Compression - can it go back together

[John_S]

Re: Compression - can it go back together

I think the Lycoming chart is just adding to confusion. The chart is pulled from some procedure, which may be adding to the confusion (we don't know the context). It appears to be testing some sort of "boost" (like a super charger?) on the engine and what an average psi reading should be, given the conditions of whatever particular test it is. The compression ratio is probably dynamic and only applicable to this Lycoming engine.

BTW: All I know about Lycoming engines are that they used in aircraft.

Outside of all our math, there can be other things that we are not considering. Does the speed of the piston within the bore have an impact? etc.

Anyway, I think I've come to enough of an understanding on the descrepencies between Jasons and our resident experts. Hopefully, this long discussion has been helpful to others.

 

[wire2]

Re: Compression - can it go back together

Another thought, been thinking about this. Very interesting.

When the intake closes, what is the pressure in the cylinder? I think it would be the same as manifold pressure, which we know is negative.

Not quite. There is flow past the valve (a restriction) while the piston is descending. The valve closes before BDC so there is less pressure in the cylinder than manifold. There would have to be a delay at BDC with the valve open for it to equal.

As for leakage past the rings, it's negligible. If you watch the gauge bump up with each compression stroke, the increase becomes less, to a peak reading.

The speed of cranking also has little effect (unless it's REALLY slow) for the same reason; each successive compression stroke reduces the error .

...

 

[Robj]

Re: Compression - can it go back together

Thanx Wire 2. I just tried to figure this out from basic principles. I agree the leakage past the rings is small. So you basically agree that the pressure in the cylinder is something less than atmospheric? My guess at manifold pressure did not consider the valve restriction, which I think would be small. But the valve closing before BDC would be more significant.....I think? To further mess this up, the air temperature would increase from the compression which would also increase the pressure. Not sure how significant this is, but do not feel like digging up my old thermo books to figure it out. What started off as a simple question turns out to be quite a complicated process.

I think with the compression tester, the reason compression pressure increases gradually is because the volume of the chamber is increased by the volume of the compression tester, that is why you do not get peak pressure on the first compression stroke. The impact of this additional volume decreases with each compression stroke until you reach the max at 3-4 strokes. That's all folks.

Have a great day,

Rob.

 

[delsol]

Re: Compression - can it go back together

my new rebuilt 5.7 has 170 psi across the board