Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Got a question about air molecules.

  1. May 22, 2010 #1
    As you know i've been asking questions about alternative fuels and honestly its getting me no where, because I desire alot more power than what our time has to offer with electricity and other things lol

    BUT if there is one thing i know about, it is a flippin car motor and how to make alot of horsepower, (I'm a hoosier boy=p)

    so i can easily get my hands on 450ft/lbs torque@3XXXRPM and 310hp@6XXXRPM for a power supply. I want to manipulate the rotation of an axial type turbofan (since its gonna be handmade and hopefully be made correctly. I'm lookin at a 48" opening with a 10" exit) I want to create something like a 9 stage turbine like alot of the higher powered jets and airliners use.

    my question is, would running the exhaust of the engine through the turbine increase the velocity? i dont know much about airflow but i think i remember something about wider spread out air molecules move more matter covering more distance...er something close to that...i suck at wording out this stuff. i'd love to have an answer soon tho

    now to the manipulation part of the idea, this axial fan along with the oiled bearings and shaft will probably weigh around 300lbs. I want to get the most out the rotation, since there is no serious friction contact (like transmissions have so they use reverse gear calcs) would it be possible for me to have a 1:5 rotation?

    example; my engine will idle at 800rpms, the fan will be rotating at 4,000rpms.

    i read that thrust (in measurement) is a little over half the horsepower required (3900lbf needs 5400hp*not exact numbers but close*) but i'm guessing that measurement was from a direct hook up with no gearing change like i'm stating above.

    so with a 1:5 ratio, that would put my torque at 2,250lbs and 1,550hp, so in theory would I be able to produce 1,119lbf of thrust?

    i mean thats alot compared to how light the vehicle is going to weigh. the b-2 bomber will actually have a higher thrust to weight ratio than this vehicle would if my theory is correct. *weighing at 370,000lbs and only using 71,600lbf of thrust while cruising at 550mph*

    I know i wont be able to go 500mph but thinking upon how an engine revs almost instantaneously, the turbine would wind up and move alot of air really fast shouldnt it??

    if you answer everything, i'll love you haha. i'm sorry for no equations, i'm a musician...not a physicist.
  2. jcsd
  3. May 23, 2010 #2
    I'm afraid none of that really makes any sense.

    1. What 'power supply are you talking about'?

    2. What do you mean by 9 stage turbine? Do you acutally mean 9 turbines, or 9 compression stages? (As turbines reduce the alow of the exhaust).

    3. What do you mean by 'manipluation of the turbine'?

    4. Things like turbines can't be hand made, they are extemely precise peices of machinery.

    5. I don't get what you mean by 'running the exhaust through the turbine', as thats how jet engines work currently.

    6. What on earth is a 1:5 rotation? 1:5 to what?

    7. If you mean just spin it 5 times fater, then you come up with the exact same problem you had in the other thread. you'll just break it.

    8. If you mean make it act like a gear, you spin it faster but torque goes down but power levle stays the same. You can't gear power from nowhere. There is a finite amount of fuel burnt.

    9. This last part confuses me, so are you trying to build a jet car? Jets are a horrible way to try and move a car about, unless you aim is to build a drag car?
  4. May 23, 2010 #3
    1. this 350 small block chevy sittin in my dads garage.

    2. compression stages

    3. gear ratio's, as in theres a big pulley on the 350 and a smaller pulley on the turbine.

    4. I thought I would run into that problem so i'll just have to find one...even tho its expensive...

    5. the Jet engine is just going to be pushing air, no combustion. the 350 will be rotating the jet engine producing thrust. i was just curious as to shoving the exhaust of the 350 through the turbine would move more air with less rotation.

    6. 1 rotation of the 350 to 5 rotations of the turbine.

    7. I don't think thats true, alot of axial turbines of jet size can reach speeds up to 70,000rpms, this one would only be hitting 30,000rpms

    8. Not for sure what you ment by the power thing but yea you need torque to start up the motor and then hp to keep it spinning. I was in our schools auto tech, our teacher did a discussion on torque vs. hp in a demonstration; car 1 has 400ft/t and 100hp, car 2 has 100ft/t and 400hp. which would win in a 1/2 mile drag race? car 1 would kill car 2 off the line but car 2 would finish first because after car 2 gained momentum from a complete stop the higher hp designated a higher traveled velocity than car 1, AKA car 1's top speed is 60mph while car 2's is 150mph. its simple powerband diagnostics.

