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

Understanding DynamometersMeasurements

  1. Apr 2, 2010 #1
    Hey there folks :smile:

    I am trying to gain a more technical understanding of dynos and how they really work. When measuring the power at the wheels of a vehicle there are really two variables being measured: Torque and rotational speed of the roller.

    I have read on the different methods of controlling the torque. Now I need to clear up some things:

    How does a dynamometer actually measure the a) Torque and b) Rotational speed?

    Let me clarify my question a little: I know different ways that one could measure these things; but I am hoping someone with a little dyno experience can step in here and tell me how dyno manufacturers actually do measure these things.

    Is an encoder built in to the roller? Or a tachometer? How is the torque actually measured? Some kind of load cell?

    Anyone know of a specific brand of Dyno that has a white paper about it that I could read? (Yes. Google is working, but I am still looking for more technical writings.)

    Any thoughts?

    EDIT: After much Googling, I am thinking that a tachometer and load cell are used for the measurements. However, the load-cell reference that I came across was in a patent filing that was never granted; so that is hardly a reliable source. I am looking for some concrete instances of a particular kind of transducer being used on practice.
    Last edited: Apr 2, 2010
  2. jcsd
  3. Apr 3, 2010 #2
    Well you can either measure the energy directly, or measure the force through distance.
    The force through distance means a load cell is the easiest way to measure the force, and for distance the speed of the rollers (RPS*2*pi*dia).

    Directly measuring the energy can be problematic, maybe a generator and measure the voltage and current to get kW ??
    I know that some use eddy current or other means for dissipating the energy.

    The other way is to measure the effect of force (torque) against the mass, by measuring it in-situ with an accelerometer on the vehicle. This is probably the most relevant and takes out the tire pressure and other concerns. But you need a (private) road and all the gear for associated measurements to be on the vehicle (e.g. AFR).
    It used to be done on at least some Le Mans type cars.

    Do the in-situ approach with an accelerometer is a project I am working on.
  4. Apr 3, 2010 #3
    Wow, I can't believe how difficult it is to find information about the actual architecture of a dyno. I was hoping to find a 'blueprint' that shows the layout and what kind of components are used...but nothing.
  5. Apr 3, 2010 #4

    jack action

    User Avatar
    Science Advisor
    Gold Member

    You need to look at the http://www.land-and-sea.com/" [Broken].

    http://www.land-and-sea.com/dynamometer/dynamometer-comparison.htm" [Broken] you have a comparison of all types of dyno.

    http://www.land-and-sea.com/dyno-dynamometer-article.htm" [Broken] they tell you how they work.
    Last edited by a moderator: May 4, 2017
  6. Apr 3, 2010 #5
    Nice jack. That's some good info at that site. Most places give a high-level description like "we measure the torques with a 'torque sensor' and the RPMs to calculate Power." That's great and all, but *what kind* of sensor is what I was looking for and this site has great info!

    Thanks again :smile:
  7. Apr 5, 2010 #6
    From what I remember:
    Dyno-mite uses a strain gage for measuring torque and uses engine RPM signal to calculate power. Dyno-mite uses water pressure in an "absorber" turbine looking thing for generating the torque.

    Mustang dynos use eddy current dissapation to directly calculate power, I think.

    Dynojet probably uses a tachometer on the drum, and calculates force based on rotational acceleration of the known intertia of the drum.

    There are also some hydraulic based units out there, don't remember the names now.
  8. Apr 5, 2010 #7


    User Avatar
    Science Advisor
    Gold Member

    All the eddy current (and AC) dynos I've ever used have still used the torque reaction (by means of a load cell) and a tachometer to measure power.
  9. Apr 6, 2010 #8
    Great info guys. I am still trying ti figure out what *flavor* of dyno they would use for a dragster? It seems that I am finding some info that conflicts (mostly because I don't understand every application).

    There are *inertial* dynos that cannot measure torque directly and there are brake dynos which can. Since a dragster cannot really be kept at a steady state load (else the engine will burn), it wold see that the inertial is better for this application seeing as *brake* dynos need to be run at steady state.

    Any thoughts on this? Is my reasoning sound?
  10. Apr 7, 2010 #9
    What kind of dragster are you thinking about? Not sure I have ever seen a commercial dyno that can handle much over 1000 hp, but someone correct me if I am wrong. For top fuel dragsters I always seem to hear "estimated hp" since there are not dynos that can handle them.

    When I have seen brake dynos run, they can certainly do "dyno pulls" by varying the load on the rollers during acceleration. I have run a water brake dyno, its load varied with RPM due to the "absorber" design and a variable valve.

    This wikipedia article has some interesting info: http://en.wikipedia.org/wiki/Dynamometer
    Last edited: Apr 7, 2010
  11. Apr 7, 2010 #10

    jack action

    User Avatar
    Science Advisor
    Gold Member

    Are you talking about this:


  12. Apr 7, 2010 #11
    Wow, there you go!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook