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HEAT ENGINES, PART III - EXTERNAL COMBUSTION ENGINESby Daniel Sweeney
External combustion engines we have already defined as those utilizing an external heat source to energize an internal working fluid. Such engines once ruled the industrial world but today many people assume that they have ceased to exist, and among alternative energy advocates, only one such design, the venerable Stirling cycle engine, excites much interest.
In truth, however, external combustion engines are still very much with us, and are used to generate most of the world’s electrical power. Where they have been eclipsed by internal combustion engines is in the area of transportation, but even there the transition came much later than many people assume and is not complete today.
The question here is should heat engines continue to play a major role in electrical generation, and are they susceptible to improvements that would regain them a place in transportation and portable power? To answer that question we must first look over the family of external combustion engines and examine their suitability to various applications.
External combustion engines can be classified either by working fluid or by mechanical design though the two are somewhat interrelated. A further division involves whether the working fluid cycle is open or closed, that is whether the working fluid must be replenished.
Principal working fluids are six in number, steam, hot air, hydrogen, helium, mercury vapor, and organic fluids with low boiling points such as Freon. Steam has been used in the overwhelming majority of engines made recently and in the past.
External combustion engines can take any of the basic forms we find in the internal combustion segment. There are piston engines, turbines, and various other kinds of rotary engines including Wankels. But there are, nonetheless, a few basic distinctions between external and internal combustion engines regarding mechanical design. First of all, there are no four stroke external combustion engines. All are two stroke because there is no need for an intake or compression stroke. Second, the aforementioned Stirling engine represents a basic mechanical design without parallel in the internal combustion category.
If we turn to applications, we find that external combustion engines are alive and well in coal fired generating plants and nuclear plants, though, strictly speaking, nuclear plants do not utilize chemical combustion to heat the working fluid. In both cases steam is normally the working fluid, and the steam itself drives a large, multi-stage turbine which itself turns a generator.
As a matter of interest, it is possible to utilize coal preparations in internal combustion turbines and the efficiency of these is higher than that of the external combustion variety. At least one company, Clean Energy Research in Sacramento, CA, is attempting to sell the industry on this approach. I wish them luck because their turbine if combined with carbon sequestration is purportedly zero emission. Still, the electrical utility industry has not been friendly to startups, and the carbon sequestration process itself would probably more than offset the gains in efficiency produced by their design.
Steam was extensively used in transportation much later than is generally assumed. Diesel powered locomotives were rarities until the late nineteen thirties and steam hung on in many locales through the fifties. In fact, steam locomotives are still built today, albeit in very small numbers, and some utilize innovative engine designs that borrow from the jet aircraft industry.
Almost all large ships used steam up through the nineteen forties including most naval vessels. World War II was largely fought with steam navies. Steam was only gradually displaced by diesel in the fifties and sixties, and steam is still used in large liquid natural gas tank ships and of course in atomic naval vessels. Elsewhere, however, diesel is absolutely dominant, though internal combustion turbines burning kerosene are used in fast naval patrol vessels and in few yachts.
Most of us have some familiarity with the steam cars of a hundred years ago and their inability to compete effectively with internal combustion, but obviously the same state of affairs did not obtain in shipping or rail transport where steam survived and flourished for another fifty years.
So why was steam eventually supplanted?
Steam engines, and all other external combustion engines, for that matter, like to run at a a steady state. That is a grave disadvantage in an automobile, though it is of less significance in trains and large ships.
Steam itself is a way of storing potential energy and the process of producing the steam is not instantaneous. If the steam in the boiler is rapidly depleted to accelerate the engine, it cannot immediately be replaced, and, in fact as one injects water into the boiler to produce a new “head of steam” the temperature plunges and the steam pressure momentarily drops. With a steam engine you can’t just “give it the gas”. The best you can do is use two or more boilers so that you always have something in reserve, but that in itself compromises efficiency. And that’s why steam cars never really had a chance. Some very ingenious attempts to address these shortcomings were made by Abner Doble, an American engineer who built steam cars of Rolls Royce quality and impressive performance in the twenties and thirties (Howard Hughes was a customer), but Doble’s very complex and expensive engines full of servo mechanisms and solenoids were never more than curiosities.
