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Engines: How Internal Combustion Works
Intake, Compression, Combustion, Exhaust
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Internal combustion engines convert the chemical energy stored in fuel into mechanical motion that can power cars, generators, and many machines. They matter because they have been one of the most important technologies in transportation and industry for over a century. At the heart of the engine, fuel burns inside a cylinder and pushes a piston downward. That piston motion is then turned into rotation by the crankshaft.
A four stroke engine completes its cycle in four piston movements: intake, compression, power, and exhaust. During intake, the cylinder fills with an air fuel mixture or just air in some designs, and during compression the piston squeezes that charge into a smaller volume. A spark plug or high temperature from compression starts combustion, causing hot gases to expand rapidly and produce the power stroke. Finally, the exhaust stroke pushes burned gases out so the cycle can begin again.
Key Facts
- The four strokes are intake, compression, power, and exhaust.
- One full four stroke cycle requires 2 crankshaft revolutions = 720 degrees.
- Pressure creates force on the piston: F = P A
- Mechanical work from the gas on the piston can be estimated by W = F d
- Engine power is related to torque and angular speed: P = tau omega
- Compression ratio = (maximum cylinder volume) / (minimum cylinder volume)
Vocabulary
- Cylinder
- The cylinder is the chamber in which the piston moves up and down during the engine cycle.
- Piston
- The piston is a sliding component that is pushed by expanding gases and transfers force to the connecting rod.
- Crankshaft
- The crankshaft converts the piston's back and forth motion into rotational motion.
- Spark plug
- A spark plug creates an electric spark that ignites the compressed air fuel mixture in a gasoline engine.
- Compression ratio
- Compression ratio is the ratio of the cylinder volume before compression to the volume after compression.
Common Mistakes to Avoid
- Thinking the piston produces rotation directly, which is wrong because the connecting rod and crankshaft are the parts that convert linear motion into rotational motion.
- Assuming combustion happens during every stroke, which is wrong because burning mainly drives the power stroke while the other strokes prepare the mixture or remove exhaust.
- Confusing intake and compression, which is wrong because intake brings charge into the cylinder while compression squeezes that charge into a smaller space.
- Believing higher compression ratio always means any fuel will work, which is wrong because excessive compression can cause knocking if the fuel does not resist premature ignition.
Practice Questions
- 1 A cylinder has a pressure of 4.0 x 10^5 Pa acting on a piston with area 0.0080 m^2 during part of the power stroke. What force does the gas exert on the piston?
- 2 During one stroke, the piston moves 0.090 m while the average force on it is 2500 N. Estimate the work done on the piston.
- 3 Explain why the exhaust valve must stay closed during the compression stroke and describe what would happen if it were open.