It therefore behoves us to make the ratio of the initial and final temperatures of the gas which does work in a gas engine as large as possible, and it is for this reason that gas engines can be made more efficient than steam engines, for in the former a momentary initial temperature of 1500° C. may be obtained by the combustion, whilst steam at 200 lbs. on the square inch is at about 1/10th of that temperature. There are practical difficulties which prevent higher initial temperatures being used, residing chiefly in the fact that at 400° C. iron is red-hot, so that any lubricant coming into contact with it is decomposed and loses its lubricating properties. Even at 300° C. most lubricating oils in contact with the air become oxidized and destroyed.

This difficulty of lubrication, by limiting the temperature, at the same time limits the efficiency, and not till some new lubricant is discovered which defies heat will there be much improvement in this direction.

Even as it is, it is necessary to cool the sides of the vessel or cylinder in which the gases expand, and in doing so we lose a great deal of heat.

Hot-air engines using ordinary air as the expansible gas have been devised from time to time, but they have not met with much success owing to their weight and the large amount of space they take up, neither are they as efficient as a good modern gas engine. We will not, therefore, study the theory of hot-air engines, but further consider the details of gas engines, whose superiority over all other heat engines we think we have sufficiently pointed out.

It seems at the present date almost impossible to conceive anything fresh in the cycle of operation of a motor using explosive gases, so numerous and varied are the already existing types. All possible combinations appear to have been considered, and even repeated, for in many recent patents old ideas have once more been brought forward which date back to the early attempts of Lebon, Barnett, Beau de Rochas. The greater number of existing types are based in principle on two or three fundamental ideas, and their improvement is rather to be found in their mechanical design than in the conception of a new cycle.

This fact enables us to classify gas engines very much more easily, because, apart from some perfection of detail, they fall naturally into several groups, which will prevent the reader from losing his way in what otherwise might be chaos. We shall therefore, in describing individual engines later on in this book, follow a systematic course, and arrange the different systems into four classes, which we shall consider in turn.

Motors using

  1. (1) coal gas.
  2. (2) carburetted gas.
  3. (3) petroleum.
  4. (4) water gases.

And in order to classify them according to the principles of their cycle of operations, irrespective of their fuel, M. Witz places them in four groups—

  1. (1) Explosion of the gases without compression.
  2. (2) Explosion of the gases with compression.
  3. (3) Combustion of the gases with compression.
  4. (4) Atmospheric motors.