Section 1 - Introduction

Vapor-cycle engines, primarily those using water as the working medium in a two-phase cycle and known as steam engines, have a long history as prime movers for stationary and mobile applications. Engines in the form of steam turbines generate most of the world's electricity. Piston steam-engines, on the other hand, have virtually disappeared from use but in the past they have powered railroad locomotives, automobiles, marine vessels and even aircraft. In stationary applications they have powered electric generators, pumps and machines in factories.

The classical piston steam-engine has been replaced by the internal combustion engine which is more efficient, smaller, lighter and cheaper to make. The idea of the steam engine has not died however and there have been many attempts at reviving it. The driving force for this revival, particularly since the 1970s, has been environmental concern and legislation, especially as regards automobile power units.

The steam engine is an external combustion engine and this bestows upon it one crucial advantage over internal combustion engines - namely that it produces much less pollution. A steam engine fired by a well-designed liquid or gaseous fuel furnace is able to meet current emissions regulations for automobiles with no exhaust after-gas treatment. By contrast, automobiles powered by internal combustion engines require catalytic converters and filtration systems, and manufacturers have been forced to make use of electronics and sophisticated injection systems to meet increasingly stringent emissions regulations.
Pollution control and fuel economy measures together represent almost half the cost of contemporary internal combustion engines. What is more, the environmental challenge facing manufacturers is getting more difficult to meet by the year. Some further reductions in emission levels from automobile engines are possible by even greater optimization of the combustion process through the use of variable valve timing, more sophisticated injection systems and further exhaust gas after-treatment. All these, however, add even more complexity and cost.

Engineers are now being forced to the conclusion that the internal combustion engine, in the way that it currently powers automobiles, will soon reach the end of its life. Because of this, alternatives which can more easily meet future emissions regulations are being actively developed. One of these alternatives, the diesel-electric hybrid, is already a reality. Another, the electric vehicle powered by a fuel-cell, is receiving much research funding but there is no certainty at this time that it will result in a cost effective solution to the problem.

The piston steam-engine has been promoted since the early 1970s as an alternative power source, in particular for automobiles. Prototype vehicles confirmed low exhaust emissions but it proved impossible to produce a steam engine with fuel economy levels approaching those of internal combustion engines given the technologies available thirty years ago. Getting these engines down to a comparable size and weight was also found to be a problem. So, although the research work done during the 1970s made many advances in the application of steam engines to automobiles, the industry ultimately rejected the idea of a revival.
Since then, new materials and more advanced control systems have motivated researchers to try again. Each time the steam engine has been improved by these methods, however, it has had to confront internal combustion engines which themselves have been improved in recent years and offer lower emissions and better performance. The fuel economy of internal combustion engines has improved by thirty to fifty percent since the 1970s. Advocates of steam power have therefore been faced with a moving target when they have attempted to close the gap with the internal combustion engine.

However, a breakthrough is now on the horizon as two separate trends emerge. On the one hand internal combustion engines are becoming increasingly complex and costly and are reaching the limit of their potential to meet future emissions regulations - they are in effect approaching a technological wall. On the other hand steam engines can now be made to run faster and can be smaller, lighter and more economical than in the past - paradoxically by using adaptations of the recent technologies which have been developed for the internal combustion engine. They now have the potential to become a serious competitor to the internal combustion engine in the small to medium power prime-mover sector.

In order to place the subject in a context that is quickly and easily understood, this monograph covers:

The classical steam engine (Section 2)
The steam automobile engine as it appeared in its final form (Section 3)
Emissions and the steam engine (Section 4)
Steam automobile engine research since the 1930s (Section 5)
The fuel-cell and other alternatives (Section 6)
Flash steam (Section 7)
Other Vapor Power Systems (Section 8)
A steam engine for the twenty first century (Section 9)

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