Lessons 28–29 · 2 lessons · P. Habibullayev, A. Boydedayev, A. Bahromov, K. Suyarov, J. Usarov, M. Yuldasheva. Physics Grade 9, revised and expanded 3rd edition. G‘afur G‘ulom Publishing House, Tashkent, 2019
28
Internal combustion engines
Textbook: pp. 81–82
GoalUnderstand the heat engine; explain the working of the four-stroke internal combustion engine, the diesel engine and the jet engine.
New words
heat engine · issiqlik dvigateliinternal combustion engine · ichki yonuv dvigatelidiesel engine · dizel dvigatelijet engine · reaktiv dvigatel
Explanation
A heat engine is a device that converts heat energy into mechanical energy. In an internal combustion engine the fuel (petrol or gas) burns inside the cylinder. It usually works in four strokes: 1) intake – the combustible mixture (fuel and air) enters the cylinder; 2) compression – the piston compresses the mixture; 3) power stroke – a spark from the plug ignites the mixture, the hot gas expands, pushes the piston and does work; 4) exhaust – the spent gases are pushed out. A diesel engine (R. Diesel, patented in 1893) has no spark plug: the air is compressed so strongly that it becomes hot, the injector sprays liquid fuel into it and it ignites by itself; a diesel has a higher efficiency (about 40 %, against 25–30 % for petrol engines). In a jet engine the fuel burns in a combustion chamber and the gas rushes backwards out of a nozzle at high speed, pushing the engine forward; a rocket carries both fuel and oxidiser, so it works even in airless space.
Worked examples
Order of the four strokes: intake → compression → power stroke → exhaust. In one working cycle the piston goes up twice and down twice, the crankshaft turns 2 full revolutions, and work is delivered only in the power stroke.
Jet propulsion: if gas is thrown backwards with force F the engine is pushed forwards with an equal force (action and reaction). That is why a rocket accelerates even in a vacuum: it needs no air to push against.
Class activity
Draw a scheme: four pictures of a one-cylinder four-stroke engine (piston position, valves), labelling each stroke and what the gas is doing. Do not work on a real engine; only a mechanic may show a car engine.
Practice
1
What is a heat engine?
A device that converts heat energy into mechanical energy.
2
Name the four strokes of the engine in order.
Intake, compression, power stroke, exhaust.
3
How does a diesel engine differ from a petrol engine?
No spark plug: fuel ignites by itself in strongly compressed hot air; higher compression and efficiency.
4
Why can a rocket move even in airless space?
It carries its own fuel and oxidiser; the gas thrown backwards produces the reaction force and needs no air.