Four-stroke cycle engine valves
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Four-stroke engines, of either spark ignition or compression ignition varieties, use poppet valves to allow air (or an air/fuel mixture) into the cylinder and exhaust gases out. Very early engines used alternative valve types such as slide valves, but these proved less satisfactory, especially for high-speed engines.
The poppet valve is pushed into the cylinder by a camshaft, usually acting on some kind of cam follower (tappet) in order to open it. It is self-closing under spring pressure, with the notable exception of desmodromic valves, mostly used on Ducati motorcycles, which are closed positively by a linkage. F1 racing car engines use pneumatic valve gears which use compressed air rather than metal springs to close their valves, for the same reason as desmodromic valves; at very high engine speeds a simple spring cannot close the valve quickly enough and the valve "floats" or loses contact with its camshaft, spoiling valve timing and reducing power. The valve seals against a seat cut into the cylinder head, which may be a simple 45 degree angle or, in modern engines, may have compound angles in order to achieve less fouling and better gas flow. New valves are generally lapped (ground) into the head using a fine abrasive in order to achieve a perfect seal.
Modern automobile engines often use more than one inlet and/or exhaust valve. The reasoning behind this is to increase the total valve area, which is a major determinant of the power the engine can be made to produce. The circular shape of the combustion chamber limits the size of two single large valves, and it is easier to fit four smaller valves into the space while still achieving greater valve area.
Valves are almost always round since they have the advantage that they can rotate harmlessly in use and, as described above, can be lapped in. However, it is easier to achieve very large valve areas with non-round valves, and heat transfer to the cylinder head is more effective due to the larger circumference per unit area.
The exhaust valve is usually the hottest part of any engine. It is exposed to the full heat of the exhaust gases and can only lose heat to the cylinder head (and hence the cooling system) when it is closed, heat transfer to the valve guide being rather slow. Special steels are required, and some valves are filled with liquid sodium in order to conduct the heat away from the face. Aluminium cylinder heads always require hard steel valve seats, but cast iron cylinder heads often had the valve seats machined directly into the head until the 1980s. With the move to unleaded gasoline (petrol), a phenomenon known as valve seat recession occurs with these engines. The hot exhaust valve micro-welds to the seat under hard running conditions and tears material away as the valve reopens. Leaded fuel prevented this action, and the remedy is either to use a suitable fuel additive with the same protective function as tetraethyl lead, or to have an engine shop insert hard steel seats.
Since the valve has its head in the cylinder and its tip is exposed in the cylinder head, the valve guide must be sealed to maintain cylinder pressure. A rubber lip-type seal ensures that excessive amounts of oil are not drawn in from the crankcase on the induction stroke and that exhaust gas does not enter the crankcase on the exhaust stroke. Worn valve seals are characterised by a puff of blue smoke from the exhaust when pressing back down on the accelerator pedal after allowing the engine to over-run, such as when changing gear.
