Valves
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Engine valves control the intake and exhaust flow to and from the combustion chamber.
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Valve Seats
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A
narrow valve seat is desirable because a thin circular contact with the
valve face forms an efficient seal. But a wider seat transfers heat
better from the valve to the cylinder head.
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Valve Seats in Cylinder Heads
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In
some cast-iron cylinder heads, valve seats are cut directly into the
edge of the valve port. Valve seat inserts are metal rings that match
the shape of the valve.
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Valve Rotation
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An
operating valve tends to rotate and keep seating in a new place. This
helps keep its face and seat free of carbon, and reduce sticking in the
valve guide. It also spreads heat around the valve seat.
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Valve Stem Oil Seals
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Coil
springs normally hold valves on their seats. Their coils are usually
closer at the bottom than the top. Oil seals on valve stems or guides
prevent too much oil passing into the combustion chamber.
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Intake Valves
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Intake
valves are usually larger than exhaust valves because pressure forcing
charge into the cylinder is much lower than that forcing the exhaust
gases out. Different engines use different valve combinations.
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Valve Trains
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An
overhead valve or pushrod system has the valves in the cylinder head and
the camshaft in the block near the crankshaft. Many engines use
hydraulic valve lifters to quieten the engine and eliminate valve
clearance adjustments.
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Valve-Timing Diagram
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The time valves in a
4-stroke engine cycle actually open and close can be measured by
angles. These angles can be easily read using a valve-timing diagram.
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Variable Valve Timing
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Valve overlap is the
amount of time the intake and exhaust valves are both open at once.
Less overlap produces a smooth idle and more slow speed torque. More
valve overlap allows better engine breathing at high speeds. |
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