Engine Vibration Analyzer

An engine repeats two kinds of event as it runs, and each shows up at its own frequency. The first is simply the crankshaft turning: it rotates RPM times a minute, so the RPM to Hz conversion for the fundamental — the 1X or first order — is f_r = RPM ÷ 60 Hz. Anything tied to rotation (an out-of-balance crank, a damaged harmonic balancer, a bent driveshaft or wheel) puts energy here. The second is combustion. On a four-stroke each cylinder fires once every two crank revolutions, so the cylinders together produce cyl ÷ 2 power strokes per revolution and the firing frequency is (RPM ÷ 60) × (cyl ÷ 2). A two-stroke fires every cylinder every revolution, so its firing frequency is (RPM ÷ 60) × cyl. The firing frequency is therefore always a fixed order of rotation — 2X for an inline-four four-stroke, 4X for a V8, 6X for a V12, and so on.

Two more orders are worth watching. Because pistons accelerate twice per revolution, reciprocating engines carry an inherent second-order (2X) shake — a classic reciprocating imbalance — that is famously strong on inline-fours and is why many four-cylinders buzz at idle. And on a four-stroke a single weak or dead cylinder fires only once every two revolutions, half as often as a healthy one, so the misfire frequency characteristically appears at the 0.5 order (and its odd multiples 1.5X, 2.5X). Belt-driven accessories spin at their own rate set by pulley size: an accessory turns at f = f_r × (crank-pulley diameter ÷ accessory-pulley diameter), so a smaller accessory pulley spins faster, and a worn alternator, A/C-compressor or power-steering-pump bearing peaks at that accessory pulley frequency independently of any engine order.

Two honest cautions. First, these are exact kinematics for an idealised, evenly-firing engine — they tell you where peaks should land, not how big they are. You must confirm your own inputs: whether the engine is four- or two-stroke, the real cylinder count, and the true pulley diameters. Second, the order → cause table is a widely used diagnostic convention, not a diagnosis. A real machine mixes many orders with engine-mount and structural resonances and with road and driveline inputs, so a peak at a given frequency only nominates a suspect to investigate. Diesel and marine engines follow the same maths (use the four-stroke or two-stroke rule to match the engine) but often run uneven firing, large flywheels and propeller/shaft orders that add their own peaks. Crucially, to truly measure vibration you need a calibrated accelerometer and a vibration analyzer, ideally order-tracked to a tachometer — a microphone reveals only the frequency content of the airborne sound and cannot report calibrated mm/s velocity or g acceleration.

A useful extension of this analysis is the Campbell diagram (speed-frequency map): plot the predicted order lines against a sweep of RPM values and you can see which orders pass through structural or mount natural frequencies as the engine runs up — those crossings are where resonance-amplified shake typically peaks. V-engines also carry inherent differences from inline engines: the cylinder banks fire alternately with an unequal angular spacing, which creates firing irregularity that introduces sub-harmonics alongside the main firing order. These are important caveats when interpreting real spectra, and are additional reasons to validate predicted frequencies against a vibration frequency analyzer with live microphone data from the engine bay.