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This is a poppet or mushroom valve. It has two main parts, a steam and a head. It fits into a port in the head. Its face makes a gas type seal against the sit. During operation, the head near the face of the valve transfers heat to the sit. Sum is conducted up into the valve steam. The steam transfers heat on to the guide. So, the steam is the valves coolest part.
The valves sit and guide are also called by colon in passages around the valve ports. When the valve does not sit properly, there is a smaller area where he transfer and can occur. That means the face will over heat.
Local hot spots can reach such extreme temperatures that the edge of the valve can actually burn. The wood of the valves sit is important. A narrow sit is desirable because a thin circular contact with the valve face forms an inefficient seal. But a wider sit is better for transferring hits from the valve to the seamier head.
A common compromise is for the inlet valve to have a narrow sit than the excurse valve.
Inlet valves pass only air or air fuel mixture so they run it much lower temperatures than excurse valves. They are usually larger than excurse valves because the pressure forcing the charge into the cylinder is much lower than the pressure forcing the excurse gasses out of the cylinder. Excurse gasses under pressure need much less space.
Different engines use different valve combinations. Having more than one inlet valve provides better breathing. In additional inlet valve allows larger inlet passages and the free of flow into the cylinder so the engine receive a better charge.
Similarly to excurse valves, mean the cylinder can be design with larger excurse ports which provide a free of flow of excurse gasses out of the cylinder.
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