Quote:
Originally Posted by drmiller100
but doesn't an air compressor look a WHOLE lot like a piston engine?
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No. Piston engines typically use poppet valves to seal the cylinder, while air compressors use reed valves. As the piston travels up in an air compressor cylinder, the resultant buildup of pressure forces the inlet reed valve shut and the outlet reed valve open, forcing the air into a pressure vessel. Similarly, as the piston travels downward, the inlet reed valve is forced open by the partial vacuum formed, and the outlet reed valve is forced shut.
For this reason, an air compressor can be modeled most nearly as an isobaric process (constant pressure) for its cycle, which is different from an adiabatic (constant heat energy) process that occurs inside a combustion engine. The only real heat buildup in an air compressor is as a result of friction as the piston travels back and forth inside its cylinder. The temperature of the air doesn't really rise all that much because the heat energy has enough time to bleed out of the compressed air's pressure vessel.
So why does water condense inside the pressure vessel of an air compressor? Simple. Remember that the compressor is compressing the incoming air. That air, in turn, is composed of water vapor at some partial pressure (probably around 1 psig on a good, hot, humid summer day). If that hot summer air is being compressed to 100 psig, the water vapor present is also being compressed to about 7 psig partial pressure. Assuming that the compressed air does not appreciably rise in temperature, which it doesn't, the water vapor concentration increases to about 6 times what the air can support, and the excess water condenses out as a liquid.
Quote:
Originally Posted by drmiller100
And doesn't a 10:1 compression ratio pretty much equal the final pressure in most compressers?
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Please see above.
Quote:
Originally Posted by drmiller100
And if you are arguing the gasoline gets boiled/vaporized, why wouldn't the water reach 100 percent relative humidity for the given pressure/ temperature and do the same?
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Are you talking about what happens inside a combustion chamber? Or in an air compressor?
Quote:
Originally Posted by drmiller100
And if the pressure is 150 psi, the ignition event happens, and the temperature sky rockets to, say, 1200 degrees or so, wouldn't the water turn to steam keeping the absolute pressure up nice and high for the entire piston stroke????
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I am going to assume you're thinking of a piston engine here.
On the compression stroke, any entrained water or gasoline droplets will in fact enter their gaseous state. This is because, as the temperature of the air rises as a result of adiabatic compression, the temperature differential will increase between the air and the entrained droplets of liquid.
This, in turn, will cause heat energy to flow from the air being compressed to the entrained droplets of liquid, causing the temperature of the droplets to rise until the temperature inside the droplets reaches the boiling point of said droplets.
Once this happens, further heat energy transfer from the air to the droplets will not cause further temperature increase, but will force the droplets to change from a liquid state to a gaseous state.
Keep in mind, that as all of this happens, the temperature of the air will not rise as much as it would if the entrained droplets of liquid were not there.
Finally, combustion temperatures approach something closer to 6000 F than 1200 F.