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Torque Required to Move a Hybrid Car

08/19/2009 6:07 PM

HELP ........ I am building a Hybrid car and I'm trying to figure how much torque will be required. I need to take into account the weight of the car, acceleration from a dead stop as well as acceleration at speed, tire resistance, surface roughness (this isn't real important), wind resistance and incline. I know that that acceleration is acceleration but acceleration at speed is important as I've got a very narrow RPM to torque ratio at high RPM's, so this is a very important calculation.

It has been a long time since college and I don't remember my physics that well. I'm planning on using Hub motors so I'm looking for Torque at the wheels. I'm at a point where I need to pick the motors. I've found a lot of info on Electric car kits but nothing that really gives me the formulas. I could just do it by trial and error but as with most projects I'm on a tight budget so I would really like to pick the right motor the first time. My design is pretty bizarre and most people just laugh at me when I tell them about it but I really believe in it and want very much to make this work. Oh and I'm not looking for HP what I want is Torque. High performance isn't very important to me. High efficiency is what I'm looking for. Use the minimum required force to get you from point A to point B with the minimum cost per mile.

Can anyone help me find these formulas?

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#1

Re: Torque required to move a Hybrid car

08/19/2009 6:57 PM

What kind of hybrid are you planning?

You got my interest with the phrase "My design is pretty bizarre and most people just laugh at me".

Well, lets have it.

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#6
In reply to #1

Re: Torque required to move a Hybrid car

08/20/2009 9:56 AM

I'm starting with a Dodge Neon something small and light, especially without the engine and tranny. I'm thinking about 1500 pounds.

My design is for a hydraulic drive train, like the UPS trucks. Not all that special but my ultimate plan is to have the hydraulic pumps being driven by a series of DC electric motors with a generator to keep the batteries charged. What everyone says is why not just use electric only and my answer is because it's there, I like the challenge.

My system is pretty complicated but I'm hoping for some pretty big gains. I've got a lot of things built into the system to take advantage of stored energy like hydraulic accumulators. Thats also why I want to run off of batteries, more stored energy. When you leave a stop sign you use the accumulators for the boast to get you going. When you need to pass someone on the highway you fire up another electric pump and draw the power out of the batteries. I am sizing the generator for a little more output then the highest cruising demand. That way there is a little extra to keep the batteries charged. And I know everyone is going to start poking holes in my design, like how do you find one hydraulic motor that can handle the full power band, well there isn't ONE thats why I'm using four different motors. 1st gear - high torque/low RPM, 2nd gear less torque a little higher RPM so on and so forth. I also like using small electric motors with a pump on each because it makes the controls easy. No need for a big expensive speed controller, I'm designing a step controller, that will stage based on pressure. I really like how simple my controls are going to be.

Well thats the bases of my design theres lots and lots of little details that I haven't gotten into like how do I cool the hydraulic fluid, thats a big deal and I'm hoping that I have a trick for that, but it does solve one thing and thats heat during the winter. I'm no a hydraulic engineer so this is pretty much by the seat of my pants. But I have talked to an engineer and he didn't laugh to hard. He said I have some pretty good ideas. The accumulators are giving me a lot of trouble those things are just plain expensive, so I'm designing my own, and yes I know they are dangerous but they can be designed with a safety factor they are just a pressure vessel and I know what it takes to build one of those.

I'm almost ready to put my 1st hydraulic motor in but I need to pick the best motor for 1st gear thats why I'm asking for help from everyone. I really want to pick the right one the first time and not have to waste money I don't have.

Thanks for what ever help anyone can give.

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#8
In reply to #6

Re: Torque required to move a Hybrid car

08/21/2009 12:34 AM

"...I am sizing the generator for a little more output then the highest cruising demand. That way there is a little extra to keep the batteries charged...."

This is such a great idea that I'm surprised no one has thought of it before.

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#2

Re: Torque required to move a Hybrid car

08/19/2009 7:39 PM

F=ma

You know the mass you need to move and presumably you have an acceleration rate in mind. That will get you the force needed. Divide by the number of driven wheels, and multiply by your tire radius (watch units). Voila: torque requirement! (Add a bit for losses!)

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#3

Re: Torque Required to Move a Hybrid Car

08/20/2009 12:04 AM

Interesting. I hope this thread develops into something good.

Keep the drive train simple and rugged. If its not absolutely necessary or can be done with a simple cheap method don't get carried away. But don't get to minimalistic on the peak power though. Efficiency doesn't equate to needing to be underpowered.

