In a four-stroke engine, which statement about valve operation is true?

• A. Valves are opened by reed valves
• B. Valves are operated by camshafts
• C. Valves are opened by pressure differential
• D. Valves open only when the piston travels upward

Answer: (B)

In a four-stroke engine, valve timing is controlled by a camshaft with lobes that push on valve lifters or rocker arms to open the intake and exhaust valves. As a cam lobe pushes, the valve is opened against its spring; when the lobe moves away, the spring closes it. This precise cam-driven action coordinates when the valves open and close with each piston stroke, ensuring proper air entry and exhaust expulsion. Reed valves aren’t used for this purpose in a typical four-stroke engine, and opening a valve isn’t done merely by pressure differences across it. Also, valves don’t open only when the piston travels upward—they open at different times in the cycle (the intake valve opens during the intake stroke as the piston moves downward, while the exhaust valve opens near the top of the compression/early power stroke as the piston moves upward).