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PHY 113 A Fall 2012 -- Lecture 32 111/19/2012
PHY 113 A General Physics I9-9:50 AM MWF Olin 101
Plan for Lecture 32:Chapter 22: Heat engines
1. Thermodynamic cycles; work and heat efficiency
2. Carnot cycle3. Otto cycle; diesel cycle4. Note – in this class, we will not focus on entropy and
the second law of thermodynamics
PHY 113 A Fall 2012 -- Lecture 32 211/19/2012
PHY 113 A Fall 2012 -- Lecture 32 311/19/2012
Thermodynamic cycles for designing ideal engines and heat pumps
P (
1.01
3 x
105 )
Pa
Vi Vf
Pi
Pf
A
B C
Din
eng
outin
eng
Q
W
QQQ
WW
:Efficiency
:system input toHeat
:engine ofWork
Engine process:
http://auto.howstuffworks.com/engine1.htm
PHY 113 A Fall 2012 -- Lecture 32 411/19/2012
P (
1.01
3 x
105 )
Pa
Vi Vf
Pi
Pf
A
B C
D
Examples process by an ideal gas:
A®B B®C C®D D®A
Q
W 0 -Pf(Vf-Vi) 0 Pi(Vf-Vi)
DEint
1-γ
)( ifi PPV 1-γ
)(γ iff VVP 1-γ
)( iff PPV 1-γ
)(γ ifi VVP-
1-γ
)( ifi PPV 1-γ
)( iff PPV 1-γ
)( iff VVP 1-γ
)( ifi VV-P
BCAB
ifif
in
eng
VVPP
Q
W
:Efficiency
PHY 113 A Fall 2012 -- Lecture 32 511/19/2012
Example from homework
BCAB
ifif
in
eng
VVPP
Q
W
:Efficiency
BCAB
DACD
in
out
in
outin
outineng
Q
Q
Q
QQQWW
1
1
:Also
PHY 113 A Fall 2012 -- Lecture 32 611/19/2012
Most efficient thermodynamic cycle -- Carnot
Sadi Carnot 1796-1832
PHY 113 A Fall 2012 -- Lecture 32 711/19/2012
Carnot cycle:
AB Isothermal at Th
BC AdiabaticCD Isothermal at Tc
DA Adiabatic
h
c
in
out
in
outin
T
Tε
Q
Q
Q
1
1
cycleCarnot of Efficiency
PHY 113 A Fall 2012 -- Lecture 32 811/19/2012
iclicker exercise:We discussed the efficiency of an engine as
Is this resultA. Special to the Carnot cycleB. General to all ideal thermodynamic cycles
iclicker exercise:We discussed the efficiency of an engine running with hot and cold reservoirs as
Is this resultA. Special to the Carnot cycleB. General to all ideal thermodynamic cycles
in
out
in
outin
Q
Q
Q
1
h
c
T
T1
PHY 113 A Fall 2012 -- Lecture 32 911/19/2012
h
c
in
out
ABDC
DcAh
CcBh
ABh
DCc
CD
AB
in
out
T
T
Q
Q
VVVV
VTVT
VTVT
VVnRT
VVnRT
W
W
Q
Q
//
process adiabaticFor
)/ln(
)/ln(
:cycleCarnot afor that Note
11
11
PHY 113 A Fall 2012 -- Lecture 32 1011/19/2012
iclicker exercise:Why should we care about the Carnot cycle?
A. We shouldn’tB. It approximately models some
heating and cooling technologiesC. It provides insight into another
thermodynamic variable -- entropy
PHY 113 A Fall 2012 -- Lecture 32 1111/19/2012
The Otto cycle
1
1
21
:efficiency lTheoretica
V
V
PHY 113 A Fall 2012 -- Lecture 32 1211/19/2012
The Diesel cycle
BC
AD
TT
TT
1
1
:efficiency lTheoretica
PHY 113 A Fall 2012 -- Lecture 32 1311/19/2012
PHY 113 A Fall 2012 -- Lecture 32 1411/19/2012
Engine vs heating/cooling designs
