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and the rocker arm when the valve is seated and the cam follower is off the cam lobe.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 609
valve core (aircraft wheel component). A spring-loaded air valve screwed into a metal tube installed in an
aircraft wheel. When the tire mounted on the wheel is being inflated, the stem of the valve core is depressed
by the air chuck, and air flows into the tire. When the air chuck is removed, the stem seats in the valve core,
and the air is trapped in the tire.
To deflate the tire, the valve stem is depressed, allowing air to flow out through the core.
valve duration (reciprocating engine specification). The amount of time, usually measured in degrees of
crankshaft rotation, an intake or exhaust valve remains off its seat.
valve face (reciprocating engine component). The portion of a poppet valve that forms a seal with the
valve seat in an aircraft engine cylinder head. The valve face is ground so it is perfectly smooth and
concentric with the valve stem.
valve float (reciprocating engine operation). The condition of engine operation in which an intake or
exhaust valve does not seat when the rocker arm is off the valve stem. It is possible for the engine to
operate at a speed at which valves are being opened at the natural, or resonant, frequency of the valve
springs. If this happens, the valve springs lose their effectiveness and will not close the valves, and the
valves will float.
To prevent floating, most aircraft engine valves are closed with more than one spring. These
springs are wound with a different pitch and of different size wire, giving them different resonant
frequencies.
valve grinding (reciprocating engine overhaul procedure). The step in the overhaul of a reciprocating
engine in which the valve faces are ground to remove all pits and the worn surface. The ground surface is
smooth and perfectly concentric with the valve stem.
valve guide (reciprocating engine component). Bronze or cast-iron sleeves installed in the cylinder head
of an aircraft reciprocating engine. The stems of the intake and the exhaust valves ride up and down in the
valve guides which hold them in the exact center of the valve seats.
valve lag (reciprocating engine specification). The number of degrees of crankshaft rotation after the
piston passes the top or the bottom center of its stroke at which the intake or exhaust valves open or close.
For example, if the exhaust valve closes 15° of crankshaft rotation after the piston passes over top center
and starts down on its intake stroke, the exhaust valve lag is 15°.
valve lapping (reciprocating engine overhaul procedure). The procedure in which the intake and
exhaust valves are given a final processing so they form an airtight seal with the valve seat.
A small amount of a very fine abrasive paste, valve-grinding compound, is spread on the valve
face, and the valve is placed in the valve guide. The valve is rotated by hand to lap the valve face and seat
together to remove any material that would prevent an airtight seal.
valve lead (reciprocating engine specification). The number of degrees of crankshaft rotation before the
piston reaches the top or bottom center of its stroke, at which the intake or exhaust valves open or close.
For example, if the intake valve opens when the crankshaft is 15° before the piston reaches the top of the
exhaust stroke, the intake valve lead is 15°.
valve lift (reciprocating engine specification). The linear distance a poppet valve in a reciprocating
engine moves off its seat when it is opened by the cam.
valve overlap (reciprocating engine operation). The portion of the operating cycle of a four-stroke-cycle
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 610
reciprocating engine in which both the intake and the exhaust valves are off their seats at the same time.
Valve overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke.
Valve overlap increases the efficiency of engine operation. The low pressure caused by the
exhaust gases leaving the cylinder helps the fresh fuel-air charge start moving into the cylinder.
valve ports (reciprocating engine components). The openings in the cylinder heads of a reciprocating
engine through which the fuel-air mixture flows into the cylinder and the burned gases leave.
valve seats (reciprocating engine components). Rings of hardened steel or bronze shrunk into the soft
cast-aluminum cylinder head of an aircraft reciprocating engine. The wear caused by the continual
hammering of the valves as they close is absorbed by the valve seat, rather than allowing the hammering to
ruin the cylinder head.
　
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