Keep some reserve headroom in the system design. If your running at 100% power output just to maintain yourself on level ground a good head wind or slight hill is going to really be embarrassing! Putting a brick on the power peddle and getting out and pushing because a slight hill and a breeze stalled you out will be the talk of the town for years to come! Especially when the grade school kids come out with string and bicycles to give you a tow!

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#4

Re: Torque Required to Move a Hybrid Car

08/20/2009 12:24 AM

I can't help you myself, but keep an eye out for post from KenFry.

This is right up his alley.

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#5

Re: Torque Required to Move a Hybrid Car

08/20/2009 1:23 AM

I'll post the formulas you need and some suggestions for putting them in a spreadsheet tomorrow. If I fail to do so, feel free to PM me through CR4.

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#9
In reply to #5

Re: Torque Required to Move a Hybrid Car

08/21/2009 2:07 AM

Yikes, it's already late, and I'm just finishing up work in my shop... but quickly...

Tractive effort is the forward push of a tire against the road to propel a car. It happens that on a vehicle the size of yours, the tire radius is very close to one foot. Therefore, if you apply 1 lb ft of torque to the wheel's axle, you get one lb of tractive force. (In your spreadsheet, you can use actual tire radius to refine this, but at this stage, it is quicker and easier to keep things simple -- and having ball park figures helps to ensure that you don't put inches in where feet belong and then buy a motor with 12 times as much torque as required (or vice versa).

At any instant, the ability of a car to accelerate depends upon there being more tractive effort available than required to maintain the current speed. So, for example, if 50 lbs of tractive effort are required to overcome aerodynamic drag at 55 mph, and you are on a 6% hill (requiring 180 lb tractive effort for a 3000 lb car) and another 30 lb of effort are required to overcome rolling resistance, then to be able to accelerate you will need more than 260 lb of tractive effort (or in our simplified case, 260 lb ft of wheel torque). How quickly can you then accelerate?

It is simple to work in "gees" because it's easy to remember that applying a force equal to the weight of the car will cause it to accelerate a 1 g. If you want .1 g of acceleration, then you need an additional .1 times the weight of the car in tractive effort. So if the car is 3000lb, and you want to be able to accelerate up this hill at .1 g, then you need 300 lb additional tractive effort, or 560 lb -- again, 560 lb ft of wheel torque.

So... First determine what grade climbing abilities you want. If you want performance like a normal car, you will want to be able to climb at least a 40% grade in first gear. At 60 mph, you will want to be able to climb a 6% grade, to maintain speed on interstates. When going 60, aero drag is working against you, and your gear ratio is also working against you.

Next, determine what you want as a 0-60 time. 60mph is 88 feet per second, and 32 ft per second per second is one g, so the average acceleration required for a 10 second 0-60 time is 8.8 fps2 or (8.8/32) .275g.

Where the acceleration time calculation starts to seem a little complicated is when you take into account the things (aside from mass) that are trying to keep the car from accelerating. If aero drag is 30 lb at 30mph, it will be be 120 lb at 60, and will be changing constantly along the way. That's where the spreadsheet comes in.

At increments of 5 mph, you do all the calculations of the drags on the car: rolling resistance, aero, transmission losses, and find out what the available tractive force is (based upon the gear ratio and performance curves of the engine or motors) to apply to accelerating the car.

The formulas to use are pretty simple:

1. Rolling resistance can be considered constant (relatively independent of speed) and is equal to the weight of the car x the rr coefficient (which ranges for .014 for sticky tires down to .006 for low rr tires).

2. Transmission losses are also relatively constant, running from about 98% efficient for a chain drive down to about 90% for a geared transmission with several reductions.

3. Grade climbing tractive effort is simply the weight of the car x the grade in percentage.

4. Aero drag is the frontal area in sq feet, times the speed squared (in feet per second) times .0012 times the drag coefficient. (The coefficient for the Neon is probably about .35, I'd guess, but I'm sure that actual values can be found on the web. The formula usually looks like this: 1/2 rho x V2 x Cd x A. (The figure .0012 is equal to 1/2 rho, rho being the mass density of air).

Torque and HP are not independent, so you cannot really think in only one or the other. Torque does not require movement... it is only a force. (You can apply 20 lb ft of torque to a bolt that you have already tightened to 40 lb ft, and nothing moves -- no work is done -- but torque is being applied and can be measured.) When there is movement, then work is done... and power is the rate at which work is done.

