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An expansion valve is a device in steam engine valve gear that improves engine efficiency. It operates by closing off the supply of steam early, before the piston has travelled through its full stroke. This cut-off allows the steam to then expand within the cylinder. This expanding steam is still sufficient to drive the piston, even though its pressure decreases as it expands. As less steam is supplied in the shorter time for which the valve is open, use of the expansion valve reduces the steam consumed and thus the fuel required. The engine (on 1875 figures) may deliver two-thirds of the work, for only one-third of the steam.〔 An expansion valve is a secondary valve within a steam engine. They represent an intermediate step between steam engines with non-expansive working and later valve gears that could provide for expansion by controlling the motion of a single valve. Expansion valves were used for stationary engines and marine engines.〔 They were not used for locomotives, although expansive working was achieved by the use of the later variable expansion valve gears. == Need for varying expansion == The pressure of the expanded steam is less than that of steam supplied directly from the boiler. An engine working with an expansion valve set to an early cut-off is thus less powerful than with the valve fully open. Accordingly the engine must now be ''driven'', so that the valve is manually adjusted as the load on the engine changes. An engine running under light load may be operated efficiently with an early cut-off, an engine under heavy load may require a longer cut-off and the cost of more steam consumption. When Trevithick supplied his 1801 engine for a rolling mill at Tredegar Iron Works, the engine was more powerful when worked without expansion and Samuel Homfray, the ironmaster, preferred to use the extra power despite the potential saving in coal costs 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Expansion valve (steam engine)」の詳細全文を読む スポンサード リンク
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