Daniel Hagen

Development of my free-piston linear 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. 

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. The engines he built had not undergone any development over all those years, everything seemed very improvised and amateurish. Frank rarely had an engine on hand that he could start or demonstrate right away, even though he had been working on 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 fluidic nightmare. The number of edges and bottlenecks is overwhelming. The short stroke and poor throttle exchange make it almost impossible to build up sufficient combustion pressure. As a result, the pistons often hit the cylinder head.


One thing was clear: If the engine only ran as its inventor designed it, it was good for nothing.


So the question was:
Can the engine be significantly optimized, or not?

Stelzer engine
Apart from the inevitable vibrations, the aspect of the uniflow scavenging outweighs 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 
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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 of the specialists I asked had trouble transforming a circular motion into a linear one. Even more so, if the reversal points of the linear movement are variable. This circumstance forced me to deal intensively with the matter myself, in order to be able to evaluate the suggestions and arguments around linear generators. I finally decided for an own (not published) design, which I wanted to realize in the smallest technically reasonable size.
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.

Due to the lack of a gear ratio, the starting energy is not sufficient to achieve the necessary compression directly. Starting a free-piston engine therefore means rocking the piston(s) using the kinetic energy and the resulting air cushion. Conversely, it can be assumed that a correctly designed electromagnet will also function as a generator.

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

Now a great advantage of this engine became apparent - at least for test setups:


The pistons, which are freely accessible from the outside, make it possible to determine their outer position in the simplest way.


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

Great importance was given to a low operating temperature of the generator. With the central installation, the heat transport from the center to the outside could be used.


The engine cylinders are only shown schematically; they were completely redesigned in terms of flow technology.​​​​​​​


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
A compression ratio of 1:24 in a gasoline-powered engine sounds breathtaking. However, thanks to the lack of a crankshaft and the resulting enormous piston acceleration, much higher values are possible.
At maximum frequency, the shutdown of the electronic ignition causes the very highly compressed gasoline/air mixture to self-ignite. Since the piston travel is not bound, this results in knock-free, stable running at very high compression (HCCI).

The very high initial piston acceleration mentioned at the beginning avoids excessive heat losses via the cylinder walls. These factors are what make the very high efficiency of free-piston engines possible in the first place, but require a long-stroke engine because of the high piston acceleration.
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