Quote:
Originally Posted by mpgmike
It is Avogadro's Law which states:
Therefore at any given temperature and pressure, whether the cylinder is filled with nitrogen, water vapor, carbon dioxide, or other, there will be a calculated number of molecules. Furthermore, 1 degree rise in temperature provides for the exact expansion rate regardless of the gas. It then boils down to how much thermal energy is required to raise the temperature of any given gas by 1 degree. This is Thermal Density. |
While Avogadro's Law seems to be generally correct, it seems to me that there may be another factor we're overlooking here, or the small inconsistencies in this imprecise Law end up causing more pronounced effects in the extreme environment of an internal combustion engine.
I say this because the equation I quoted before is used in many places including many text books. It normally is used to identify the thermodynamic efficiency of a certain compression ratio. But it needs the heat capacity ratio of the working fluid (also known as the adiabatic index, the ratio of specific heats, or Laplace's coefficient) in order to work. Most people just set this to 1.4, the heat capacity ratio of Air or Nitrogen, but when you set it to a lower HCR, such as that of water vapor or CO2, the efficiency decreases in the calculation.
Another thing is that I've read in several places that Noble Gasses make the best working fluids for Stirling Engines. This places Neon, Argon and even Krypton at an advantage over other gasses such as water vapor and Nitrogen. Of course noble gasses have only one atom per molecule, but even though N2 should be better for a Stirling engine than Argon or Krypton, it apparently isn't.
The fact that different gasses can have different heat capacity ratios, that is, ratios of the heat capacity at constant pressure (CP) to heat capacity at constant volume (CV), suggests to me that Avogadro's Law may be overly simplified for use in every situation, especially calculating the efficiency of a working fluid in a combustion engine.