When the piston starts going downward, the intake valve is opened by the camshaft, and air-fuel mixture is sucked into the combustion chamber. As the piston reaches the bottom of the combustion chamber, valve closes, and the air-fuel mixture is trapped inside the combustion chamber.
Piston then going upward, the air-fuel mixture gets pushed and squeezed to a reduced volume. At the end of this stroke, the mixture usually reduced to 1:10 of its original volume. At this moment, the crankshaft completes one revolution.
The spark plug mounted on the cylinder head then fires to ignite the compressed mixture, which produces massive amount of energy to push the piston downward.
As the crankshaft rotates, piston is guided back, and it is pushing upward, forcing the used exhaust gas to expel from the valve. The cycle completes with two rotation of the crankshaft.
The engine is a well-designed mechanism. All parts in this system works to getter to perform a combustion cycle at a high rate of speed. As a result, automobile gain the ability to move and travel with long distance, benefitting human beings in decades. Understanding the basic of engine is not only learning new knowledges, but also a chance to open minds. Engines does not have to be in this shape forever. A more powerful, more energy efficient, less polluted engine will showed up in the coming years. And it is the chance for next generations, which are curious, creative, and ambitious, to bring it to the world sooner.