Transcript Objectives

Objectives
Describe thermodynamic processes on
P-V diagrams.
State and apply second law principles.
Apply the laws of thermodynamics to
the Otto cycle.
Today’s Plan
Finish efficiency discussion.
Discuss Thermodynamics.
Discuss the Otto cycle.
Homework problems due Monday!
Quiz Friday.
Do Now (11/5/13):
Rank the following diagrams in order
from highest to lowest in terms of the
work done. Justify your response.
A.
B.
C.
Thermodynamics
Study of processes in which energy is
transferred as heat and as work.
System vs. environment
Closed system: no mass enters
Open system: mass may enter or leave
Isolated system: no energy passes
across boundaries
Thermal Energy
Internal energy—sum of all energies in
a system. U=KE + PE (molecular level)
U=(3/2)NkT for an ideal monatomic gas
where k=1.38 x 10-23 J/K
U=(3/2)nRT, where R=8.315 J/mol-K
First Law
Energy is neither created nor
destroyed; changes from one form to
another.
Transferred by work or heat.
U can change only if system is not
isolated. (Energy moves between
system and surroundings)
Work
Energy transfer between system and
surroundings due to organized motion
in the surroundings. (rubbing a block
of wood vigorously, stir a glass of
water, allow a gas to expand against an
external pressure.)
Work on/by an Enclosed Gas
Ideal gas in moveable piston.
Expands slowly so it remains near
equilibrium throughout. Isobaric
process.
Pressure inside equals pressure exerted
by piston (ie from outside)
Wby gas = - Won gas by the piston
Pressure-Volume Work
P= F/A
F=P*A
Wby = F*d = PV
Graphical representation.
Area under curve = work done by gas
Heat
Energy transfer between system and
surroundings as a result of random
motion in the surroundings.
Flows spontaneously from high temp to
low temp. Work can be used to make
heat flow opposite natural flow
direction.
First Law
U=Won + Qinto
Where U=internal energy, Q=net heat added to
system, W=net work done on the system.
Conventionally, heat added is +,
lost is negative
Work done on system is +, done by
system is negative
Statement of energy conservation
Thermodynamic Systems
Isothermal—work done by the gas
equals the heat added to the gas. U=0
Adiabatic—no heat is allowed to flow
into or out of the system. Q=0
therefore, U=Won.
Isobaric—pressure is constant
Isochoric—volume is constant
Isothermal
Constant temperature
PV=constant
Graphical representation and isotherms
As heat is added slowly, gas expands at
constant temperature. Work is done by
the gas. U=0, so Wby = Qin.
Adiabatic
No heat is exchanged between the
system and surroundings. Q = 0.
U = Won
Internal energy decreases as gas
expands. (U=3/2 NkT, so
temperature will decrease.)
Isobaric
Pressure of system remains constant.
W= PV
Isochoric
Volume remains constant
W = 0.
Second Law
Heat flows naturally from a hot object
to a cold object; heat will not flow
spontaneously from a cold object to a
hot object.
http://www.entropylaw.com/
Heat Engine
Carnot Cycle
Refrigerator/Heat Pump
Refrigeration Schematic
Refrigeration Cycle
Entropy
Disorder
S=Q/T where T=absolute temperature
The entropy of an isolated system never
decreases. It only stays the same or
increases.
If not isolated, the change in entropy of
the system plus in the environment is
greater than 0.
Second Law Restatement
Natural processes tend to move toward
a state of greater disorder.
Nikolaus Otto
The four stroke engine was first
demonstrated by Nikolaus Otto
in 1876, hence it is also known as
the Otto cycle. The technically correct term is
actually four stroke cycle. The four stroke
engine is probably the most common engine
type nowadays. It powers almost all cars and
trucks.
starting point
Position 1 being the beginning of the
intake stroke of the engine. The pressure
is near atmospheric pressure and the gas
volume is at a minimum.
Intake Cycle
Position 12
Copyright 2000, Matt Keveney. All rights reserved.
Between Stage 1 and
Stage 2 the piston is
pulled out of the
cylinder with the
intake valve open.
The pressure
remains constant,
and the gas volume
increases as fuel/air
mixture is drawn into
the cylinder through
the intake valve.
Compression
Cycle
Position 23
Copyright 2000, Matt Keveney. All rights reserved.
Stage 2 begins the
compression stroke of
the engine with the
closing of the intake
valve. Between Stage 2
and Stage 3, the piston
moves back into the
cylinder, the gas volume
decreases, and the
pressure increases
because work is done
on the gas by the piston.
Stage 3
Stage 3 is the beginning of the
combustion of the fuel/air mixture. The
combustion occurs very quickly and the
volume remains constant. Heat is
released during combustion which
increases both the temperature and the
pressure, according to the equation of
state.
Power Cycle
Position 45
Copyright 2000, Matt Keveney. All rights reserved.
Stage 4 begins the
power stroke of the
engine. Between
Stage 4 and Stage
5, the piston is
driven towards the
crankshaft, the
volume in
increased, and the
pressure falls as
work is done by the
gas on the piston.
Stage 5
At Stage 5 the exhaust valve is opened
and the residual heat in the gas is
exchanged with the surroundings. The
volume remains constant and the
pressure adjusts back to atmospheric
conditions.
Exhaust Cycle
Position 61
Copyright 2000, Matt Keveney. All rights reserved.
Stage 6 begins the
exhaust stroke of the
engine during which the
piston moves back into
the cylinder, the volume
decreases and the
pressure remains
constant. At the end of
the exhaust stroke,
conditions have
returned to Stage 1 and
the process repeats
itself.
Practice:
Work with your group to complete the
conceptual questions.
One paper per group will be collected
Each person should use a different color
writing utensil
Each person should have written roughly
the same amount of work