On ships and locomotives steam’s liabilities in this regard were not very significant, but there the relative inefficiency of steam compared to diesel internal combustion engines doomed the older technology. Shippers and railroads make more profit when they spend less money on fuel.
Can steam ever make a comeback in transportation? We think it’s highly unlikely unless the shipping industry, facing a severe shortage of fossil fuel, takes another look at nuclear propulsion (for various reasons hydrogen powered fuel cells on ships appear to be an enormous stretch). The safety and security problems that would be associated with marine atomic propulsion on a massive scale would be of such magnitude that we don’t see it happening, at least with conventional fission reactors.
We would point out, however, that a few companies have made serious efforts to develop modern steam engines recently. A Germany company calling itself Enginion developed what it calls a porous boiler with claimed thermal efficiency greatly exceeding the prior art and an accompanying rotary engine of undisclosed design. Enginion claims an overall efficiency exceeding 60% which we find very difficult to believe. Enginion designed and built a pilot run of steam automobile engines which were extensively tested in vehicles by Skoda of the Czech Republic but never put into production. A large German truck engine maker whose name escapes us also prototyped and tested a steam propulsion engine, but, again, no commercialization.
A very interesting rotary steam engine called the Henry engine after its inventor was briefly sold a few years ago. The inventor, since deceased, was literally residing in a lunatic asylum at the time he conceived the project but the part of his brain devoted to mechanical engineering was in top working order (he was also a chess prodigy). It’s truly an ingenious design but in the saturated stationary power market it went nowhere.
The Stirling Cycle Engine
The Stirling engine is named after its inventor, one Robert Sterling, a Scottish minister and amateur inventor (1780 – 1878). Stirling led his flock in the industrial region of Scotland and was dismayed by the number of accidents that befell the members of his congregation involving burst boilers on factory steam engines. Fortunately, he had devised an alternative while barely out of his teens.
Stirling’s engine represents extraordinarily original thinking and a radical departure from all prior heat engine design. Its simplicity is staggering. In its most basic form it consists of only two sections, a power cylinder and power piston, joined to a displacer cylinder and piston. Here’s how it works.
The whole system is self contained and gas tight. Stirling’s own engine contained trapped air but most contemporary designs use helium. An external furnace communicates with one end of the power cylinder and heats the gas within, causing it to expand and push the power piston on the power stroke. A displacer piston, generally but not always actuated by a camshaft, rises to the top of its own cylinder which communicates with the power cylinder, sucking in the heated gas. The displacer cylinder is not heated and is generally provided with some form of heat sinking. It cools the gas, causing it to contract, where upon it is pushed back into the power cylinder by the down stroke of the displacer piston and then compressed by the power piston. Then the cycle begins again.
In a sense the Stirling might be considered a kind of quasi-four stroke engine because there is only one power stroke per four movements of the two pistons, but the analogy is inexact because two pistons rather than one are involved in the process.
Stirlings have theoretically very high thermodynamic efficiency, approaching the Carnot limit, are inherently safe and robust, and are mechanically simple. They’re also dead quiet. So what’s not to like, and why aren’t they used to any extent today?
It turns out that there are all manner of not so obvious design problems that aren’t very amenable to easy solutions.
First of all, Stirlings require hydrogen or helium to operate efficiently because air is not thermally highly conductive and doesn’t transfer thermal energy effectively to the displacer cylinder. That means they have to be sealed very tightly which is very difficult if the cylinders are communicating with drive mechanisms through the cylinder walls. Kockems of Sweden, one of the few manufacturers, reportedly spent over $100 million researching seals.
