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Thermodynamics & Energy Systems - Leejohnston - 11-17-2025
Thread 2 — Thermodynamics & Energy Systems
Understanding Heat, Work, Efficiency & How Engines Really Work
Thermodynamics is the science of energy — how it moves, transforms, and powers machines. Every engine, power plant, refrigerator, heater, and even your own body obeys thermodynamic laws. This thread introduces the essential principles mechanical engineers use to design efficient, powerful, and safe systems.
1. What Is Thermodynamics?
Thermodynamics studies:
• energy
• heat
• work
• temperature
• efficiency
• entropy
Mechanical engineers use thermodynamics to design:
• engines (cars, jets, turbines)
• refrigerators & heat pumps
• boilers & power plants
• HVAC systems
• industrial heating & cooling
It answers one big question:
“How can we turn energy into useful work?”
2. The Zeroth, First, Second & Third Laws
These four laws describe everything about energy and heat.
Zeroth Law — Temperature Exists
If A is the same temperature as B, and B is the same as C,
→ A = C.
This enables thermometers and the idea of “thermal equilibrium”.
First Law — Energy Conservation
Energy cannot be created or destroyed — only transformed.
ΔEnergy = Heat In − Work Out
Example:
Fuel’s chemical energy → heat → mechanical work → motion.
Second Law — Entropy Always Increases
Heat naturally flows from hot → cold.
You can’t build a 100% efficient engine.
Third Law — Absolute Zero Is Unreachable
At 0 K (−273.15°C), entropy becomes minimal but can never be perfectly zero.
3. Heat, Work & Internal Energy
Heat (Q): energy transferred because of temperature difference.
Work (W): energy transferred by force × distance.
Internal Energy (U): microscopic energy stored inside molecules.
The First Law in equation form:
ΔU = Q − W
(add heat, internal energy increases; do work, internal energy decreases)
4. The Four Thermodynamic Processes
Mechanical systems often operate in one of these modes:
1. Isothermal — constant temperature
2. Adiabatic — no heat transfer
3. Isochoric — constant volume
4. Isobaric — constant pressure
Example:
Car engines use alternating **adiabatic** and **constant-volume** processes.
5. Heat Transfer — Conduction, Convection & Radiation
Conduction — heat moves through solids
Example: touching a hot pan handle.
Convection — heat moves through fluids
Example: boiling water, warm air rising.
Radiation — electromagnetic waves
Example: sunlight heating the Earth.
Engineers must combine all three to design safe thermal systems.
6. The Ideal Gas Law — The Engine Equation
PV = nRT
This describes the relationship between:
• pressure (P)
• volume (V)
• temperature (T)
• amount of gas (n)
Engines, compressors, air tools, and pneumatics all rely on this.
7. Heat Engines — Turning Heat Into Work
A heat engine absorbs heat from a high-temperature source, produces work, and rejects waste heat to a cooler sink.
General flow:
Hot Source → Engine → Work Output → Cold Sink
Examples:
• car engines
• jet engines
• steam turbines
• nuclear power plants
Thermal efficiency:
η = (Work Out ÷ Heat In) × 100%
No engine is 100% efficient due to the Second Law.
8. The Carnot Cycle — Maximum Possible Efficiency
The Carnot engine is a theoretical perfect engine.
Its efficiency depends only on temperature:
η = 1 − (Tcold ÷ Thot)
This sets the limit for all real engines.
If you want higher efficiency → increase temperature difference.
9. Refrigerators, Freezers & Heat Pumps
These machines run in reverse:
Work In → Moves heat from cold → hot
This requires energy because it goes against nature’s direction.
Performance is measured by the Coefficient of Performance (COP).
Refrigerators use:
• compressors
• evaporators
• condensers
• expansion valves
Heat pumps are becoming crucial in sustainable energy engineering.
10. Real Engineering Applications
Thermodynamics is used in:
• engine cycles (Otto, Diesel, Brayton)
• turbochargers & compressors
• air conditioning design
• battery thermal management
• spacecraft thermal control
• industrial furnaces
• geothermal & solar energy
Mechanical engineers constantly ask:
“How can we move heat or convert it to useful work more efficiently?”
End of Thread — Thermodynamics & Energy Systems