Silly Charging Idea No3

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Paul9
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Silly Charging Idea No3

Post by Paul9 » Sat, 13 Jun 2009, 03:03

Here I go again. Again this is not so much a charging idea as a reduction in power drawn idea. It was explained to me that spoilers on the back of race cars have a fairly pronounced effect on keeping the back of the car down on the road. This is apparently different from many spoilers on the back of road cars which are simply there to look "good". As the main(?) object of EV design is to reduce the weight of the vehicle, I wondered why no one has put a "reverse" spoiler on an EV. By a "reverse" spoiler I mean a spoiler who's leading edge is slightly higher than its trailing edge rather than a normal spoiler who's leading edge is slightly lower (relative to wind direction) than its trailing edge. A lower leading edge gives downward pressure thus increasing weight (downward force on road) whereas a higher leading edge would (?) give a lower weight (lower downward force on road).

In my younger (much younger!) days I flew light planes and so am somewhat familiar with aerodynamics. By imitating the effect of a plane's wings, by using a spoiler, could one lighten the "weight" of an EV? Obviously the actual weight of the EV (same as a plane) does not decrease but the force it exerts on the ground does decrease and also continues to decrease as the plane/EV increases in speed.

I have read that the power required to maintain a speed of 70km/hr is 4 times the power required to maintain a speed of 35km/hr. In other words the power required at double the speed is not double but quadruple the power. The good thing with a "reverse" spoiler is that the lift it would produce is also much greater at 70km/hr than 35km/hr.

Anyone got a wind tunnel they are not using at present?

This one is probably sillier than my earlier ideas but I won't learn unless I ask!
Thanks in advance
PAul

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Post by Johny » Sat, 13 Jun 2009, 05:02

While your spoiler idea would reduce down force and therefore reduce drag due to the deformation of the rear tyres very slightly, it would increase air drag - the main contributor to speed related drag. Better to use low-rolling resistance tyres and remove any obstacles to smooth air-flow entirely.

I also would think it would create instability should you get up a bit of speed.

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Post by Squiggles » Sat, 13 Jun 2009, 05:21

There is probably something to think about here in terms of improving air flow at the rear of an older style car with add on bits. Suspect they would not look so 'cool' but may improve efficiency.....now about that wind tunnel.

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Post by Thalass » Sat, 13 Jun 2009, 12:25

the weight is not what the motor has to fight during acceleration, it's the mass. So while the car will be lighter at speed, it's mass will be the same. And when stopped the weight will be the same as the mass. And the wing will create drag which will mean you will use more power to maintain that speed.

Also: if the car is lifting off of the road at speed, it's handling will suffer. It will be all floaty, and hard to steer. Which is bad.

This is how you improve aerodynamics hahaha
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Post by Tritium_James » Sat, 13 Jun 2009, 14:28

Like Johny said, your main drag contribution is aero drag, not rolling resistance. So an upside down spoiler reduces rolling resistance at the expense of aero drag, which really isn't what you want.

Also for your power example, reality is actually worse than that. Aero drag goes up proportional to the CUBE of velocity, not square. So doubling your speed from 35 to 70km/h actually makes the aero drag (and power required) go up by 2³ = 8 times.

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Post by weber » Sat, 13 Jun 2009, 16:36

Tritium_James wrote: Also for your power example, reality is actually worse than that. Aero drag goes up proportional to the CUBE of velocity, not square. So doubling your speed from 35 to 70km/h actually makes the aero drag (and power required) go up by 2³ = 8 times.


Yes. Totally right. But total power may just happen to go up only 4 times with a particular doubling of speed (e.g. 50 km/h to 100 km/h) because the rolling resistance makes up a larger proportion of the power at the lower speed, and rolling resistance power only goes up linearly with speed. e.g. See this graph of what we estimate the MX-5 requires.



Image
Last edited by weber on Tue, 10 Jul 2012, 20:24, edited 1 time in total.
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Post by Squiggles » Sat, 13 Jun 2009, 16:47

What actually causes rolling resistance to increase?

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Post by weber » Sat, 13 Jun 2009, 17:04

Squiggles wrote: What actually causes rolling resistance to increase?

The frictional forces are constant and power is force times speed so the power goes up linearly with speed.

Similarly, the air drag force goes up as the square of the speed, so the air drag power goes up as the cube of the speed.
[Edit: Added sentence about air drag above]
Last edited by weber on Sat, 13 Jun 2009, 08:01, edited 1 time in total.
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Post by acmotor » Sat, 13 Jun 2009, 18:55

Yep, so under 100kmph don't get too knotted about aerodynamics unless you plan a lot of cruising ! As per the Aerocivic.
After all, most vehicles on a city cycle will only average 30kmph.