All that mechanical movement not involving the power stroke ain’t good either and spells poor mechanical as opposed to thermodynamic efficiency. Moreover, the Stirling tends to be large and heavy for its output—a problem with all external combustion units—because the combusted gas and the working fluid are not one in the same, and reside in different compartments.
Another problem is the rather lengthy interval—up to several minutes—required for the engines to reach full power output. In this respect they suffer similar liabilities to steam engines but liabilities that are more intractable because a flash or fire tube boiler of the sort used to raise steam quickly can’t be used to heat air in a cylinder. Simply put, it’s difficult to put enough heating surface inside a cylinder to obtain very rapid heat transfer to the working fluid.
Many improvements on the basic design have been proposed or attempted, including free piston designs where the displacer piston is resiliently connected to power piston within the closed cavity containing the working fluid, and linear generators which utilize magnetic rather than mechanical coupling to draw power from the power piston. Much work has also been done of recuperators for transferring waste heat to the air intake of the combustion chamber.
Stirlings have continued to fascinate mechanical engineers through the nearly two centuries since the good Reverend set thoughts to paper, and countless variants have been devised. Only a few have been commercialized however. Kockems makes large units for use on naval submarines where their silence is appreciated. The company is also seeking a market in solar electrical generation where solar concentrators would be used to heat the helium without the necessity of a boiler. Solo of Germany, a major manufacturer of two cycle gasoline engines, also makes Stirlings intended for solar installations. The Kockems and Solo products are large and very expensive.
A few companies make smaller Stirlings. STM, an American startup, has been making medium output units for distributed onsite power, while WhisperTech and Victrex make very small units for off grid power and for use on boats. American Sterling makes industrial units that use waste heat from industrial processes as a thermal source.
In the past larger companies have taken an interest in the technology. Dutch Philips, the giant electronics firm, sank tens of millions of research dollars into Stirling technology in the seventies with a view to creating a new automotive power plant and also a solar generator. They abandoned the project but sold some of their intellectual property to GM which actually built several prototype vehicles, prompted by fears of fuel shortages in the wake of the seventies gas crises. GM Stirlings worked after a fashion but predictably exhibited poor acceleration and proved costly to manufacture. One could conceive of a Stirling working satisfactorily in a hybrid vehicle, but the design has thus far proved physically incapable of delivering the surge of power most drivers expect and demand. It’s really a steady state machine, right at home in a submarine or solar generating plant, but not on the highway.
One American company, Quiet Revolution Motor Company, has designed a Stirling for light aircraft. The design, which places the power and displacer pistons in a nested configuration within the same cylinder and uses hydraulic fluid both to seal the cylinder and to operate a fluid transmission is extremely ingenious, and would appear to ameliorate at least one of the Stirling’s principal problems, it’s bulk and consequent low power density. Quiet Revolution has suspended efforts to commercialize the aircraft unit, however, and is now focused on the stationary power market.
The Quiet Revolution machine appears to have great merit, but execution is everything and only field testing could reveal if it fulfills its promise. Our own guess is that an aircraft Stirling is a real long shot. The most significant innovation in light aviation today is the supercharged, two stroke, direct injection diesel, a design which offers excellent power density and fuel efficiency, and can be made very quiet and low in emissions. Several manufacturers are introducing these units at this time and we believe that they will capture a large part of that market as high octane aviation fuel becomes less and obtainable. We believe that subsequently these same designs will spread to marine and stationary power markets, anywhere where the size and weight of four stroke diesels puts them at a disadvantage.
Where Stirlings are most likely to find a home is in large solar concentrating generators which have yet to be commercialized on any scale but which are subject to pilot implementations both in the U.S. and in Spain. If this technology achieves a cost parity with wind it could play a major role in alternative energy generation, but that remains to be seen.
Our next installment will deal with other types of external combustion engines. After heat engines we aim to tackle the weighty subject of the hydrogen transition and what it might entail in terms of modifications in the electrical grid.