    9. and yes I am building a jet car. I had an idea that if my motor could sit at idling speed and yet make the turbine produce enough thrust to move the car, i could use the brakes as an accelerator and deccelorator, then when i hit the gas the turbine will react with the pulley of the 350 inducing lots of thrust.

    the reason why I wanna do this is because I could travel anywhere while just having my engine idle. no acceleration. and then kill competition when i floor it XD.
  5. May 24, 2010 #4
    Ok so you weren't doign what I was thinking. You are doing something of a 'reverse supercharger'. but not intending to burn any more fuel.


    If you aren't going to be burning anything, then you aren't going to get much thrust. You are infact just going to end up with something very very slow. As you are compressing massive amounts of air for no gain what so ever. You are proposing adding huge amounts of ram drag and weight and not really utilising the comressed air for anything useful.

    I thought the 'power plant' thing you were talking about was a gas turbine not a standard engine, and you were just trying to get more from it.

    I mean you take a 350HP engine. You apply a gear ratio to it, you don't incerase both the torque and the power by a factor of 5 (as you did before). If this were the case we would all be driving round in 1hp cars, and gear it up to any power level you wanted. If you are putting in a set amount of energy, thats all the power you have availalbe. As you are adding no more fuel to the compression in gas turbine, you are getting no more power.

    So you are proposing substiuting a transmission system from the 350 hp IC engine to the wheels that is quite efficienct for movement. To 350hp engine being used to power a compresser to produce thrust, which is a dreadfully inefficienct way of moving a car around.

    This idea will only work and produce more power if you used the comressed air in the gas turbine to burn more fuel.

    Even if we assume that the compressed air, after being accelerated though the nozzle will produce as much trust as the exhuast from a standard gas turbine (which it won't), we can use the rule of thumb of thrust pounds = power (hp) /2.5 (this is about right). So the very best you can hope for is about 140 lb of thrust. 622N.

    That 350 hp engine is putting out 450 ftlb torque, 610Nm. If you run this figure through the gearing and use the rolling radius you will find you get FAR more thust using the engine conventionally.
    Last edited: May 24, 2010
  6. May 24, 2010 #5
    okay bud, we need to talk then. explain to me how a helicopter produces so much thrust that it is able to basically defy gravity and float? it has no wall structure or compression like housing to produce more thrust, and yet, the damn thing flies. To make an object descend upward in aviation, you need a matching force compared to the weight of the vehicle. being a helicopter. thats alot of weight. opposing this weight. you need alot of thrust. Explain how a 4 bladed fan,with NO exhaust what so ever. could produce far more thrust than 9 stage compression turbofan, with NO exhaust what so ever. at the same rpm?

    the 350 is actually the CID of the v8 but its all good haha. everything you stated about the power is correct. but you forgot what i said about the 1:5 pulley ratio. hence boosting my 310hp to 1,550hp.

    now this is why i mentioned all that torque. if i have a rotating, oiled, axial fan that weighs 300lbs, and i have a motor that produces 450lbs/t. the motors torque could move the weight of this turbofan without much effort, enabling the 350 to idle at 800rpms and the turbine to idle at 4,000rpms.

    in perspective its like a water wheel. you have a big wheel flowing by current then the rotation of the water wheel is being manipulated by making a very small gear spin very fast that is hooked up to a generator. producing more power than it would be producing if the water wheel and the generator gear were equal sizes.

    i honestly dont think you can go wrong with this idea. guys see 410lbs/t torque and they cream their pants. japanese and europeans see 610Nm in a car and think its a mythical dragon.

    and even WITHOUT the gear ratio, 140lbs of thrust would still actually be somewhat fast, but thats why i mentioned using that pulley system to manipulate the power.

    hope this cleared up alot of stuff i probably worded wrong. thanks for the replies.
  7. May 24, 2010 #6


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    A few points:
    1. It's well know that helicopters hover very inefficiently. As they are never climbing at much of a speed, their useful Power (in climbing) is small.
    2. Pulleys can't increase Power - they can only increase Force and reduce Speed. Force times Speed is Power - so no gain but a loss from friction.
    3. Sort out a few basics before you waste a lot of time an money on trying to put your plan into action.
  8. May 24, 2010 #7
    Helicopters don't use thust in the way you are thinking to hover. They use a powerful engine to power rotors. Which are 4 aerofoils (wings) that move through the air and generate lift. The lift is a function of the swept area covered by the rotor.

    You need to go and read up on this. Gears/pulleys cannot make power, they alter torque and rpm. The power is always the same.

    It may be turning at 4000rpm, but it isn't producing any power. Lets say it takes 200hp to run a compressor, all you are doing is compressing 200hp worth of air. You can try to use that air to move around, but it would be more effective to simply use the 200hp connected to the road wheels.

    It's just like running a supercharger. They take power to run, but don't acutally give you any power unless you burn more fuel.

    Utter, utter rubbish. The power produced is exactly the same. The large wheel would produce far more torque at lower rpm.

    It'll be a hell of a lot slower than using the engine directly connected to the roadwheels via a gearbox. If you are running your gas turbine, all you will get is 622N thrust.

    Just to show how poor it is. A car with 250Nm torque (turbo motor so a steady flat torque curve - as you have a compresser so can I :P). 2nd gear ratio of 2.06 and a final drive of 3.8. This gives a wheel torque of 1950Nm. It has 16inch wheels and 210/50 tyres. So a rolling radius of .6214m.
    This will produce a wheel thrust of 3250 N. OR 5x that of the gas turbine.

    Now we know that the compessor will provide steady thrust and only get that at higher speeds. So we'll compare that to top gear in the car. 6th 0.73 * FD 3.8 = 2.774

    2.774*250 = 693Nm rear wheel torque.
    = 1116 N thrust.

    So even in top gear a car with lower torque figures than that V8 is still putting out more thrust.

    I don't wish to be mean, or appear rude, but you need to go away and read up on some of the things discussed. As they weren't wrongly worded, they were just wrong.
    Last edited: May 24, 2010
  9. May 24, 2010 #8
    I'm so lost then...how can power be manipulated by gear changes like u said above and also in figure, but yet not produce more power...?

    if 2.5hp is able to produce 1lbf thrust at a 1:1 ratio. then with a 2:1 ratio wont that double the thrust while keeping the hp the same, but in theory actually double the hp on the turbine end?

    its in gearboxes everywhere... how do you think a corvette does 0-60mph in 3.7 seconds? hooking the motor directly to the wheels wont do it... it uses that 2.6:1 and 3.8:1 ratios u were just talking about above...

    it takes that torque, multiplies by a transmission, then multiplies it again by a differential. and in the end you have 3000 some lbs of torque pulling that vette to 60mph in first gear... thats exactly what i'm talking about with hooking this 350 up to a turbofan... theres no gears its just a gas and brake pedal and an e-brake... no transmission or differential to the wheels... the 350 will power the turbine using a gear ratio *like a transmission* to make the turbine spin faster. isnt the rotation of the turbine is what causes thrust? or am i not understanding how a fan works...=/

    and i cant go away because there is no other place that explains this stuff as you guys do...because what im readin is saying this is clearly possible and is argueing with everything you 2 are saying...

    i was just simply asking if shoving air thats 40 times hotter than a mild summer day through the turbo fan would increase the velocity of matter...
  10. May 24, 2010 #9
    Becuase it reduces the RPM. So lets do an exmaple with nice round numbers.

    The equation for Power proportional to torque * angular velocity. In imperial the equation is
    Power = Torque * RPM / 5250

    An engine produces 100 ft.lb torque at 100rpm. If it's hooked up to a wheel 1:1 with the engine the wheel will spin at 100rpm.

    The power is: 100*100/5250 = 1.9Hp

    By using a gear ratio of 2:1, For every 2 revolutions of the engine, the gear revolves once. MEaning the wheel revolves then at 50rpm. So the speed is halved. However the torque is doubled. So instead of 100 ft.lb at the wheel, there is now 200 ft.lb.

    The power is: 200*50/5250 = 1.9Hp

    This is why lower gears priovide better acceleration, but can't attain the same speed as higher gears. The power output remains exactly the same.

    And thrust from a gas turbine is the speed of the exhaust * mass of stuff flowing through it. This is why combusting stuff in it increases thrust. It certaily will produce thrust by just compressing air, but that level of thrust will be much much less than just connecting the engine up to a gearbox and using it normally.

    EDIT: Putting exhuast gasses through it will have the potential to make more thrust. However you'll never get enough exhuast gas to feed it constantly. However it could also be mixed with a frech air feed. (I doubt the gains would be significant though).
    You also have the problem that doing this will create enormous amount of back pressure and destroy the pressure waves in the exhaust. (Basucally the engine will run like a bag of spanners)
    Last edited: May 24, 2010
  11. May 24, 2010 #10


