Dynamometers

[johnh875]

Silly question of the day time, but can anyone tell me what a dyno actually measures? Is it force or torque??? I would have thought being a rotating device it is torque, but I could also convince myself the opposite too, especially since the "torque" measurement is usually given in Newtons (not Nm) and described as "tractive effort".

[Revracing]

A simple engine Dyno will measure the "braking" torque of an engine at a given RPM, more complex systems are able to generate Brake torque curves. Power is derived from torque and RPM.

[Dave Brand]

Originally Posted by johnh875

Silly question of the day time, but can anyone tell me what a dyno actually measures? Is it force or torque???

The basic measurement will be force - the very primitive dynos I used at college in the 1960s used a spring balance on the end of an arm. The measured force mutiplied by the length of the arm gives torque, add in rev/min & you get bhp.

I would have thought being a rotating device it is torque, but I could also convince myself the opposite too, especially since the "torque" measurement is usually given in Newtons (not Nm) and described as "tractive effort".

Tractive effort & torque are two different things. Tractive effort was commonly used for measuring the performance of steam locos where a dyno car was coupled to the loco; tractive effort is the force at the coupling.

In car terms, tractive effort can be a useful measure - it is the force seen at the tyre/road interface, & is therefore a direct measure of the force available to overcome the various resistances & to accelerate the vehicle.

[johnh875]

Ok, that's the understanding I had of the dyno operation. So the tractive effort measurement would have to be derived from the torque measured (easy as you know the diameter of the roller). Given you are usually interested in the output of the engine the torque reading would be of more use even if you have to calculate back through the final drive & gearbox ratios to get to the engine figure (and allow for parasitic losses which is where the whole thing gets hazy just like deriving engine output from chassis dyno readings…)

Dave that’s an interesting bit of trivia. I read about testing done a government agency that used to test tractors here in Aust and give official power ratings for both drawbar hp and belt hp – back when it was common for tractors to be used to drive machinery via belts, ie long before the term “government agency” was invented!

[dtype38]

Originally Posted by johnh875

Given you are usually interested in the output of the engine the torque reading would be of more use even if you have to calculate back through the final drive & gearbox ratios to get to the engine figure (and allow for parasitic losses which is where the whole thing gets hazy just like deriving engine output from chassis dyno readings…)

Last rolling road run I did had an electic dyno on it. The car was run up to full power then the tester lifted off and jabbed the clutch in. The dyno then measured the torque absorbed on the overrun as the power train slowed down again. The dyno computer then back calculated what the engine torque and power must have been. A reasonably accurate figure I think if the machine is well calibrated.

[RWC]

Put simply they measure torque
From that (and knowing the revs) HP can be calculated