Measuring Power Manually
Shaft output power can be measured manually in one of two ways:
| 1. | Raise a weight from floor level to the bottom of the engine and time how long this takes; or |
| 2. | Suspend a thread from the scale and wrap it around the output shaft in such a way that the spinning output shaft wants to raise the weight and measure the resulting engine RPM. |
Measuring Power by Lifting a Weight
Place a measured weight (in the range of 10g to 30g.) on the floor, tie a thread to it, and run the thread up through the access hole in front of the engine tower. Place the weight directly below hole, on the floor, and allow roughly 30 cm.of thread beyond that needed to reach the shaft. Wind the thread around the spinning shaft until it catches and then carefully guide the thread onto the spinning shaft to make sure that it does not bind or fall off the end of the shaft. Have someone measure the amount of time that it takes to raise the weight from the floor until the weight hits the bottom of the engine and stalls it.
It is acceptable to unwind the string by gently pulling on it and reversing the direction of the engine as it unwinds.
Vary the weight used from one so light that the engine hardly slows down to one so heavy that it almost stalls the engine and takes quite a while to raise the weight.
Set the engine up at a given hot and cold temperature, start it running, and lift one of your weights and time how long it takes to lift the weight. Then proceed to the next of your weights, collecting a series of weight/time data pairs. Then use the following formulas to compute power for each of the weights you used. You should collect 10-20 data points using different weights.
In this case power is mass * gravity * (height raised/elapsed time)
We have prepared an Excel spreadsheet to help you perform these computations easily:
"Computing Power by Lifting Weights.xls"
Measuring Power with a String Brake
Tie a thread to the bottom of the triangle and hang the triangle from the digital scale.
Drop the thread down to the power output shaft, and and wrap it three times around the shaft so the spinning shaft wants to lift the weight, as shown. Then drop the thread down through the hole and hang your weights at the end of the string.
Set the engine up at a given hot and cold temperature, start it running, and hang one of your weights at the end of the string, so that it is hanging in the air. Measure the RPM of the engine using the Grab feature in the software, and record the RPM as well as the mass of the weight and the reading from the digital scale. Note that each time you turn the digital scale on, it will assume that the current weight is zero. Thus, you will have to pull the string up from the power output shaft slightly, to allow the scale to weigh the triangle as "zero", and then drop the string, so that the scale gives you a reading for just the weights, and not the triangle. Then proceed to the next of your weights, collecting a series of weight/digital-scale-reading/RPM data triples. Then use the following formulas to compute power for each of the weights you used. You should collect 10-20 data points using different weights.
This approach is a good method for measuring power (you have just built a simple "prony" brake). It is quite easy to use although not as emotionally satisfying as watching the weight being raised from the floor.
Torque = (W - S) * A * R
where
W = mass of weight (in kg)
S = digital scale reading (in kg)
A = acceleration of gravity = 9.8 m/sec2
R = shaft radius (in meters) = 0.0016
Power = T * engine-radians-per-sec
where
T = Torque
engine-radians-per-sec = RPM * 2 * pi / 60
We have prepared an Excel spreadsheet to help you perform these computations easily:
"Computing Power Using a String Brake.xls"