Step one
![conventional port fuel injection four-stroke petrol engine](/en_cn/motorists/motoring-tips-and-advice/looking-after-your-vehicle/anatomy-of-an-engine-heart-condition/_jcr_content/par/tabbedcontent/tab/textimage/image.img.960.jpeg/1459541014971/conventional-port-fuel-injection-four-stroke-petrol-engine.jpeg?imwidth=960)
Let’s get to grips with how a conventional port fuel injection four-stroke petrol engine works.
1. The inlet valve opens as the piston is about to descend, sucking in the mixture of fuel and air.
Step two
![Anatomy of an engine showing the piston rising to squeeze together the fuel and air](/en_cn/motorists/motoring-tips-and-advice/looking-after-your-vehicle/anatomy-of-an-engine-heart-condition/_jcr_content/par/tabbedcontent/tab_0/textimage/image.img.960.jpeg/1459541015038/anatomy-of-an-engine-showing-the-piston-rising.jpeg?imwidth=960)
2. With both valves now closed, the piston rises upwards, squeezing the mix of fuel and air.
Step three
![Anatomy of an engine showing the fuel and air mixture being ignited by the spark plug](/en_cn/motorists/motoring-tips-and-advice/looking-after-your-vehicle/anatomy-of-an-engine-heart-condition/_jcr_content/par/tabbedcontent/tab_1/textimage/image.img.960.jpeg/1459541015105/anatomy-of-an-engine-showing-the-fuel-and-air-mixture.jpeg?imwidth=960)
3. Just before the piston reaches the top, the fuel and air mix is ignited by the spark plug.
Step four
![Anatomy of an engine showing the exhaust pipe opening to expell the burnt gases](/en_cn/motorists/motoring-tips-and-advice/looking-after-your-vehicle/anatomy-of-an-engine-heart-condition/_jcr_content/par/tabbedcontent/tab_2/textimage/image.img.960.jpeg/1459541015174/anatomy-of-an-engine-showing-the-exhaust-pipe.jpeg?imwidth=960)
4. The exhaust valve opens and the piston rises back to the top, expelling the burnt gases.