It is worth noting that the constants (like rolling resistance) will cause the same drag at low speed as at high speed. However, to overcome these drags requires twice the HP if the speed is doubles (because the rate of work has doubled). Aero drag sets a limit on top speed, because doubling the speed quadruples the drag, and therefore octuples the hp requirement to overcome that drag.

It is important to have performance curves of the motors to be used, because you want to keep them from operating where they are not efficient.

That should get you started. If you have questions, just ask.

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#14
In reply to #9

Re: Torque Required to Move a Hybrid Car

08/21/2009 11:57 AM

Wow so much information. Thats why I love CR4 people are so willing to share their knowledge. Thank you very much this is going to help me a lot. It is going to take me a little while to digest every thing you wrote.

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#21
In reply to #9

Re: Torque Required to Move a Hybrid Car

08/21/2009 5:22 PM

Fishpipes and Blink:

I have a spreadsheet. This is for a flat road, no acceleration...

Putting in the weight 2000 lbs (empty weight 1500 plus cargo of 500...) and flat plate drag about what a Prius is (6.25 sq ft).

Hope that helps.

SpeedRolling DragAero DragTotal DragRWHP
(mph)(lbs) (lbs)(lbs)
1016.041.5217.550.47
1516.103.4119.510.78
2016.216.0622.271.19
2516.369.4725.841.72
3016.5713.6430.212.42
3516.8518.5735.413.30
4017.1824.2541.434.42
4517.5830.6948.275.79
5018.0637.8955.957.46
5518.6245.8464.469.45
6019.2554.5673.8111.81
6519.9764.0384.0014.56
7020.7874.2695.0417.74
7521.6885.25106.9321.39
8022.6896.99119.6725.53
8523.77109.50133.2730.21
9024.96122.76147.7235.45
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#24
In reply to #9

Re: Torque Required to Move a Hybrid Car

08/05/2010 1:28 PM

Hi I was trying to solve a problem very similar to the question at hand. I'm not very familiar with physics (or cars), but I'm trying my hand at it. So to the point. My question is how much torque and Horsepower does a car (say 4000lbs) need to reach 60 miles an hour in 10 seconds (Assuming wheel's radius equals 1 foot).

Now, I know there is a lot of things to keep in mind as far as resistance, but I'm ignoring that part for now.

60 mph = 88 fps

over 10 seconds = 8.8fps^2

g's = 8.8fps^2 / 32.15 fps^2 = .274g

torque = 4000lb * .274g = 1094.9 lbs - ft

Horsepower = (torque * RPM) / 5252

60 mph * (5280 ft/ 1 mile) * (1 Rotation/6.283 ft) * (1 hour/60 minutes) = 840.4 RPM

(1094.9 * 840.4) / 5252 = 175.193 HP

But this is where my problem comes in. What next? My lack of knowledge is keeping me confused. I don't understand how a car can have 1094.9 lb-ft of torque when most of the cars I've looked at only have 130-140 lb-ft of torque on average. Then because of my confusion on the torque. The horsepower seems to be too high. Is it because I didn't divide by the number of "driving tires?" Still that even then it seems high for acceleration without any resistance.

Sorry for a dumb question, but I've been working on this for a few days and I can't come up with a solution.

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#25
In reply to #24

Re: Torque Required to Move a Hybrid Car

08/06/2010 9:37 PM

Why not use this formula to calculate power:

Kinetic energy due to translation= .5 M V^2 = W * V^2 / (2 g )

EK = 4000 lbs * (88 fps)^2 / ( 2 * 32 ) = 484,000 ft lbs,

P = EK / t = 484,000 / 10 s = 48 400 / 550 = 88 HP.

That is the theoretical HP you need. It is not torque but hp that makes you accelerate and maintain speed. Torque is a result of power and rpm of the wheel and it changes constantly with speed.

If you want to know the torque as well you would need to know when.

T = HP * 5252 / n = 88 * 5252 / 840 rpm ( your number) = 550 ft lbs

That is the torque at 60 mph. At any other speed you need to recalculate the torque that exists at the speed you select. At zero speed it would be infinity in theory.

In praxis is depends on your gear ratio and clutch.

In general, such calculations are mostly theoretical as if the power would be applied constantly with an infinitely variable transmission. Its only school math. In real life we have gear ratios and need time to shift. Power is not constant either as it changes with engine speed as well. You better add some hp if you want to accelerate in 10 seconds to 60 mph.

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#26
In reply to #25

Re: Torque Required to Move a Hybrid Car

08/06/2010 11:00 PM

Looking again at your calculation, look at your following line:

" torque = 4000lb * .274g = 1094.9 lbs - ft "

This should read: Force = 4000 lb * 8.8 ft/s^2 / 32 ft/s^2 = 1100 lbf. (not ft-lbs, it is a force not torque.) F (in lbs) = m * a.

