Valves

Valves are the precisely timed gates that govern the flow of gases into and out of an engine's cylinders, opening to admit the fresh air-fuel mixture and to expel spent exhaust, then sealing tightly so combustion can take place under pressure. They are fundamental to the four-stroke cycle: without their accurate, repeatable operation, an engine could neither breathe in the charge it needs nor clear the burnt gases that would otherwise choke it. Each cylinder has at least one intake and one exhaust valve, and their behaviour is central to how an engine performs.

The typical poppet valve is a mushroom-shaped component with a flat or conical head and a long stem. Its head seats against a machined ring in the cylinder head to form a gas-tight seal when closed. Opening is driven by the camshaft: rotating lobes push on the valve, directly or through followers, rockers and tappets, forcing it down into the cylinder against resistance. Closing is handled by a stiff valve spring, which returns the valve firmly to its seat the instant the cam lobe rotates away. The timing of these events is geared precisely to the crankshaft so that valves open and close at exactly the right point in each cycle.

The number, size and timing of the valves strongly shape an engine's character. More valves per cylinder — four being the common modern choice, with two intake and two exhaust — provide greater total opening area for a given bore, letting the engine breathe more freely and develop higher power, particularly at speed. Their timing and the duration for which they remain open determine whether an engine favours low-end torque or top-end power. Engineers thus tune valve count, lift and timing carefully to balance performance, efficiency and emissions for the engine's intended use.

Exhaust valves in particular endure ferocious conditions. They are repeatedly exposed to combustion gases that can exceed 700 to 800 degrees Celsius, with relatively little opportunity to shed heat except through their seats and stems. They are made from heat-resistant alloys, and some are sodium-filled to conduct heat away from the head, or fitted with hardened seats and stem seals. Intake valves run cooler, helped by the incoming charge, but all valves must seal precisely thousands of times a minute throughout the engine's life.

Practical considerations follow from this demanding duty. Valve clearances must be correct — too tight and a valve may fail to seat and burn, too loose and the valvetrain becomes noisy and worn — which is why many engines use hydraulic adjusters to maintain clearance automatically. Carbon deposits, seat recession and weakening springs can degrade sealing over time. Valves work intimately with the camshaft, and modern variable-valve-timing and multi-valve designs build directly on their basic principle to extract ever more efficiency and flexibility from the engine.