Quickly determine Engine RPM from a Picoscope waveform screenshot

Dmitriy Analyst Ontario Posted  

Someone is showing you a waveform printout, and you are wondering what the engine RPMs were during the capture... so you need to figure that one out.

Automotive Picoscope software has an RPM readout in the bottom-right corner of the screen. But all it does is taking the Hz value obtained from time cursors and multiplying it by 60. If you are lucky to have the time cursors placed on two consecutive sync notches of the CKP waveform, or if they span half the distance between two consecutive compression towers, the program will show you the engine RPM:


However, the time cursors may be in use for other tasks, and the displayed value will not match the engine RPM. Nevertheless, there are a few ways:

I. If the time cursors are placed on the compression towers, the displayed RPM is for a camshaft (camRPM). To get the engine RPM, use formula engineRPM = 2 x camRPM = 2 x 248 = 496 rpm


II. If the time cursors are placed anywhere on the waveform, but the 0-360deg phase rulers are used on CKP edges or the 0-720deg phase rulers are used on CMP edges or compression towers, it is still easy to determine the engine RPM. Take the displayed RPM value, multiply it by degree delta between the time cursors (see the cursors box), and divide by 360:

6203.8 x 51.79 / 360 = 892 rpm


III. If all else fails, it is time to count cells on the grid... In the previous waveform, each cell is about 30 ms, as seen on cells that are not covered by the phase rulers. The 0-360 spans around 2 cells, that is 60 ms. To convert ms into rpm, take 60000 and divide by the ms value: 60000 / 60 = 1000 rpm. As we counted cells approximately, this is a rough estimate.

Do you know any other RPM tricks or something that works for screenshots from other scopes?