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

Small engine dyno

  1. Feb 11, 2008 #1
    Hi guys could you help. I am building a small engine dyno using an car alternator. MUT is rated 6.5kW. I was thinking of using the output from the alternator to power the laptop through an dc to ac inventor or heat loss through heat coils from hairdrier. How would i control the trottle feed, varying alternator using a computer. Please help.
  2. jcsd
  3. Feb 11, 2008 #2


    User Avatar
    Science Advisor
    Gold Member

    What's MUT?

    What's your engine rated at? Speed? Power? What are you doing this for? A car alternator could be way too small.

    Most commercial load banks are computer operated these days. They basically work by switching on and off heater elements. The control system just needs to know which elements correspond to which loads at a given voltage, and switch them accordingly. And obviously dissipate heat effectively.

    Control throttle feed? This would be done separately by your engine control system.
  4. Feb 11, 2008 #3
    MUT simple means motor under test, the engine is rated at 6.5kW go kart engine. I am doing this a to measure rpm, torque and air fuel flow rate. Wat about a 24v alternator. This is for my mech final year project. The auto lab has no small engine dyno.
    Last edited: Feb 11, 2008
  5. Feb 11, 2008 #4
    Just added something
  6. Feb 11, 2008 #5
    You might be able to use a data logger as your central hub for receiving information. You would plug in different sensors to the data logger, then record that information to something like data to store the info.

    Someone told me that if you use the right data logger, you can control something from the computer through Excel. I am not sure how to do that yet, but if you are in school, you should be able to find someone who has a background on these.

    Hope this helps somewhat.....
  7. Feb 11, 2008 #6


    User Avatar
    Science Advisor
    Gold Member

    MUT, I've never heard that, despite testing engines every day! Thanks!

    You'd need a bloody big battery-charging alternator to manage 6.5kW. I'd try and get hold of a small commercial alternator from someone like Newage Stamford or Leroy Somer.

    Measurements will be easy; rpm will come straight from your load (frequency) or from the flywheel. Torque can be measured (mechanical or strain gauge) or calculated from the alternator output. All your flows can be calculated reasonably easily from pressures, or measured directly.
  8. Aug 1, 2008 #7
    hi Bongz,

    You still working on this project or is it done? I did this same thing for my BScEE thesis: an "active" dyno for an SAE supermileage car using an alternator.

    Here's what you need to do:
    Pick an alternator which is rated greater than your motor, or downsize your motor, or physically chain a couple alternators together. A 90A alternator putting out 13.2v = 1188watts.

    1188Watts/ 746W-per-HP = 1.6HP

    Because the conversion from mechanical to electrical isn't 100% efficient, to produce this 1188Watts would load an engine by, say, 2HP.

    Find someone who knows electronics. :cool:

    Alternators have a coil on the spinning armature, which is fed by a control circuit on powered by the 3-phase stator coils on the housing. By varying the current thru the armature, the unit's field controller keeps a constant ~13.2V coming out of the stator, no matter what the engine speed, to correctly charge a battery.

    Rip out the field control circuit, to be replaced by your own, described below


    Create a purely resistive load. You can buy 12v resistor load banks from a car battery shop (they use em to test batteries cold-cranking-amps). You may also use coat hangers, coz they're low-grade metal. Make sure you have enough wattage for your motor! A resistive load is linear, and will suck proportionally more power for the current you feed it.

    If your alternator still has a diode bridge to rectify the generated AC into DC, you can hook your load on the outputs of this bridge. Sometimes the field controller and bridge are combined into one circuit. Phone an alternator shop if you need just a bridge.


    To build an "interesting" dyno, what you want to do is precisely control the speed of your engine on a rotation-by-rotation basis. You'll need some kind of angular sensor to know your MUT's RPM.

    Your control software or circuit needs to set the alternator's DC field current to 0A if you're below the desired speed. This causes no current output from the alternator, and hence no load...causing the engine to speed up.

    If you're above your desired speed, have your control software or circuit set the *DC* current on the armature to the maximum rated (not over coz it will overheat!) This induces maximum load from the alternator, and causes the engine to slow...provided the alternator is rated greater than your engine!!!

    That's it. I found the reaction time of the alternator so fast you couldn't possibly get the engine to go any other speed, even slamming the throttle from end-to-end...unless you completely starved it of gas.

    For our super-mileage car, by fixing the engine speed on the dyno, you could then tweak the amount of gas and spark angle to find the greatest output voltage from the alternator for the least amount of gas. Because the resistive load is linear, greater output voltage means greater engine output.

    It then becomes a question of stepping through all possible engine RPMs and tabulating the precise fuel and spark angles.

    NOTE: A rubber "spider" between the engine and the alternator might extend the life of your dyno.

    hope it helps!
  9. Aug 5, 2008 #8
    Possibly because there is a small, but important distinction between a motor and an engine. (at least from the perspective of an aerospace engineer)

    a motor, otherwise known as a solid motor, is a rocket motor that is powered by a solid propellant. basically a tube with a nozzle and propellant - no moving parts

    an engine, on the other hand is like a liquid engine. they have turbo pumps, injectors, and various thousands of other parts, lots of them moving.

    i have always been taught and told that engines have moving parts, motors do not.

    i suppose from an automotive engineering standpoint they are quite interchangeable, as there really isn't a (conventional or reasonable) propulsive device for cars that has no moving parts

    just FYI
  10. Aug 5, 2008 #9


    User Avatar
    Science Advisor
    Gold Member

    From a Mechanical Engineering standpoint, "motors" run off of electricity (standard electric motors with rotors, windings, etc.), where as an "engine" runs off of a combustible fuel or some working fluid using a thermodynamic process (car engine, steam engine, etc.)
  11. Nov 23, 2010 #10
    Hey Bongz...do you have a link to the project that you did?
  12. Nov 30, 2010 #11
    Hi Bongz

    We also are planning to build a small dyno to test our hybrid drivetrain for a vehicle. Would like to know how you did and what was required. Little bit of guidance would be appreciated
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook