Four-stroke cycle engine valves are

• A. Operated by camshafts
• B. Operated by pressure differential
• C. Often called reed or channel valves
• D. Open whenever the piston travels upward

Answer: (A)

Valves in a four-stroke engine are opened and closed by a camshaft-driven mechanism. The camshaft, connected to the crankshaft by a timing belt or chain, carries lobes that push on valve lifters or rocker arms to lift the valves against their springs. When a lobe pushes, the valve opens; as the lobe rotates away, the spring closes it. This timing is coordinated so the intake valve opens to admit the air–fuel mixture at the right moment and the exhaust valve opens to vent exhaust gases at the appropriate part of the cycle.

Other options don’t fit because valves aren’t operated by pressure differences, reed/channel valves are typical of some two-stroke designs (not standard four-stroke cylinder head valves), and valves do not simply stay open whenever the piston moves upward—the opening and closing are precisely timed with the engine cycle.