A free piston machine

The invention relates to a simulator for use as a sound generator to simulate the sound of gunfire.

It is known from GB-B-2250333 to provide a gunfire simulator comprising a combustion chamber, means for admitting fuel gas to the combustion chamber, a flap valve for admitting air to the combustion chamber, means to force ambient air into the combustion chamber through the flap valve, ignition means for igniting fuel gas in the combustion chamber to cause an explosion, an exhaust port in the combustion chamber and outlet valve means for closing the exhaust port and arranged to open rapidly and with audible result in response to explosive pressure rise within the combustion chamber, the outlet valve means being in the form of a frangible diaphragm of thin sheet material.

It is an object of the invention to provide a gunfire simulator having a novel rapidly opening outlet valve closing an exhaust port.

It is a further object of the invention to provide a gunfire simulator having a novel rapidly opening outlet valve closing an exhaust port which obviates the need for a frangible diaphragm which must be replaced each time the simulator is used.

According to the invention there is provided a gunfire simulator having a combustion chamber, means for admitting fuel gas to the combustion chamber, means for admitting oxygen or an oxygen containing gas to the combustion chamber, means for igniting fuel gas in the combustion chamber, an exhaust port in communication with the combustion chamber and outlet valve means for closing the exhaust port and arranged to open rapidly in response to a pressure rise in the combustion chamber for rapidly exhausting gas under pressure to atmosphere to simulate an explosion, characterised in that the outlet valve means comprises a cylinder having first and second generally closed ends respectively, a piston freely slidable in the cylinder between the first and second ends, a first conduit communicating between the combustion chamber and the first end of the cylinder, a second conduit communicating between the first end of the cylinder and atmosphere, valve means controlling flow through the second conduit, a piston-controlled port in the cylinder adjacent to the first end and communicating between the cylinder and the combustion chamber, and a piston-controlled exhaust port disposed in the cylinder between the inlet port and the second end, the arrangement being such that the piston closes the inlet port when disposed at the first end of the cylinder and opens the exhaust port when disposed at the second end of the cylinder. Preferably the first conduit is in the form of one or more bleed holes of small diameter relative to the size of the piston-controlled port. With such an arrangement the piston will remain stationary at the said first closed end of the cylinder while the valve means controlling flow through the second conduit, which is preferably larger in diameter than the diameter of the first conduit, remains open since gas under pressure entering the first closed end of the cylinder through the bleed holes is exhausted to atmosphere through the second conduit. On closing the valve means, the piston will move relatively slowly away from the first closed end of the cylinder under the action of the limited quantity of compressed gas bleeding through the first conduit until the piston-controlled port is uncovered, at which stage a large volume of compressed gas will enter the cylinder and will cause the piston to move rapidly along the cylinder until the exhaust port is uncovered when the compressed gas is rapidly expelled to atmosphere to simulate an explosion.

Means may be provided at the second end of the cylinder for arresting the piston. The piston-arresting means may comprise means arranged in association with the piston and the second closed end portion of the cylinder to form an air cushion. Preferably the air cushion in the second closed end portion of the cylinder comprises one-way valve means vented to atmosphere to control, reduce or prevent bouncing of the piston at the second closed end of the cylinder.

The arrangement may be such that the cylinder is disposed substantially vertically with the second closed end uppermost, in which case the piston may return to the first closed end under gravity. Alternatively resilient means, e.g. a spring, may be provided for returning the piston to the said first closed end of the cylinder.

The valve means controlling the second conduit may be arranged to close in response to a predetermined degree of pressure rise in the combustion chamber. Preferably the cylinder is arranged to project into the combustion chamber such that the first end of the cylinder including the inlet port is disposed in the chamber.

The invention is diagrammatically illustrated by way of example in the accompanying drawing which is a cross-sectional view of a gunfire simulator.

In the drawing, a gunfire simulator of the kind generally described in U.K. patent application GB-B-2250333 is shown, but in the present case the exhaust system is in the form of a free piston device.

The simulator comprises a generally cylindrical combustion chamber (1) vented to atmosphere through a radial series of piston-controlled exhaust ports (2). The exhaust system comprises a cylinder (3) having an opposed first and second closed ends (4,5) respectively, the first end (4) projecting into the combustion chamber (1). The cylinder end (4) in the combustion chamber is formed with a first conduit in the form of one or more small bleed holes (6) in its axial end (7) which communicate between the interior of the combustion chamber and the interior of the cylinder. A piston (8) is freely slidable in the cylinder (3) with its head (9) towards the combustion chamber and with a cylindrical skirt (10) of sufficient axial length to cover a radial array of inlet ports (11) in the curved surface of the cylinder adjacent to the end (4) and which communicate between the combustion chamber interior and the interior of the cylinder.

The interior of the first end (4) of the cylinder is vented to atmosphere via a second conduit (12) which is of greater cross-sectional area than that of the bleed holes (6). Flow through the second conduit (12) is controlled by a normally open solenoid valve (13) which is linked to a pressure sensor (17) in the combustion chamber such that the valve is closed in response to a predetermined degree of pressure rise in the combustion chamber. Thus the valve may be arranged to close when the pressure in the chamber approaches its maximum value, typically around 9 bars.

Externally of the combustion chamber, the cylinder is formed with a radial series of exhaust ports (2) which are uncovered when the piston (8) moves towards the second end (5) of the cylinder.

At the second end (5) of the cylinder there is formed a piston arresting mechanism formed by the closed end (5) of the cylinder and comprising a plug (14) which reduces the dead volume between the piston skirt and the cylinder and as the piston approaches the outer end of the cylinder so that an air cushion is created. To prevent or reduce the piston from bouncing off the air cushion the second end of the cylinder is vented to atmosphere via small ports (15) formed in the axial end (5) of the cylinder and covered by a flexible diaphragm (16) which forms a one way valve which closes in response to a pressure drop at the end (5) of the cylinder caused by the piston bouncing away from the cylinder end (5). The diaphragm is shown dotted in its open position and in full in its position closing the parts (15). Thus the piston arresting and damping is effective in both directions of piston travel.

The operation of the simulator is as follows:-

• 1. Combustion of a gas charge in the combustion chamber (1) raises the pressure therein to around 9 bars.
• 2. The piston (8) remains stationary in the cylinder (3) during this pressure rise due to inlet port(s) (10) being closed, and since the combustion gases entering the cylinder through the bleed hole (6) exits to atmosphere through the second conduit (12) and normally open solenoid valve (13).
• 3. When the combustion pressure approaches its maximum value the solenoid valve (13) closes under the influence of the pressure switch (17).
• 4. Gas entering bleed hole (6) moves the piston (8) slowly away from the first cylinder end (4).
• 5. Inlet port(s) (10) are then uncovered and the piston (8) accelerates rapidly under the action of the combustion peak pressure.
• 6. Piston travel uncovers the exhaust port(s) (2) producing a supersonic bang from the combustion pressure exhausting to atmosphere.
• 7. Further piston travel covers the exhaust ports (2) causing a braking effect due to pressure build-up ahead of the piston as it approaches the second closed end (5) of the cylinder.
• 8. The speed of deceleration is controlled by controlled release of pressure through the port(s) (15).
• 9. Pressure ahead of piston causes the piston (8) to stop and then bounce back causing a vacuum which closes the diaphragm flap valve (16) to damp the piston bounce.
• 10. The piston returns slowly to the first end (4) of cylinder under gravity or by spring means.

The invention thus provides a simple device for use as a gunfire simulator and which can be used repeatedly without the need for frequent maintenance or replacement of parts.