Are tests for efficiency of the IC piston engine accurate !

What you say is absolutely true for a 1 cylinder 4 stroke engine. We are discussing a 4 cylinder 4 stroke engine

@Dilip - you have obviously misinterpreted the video, and no. of cylinders has no bearing on the no. of power strokes per cylinder. For a 4 cylinder engine, you still have one power stroke per cylinder, but the cylinders are so sequenced that there is one stroke available for the crank (from one cylinder)  for every quarter rotation of the crank.

Hi Rohit,

 I don’t think it a misinterpretation of the video as much as wrong wording on my part. Yes, regardless of the number of cylinders there is still only 1 power stroke per cylinder. My point is that while cylinder 1 reaches BDC on the power stroke, cylinder 2 has compressed air/fuel and the next stroke in cylinder 2 is a power stroke. So while cylinder 1 is traveling from BDC to TDC on the exhaust stroke it is in effect getting an assist from the fact that cylinder 2 is on a power stroke. This is as applicable for a 4 cylinder 4 stroke. So taking the engine as a whole there are two power strokes for every revolution of the crankshaft, therefore at 4000 rpm there are 8000 power strokes. 

Well, if one cylinder has the power stroke which is 1/4th the crank's rev, the best "overlap" you can have is to fire each cylinder such that as the previous one's power stroke ends the next one's begins. So this way you have one power stroke for every quarter rotation of the crank. Seen from the crank's point of view, as it completes 1 rev, it has 4 power strokes (a quarter stroke each contributed by 4 cylinders). Each cylinder has 1 power stroke every 4 revs and the crank has 4 power strokes for each rev.

The power stroke is something that happens in the cylinder, so if it lasts only for 1/4th the crank's rev then at 4000 rpm, each cylinder will have 1000 powerstrokes each of which that lasts 60ms. At 8000 rpm, each cylinder's power stroke will last 30ms. From the crank's view, at 4000 rpm, since each cylinder contributes 1000 power strokes and there are 4 of them, then the total power strokes contributed by all cylinders is also 4000.