Announcements 1/28/11
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Prayer
HW 8: one with invalid CID
Lab 2 due tomorrow
Exam 1 starts in a week and a half
Special Processes: Review
Constant volume (isovolumetric)
 Constant pressure (isobaric)
 Constant temperature (isothermal)
 No heat added (adiabatic)
 General process?
P
a. Eint = ?
A
b. Won gas = ?
c. Qadded = ?
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B
V
Last Special Case: Cyclical Processes
P
State 1
V
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Eint = ?
net Wby gas or Won gas? |W| = ?
Qadded = ?
Engines: Energy Transformation
work
heat in
(higher T)
engine
exhaust out
(lower T)
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Qin = work by gas + Qout
Notation: Qh, Qc, Th, Tc, |W|
Qh = |W| + Qc
Heat Engine Example
P
(a) - piston at
room temp and
atmospheric
pressure
a
V
Heat Engine Example
P
(a-b) - gas
heated to keep
volume
constant as
cart rolls onto
piston
b
a
V
Heat Engine Example
P
(b-c) - gas
heated to
increase
volume and lift
the cart
c
b
a
V
Heat Engine Example
P
(c-d) - gas
cooled to keep
volume
constant at
cart rolls off
piston
b
c
a
d
V
Heat Engine Example
P
(d-a) - gas
cooled to
reduce volume
back to initial
level
b
c
a
d
V
Efficiency
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How good is your engine?
Definition
Worked Problem
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A car engine produces 5000 W of power, at
20 cycles/second. Its efficiency is 20%. What
are |W|, Qh, and Qc per cycle?
What do those quantities represent?
Reading Quiz (graded)
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What is the name we use for the cycle which
models how a gas engine works?
a. Carnot cycle
b. Brayton cycle
c. Diesel cycle
d. Otto cycle
e. Tri cycle
Otto Cycle: Gas Engines
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(Starting at 2) Piston
is compressed
quickly
Heat is then added
quickly by igniting
fuel (const volume)
Piston then expands
quickly
Heat is then
expelled quickly (by
getting rid of old air)
→ In our approxiImage credit: http://www.grc.nasa.gov/WWW/K-12/airplane/otto.html
mation we pretend
like same air is
“Compression ratio” r = Vmax/Vmin
reused
Derived in book:
eOtto  1 
1
r
 1
Thought Question
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If I replaced all of the nitrogen (N2) in the air
with carbon dioxide (CO2), what do you think
would happen to the efficiency of car
engines?
a. They would get more efficient
b. They would get less efficient
c. The efficiency would not change
eOtto  1 
1
r
 1
Diesel Cycle: Diesel Engines
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What’s the main difference between gas and
diesel engines?
Change to our PV-diagram model
Diesel cycle details… done in HW problem
Worked problem: Class designed
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Make up a “three-legged cycle”.
What is the efficiency of this cycle?
Game plan:
a. Calculate Q for each leg
b. Calculate Qin, Qout, |W|
c. e = |W|/Qin
d. [Next class period: Test to make sure e < emax]