It’s already apparent that the gear oscillator allows to pack a large number of cylinders in a small space. In part this is due to the double acting pistons, that take up a little more room than a single cylinder to allocate two. On this account only, it already has a high power density. Only the old radial engines with their contrived connecting rods approached this cylinder density. And we can even put in more cylinders if we build more than one “donut”, or we can dispose the cylinders in a more compact manner if we offset them or attach the pistons directly to the rotor. In the end it seems that the limiting factor will be the gear drive capacity, and in this respect, the robustness of planetary gear sets is well known.
But there’s a second reason why using a bigger number of small cylinders as opposed to a smaller number of big cylinders may contribute to increase power density. As the late MIT Professor C. F. Taylor wrote:”It is apparent that the use of small cylinders is a very powerful method of reducing size and weight for a given output (…). In practice, this relationship has not always been fully appreciated (…)”. To say the truth, this relationship is not so apparent. The key to understand it is mean piston speed. Machines of very different size, from tiny model engines to huge marine diesels, tend to work optimally in a relatively narrow range of mean piston speeds (around 15 m/s). Reciprocating compressors also operate at a narrow range of piston speeds. So if we have some small pistons as opposed to fewer larger pistons to produce a given output, the total displacement of the small ones will be less than the bigger ones because they can operate at higher rpms keeping the same mean piston speed. Further, the gear drive can be scaled down as well, since the stroke will be shorter and the torque sustained by the planet carrier lower. Thus we reduce size and weight overall, i.e. we’ll get more power density.
Of course, there’s a trade-off. Citing Taylor anew:
“Sacrifices which have to be made when the size of cylinders is reduced and the number of cylinders is increased include the following:
In the case of Diesel engines, more expensive fuel may be required.
A larger number of parts will have to be serviced.
The life of wearing parts will be shorter.
A reduction gear may be required because of increased rpm.
If only one engine is used, it may be necessary to use complicated cylinder arrangements such as the multirow radial. ”
This is in a conventional engine. It must be noted that he was assuming a constant bore-to-stroke ratio. In a gear oscillator engine or compressor, when we increase the number of cylinders and reduce its size:
There are no more moving elements added because the new pistons are fixed to the rotor.
There’s only residual friction in the pistons and they will operate at similar piston speeds, so we don’t expect increased wear on this account. The rotor oscillates faster but along a shorter angle, so the inertial forces stay the same. It’s just the planet gears and the planet carriers (see next point) that rotate faster.
An increased ratio of oscillation translates only into one third or less rpm increase at the main shaft, depending on the number of planet gear pairs used.
Increasing the rpm, the power delivery becomes more steady because the power strokes are shorter and more frequent.
The arrangement is already radial.
In conclusion, the the gear oscillator is clearly better suited to increase power density this way than a conventional piston machine.
No forces between piston and cylinder
In a conventional piston-crank mechanism, there’s a force between the piston and the cylinder walls that balances the other forces acting on the piston (see red arrow on the figure). This is the cause of wear on the cylinder walls and requires their abundant lubrication in order to minimize it. This is particularly critical in the case of compressors where an oil-free working gas is needed. In such cases, several solutions are implemented to circumvent this problem, but none of them is particularly compelling.
In a gear oscillator system, there’s no force between piston and cylinder aside from the residual friction between the sealing rings and the piston walls, and it can even be nullified. In the case of compressors, this permits an oil-free cylinder, a most sought-after feature.
In the case of engines, the greatly reduced friction requires less oil for lubrication and increases the lifespan of the parts. In the words of the Report of the Comitee on Ceramic Technology for Advanced Heat Engines (National Research Council, US): “Cooling is required in conventional engines primarily to preserve the lubricating oil film between cylinder and piston rings. If this problem could be eliminated, engines would be routinely designed without cooling systems, because they are heavy, bulky, require power to run (…)”(pg. 54) .
Thus, this feature makes a gear oscillator engine better suited for experimentation with ceramic materials (adiabatic engines).