In Chamber Testing

[scotty305]

Originally Posted by vex

For DAQ's it is a good rule of thumb to sample at twice the frequency than what your expecting to be as your function. IE: if you are ever going to see 48khz in a function you'd want to sample double that to ensure you don't have window the function.

I don't mean to be contrary, but I received the same 'sample at double the input signal frequency' advice from textbooks and some professors. Other professors with more real-world industry experience advised that data will be much more useful when sampled at no less than 5-10x the signal frequency. Try sampling a 100Hz sine wave at 200Hz, the plotting the results... it's not going to look much like a sine wave. If you logged a fuel injector its operating frequency, the indicated duty cycles would always be 0%, 50% or 100%... Personally I'd like to at least know +/-10%.


80kHz sounds like a very reasonable sampling rate. Assuming the sensor responds quickly enough, an 80k sample rate will give you one datapoint every ~0.5 eccentric shaft degrees at 6500 RPM.

Compare this to the RX-7 'crankshaft' position sensor which has one tooth every 30 degrees... I wonder if your ECU's ignition timing accuracy is better or worse than +/- 0.5 degrees? If your logger has a spare input it would be very interesting to monitor the ignition trigger signals in addition to chamber pressure, either by tapping into the 0/5V signal going from the ECU to the ignitor or by tapping the wire between the ignition coil and ignitor (careful, this will be over +20V due to inductive flyback... make sure the logger can handle it).