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Reinventing the engine

Nature volume 399, pages 303–305 (1999)Cite this article

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The operation of the internal-combustion engine depends on the proper phasing of the compression, ignition, expansion and exhaust stages of the power cycle. This phasing is maintained by mechanical means. Modern car engines may require as many as 24 valves (each with a rocker arm, spring, push rod, cam and so on) to produce the required phasing. The thermoacoustic Stirling heat engine, described by Backhaus and Swift on page 335 of this issue1, has a thermal efficiency (ratio of net work produced to fuel energy used) of 30%, comparable to that of a car engine (25-40%). They use carefully shaped acoustic ducts to provide the required phasing, thereby eliminating any moving parts, other than the acoustic oscillations of the gas. There are no sliding seals or machined parts requiring tight tolerances and lubrication. The working fluid is pressurized helium, an inert gas, which neither depletes stratospheric ozone nor contributes to global warming. The simplicity and efficiency of their thermoacoustic approach could provide new, environmentally friendly options to industries as diverse as transportation, natural-gas liquefaction, refrigeration and air conditioning (see Box 1, overleaf).

Over the past two decades, substantial progress has been made in thermoacoustic engine and refrigerator development. Thermoacoustic devices have flown on the space shuttle2, cooled radar electronics on a US Navy destroyer3, and liquefied 140 gallons of natural gas per day4. In these earlier devices, the gas used as the working fluid is placed in thermal contact with a solid medium, called a ‘stack’ owing to its resemblance to a pile of plates. During periodic fluctuations in gas pressure, the gas passing through the stack in an engine is heated at the proper phase in the acoustic cycle to amplify the oscillations. This is similar to the way light waves are amplified as they reverberate within the optical resonator of a laser.

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Figure 1: The workings of the thermoacoustic Stirling engine.

References

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  1. the Graduate Program in Acoustics, Pennsylvania State University, PO Box 30, State College, 16804, Pennsylvania, USA

    Steven L. Garrett

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Garrett, S. Reinventing the engine. Nature 399, 303–305 (1999). https://doi.org/10.1038/20546

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