What closes the valves of a four-stroke single-cylinder petrol engine?

In a four-stroke single-cylinder petrol engine, the valves are primarily closed by springs. These springs are specifically designed to provide the necessary force to return the valves to their closed position after they have been opened, usually by the action of the camshaft during the engine's operating cycle.

When the camshaft rotates, it pushes against the valve lifter or rocker arm, which in turn opens the valve against the tension of the spring. Once the camshaft moves past the peak and the force is removed, the inherent tension in the spring quickly closes the valve. This action is critical to maintaining the engine's cycle of intake, compression, power, and exhaust strokes.

Other options like hydraulic lifters are used to adjust the play in the valve train and can influence valve timing and operation, but they don't directly perform the closing action of the valves themselves. Pneumatic actuators and solenoids are not typically used for closing valves in traditional petrol engine designs; they are more common in specialized applications or other types of mechanical systems.