Now calculate the distance traveled: Dist = .5 * a* t^2 = .5*8.8*10s^2 = 440 ft.

From this you calculate the work or energy: E = F * D = 1100 lb * 440 ft = 484,000 ft-lbs.

The power is energy or work / time: 484,000 ft lbs /10 s = 48400 ft-lb/s,

divide that by the constant "550" for HP = 48400 / 550 = 88 hp. Same result as per my first answer.

Torque T = 88 * 5252 / 840 = 550 ft lbs. (coincidentally the same number as the conversion constant)

Now, I made an error in the first answer in saying that the HP is constant and the torque changes. In reality the torque stays more or less constant and the hp changes with speed in such a calculation. That means you need 550 ft lbs of torque on the wheels at all times, after all other losses, to get to 60 mph in 10 seconds with the 4000 lb car, not considering shifting and time delays, wind and rolling resistance etc. . The hp is then calculated by using the torque number times the rpm of the wheels:

HP = T * n /5252. The hp needed rises with increasing speed at constant acceleration.

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#27
In reply to #26

Re: Torque Required to Move a Hybrid Car

08/07/2010 1:13 AM

First and foremost, Thank you very much for the suggestion and help.

My appologies on this one, the line "torque = 4000lb * .274g = 1094.9 lbs - ft"

should read

"torque = 4000lb * 0.274g * 1 ft(radius of wheel) = 1094.9 lbs - ft"

Which leads into the next question, what does "lbf" stand for? pounds-Force?

With a little studying up on my physics I think I might be able to grasp the idea you are showing me. So does that mean my approch was incorrect? Or was my math horribly off?

Thank you again for everything Floram!

-PM

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#28
In reply to #27

Re: Torque Required to Move a Hybrid Car

08/07/2010 1:27 AM

4000 * .274 = 1096 ... lol

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#29
In reply to #27

Re: Torque Required to Move a Hybrid Car

08/08/2010 10:14 PM

Indeed, lbf stands for pounds-force.

Your line F * r = 4000 lb * .274 g * 1 ft radius (wheel) = 1100 ft lbs of torque is also correct. Not sure what the other guest is lol about.

Why do I calculate 550 ftlbs of torque and you have 1100, you may ask.

As I said before, it is all school math. 550 ftlbs of torque exists at the final speed and at 88 hp, and 1100 ftlbs of wheel torque exists at half speed and 88 hp. At a constant hp torque rises with reduced speed.

Also note that the 88 hp is the "average" hp at a constant torque of 1100 ftlbs. The maximum hp at constant 1100 torque is twice the 88 or 176 hp at maximum speed of 88 fps (60 mph) and 1100 ftlbs torque, which you don't need. Confused? you bet.

Use a spreadsheet and calculate the following:

Set the first column at selected wheel rpm, pick say 10, 50, 100, 200 in 100 hundred increments to 800, add 840 as max and 420 as half speed.

In the second column calculate hp at a given constant torque of 1100 ftlbs. (hp = 1100 * n /5252). You will see the max hp is 176 at 88 fps, twice as much as needed.

In he third column calculate Torque from a given hp of 88 hp. (T = 88 * 5252 / n).

The first column displays the hp needed at a constant torque. (school math). Reality is different, it is somewhere in between but closer to the answer in the second column.

At top engine hp, really at read line engine speed the top gear ratio is such to have 840 wheel rpm and 550 ftlbs of torque on the wheels, that is all that is needed. As you can see, at lower speeds the torque on the wheels is higher which is normal due to the lower gear ratios. If we would have a constantly variable gearing the results of the second column would be approached.

Since we have gear ratios we are in between both results but close to the numbers of the second column. Trust that helps.

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#30
In reply to #29

Re: Torque Required to Move a Hybrid Car

08/09/2010 1:13 PM

So my math was sorta right, but my logic was wrong. So it lead my answer to be wrong.

Every time I think I understand what you're saying my mind fools me.

The way "I'm" thinking about it is...

"I need 1100 ft-lbs of torque to move from 0 - 60 in 10 seconds," but what I really need to be thinking about is "I need 88hp to get from 0-60 in 10 seconds."

So the mistake I made was in the equation "HP = torque * rpm/ 5252" the rpm isn't the final speed I want is it?

