Stelzer engine

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The Stelzer engine , named after its inventor Frank Stelzer is a two-stroke - free-piston engine in cylindrical-stretched, bilaterally symmetrical design.

In the Stelzer engine, three pistons connected by a continuous piston rod are moved back and forth. The piston rod has different diameters (stepped piston) and controls the inlets of the two working cylinders. In the middle of the housing is the central flushing pump (pre-compression chamber) with a large diameter and a central inlet opening, followed by a round overflow channel to the outside, which is released by the stepped piston rod just before bottom dead center and is closed during compression and work. Adjacent to this are the working cylinders with ring-shaped combustion chambers , a spark plug and an outlet opening at the outer end. A matching piston belongs to each cylinder.

One side workflow

The middle piston is a double-acting disc piston. In its movement it connects the inlet opening alternately to the left and right side of the pump cylinder and always to the side whose volume is currently increasing. This sucks in the mixture. After driving over the inlet opening, the enclosed volume is reduced and the mixture is thereby pre-compressed.

Shortly before the bottom dead center on one side, the thinner part of the piston rod reaches the overflow opening, thereby exposing an annular cross-section, the gas is pressed into the working cylinder and flushes the exhaust gases out through the outlet openings that are open at the same time.

The Stelzermotor in motion

In the back swing, the working piston closes the outlet opening and the piston rod closes the overflow opening. The working piston moves further to the center in the cylinder and compresses the mixture here until ignition and reversal of movement. Now the piston is doing work during the renewed outward movement. At the same time, the fresh mixture is pre-compressed in the flushing pump. Shortly before bottom dead center, the outlet opens first, followed by the overflow opening.

This workflow takes place alternately on both sides. The big advantage of the Stelzer motor is the small number of components. Since it works without a crankshaft and connecting rod, there is less friction on the cylinder walls because there is no connecting rod side force. There are various ways of using the kinetic energy of a Stelzer motor. The free ends of the working pistons, which protrude from the engine block, can pump a liquid and thus act as a pump for a hydraulic system, or they can carry magnets and work as a linear generator to generate electrical energy.

A mixture of both technologies is also possible so that electrical and / or hydraulic energy is available.


Air pump

If the piston ends are extended beyond the housing, it can be used as a compressor . This then works as a double-chamber piston air pump acting on both sides.

hydraulic pump

A Stelzer motor can accelerate fluids with a longitudinally pierced piston and build up hydraulic pressure in a circuit . It then acts as a hydraulic pump . A practical test of this application by Stelzer is not known.

Power generator

If the piston ends are extended beyond the housing, it can be used as a power generator - the Stelzermotor then works as a linear generator . For this purpose , the piston or its protruding ends are provided with magnets that are immersed in the windings and induce an electrical voltage through their movement . Several prototypes of this linear generator were tried out.


The basic principle is that of the free piston machine . It has been known for about 100 years. Today's applications are limited to specifics. With the exception of the assessment cited below, there are no independent sources or reports for the Stelzermotor's claimed advantages. Nor has a prototype been independently tested.

The combustion chamber of the Stelzermotor has an annular combustion chamber because the working piston is firmly connected to the compressor piston by a central rod. The engine has problems similar to double-acting two and four-stroke engines , which have been practically out of use since the 1950s.

According to, key issues such as vibrations, ignition and exhaust gases were never resolved.


  • Brockhaus Encyclopedia - in twenty-four volumes . Verlag FA Brockhaus, Wiesbaden 1996, ISBN 978-3-7653-1100-0 , Volume 16, pp. 155/156.
  • Reinhold Popp, Elmar Schüll: Future research and shaping the future . Contributions from science and practice (=  future and research . Volume 1 ). Springer, 2008, ISBN 978-3-540-78563-7 , pp. 335 ( limited preview in Google Book search).

Web links

Commons : Stelzer Motor  - collection of images, videos and audio files

Individual evidence