Re: free piston
Posted by:
Peter Heid (IP Logged)
Date: February 06, 2002 09:08PM
<HTML>The free piston engine of which I am refering to is an engine with no crank shaft or connecting rods. The source of power and the application of the engine do not concern the definition. The piston or pistons use a gas compression spring to afford the return travel after the power stroke. the piston is normally a double ended afair with one end being the power piston and the other is the gas spring piston (bounce piston) or in some applications such as pumping, it is also called thework piston. There many designs of free piston engines but so far as I know the sterling engine is the only external combustion free piston engine. A company called sunpower out of Athens Ohio developed a 6 Kw single, free piston sterling engine-alternator combination for Cummins that uses a solar collector for the heat source and permanent magnets in the linear alternator.
The free piston engine seems to have been developed for gas generation in the gas turbine as shown in the 1857 brittish patent, number 1655, issued to F. Matteucci and E. Barsanti of florence Italy for a free piston atmospheric turbine. In 1864 Langen and Otto progressed with work on the same principal, followed by others but it was not until the efforts, started in 1922, by frenchman Pescara that the modern free piston engine took form. His first engine, an air compressor, was a gasoline engine completed in 1925. His second engine was operated on the diesel cycle and was completed in 1928. Pescara's original conception of the free piston engine is the basis for most engines built since then. Ford and GM have spent great sums of money on development programs involving the free piston turbine.
The Pescara design uses two power pistons each mated to a work piston, the power pistons share the same cylinder and compression is between them as in some two piston diesel engines. Compression is acheived by forcing the two pistons toward each other. The frequency or cycles per minute is a function of piston mass and gas spring math, which I will not bore you with here since there a couple of books that would do a better job of it than I could. The range of operation seems to be in the 360 to 3600 CPM for most engines produced. The sunpower sterling runs at 3600 CPM with a stroke less than 1 inch and a bore size around 5 inches to provide 60Hz power. The speed or frequency can be varied somewhat by varying the gas spring pressure. In the IC version, one piston would uncover the exhaust port and the other piston the inlet port near the end of the power stroke. The pressure in the bounce chambers must be equalized or a linkage must be used to provide for continueous synchronization of the pistons. With the diesel design, the linkage is used to operate the fuel injector. It is interesting to note that some free piston turbines were started in synchronous mode and after running breifly they were operated out of sync to even the pumping flow. Varying the fuel charge will effect the the outer dead point in the piston travel and compression including any pumping influences the inner dead point, TDC if you will. This allows a compression ratio that varies with output, the minimum with the shortest stroke and the maximum with the longest. As an IC engine, the free piston engine has exibited compression ratios from 10:1 to 50:1 at full output and thermal efficiencies in the range of 45 to 50%. The highest compression pressures at full out put run between 1000 and 1200 psi with initial combustion pressures around 2000 psi.
sorry it is draging out so long.
I must finish tomarrow.
Peter heid</HTML>