Is the RPM the last thing I should solve for? I should solve for Torque(1100lb-ft). Then use the given equation ("P = E/t" WHERE "E = 0.5 * mass * Velocity^2") to solve for HP (88HP). Which then allows me to use the HP and the Torque to solve for the RPM (420rpm)?

So part of this makes sense and part of it doesn't. So do my RPM's increase(0 to 840) over 10 seconds because I'm increasing in speed? Then that means there is a proporonal decrease in torque(46,217lb-ft to 550lb-ft) over that same 10 seconds?

Now once I reach 60 mph I want my RPM to equal 840 in order to stay at 60mph?

So I was calculating 1100 torque while I was at 60mph. Which isn't correct. I just want 1100 torque to GET TO 60mph. After I get to 60mph do I just need enough torque to overcome the "resistances?"

So because this is school math, in reality what you are saying is when starting off at 0 mph I cant go to 10 rpm and have 46k torque (to equal 88hp). So my torque and RPM start off low and build up (meaning my HP grows too 88hp). Which then because of the increase in speed the torque can become lower and I can still stay at 88hp, which will lead to my 60mph.

Thanks again.

-PM

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#31
In reply to #29

Re: Torque Required to Move a Hybrid Car

08/09/2010 1:30 PM

So not going to far into gear ratio, but just using them as a reference/additional support.

A car still has to build up to the required HP, but growth will be quicker because of gear ratios.

Because torque decreases with speed as the car gets faster you can shrink the gear ratio to get more RPM's, but you can only do this because less torque is needed.

A car can be at a Low RPM and have a high gear ratio (4:1, 3:1, ...) [low gear] and have a high torque

but...

A car can be at a high RPM and have a low gear ratio(0.95:1, ...) [high gear] and will have a low torque.

Is this also more or less true?

-PM

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#32
In reply to #31

Re: Torque Required to Move a Hybrid Car

08/09/2010 4:23 PM

Hello Guest,

My last answer was as "guest" as I was at another computer. It would be nice of you, I think, if you would join. That way we know who you are and you get better responses. Generally I ignore guests, you are an exception. We then can also exchange private messages.

Your thinking is not far off. It is a bit difficult as reality is different from school math. In practicality you cannot accelerate with constant hp, here 88 hp, as the torque at start up would be too high and can not be put on the ground, the wheels would spin and we don't have such a low gear (high ratio between engine and wheels).

On the other hand we can accelerate with a constant torque of 1100 ftlbs, however we would need twice or 176 hp when reaching 60 mph. But this is not necessary. We apply more hp and torque at lower speeds in real cases.

The calculated hp of 88 is the "average" hp as obvious, it is the average between zero and 176 hp.

Our car engine can produce 88 hp at all times also when going slow. If you made these tables I had suggested (you did) you will see that at the beginning of the run to 60 mph we have lots of unused hp. In real life more hp is applied at slower speed and less at higher speed (I mean less than 176). With a stick shift, revving up the engine and letting the clutch out slowly in first gear we can produce enough torque to spin the wheels. The clutch might not last too long but it can be done.

Full hp at 10 rpm and a torque of 46,218 ftlbs is not realistic. The ratio between engine output shaft and wheels would be 840 / 10 = 84 :1, too high, too much torque, too much stress, etc. This is only to illustrate what would be.

A car engine accelerating from zero car speed may have only about 25 - 30% of its maximum power and the gear ratio overall is likely under 20:1, ~ 4:1 engine to first gear and ~ 4:1 in the transaxle (differential). Now 840: 16 = 52 rpm = 8800 ftlbs * .25 = 2200 ftlbs, or * 30% = 2640 ftlbs which is more realistic and can be transmitted through the wheels to the ground.

As you can see, the car-, engine- and gear box designers need to work together to match the three for optimum car performance.

To make the 0-60 mph in 10 seconds you need more than 88 hp from the engine but not 176. The more gears you have and the higher rpm the engine can run between gear shifts the less max hp you need, but it will be more than 88. See also this link, (among many other).

http://www.allpar.com/eek/hp-vs-torque.html

Your other question what happens after you reach 60 mph, you have to separate the acceleration phase from a continuous speed phase.

So far we only looked at the acceleration part and ignored all other resistances. Once you reach 60 mph acceleration stops and we now need to consider wind resistance and rolling resistance and road slope if any. Mind you, these factors exist as well during acceleration and need to be added to the hp number of the engine.

There are other sites that give data on rolling and wind resistance. The power for the slope is simply the potential energy / time.

