170 likes | 171 Views
Announcements 9/21/11. Prayer Exam 1 starts a week from Saturday! 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). Video.
E N D
Announcements 9/21/11 • Prayer • Exam 1 starts a week from Saturday! • 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)
Video • Adiabatic cotton burner • Mist from pressurized bottle
Special Processes: Review • Constant volume (isovolumetric) • Constant pressure (isobaric) • Constant temperature (isothermal) • No heat added (adiabatic) • General process? • Eint = ? • Won gas = ? • Qadded = ? P B A V
P State 1 V Last Special Case: Cyclical Processes • Eint = ? • Is it Wby gas or Won gas? • Qadded = ?
work heat in (higher T) engine exhaust out (lower T) Engines: Energy Transformation • Qin = work by gas + Qout • Notation: Qh, Qc, Th, Tc, |W| • Qh = |W| + Qc
Heat Engine Example a (a) - piston at room temp and atmospheric pressure P V
Heat Engine Example (a-b) - gas heated to keep volume b constant as cart rolls onto piston P a V
Heat Engine Example c (b-c) - gas heated to increase b volume and lift P the cart a V
Heat Engine Example (c-d) - gas cooled to keep volume constant at cart rolls off piston d c b P a V
Heat Engine Example c b (d-a) - gas P cooled to reduce volume d back to initial a level V
Efficiency • How good is your engine? • Definition
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? • Brayton cycle • Carnot cycle • Diesel cycle • Otto cycle • 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 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? • They would get more efficient • They would get less efficient • The efficiency would not change
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
Worked problem: Class designed • Make up a “three-legged cycle”. • What is the efficiency of this cycle? • Game plan: • Calculate Q for each leg • Calculate Qin, Qout, |W| • e = |W|/Qin • [Next class period: Test to make sure e < emax]