After reading about the Kinetic Molecular theory I returned
to the Stirling Engine to see how these concepts can be applied to explain the
Engine. Like other parts of this project instead of reading about how the
theory applied to the engine I was able to discover it myself and confirm my
knowledge through observations and talking to my father. It’s interesting to
note that since Robert Stirling patented the Stirling Engine in 1816, before
the final development of the kinetic molecular theory there was a time when no
one could fully explain how the engine worked. Earlier I explained the motions
of the four phases of the Stirling Engine. In these phases the gas is heated and
cooled to move the working piston and in turn rotates the flywheel and
displacer. The kinetic Molecular theory can be used to explain the flow of heat
in the Engine.
During the heating phase of the cycle the displacer allows
the gas to have contact with the heat source and the piston has fallen to its
rest position. The particles in the heat source carry a high amount of thermal
energy and collide with the wall of the cylinder. This heats the cylinder transferring
the thermal energy of the heat source to the cylinder. The particles of the
cylinder then collide with the particles of the cooled gas, transferring the
thermal energy to the particles. Millions of collisions occur over a period of
a few seconds and the gas is heated. By this point the pressure from the
increase in average kinetic energy of the particles has increased to the point
that the working piston is pushed upwards.
Once the piston is pushed upwards the displacer then moves
allowing the gas to come into contact with the cooler side of the cylinder. The
fast moving particles in the gas collide with the cool slower particles of the
cylinder. This slows the particles of gas. The result is the thermal energy is
transferred out of the system. During this phase efficiency is lost. This is because
to be 100% efficient all of the thermal energy added to the system must be
converted into mechanical energy. Since some energy is transferred out of the
system during the cooling phase efficiency is lost.
This theory explains why a difference in temperature is
required for the engine to work. It also explains why a greater difference in
temperature will result in a faster engine. First, in the event of a thermal
equilibrium the kinetic energy of each particle is the same. This means that
when the two particles collide they would gain the same amount of energy as
they lost. As a result during the heating phase of the engine there would be no
increase in kinetic energy per particle. As a result, the gas would not heat up
and there would be no change in pressure. This means the working piston would
not move and the cycle could not be completed.
To explain why a greater difference in temperature results
in a faster engine I would like to return to the analogy of cars colliding with
each other. Imagine one car is not moving while the second car is going very
fast. When they collide, the car that isn’t moving gains a lot of speed and the
car going very fast slows down dramatically. Now imagine the car moving very
fast is only going half the speed. When the cars collide the force of the
collision is smaller. The car that is not moving gains less speed. When this is
applied to the particles in the engine if the particles have a greater
difference in thermal energy more energy would be transferred during each
collision. This means the temperature can be changed quicker and the engine can
operate faster.
Sources:
Hooper, C & Reader, T. G. (1983). STIRLING ENGINES. New York, NY: E. & F. N. Spon.
Dick G, Geddis A, James E, McCaul T, McGuire B, Poole R, Holzer B. (2001). Physics 11. Toronto, ON: McGraw-Hill Ryerson Limited.
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