Quiet renaissance of the `hot air' engine

JIM Senft doesn't look like a Rube Goldberg sort of guy. But this University of Wisconsin mathematician has built a strange-looking machine. At first blush it appears just the kind of contraption Goldberg would have appreciated. But it is the machine's, rather than Dr. Senft's, looks that are deceiving. His glass and aluminum device is actually a modern version of an antique engine that is staging a quiet comeback.

The basic idea originated with a Scottish minister named Robert Stirling in 1815. The device, known as the ``hot-air engine,'' was produced by the thousands in the late 1800s and early 1900s to pump water and drive fans and other low-power machines. It was soon displaced, however, by the internal combustion engine and electrical power.

Even in eclipse, though, the hot-air engine's unique characteristics assured it of a small but devoted band of enthusiasts. To the uninitiated, operation of the Stirling engine seems almost magical. Even scientists and engineers who have devoted a lifetime to its study admit they frequently find it baffling. ``It's a tricky little engine,'' says one of its devotees, G. (Joe) Walker, shaking his leonine head ruefully. Dr. Walker is a mathematician at the University of Calgary.

Senft's machine dramatically illustrates the point.

To start it up, the scientist first pours two cups of hot, steaming water into what looks like a metal cake pan nested in a block of Styrofoam. Next he places the engine over the pan. Finally, he places a second pan on a large cylinder at the top of the machine and fills it with ice water. After a few seconds, the machine's piston begins beating, and the flywheel begins to twirl. ``It will run for about two hours on the heat in two cups of hot water,'' the mathematician explains. It takes a temperature difference of only 12 degrees F. between the hot and cold sides of the engine to run: a thermodynamic first.

The Stirling engine is an external-combustion engine. That is, the heat is applied from the outside. The engine is totally sealed. When one area of the engine is heated and another is cooled (typically with a radiator), the gas inside expands and contracts, causing a pressure oscillation that pushes the piston up and down, producing useful work.

Senft's little engine demonstrates some of the Stirling's chief virtues: It can be used with any heat source. It is highly efficient (Stirlings have achieved 39 percent thermal efficiency, compared to the 26 to 30 percent efficiency of internal-combustion engines). It runs smoothly, with little noise.

The engine is striking in another way: Its hot side gets very hot while its other side gets quite cold. As a result, the Stirling has carved out a multimillion-dollar niche for itself as a cryogenic cooler, liquefying gases such as nitrogen, neon, and helium as well as cooling sensitive electronic components in missile guidance systems.

The 1930s saw the beginning of the Stirling renaissance. The N. V. Philips Company studied the engine as a portable power source for its radios. It transformed the massive, low-powered, cast-iron Stirling engines then in existence by building new versions out of modern materials and then filling them with pressurized helium rather than air.

These improvements sparked increasing interest. General Motors studied the engine from 1958 to 1970, during the height of the nation's concern about air pollution. Because it uses an external burner, the Stirling can operate with very little pollution. It was the energy crisis of the late 1970s, however, that caused the engine's current comeback.

``The 19th century belonged to the steam engine. The 20th century belongs to the internal-combustion engine. But next century belongs to the Stirling,'' proclaims Dr. Walker with a bit of overstatement.

While the Stirling is more fuel-efficient than conventional engines, it also costs more to build. In addition, high-performance models pose ticklish technical problems.

In the past, research on the Stirling was centered in Europe and North America. But in the last few years the Japanese and Chinese have taken an intense interest.

In 1982, the Japanese launched a $43 million, six-year program. ``It is undoubtedly expected that these engines completed will make significant contributions toward the promotion of energy conservation and oil-fuel replacement . . . ,'' summarized a report by Japanese researchers at an engineering conference last year.

And Chinese scientists from the Shanghai Marine Diesel Engine Research Institute see a bright future for the Stirling in their country because it can minimize the need for oil imports. Sunpower Inc. of Ohio has built a prototype engine, powered by burning rice hulls, designed for manufacture in Bangladesh. The engine produces up to 5 kilowatts of power.

Even its enthusiasts predict the Stirling engine won't crowd out the internal-combustion engine in the field of transportation.

In other fields, however, the Stirling could have a distinct edge. Stirling engines have been built to generate electricity from solar energy as well as from natural gas and other heat sources. General Electric is exploring using the engine as a heat pump to heat and cool buildings using waste heat from industrial processes, and a British firm, HoMach, is marketing a Stirling-powered generator for use in remote installations such as lighthouses.

More exotic applications include using Stirling engines to power space stations in orbit, artificial hearts in humans, and submarines. In the latter two cases, the power source is a molten salt heat battery, which outperform electrical batteries in these applications.

Stirlings have also successfully powered a bus, a light truck, and several autos on an experimental basis. General Motors and the Ford Motor Company have both studied the concept but eventually dropped their projects. More recently Toyota has been experimenting with it.

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