If you have to tweak, then lower the rolling resistance. e.g. lose the gearbox and motor brushes, go for well inflated LRR tyres and one day use hub motors. Go for efficient motor and drive, then tackle the bodywork.

The big power requirement is to accelerate your brick and the smarts are to recover as much of the kinetic energy as possible when slowing down again.

Paul9, if spoilers were actually good for aerodynamics, there would be one on a prius ! Not even the Aerocivic has one.
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Post by Tritium_James » Sat, 13 Jun 2009, 19:29

Weber, that rolling resistance in your curve seems quite high. What's the weight of the MX-5? Here's the plot I've done for my Mira, this data comes from the total road force vs gradient vs speed curves in the workshop manual, which I've used to calculate back to rolling + aero drag power figures.

Image

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Post by weber » Sat, 13 Jun 2009, 19:56

Tritium_James wrote: Weber, that rolling resistance in your curve seems quite high. What's the weight of the MX-5?

I see it's more than double yours. I used a GVM of 1365 kg.

I estimated the Crr at 0.025 by using the quoted Cd of 0.38 and estimating the frontal area at 1.87 m^2 from width * (height - ground_clearance) . Then I adjusted the Crr until the total power curve agreed fairly well with the top speeds and peak powers of the 1.6 L and 1.8 L engines (using their respective GVMs).
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Post by Tritium_James » Sat, 13 Jun 2009, 20:11

OK, factor in some wider tyres than I've got and your rolling resistance number is pretty believable! It scales with mass. Bummer... I was hoping I'd spotted an error in your curve, and you were going to get better performance than expected Image

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Post by zeva » Sat, 13 Jun 2009, 20:23

weber wrote: I see it's more than double yours. I used a GVM of 1365 kg.

Holy moly what are you and Coulomb doing to that poor little MX5!? Image (Curb weight of the 1st Gen was 940kg)

Also Crr of 0.025 sounds unusually high to me, I was under the impression that modern tires should all be under about 0.015 (with the latest LRR tires being about 0.007 - plug that number in and see what a difference it can make!)
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Post by acmotor » Sat, 13 Jun 2009, 20:36

Maybe just do a tow test at 30kmph with a load cell in the line. Flat road. Tow both directions take average. It will all be RR. Take it from there. At least you will have DATA for the graph.
There is nothing like real world data ! Right now you guys are just playing with numbers.

Hoops, your MX5 is emotored up ready to go ? (or at least tow ?)
That would give weber the numbers for his graph.

You could tow with another EV and measure the kW change, but that does not allow for the drive efficiency etc in the towing EV.
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Post by weber » Sat, 13 Jun 2009, 22:44

zeva wrote:Holy moly what are you and Coulomb doing to that poor little MX5!? Image (Curb weight of the 1st Gen was 940kg)
Poor little MX5? It's incredible what those things can take, as I'm sure you know. e.g. V8 engines. http://www.monstermiata.com/ Of course, what we're doing to it is to put in enough motor, batteries and controller to pump out 100 kW for long enough to get arrested. Image You'd better get your finger out if you're gonna have the fastest EV conversion in Australia. You realise this is AC vs DC? Image

With power steering and aircon and fuel ours was 983 kg before we started on it. But that's Kerb Mass. We think the KM will end up at about 1200 kg to 1250 kg. Then there's the NCOP allowance of 68 kg per person and 13.6 kg for their luggage for the 2 seating positions (payload 163.2 total).
Also Crr of 0.025 sounds unusually high to me, I was under the impression that modern tires should all be under about 0.015 (with the latest LRR tires being about 0.007 - plug that number in and see what a difference it can make!)
Yes. Quite a bit. Probably just putting more air in the tyres would make a significant difference. I was amazed at how low the recommended pressure was. But the fact is, we're going prmarily for performance here, not efficiency. So do LRR tyres have less grip? I'm guessing they might because I understand they use a proportion of silicone rubber which I think of as more slippery than isoprene rubber. But then it could be layered so the tread is still the old stuff.

I think we've hijacked this thread somewhat. Sorry.
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Post by weber » Sat, 13 Jun 2009, 22:47

Tritium_James wrote: OK, factor in some wider tyres than I've got and your rolling resistance number is pretty believable! It scales with mass. Bummer... I was hoping I'd spotted an error in your curve, and you were going to get better performance than expected Image

Bummer. I was hoping so too. But as Zeva says, some LRR tyres might do wonders. But of course it's not just the tyres. As ACmotor is fond of reminding us, there's significant drag in the gearbox too (which we're keeping).
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Post by Nevilleh » Thu, 18 Jun 2009, 16:28

Just some stuff I thought about:

Total drag = aerodynamic drag + rolling resistance
= Cd x A x V^2 x 1.22/2 + Crr x M x 9.81

where Cd = aero drag coeff, A = frontal area, Crr = coeff rolling res, M = weight of vehicle. 1.22 = air density at sea level and 9.81 = Acceleration due to gravity.
Say Cd = .35, Crr = .015, M = 1500 kg, A = 1.8 m^2 and V = 27.8 m/s (100 kph)
then total drag = 518 Newtons.

