'Reprinted from the January 10, 1970, issue of Business Week by special permission. Copyrighted (c) 1970 by McGraw-Hill, Inc.'
We thank Mr. Paul B. Finney, Managing Editor for kind permission to use this article in our Gas Engine Magazine. Our appreciation also to Mr. Denis McCormack, 404 West Timonium Road, Timonium, Baltimore Co., Maryland 21093, for his efforts in acquiring this article and permission to use same.
One of these days, a Dutch engineer named R. J. Meijer expects to step out of his lab in Eindhoven, the Netherlands, and tool off in a revolutionary new car powered by a silent, vibration-less, pollution-free engine that will run on anything from alcohol to salad oil.
That day, he admits, is still down the pike. But Meijer's employer, the giant Dutch electronics company Philips' Lamp Works, has made a 30-year wager that the Stirling cycle engine Meijer has been puttering with for 22 years will eventually do to the diesel what the gasoline engine did to the horse and buggy. By 1975, backers predict, the Stirling will be operating in buses, boats, ships, submarines, trucks, and even locomotives. A nuclear-fueled Stirling for space travel is not too far beyond that, nor is Meijer's hot rod. In fact, he already has a cabin cruiser outfitted with a Stirling.
The idea Philips is betting on is a thermodynamic phenomenon discovered over 140 years ago by a Scottish clergyman named Robert Stirling. His own 'air engine' proved too bulky and inefficient compared with steam and internal combustion engines, and did not gain widespread use. Like the internal combustion engine, the Stirling relies on the principle that gas expands when heated, and this expansion can do work. However, instead of drawing in gas with air and exploding it inside a cylinder to drive a piston, the Stirling engine is completely closed. Gas--helium or hydrogen in Philips' versions-is heated in a chamber surrounding the cylinder, and never mixes with the atmosphere. The heated gas drives a piston in a leak-proof cycle.
The result is an engine free of rattling valves and noisy explosions. Fuel is not consumed inside the cylinder, so many of the complications of reducing noxious pollutants are eliminated.
It took two major innovations by Meijer himself, however, as well as the evolution of better heat-conducting and heat-resistant materials not available in Stirling's day, before an efficient Stirling engine could be developed, even in prototype. The problem was to keep gas from escaping from the cylinder. Working almost sin-glehandedly, Meijer developed the so-called rhombic drive, a Rube Goldberg configuration of gears, counterweights, rods, and shafts, which limits the motion of the Stirling's piston rod to a simple up-and-down motion. This cut down the area where gas could escape most easily.
Then, a few years ago, Meijer succeeded in making the cylinder leak-proof when he came up with the roll-sock seal, a polyurethane diaphragm that fits snugly around the piston rod and operates like the rolling of a stocking up and down. The result is a much more durable engine.
'Our goal is an engine that will run 10,000 hours without maintenance,' says Meijer. One 10-hp prototype has already logged more than a month and a half of continuous operation, and is still going strong. That is equal to an engine driving a car steadily at 100 mph for 120,000 mi. This engine, like two others Philips has made, has only one cylinder. But the first of 11 four-cylinder, 200 hp prototypes, suitable for use in buses, should be operating this month. These will be the same size, weight, and have about the same power-to-weight ratio as comparable diesels, which are most vulnerable to the challenge which the Stirling presents.
'The gas turbine will probably replace the gasoline engine,' says Stig Carlqvist, president of United Stirling Co., a Philips' licensee in Malmo, Sweden, 'and the Stirling will probably replace the diesel. At least that's how it looks to us now.'
Philips' technicians now are also putting together the first versions of four-cylinder engines suitable for passenger cars. In these 'double-acting' engines, gas is interchanged from one cylinder to the next, instead of having a separate closed system for each cylinder. This eliminates several working parts and permits a lighter, more compact design. The prototype, which may be in operation within two months, will be capable of generating 60 hp and will weigh about 250 lb., roughly the same as an internal combustion engine of similar horsepower.
Carlqvist, whose United Stirling was formed by three Swedish manufacturing companies, thinks the initial market for the Stirling will be for stationary units in the range of 10 to 200 hp. These would be ideal in places where it is vital that noise, vibration, and pollution be suppressed-in mines, for example. United Stirling, formed in July, 1968, expects to sink $8-million into research over the next four years.
Carlqvist, like Philips, has his eye on the bus market. 'For that,' he says, 'you have to have customers. You have to start someplace.'
