Hipo AWD project

Technical discussion on converting internal combustion to electric
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Post by MikeG » Tue, 14 Oct 2008, 16:44

I'd like to know how much it would cost to do a conversion given a rolling chassis for a Nissan Bluebird SSS Attesa (AWD) Ltd.

I want to retain the back seats and boot space.

I want to go a high performance motor to attain 0-100 in under 5 seconds, and with a top speed of 110km/h.

I want a range of at least 100km driven carefully (with or without regenerative braking). I was hoping for 200km but I can cope with a gen trailer I guess.

I want included protection for overrevving (std in good quality motors I gather from my research).

I want to try and keep a 50/50 weight distributions left to right and front to back.

My thoughts are that a large (AC) motor be mated directly to the centre viscous limited slip differential at the front or through a custom heavy duty two speed gearbox. The drivetrain of this vehicle can withstand over 600Nm and 250kW.

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Last edited by MikeG on Tue, 14 Oct 2008, 05:48, edited 1 time in total.

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Post by woody » Tue, 14 Oct 2008, 22:51

My spreadsheet only does direct drive industrial AC, but anyway:
For a 920kg rolling shell with:
4.1:1 diff
90% efficient drivetrain
Cd of 0.32
Frontal Area of 2.14
Tyres with normal rolling resistance (0.015)

Adding a 290Kg ABB 4 pole premium performance 45kW nominal induction motor rewound for 400V 85Hz Star / 400V 150Hz Delta
Adding 220 40Ah Thundersky lithium batteries for 330Kg / 28kWh
gives you kerb weight of 1550kg.
Torque should be a flat 931 Nm up until 200kW mark at 55kph/1900 RPM
200kW flat from 55kph to 163kph(5650 RPM) if your motor is balanced
After that the power drops off somewhat.

0-60kph in 2.5s
0-100kph in 5.0s
0-100mph in 12.3s
400m in 13.37s @ 166
top speed 216 kph

7kW cruising at 60kph gives you 280km range @ 70% DoD
11.5kW cruising at 80kph gives you 180km range @ 70% DoD
21.5kW cruising at 110kph gives 90km range @ 70% DoD

your controller will need to do about 700 amps and 200kW, I'm not sure where to get one of those which is smaller than a fridge :-(
The danfoss model which will do that is a VLT5352 which weighs 221 kg and is 155x59x50 cm.

General Comments:
I'm guessing the 600Nm limit is in front of the gearbox, because you can only just get 0-100 in 5 secs with a flat 600Nm torque curve and 4.75:1 diff.
0-100 in 5 secs is really fast.
LiFePO4 Batteries which can deliver 200kW for a few seconds are probably going to give you the range you want.
You might want to get a commercial AC solution like AC-150, but that is only about 150kW, you'd want a lighter car.
You might look at A123 lithium batteries which are higher energy density and price.

Edit: You wanted price, I'm guessing: (pre-AUD$ crash)
Motor: $5000 + $1000 rewind
Controller: Danfoss Fridge size one (VLT5352) is probably $20,000+ new
Batteries: $15,000 for Thunder-Sky AHA 40s
Other bits: $3,000

cheers,
Woody
Last edited by woody on Tue, 14 Oct 2008, 12:11, edited 1 time in total.
Planned EV: '63 Cortina using AC and LiFePO4 Battery Pack

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Post by Richo » Wed, 15 Oct 2008, 01:12

40Ah batteries maybe a bit small to pull 700A from.
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Post by antiscab » Wed, 15 Oct 2008, 04:10

actually for 200kw you would need "only" 350A, which would be fine if we were talking about 90AH:p
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Post by MikeG » Wed, 15 Oct 2008, 14:08

Guys, this sounds pretty good - thanks. The down side is to fit all this stuff in the car, we're probably getting rid of a lot of cargo and possible back seat space. :(

The original car does 0-100 in around 6.5 sec, so it's not a stretch for this sort of car.

Halve the batteries if this car is not for freeway duty, and suddently we've got less weight and can probably downgrade the motor slightly.

Usual duty for this car will be daily drive to/from work (20km round trip, top speed 60km/h.

It will be used for some 80km/h driving after hours and on weekends. it will almost never be used for freeway driving, but with a suitable gen trailer, the battery capacity becomes almost irrelevant.

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Post by woody » Wed, 15 Oct 2008, 14:58

MikeG wrote: Halve the batteries if this car is not for freeway duty, and suddently we've got less weight and can probably downgrade the motor slightly.
You can't really halve the thundersky pack and keep the performance. Thunderskys are good for 3C (= 3 x Capacity), and maybe 5-10C for a little bit. So if you halve your pack from 28kWH, you are halving your power input to the controller and hence your power output.

