Download ASEN 3113 - Power Cycles

This Week > POWER CYCLES
Definition of a thermodynamic
cycle?
• Carnot Cycle (Max
Efficiency Cycle)
• Otto Cycle (Spark Ignition
Engine)
• Diesel Cycle (Compression
Engine)
• Brayton Cycle (Gas-Turbine
Engine)
Analysis of Power Cycles
• Energy balance of a
thermodynamic
cycle
• Because system
returns to initial
state - no net
change in energy
• Power Cycle - Net

work to surrounding
from net heat
transfer to system
Ecycle  Qcycle Wcycle
W cycle  Qin  Qout
Power Cycles
• Energy conversion
from heat to work thermal efficiency

W cycle
Qin
Qin  Qout

Qin
 1

Commonly employed assumptions and idealizations:
1) Cycle is frictionless - no pressure drops in flow
2) Expansion and compression quasi-equilibrium
(meaning what is in equilibrium?)
3) Heat transfer through connecting pipes is negligible
Carnot Cycle
• Devised by Nicolas Leonard
Sadi Carnot (1796 - 1832)
• Research was concerned with
determining the motive power
of heat (relation between heat
and mechanical energy)
• First to show that even under
ideal conditions an engine
cannot convert all of the heat
energy supplied to it
• His work was a prelude to
Joule and Kelvin
Carnot Cycle as a Heat Engine
• Most efficient cycle for
converting a given
amount of thermal
energy to work
• Two isothermal and two
isentropic steps
• Work in or out? What if
you reverse?
Carnot Engine
1) Reversible isothermal expansion of the gas at the "hot"
temperature, TH (isothermal heat addition). During
this step (A to B on diagram) the expanding gas
causes the piston to do work on the surroundings
(move down). The gas expansion is propelled by
absorption of heat from the high temperature
reservoir.
2) Reversible adiabatic expansion of the gas. For this step
(B to C on diagram) we assume the piston and
cylinder are thermally insulated (or the heat source
is removed), so that no heat is gained or lost. The
gas continues to expand while cooling (until TC is
reached), doing work on the surroundings.
Carnot Engine
3) Reversible isothermal compression of the gas at
the "cold" temperature, TC. (isothermal
heat rejection) (C to D on diagram) Now
the surroundings do work on the gas,
causing heat to flow out of the gas to
the low temperature reservoir and the
gas to recompress.
4) Reversible adiabatic compression of the gas. (D
to A on diagram) Once again we
assume the piston and cylinder are
thermally insulated. During this step, the
surroundings continue do work on the
gas, compressing it further and causing
the temperature to rise to TH. At this
point the gas is in the same state as at
the start of step 1.
Play movie!!!!!!!!!!!
Carnot Efficiency
• Starts from the cycle efficiency and a form of the second law
Sengine  Ssurroundings  0
• Engine returns to original state and all processes are reversible, so
change in entropy of engine is zero
• Hot reservoir delivers entropy to the engine and is rejected to the

cold reservoir in equal amounts, so:
QH QL

TH TL
• Combine with cycle efficiency to get:

TL
C  1
TH
Internal Combustion Engine - Otto Cycle
• Conceptualized by
Nikolaus August
Otto (June 14, 1832 January 28, 1891)
• Four stroke is more
fuel efficient and
clean burning than a
two stroke cycle
• Otto cycle consist of
strokes:
–
–
–
–
Intake stroke
Compression stroke
Power stroke
Exhaust stroke
Reciprocating Internal Combustion Engine
It is important to realize that an internal
combustion engine operates on a
mechanical cycle because the piston
system goes to the same initial points.
However from the thermodynamics stand
point this does not occur because new air
and fuel enters the engine in order to
initiate the combustion process. Thus
internal combustion engines operate in
Internal cycles.
Compression Ratio Defined as volume at
bottom dead center
divided by volume at
top dead center
Actual and Ideal Cycles in Spark-Ignition
Engines and Their P-v Diagram
In summary the Otto Cycle is internally reversible, so the area
Underneath the P-v diagram represents work and the T-s
Represents heat.
Also the cycle has:
• 2 Isentropic Processes when work is produced or input
• 2 processes at constant v, when heat is added or removed.
• Air Standard Otto Cycle
– Otto, Diesel, and Brayton cycles are gas power cycles working fluid remains a gas throughout the cycle
– Actual gas power are very complex - to simplify we
approximate > air standard assumptions
• Working fluid is air (neglect combustion products)
• Air circulates in a closed loop acting as an ideal gas
(constant specific heats)
• All processes are internally reversible
• Combustion is replaced by a heat addition process from
an external source
• Exhaust is replaced by a heat rejection process that
return working fluid to initial state
Air Standard Otto Cycle
The air standard Otto Cycle is an ideal
cycle that approximates a spark-ignition
internal combustion engine. It assumes that
the heat addition occurs instantaneously
while the piston is at TDC.
Process
(1-2) Isentropic Compression
Compression from ν1 => v2
↓
↓
BDC(β=180º )
TDC (θ=0º)
(2-3) Constant Volume heat input: QH
•While at TDC: umin
•Ignition of fuel (chemical reaction
takes place)
(3-4) Isentropic Expansion
•Power is delivered while s = const.
(4-1) Isentropic Expansion
•QL at umax=constant (BDC, θ =180º)
To the board for Otto cycle efficiency
• Parameters affecting thermal efficiency
– Octane rating - measure of the resistance to
autoignition of the fuel
• Unleaded vs. leaded
• Leaded fuel resistant to autoignition - unleaded restricts
engines to a compression ratio of around 9
– Specific heat ratio
• Thermal efficiency degrades as the molecules in the fluid
get larger
• Efficiencies of actual engines range from
around 25 to 30
• Now - sample problem
• Thur. - compression ignition cycle - no
knocking problems