Exhaust Valve - MINI Cooper Base / R55 / R56 / R57 / R58 / R59 / R60 / R61 | 11347533885-INT

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.

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).