Think of it as a petrol tank with a small outlet :-)

Other format lithiums (e.g. A123) have a higher C rating, so you could go with them, I think they are 2-5 times the price though.
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Post by woody » Wed, 15 Oct 2008, 16:23

Richo wrote: 40Ah batteries maybe a bit small to pull 700A from.
You're right, I'm pushing it a bit, but I don't think it's as much as you think.
A 700V pack of 40Ah batteries is 28kWH. At 5C discharge, this is 140kW.

So the controller should be able to deliver 700A up to about 200V.

But I've done a bit more development and put in a limit for the discharge on the Thunderskies in my spreadsheet, so now I reckon:

Same motor rewound for 340V star/195V delta @ 50Hz
Danfoss 5202 Controller (132kw * 160% = 211kW)
Limit TS-LFP40AHA Battery discharge to 6.5C
gives:
Smaller, lighter controller (112kg, 133x408x373cm)
0-100 5.3s
400m: 14s

But this is all worked out with a few electrons wizzing around in a laptop, the rubber hits the road when lots of electrons start flowing.

cheers,
Woody
Planned EV: '63 Cortina using AC and LiFePO4 Battery Pack

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Post by MikeG » Wed, 15 Oct 2008, 16:53

I would have thought reducing the weight by 120kg would reduce the need for such a big motor, and hence you need less power... The volume (space) occupied by the batteries is a concern IMHO too...

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Post by antiscab » Wed, 15 Oct 2008, 17:34

ah woody, you are neglecting voltage sag.
when a 40AH cell is toasty (ie 50 deg c+) it *might* be able to do 200A, but at a voltage of 2.5v.
that would give 550v for 220 cells or 110kw.

MikeG,
The price per peak kw is the same for thunderskys, lifebatt and just about any other LiFePO4 battery i have come across.
It just happens to be that the price per kwh is far lower for thunderskys, so you get big range for the same money.
IMHO, only go the really expensive hi power battery option if you cant take the weight.

Matt
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Post by woody » Wed, 15 Oct 2008, 20:07

From a purely mechanical/aerodynamic point of view, a 1650 kg car with a frontal area of 2.14m and a Cd of 0.32 at 80km/h needs:
7kW to overcome rolling resistance
4.6kW to overcome air drag

A 1650 kg car @ 100kph has 1/2mv^2 = 825*28^2 = 636kJ
kW is kJ/s
So accellerating that to 100kph in 5 seconds is an average of 636/5 = 127kW

This is way more than the drag or the rolling resistance.

The way to reduce this number is either less weight or more time.

But you've pushed me to look at things a different way.

The way to get this sort of power from a small AC motor is to spin it to higher revs and drive it through a high ratio (~10:1) diff.

Unfortunately this goes away from the easily available industrial motors, which aren't balanced to do 7,500+ RPM.

You'd need to go to Metric Mind for a specialised motor + controller, or AC Propulsion's AC-150 which is US$25,000 :-(
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Post by MikeG » Wed, 15 Oct 2008, 20:26

What are the average drivetrain losses on an AWD? I'm guessing they're up around 25% of you go through a gearbox, or say 10% if the motor is mounted to the front diff.

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Post by Johny » Wed, 15 Oct 2008, 20:36

Does the Nissan have the diffs mounted to the body and have independent suspension on the drive axles?

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Post by Johny » Wed, 15 Oct 2008, 20:43

Have a look at this rather expensive AWD twin motor setup:
http://www.metricmind.com/audi/main.htm

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Post by MikeG » Wed, 15 Oct 2008, 20:44

Yes, from memory it does.

Suspension system, front: Independent suspension strut type
Suspension system, rear: Independent suspension parallel link strut system (sub frame attaching)

(all axles are drive axles, obviously)


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Post by MikeG » Wed, 15 Oct 2008, 21:12

Johny wrote: Have a look at this rather expensive AWD twin motor setup:
http://www.metricmind.com/audi/main.htm


Yes, quite silly IMHO.

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Post by MikeG » Wed, 15 Oct 2008, 21:15

I've also been thinking about the possibility of fitting the motor at the rear (in the boot) and reversing the diffs and driveshaft, so the car initially runs the rear wheels, bringing the front wheels into action when needed (like in the Skyline GTR, etc)

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Post by a4x4kiwi » Fri, 17 Oct 2008, 14:23

I always like your analysis Woody.

This explains why the EV1(or is it teh Tesla) uses a 10,000RPM motor.

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Post by tsport100 » Sun, 23 Nov 2008, 07:47

Comparing engine Dyno results to rolling road dyno results a 2WD transmission has a 30% power loss between flywheel and tire while a 4WD transmission has a 40% loss. This is why there is so much interest in Wheel Motors for EVs.

On the subject of ACPs 150kw motors that turn at up to 13,000rpm. There's no reason why a standard rotor couldn't be balances to handle that rpm. The only trick that ACP seem to pull is designing the windings to handle up to 400hz without hitting the reactance wall.

