Power Loss measurment on Eddy current dynamometer

[serbar]

Hello.

I want to know more about dynamometers, namely Eddy current dynamometers.

My issue is Power Losses Measurement, i know that after reach max desired rpm one should press the clutch till the car stops and the computer in the dyno take the power loss values.

But when you press the clutch the engine stops transmiting torque to the engine rollers, so the load cell isn t submited to any force in that period...i think...

My question is:

How exactly the Eddy current dyno measures Power Losses when you press the clutch?

Thanks.

 

[serbar]

Wanted to say dynamometer rollers.

Sorry.

 

[patprimmer]

They don't measure power when the clutch is disengaged as there is no power transmitted.

Roll down time can be used to measure inertia in the system downstream of the flywheel or drag in the system downstream from the flywheel.

Regards
Pat
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[serbar]

So roll down time and variation of roller speed (acceleration) can be used to calculate kinetic energy and then power losses.

The dynamometer computer do this if we input car and driver weight is it right?

 

[patprimmer]

An eddy current dynometer depends on the load from the eddy current to act as a brake to hold the engine at steady speed when you measure the torque reaction against the brake to calculate power.

It is inertia type dynos that calculate power from rate of acceleration of a heavy wheel or roller. They use a coast down procedure to calibrate them, BUT the engine should be in gear as that is done so the inertia of the engine is also measured.

Regards
Pat
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[serbar]

Yes i understand, but i have seen people in Eddy dynos doing the "coast down procedure" after a test, and i want to know what kind of data and what is behind the procedure to calculate power losses.

Can you clearly tell me that?

Thanks.

 

[KiwiME]

Clearly the coast down will take a shorter time if the vehicle's drive train friction is added to the coast down of an un-occupied chassis dyno. With the clutch pressed in that would give an estimate of the sum of component friction between the clutch and the tires. The fact that the dyno is an eddy current type is of no consequence.

Many years ago I worked for a well-known UK manufacturer of DC-motor chassis dynos and developed a coast down procedure to calibrate the roller bearing friction out of the equation. Bearing temperature makes big difference to accuracy.

 

[serbar]

Ok. From what you said,

My conclusion: The transmission losses are estimated ( calculated) in every chassis dynos based on coast down time and speed of the rollers among others.

Am i right?

Thanks.

 

[GregLocock]

One reason to use a coastdown test is to calibrate the vehicle model for fuel consumption and emissions type tests.

If you do a coastdown on the track, and get the same speed vs time curve as you do on the dyno, then you are telling fibs, but it is possible to get it pretty close. One problem is that a real coastdown test is a great way of measuring the gradients and local variations of wind speed and direction on your test track.

Cheers

Greg Locock


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