    User Avatar
    Homework Helper

    Ignoring power losses in a transmission, gearing multiplies torque and divides rpm or vice versa, without changing the overall power. Converting this concept into linear force and speed, a 1 horspower motor can produce 1 lb of thrust at 550 feet / second, or it can provide 2 lb's of thrust at 275 feet / second, or it can provide 1/2 lb of thrust at 1100 feet / second, but those are all examples of 1 horsepower.
  12. May 24, 2010 #11
    along with what you said about a helicopter. sure its inefficient but the fact still stands that you need the same amount of upward thrust to go up. lol. i weigh 150lbs. i'm pretty sure i can't even make myself lift if i spun around with big fans on my arms. its still alot of thrust to lift a helicopter. isnt that what i'm producing with this design? my car isnt gonna lift off the ground. its just going forward. there are airplanes with 150hp motors that use 4 fan blades that fly. producing wayy less thrust than what im talking about. you cant tell me this idea of a turbofan car would be slow and inefficient. its opinion vs fact.
  13. May 24, 2010 #12
    so if i want to travel 60mph, i need to go 88 feet / second correct? so i take 550 divide it by 88 and that gives me 6.25lb of thrust at 1hp.

    something tells me thats not correct...
  14. May 24, 2010 #13


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I can see your confusion. A real muddle in your head at the mo' I think.
    Petrol engines don't work at zero speed or even low revs - they have an optimum range of revs to work at. The gearbox allows you to use that range of revs over a much bigger range of road speeds.
    'Extreme' cars use the wheelslip as a sort of clutch and first gear can be a bit higher than in a conventional car - hence the 0-60 in first figure.

    What do you mean by "multiplying torque"?
    Power is the fundamental quantity for an engine. You can't get more power out than it can deliver.
    Torque = Power/revs and the road wheels rev slower than the engine so there is more torque at the wheels than on the crankshaft. But that isn't relevant. You are limited by the Power, in the end. More torque is only available by using lower revs of the roadwheels. Power hasn't changed.

    The torque on the road wheels is less in high gears (wheelspin happens in first for almost any car, at times).

    I really can't understand how you expect your turbine / fan / thingey to be producing extra power except, perhaps, to make use of the Kinetic Energy of the exhaust gases. Unfortunately, this would involve a finite amount of back pressure which would spoil the breathing of the engine (turbochargers actually take some of the power from the engine but you have a net gain because they stuff more fuel / air into the engine). Blah blah . . but you don't want to hear that, I suspect.

    I have a feeling that the subjective effect of a massive roaring engine is weighing more in your mind than the basics of physics (which always work, where engines are concerned).
  15. May 24, 2010 #14


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    How is it not correct? The sums are correct; it's just your appreciation of what they mean that's adrift.
    You would need a very low drag factor to enable a vehicle to travel at 60mph with only a 1hp engine.
  16. May 24, 2010 #15
    I had the image of him using the gas engine as a 'reverse supercharger'. Using the V8 to spin the compressers and then add extra fuel to the gas turbine to get more power. It's basically the same operation as a gas turbine, only with a V8 powering the compressor instead of a turbine.

    A true jet car.
  17. May 24, 2010 #16
    ugh. this garbages gay. this is all theory...i wish i was rich so i could just try it. i just took calculations, found their outcomes of greater turbo fans and dwindled their power bands down to a reasonable horsepower rating that i could obtain inside a lighter vehicle... so does this prove that everything written down about power specs is false?
  18. May 24, 2010 #17
    No just you are using the tools wrong.

    You have a fundamental misunderstanding of how the power torque and gearing works. You also seem to have a misunderstanding of how a gas turbine produces it's power and trust.

    It's this misunderstanding that is leading you to doing wrong sums, and making wrong conclusions from correct sums.#

    don't worry about it though, if you want to learn where you are going wrong, we are here to help and can point you to some reading material.

    I think this thread should be killed off, as the original content isn't worth discussing currently. If you'd like to start a new thread wehre we can deal with a single topic at a time, it'll be less confusing.
    Last edited: May 24, 2010
  19. May 24, 2010 #18
    but that means if i wanted to go 60mph with a 310hp motor i'd be making 1,937lbs of thrust. which is obviously impossible because every is saying so.

    i have no concept of thrust. i'm just using calculations and what others are saying. i've never used a turbofan before.
  20. May 24, 2010 #19


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Your example of a helicopter is totally irrelevant. If you want to compare fans with driving wheels on the ground, look at the 0-60 figures for a sports car and a light aircraft. The car always wins. In a plane, you're stuck with a propellor because you can't apply traction to the ground.
  21. May 24, 2010 #20


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Do you actually drive a car?
    If you did, you would remember that, once you have reached 60mph (after Accelerating with full thrust), you back off the accelerator and cruise with just enough power to keep you going. With your 1937lbs of thrust you would still be accelerating through 60 at a considerable rate until you reached 180mph or something, at which point the drag forces would balance your thrust and you would have reached your terminal speed.

    You seem to be shouting rather loud and just not really thinking about this!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook