Thermodynamic Cycles

Alicia Ma. Esponda Cascajares

Power Cycles• Cycles which convert a heat input into a

mechanical work output.• Power cycles can be divided according to the

type of heat engine they seek to model.• External combustion engines:

• Rankine cycle• Brayton cycle

• Internal combustion engines:• Otto cycle• Diesel cycle

External combustion engines

• They use steam or gas turbines.• Combustion takes place outside the

heat engine itself.• They are complete thermodynamic

cycles.

Internal combustion engines

• Combustion takes place inside the heat engine as one of the main processes.

• Strictly speaking, they aren’t a real thermodynamic cycle, but they are model as one.

Rankine cycle

• Typical cycle for electric power generation.• Its common heat sources are coal

combustion, natural gas combustion, oil combustion or nuclear fission.

• It’s very similar to Carnot cycle, except that it uses two isobaric processes instead of isothermal processes.

Rankine Cycle

• In an ideal Rankine cycle the pump and the turbine would be isentropic.

• Pump and turbine would generate no entropy and the net work output would be maximize.

Rankine Cycle

Rankine Cycle

• 1-2) Adiabatic compression

The working fluid is pumped from low to high pressure requiring very little energy input.

• 2-3) Isobaric heat transfer

High pressure liquid enter a boiler where it’s heated at constant pressure by an external heat source to become a dry saturated vapor.

Rankine Cycle

• 3-4) Adiabatic expansion

Dry saturated vapor expands through a turbine, generating power and decreasing the vapor’s temperature and pressure.

• 4-1) Isobaric heat transfer

Wet vapor enters a condenser where it’s condensed at a constant pressure to become a saturated liquid.

Rankine Cycle

Rankine Cycle

Normal Air Cycles Open cycles are usually modeled as normal air

cycles. Normal air cycles use the following rules:

Working substance has a constant mass during the whole cycle.

Air is modeled as an ideal gas.

Any combustion process occurring during the cycle is replaced by a positive heat transfer process from an external source.

The cycle is closed by a rejection heat flow process to the surroundings instead of the real products rejection process.

All processes are internally reversible.

Air is considered to have a constant specific heat capacity at room temperature.

Brayton cycle

• Typical cycle used in gas turbines basis of the jet engines used in airplanes, space-shuttles, etc.

• It’s similar to Rankine cycle, except that it doesn’t have phase changes in its working fluid which remains always a gas.

Brayton Cycle (real)

Brayton Cycle (modelled)

Brayton Cycle

• 1-2) Adiabatic compression

Ambient air is drawn into the compressor, where it is pressurized.

• 2-3) Isobaric heat transfer

The compressed air then runs through a combustion chamber, where fuel is burned, heating that air at a constant-pressure, since the chamber is open to flow in and out.

Brayton Cycle

• 3-4) Adiabatic expansion

The heated, pressurized air then gives up its energy, expanding through a turbine. Some of the work extracted by the turbine is used to drive the compressor.

• 4-1) Isobaric heat transfer

Heat is rejection in the atmosphere.

Brayton Cycle

Brayton Cycle

Reciprocating Engines Many of the air cycles use piston – cylinder

combinations for cycles and are known as reciprocating engines.

For this kind of engines you must understand the following terms:

• Stroke (Carrera) length of a piston travelling the full length of its engine cylinder in one direction or full travel of the piston from Top Dead Center to Bottom Dead Center.

• Top Dead Center (Punto Muerto Superior, PMS) is when the piston is farthest away from the axis of the crankshaft.

• Bottom Dead Center (Punto Muerto Inferior, PMI) is when the piston is nearest away from the axis of the crankshaft.

Reciprocating Engines

• Clearance Volume (Volumen libre de compresión) is the volume of the combustion chamber and is the minimum volume of the space at the end of the compression stroke.

• Engine Displacement (cilindrada) is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre.

Reciprocating Engines

• Compression rate (Razón de compresión, r) is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.

• Mean Effective Pressure (Presión Media Efectiva, PME) is the measured power output of an engine, it does not reflect the actual pressures inside an individual combustion chamber and serves only as a convenient measure of performance.

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rPMS

PMI

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Otto Cycle

• Common cycle in all gasoline engines.• Closest resemblance to the four-stroke-

engine.• Mechanically and thermodynamically

consists of 4 steps.

Otto Cycle

Otto Cycle

• Stroke 1 "Intake": the piston descends from the top of the cylinder to the bottom of the cylinder, reducing the pressure inside the cylinder. A mixture of fuel and air is forced by atmospheric (or greater) pressure into the cylinder through the intake port. The intake valve(s) then close.

1-2) Adiabatic compression • Stroke 2 "Compression": With both intake and

exhaust valves closed, the piston returns to the top of the cylinder compressing the fuel-air mixture. This is known as the compression stroke.

Otto Cycle

2-3) Isochoric heat transfer• Stroke 3 "Power": While the piston is at or close to

Top Dead Center, the compressed air–fuel mixture is ignited by a spark plug.

3-4) Adiabatic expansion• The resulting massive pressure from the combustion of

the compressed fuel-air mixture drives the piston back down toward bottom dead center with tremendous force. This is known as the power stroke, which is the main source of the engine's torque and power.

Otto Cycle

4-1) Isochoric heat transfer• Stroke 4 "Exhaust": During the exhaust stroke, the

piston once again returns to top dead center while the exhaust valve is open. This action evacuates the products of combustion from the cylinder by pushing the spent fuel-air mixture through the exhaust valve(s).

Otto Cycle