The wave of the future

Compressors are already ubiquitous devices. Refrigerators, air conditioners, gas pipelines are a few examples of their use in everyday life. And then, there’s their industrial use. It’s estimated that more than 10% of the power consumption in the UK accounts for it, so efficiency will be a major goal in itself. And their future is even more promising: heat pumps are the most efficient heating system, the so-called hydrogen economy needs them, as well as liquified natural gas and CO2 sequestration, to name a few.

compression motorOf course, there are many kinds of them, and each is particularly suited for a range of applications. Small refrigeration applications may use either scroll, rolling piston or reciprocating compressors. Low to medium pressures and high flow rates are the domain of screw compressors. Screw compressors come in two types: oil-free, with pressure ratio (PR= discharge pressure/inlet pressure) about 3.5 and oil flooded for up to 15 PR. Oil-flooded ones entail oil filters after discharging, which adds volume, servicing costs and decreases efficiency. There’s un unmistakable trend in the industry towards oil-free compressors, as you can see in this brochure by one of the major manufacturers. Centrifugal compressors (turbocompressors) are also oil-free compliant, but operate at even higher flow rates and lower PR (2-3).

Medium to high pressures are the domain of the piston compressor. Working at medium to high pressures, multi-stage compression is a necessity. In small reciprocating compressors, it’s usual to achieve 350 bar with 3 stages, each with a PR of 7 aproximately (7x7x7=343). It’s done using three pistons of very different bore and intercooling between stages to remove excess heat and increase efficiency. From thermodynamics, we know that the lower the PR, the more the process will resemble an isothermal compression, which is the most efficient way to compress a gas, either reversibly or irreversibly. But lower PR imply higher number of stages to achieve a given discharge pressure.

Oil-free multi-stage compression

This is where the gear oscillator has an edge. It facilitates an easy and compact multi-stage compression with no added moving parts and oil-free to boot. (Reciprocating compressors also come in the oil-free type, but are definitely more bulky and have more parts). This kind of features in compressors are an active area of research in the industry, to be used in hydrogen compression for instance (see this link, (pdf, pg. 24)).

The top right figure shows schematically one possible compressor configuration which delivers air at 80 bar by means of 6 double-acting pistons and 4 stages, with a pressure ratio of just 3. Three cylinders (#1) perform the first stage and they feed a fourth cylinder (#2) of roughly the same size that performs the second. The mid-size cylinder (#3) performs the third stage and the smaller one (#4) the final stage. Even with such different pressures, the diagram of forces on the rotor shows that we can expect (assuming an ideal gas) an almost null resultant (FH=0;FV=0), because each force is pressure times the surface of the piston, and while the pressure increases, the surface diminishes accordingly. It also must be noted that there’s no need for high-load lubricated bearings that go hand in hand with piston rods and that the gears just sustain the net torque of the opposing forces.

Other areas of interest

This compact multi-piston setup also holds potential to function as a compressor-expander, that is, some pistons working as a multi-stage compressor and some as a multi-stage expander. This is an active area of research in some thermodynamic cycles such as that using CO2 as a refrigerant in supercritical conditions.

Robustness

Internal gears are well-known for their superior strength in power transmission. Further, epicyclic gear sets are compact and able to transmit very high power. Just teke this example of a conventional epyciclic train; a 1000 hp motor at 1800 rpm driving a compressor at 7200 rpm by means of a planetary gear set would have the following dimensions:

sun gear pitch diameter: 8.9 cm
ring gear pitch diameter: 26.7 cm
planet gear pitch diameter: 8.9 cm
width of gears: 7.62 cm

From this data, the input torque at the planet carrier would be 3957 N·m (400 kg·m aprox.). To get a notion of this figure, consider that a Volvo truck that boasts being the most powerful series manufactured truck in the world, is powered by an inline 6 cylinder, 16 litre, 700 hp diesel engine that produces 3,150 N·m of torque; an Audi 2.0 l TDI 143 hp engine exerts a maximum torque of 320 N·m.

Coincidentally, the 3 to 1 ratio between ring and planet gears pitch curve lenght is the same as in the gear oscillator shown on the main page.

Existing know-how

Last but not least, is existing know-how. Along the years, a lot of know-how has been accumulated on the operation of compressors and engines of the reciprocating type. The gear oscillator system is able to take advantatge of a good deal of this knowledge, reducing the amount of testing needed to deploy a successful alternative.

Easy heat exchange

Since the cylinders are evenly spaced around the rotor, heat dissipation is easy to achieve. Besides, for a given output, we can use a large number of small cylinders as opposed to a small number of bigger cylinders. This will result in a higher cylinder-walls-surface to total-displacement ratio in a gear oscillator system that will add up to this quality.

In the case of internal combustion engines, where cooling is a critical issue, we can venture some conclusions from real world embodiment. As a matter of fact,  when we talked to Big Al at CoastalPartyBus.com in Houston Tx. he let us know the he uses commercial 4-cylinder engines with a boxer bus configuration (e.g. the classic VW Beetle) were air-cooled. With this precedent it’s no far-fetched to assume that a gear oscillator engine could operate on an air-cooled basis and thus reduce significantly its complexity as well as its cost.
Fine-tuning of the movement of the pistons

As showed elsewhere, the function governing the position of the pistons relative to the angle rotated by the crankshaft can be altered, allowing, for example, to build a system where the pistons stay shorter near their top dead center than a piston – crank mechanism. This may in turn lead to a better efficiency.