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航空術(shù)語(yǔ)詞典Dictionary of Aeronautical Terms 下

時(shí)間：2011-03-11 23:18來(lái)源：藍(lán)天飛行翻譯 作者：航空 點(diǎn)擊：次


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 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.
Valve seats are ground with a special seat grinder so that they are perfectly concentric with the valve guide. Worn valve seats can be removed from the cylinder head and replaced with new seats.

valve springs (reciprocating engine components). Helically wound steel-wire springs used to close the poppet valves in the cylinders of an aircraft reciprocating engine. Almost all aircraft engines use more than one spring to close each of the valves.
valve spring tester (reciprocating engine overhaul equipment). A tester used to determine the amount of force needed to compress a valve spring to a specified height. Weak valve springs do not close the valves tightly enough, and they must be discarded when the engine is overhauled.
valve stem (reciprocating engine component). The portion of a poppet valve that rides in the valve guide in the cylinder head of a reciprocating engine.
The rocker arm presses on the end of the valve stem to open the valve.
valve stretch gage (reciprocating engine overhaul tool). A special gage that measures the radius between the valve head and the valve stem. If this radius is greater than that of the gage, the valve has been stretched and it must be discarded.
valve timing (reciprocating engine specification). The relationship between crankshaft rotation and the opening and closing of the intake and exhaust valves in the cylinders of a reciprocating engine.
When the valves are properly timed, the intake valve opens a specified number of degrees of crankshaft rotation before the piston reaches the top of the exhaust stroke. The intake valve closes a specified number of degrees after the piston starts moving upward on the compression stroke. The exhaust valve opens a specified number of degrees before the piston reaches the bottom of the power stroke, and the exhaust valve closes at the correct position of the crankshaft after the piston has started down on its intake stroke.

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