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

Last Special Case: Cyclical Processes

Eint = ?

net Wby gas or Won gas? |W| = ?

Qadded = ?

P

V

State 1

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)

(a) - piston atroom temp and

atmosphericpressure P

Va

Heat Engine Example

(a-b) - gasheated to keep

volumeconstant as

cart rolls ontopiston P

Va

b

Heat Engine Example

(b-c) - gasheated toincrease

volume and liftthe cart

P

Va

b c

Heat Engine Example

(c-d) - gascooled to keep

volumeconstant atcart rolls off

piston

P

Va

b c

d

Heat Engine Example

(d-a) - gascooled to

reduce volumeback to initial

level

P

Va

b c

d

Heat Engine Example

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 |W|, 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. Carnot cycleb. Brayton cycle c. Diesel cycled. Otto cyclee. Tri cycle

Otto Cycle: Gas Engines (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 approxi-mation we pretend like same air is reused

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

Worked problem: Class designed

Make up a “three-legged cycle”. What is the efficiency of this cycle? Game plan:

a. Calculate Q for each legb. Calculate Qin, Qout, |W|

c. e = |W|/Qin

d. [Next class period: Test to make sure e < emax]