Ep = mgh, here 4000 lbs * 32 * h

Pp = mgh/t = mgv, where v is the vertical velocity. You simply multiply the horizontal vel times the gradient in %. 88 fps * 5% = 4.4 fps vertical velocity in this example.

I trust I answered your questions.

Regards

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#7

Re: Torque Required to Move a Hybrid Car

08/20/2009 11:15 PM

For the moment forget about torque. It will just confuse you. Horsepower is what drives the vehicle. It pushes it against the air and friction and makes it climb hills. Do your calculations in terms of energy or power (energy per unit time). Remember that although friction drag (drive train friction and tire friction are roughly linear functions of speed aerodynamic drag is a third power function of speed. Torque in hydraulics is a linear function of hydraulic pressure. Since in the first order your maximum hydraulic pressure is the same regardless of pump/motor speed you will have plenty of surplus torque at lower speeds for acceleration or hill climbing even if you have no gear reduction mechanism.

A 40 horsepower VW beetle was good for about 65 or 70 mph and had a reasonable hill climbing ability in 3rd gear. Your Neon is probably a bit better aerodynamic drag coefficient; but has more frontal area. Fitted out with all that hydraulics stuff plus an IC engine it will likely weigh in the 2500 pound range (about 20% more than the VW). Dig around on the internet to refine these numbers. Neons were popular in SCCA closed course racing some years back; so there is likely a good bit of data available on things like aerodynamic drag. All the hydraulics will have efficiency in the around 80% as compared to 90% for a conventional auto drive train. The upside is going to be the braking energy that your hydraulic motors and accumulator will capture for reuse.

So pick your components including consideration of maximum rpms and hydraulic pressure and the IC engine as the mechanical energy source based on how fast you want to go on pure hydraulics taking into account all the efficiency losses, friction and aero drag losses and then add 10% to that for a component wear margin.

Ed Weldon

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#12
In reply to #7

Re: Torque Required to Move a Hybrid Car

08/21/2009 11:28 AM

"All the hydraulics will have efficiency in the around 80% as compared to 90% for a conventional auto drive train."

Please provide references to support this claim.

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#13
In reply to #12

Re: Torque Required to Move a Hybrid Car

08/21/2009 11:54 AM

I agree I've never heard any numbers for a conventional drive train being 90% eff. Now the 80% for the hydraulics seems close enough. I've read anywhere from 70% to 90%, so 80% is a nice average. Now you have to take into account the lose on the whole system the pumps, motors, valves and piping. I haven't read what the system eff is but the motors and pumps both run about 80%. The best I've heard an automatic transmission can do is 70% and we don't even want to talk about a gas engine they have gotten it up to what 30%.

Have you seen how eff the little Honda generators are. A 2000w generator they claim can run for 15 hours on a gal of gas. I wish I could afford one to test it out. You can do a lot with 2000 watts anymore, led lighting, lcd TV's low power laptops. I could run my house with 2000 watts in the winter at least.

I know this is off subject but have you seen the Waterheaters that heat the water with a little built in Heatpump. It uses the room air as the heat source. When I read about them it got me thinking that what if you used the condenser coils from your refrigerator and put them in your water heater tank. Free hot water. Of course you don't use enough hot water so you would still have to have a normal set of coils for when your water gets up to temp.

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#22
In reply to #13

Re: Torque Required to Move a Hybrid Car

08/22/2009 1:14 AM

The best I've heard an automatic transmission can do is 70%

Actually, every conventional automatic transmission has a torque converter lockup clutch. With the converter locked up, an automatic is as essentially as efficient as a manual - 95% or better.

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#16
In reply to #12

Re: Torque Required to Move a Hybrid Car

08/21/2009 12:27 PM

Sorry RZV717.....I'm not going to take the time to research and write a book for you on the complex subject of drive train and hydraulic efficiencies. We've beat this subject around before. So let's just say my references are previous CR-4 discussions. Suggest you go to the search block on this page and start digging there.

Anyone planning to undertake the building of a car with an unusual drive configuration, especially one without prior building experience, is well advised to do a lot of careful research and design work before throwing resources at the project. My comments are very general at this point and are meant to motivate a rough back of the envelope type of analysis.

Ed Weldon

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#17
In reply to #16

Re: Torque Required to Move a Hybrid Car

08/21/2009 1:16 PM

Sorry, I didn't mean to come off harsh, just disagree slightly with the comment.

The 80% for hydraulics, can be accurate for some parts, and definitely for the whole system. Although, Some parts can be as high as 95-97%, such as a good bent axis motor.

Its the 90% efficiency for a conventional drive train that I have a hard time digesting.

