What are the different ways we can measure torque and power from an model engine (dynamometer) with props on the engine??

Do you have any tips on designing a dynamometer from scratch?

From : Don Stackhouse

In most cases folks mount the engine mount and the supporting equipment (fuel tank, etc.) on a shaft, which is mounted in bearings. The bearings need to have very low friction, so that means ball bearings, preferably without seals (the seals can add quite a bit of friction). The shaft is restrained from turning either by a lever and spring affair of some sort, or an arm and weight to oppose the torque of the engine.

In the case of the spring method, the deflection of the spring under load times the length of the arm gives you the torque. Sometimes the spring is nothing more than a simple fish scale.

In the arm-and-weight approach the weight times the arm length provides the measurement. The system works like a beam balance, with either the weight sliding along the arm till it matches the torque of the engine, or the weight is varied at some fixed arm length.

In both of these methods you may need to add damping of some sort to take out the effects of the engine's torsional vibration.

The bearings required, the size of the shaft, the length of the arm, etc., are all very dependent on the characteristics of the engine you are trying to test. It will be very difficult to design a dyno that does a good job of testing a wide range of engine sizes. If you plan to test a variety of engines, it's best to design and build a variety of dynos, small ones with the sensitivity to test small engines and larger ones for the strength to handle larger engines. Trying to test a 1/2A .049 engine on a stand capable of testing a 100 CC giant scale engine is not likely to give very accurate measurements.

Another method that is commonly used for testing electric motors is to use a second electric motor coupled to the test motor to act as a generator. A variable resistor across the terminals of the generator allows you to apply different loads to the test motor at your desired test rpm. You measure the voltage and the current from the generator, which from your calibration tests you ran when you built the test rig will give you the torque at that rpm, and therefore the power it's absorbing.

You could also just mount the generator case on bearings and measure the torque directly like in the systems described above. This is probably a good way to calibrate the system, but once that is done, the voltage and current method is probably better. For example, it would not be very difficult to feed the rpm, voltage and current directly from simple sensors in the generator wiring into a PC for automated data logging and subsequent analysis.

There are quite a few advantages to this system as far as accuracy of measurement and ease of setting up different loads on the test engine. However, you will need to come up with an efficient way to couple the test engine to the generator, and if you plan to test a gas engine with this rig, you will also need a separate electric motor and fan to supply cooling air to the motor, since it will not have a prop to do that. None of those are insurmountable problems, and there are enough other advantages (such as having a nice, rigid mounting for the test engine) that this might be a better way to go in the long run.

Don Stackhouse
DJ Aerotech