Compressible Fluid Thermodynamics

In summary, the conversation discusses the relationship described by the ideal gas law and how it applies to the compression process in an internal combustion engine. The speaker questions their understanding and calculations of the resulting temperature in the compressed air. They also mention concerns about the effects of this high temperature on engine components. Ultimately, they ask for clarification and direction on the issue.
  • #1
iridium889
2
0
Hi, I am new to physicsforums, and have no higher education in math or physics; but have much interest.

I do not think I understand correctly the relationships described under the 'deal gas law' PV=nRT. Specifically, my question is:

If pressure and temperature are directly proportional when volume and mass are constant, and volume and pressure are inversely proportional when mass is constant, then how exactly does one describe the thermodynamic relationship in say a compression action of ambient air? Such as the compression stroke of an internal combustion engine?

Ambient air (say 295K) compressed with a 10:1 compression ratio in an engine, should then produce a charge (before ignition) of about 2928*C? This is assuming on heat loss through conduction to the cylinder, but still, how much of this astronomical temp could be lost to the cooling system?

(273 + 22) * 10 - 22 = 2928*C.

Am I way off the mark here? This seems like a truly obscene temperature, especially when one considers the autoignition temperature of the air/fuel mixture involved being only around 750*C or so. And the fact that economy engines often use around 10:1 compression ratio with aluminum cylinder liners, heads, and cast aluminum pistons, which should easily melt in such an environment.

This leads me to believe that my understanding here is fundamentally flawed, and I would appreciate any and all direction regarding this issue.

Thank you for your time.
 
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  • #2
I think your conclusion is correct.

(273 + 22) * 10 - 22 = 2928*C.

It is not at all clear to me where you got this calculation!
 
  • #3
The equation is derived from:

273 + 22 is a correction to turn ambient temperature in degrees celsius to kelvins, multiplied by the compression ratio (10), which then has 22 subtracted from it to convert back to degrees celsius.
 

What is compressible fluid thermodynamics?

Compressible fluid thermodynamics is a branch of thermodynamics that deals with the study of fluids that can change in volume when subjected to changes in temperature and pressure. This includes gases and fluids at high pressures and temperatures.

What are some applications of compressible fluid thermodynamics?

Compressible fluid thermodynamics has many applications in engineering and science, such as in the design of jet engines, gas turbines, and rocket propulsion systems. It is also used in the study of atmospheric phenomena, like air flow over aircraft wings and weather patterns.

What is the equation of state for compressible fluids?

The equation of state for compressible fluids is a mathematical relationship that describes the state of a fluid in terms of its temperature, pressure, and density. It is typically represented by the ideal gas law, which states that the product of a fluid's pressure and volume is proportional to its absolute temperature.

How do compressible fluids behave differently from incompressible fluids?

Compressible fluids behave differently from incompressible fluids because they can change in volume when subjected to changes in temperature and pressure. This means that the density of a compressible fluid can vary, while the density of an incompressible fluid remains constant.

What is the Mach number and why is it important in compressible fluid thermodynamics?

The Mach number is a dimensionless quantity that represents the ratio of the speed of a fluid to the speed of sound in that fluid. It is important in compressible fluid thermodynamics because it helps determine the compressibility of a fluid and its behavior at high speeds. It is also used to describe the flow of gases through different types of nozzles and in supersonic aircraft design.

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