Scientific American Supplement, No. 484, April 11, 1885 | Page 6

or 1/8 to 1/2 horse, the most widely known of this kind being the "Bischoff," which is very largely used; its consumption of gas is even greater than the Lenoir, being 110 cubic feet per horse power per hour, as tested with a half-horse engine at a late exhibition of gas apparatus at Stockport.
So large a consumption of gas prevented these engines coming into extended use for engines of moderate power, and led inventors to work to obtain better results. The force generated by the explosion of a mixture of gas and air is very short lived, and if it is to be fully utilized must be used quickly; a high pressure is produced, but it very quickly disappears.
The quicker the piston moves after the maximum pressure is reached, the less will be the loss of heat to the sides of the cylinder. The flame which fills the cylinder and causes the increase of pressure rapidly loses heat, and the pressure falls.
The idea of using a free piston was proposed as a remedy; it was thought that a piston connected to a crank in the ordinary manner could not move fast enough to utilize the pressure before it was lost. Many inventors proposed to perform work upon a piston free from any direct connection with the crank or shaft of the engine; the explosion after attaining its maximum pressure expends its force in giving velocity to a piston; the velocity so acquired carries it on against atmospheric pressure until the energy is all absorbed, and a vacuum or deficit of pressure exists in the cylinder instead of an excess of pressure. The return stroke is accomplished by the atmospheric pressure, and the work is now done upon the engine shaft on the return only. The method of connecting on the return stroke while leaving the piston free on the out stroke varies, but in many engines the principle was the same.
Barsante and Matteucci, year 1857, British patent No. 1,625, describe the first engine of this kind, but Messrs. Otto and Langen were the first to successfully overcome all difficulties and make a marketable engine of it. Their patent was dated 1866, No. 434. To distinguish it from Otto's later patents, it may be called the rack and clutch engine.
The economy obtained by this engine was a great advance upon the Lenoir. According to a test by Prof. Tresca, at the Paris Exhibition of 1867, the gas consumed was 44 cubic feet per indicated horse power per hour. According to tests I have made myself in Manchester with a two horse power engine, Otto and Langen's free piston engine consumes 40 cubic feet per I.H.P. per hour. This is less than one-half of the gas used by the Hugon engine for one horse power.
The igniting arrangement is a very good modification of Barnett's lighting cock, which I have explained already, but a slide valve is used instead of a cock.
Other engines carried out the same principle in a different manner, including Gilles' engine, but they were not commercially so successful as the Otto and Langen. Mr. F.H. Wenham's engine was of this type, and was working in England, Mr. Wenham informed me, in 1866, his patent being taken out in 1864.
The great objection to this kind of engine is the irregularity and great noise in working; this was so great as to prevent engines from being made larger than three horse power. The engine, however, did good work, and was largely used from 1866 until the end of 1876, when Mr. Otto produced his famous engine, now known as "The Otto Silent Gas Engine." In this engine great economy is attained without the objectionable free piston by a method proposed first by Burnett, 1838, and also by a Frenchman, Millein, in 1861; this method is compression before ignition. Other inventors also described very clearly the advantages to be expected from compression, but none were able to make it commercially successful till Mr. Otto. To him belongs the great credit of inventing a cycle of operations capable of realizing compression in a simple manner.
Starting from the same point as inventors did to produce the free piston engine--namely, that the more quickly the explosive force is utilized, the less will be the loss, and the greater the power produced from a quantity of burning gas--it is evident that if any method can be discovered to increase the pressure upon the piston without increasing the temperature of the flame causing this pressure, then a great gain will result, and the engine will convert more of the heat given to it into work. This is exactly what is done by compression before ignition. Suppose we take a mixture of gas and air of such proportions as to cause when exploded, or rather ignited (because explosion