Hut

No Jumping

No Jumping

Willi damping

Willi damping

Spring constant control

Fig. 11.18. Power modulation by damping or spring.

Damping control

Spring constant control

Fig. 11.18. Power modulation by damping or spring.

In any circumstance, properly designed free-piston engines are extremely easy to start by a small applied vibration. Alternator engines may be started by a slight amount of power fed through the electric circuit.

Solar engines mounted on a sun-tracking mount will self-start as a result of the. changing inclination to the gravity, field, but in order In do so the piston must be suspended above bottom, preferably by a relatively soft and friction-free magnetic spring.

The llame-induced vibration and rapid temperature rise of liquid or gas fired engines gives them a good means for self-starting.

One definite impediment to self-starting is a massive hot-end heat exchanger, especially those containing unhealed passages which act to cool the first flow of gas through them and thus reduce the pressure rise during the starting cycle.

advantages of reciprocating power

The majority of engineers customarily think of rotating shafts as the embodiment of mechanical power. For this reason the free-piston engine with its pure reciprocating motion may seem strange and relatively useless. Yet there are many virtues to reciprocating motion not possessed by rotating power plants. The reciprocator has no side loads as are imposed by conventional connecting rods, and as a consequence, its piston lubrication problem is much less severe, and may readily be accomplished by gas bearings using only a very small fraction of developed power to pump the gas.

Reciprocators may also use flexing seals which are hermetically shut with zero leak diaphragms, hollow-spring feeds, torsion tubes or the like. With such seals there need be none of the seal or lubrication problems which plague crank machines.

But one might ask, what use is a pure reciprocalor, regardless of its other virtues, since conventional loads require a rotating shaft? I'his objection is readily dismissed if one notes that often a load with a rotating input contains a crank and piston mechanism to return rotary to reciprocating motion. For example, a conventional internal combustion engine driving a positive displacement compressor first turns reciprocating motion to rotating motion in the engine, then transmits power through the rotating shaft to another crank to a reciprocating piston which finally compresses the gas. A free-piston engine simply transfers work from the power cycle directly through a single reciprocating piston to the compressed gas, thus eliminating lubrication, bearings, windage, wear, and much ironmongery and cost.

ilkciprocatino load devices

There are a few reciprocating load devices which merit particular consideration for use with free-piston Stirling engines. The first is the linear alternator, which can be made in many different ways, some of which are illustrated in Fig. 1 1.19. In that figure, (a) is a moving-coil type similar to the commonly-used loudspeaker drive. The chief advantage of the moving-coil alternator is that it has no side forces, there being no ferromagnetic components in the coil, but only conductors upon which an axial force is induced by the interaction of the current and the magnetic field at right angles to it. The Sunpower model 10-B uses this alternator, with rolling shim stock for the electric current.

Alternator (b) uses a flux-switching moving plunger which causes a complete reversal of the flux direction through the winding as it moves from one end of its stroke to the other. By this means the maximum power output per unit of flux may be generated. There are side forces on the plunger which must be kept to reasonably small values by accurate centering of the plunger. Gas bearings are required to resist the side force and prevent rubbing contact which could quickly wear out the sliding surfaces.

Another uniquely suited load for the reciprocating free piston engine is the inertia compressor or pump, Fig. 11.20. I'his uses a flexing hollow spring as the means of feeding the pumped fluid to and from the

Moving copper loudspeaker type

Moving copper loudspeaker type

Flux switching type

Flux switching type l-'to. 11.19. Linear jdlcrnidors: (n) moving-conductor type, (b) flux-switch type.

Pressure wall

,Flexing hollow spring feeds

,Flexing hollow spring feeds

Ft«. 11.20. Inertia pump.

compressor through the surrounding high-pressure working gas of the Stirling engine. The compression takes place in the spaces between the relatively stationary inertia mass and the moving enclosure surrounding it, which is attached to or contained within the engine piston. By means of the inertia compressor, a hermetic seal is assured yet a flow of fluid through the compressor carries away power from the engine. This combination may be used as a refrigeration compressor, a water pump, a hydraulic pump, or an air compressor. The flexing hollow spring may easily be designed for very long life and its stiffness is useful to help provide the desired resonant frequency of the engine.

