Few engine components are subjected to such heavy stresses like the exhaust valves, lapped by corrosive gases at high temperature. They can cool only by giving heat to the head through contact with the seat, when they are closed. The heat transfer occurs also between the valve stem and its guide, but to a much less amount (about 20% of the total).
For intake valves the problem does not exist, as they are effectively cooled by the air-fuel mixture (or by the air alone, in direct injection engines) that enters the cylinder. To get an idea of the working conditions of the valves, consider that when the engine works at 6000 rpm, each valve opens and closes 50 times per second.
Racing bikes engines run much faster: for those with four cylinders, 12,000 rpm are a more than normal working speed, at which each valve is brought from the closed position to the maximum opening in times of the order of 1.8 milliseconds.
Even closed, the valves are subjected to high stresses. With a maximum combustion pressure of 80 bar (normal value in modern high-performance engines), on the of 36 mm diameter valve head there is a force of over 8,000 newtons (more than 800 kg)!
For exhaust valves we use austenitic steels (having the elementary crystalline cell different from that of normal case-hardening or tempered steels, used for connecting rods, shafts, gears, etc., …), containing a high percentage of chromium, nickel and other elements, like manganese and molybdenum.
In some extremely fast engines, the valve temperatures are so high that even these austenitic steels are not adequate: we therefore switch to nickel-based superalloys, originally developed for jet engine turbine blades (Nimonic, Inconel).
The intake valve is not made of common steels, but with special martensitic steel alloys containing chromium and silicon. This steel can be thermally hardened (tempered), in order to make the valve seat and the terminal more resistant.
The exhaust valve is not made of common steels, but with special hi-tech austenitic steel alloys, containing chromium, manganese and nickel. For exhaust valves, it is often necessary to use two different materials, one for the stem (upper part of the valve), suitable for hardening with heat treatment, the other for the head, suitable for greater heat resistance.
If necessary, the valve seat can be reinforced with a stellite coating, in order to further improve mechanical and heat resistance. For high-performance engines, this is not enough. In these cases, superalloys based on nickel or titanium are used. In some cases, the valve stem can be filled with sodium, which helps to speed up the heat dispersion, reducing the valve temperature by over 100 ° C.
Over the years we have developed over 5,000 valves, increasing our experience with materials, processes and quality controls.
Starting from a drawing or, in the simplest cases, with a sample, we can develop a product on specific request and also support the development with suggestions based on our experience.