 kWh = 525 kg = 54,000 EUR.

If I have 4 groups of 45 x 15 that would be 2700 cells, 32 kWh, 130 kg and 13,000 EUR.

These could be switched in series to give 650 V and 100 A, or parallel to give 162 V and 400 A or 2x2 giving 200 A at 325 V.

I'm surprised that I can't find larger/better batteries than 18650s! 15 in parallel would give me a 3.6 V, 50 Ah [180 Wh] cell at 0.73 kg and 75 EUR.

A GWL 3.2 V, 20 Ah LiFePO4 prismatic cell is 64 Wh at 0.65 kg and 25 EUR.

200 => 12.8 kWh, 130 kg, 5000 EUR.
 

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Thanks for the math.
I'll need to convert it to non metric numbers, but I can handle that!
please allow me to introduce myself, I'm a man of wealth and taste..

No,
my name is Paul an retired gear head but pistons, not amps!
my project car is a 1972 SAAB Sonett iii.
I rebuilt it ( stock ) from 2001, to 2003.
raced it Scca solo stock and slowly modified it to prepared.
not street legal. Until 2005.
my hair brained goal was, of course, more power!

My experience, was in Chevy v8s. From my first race car, a 500 plus HP
gas dragster, to building dirt track cars.
selling open chamber sbc heads.
for oval and drag use!

Crewing on all types of racing all over the mid west.
so of course, I imagined a sbc in the Sonett!

Off came the fiberglass body,
and a new tube frame... et AL
and in goes a sbc

Sorry for the long story..
the short, I have given up the sbc. And seeing the results of EVs,
I think I have seen the ( electric ) light!

The only problem, I know almost nothing of what I may be planning! .?

My experience? Rebuilding starter motors or alternators!
I can wire a lamp or a house, but know nothing of controlers!

I hope I can find help here!

My goal?
a drivable car. With enough power to embarrass the kids down the street with fart cans on their Hondas!
and make female passengers pee their pants!
Enough fun for a 64 year old retired gear head!

Any help?
any one in Columbus Ohio ?
or nearby?
thanks in advance,
Paul
 

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interesting idea and in theory you can get pretty good one out of it.
few things to clarify.
1, less than 500kg as a total weight sounds almost impossible with 200km range

2, 16kw pack wont be able to achieve 200km.
my calculation says 185km given 80% DOD

3, little thing but 0.01 rolling resistance seems too good. (I usually use 0.015)

best of luck!
 

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Hi Guys
My "Device" is a crude and primitive version of what you are thinking of

http://www.diyelectriccar.com/forum...dubious-device-44370p15.html?highlight=duncan

But it is great fun!

If I was doing it again I would be thinking about a Leaf motor/transmission - with one of the DIY controllers that the electronic guys are working on - or even a Tesla power unit

The two requirements of simple lightweight and good aerodynamics conflict a bit - I went for the simple but I did overbuild it a bit so it is a bit too heavy
 

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I put your numbers into my EV Designer and if you choose a 3:1 final drive, the results look quite good, about 160k/h top speed, 0-100k/h in about 4s

Text Line Parallel Font Circle


(I assumed the mid voltage motor with numbers extracted from emrax power/torque graph)

Are you sure your CDA will be 0,25? Tesla is very slippery and is 0.58?
Also CRR of 0.01 is lower than Tesla @ 0.014

Using the Tesla CRR,CDA as an example, assuming 2 Chevy Volt packs in series for about 630V, range would be about 150miles

Not sure this pack size is realistic with your gross weight of 500kg tho...
 

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My current car is 530kg with 7Kwh.

My project is to build another frame to reduce the wieght down to about 430Kg with the same mechanicals. This will be a single seater with no lights, blinkers etc.

If your targets are just that, targets and not an absolute must then I think you are on the right track. But if any of those are absolute must haves then I think you might be a little disappointed.

Will yours be a single seater, will it be road registered. Every little component adds weight which will bump you up over your targetted 500KG. The weight you have quoted for your batteries would be the unassembled weight. As soon as you add conductors, mounts cases you will be adding to that weight. But again as a round figure I think it is good to be a little optimistic.
 
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