Gasoline engines must control the flow of combustible mixture they take in, and when it goes in.

In small 2-stroke gasoline engines, this is done by ports that are opened and closed by the piston skirt during the engine cycle, or by pressure-operated reed valves.

Diesel engines are different. Their power and speeds are controlled by the amount of fuel injected, so it isn’t necessary to control airflow into the intake manifold.

Almost all 4-stroke gasoline and diesel engines use valves, which are located in the cylinder head. This is also true for 2-stroke diesel engines that are used in road vehicles.

Valves experience enormous stress even in normal conditions. In a 4-cylinder car driven at around 90 kph, each valve opens and closes about 30 times a second. Exhaust valves withstand huge temperatures and they can become red-hot.

A valve must not soften at high temperatures. It needs good hot strength to stand up to being forced against the seat, and to prevent tensile failure in the stem. It needs good fatigue properties to overcome cracking.

Various surface treatments are used to help the valve resist wear, burning and corrosion.

Inlet valves are made of steels mixed with chromium or silicon to make them more resistant to corrosion, and manganese and nickel to improve their strength.

Exhaust valves are made of nickel-based alloys.

Some high performance applications use especially hard-wearing titanium alloys.