Air Engines: The History, Science, and Reality of the Perfect Engine
3 Later Single-Cylinder Engines
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It is evident that a closed hot-air engine with a power cylinder separate from the displacer cylinder works with a lower compression ratio than one with a single-cylinder design, due to the presence of a disproportionately large clearance volume. This considerably reduces the specific power, so that engines of the type described in the last section could hardly be suitable for applications other than mechanical toys or small models where efficiency did not matter. Although a patent registered in 1845 by Franchot described an engine with piston and displacer working in the same cylinder, Stirling's original design seemed to have been entirely forgotten for the next 50 years until the first engines by Lehmann were produced on the European continent. These engines were practically identical to Stirling's first design, except that they had a horizontal cylinder, direct contact between cylinder bottom and flames, and a more compact link mechanism to reduce overall size (Eckerth 1869a, Röntgen 1888).
An early model of the Lehmann engine is shown in Fig. 3.1. A very long cylinder ABCD was used, closed at the hot end and with the power piston UU reciprocating in the open end, which was kept cool by a water-jacket LMNP. About a third of the cylinder was enclosed by the furnace. The hot gases surrounded the lower end of the cylinder and kept it hot, after which they escaped through the smoke stack RR. The airtight hollow displacer was strengthened by a riveted plate KK to an extension of which the piston rod was attached. EF was a wooden plate closing the front of the displacer, and HG a domed metal cap closing the hot end. The weight of the displacer was taken on a support roller O. Between the displacer and the cylinder there was a narrow annular space through which the working fluid passed when transferred from one end of the cylinder to the other, but apparently no regenerator was fitted. The working piston UU was constructed similarly to a bicycle pump with a dished leather washer, which allowed additional air to enter whenever the internal pressure fell below atmospheric. The driving mechanism, by which the motion of the displacer was advanced by about 65° with reference to the working piston, was very similar to that employed on the Ericsson engine.
According to Eckerth such an engine with piston diameter 0.349 m and stroke 0.244 m, gave 0.984 h.p. at 97 r/min, with a thermal efficiency of 4 per cent, which corresponds to a specific power of 1.2 h.p. per cubic foot swept volume. The mechanical efficiency was quoted as 65 per cent (Delabar 1864, Eckerth 1869b, Knoke 1899).