announcements 9/21/11 prayer exam 1 starts a week from saturday! a. a.doodle survey about exam...
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Announcements 9/21/11 Prayer Exam 1 starts a week from Saturday!
a. Doodle survey about exam review times, vote for times before Friday
If you emailed me about Mathematica issues on HW 7 and still need to turn it in, do so by Friday
Regarding showing your work for the TA: (Foxtrot)
Special Processes: Review
Constant volume (isovolumetric) Constant pressure (isobaric) Constant temperature (isothermal) No heat added (adiabatic) General process?
a. Eint = ?
b. Won gas = ?
c. Qadded = ?
A
B
P
V
Engines: Energy Transformation
Qin = work by gas + Qout
Notation: Qh, Qc, Th, Tc, |W|
Qh = |W| + Qc
engineheat in(higher T)
work
exhaust out (lower T)
Worked Problem A car engine produces 5000 W of power,
at 20 cycles/second. Its efficiency is 20%. What are |Wnet|, Qh, and Qc per cycle?
What do those quantities represent?
Reading Quiz (graded)
What is the name we use for the cycle which models how a gas engine works?
a. Brayton cycle b. Carnot cyclec. Diesel cycled. Otto cyclee. Tri cycle
Otto Cycle: Gas Engines (Starting at 2)
Piston is com-pressed 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)
→ We pretend heat is added without changing gas
Image credit: http://www.grc.nasa.gov/WWW/K-12/airplane/otto.html
“Compression ratio” r = Vmax/Vmin
1
11Ottoer Derived in book:
Thought Question
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 efficientb. They would get less efficientc. The efficiency would not change
1
11Ottoer
Diesel Cycle: Diesel Engines
What’s the main difference between gas and diesel engines?
Change to our PV-diagram model Diesel cycle details… done in HW
problem