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.
The following image is from a Hyundai Terracan 2.9l diesel engine.
It is a 4 cylinder, 16 valve engine. The image was taken whilst cranking only. The valve timing was confirmed correct.
This is fairly typical of a diesel cylinder profile. I have noticed that the exhaust valve opening does vary around the 180° marker, from one engine to another. But it is always close to the 180° point.
Because it is diesel and so does not have a typical throttle butterfly, there is very little detail in the valve train profile. This can be improved, by choking the intake, to increase potential pressure drop, for analysis purposes.