Heat-Engine Cycles, Energy Balance, and Efficiency

All flashcards

What is a heat engine?

A device that absorbs heat from a hot source, converts some of it into useful work, and releases the remaining heat to a colder sink.

What is the energy balance for one complete heat-engine cycle?

Since the net internal-energy change is zero, the work output is $W=Q_h-Q_c$.

How is heat-engine efficiency defined?

Efficiency is the fraction of absorbed heat converted to work: $e=W/Q_h=1-Q_c/Q_h$.

How are work and average power related?

For average power $P$ over time interval $\Delta t$, work is $W=P\Delta t$.

How can discharged heat be calculated from work and efficiency?

For work output $W$ and efficiency $e$, discharged heat is $Q_c=W(1/e-1)$.

Quiz questions

1. A heat engine absorbs $900\,\text{kJ}$ and releases $650\,\text{kJ}$ in one cycle. How much work does it deliver?Answer: $250\,\text{kJ}$
2. An engine delivers average power of $2.0\,\text{kW}$ for $30\,\text{s}$. What work does it deliver?Answer: $60\,\text{kJ}$
3. A heat engine delivers $120\,\text{kJ}$ of work at efficiency $e=0.30$. How much heat does it discharge?Answer: $280\,\text{kJ}$
4. Which equation gives the efficiency of a heat engine in terms of absorbed heat $Q_h$, discharged heat $Q_c$, and work $W$?Answer: $e=W/Q_h=1-Q_c/Q_h$
5. In a steam power plant, what role does water play as the working substance?Answer: It is heated into vapor, drives a turbine, and is condensed back into liquid.