A bash valve is a

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...

within a

piston engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common feat ...

, used to control the admission of the

working fluid For fluid power, a working fluid is a gas or liquid that primarily transfers force, motion, or mechanical energy. In hydraulics, water or hydraulic fluid transfers force between hydraulic components such as hydraulic pumps, hydraulic cylinders, a ...

.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 gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing ...

. The valve is usually constructed as a circular

poppet valve A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into an engine. It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usua ...

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 In a reciprocating engine, the dead centre is the position of a piston in which it is either farthest from, or nearest to, the crankshaft. The former is known as Top Dead Centre (TDC) while the latter is known as Bottom Dead Centre (BDC). ...

, 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 engine A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...

s, although they are known. Most examples were applied to some form of

uniflow steam engine The uniflow type of steam engine uses steam that flows in one direction only in each half of the cylinder. Thermal efficiency is increased by having a temperature gradient along the cylinder. Steam always enters at the hot ends of the cylinder and ...

; 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 An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in ...

. 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. Animation of Advanced Uniflow Steam Engine

Animation of Advanced Uniflow Steam Engine

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 engine A small engine is the general term for a wide range of small-displacement, low-powered internal combustion engines used to power lawn mowers, generators, concrete mixers and many other machines that require independent power sources. These eng ...

s, such as

lawnmower A lawn mower (also known as a mower, grass cutter or lawnmower) is a device utilizing one or more revolving blades (or a reel) to cut a grass surface to an even height. The height of the cut grass may be fixed by the design of the mower, but g ...

s, 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.

Pneumatic motors

Arc valve tappet rock drill, section (Rankin Kennedy, Modern Engines, Vol VI)

Arc valve, tappet and valve chest (Rankin Kennedy, Modern Engines, Vol VI)

One successful application for bash valves has been to

pneumatic motor A pneumatic motor (air motor), or compressed air engine, is a type of motor which does mechanical work by expanding compressed air. Pneumatic motors generally convert the compressed air energy to mechanical work through either linear or rotary ...

s. Owing to the characteristics of compressed air pneumatic power, their simplicity is valuable and their inefficiencies with other fluids are less important. Compressed air is supplied cold to the motor.Although air becomes heated during compression, there is still little usable heat in the supplied air, compared to steam. Energy is represented solely by the pressure of the air not, unlike steam, by the combination of pressure and temperature. Efficient operation of a steam engine relies upon expansion of the steam during the piston stroke, which relies upon accurate valve timing and an early closure of the valve. During the expansion phase of the steam it does not expand in a simple

isothermal In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, an ...

fashion, but does so

adiabatically Adiabatic (from ''Gr.'' ἀ ''negative'' + διάβασις ''passage; transference'') refers to any process that occurs without heat transfer. This concept is used in many areas of physics and engineering. Notable examples are listed below. A ...

, much of the energy having been supplied as heat rather than pressure. The compressed air motor is thermodynamically simpler. It uses simple isothermal expansion.Strictly this is an adiabatic expansion too, but it is approximately isothermal, at least compared to the steam engine. This means that expansion is less important, valve timings are thus longer and less crucial and so a simple valve may be adequate. To provide long opening times, the bash valve normally incorporates some form of

tappet A tappet is most commonly a component in an internal combustion engine which converts the rotating motion of the camshaft into linear motion of the valves, either directly or indirectly. An earlier use of the term was for part of the valve gea ...

mechanism. Rather than a valve that is held open by the piston directly, the valve becomes double-acting and is opened by the piston's impact at one end of the stroke and closed by a further impact at the other end of the stroke. The tappet and valve are commonly separate, allowing the valve to remain in a well-defined fully open position throughout the stroke, however the tappet is bounced around by the piston. Where a reciprocating action is produced, such as for a

rock drill Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...

, the valve may be actuated either by

inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...

of the frame or by the movement of the working piston. As the piston hammers back and forth, it impacts a small tappet, which in turn moves the air valve and so reverses the flow of air to the piston. One form of this, the arc tappet valve, was an important feature of the Ingersoll rock drill, the first successful compressed air rock drill for use in mining and tunneling. This used a valve that rotated in a slight arc, rather than sliding. The valve was double-acting, controlling the air supply for both the power and the return stroke. The innovation that made this valve so reliable, thus successful, was a separate tappet that was actuated by the piston in passing at the middle of the stroke, rather than being hammered by a jarring direct impact of the piston. The mid-stroke actuation also opened the valve passageways earlier, before top dead centre, allowing in air that provided a cushioning effect. This further improved the action of the drill, giving a powerful stroke on the working piston and drill rod, but with less damaging hammering to the frame of the drill.

Internal combustion engines

Bash valves are not used in such engines. The cylinder peak pressures of an

Otto cycle An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine. It is the thermodynamic cycle most commonly found in automobile engines. The Otto cycle is a description of what hap ...

engine are too high for such a valve to remain on its seat.

Single-shot valves

Bash valves are also used in a 'single-shot' situation, where a valve is opened once and then remains open until the contents of a

pressure vessel A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. Construction methods and materials may be chosen to suit the pressure application, and will depend on the size o ...

are released. Such valves are used in some

fire extinguisher A fire extinguisher is a handheld active fire protection device usually filled with a dry or wet chemical used to extinguish or control small fires, often in emergencies. It is not intended for use on an out-of-control fire, such as one which ha ...

s and pre-charged air rifles. These valves are arranged so that once lifted off their seat, pressure underneath the valve becomes sufficient to keep the valve raised and so it remains open until the pressure reservoir is empty. The valve is then closed by a light spring.

Notes

References