Chapter 9 POWER ENGINES AND REFRIGERATION CYCLES (Thermal Cycles)
Carnot cycle => totally reversible cycle (with both internally and external reversible
processes) => have the highest thermal efficiency of all heat engines operating
between the same temperature levels.
Ideal cycle => When the actual cycle is stripped off all the internal irreversibilities and
complexities, we end up with a cycle that resembles the actual cycle closely but
is made up totally of internally reversible processes. Such a cycle is called an
ideal cycle. (May need externally reversible processes)
Cycle Modeling => provides great insight and simplicity at the expense of some loss in
accuracy
The idealizations and simplifications in the analysis of Cycle Modeling
1.
2.
3.
Not involve any friction. Therefore, the working fluid does not experience any pressure drop.
All expansion and compression processes take place in a quasi-equilibrium manner.
A system is well insulated, and heat transfer among components and soundings is negligible.
On both P-v and T-s diagrams, the area enclosed by the
process curve represents the net work of the cycle. Any
modification that increases the ratio of these two areas
will also increase the thermal efficiency of the cycle.
Carnot (power engine) cycles
The Carnot cycle is composed of four totally reversible processes:
(1) isothermal heat addition,
(2) isentropic expansion,
(3) isothermal heat rejection,
(4) isentropic compression.
For both ideal and actual cycles:
Thermal efficiency increases with an increase in the average temperature at
heat is supplied to the system or with a decrease in the average temperature
which heat is rejected from the system.
Gas power cycles (Air-standard cycle and assumptions)
The combustion process is
replaced by a heat-addition
process in ideal cycles.
Air-standard assumptions:
1. The working fluid is air, which continuously circulates in a closed
loop and always behaves as an ideal gas.
2. All the processes that make up the cycle are internally reversible.
3. The combustion process is replaced by a heat-addition process
from an external source.
4. The exhaust process is replaced by a heat-rejection process that
restores the working fluid to its initial state.
Cold-air-standard assumptions: When the working fluid is considered to
be air with constant specific heats at room temperature (25°C).
Air-standard cycle: A cycle for which the air-standard assumptions are
applicable.
which
at
Chapter 9 POWER ENGINES AND REFRIGERATION CYCLES (Thermal Cycles)
The assumptions for Reciprocating Engine Analyses
(The overview Reciprocating Engines)
Example
 Spark-ignition (SI) engines
 Compression-ignition (CI) engines
Otto cycles (Spark-ignition gasoline engines)
4 Thermal processes
1-2 Isentropic compression
2-3 Constant-volume heat addition
3-4 Isentropic expansion
4-1 Constant-volume heat rejection.
Four-stroke cycle
1 cycle = 4 stroke = 2 revolution
Two-stroke cycle
1 cycle = 2 stroke = 1 revolution
less
efficient + simple
high
power-to-weight ratio
power-to-volume ratio.
V2=V3, V4=V1,
,
Diesel cycles (Compression-ignition engines)
,
,
4 Thermal processes
1-2 Isentropic compression
2-3 Constant-volume heat addition
3-4 Isentropic expansion
4-1 Constant-volume heat rejection.
 In diesel engines, the spark plug is replaced by a fuel injector, and
only air is compressed during the compression process. eliminating
the autoignition (engine knock).
 It can be designed to operate at much higher compression ratios
than SI engines, typically between 12 and 24.
For the same compression ratio
Chapter 9 POWER ENGINES AND REFRIGERATION CYCLES (Thermal Cycles)
The assumptions for Gas Turbine Engine Analyses, (Gas turbine cycles => Brayton cycles)
(The overview of an ideal Brayton cycle for gas-turbine engines simplification and analysis)
An ideal Brayton cycle
An actual open-cycle
gas-turbine engine
A closed-cycle
gas-turbine engine
Simplify
4 Thermal processes
1-2 Isentropic compression
2-3 Constant-volume heat addition
3-4 Isentropic expansion
4-1 Constant-volume heat rejection.
 The combustion process is replaced by a
constant-pressure heat-addition process from
an external source, and
 the exhaust process is replaced by a
constant-pressure heat-rejection process to
the ambient air.
Deviation of Actual Gas-Turbine Cycles from Idealized Ones => Brayton cycles with regenerations
In gas-turbine engines, the temperature of the exhaust gas leaving the turbine is often considerably higher than the
temperature of the air leaving the compressor. Therefore, the high-pressure air leaving the compressor can be heated by
the hot exhaust gases in a counter-flow heat exchanger (a regenerator or a recuperator). The thermal efficiency of the
Brayton cycle increases as a result of regeneration since less fuel is used for the same work output.
Under cold-air standard assumptions
Chapter 9 POWER ENGINES AND REFRIGERATION CYCLES (Thermal Cycles)
Vapor power cycles (the working fluid is alternately vaporized and condensed)
Carnot vapor cycle
4 Thermal processes
1-2 Isothermal heat addition in a boiler
2-3 Isentropic expansion in a turbine
3-4 Isothermal heat rejection in a condenser
4-1 Isentropic compression in a compressor
The impracticalities associated with the Carnot cycle
can be eliminated by superheating the steam in the
boiler and condensing it completely in the condenser.
The cycle that results is the Rankine cycle.
Rankine vapor power cycle (Ideal Rankie)
Increase the cycle thermal efficiency.
Reheat vapor power cycles.
Vapor-compression refrigeration cycle.
The operation of refrigeration and heat pump systems
4 Thermal processes
1-2 Isentropic compression in a pump
2-3 Constant pressure heat addition in a boiler
3-4 Isentropic expansion in a turbine
4-1 Constant pressure heat rejection in a condenser
Actual vapor power cycles
Steady-flow energy equation
The thermal efficiency can be interpreted
as the ratio of the area enclosed by the
cycle on a T-s diagram to the area under
the heat-addition process.
Fluid friction and heat loss to the surroundings are the two common sources of irreversibilities.
How can we increase the efficiency of the rankine cycle?
1. Lowering the Condenser Pressure (Lowers Tlow,avg)
2. Superheating the Steam to High Temperatures (Increases Thigh,avg)
3. Increasing the Boiler Pressure (Increases Thigh,avg)
Refrigerators and heat pumps
The objective of a refrigerator is to remove heat (QL) from the cold
medium; the objective of a heat pump is to supply heat (QH) to a
warm medium.
for fixed values of QL and QH