Pistondisplacer Versus Multiplepiston Engines

ll has been shown above that many different arrangements of piston-displacer machines and multiple-piston machines are possible, some of which have been developed to a commercial degree. There is no one ¡irfMiiocmnni that excels above all others in every case, but there are a

Fio. 6.8. Alternative arrangement ol single-acting two-piston engine.

larger machines, the choice may lie between multiple, single-cylinder, single-acting, piston-displacer machines on a common crankshaft, or multi-cylinder, double-acting Siemens machines. There is every possibility that the simplicity of the Siemens arrangement will prove compelling. The reduced number of reciprocating elements can result in substantial economies in manufacture.

An important reason for the preference of piston-displacer machines over multiple-piston machines is that, in the former, it is somewhat easier to deal with the problem of reciprocating seals. On all machines, at least two dynamic-fluid seals are required. In the case of the three machines Shown in Fig. 6.3, fluid seals are required on all the pistons, two in the case of the two-piston machine, and one each in the two other piston-displacer machines. An additional fluid-seal is necessary on the displacer

Power shaft

Heater tubes

Combustion region

Water-cooled

Expansion

Regenerative heat- machine exchanger \ /

I-u; h.'.> Zwiauer-Wanket configuration of rotary Stirling engine.

displacer rod is much smaller than the seal around the piston, with proportionally less leakage and friction. This is. perhaps, the most singular advantage of displacer engines, since the problem of reciprocating seals is particularly difficult, especially when working fluids other than air are used. A further advantage of piston-displacer machines is that the total reciprocating mass can be less than in multiple-piston machines. This facilitates balancing, and reduces vibration problems. The displacer does no work, arid has to withstand merely the gas forces (arising from aerodynamic-flow losses) and its own inertia forces. Therefore, it can be structurally light, and requires correspondingly smaller rods, links, and bearings, so that appreciable savings in weight and mechanical-friction losses can be made.

The power output of a Stirling engine is, to a first approximation, a linear function of the pressure of the working fluid. Thus, one way to immediately increase the specific output is to pressurize the engine. On small engines, it is advantageous to pressurize the crankcase: this not onlv reduces the duty on the reciprocating fluid-seals, hut also reduces the structural strength requirement of the piston and connecting rod assembly, including bearings. This arises from the fact that, with a pressurized crankcase. the pressure difference across the piston is reduced to (Pcylinder Pcmnkcanc)* hatead Of (Pcyltndcr ~ P,.mo.phcr.) With ail U llprCSSUrized crankcase. Savings in weight, mechanical bearing friction, and seal friction may be gained thereby. These are offset by the fact that (a) the crankcase is now a pressure vessel with an increased strength requirement, and (b) that at least one dynamic seal is involved if the crankshaft is required to exit from the crankcase.

The problem of a rotating crankshaft-seal is less rigorous than that of a reciprocating piston-seal, and further, may be eliminated by combining the electric motor, or generator driven by the engine, into the crankcase; however, this may cause appreciable 'windage loss' in a highly pressurized engine.

The use of a pressurized crankcase is limited to small engines. On large engines the weight of the crankcase becomes excessive if it is pressurized to the minimum cycle pressure.

SINGLE-CYLINDER VERSUS TWO-CYLINDER I'IST ON-D IS IM. ACE R M A CM INKS

It is almost intuitively obvious that, for a pressurized engine, the single-cylinder pistón-displacer configuration leads to a crankcase of minimum size and weight. As the engine power rating increases, the crankcase becomes a dominant fraction of the total engine weight, and for large engines, the simple expedient of a pressurized crankcase must be abandoned.

There arc two other advantages of the single-cylinder piston-displacer engine over the machine with separate cylinders. In the two-cylinder machine. (Tig. 6.3(b)), the compression space is divided between the displacer cylinder and the piston cylinder, and includes the port connecting the two. This space can never be reduced to zero, so that (in effect), the compression space has a large clearance volume. This clearance volume must be included with the dead space. X. and as we have seen earlier, any increase in X results in a decrease in power output.

The second advantage of the single-cylinder piston-displacer engine is that, in every revolution, the displacer and piston both sweep the same part of the cylinder, although at different times. This overlap of strokes is shown clearly in Fig. 6.10 and represents a most efficient utilization of the available engine-cylinder volume.

The advantages of the separate-cylinder piston-displacer machine are:

(a) the increased flexibility for production-engineering design of the crankshaft and connprtmo-rMH «icinni

Cr.-inl; angle

Cr.-inl; angle

Fig. (».10. Piston and displace; motion in a single-cylinder engine.

(bj the separation of the displacer-rod seal to a fixed location in the displacer cylinder, rather than the more limited environment of the piston crown.

In practice, these are very important advantages. An attractive arrangement of a piston-displacer in separate cylinder machine is shown in Fig. 6.11. The engine is arranged in Vec form. Engines of this configuration in a power range of about 10 kW are at an advanced stage of development al the Swedish company. F.F.V. (a part-owner of United Stirling). These engines are soon to be introduced for commercial use (Johannson 197S).

DESIGN VARIANTS. Ol l-RICSSON ENGINES

Regenerative engines of the type where the flow is controlled by valves (called here Ericsson engines) are found, like their Stirling cousins, in a wide variety of types, shapes, and sizes. Sometimes engine arrangement for both types can be very similar; the only distinction between them is the existence (Ericsson) or non-existence (Stirling) of valves which allow the passage of fluid through the working space in a cyclic manner, and generally control the flow of the working fluid. In this distinction it is important to note tiiat we exclude the gas valves used on an intermittent basis as pari of the control system on Stirling engines to vary the working fluid pressure.

The families of Ericsson engines are nol considered here in detail, but a brief guide to the principal types might be in order, so as to assist in identification. The degree to which any of the theoretical material or onoi noonnii ,ivni>rti»nr«i» flirr'MPO/»/! Y\tp ft* rriniKl K/« *innln«rl t A

Fig. 6.12 is a 'family tree' of Ericsson engines. In most cases, they can be classified either as displacer machines or as piston machines. Each ol these principal groups can be further subdivided. Displacer machines may have either a constant working-volume or a variable working-volume. Piston machines may be classified into single-piston machines or twci-piston machines.

Fig. 6.13 shows some of the design variants of displacer machines, and identifies some of the better-known arrangements by the names of their inventors. Of the variable working-volume type, only one example is shown. This was first used by John Ericsson, and contains both a piston and a regenerative displacer, coupled together (but moving out of phase) by means of a crank-connecting-rod system. The arrangement may be equipped with gas-operated (or mechanically-operated) valves. This arrangement is potentially attractive for very large nuclear-reactor installations, where the working fluid could be passed as the coolant through the reactor core.

There is a larger ranee of possibilities in machines of constant workine-

Ericsson engines (How control by valves)

Displacer machines

Piston machines

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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