I'm studying the possibility to read an engine RPM by reading the vibrations of it. Basically an engine is "oscillating" on a car chassis on all 3 axes (X, Y, Z) but mostly on the Y one. When ignition is produced on a cylinder, then there should be like a "spike". Considering for a 4 stroke engine that the crankshaft has 2 revolutions per ignition, if I can "read" ignition then I can do the math and get RPM. Because my expertise in electronics is quite limited I ask here for an advice of how to approach this. Which accelerometer to use? What is range and sensitivity / resolution that I should look for? Any advice / hint is really appreciated Thank you in advance
A piezo electric accelerometer would work here. Most general purpose accelerometers would be fine. Resolution is probably not that important if all you care about is the frequency. You'll also need a charge amplifier. A used accelerometer and charge amp could be had on eBay for pretty cheap. But then you'll also need either an oscilloscope/spectrum analyzer to read and display the signal (may be expensive even for a used one) or buy a data acquisition board and write software to do the signal processing (could be very challenging if you are a beginner). Probably best route is to try to take a class (maybe at local community college) or buy an educational electronics starter kit. Either one will get you some experience and get you closer to being able to figure out this project
In general, it doesn't work reliably. The vibration on an engine block is related to multiples of the firing frequency but is also affected by resonances. So you don't get a nice clean waveform at all speeds. The usual solution is a blob of white paint on the crankshaft pulley, or the timing signal from the EEC, or an HV pickup on the ignition leads. For diesels we used to use a strain gauged clamp on the injector lines.