1. Field of the Invention
This invention relates generally to free piston machines, and more particularly to an apparatus for reducing excessive amplitude of oscillation of a piston in a free piston machine.
2. Description of the Related Art
Many different kinds of machines operate by reciprocating massive objects along a path, causing vibration. For example, a conventional internal combustion engine has one or more cylindrical pistons slidably mounted within a sealed cylindrical cavity. As a flammable gas is injected into one end of the cavity and ignited, the resulting combustion reaction forces the piston along a path of reciprocation toward the opposite end.
The piston of the conventional internal combustion engine is linked to a crank shaft, and upon reaching the limit of its travel, the piston stops and begins to move in the opposite direction. This continues in a cycle. The cyclical stopping and starting of the piston in a conventional internal combustion engine causes vibrations that are transmitted from the engine to the structure to which it is mounted.
Other machines, such as free piston Stirling cycle engines, also create vibrations as they convert thermal energy to mechanical energy. Free piston Stirling cycle engines have a working gas that is moved between a warmer space and a cooler space by a reciprocating displacer to drive a reciprocating piston.
It is conventionally known that it is desirable to reduce vibrations caused by free piston Stirling cycle machines, such as engines. An apparatus for accomplishing this purpose is described in U.S. Pat. No. 5,895,033 to Ross et al., which is incorporated herein by reference.
Ross et al. describe an apparatus that is mounted to the housing of a free piston Stirling cycle machine. The apparatus includes a combined spring and mass. The spring/mass combination is tuned to a natural frequency that causes it to reciprocate in a phase relationship with the free piston and displacer of the machine, to reduce significantly the reciprocation of the housing. For example, the mass generally moves in opposition to the combined inertia of the internal moving parts of the machine. Thus, the result of the Ross et al. invention is a reduction of the vibration transmitted from the housing of the machine to the surrounding structures.
One difference between a free piston Stirling cycle machine and a conventional internal combustion engine is that the piston of the conventional engine described above has a predetermined amplitude of oscillation that cannot be exceeded unless a part of the engine such as the crank shaft, breaks. On the contrary, the piston in a free piston Stirling cycle machine is not so structurally limited in the distance which it normally travels in its path of reciprocation. Under normal conditions, a free piston will reciprocate within a predetermined amplitude range. However, the amplitude can exceed this range under some circumstances, thereby resulting in what is commonly known as “overstroke”. Overstroke is the condition in which the piston exceeds a predetermined amplitude range. If overstroke cannot be stopped before it becomes excessive, harm can come to the engine.
Although dynamic vibration absorbers, such as described by Ross et al., help to reduce the vibration transmitted to the surrounding structures, there is also a need to prevent or stop overstroke in free piston machines.