What keeps the intake and exhaust valves closed?

The intake and exhaust valves in an internal combustion engine remain closed primarily due to the force of valve springs. These springs exert pressure on the valves, ensuring they stay seated against their respective seats when not activated. Additionally, the camshaft controls the timing of the valve openings, pushing the valves open at precise intervals during the engine’s cycle, while the springs pull them closed again when the camshaft lobe rotates away.