The image below is a typical in-cylinder pressure waveform, taken from an 8 valve petrol engine. It was captured during warm engine idling.
It represents a typical pattern for a fixed timing engine. It is also a good baseline for most variable valve timing 16 valve engines.
The vertical rulers show the degrees of crankshaft rotation over a 4 stroke cycle.
This next waveform illustrates how useful the In-Cylinder pressure analysis is for quick valve timing issue checks.
It was taken from a 3 cylinder VW Polo, that had failed to start after stopping. It is not an uncommon complaint, due to the timing chain assembly stretching and jumping.