2 Feb 2010

Theory and practice are two different things - part 1

The writer C P Snow famously said that an unbridgeable gap had developed between the arts and the sciences, and the mark of the scientist was that he (and it usually was a "he" when he made his comment, in the late 1950s) could explain the Second Law of Thermodynamics. This was part of the physics syllabus and was usually and confusingly explained with diagrams like the one on the left. The simplest way of putting it is to say that heat does not flow of its own accord from a cold object to a hot one, which everyone knows.

When the Law is applied to applied to engines which work through the expansion of a hot gas, it turns out that they are more efficient if the gas starts off as hot as possible and is cooled to the lowest possible temperature. Steam locomotive engineers knew about this early on but it was not always easy to put into practice. The boiling point of water increases with pressure and progress consisted of running boilers at higher pressures - which was dependent on the state of contemporary metallurgy. Superheating, which involves sending the steam back through extra banks of tubes into the fire, led to a dramatic increase in efficiency but did not become widespread until quite late in the history of the steam locomotive. The delay was due to the lack of suitable lubricants which were not carbonised by the hot steam. Compounding was another device meant to improve efficiency, the steam being allowed to expand in successive stages in two or more sets of cylinders or turbines. Condensing the steam back to water is another way to increase thermal efficiency, and although it was usual on ships and always applied in power stations, it was only used on steam locomotives in special circumstances.

But these efficiency improvements came at a price, since they added complexity and cost, and could come at the expense of reliability. It was also essential that the engineer knew what he was doing, which was not always the case.

Steam engines are not obsolete, since most electricity is generated using steam power, but what is practicable in a fixed power station is not always so in a mobile unit, and the overall efficiency of the average steam locomotive was less than 10%, compared to perhaps 50% in a power station.

Internal combustion arrives
Internal combustion engines are inherently more efficient, since the hot gas is produced in the cylinders or turbines themselves by burning a mixture of fuel and air. In most circumstances the hot gas is hotter than the hottest steam can be, and the machine is bound to be more efficient, in accordance with the theory. Stationary engines using ordinary town gas had become quite common at the end of the nineteenth century, and then liquid hydrocarbon fuels made it possible to build small and power engines capable of driving road and rail vehicles and aircraft.

However, because the power of an internal combustion engine depends on the speed at which it is running, and great forces must be exerted to start a load as heavy as a railway train, complex and expensive electrical or mechanical transmission systems are needed. Steam traction remained dominant on railways worldwide until the 1960s, despite its thermal inefficiency and the added disadvantage that the coal fuel normally used was dirty and difficult to handle.

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