12-03-2012, 03:07 PM
Thermodynamic Analysis of Compressed Air Vehicle Propulsion
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Introduction
The air flow of a compressed air car is schematically shown in Figure 1. The
following two questions need to be answered.
E-Motor Compressor
Schematic of air compression, compressed air transfer to car and
the use of compressed air for vehicle propulsion
Air Compression
Three compression processes are illustrated in a pressure-volume diagram
(Figures 2) and a temperature-entropy diagram (Figure 3). Both presentations
Isothermal Compression
During the idealized reversible isothermal compression process the temperature
is considered to remain unchanged. The initial temperature T1 is also the final
temperature T3. All compression heat must be removed during the compression
process by heat exchange with the environment (e.g. by transfer to a colder
medium). In reality this is impossible for practical system designs.
are commonly used for thermodynamic analyses.
Polytropic Compression Followed by Isochoric Cooling
The polytropic change of state follows the isentropic laws. However, the
isentropic coefficient (γ = 1.4 for air) is replaced by a polytropic coefficient n.
The value of the coefficient n may vary between 1.4 for isentropic and 1.0 for
isothermal expansions. Air is treated as an ideal gas.
In the isentropic case, no heat is neither exchanged with the environment nor
generated internally by friction or poor aerodynamics, while in the polytropic
case some heat is exchanged with the environment or available form internal
friction losses. The isothermal case is the second idealized limit for real
compression or expansion processes. However, a polytropic compression
process is always associated with an increase of entropy as seen in the T-s-
Diagram