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What is the best way to plot motor power output graphs? Does anyone know how we can plot this graph for motors and engines?

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  • $\begingroup$ Flywheel horsepower against rpm or horsepower against fuel consumption in litres per hour or grams per second - whatever you want to show. BSFC etc $\endgroup$
    – Solar Mike
    Dec 17, 2022 at 15:02

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The classic way is to make a chart with RPM on the x-axis and torque on the y-axis. Often the experiment is set up in such a way as to ensure an impedance match between the engine being tested and the load being imposed upon it to furnish a baseline "operating point" for the engine, in the following way:

First, remember that (power) = (torque x RPM) in all cases.

The engine is disconnected from the load and the throttle is opened all the way. at the resulting (high!) RPM, all the engine power output is being dissipated as internal friction, noise, heat, etc. This is the no-load case.

Then we apply a brake to the output shaft to load the engine down to the point where the engine's RPM has been cut in half, and measure the torque applied by the brake. This is the nominal operating point of the engine and represents the peak of the torque vs. RPM curve.

To get RPMs greater than this, the engine has to be partly unloaded and its torque output will fall until you get all the way over to the no-load case, where the engine is at maximum RPM but there is no power output at all and the engine is dissipating all its power output in internal friction.

Back to the nominal operating point. To get RPM's less than this, you have to haul harder on the brake which will slow the engine RPM's down. Remember in the case of 0 RPM, the power output is identically zero and so the power output of the engine will fall as you lug the engine down progressively towards zero RPM.

After an hour's worth of light, pleasant work, you will have enough data to plot the full curve.

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  • $\begingroup$ What I don't understand is the part of the curve where RPM les lower than the nominal operating point. You press harder on the brake, the rpm goes down, but shouldn't the torque go up because the torque is equal to the amount of force applied on the brake (modulo the moment etc.)? Whereas for most ICE, the torque curve goes lower than peak torque in this part. $\endgroup$ Jan 22 at 17:44

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