Other features

Crankshaft

The crankshaft is usually a traditional crankshaft such as that found in an internal combustion engine. It is a rotating shaft with an offset crank to which the power piston is attached via a connecting rod or con-rod. The length of the crank determines the stroke of the power piston. All power produced by the engine must be transferred to the crankshaft through pressure acting on the piston.

Flywheel

As only one of the four phases of the Stirling cycle produces power, a flywheel is needed to keep the engine in motion during the two non power-producing phases and one power — absorbing phase. This is critical in low RPM engines typically associated with low

Temperature differences and low power where the angular momentum of the flywheel is needed to keep the engine moving smoothly, or even at all, through the cycle.

Working Gas

The working gas, or working fluid, is the gas that fills the engine. It is in a sense the fuel for the engine as all work performed on the piston (and hence by the output shaft) is performed by the gas. There is much literature that describes the benefits in using lighter-than-air gases as the working fluid to improve performance. The lighter gases have lower viscosity, resulting in less flow losses, as well as a greater specific heat capacity, cp, and a higher gas constant value, R.

A 71

Efficiency

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250

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1

1500

500

2000

500 Air

He

Lium

. 2500 Hyc

Irogen

J

!000

‘3

O

O

O

0

30 1————————————— 1—————————————

0 20 40 60 80 100 120 140 160

—— ► specific output, hp/liter piston swept volume

Figure 27: Graph of efficiency vs. specific power for air, helium and hydrogen [14]

The graph of Figure 27 is a widely reproduced graph, produced by computer simulation of a Philips beta engine. It shows that performance is clearly improved by using helium or hydrogen over air. There is however some debate over the authenticity of these results — Finklestein [14] argues that the same performance should be achievable from air as with hydrogen, with only a re-design of the gas circuit to suit the different flow characteristics of the different gases necessary. This theory has however never been proven and in all practical tests the lighter gases out-perform air or nitrogen.

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-34

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10

KW

0.2

TJth

0.1

0 1000 2000 3000 4000

RPM

Figure 28: Graph of power (solid lines) and efficiency (dotted lines) for engine under different pressure levels [27]

As illustrated by the graph in Figure 28 showing measured performance of a General Motors GPU-3 engine on hydrogen, it is not just the gas type but also the gas pressure that affects the performance of the engine. The dotted lines represent efficiency and the solid lines power, for four different pressure levels. It is clear that there is a near linear increase in power with pressure, and a non-linear but still positive increase in efficiency.

Phase Shift

There is always a phase shift of about 90° between the power piston and the displacer piston. This is typically accomplished with mechanical linkages that connect the crankshaft to the displacer. The phase shift is necessary for the gas to flow to the right parts of the engine at the right time. Depending on the actual design of the engine and the gas flow path, the ideal phase difference may be less or more than 90°, sometimes significantly. A numerical example of this phenomenon is illustrated graphically in Figure 46, Section 3.1.1.

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