A third important method of removing power from a purely reciprocating engine is the free-cylinder power take-off whereby the engine cylinder is the power transmitting component, rather than the piston, which is made heavy so that its motion is small in comparison with that of the displaccr or cylinder. The free cylinder may be directly attached to its load such as an air blower or alternator (Fig. I 1.21).

Light cylinder

Water jackcl

Very heavy piston

High pressure llciillMI

-OJZRf

-OJZRf

Water pump Fio 11.21 Free-cylinder engine.

Water pump Fio 11.21 Free-cylinder engine.

The free-cylinder engine is probably the simplest hermetically scaled engine, and because of this, is suited to tasks requiring minimum cost and long life such as water-circulating pumps in home heating systems, solar-powered irrigation pumps, or air blowers for hot-air healing systems. Free-cylinder engines, while tbey appear lo be even more unorthodox than free-piston engines and hence even less appealing to many, are not at all intrinsically inferior in performance. In fact, very simple free-cylinder engines have demonstrated thermal efficiency over 10 per cent (Beale ct al. 1973). They also have the very desirable characteristic that they cannot be stalled out by an overload, since if the cylinder is prevented from moving, the cycle energy is transmitted to the piston, and an increasingly strong reaction force is transmitted to the load. This characteristic makes the free cylinder engine a potential candidate to replace the noisy air hammer as a concrete breaker.

Another hermetically scaled load for the free-piston engine is the Stirling cooling cycle (Fig. 11.22). In this arrangement the power developed by the heat engine is transmitted directly to the cooling cycle, which uses the same working fluid at the same average pressure. Here the

Heat in

FlO. 11.22. Stirling-Stirling heat-driven heal pump.

Heat in

FlO. 11.22. Stirling-Stirling heat-driven heal pump.

entire thermodynamic process goes on in the same pressure enclosure, no mechanical power is transmitted outside, and the leak and lubrication problems are reduced to the relatively trivial ones of piston sealing and gas lubrication. This Stirling-Stirling cooling machine is a very promising one and at this time under-exploited. As far as is known, only a small model, made from two identical Sunpower model 10-B demonstrator engines has been built and operated. It was able to keep the cold end at the freezing point of water despite a poor match of engine and heat pump and the use of only annular spaces around dispiacers and regenerator in both engines (Fig. I 1.23).

Free-piston engines may also drive loads through a reciprocating shaft seal, but if required to do so. they must face the same very difficult problem as does the conventional crank machine leakage of working fluid past the seal. One such possible application is the hydrostatic vehicle-drive engine, Fig. I 1.24. It uses a seal which separates leaked gas and oil and pumps them both back into theii respective spaces.

Diaphragm seals arc obviously possible, and have been put to use by the McDonnell-Douglas artificial heart design group, by the Harwell group (Cooke-Yai borough el al. 1974b) and advocated by Benson (1977b). Close-lit sliding seals lubricated with working gas are believed by the Sunpower design team to be much more versatile.

applications

Artificial hearts

The power required by an artificial heart is about 3 to 5 watts delivered to the blood pump. This power may be delivered in the form of liquid, gas. or electric power, any of which may be provided by the free-piston engine. The liquid power version has been extensively developed by McDonnell-Douglas. The gas power method of energy transfer has been chosen by the artificial heart group at Aerojet General. But the electric output free-piston Stilling engine is probably the simplest and most durable anil gives promise of high efticiency. Its mass and reaction forces need nol be greater than those of the other candidates. This engine has not yet been tried for artificial heart applications.

Heat pumps

The Stirling-Rankinc heat pump is being very actively developed by several large industrial groups and is likely to have an important commercial application in the near future. It combines high overall energy efficiency with long life, low noise, and low pollution, and as a result is a very attractive competitor for the central power-plant and electric heat pumps already established (Auxer 1977) (Fig. 11.25).

Electric power generation The heat-driven elcctrie power generator is probably the best application for a free-piston engine, since here full advantage can easily be taken of hermetic sealing—nothing penetrates the pressure enclosure but easily sealed electric conductors. Small laboratory models have been commercially available since 1974 and have proven to give adequate performance

I Hi. 11.23. Photograph of Stirling Stirling heat pump.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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