Section 3- The steam automobile engine as it appeared in its final form

The steam automobile engine as it appeared in its final form is mentioned here because it represents a peak of development for the steam engine in the small power ranges. It serves as a reminder that the steam engine was indeed made in a practical and relatively compact form before it was ousted by the internal combustion engine.

Steam automobile and even aircraft engines were made until the 1920s and 1930s by Doble (USA), Stanley (USA), Serpollet (France), and others. These used very high pressure steam at 6500kPa and at a temperature of 700ºK which was generated in a mono-tube steam generator.

Fig.1 Mono-tube steam generator

The mono-tube steam generator comprises a burner, generally fed by a liquid fuel such as kerosene, from which hot gases pass over a coiled tube (Fig 1). Pressurized water is pumped in at one end of the tube while superheated steam is admitted into the engine cylinder from the other end via a valve. There is no boiler as such, the water in the tube being progressively heated along its length as it turns to steam. Although they comprise one continuous tube, mono-tube steam generators of this type can be considered as containing three distinct sections which are not physically separated, the first contains water, the second saturated steam and the third superheated steam. Such a steam generator is much smaller than a classic steam boiler, suffers fewer thermal losses and is capable of generating steam within a minute or so. This latter point is also significant when confronting another argument used against steam engines - that they require a long time to fire-up. For a coal-fired locomotive, with a traditional boiler, this is a correct assumption. For a liquid-fuelled mono-tube steam generator it is not.

The steam automobile engine of the 1920s and 1930s nevertheless still suffered from the disadvantages of slow rotational speed, larger size, greater weight and higher first-cost than the internal combustion engine of the day. In its final production form, it could not keep up with the improvements being made to the internal combustion engine and particularly to advances in the diesel engine. It gradually disappeared from low to medium power applications. Firstly from automobiles, then from marine, railway and stationary power use. It was increasing concern with exhaust emissions from internal combustion engines (particularly in automobiles) which sparked new interest in steam for low to medium-power applications in the 1960s and 1970s.

>> Emissions and the steam engine (Section 4)

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