What happens to the piston after the fuel/air mixture is ignited?

When the fuel/air mixture in an internal combustion engine is ignited, a rapid combustion process occurs, producing high-pressure gases. This sudden increase in pressure causes the piston to move downward in the cylinder. As the piston descends, it is connected to the crankshaft; this downward motion transfers energy, effectively turning the crankshaft. This movement is crucial because it is what ultimately provides mechanical power to the vehicle or machinery. The design of the engine ensures that the conversion of the piston’s linear motion into rotational motion is efficient, which is fundamental for the ongoing operation of the engine.

In addition, it's important to understand how the other options relate to the function of the piston in an engine. For example, if the piston were to remain stationary, it would not generate the power needed to drive the vehicle. Similarly, if it were to move sideways, it would not effectively convert the combustion energy into usable mechanical energy, as the engine mechanics depend on the vertical movement of the piston. Therefore, the choice indicating that the piston moves down and turns the crankshaft accurately describes the fundamental operation of the engine during the combustion stroke.