Operational Stability

Getting freshly involved in free-piston technology, one is often confronted with the argument that such motors are unstable and difficult to be controlled. Even studies of renowned research institutes refer to such alleged facts, and that further fundamental research is needed to control the movement of the free floating pistons. 

However, the ancient free piston engines (e.g. Junkers, Pescara) ran absolutely stable without active control mechanism, nor electronic equipment. It is a question of how the engine is constructed: Is it self-centering after load change, or does it run out of control. If it runs out of control, then the second best way is to add expensive and complicate active stabilizing equipment. 

But the best way is to adapt the engine's design, so that it does readjust itself to standard operation after each interruption of normal running. This is the most difficult subject of a free piston engine, to be solved by classic and proper engineeering. No computer-aided motion control is necessary at all to keep the engine running, same as for conventional crankshaft engines.

Let us see what "Mr. Free Piston" Robert Huber says about the operational stability of free piston engines.
We talk specifically about opposed piston engines of the inward compressing type. For the operational stability, it does not matter in which manner the power is taken from a free-piston engine. Nor does the size matter, since the engines were built in a range of approximately 10 to 1200 kW based on the same formula.

Robert Huber's key phrase:
"Among the problems to be solved in the course of development of these machines, one of the most difficult was concerned with the absolutely uniform and constant operation of a piston not attached to an assembly consisting of connecting rod and crankshaft.

At first sight it may appear, that elimination of the classical connecting rod and crankshaft linkage, used at present in all internal combustion engines would necessarily culminate in a sensitively balanced operation, and that minor disturbances or misadjustments would cause the engine to operate irregularly and even cause it to stall, i.e., would "kill" the engine.

However, many free-piston engines of this type were built for decades and have run for thousands of operating hours without a single incident attributable to the absence of wrist pins, con-rods and crankshafts."
​The engines used for the tests to be discussed herein have been described elsewhere in great detail. Therefore, it will be sufficient in the following to consider only briefly the fundamental concepts involved and the operation of the unit, while measures taken to insure stable operation will be studied in greater detail. The engine arrangement finally adopted after testing various other types is shown in Figure 1.
Pescara Free Piston Gas Turbine
Pescara Free Piston Gas Turbine
The free-piston gasifier A contains two opposed step pistons (1) of symmetric stroke. The engine cylinder (2) is in the center.

It operates as a two-stroke supercharged Diesel engine. The two single-acting compressor cylinders (4) are located on both ends of the central housing. The cushion cylinders (3) store the energy for the return stroke. The compressed air is used for scavenging and charging of the engine cylinder.

Fresh air is aspirated through suction valves (5) and is discharged through delivery valves (6) into the case surrounding the engine cylinder. Fuel is injected by fuel injectors (7), mounted in the center of the combustion chamber.

The hot combustion gases, mixed with the excess scavenging air, exhaust into receiver (B) and then flow through the gas turbine (C) which alone produces the useful power, i.e., is the prime mover of the unit.

"The records of successive piston strokes offer a very accurate means for analysis and study of the operation of the engine without parallel or equivalent in crank-connected machinery.

These records show the position of the dead-centers, both inner and outer, very accurately with respect to the center of the machine from which may be determined the compression pressure within the power cylinder, the instantaneous load carried, as well as the regulation of successive cycles."
A stroke diagram with constant fuel injection is reproduced in Figure 6. It will be noted from this diagram that the regularity of stroke is very high.

The maximum deviations of the outer dead-center with respect to the mean position do not exceed 1.4 mm. For these positions of the outer dead-center, the effective pressure in the bounce cylinder is equal to 4.5 kg/cm2.

The energy variation due to these irregularities is 41 kgm and, since the indicated output equals 3430 kgm, these fluctuations will be of the order of +/- 1.2%. In reality the power output varies even less, since the displacements of the outer dead-center are not caused solely by the small differences of gasifier output from one stroke to the next, but include also the variations of the compressor work and friction losses.

"Stroke records constitute, moreover, a very practical means of tracing the changes resulting from momentary disturbances, notably injection troubles. For test purposes, such disturbances are reproduced by varying the amount of fuel injected for a period of several cycles."

The stroke diagram shown in Figure 7 shows the records taken under such conditions.
P denotes the case for an increase of the stroke, whereas M shows a decrease of the latter. The displacement of the outer dead-center was in both cases approximately 40 mm. Moreover, at the instant of cessation of this disturbance, the original, normal dead-center is restored within a few strokes without the least instability effect, causing, for example, a fluctuation of the outer dead-center about some mean position.

These diagrams show, therefore, that there is a very pronounced tendency on the part of the engine to readjust itself to standard operation after each interruption of normal running.

The degree of stability of a gasifier depends on the arrangement and on the dimensions of the power compressor and bounce cylinders.
The time required for the bounce cylinder pressure to adapt itself to a new position of the outer dead center can likewise be read off the stroke diagram.

As Fig. 11 shows, the shorter strokes last through several cycles.

The fuel injection was reduced at point A by about one-third. The inner dead center first shifts outward, but after about six cycles it returns practically to its former position, thus indicating that the pressure adjustment in the cushion is complete.

The time necessary for this adaptation was about 0.6 second.

A displacement of the position of the inner dead center of the same order of magnitude but of inverse direction is shown between points C and D when the fuel injected has again been increased to its original value.

The (partially cited) study above of operational stability shows that free piston engines will operate with perfect reliability and uniformity despite the absence of crankshaft restraint.

Collision of the two power pistons or impact between the compressor pistons and cylinder heads is (when properly designed) impossible. Even in the case of temporary disturbances due to faulty fuel injection, excessive friction, or leaky valves, normal operation undergoes scarcely any change whatever.
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