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Flywheel weight,power and torque

  1. Sep 22, 2007 #1

    I've been reading this forum for a few days now and I'm impressed by how the questions are answered.Short,simple and easy to understand.
    So I thought I could get a good answer to my question here.

    One day,I was talking with someone who studied mechanical engineering.I told him that my passion is cars and that I'm going to study mechanical engineering too.
    So he decided to "test" my knowledge.He showed me an engine performance graph like this one:
    As you can see,this engine has its maximum torque at 4000 rpm and peak horsepower at (more or less) 5800 rpm.

    His question was:
    Why does the power increases after 4000 rpm eventhough the torque decreases?

    I answered that it was easy. Power = RPM*Torque,so even if the engine has less torque after 4000 rpm,it still has more power because of the rpms.

    And then the shock: he said I was wrong :confused:

    He said,it has something to do with the flywheel.He even said that some engines have a lot of torque because of their heavy flywheel and that race engines don't have a lot of torque because those engines have a very light flywheel.

    I said,it was non-sense.He got a bit angry and asked me other questions to prove that I know nothing about this stuff and kept saying that HE was the engineer and I was a simple trainee.I couldn't answer any of his questions because I was literally in shock because I thought I knew this kind of stuff for years and all of a sudden someone tells me I was wrong the whole time.

    So what do you guys think? Who's right and who's wrong?
    Mathematical formulas are very welcome!

    Sorry for the long post...I hope I was clear enough.
    Thanks :smile:
  2. jcsd
  3. Sep 22, 2007 #2
    hi The_Trainee-
    well,thats agood point what u opened to discuss here.
    your mathematical formula for the Power=torque*w(angular velocity)-is correct.
    But do u know how many hps are considered when u want to design an engine?
    1-theoretical hp and thats what u showed in ur formula
    2-Bhp and thats what showed in the graph.
    3-friction hp.
    4-Indicated hp which equals=Bhp+friction hp
    5-Rated hp,and this is what the manufacturer tells how much hp the car would give at a certain RPm.
    now,what the engineer was telling u about is the Bhp which is the power generetaed by the flywheel,so,it hase an effect on the power,when u use a light flywheel or heavy-to see the difference jus read this-(http://www.thumperfaq.com/mxa_flywheel.htm).
    so,the graph wasnt showing the theoretical hp which u could calculate from ur formula,if it was theoretical then ur right,about ur answer u give to the engineer,when the torque decreased cuase the Rpm increased then it would keep the power increasing.
    thanks in advance,and please let me know if im wrong-:)
  4. Sep 23, 2007 #3


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    Some people can't tell the difference between

    (1) engine power and torque
    (2) vehicle performance and "driveability".

    The OP's graph is about (1) and the understanding about the relation between power and torque are correct. If you read the numbers off the graph and convert one to the other, the two curves are exactly equivalent.

    Machinest's link is about (2) - and some of it (e.g. using nonsense terminology like "gyroscopic reverberation", and the garbled description of how to measure inertia) is BS.
  5. Sep 23, 2007 #4


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    You're correct. Unless you're seriously misrepresenting/misunderstanding this guy's arguments, he should return his degree.
  6. Sep 23, 2007 #5
    Thanks for the answers! :smile:

    I told him that it can't be like he said but he continued to say that I was wrong.

    I can't believe that someone that spend so much time at the university could be so wrong about something that is thaught in school.That's why I had so many doubts.

    I do have 2 more questions (could you please answer it?):

    -If that engine (the one of my example) had a lighter flywheel,would the torque curve drop much quicker after 4000 rpm?
    -Can a flywheel store enough energy to be able to accelerate an engine? (for example,when you're going at full throttle and then lift the throttle just a little bit)

    Thanks again!
  7. Sep 24, 2007 #6


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    Just to echo what was already said, this guy is a tool. His own plot proves him wrong. Tell him to take two minutes to do a couple of quick calculations to see that your answer matches exactly with the plots. I cringe whenever I hear anyone come out with that famous line "I'm the engineer. I must be right."
  8. Sep 27, 2007 #7
    Torque is increased either by increasing the radius of the flywheel or applying more force. Which, in turn effects the power. If you actually took any 2 sets of data point from that graph for Torque and converted to HP, you'll get what's shown on the graph. The only thing making the flywheel lighter will do is effect it's inertia. It'll effect vibration and damping issues as well.
    Last edited: Sep 27, 2007
  9. Oct 7, 2007 #8
    With a lighter/lighten flywheel, it will "feel" like you've gained 20 more hp. This not true in reality, it just means the power is transfered to the wheels a lot faster than the stock flywheel did. Revs are faster and revs also drop faster. The car will surely accelerate and de-accelerate a lot faster than before, making it really fun to drive...but it also hampers street driving, in stop and go traffic. I build Hondas as a hobby and im also an administrator at www.preludezone.com.

    -Can a flywheel store enough energy to be able to accelerate an engine? (for example,when you're going at full throttle and then lift the throttle just a little bit)
    Yup. Flywheels are made from strong chromoly metals, or cast iron. If the HP is increased on a motor(for example via forced induction - turbo/supercharge), the clutch and flywheel must be upgraded to handle all that power, or energy - for all sorts of applications.

  10. Oct 7, 2007 #9
    Oh and looking at the dyno sheet, im really semi-impressed on how much that 2 liter pumps out HP, even with the turbo. It looks like that motor is starving for more air :P

    oh and to back up Cornellians response, that 2 liter motor is equipped with balancer shafts and probably an engine dampener.
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