Heat Engines and Thermodynamic Cycles: A Technical Analysis
Heat Engines
A heat engine is capable of producing useful work from the thermal energy stored in a fluid. Thermodynamics studies the processes of transformation of work into heat and vice versa, by first setting the equivalence between work and heat and determining then how working conditions can be obtained from thermal energy.
- The process for converting heat to work or vice versa, there is a constant relationship between the work done and the heat consumed (mechanical equivalent of heat, and = 4.18 J / cal.) Where the final state of the system is equal to the initial: W / Q = cte
Heat cannot be converted entirely into work, because by heating it transforms a part of heat into work, and the rest is intended to modify its internal energy (U):
Q = W + U -> U = Q – W
Isocore (V = constant), isobars (p = cte), isotherm (T = constant), adiabatic (Q = constant)
- Built-in state: They depend on the initial and final states, not the path taken to get there (P) (V) (T) (U)
- Equation of state pV = nRT
- Variables not state function: Depend on the conditions in which the thermodynamic process unfolds (Q) Work (W)
Second Law
- A heat engine can only do work if it absorbs heat from a hot source and gives it to another colder part.
- Heat cannot be spontaneously transferred from a colder body to a hotter one. W = Qc-Qf
Carnot Cycle
- Ideal
- It is reversible, therefore a refrigerator can operate.
- Yield: The maximum that can produce a heat engine operating between the foci Qc and Qf, n = 1 – Qf / Qc n = 1 – Tf / Tc
Steam engine: The cylinder moves so alternately that it arrives by steam boiler, transformed its linear motion into rotary by a rod system that is part of the crank flywheel.
4 Stroke Engine
Parts: Pistons, spark plugs, rings, oil seals, bolt, rod, cylinder, crankshaft, valve, balance, spring.
Operation: Performed in 4 changes of movement of the piston (two laps from the crankshaft)
Time 1: Intake: The piston descends from the top dead center, creates a vacuum in the cylinder that makes it suck air through the intake valve that is open.
2nd Compression: The valve closes when the piston reaches bottom dead center, at which this starts to rise again compressing the load to reach the top dead.
3rd Expansion: Before compression, ignition of the fuel occurs, increasing P and T. The piston moves downward, producing the work.
4th Escape: Once the piston is at bottom dead center, the exhaust valve opens, the piston rises and the gas flows out.
2 Stroke Engine
No valves: Intake port, exhaust port, transfer port.
TIME: INTAKE-COMPRESSION-EXPANSION-EXHAUST (1 crank back)
Advantages: Simplicity of construction – Removal of valves – Increased power (useful work at every turn of the crankshaft, while that of 4t every 2 laps) Improved transmission.
Disadvantages: Minor mechanical performance – Higher operating temperature – Increased wear of its parts – Increased pollution (fuel oil).
First Time
When the piston is at top dead center ignition occurs, the gases expand to open the exhaust port where it exits the high pressure gas. Once the piston compresses the mixture of low and carter, it opens a communication between the cylinder and carter. This fluid enters the cylinder expelling the gas to the exhaust port.
Second Half
The piston starts to rise from the bottom dead center to the stage of admission to sweep and closing the intake port and then escape. In compression, it starts until the GPA, opening the skylight and into the fluid the carter.
Otto Cycle
Stages: 0-1: Admissible 1-2: Compression 2-3: Explosion 3-4: Expansion 4-1: Escape 1-0: Exhaust Gases
Four Stroke Compression Ignition Engine
Diesel Cycle: Stages
0-1: Admissible 1-2: Compression 2-3: Explosion 3-4: Expansion 4-1: Escape 1-0: Exhaust Gases
Comparison with Diesel and Gasoline Engines
Advantages: Much higher compression ratio – Higher thermal efficiency (higher heat transformed into work) – Less fuel consumption – Increased engine life.
Disadvantages: Less Strength – Increased pollution (more NOx, SOx, soot) – Heavier engine – Increased construction costs – Increased noise from loud explosions.
Gas Turbine
Parts: Compressor: Raises air pressure. – Fuel Chamber: It mixes the compressed air with injected fuel. – Turbine: An element that transforms the kinetic energy of exhaust gas into mechanical energy (rotation). – Exhaust Nozzle (for directing the gas jet machines) Applications: Autonomous generators. Aircraft Propulsion.
Steam Turbine
Parts: Pump: Driving the liquid – Heater: Heats the liquid water and passes steam – Power: Steam produces work when the incident on their blades – Condenser: Wet steam condenses in its entirety – Applications: – Power production of electricity – Ship Propulsion.
Efficiency of the Refrigeration Steam Engine
Efficiency is defined as the ratio of heat absorbed and work done. Efficiency = Q2 / W = Q2/Q1-Q2
Efficiency = T2/T1-T2
Components of a Refrigeration Installation
Compressor: This is the element that supplies power to the system. The refrigerant in a gaseous state reaches the compressor and its pressure increases.
Condenser: The condenser is a heat exchanger, which dissipates the heat absorbed in the evaporator (below) and the energy of the compressor. In the condenser the refrigerant changes phase from gas to liquid.
Expansion System: Liquid refrigerant enters the expansion device which reduces pressure. By reducing its pressure suddenly reduces its temperature.
Evaporator: Refrigerant at low temperature and pressure passing through the evaporator, which like the condenser is a heat exchanger and absorbs heat from the room where is located. The liquid refrigerant entering the evaporator is transformed into gas by absorbing heat from the enclosure.
Types of Systems for Gas Refrigeration
- Usually air
- Reverse Brayton Cycle
- Implementation in aviation
- By Steam: Cryogenic Fluids (fluorocarbons)
- Reverse Rankine Cycle
Efficiency of a Heat Pump
Efficiency is defined as the ratio of the heat transfer and work done. Efficiency = Qc / W = Qc / Qc-Qf Efficiency = Tc / Tc-Tf.
Less energy (electrical) resistance heating
Gas Turbine (Internal Combustion Engine)
Brayton cycle
Steam Turbine (External Combustion Engine)
Rankine Cycle