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#20
In reply to #17

Re: Torque Required to Move a Hybrid Car

08/21/2009 1:38 PM

We pay a lot of attention to that in land speed racing. So do the drag racers. Synthetic oils and some tricks with transmissions and rear ends help a lot, Ditto tires. 90% is kind of a rule of thumb for relating chassis dyno results to engine brake horsepower.

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#23
In reply to #17

Re: Torque Required to Move a Hybrid Car

08/22/2009 2:19 AM

Its the 90% efficiency for a conventional drive train that I have a hard time digesting.

That figure is about right, when considered from flywheel to drive wheel axle shaft. Typically, each gear mesh is about 97% efficient, or so, depending upon the gear cut (with straight cut gears being more efficient but noisier). Old manual three speed transmissions (and many four speeds) had straight-through high gears, in which there were no loaded meshes in top gear, just direct coupling of input shaft with output shaft. These transmissions were 99% efficient in top gear, with the only losses coming from slinging around some oil and meshing the unselected gears under essentially no load. (The final drive losses would reduce this to 94% or so.) In a typical current car, there are three loaded meshes, two in the gearbox and one in the ring and pinion, so .97 x .97 x .97 is a good estimate: 91.3%

A standard v-belt drive is more than 95% efficient. Toothed belts are about 98% efficient, and a properly lubed chain is similar, or slightly better.

Hydrostatic drives as used in lawn mowers (a hydraulic pump-motor pair) could be moderately efficient, but typically are not. I've looked at performance charts which showed efficiencies as low as 50% under some conditions. In the second picture down on this page, you can see the indications of low efficiency: cooling fan and finned housing. 80% is probably a reasonable average for a pretty good hydrostatic transmission.

The places where hydraulic transmissions are used most points up their strong and weak points. You will never find them on solar racers, or bicycles, or anywhere else where efficiency and light weight is the first consideration. Instead you find them on lawn mowers and construction equipment, where infinitely variable gearing can give a control advantage, (such as the ability to crawl along with abundant torque) at the expense of efficiency. In the Fed X and UPS trials, it looks like, even for frequent stops and starts, the electric hybrids work better, and that situation is likely to evolve in ways that will favor electric hybrids more strongly.

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#19
In reply to #16

Re: Torque Required to Move a Hybrid Car

08/21/2009 1:27 PM

It is to bad you are so closed minded about someone thinking outside the box. This is my hobby, I've asked for some help with some of the math. I still think I'm going to end up with a nice little car I can tool around town with, and something I can be proud to say I built that. There have been a lot of positive statements made. I really hope this discussion doesn't turn into the name calling like some discussions do.

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#15
In reply to #7

Re: Torque Required to Move a Hybrid Car

08/21/2009 12:18 PM

Thanks for the encouragement and all the helpfull info. But I just don't agree with your eff numbers with a conventional drive train. The best I have heard for a car engine is 30% and thats on the new engines. I also don't agree with the weight. The Neon only weights in at 2100 pounds and I'm taking out 500 to 700 pounds in engine tranny. I'm not going to have a lot of hydraulic fluid on board because I'm going to cool it through a radiator so I'm only going to have 5 to 10 gallons, I'm not going to cool it in the tank the way some systems do. And it really depends on how big I can make my accumulators as to how much I'm going to need, the bigger the better in my mind.

I've got these cheap little 2 HP 130volt DC motors that I'm going to use to power my alum pumps. I think I'm going to be using D-cell batteries, I wish I could afford Li-Ion but they are still pretty expensive. Did you know that the 1st gen Prius used D-cells? I'm not looking to run on batteries just use them to store energy for a boast for passing or hills. I've got a little 250 honda motorcycle motor I'm going to use and that only weights in around 75 pounds. The accumulators are going to be the heaviest. All in all I'm really hoping to keep the weight under the 500 pounds I'm taking out.

Thanks for the input.

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#18
In reply to #15

Re: Torque Required to Move a Hybrid Car

08/21/2009 1:24 PM

Fishpipes -- I think the confusion over efficiency is due to what we mean as "drivetrain". I was thinking of the part that includes transmission, driveshafts, differntial, and the various included bearings, seals and couplings. But not the engine. Best to separate the two when talking about efficiencies although lots of people think of it as one system especially when comparing automobiles with other forms of transportation that don't use heat engines for propulsion. The wrinkle here is that all heat engines whether they be gas, diesel, gas turbine or steam are limited in thermal efficiency by thermodynamic laws to the 20-30% range at best. So best to separate the discussion of the engine and the rest of the drive system.

