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A bash valve is a valve within a piston engine, used to control the admission of the working fluid.〔(Bash Valve ) - description〕 They are directly-actuated valves, operated by contact between the piston and the valve tip. Bash valves have the advantage of great simplicity, for manufacture and operation. Their disadvantages are that their opening and closing times are relatively crudely controlled, compared to other types of valve gear. The valve is usually constructed as a circular poppet valve with a conical seat, inserted into the cylinder from the outside. A protrusion on the inside is hit by the piston as it approaches top dead centre, forcing the valve open. Bash valves are usually held closed by the pressure of fluid in the reservoir behind them. There may be a light spring to assist closing when the reservoir is empty. For this reason they are used as inlet valves, not exhaust. An exhaust bash valve would have the cylinder pressure and the piston actuation both acting to open it, with nothing to close it. == Steam engines == Bash valves are not widely used in steam engines, although they are known. Most examples were applied to some form of uniflow steam engine; unlike the more common slide and piston-valved engines with their bidirectional-flow ports, uniflow engines use inlet ports at the cylinder ends and an exhaust near the centre. Although the opening time of a bash valve is fixed, imprecisely controlled and always occurs near top dead centre, this is not a major drawback for a steam engine. A more important requirement is the ability to accurately control the closing time of the valve, and for its duration to be adjustable in order to 'drive' the engine, according to varying load. Some designs of uniflow engine have used a combined mechanical and electromagnetic valve to do this. The valve is opened mechanically, then held by an electromagnet. This requires less electrical power to merely hold the valve than to open it. A patent for such an engine was granted to Sturtevant in 1968. The same idea has recently been revived as the main feature of an Advanced Uniflow Steam Engine. In this engine, a second valve is used for exhaust purposes in the later part of the cycle too, although this one is bashed shut, rather than opened. Bash valves have also been used for the ''ad hoc'' conversion of commonplace petrol small engines, such as lawnmowers, into hobbyist steam engines. The original petrol engine sparkplug mounting hole is used as the location for a new piston-actuated bash valve, together with the original exhaust valve. Performance and efficiency are not a need of such projects.〔 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「Bash valve」の詳細全文を読む スポンサード リンク
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