When giving consideration to a form of power for small to medium-scale applications other than the internal combustion engine, several possible candidates present themselves apart from the steam engine.
Electrically powered automobiles which generate electricity from on-board fuel-cells are currently receiving much attention. They are seen by some as the ultimate solution to the emissions problem. They do represent a much bigger technological leap forward than any other proposed solution to the problems besetting the internal combustion engine. Hydrogen- powered fuel-cells already power city buses in an experimental program in Germany. These require large pressurized hydrogen cylinders mounted on the roof. It takes a big leap of imagination to see hydrogen fuel cells being applied on a large scale to passenger cars or used in other prime-mover applications because of space constraints, safety concerns and the difficulties of providing a hydrogen supply infrastructure. It should also be noted that while fuel-cells working directly from hydrogen solve the emissions problem from the vehicle itself, the hydrogen production process may cause its own emissions thereby simply transferring the problem somewhere else. Most hydrogen is currently produced by steam reformation of natural gas – a process which makes hydrogen about sixteen times as expensive as gasoline fuel. New technologies are being worked on for hydrogen production but the cost of the resulting fuel as well as the environmental effects of producing it are, as yet, unknowns.
A more likely way forward for fuel-cell powered automobiles is to generate hydrogen by on-board conversion from methanol or gasoline and research in this direction is being conducted all over the world on a massive scale. The problems of building a methanol supply structure are much lower than building one for hydrogen and prototype electric vehicles powered by fuel cells working on this principle have been demonstrated. It is, however, still far from certain (in 2004) that a practical system meeting the overall requirements of vehicle power plants – competitive first-cost, compactness, good power-density and very low emissions can be met in the foreseeable future. It is entirely possible that when practical production versions of cars powered by electric motors and fuel cells become a reality their energy conversion efficiencies will struggle to compete with either internal or external combustion engines.
Another idea under investigation is to power internal combustion engines by hydrogen and so remove the need for a hydrocarbon fuel which is the source of the pollution problem. The sole emission which result from burning a hydrogen/air mixture is water vapor. An experimental filling station providing pressurized hydrogen to a small number of vehicles is operating in Reykjavik. Iceland is rich in cheap geothermal electric power and the production of hydrogen by electrolysis in that country may be a realistic economical and environmental proposition. The circumstances in Iceland are, however, exceptional and most hydrogen fuel would have to made by processes which are expensive or yet to be developed, as already explained.
The main point of the fuel-cell research programs as well as the program to power internal combustion engines directly by hydrogen is to eliminate fossil-fuel burning and through it to stem the production of carbon dioxide ‘greenhouse’ gas. Here the steam engine has an answer too. It can run on hydrogen just as easily as on fossil fuels and produce exhaust comprising of nothing more than water vapor. If there is ultimately a ‘hydrogen economy’ to replace the ‘hydrocarbon’ economy we now have then steam engines could be well placed to serve it.
The steam engine is, of course, not the only prime mover which utilizes ‘external’ combustion and benefits from this clean burning process. Both the gas turbine and the Stirling cycle engine share this same feature. The gas turbine is a highly developed machine and it is well proven that it has a clean exhaust without supplementary emissions-control devices. It is very difficult, however, to recapture effective quantities of waste-heat from the exhaust gas in this type of engine and because of this its fuel consumption is high compared to internal combustion engines. For this reason and also because of high manufacturing costs it is unlikely that the gas turbine will ever compete with the majority of small to medium power applications and will remain confined to specialized service in aircraft power-plants and stand-by electricity generators, for example. The Stirling engine has also attracted interest from time to time and Philips (Netherlands) built a modern prototype unit in the nineteen seventies. It proved impossible, however, to build to a comparable size and weight as an equivalent power internal combustion engine and the Stirling engine remains largely a curiosity.
It will be seen from this review of alternatives that the steam engine could still be a candidate for a potential replacement of the internal combustion engine.