You're looking at a lightweight IC engine. So find the most efficient one that will give the power you need at the rpm your hydraulic components can stand. A substantial difference in those RPM numbers will mean that you'll need to gear down the rpm's of the motor. Hence the wisdom of using a motorcycle engine with its built-in gearbox. But I suspect that the 250cc engine will produce only enough horsepower to keep the Neon body going about 40-45 mph once the stored enercy in batteries and or accumulators runs out.

(You should test my numbers with your own calculations...you can get decent data with a coast down test and basic F=ma calculations if the neon is still in running shape.....a whole series of experiments with tire pressures, brake adjustments, light viscosity gear lubes might prove useful .)

I suspect a venture with a stock 4 stroke 250cc engine on to a class 1 highway will have you searching for an exit after a very few miles. But that may be fine for what you want the vehicle to do. Just go into your project with your eyes wide open. If you want to stick with the 250cc engine to start leave some latitude and space in your powertrain layout for a larger engine and/or a turbocharging option. The weight difference in these small bike engines is not that great as you go up in displacement.

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#10

Re: Torque Required to Move a Hybrid Car

08/21/2009 4:51 AM

Ref your design approach, have a look at this site;

http://www.epa.gov/OMS/technology/recentdevelopments.htm

This link;

Cost Effective Hybrids

And this link;

Hydraulic Hybrid Technology: A Proven Approach (PDF)

Now if you're thinking "car" note the mass of 'un-sprung weight' in the UPS truck embodiment. (dia in link 1) This is not good for 'handling', and if I am reading your proposition correctly, your intention is to add the mass of electric drive motors to the already fairly chunky hydraulic units.

Also you should consider 90 % (battery) x 90% (electric motor) x 90% (hydraulic motor) is 72% overall and 80% without the 'extra' electromagnetic transmission.

Note in link 2;

"The hydraulic system offers great advantages for vehicles

operating in stop-and-go conditions because the system can

capture large amounts of energy when the brakes are applied.

This energy is subsequently used to propel the vehicle."

This indeed capture is the major advantage over electric re-generation ('potentially' 50%)

Focus on "the brakes".

Figure out how braking may best work. Do the ½ MV2 numbers x your system capture %. Apply that "torque figure" to your 'motor selection'.

It is vastly greater than "drive" torque.

Accumulators are vastly over "50%"

"Complex control system"? "3 motors"?

Research how excavators, mining machinery and container crane hydraulic systems work and what type pumps and motors and control systems /produce "variable" and "stepped variable RPM" - and/or proportional speed to torque - and/or constant torque - and/or whatever you want. Have a harder look at what your output 'motor' is doing, and likely you will find 'there is a pump/motor 4 U !!!'

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#11

Re: Torque Required to Move a Hybrid Car

08/21/2009 10:01 AM

Its is not torque but HP you have to select.

Power is the product of Torque and Rpm. In other words the torque is only half the criteria, the other is the rotational speed. The torque number changes with speed, rpm.

I know a lot of people think that it is torque you need for acceleration. Not so, it is HP. You need torque and rpm which is Power.

I am sure you have seen this formula:

Power in kW = Torque in Nm * n rpm / 9550, or

Torque (Nm) = Power (kW) * 9550 / n (rpm)

I have only glanced at all the other answers. Perhaps this was said before, but not in a short statement.

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#33
In reply to #11

Re: Torque Required to Move a Hybrid Car

12/08/2010 10:14 PM

So how much torque is needed

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#34
In reply to #11

Re: Torque Required to Move a Hybrid Car

12/08/2010 11:15 PM

Depending on the inflation of the tires and assuming a flat, hard horizontal surface the torque will be in the range 20 to 50 foot pounds applied at the centerline of one wheel for the average passenger vehicle. Note that a momentary force in that range may produce no forward motion, only a slight horizontal oscilation due to the complex elastic behavior of pneumatic tires.

Note that the velocity achieved will depend on the amount of power (not torque) applied, the weight of the vehicle and the forces acting in a direction opposite to the vehicle movement (friction, wind resistance , etc.)

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#35
In reply to #34

Re: Torque Required to Move a Hybrid Car

12/27/2010 11:58 AM

Yes, I agree, its is power not torque that determines the velocity achieved. This applies to accelertion as well. Power will determine how fast you can accelerate not torque.

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#36
In reply to #35

Re: Torque Required to Move a Hybrid Car

12/30/2010 11:57 AM

So is that on a hill with .1 g acceleration?

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