Re: Does psi compression relate to compression ratio?
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the computer program
Engine Analyzer 3.2 does a pretty good job of taking every thing into account and calculating a cranking compression PSI
http://performancetrends.com/download.htm#ea3
Long answer:
If the camshaft closed the exhaust valve at top dead center and the intake valve at bottom dead center
And
The rings leaked no air
And
The cylinder head, bore walls and piston were perfect insulators
And
There was no humidity in the air the engine drew in
And
you knew the "ratio of the specific heats" of the air that is given the nickname Gamma,
... Then you could calculate the compression PSI
by hand with a modestly complicated "Power Law" math equation
Details of this at
http://en.wikipedia.org/wiki/Adiabatic_process#Example_of_adiabatic_compression
It is inaccurate to simply multiply
the 14.7 PSI of the air around us
by the compression ratio of the engine,
but that gets less inaccurate
the colder And drier the air is.
If you were working on snowmobile engines at the South Pole you would not be far off that way.
Short answer:the computer programEngine Analyzer 3.2 does a pretty good job of taking every thing into account and calculating a cranking compression PSILong answer:If the camshaft closed the exhaust valve at top dead center and the intake valve at bottom dead centerAndThe rings leaked no airAndThe cylinder head, bore walls and piston were perfect insulatorsAndThere was no humidity in the air the engine drew inAndyou knew the "ratio of the specific heats" of the air that is given the nickname Gamma,... Then you could calculate the compression PSIby hand with a modestly complicated "Power Law" math equationDetails of this atIt is inaccurate to simply multiplythe 14.7 PSI of the air around usby the compression ratio of the engine,but that gets less inaccuratethe colder And drier the air is.If you were working on snowmobile engines at the South Pole you would not be far off that way.
Originally Posted by
William the Third
Originally Posted by
The direct injection engines used in the Aprilia SR50 are low compression engines, so 90~110 PSI is ideal (even with the compressor attached, as the amount of air injected is trivial). Too low is a problem for obvious reasons (one you get below 75~80 PSI, it will be hard to get the engine to start), but too high (110+) is also a problem. The fuel/air mix is injected by the air injector at ~70 PSI (fuel pressure is ~105 PSI stock, but that is injected into the air injector, which is fed by the compressor ~70 PSI). So if cylinder compression is too high, rather than the fuel/air mix being injected into the combustion chamber when the air injector opens, there won't be a large enough pressure differential for proper injection to occur, or even worse air from the combustion chamber will be pushed into the injector. Since I know some of you will be curious as to how it's even possible to inject at ~70 PSI when cylinder compression is 90~110 PSI, this is because the air injector opens well before top dead center when peak compression happens. When the air injector opens pressure in the cylinder is still well below 70 PSI. However as you raise peak compression, so too do you raise the pressure in the cylinder when the injector opens. Get too close to, or exceed, 70 PSI, then you'll have problems.
Mind you, not all compression testers are equal, so unless it's been recently calibrated, the number it reads is at best a good estimate.