Heat Engines, Efficiencies, Carnot Cycle, and the Second Law of Thermodynamics Flashcards
(26 cards)
What is a heat engine?
A device that absorbs heat and performs useful work on the surroundings during a cycle.
What must happen to the working substance in a heat engine cycle?
It must return to its original thermodynamic state.
What are common working substances in heat engines?
Air, steam, or fuel vapours.
What is the work done by a heat engine during expansion?
W=−∫ P dV
What type of combustion does the Otto cycle use?
Isochoric (constant volume) combustion.
What type of combustion does the Diesel cycle use?
Isobaric (constant pressure) combustion.
What do both Otto and Diesel cycles have in common?
Both involve adiabatic compression and expansion.
What makes the Stirling engine unique?
It’s a closed system operating between small temperature differences.
What is the order of processes in the Stirling cycle?
1 → 2: Isothermal compression
2 → 3: Isochoric heating
3 → 4: Isothermal expansion
4 → 1: Isochoric cooling
What is the general formula for engine efficiency 𝜂?
η = W1 / Q1 = 1− Q2 / Q1
When is efficiency maximised?
When 𝑄2 (wasted heat) is minimised.
What is special about the Carnot cycle?
It is the most efficient reversible heat engine possible.
What are the four steps in the Carnot cycle?
1 → 2: Isothermal expansion at 𝑇1
2 → 3: Adiabatic expansion
3 → 4: Isothermal compression at 𝑇2
4 → 1: Adiabatic compression
What is Carnot efficiency 𝜂𝐶?
𝜂𝐶 = 1 − 𝑇2/𝑇1
What type of working substance is assumed in the Carnot cycle?
Ideal Gas
Equation for Heat In / Out the system
Q =−∫C dT
What is the Kelvin-Planck statement?
No engine can fully convert heat from a single reservoir into work in a cycle.
What is the Clausius statement?
Heat cannot flow from a cold body to a hot body without doing work.
What is efficiency for any real engine?
𝜂 < 1 for any real engine.
How does a refrigerator work thermodynamically?
It’s a heat engine in reverse, moving heat from cold to hot with work input.
What is refrigerator efficiency (coefficient of performance)?
ηR = Q2 / Q1 - Q2
What is heat pump efficiency?
ηR = Q1 / Q1 - Q2 = T1 / T1 - T2
What is the Clausius equality for reversible cycles?
∮ dQ rev / T = 0
What is the Clausius inequality for irreversible cycles?
∮ dQ rev / T ≤ 0
