A First Course in Physics · Ch. IX, §§ 252–263 · Chapter 9: Work and Heat Energy
29. Heat engines: steam, gas and turbines
Goal: Your student can explain how a steam engine, a four-stroke gas engine and a steam turbine turn heat into work, and why no heat engine is very efficient.
You'll need
- The book, open to these pages
- A pinwheel (to show the turbine idea by blowing on it)
- Paper and pencil to draw the four strokes
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Watt's steam engine and its valve; condensing and noncondensing engines; the eccentric, boiler, draft, governor and reversing gear; compound engines; the efficiency of steam engines; the four-stroke gas engine; the steam turbine. Seven problems follow.
The lesson, step by step
Hook question
SayA car engine burns gasoline. Where does most of that energy end up? (Mostly as heat out of the exhaust and radiator.)
Read together
Read §§ 252–263, studying Figs. 188 and 194 closely. The book's point about efficiency:
it is only a comparatively small fraction of the heat developed by combustion which can be transformed into work.
Trace the steam engine
With Fig. 188 open, have your student trace the steam: boiler, pipe, valve chest, one side of the piston, then out to the exhaust, while the sliding valve switches sides. Ask what the heavy fly wheel is for.
Draw the four strokes
Draw four boxes for Fig. 194: 1, draw in the fuel and air; 2, squeeze it; 3, the spark fires and pushes the piston (the only power stroke); 4, push out the burned gases. Most car engines today still work this way.
Try it: a turbine
Blow on a pinwheel: steam turbines (§ 263) work the same way, with jets of steam on rows of blades. Most electricity today is still made by turbines turned by steam, water or wind.
What we know now
Engines are much better now: large modern power plants turn about 40–60% of their fuel's heat into electricity, and car engines about 30–40%. But no heat engine can ever reach 100%, exactly as § 260 explains: some heat must always be thrown away.
Say it back
Your student explains why the gas engine needs a heavy fly wheel.
Problems
Work problems 1–7 on book page 195. Use 427 gram meters per calorie and 76.05 kilogram meters per second for one horse power (550 ft. lb. per second in English units).
Afterwards
With an adult, open the hood of a car and find the engine, radiator and exhaust. Where does the waste heat go?
Answers to the book's problems
1. 1500 kg. m (150,000 kg. cm) of work; about 3510 calories. 2. About 935 kg per hour. 3. About 335 kg. 4. About 274,000 kg (about 274 metric tons). 5. About 40,000 kg (40 metric tons) per hour. 6. About 374 H.P. for a metric ton of coal (about 340 H.P. for a ton of 2000 lb.). 7. 15,000 lb.
Worked out for this site from the scan. If a number in your copy differs, trust the book.