Four valves per cylinder

Four valves per cylinder is an engine breathing layout in which each combustion chamber is served by two intake valves and two exhaust valves, rather than the single pair found in older designs. It has become the default arrangement for modern petrol and diesel passenger-car engines because it markedly improves how freely an engine can draw in air and expel burnt gases. The configuration is a deceptively simple idea with far-reaching benefits for power, efficiency and emissions.

The rationale rests on valve area and flow. An engine's ability to make power depends heavily on how much air it can move through the cylinder, and that is limited by the open area of its valves. Within the fixed circular space at the top of a cylinder, two smaller valves of a given type provide more combined opening area and circumference than one large valve could, and they can also open and close faster because each individual valve is lighter. The engine therefore breathes more easily, particularly at high revolutions where the available time for each intake stroke is very short.

A second structural advantage concerns the spark plug. With two intake and two exhaust valves arranged symmetrically around the chamber, the spark plug can sit in the centre, directly above the piston. A central ignition point means the flame front travels a shorter, more even distance to the chamber walls, producing faster, more complete and more consistent combustion. This improves efficiency, reduces the tendency to knock and lowers emissions of unburnt hydrocarbons.

The four-valve concept is not new — it appeared on racing and aero engines early in the twentieth century — but it became mainstream only as manufacturing precision and the popularity of overhead camshafts made it economical for ordinary cars. It pairs naturally with the double overhead camshaft (DOHC) arrangement, in which one camshaft operates the intake valves and another the exhaust valves, giving clean actuation of all four. Alternative layouts exist, including three-valve designs with two intake and one exhaust valve, and high-performance five-valve heads, but four valves has proven the best balance of breathing, cost and packaging.

In practice the layout brings tangible benefits to the driver: a broader, more usable spread of power, a higher safe rev limit, better fuel economy and cleaner exhaust, all from the same displacement. The added complexity — twice as many valves, springs, seats and actuating components — raises manufacturing cost and the parts count slightly, but reliability has proven excellent and the trade has long been considered worthwhile.

This layout is best understood in relation to the valvetrain components it depends on. It builds directly on the function of the valves themselves and the camshaft that drives them, is most commonly realised through a DOHC cylinder head, and contrasts with the simpler three-valve technology it largely superseded.