Free Piston Stirling Engines

The Stirling engine is a heat engine that is vastly different from the internal-combustion engine in your car. Invented by Robert Stirling in 1816, the Stirling engine has the potential to be much more efficient than a gasoline or diesel engine. But today, Stirling engines are used only in some very specialized applications, like in submarines or auxiliary power generators for yachts, where quiet operation is important. Although there hasn't been a successful mass-market application for the Stirling engine, some very high-power inventors are working on it.

A Stirling engine uses the Stirling cycle,­ which is unlike the cycles used in internal-combustion engines.

• The gasses used inside a Stirling engine never leave the engine. There are no exhaust valves that vent high-pressure gasses, as in a gasoline or diesel engine, and there are no explosions taking place. Because of this, Stirling engines are very quiet.
• The Stirling cycle uses an external heat source, which could be anything from gasoline to solar energy to the heat produced by decaying plants. No combustion takes place inside the cylinders of the engine.
  
  Free Piston Stirling Engines:
  
  
  "Free piston" Stirling engines include those with liquid pistons and those with diaphragms as pistons. In a "free piston" device, energy may be added or removed by an electrical linear alternator, pump or other coaxial device. This avoids the need for a linkage, and reduces the number of moving parts. In some designs, friction and wear are nearly eliminated by the use of non-contact gas bearings or very precise suspension through planar springs.
  Four basic steps in the cycle of a “Free piston” Stirling engine,
  1. The power piston is pushed outwards by the expanding gas thus doing work. Gravity plays no role in the cycle.
  2. The gas volume in the engine increases and therefore the pressure reduces, which will cause a pressure difference across the displacer rod to force the displacer towards the hot end. When the displacer moves the piston is almost stationary and therefore the gas volume is almost constant. This step results in the constant volume cooling process which reduces the pressure of the gas.
  3. The reduced pressure now arrests the outward motion of the piston and it begins to accelerate towards the hot end again and by its own inertia, compresses the now cold gas which is mainly in the cold space.
  4. As the pressure increases, a point is reached where the pressure differential across the displacer rod becomes large enough to begin to push the displacer rod (and therefore also the displacer) towards the piston and thereby collapsing the cold space and transferring the cold, compressed gas towards the hot side in an almost constant volume process. As the gas arrives in the hot side the pressure increases and begins to move the piston outwards to initiate the expansion step as.
  In the early 1960s, W.T. Beale invented a free piston version of the Stirling engine in order to overcome the difficulty of lubricating the crank mechanism. While the invention of the basic free piston Stirling engine is generally attributed to Beale, independent inventions of similar types of engines were made by E.H. Cooke-Yarborough and C. West at the Harwell Laboratories of the UKAERE. G.M. Benson also made important early contributions and patented many novel free-piston configurations.
  
  The figure above shows v
  arious Free-Piston Stirling Configurations. F."free cylinder", G. Fluidyne, H. "double-acting" Stirling (typically 4 cylinders)
  
  
  What appears to be the first mention of a Stirling cycle machine using freely moving components is a British patent disclosure in 1876. This machine was envisaged as a refrigerator (i.e., the reversed Stirling cycle). The first consumer product to utilize a free piston Stirling device was a portable refrigerator manufactured by Twinbird Corporation of Japan and offered in the US by Coleman in 2004.