It’s not that hard to actually measure the drag.
Get your girlfriend to drive and accelerate up to 110 kph on a flat, level road with no wind. Get your stopwatch ready. Tell her to put the car out of gear and let it coast while you watch the speedo. When your speed drops to 105 kph, start your stopwatch and stop it again when you reach 95 kph. If you do several runs in each direction, you can take an average for best accuracy.
Newton’s second law says F = M x a where M = the mass of your car and a is the (de) acceleration you just measured.
Let’s say the average time you measured was 8 seconds. You slowed by 10 kph, which is 2.78 m/s, so your deceleration is .348 m/s/s
You need to know how much your car weighs, assume 1500 kg for this exercise.
So the force required to slow your car is 1500 x .348 = 522 Newtons. Note that this includes aero drag as well as rolling resistance.
The rolling resistance could be measured by towing your car with a spring balance in the towrope and the speed doesn’t matter as it is pretty constant, so you could just do it in your (flat!) driveway, but the formula above is a reasonable estimate. Assume Crr is .015 then for a mass of 1500kg the rolling resistance is .015 x 1500 x 9.81 = 221 Newtons.
That means the aerodynamic drag must be 522 – 221 = 301 Newtons.
If you measure your car’s frontal area, you can then calculate the drag coefficient Cd. i.e.
Cd = Aero drag divided by (A x V^2 x .61)
Assuming an A of 1.8 m^2 gives a Cd 0f .35.

The mean speed was 100 kph, or 27.8 m/s, so the power required = force x speed = 522 x 27.8 = 14.5 Kw.

Doesn't seem like much but if you want to climb a hill or accelerate it all changes!
If you want to accelerate your 1500 kg vehicle from 0 to 100 kph in 6 seconds say, then applying Newton’s second law again gives the following:
Acceleration =27.8 m/sec in 6 seconds, or 4.6 m/s/s, so the force required is 1500 x 4.6 = 6900 Newtons. Add on the rolling resistance of 221 Newtons and the aero drag at 100 kph of 300-odd and you get 7422 Newtons. Take the mean speed of 27.8/2 = 13.9 m/s and multiply by the force to get 7422 x 13.9 = 103 kw! And this is what is required at the wheels.

If you want to climb a 10 degree (not all that steep) slope at 100 kph, a little trigonometry tells me that is equivalent to doing 4.8 m/sec vertically. So the power required is 1500 x 4.8 x 9.8 which comes to just over 70 kw.

The energy content of a litre of petrol is about 36 MegaJoules (MJ). Diesel is a bit higher. One kilowatt-hour of electricity is approximately 3.6 MJ, so a “unit” of electricity is equal to 1/10 of a litre of petrol. But an internal combustion engine is pretty inefficient, down to only about 20%, so you have to burn about 5 litres of petrol to produce the equivalent of 10 units of electricity. The point of this is that travelling at 100 kph for an hour, which is equivalent to 14.5 units of electricity, we should use about 7 litres of petrol! (If you use more than that, it’s because the terrain is not flat, you have to accelerate and slow down and your car is not as efficient as it might be).

14.5 units of electricity cost about $2.50 whereas 7 litres of petrol is about $11 (NZ prices)
The case for the electric car is plain!


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Post by Nevilleh » Thu, 18 Jun 2009, 16:36

I recall an episode of "Top Gear" where the boys had to build an amphibious vehicle, drive it to a lake and then drive across the lake. James May did it with a Triumph Herald convertible to which he added a mast and sails. Maybe that would work on the road too?
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Post by Johny » Thu, 18 Jun 2009, 16:40

Nah - won't work. I haven't got a girlfriend - will a wife do?
Otherwise - good post - thanks.
There is also an article and support spreadsheet on measuring drag (boith types) at
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Post by Johny » Thu, 18 Jun 2009, 16:42

Nevilleh wrote: I recall an episode of "Top Gear" where the boys had to build an amphibious vehicle, drive it to a lake and then drive across the lake. James May did it with a Triumph Herald convertible to which he added a mast and sails. Maybe that would work on the road too?
Image
Other drivers would get a bit edgy when you tack.

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Post by Nevilleh » Fri, 19 Jun 2009, 13:19

Johny wrote:
Other drivers would get a bit edgy when you tack.


Yes, but imagine charging down the motorway with a big spinnaker up! Could have AEVA emblazoned on it.......

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