Daniel Hagen

Development of my free-piston linear alternator
Simplified free-piston alternator
To start with free-piston engines, those with only one movig part are easier to build and control- knowing that a serial engine would be an inherently balanced opposed piston engine. The following movie shows the principle of a simplified free-piston linear alternator.

Now from Robert Huber, I learned that the running characteristic of such engines is completely different than that of crankshaft-bound motors. The piston acceleration is substantially higher, and the timing can not be compared with conventional two stroke engines. 

Therefore, the use of commercially available two-stroke engine cylinders was out of the question due to the predetermined engine timing, that is incorrect for the foreseen use in a free-piston engine. Furthermore, cross-flow scavenged engines have no future due to emission related reasons. 

In addition, it was much of interest to me to examine the so-called Stelzer Engine more closely. Since the early 80's, one read frequently about this engine- but rather about cheated investors, than about a breakthrough in this so-called "revolutionary technology". The engine should now be analyzed from a strictly technical point of view. 

As I visited Frank Stelzer, I expected clarification about this obsolete engine and its inventor. Unfortunately, Frank Stelzer turned out to be a hobbyist. His engines revealed no technological breakthrough or at least any progress through all the years, it all seemed very improvised and amateurish. Frank Stelzer had rarely an engine on hand that he could start or demonstrate right away, although he worked with this engine for decades. Those who knew him personally, know what I mean: He was not of this world. 
1:1 cutaway model of a Stelzer engine
1:1 cutaway model of a Stelzer engine
1:1 cutaway model of a Stelzer engine

Intake and exhaust ports are strikingly small. The engine is over square, even if the center rod is subtracted from the piston surface.

1:1 cutaway model of a Stelzer engine
1:1 cutaway model of a Stelzer engine
1:1 cutaway model of a Stelzer engine


Extremely remarkable: The doughnut-shaped combustion chamber, and the uniflow scavenging system. 


1:1 cutaway model of a Stelzer engine
1:1 cutaway model of a Stelzer engine

The engine in its original state is a fluid dynamic nightmare. The number of edges and bottlenecks is overwhelming. Due to the short stroke and poor gas exchange, a sufficient combustion pressure is difficult to achieve. As a consequence, the pistons often collide with the cylinder head.

One thing was clearly visible: If the engine just ran the way it was designed by its inventor, then it would be good for nothing.

The question was:

Does the engine have sufficient potential for optimization, or not.


Stelzer engine
Apart from the inevitable vibrations, the aspect of the uniflow scavenging weighs on many disadvantages of this engine design.
Instead of building a "conventional" double-acting free-piston engine using e.g. chainsaw cylinders, the more challenging task was to investigate this remarkably engine to reveal its potential, and to test an optimized version - and finally, to integrate a linear alternator in an advanced third stage.
D. Hagen, 1st engine
D. Hagen, 1st engine
First engine 

This is the first self-built engine. The gas exchange was fundamentally revised. Spark ignition is controlled through an adjustable light barrier. A striking feature of free-piston engines is the almost immediate throttle response, like a racing engine. The engine also stands several misfires, without stopping.
Linear alternator principle
Linear alternator principle
Parallel to the first prototype, I worked on a linear alternator that I wanted to integrate into a second but completely redesigned engine.

Many specialists struggle with the transformation of a circular into a linear motion that has variable reversal points - the same phenomenon in electrical machinery, as well as in combustion engines. This situation forced me to deal intensively with the entire matter, so that I could assess the value and feasibility of any proposals and arguments around linear alternators. I finally decided to go for my own (not published) design.
Starting procedure
Starting procedure
Linear alternators had already been sufficiently calculated in theory. I focused on the the starting procedure, respectively how should the linear alternator be designed so that it generates sufficient electromagnetic power to start the internal combustion engine.

The starter energy is, however, not strong enough to achieve the necessary compression at once Because the starter has no transmission gear as in a conventional crank engine. 

Therefore, starting of a free-piston engine means to swing-up the engine piston(s) taking advantage of the kinetic energy and the air cushions. In contrary, one can assume that a properly designed solenoid works well as an electric power generator.

1:1 cutaway model of a Stelzer engine
1:1 cutaway model of a Stelzer engine

Now a significant advantage of this engine turned out- at least for experimental setups:

The the pistons are freely accessible from the outside. Therefore, determination of the outer position of the same is very simple.


D. Hagen, 2nd engine
D. Hagen, 2nd engine

Care was taken to a low operating temperature of the generator. Located in the engine center has the advantage to use the heat transfer from the center to the engine's outside.

The engine cylinders are shown schematically only. They were completely redesigned for improved fluid dynamics.


Proof of Concept
  1. An engine design has been selected that has evolutionary potential and is relatively easy to build.

  2. Great attention was paid to use an engine that works with uniflow scavenging, due to emission related reasons.

  3. Aspects of balancing oscillating masses were excluded, as finally an opposed piston engine will be used.

  4. A linear alternator was developed, which - as a solenoid - had the necessary power to start the engine, while taking account of size and weight.

  5. A strategy had been developed to ensure a reliably starting procedure

  6. The individual elements were merged into a newly designed engine and the achievement of objectives has been verified.

  7. The project was privately funded.
The result of the work

The engine has a displacement of 2 x 15 ccm, it runs at 160Hz. The linear alternator generates 15 or 30 VDC through bridge rectifiers. The compression ratio is 1:24.
Proof of Concept Prototype
The mentioned compression ratio of 1:24 sounds amazing for a gasoline-powered engine. But due to the absence of a crankshaft and therefore an enormous piston acceleration, substantially higher compression ratios are ​​possible. The combustion is ignited rather early. At maximum frequency, disconnection of the electronic ignition leads to auto-ignition (HCCI) of the highly compressed fuel/air mixture. As the piston stroke is not bound, a stable and knock-free running results at a very high compression ratio.

The above-mentioned high piston acceleration avoids excessive heat loss through the cylinder walls. These factors mainly cause the very high efficiency of the free-piston engines. But - due to the high piston acceleration - a long-stroke engine design is required.