Logging the data received from the dynamometer
On old-fashioned dynamometers, an observer had to record the simultaneous tachometer and torque gauge readings with a pencil and paper.
Today, most dynamometers replace the observer’s notes with computerized data acquisition electronics.
Because of the very loud noise generated by the dynamometer and the engine, a performance test is stressful to anyone who’s watching it. In these given conditions, it is very difficult to manually record all the data. Even worse, the readings are sometimes “rounded up” by the biased engine builder, instead of being recorded properly.
A good computerized data acquisition system should be considered mandatory for any real testing. Fortunately, today it is possible to get recording, control, and playback capabilities in a $2000 palm sized package that years ago would have cost the price of a house and filled a small room.
A suitable computerized data acquisition system should have a fast sampling rate, especially for testing 4-stroke, single cylinder engines. For obtaining quality data, you will need at least 100 samples, of all sensor channels, per second (100Hz). A 200Hz logging rate is a bit better still. It is important to remember that between spark plug firings there is a measurable drop in the instantaneous crankshaft torque and rpm. The crankshaft gets accelerated in the moments after combustion and then begins to slow until almost two revolutions later the plug fires again. You can’t feel these rapid highs and lows when driving around the track (with all that vehicle inertia), but the dynamometer will! Therefore, a 200 Hz logging rate for your data acquisition system should do the trick, as well as an appropriate dynamometer.
However, if you sampled at only 50Hz, that’s only a single torque and rpm sample every other revolution (at 6000 rpm). Periodically, a series of samples will fall in synch with the firings of the plugs, while at other times sampling will fall in synch with the lower power compression strokes. By using a fast acquisition system to read each firing cycle multiple times, enough data is captured to average out this phenomenon.
Averaging the data is a very important step in data acquisition.
At this point, while experienced dynamometer operators see the same power curve in both graphs, inexperienced dyno operators expect to see a smooth, clear line.
The reason why the ability to average and dampen the data is essential for the acquisition system is that you don’t have to deal with so many printings.
For instance, at 200Hz you’re getting 2000 lines of data for even a ten-second dynamometer pull. You might choose not to search for any small changes in 40 pages of data every time you want to prepare another run.
Averaging both eliminates transient “noise” and produces more practical half-page printout, and thus it is very useful in the process of logging the data from the dynamometer.
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