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Heat Engines


For many thousands of years motive power was provided by humans and animals. During the early middle ages (not always as dark as has been made out as lots of progress in the arts and sciences took place) proliferation of mills and forges  encouraged conversion  of the energy of flowing water and wind to rotary motion. A quantum jump occurred in the early eighteenth century, when Newcomen built the first machine to convert heat energy into rotary motion. This first usable heat engine, using steam, was put to pumping water from the tin mines of Cornwall, around 1726.

It was terribly inefficient. Steam was injected under a piston to raise it and then rapidly cooled by injecting water. A partial vacuum was thus created and the piston forced down by atmospheric pressure on the top. The steam helped the weight of the pump to push it up and the process began again. A small boy originally turned the taps to let the steam in and out, but according to legend he eventually replaced himself with an automatic valve and wandered of to play.

Some fifty years later, James Watt had the idea of doing away with this successive heating and cooling, which was obviously inefficient, by making steam the driving force, which  once expanded while pushing down a piston was condensed and reheated. This produced a big jump in efficiency: the engine could turn faster, more than twenty strokes per minute rather than just five or so, and with much more power per stroke. It was still too heavy to be mobile and it was left to Trevethick, a somewhat erratic but brilliant inventor, to develop a light high pressure machine which could move itself around. Incidentally, the French inventor Cugnot should not be forgotten. He built a machine as early as 1770 which did move itself successfully. Unfortunately, by demolishing a wall he also invented the first motor vehicle road accident. His machine can still be seen in Paris.

Two development branches for mobile steam power were thus opened in the early 19th century: one led to the railway locomotive; the other to the lighter steam road vehicle . The former essentially won, and steam stayed runner-up to the horse far as road vehicles were concerned, although it did become quite popular for trucks right up to the second world war. Of course, it remained popular for ships for many years and is still the energy source for turbines of electric power plants and some specialised vessels. A steam powered aircraft even flew in the early thirties.

The gas engine, invented by Lenoir in 1860, ushered in the possibility of combustion inside the engine itself rather than in a separate boiler, enabling it to convert a greater proportion of heat energy into motive power. However, since the gas was ignited at atmospheric pressure, its efficiency was rather low. Nevertheless by dispensing with the boiler and its enormous quantities of water and coal and using instead town gas made from coke, piped in from the mains, it saved a lot of space in factories, as well as being clean and completely silent. It was not however, mobile, being heavy and attached to its energy source.

With the formulation of the four stroke cycle (compression of an air/fuel mixture, combustion, expansion and exhaust), and the exploitation of the high specific energy and portability of crude oil derivatives, the first internal combustion engines attached to hastily adapted carriages started puttering around towards 1884. Benz is generally considered to be the key figure. However, as usually happens at key turning points, many others were also successful. From then on it was a question of finding better ways of vaporizing fuel, mixing it with air, getting it into the engine at the right time, and finally igniting it. For many years afterwards, with tyres as a welcome change of subject, carburettors and sparks ( fat or thin; present or absent) were the stuff of motorist conversations as well as the cause of long and grubby hours spent by the roadside.

The internal combustion engine is now mature, reliable and more and more economical. However, its dependence on oil must lead to its eventual decline in the developed countries, slowed perhaps by alternative energy sources, such as hydrogen. Electricity, with hybrids as a transitional form, will surely replace it for road vehicles. Electric motors do not have to convert recriprocal to rotary motion, a rather inefficent and harsh process from a mechanical standpoint. Not only non-polluting, they are quieter, smoother, and almost maintenance free, since they dispense with those costly and friction-generating sub-systems such as transmissions,  fuel pumps, injectors and emission controls, and very powerful brakes that come with the internal combustion engine.

It can be expected that poorer countries will depend on it for many years longer. Conditions there force the use of a simple well-tried technology. For this reason the burden of slowing global warming must fall on the developed world who caused it. As the internal combustion engine becomes obsolete in developed countries, the volume of used vehicles exported from them could increase. Unfortunately, given the harsh operating conditions in rural areas, the growing complexity of the electronics destined to reduce petrol consumption and emissions, particularly in hybrids, could present a serious maintenance problem. The simpler and smaller petrol or diesel vehicles of China and India could provide a viable alternative as well as the use of small electric motors in urban areas.