After buses, ships are likely candidates for Stirlings, says Otto Syassen, head of another outfit licensed by Philips and composed of West Germany's two competing diesel makers, Maschinenfabrik Augsburg-N?rnberg and Motorenwerke Mannheim. Syassen's group is tinkering with five 30-hp Stirling engines of its own design and expects to have them in operation any day now. These will be single-cylinder units, but Syassen is sure he can turn out a mul-ticylinder version in a year or so.
In the U. S., General Motors Corp. has been toying with the Stirling engine since 1958, when it signed an agreement with Philips to cooperate on research. While GM gives the Stirling high marks for efficiency, output, quiet operation and clean exhaust, the company does not think it will ever replace the internal combustion engine in cars. 'The Stirling is still bulky, heavy, complex, and expensive,' a spokesman told a recent Congressional subcommittee. GM feels that the potential for the engine lies in boats, engine-generator sets, submarines, torpedoes, and other quiet, lightweight power-plant uses.
The Europeans are far more optimistic. Carlqvist says it is technically possible to put a Stirling engine in a bus in as little as two years, but it would probably cost up to twice as much as the diesel, largely because of higher costs for special heat-resistant materials. Philips uses a specialty steel in the engines cooling tubes that is about five times as expensive as ordinary steel.
Meijer thinks concern over noise and air pollution will muffle the cost difference. He is also confident about shrinking the radiator, which is about two and a half times the size of a standard radiator. Meijer explains that the Stirling requires a bigger radiator because the engine is more efficient, and less heat is lost in the exhaust than in the internal combustion engine.
This need for a jumbo radiator is a major drawback in fitting the Stirling into trucks, say some critics. But Meijer insists radiator space is not a problem: 'There's no reason to believe designers can't design around a bigger cooling system.'
Perhaps the biggest problems still facing the Stirling, however, are starting it and getting it to respond faster. The engine must be preheated to about 1,300 F to start up, which takes at least 15 seconds even with a quick-firing kit. And since power is adjusted by varying gas pressure or temperature, not simply fuel, critics say response is naturally slower. 'Philips is talking in tenths of a second,' says a competitor. 'The ordinary automobile engine responds in hundredths.'
Despite these drawbacks, the Stirling is getting a stronger push than ever. It is as quiet as an electric motor, and the only contaminants it releases come from the external heating chamber, where it is far easier to reduce noxious elements. In the internal combustion engine, firing takes place with the cylinder, and tampering with the fuel-air mixture to trim pollutants can impair efficiency.
Meijer is confident that the Stirling engine right now can beat the most stringent U.S. pollution standards. A single-cylinder engine tested recently released only 0.1 milligram of carbon monoxide per second per hp, compared with 2 mg for the gas turbine and 0.2 for the diesel. Under greater load, the diesel spews out 5 mg per second, while the Stirling puts out only 0.3.
The Stirling also releases only traces of nitrogen oxides, the peskiest of pollutants. And Meijer is now patenting a technique which will cut these emissions to .025 mg per second per hp, well below existing or even proposed standards.
Indeed, since the engine can operate on alcohol, crude oil, butane, natural gas, gasoline, and salad oil, pollution can be reduced merely by the selection of fuel. It is even possible to use heat sources that do not involve combustion.
Besides a nuclear-heated Stirling that Philips foresees, technicians have produced engines that get their heat from electric cables immersed in a heat reservoir of lithium fluoride. A heat pipe filled with liquid sodium transfers the heat from the storage tank to the engine.
It is ironic that Philips is now trying to use electricity to drive the Stirling: The company first developed the engine to generate electricity. Some 30 years ago, it stumbled on the Stirling while hunting for a cheaper, multifuel generating source to power radios where electricity was not available. That pursuit ended in the early 1950s, when the transistor made it more practical to use batteries.
How the Stirling engine kicks over
Hot gas is what makes the Stirling engine work. A source of constant heat warms the gas, such as hydrogen or helium, that is kept in a closed cylinder. The hot gas expands and forces the displacer down. The displacer squeezes cooler gas above the piston, which then forces the piston down. The displacer then moves back to its original position and the piston follows it, driving cool gas through the heat regenerator and the heater, and the cycle starts over.
The rhombic drive in the crankcase translates the piston's motion into a circular movement, so wheels can be turned.
The Stirling does not emit the pollutants turned out by the standard engine. But it needs a huge radiator to keep cool.
The project staff was pared down to Meijer and one assistant, and little hope was held out for a practical Stirling--until Meijer came up with his rhombic drive and the roll-sock seal. These boosted the engine's chances to make the Stirling a real threat to the internal combustion engine in the years ahead.
Philips' Lamp of Holland expects its Stirling engine to rival diesels by 1975
Europeans see Stirlings for autos as well as buses, but GM is skeptical