When run through an 8:1 diff they get a max speed of just over 100mph.(1500 rpm on a 17" wheel is approx 100mph) So if you only want to drive to the legal limit in Aust with the same 8:1 diff you'd only need 7500rpm out of a motor. drop the diff ratio (if you can live without 0-100 in 3.6 sec performance) and motor rpm requirements are even less.

The ACP motor is really only a 50kw constant 3 phase AC motor. They always quote it's Peak power in PR blurbs.

BTW the spec sheet on 40ah Thundersky cells says 10C peak, good enough for 400amp bursts.

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Post by acmotor » Mon, 24 Nov 2008, 06:57

Nicely summed up on all points.
ACP motor seems to be wound for 80V? Not actually reactance wall - just maintaining V/F
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Post by tsport100 » Mon, 24 Nov 2008, 13:35

Wound for 336-360V - 450v Max

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Post by MikeG » Mon, 24 Nov 2008, 16:59

tsport100 wrote: Comparing engine Dyno results to rolling road dyno results a 2WD transmission has a 30% power loss between flywheel and tire while a 4WD transmission has a 40% loss. This is why there is so much interest in Wheel Motors for EVs.


It might be worth considering the fact that FWD has less losses than RWD which in turn has less losses than AWD.

As a live example, a 105kW SR20DE out of a Pulsar SSS will produce around 78kW to whe front wheels on the dyno. The 105kW SR20DE out of a RWD silvia will make around 70kW to the rear wheels. You can up the losses a little more for AWD transmissions.

Where it gets interesting, is that the loss is not linear. If you increase the power by say 50kW to the crank (engine modifications), you'll see most of that 50kW transferred to the wheels. Losses are constant, and vary depending on the specifics of the drivetrain.

Remove the gearbox from the car, and you'll see some of the losses disappear. I think if you're converting an older car, upgrading the diff(s) to state of the art ones will reduce your power losses.

Just my 2c worth.

Mike.

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Post by acmotor » Mon, 24 Nov 2008, 17:43

Interesting suggestion of wheel power in RWD vs FWD.

There are the same drive components involved i.e. various ratio gears, differential and bearings carrying these.
So any difference does not come from these actual drive train components.
The FWD contains one more 180deg power train turn and the RWD a 90deg turn and the FWD contains CVs that lose power when not straight. 180deg is more efficient in hypoid gears than 90 deg.
But all that considered, I think it may be sweeping to consider one system more efficient than the other.
It would be more related to the size and strength of the two systems being compared.

The biggest increase in drive train efficiency is to lose the gearbox and go direct drive or eventually to hub motors. This does require motors matched to the driving torque requirements.

In all reality though, don't make an EV conversion decision on efficiency alone. If you go DC lead acid and gearbox it will still be on the road and a working EV.... even if the efficiency from the power plug is less than 50% (quite likely). If it were hub motors and lithium it could be 70% and cost as much as your house.
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Post by tsport100 » Mon, 24 Nov 2008, 19:04

In my humble opinion a DC / lead Acid / gearbox car is so compromised as to not be worth the expense/effort.

Overall system efficiency ultimately dictates the range, power consumption & performance of any EV. Compromise that even further by using low energy density lead acid and that kind of EV can only ever be a very heavy, slow, short distance car.

I think brake regeneration is one of THE best features of an EV. For sure there are no commercially available EV wheel motors on the market at present but I own a company that is working on them and they don't cost anything like a mortgage to build.

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Post by MikeG » Mon, 24 Nov 2008, 20:49

A gearbox insitu in a FWD has less losses than a RWD setup due to less rotating mass for starters... There's no drive shaft in a FWD... This from my own motor racing, performance modifications & general automotive experience... But there are other references to this:

"Improved drivetrain efficiency: the direct connection between engine and transaxle reduce the mass and mechanical inertia of the drivetrain compared to a rear-wheel drive vehicle with a similar engine and transmission, allowing greater fuel economy." [source wikipedia & http://www.edmunds.com/insideline/do/Fe ... leId=43847]

Imagine the losses in the new GTR, that has its gearbox at the back of the car and TWO driveshafts... One from the engine to the gearbox, and one back up to the front diff.

MikeG

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Post by acmotor » Tue, 25 Nov 2008, 02:22

My EVs are AC direct drive to diff and I am stunned by the regen braking so I understand your thinking compared with DC and gearboxes and lead acid. Still not as good as hub motors and ultra caps.

I still say though that a DC lead acid EV with gearbox is better for you and the world around you than a hummer !!

Don't be fooled by the sales blurb of a wiki. Rotating mass (of say a drive shaft) is of little consequence to efficiency and consumes no power when cruising anyway. The drive shaft inertia is maybe 0.1% or less compared with the vehicles inertia. It is gears first and CVs then bearings in that order that waste power.
A good diff may be 85 to 90% efficient and the tailshaft 99.99% efficient. Its only loss being wind drag (tiny) and some uni joint loss if they are not straight. (still less than a CV at 30 deg).

"improved drive train efficiency" doesn't occur if the same mechanicals (gearbox and diff) are involved.
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