Gas and Petroleum Engines

by H. de Graffigny

Forest motor (Figs. 18, 19, 20).—We have already described the earlier attempts of M. Forest to produce a practical gas engine. The idea embodied in the motor depicted in Fig. 20, if not particularly advantageous, is none the less highly original. The ends of the single cylinder are open, and within it are two pistons, between which the explosive mixture is introduced. The explosion drives these pistons out in different directions, but by means of a suitable mechanism they are each connected to one of two cranks on the same shaft. The whole forms a very neat and compact arrangement. In another type by the same inventor (Figs. 18, 19), also working on the Otto cycle, the cylinders are vertical, there being one, two, or four placed side by side. In the last case two explosions per revolution are obtained, ensuring a very constant torque throughout. M. Forest has, however, particularly applied himself in collaboration with M. Gallice to designing petroleum engines for small boats. These two inventors have conceived a highly ingenious arrangement for reversing the direction of rotation of the engine, which is absolutely necessary in the propulsion of boats. All the cams which actuate the valves are grouped together on a light shaft, and by a simple twisting of this shaft from one end by a handle the engine is reversed. The engines are self-starting, and are so especially suitable for the propulsion of boats that they have been adopted by the French navy, who at the present date have a number of small vessels propelled by this motive power.

Fig. 18. De Forest Gas Engines. Fig. 19.

Fig. 20.—De Forest Double Piston Motor.

Cuinat gas engines.—These engines are constructed in four types. The A type is vertical with the cylinder above and fly-wheel and shaft below. This is a more stable arrangement than placing them in the reversed order, as is more frequently done. For small engines the vertical type is undoubtedly the best, space, or rather lack of space, being very often an important consideration to the purchaser. The B type is similar to the previous one, except that two fly-wheels are provided so as to make it suitable for small electric light installations. The C type is horizontal, having all the valves placed vertically, which plan seems to work better for engines of power greater than 10 horse-power. The D type is also horizontal, but has two fly-wheels, having been specially designed for electric lighting purposes. In order to take up as little space as possible the dynamo is situated underneath the projecting cylinder, both the engine and dynamo being bolted down on to a special bed-plate. This arrangement is more stable than placing them on separate foundations. In the petroleum engines an electric spark is used to ignite the gases, and in the gas engines both electricity and a gas flame.

It is best not to use animal or vegetable oils for lubricating the inside of the cylinder, because they decompose, forming fatty acids which have a corrosive action. Besides this, when they have fulfilled their function of lubrication they settle down to a thick paste, which has a most injurious effect on the working of the engine. It is best, therefore, to use nothing but perfectly pure mineral oils and to avoid all others. This does not only apply to the type of engine we have just been describing, but to all gas engines. The Cuinat gas and petroleum motors do not mix a portion of their exhausted gases with the fresh charge as is frequently done, but completely sweep away the products of the explosion before admitting a new charge. The result is, that combustion is more complete but at the same time rather more violent. An examination of an indicator diagram taken from one of these engines shows that the combustion takes place at once as an explosion, and that the final expanded pressure is as low as it is possible to get it. These being the conditions necessary for a high efficiency, it is needless to state that the consumption of fuel in these engines is as low as in any other engine.

Noël motor.—This type, constructed at Provins, has the advantage of being exceedingly simple. The entries to the cylinder are controlled by spring-valves, and the gases are ignited by electricity. From the ¼ up to the 2 horse-power size the cylinder is not provided with a water jacket, and the general arrangement is either vertical or horizontal. These engines work equally well with coal gas or carburetted air; in the latter case the carburator is placed inside the cast-iron frame of the motor. The guaranteed consumption of fuel is about 900 litres of gas or 500 grammes of gasoline per horse-power hour, which is quite satisfactory.

Tenting motor.—This is a horizontal motor with the cylinder cooled atmospherically, and the gases ignited by electricity. The governor acts upon the exhaust valve; the products of the explosion remain in the cylinder if the speed is too great, and then the admission valve, which is automatic, no longer rises to admit a fresh charge until the speed has once more fallen to the normal. This little motor is one of the most practical small-power engines existing, partly because of the great simplicity of the mechanism. It works well with carburetted air, and the vertical type has been successfully applied to the propulsion of small pleasure-boats.

Atkinson motor (Figs. 21, 22).—This apparatus, constructed by the British Gas Engine Company, was until recently the most efficient heat engine in existence, its indicated efficiency being 22·8%. Mr. Atkinson, the inventor, has arrived at this result by making the gases burn gradually and by shortening the compression stroke. The discharge of the residual gases is complete, this being directly opposite to Dr. Otto’s procedure, for he diluted his explosive mixture largely with the exhausted gases. In order, however, to make the piston execute two strokes relative to the cylinder of different lengths, the inventor has had to devise a rather complicated mechanism (Fig. 21). The result, however, quite neutralizes this slight disadvantage.