Understanding Cylinder Compression in Engines: Calculating Pressure Based on Compression Ratio

Question

Compression ratio and compression. Hello. I have a question. I wonder how do you know what compression on the cylinders should the engine have if the compression ratio is given? I've read a lot about it on the internet and I haven't really found a meaningful answer. A lot of people think...

tzok · Answer

The compression ratio is the ratio of the volume of the combustion chamber to the sum of the volume of the combustion chamber and the volume of the cylinder above the piston in its lower position.
The compression pressure is the pressure that the piston builds up in the combustion chamber at the end of the compression stroke.

This is quite clearly explained here.

http://zssplus.pl/publikacje/publikacje37.htm

There is no universal and accurate formula for converting these two quantities. The conversion rates given are an average and approximation only.

robo1973 · Answer

If the motorcycle has 4 pistons that work two at the same time and each of them is, for example, 5 bar, do we add up the pressure? Does what came out of the above formula apply to one cylinder?

kkknc · Answer

It means the engine is scrap. You don't add up anything.

andrzejsdl · Answer

You would have to do an oil test because it will explain a lot.

tzok · Answer

In a 4 cylinder 4 stroke the pistons usually move in pairs, but when one of the pairs has a compression stroke, the other has the exhaust stroke ...

Anonymous · Answer

What will explain ?? only that something is leaking .. But we already know this after the pressure ..

4495redberet · Answer

The nn oil test will give some idea of what is weak. If the compression jumps after the oil, you have problems with the bottom. If nothing changes, the head up.

andrzejsdl · Answer

That's what I meant.

SKM 1964 · Answer

This formula has nothing to do with compression and compression. What about diesel engines ?? I will give an example, compression ratio 17, measured pressure 29-31 bar.

Anonymous · Answer

Faster measurement. The first are 100% open throttles, engine warmed up, battery and starter in full efficiency, and a measuring instrument with a check valve, an efficient manometer with thin tubes and a tight connection with the head.

SKM 1964 wrote:
What about diesel engines ??

If you don't know anything about a given engine, you multiply the compression ratio by 0.21. See the literature with the basics of engine diagnostics.

0.125 applies to engines ZI ...

Anonymous · Answer

Oil rubbish into the cylinder chamber and turning the shaft with a starter splashes oil all over the chamber, sealing up and down ..

4495redberet · Answer

I guess too much sun lately.

Anonymous · Answer

Not the sun but the experiences. If you think that you will pour oil and only seal it down, you are wrong.

4495redberet · Answer

Man, this is only a temporary improvement.
If you have weak rings, for a few turns of the shaft it is enough to raise the compression.
If the valves are bent / bent, there is no chance of increasing the compression, and certainly not to the same extent as on the rings.
Also from experience.

Anonymous · Answer

And how is the valve guides made and instead of the valve closing, it slides? I have had a few such cases, compression below 40%, the oil test gave a positive result and damaged molded valve guides turned out to be. Of course, you would write this engine to the garbage and it was enough to regenerate the heads.

PS. There are other methods that are more reliable ..

4495redberet · Answer

These were not pre-made guides, but they were not there. Anyway, this is only a rehearsal. The rest is verified organoleptically.

Understanding Cylinder Compression in Engines: Calculating Pressure Based on Compression Ratio

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