You already know starting a car is as easy as turning a key, but have you ever wondered what really happens under the hood?

When your body needs fuel, you feed it food. When your car needs fuel, you “feed” it gasoline. Just like your body converts food into energy, a car engine converts gas into motion.

The process of converting gasoline into motion is called “internal combustion.” Internal combustion engines use small, controlled explosions to generate the power needed to move your car all the places it needs to go.

If you create an explosion in a tiny, enclosed space, such as a piston in an engine, a huge amount of energy is released as expanding gas. A typical car engine creates such explosions hundreds of times per minute. The engine harnesses the energy and uses it to propel your car.

The explosions force pistons in the engine to move. When the energy from the first explosion has almost run out, another explosion occurs. This forces the pistons to move again. The cycle continues again and again, giving the car the power needed to run.

Car engines use a four-stroke combustion cycle. The four strokes are intake, compression, combustion and exhaust. The strokes are repeated over and over, generating power. Let’s take a closer look at what happens during each phase of the combustion cycle.

Intake: During the intake cycle, the intake valve opens, and the piston moves down. This begins the cycle by bringing air and gas into the engine.

Compression: As the compression cycle begins, the piston moves up and pushes the air and gas into a smaller space. A smaller space means a more powerful explosion.

Combustion: Next, the spark plug creates a spark that ignites and explodes the gas. The power of the explosion forces the piston back down.

Exhaust: During the last part of the cycle, the exhaust valve opens to release waste gas created by the explosion. This gas is moved to the catalytic converter, where it is cleaned, and then through the muffler before it exits the vehicle through the tailpipe.


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    • Thanks for stopping by this Wonder of the Day®, Steven! Those are both awesome questions you ask! Part of the fun of visiting Wonderopolis is that Wonder Friends (like you!) get to learn a little bit about something they WONDER about (like how an engine works), and then take that little bit of knowledge and go on a personal quest to find out even MORE! We hope you’ll WONDER some more on your own about your questions and let us know what you find out…we LOVE to learn new things, too! :-)

    • Well thank you for telling us how much you enjoyed this Wonder, John! We’re glad you visited us today to Wonder about engines. Combustion is SUPER cool to Wonder about! :)

    • Great question, Maan singh! Maybe you can do more research at your library. Your WONDER friends would love to hear what you find out! We currently do not have a WONDER about turbo systems, but we encourage our WONDER friends to submit ideas for future WONDER topics. “What are you WONDERing?” Keep up the WONDERful WONDERing! :)

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Have you ever wondered…

  • How does an engine work?
  • What is internal combustion?
  • What are the four phases of the combustion cycle?

Wonder Gallery

Try It Out

Pop the hood of a vehicle, so you can get a closer look at the engine. Can you believe how many parts there are in a modern engine?

If possible, compare the engine in this vehicle to another type of engine, such as that found on a lawnmower or other power tool.

When you’re ready, get out from under the hood and check out the Anatomy of an Automobile. Learn more about the other parts of a car and what they do. Can you identify each part on a vehicle?


Still Wondering

Automobiles have had a dramatic impact on history since their invention. However, not everyone has the luxury of traveling by automobile. Explore the history, evolution and social benefits of the automobile with Science NetLinks’ Technology and Inequality lesson plan.


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