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Thermodynamics
System Integrations:
gasoline engine driven air compressor
Gasoline fuel
to engine,
$3.50/gallon
Gasoline
Engine
Air Compressor
Air,
0.05kg/s,
100kPa
25deg C
Air,
650kPa
Assumptions for Engine:
Ideal Otto Cycle, cold air
standard assumptions, r =
8
Assumptions for
compressor:
Adiabatic, no heat
losses or gains
Neg. changes in KE
and PE for the air
Isentropic
compression
Ideal gas assumption
Find hourly cost to operate
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Thermodynamics
Strategy
Analyze compressor and get power input to provide the flow rate and conditions.
Analyze the engine to get its efficiency and with power output of engine = to
power input to compressor, get Qin for the engine and then using heating value
for gasoline get fuel use rate and then cost per hour to operate.
))
(
)
2
/
)
((
)
((
1
2
1
2
1
2
2
2
z
z
g
V
V
h
h
m
W
Q
other

+

+

=

)
(
1
2
h
h
m
W
other

=

Energy equation for compressor:
Energy eqn for compressor with assumptions:
With the isentropic assumption T
2
/T
1
= (P
2
/P
1
)
(k1)/k
T
1
= 25 + 273 = 298K
T
2
/298K = (650/100)^0.286
therefore T
2
= 509K , assuming ideal gas:
The power input = mass flow rate * Cp(T
2
– T
1
) = 0.05kg/s * 1.005kJ/kg K
*(509298)=
10.6 kJ/s or 10.6 kW
(14.1 Hp)
If you use the ideal gas table for air (table A7.1), and use h at the temps.
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This note was uploaded on 08/03/2011 for the course EML 3100 taught by Professor Sherif during the Summer '08 term at University of Florida.
 Summer '08
 Sherif

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