Gas Power Cycles
Power Cycles
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Ideal Cycles, Internal Combustion
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Otto cycle, spark ignition
Diesel cycle, compression ignition
Sterling & Ericsson cycles
Brayton cycles
Jet-propulsion cycle
Ideal Cycles, External Combustion
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Rankine cycle
Modeling
Ideal Cycles
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Idealizations & Simplifications
 Cycle does not involve any friction
 All expansion and compression processes are
quasi-equilibrium processes
 Pipes connecting components have no heat
loss
 Neglecting changes in kinetic and potential
energy (except in nozzles & diffusers)
Carnot Cycle
Carnot Cycle
Gas Power Cycles
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Working fluid remains a gas for the entire
cycle
Examples:
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Spark-ignition engines
Diesel engines
Gas turbines
Air-Standard Assumptions
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Air is the working fluid, circulated in a closed
loop, is an ideal gas
All cycles, processes are internally reversible
Combustion process replaced by heat-addition
from external source
Exhaust is replaced by heat rejection process
which restores working fluid to initial state
Cold-Air-Standard Assumption
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Air has constant specific heats, values are
for room temperature (25°C or 77°F)
Engine Terms
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Top dead center
Bottom dead center
Bore
Stroke
Engine Terms
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Clearance volume
Displacement volume
Compression ratio
Engine Terms
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Mean effective
pressure (MEP)
Otto Cycle
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Processes of Otto Cycle:
Isentropic compression
Constant-volume heat addition
Isentropic expansion
Constant-volume heat rejection
Otto Cycle
Otto Cycle
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Ideal Otto Cycle
Four internally reversible
processes
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1-2 Isentropic compression
2-3 Constant-volume heat
addition
3-4 Isentropic expansion
4-1 Constant-volume heat
rejection
Otto Cycle
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Closed system, pe, ke ≈ 0
Energy balance (cold air std)
Otto Cycle
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Thermal efficiency of ideal Otto cycle:
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Since V2= V3 and V4 = V1
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Where r is compression ratio
k is ratio of specific heats
Otto Cycle
Spark or Compression Ignition
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Spark (Otto), air-fuel
mixture compressed
(constant-volume heat
addition)
Compression (Diesel), air
compressed, then fuel
added (constant-pressure
heat addition)
Diesel Cycle
Diesel Cycle
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Processes of Diesel cycle:
Isentropic compression
Constant-pressure heat addition
Isentropic expansion
Constant-volume heat rejection
Diesel Cycle
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For ideal diesel cycle
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With cold air assumptions
Diesel Cycle
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Cut off ratio rc
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Efficiency becomes
Brayton Cycle
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Gas turbine cycle
Open vs closed
system model
Brayton Cycle
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Four internally
reversible processes
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1-2 Isentropic
Compression
(compressor)
2-3 Constant-pressure
heat addition
3-4 Isentropic
expansion (turbine)
4-1 Constant-pressure
heat rejection
Brayton Cycle
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Analyze as steady-flow process
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So
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With cold-air-standard assumptions
Brayton Cycle
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Since processes 1-2 and 3-4 are
isentropic, P2 = P3 and P4 = P1
where
Brayton Cycle
Brayton Cycle
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Back work ratio
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Improvements in gas
turbines
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Combustion temp
Machinery component
efficiencies
Adding modifications
to basic cycle
Actual Gas-Turbine Cycles
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For actual gas
turbines, compressor
and turbine are not
isentropic
Regeneration
Regeneration
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Use heat exchanger
called recuperator or
regenerator
Counter flow
Regeneration
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Effectiveness
For cold-air
assumptions
Brayton with Intercooling,
Reheat, & Regeneration
Brayton with Intercooling,
Reheat, & Regeneration
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For max performance
Ideal Jet-Propulsion Cycles
Ideal Jet-Propulsion Cycles
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Propulsive power
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Propulsive efficiency
Turbojet Engines
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Turbofan: for same power, large volume of
slower-moving air produces more thrust
than a small volume of fast-moving air
(bypass ratio 5-6)
Turboprop: by pass ratio of 100
Jets
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Afterburner: addition to turbojet
Ramjet: use diffusers and nozzles
Scramjet: supersonic ramjet
Rocket: carries own oxidizer
Second Law Issues
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Ideal Otto, Diesel, and Brayton cycles are
internally reversible
2nd Law analysis identifies where losses
are so improvements can be made
Look at closed, steady-flow systems
Second Law Issues
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For exergy and exergy destruction for
closed system:
For steady-flow system:
Second Law Issues
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For a cycle that starts and end at the
same state: