Gas engines are in most cases mounted on a metal bed-plate, and for small engines the weight of the machine is very often sufficient to keep it in place; it is more satisfactory, however, to bolt it firmly down to a bed of concrete or stone. Plenty of room must be allowed round about to enable cleaning and repairs to be executed; space is also necessary for turning the fly-wheel round if no self-starting mechanism is provided.

Above two or three horse-power water must be used for cooling the cylinder, mere cooling by air circulation being insufficient for carrying off the waste heat except in very small motors. It occasionally happens that water is laid on, which can be used for the water jacket, the heated water being simply allowed to run to waste. Unless the water is exceedingly cheap this arrangement is far too expensive, and it becomes necessary to erect a separate circulating system, in which the water is used over and over again. Makers provide for this purpose galvanized iron tanks, which are connected by two pipes to the water jacket. The tank being filled with water, and the engines started, circulation takes place in a manner similar to the ordinary household hot-water systems. The lower opening of the water jacket is connected to the lower pipe from the tank, and the second pipe, starting from a point near the top of the tank, is connected to the upper opening of the cooling jacket. The difference of density between hot and cold water causes the former to rise and flow back to the reservoir along the upper pipe, and cold water rushes in to take its place.

About 400 to 500 litres of cold water are required per horse-power, but it largely depends on the rate at which the tank cools; in some cases only 200 litres being found quite sufficient. The temperature of the water leaving the jacket should be about 60° C. It is more economical to let it rise to 80° C., but in this case very good lubricating oil is required for the cylinder. A tap in the lower pipe, and a thermometer placed against the upper one, will enable the attendant to adjust this temperature.

Starting and stopping.—Starting is an operation requiring a little practice. First of all the igniting burner must be lit, and time allowed for it to red-heat the ignition-tube; if electricity is used this wait of a few minutes is obviated. The lubricating arrangements are then attended to, so as to make sure of them being in satisfactory working order. If no self-starting arrangement is provided, the supply of gas to the cylinder is partly turned on, and a few quick turns of the fly-wheel by hand are sufficient to set the engine in motion. Having attained the normal speed the gas can be turned full on. If the motor refuses to start, the tap regulating the gas supply should be examined as it may be too full on; the ignition apparatus must then be inspected. Sometimes the reason of the refusal to start lies in the exhaust-valve which leaks; in this case no compression takes place. This can easily be verified by turning the fly-wheel backwards, and seeing whether the piston tends to resist the motion, which it would do if the compression was taking place properly. When running it is a mistake to flood the engine with oil; a barely sufficient quantity of good mineral oil is all that is required. If the valves require cleaning, then of course plenty of oil should be used to wash away the deposit, but they should be well wiped before starting. Many persons have become erroneously imbued with the idea that a gas engine requires a skilled mechanic to attend to it. As a matter of fact any person without previous knowledge is, with a few days’ practice, competent to take charge of a motor provided that he is intelligent and careful. Under these conditions a gas engine will last as long as a steam engine, and will be generally found much more satisfactory.

To stop the machine, the self-starting mechanism must first be allowed to operate, and then the supply of gas is shut off. After this the burner or electric ignition must be cut off, and the cooling water too, if it is not a circulating system. If the machine is to remain stopped for some considerable period, turn the fly-wheel round till the piston is in its most forward position; this will prevent dust and grit from getting into the cylinder, and scoring the inside when the engine is re-started. These are the general rules which must be obeyed in the maintenance of the gas engine. The attention they require is therefore small, and this quality has contributed not a little to their success. No stoker or engine-driver is required; any person can, with very little instruction, take charge of them. If the engine is properly erected by skilled workmen, and has been running sufficiently long to have arrived at a condition of regular lubrication, etc., a person in constant attendance can be dispensed with, and it is sufficient to look in occasionally and see that everything is all right, that there is a sufficient supply of oil, and that the cylinder is cool, because if it became overheated it might bind and destroy the interior.

Every month a complete cleaning should be undertaken, the valve mechanism and cylinder being washed with mineral oil to dissolve the deposit of carbon and tarry substances in them. Every six months the valves should be re-adjusted, and if necessary re-ground into their seatings. The time and trouble necessary for these periodical cleanings will be amply repaid by the satisfactory working of the engine; nothing is more annoying than a breakdown due to accumulation of dirt, and to the continual postponement of the cleaning out.

Maintenance of oil engines.—These machines require the same careful treatment as gas engines, but they have in addition a carburator or a vaporizer which require special attention, and render the cylinder particularly liable to become fouled by the heavier products of the vaporized oil. We have already described the principal systems of vaporizers and carburators, and we shall only return to them to state, that for efficient working they should be cleaned out once a week. At the very outside not more than a month should elapse between the removals of incrustations and deposits of carbon. The lamps used for supplying heat to the vaporizer should also be attended to from time to time, and occasionally thoroughly cleaned out.

Ignition of the explosive mixture.—As we have already seen, several different methods exist of setting fire to the explosive gases, and each of them has its advantages and drawbacks. Ignition by an incandescent tube of iron or porcelain seems to be the most simple, and works as well as any other system; the sliding valve containing the igniting jet is somewhat more complicated.

Electric ignition, if effected by a magneto generator driven by the engine, is certain in its operation, but it is awkward at starting. For petroleum and carburetted air motors batteries both primary and secondary are better, especially when the motors are used for propelling vehicles.

Looked at from the economy point of view, it is very hard to decide which is the best system of ignition: red-hot tubes require gas, accumulators require electric energy to be put into them, magneto machines are expensive and use up power, in fact, they are all about the same as far as cost is concerned. In France electric ignition is in much greater vogue than in this country, where hot tubes reign supreme. The electric system requires a few words concerning batteries and induction coil.

If primary batteries are used the chromic acid type is the best. These are manufactured in a large number of varieties; some of the French makers, TrouvÉ, GuÉrot, Radiguet, and others, have paid special attention to their application to gas and petroleum engines. The induction coil should have a rather thicker secondary winding than usual; it is necessary to have as hot a spark as possible to ignite the gas, and this is not obtained if the spark is of exaggerated length.

The spark-gap also requires occasional attending to. It consists usually of a porcelain rod with two points between which the sparks pass, either intermittently or governed by an automatic switch, or else continuously. In either case the points often become coated with a deposit of carbon, which decreases their efficiency. They must therefore be wiped clean about once a week.

Maintenance of engines using poor gas.—These engines differ very little from the ordinary coal-gas engine. The arrangement of valves which will do for the one is also suitable for the other. We shall therefore only give a few hints on the management of the gas-producing plants which forms their only point of difference; to do this we shall consider a “Simplex” plant.

The firm of Matter et Cie. have received many letters from other firms to whom they have supplied their apparatus, replacing existing steam engines, in which they state that the whole management and maintenance of their new source of power is much more simple than the old. This is not to be wondered at, because the whole plant is perfectly automatic. There are no pressure or water gauges to attend to, because no more gas is ever produced than is absolutely wanted. A steam boiler may require to be stoked once every quarter of an hour, whilst a poor gas generator only requires charging about once every six hours, according to demand for power. The Simplex motor itself requires very little attention, and all bearing surfaces are made as large as possible to avoid the risk of heating.

Fig. 51 shows a complete plant in section. A is the generator filled with coke or anthracite; the opening of the hopper, N, and the door, D, of the hearth are closed. Air arrives from the centrifugal pump through a stop-valve, I, and water through W. The air passes through the mass of incandescent carbon forming carbonic acid gas, which is again reduced by the excess of carbon to carbon monoxide. The water becomes vaporized, and is dissociated by the action of the heated carbon forming hydrogen and oxygen, which combines with the carbon forming more carbon monoxide. The gas produced consists therefore of carbon monoxide, hydrogen, nitrogen, and a small proportion of hydrocarbons and carbonic acid gas.

Passing by the pipe S to the base of the washing-tower the gases meet in their ascent water from the pipe Z, which cleanses them of all impurities, after which they pass by the pipe Y to the gasometer, and from there to the engine. We close this description of gas and oil engines, hoping that we have said enough to put before electrical engineers who are thinking of making use of this economic form of motive power, the exact state of affairs, what results have been obtained, and especially the considerations which govern the choice of an engine.

We further hope that this systematic examination of all engines which have received the sanction of practical experience, will prove of some service to those who wish to become better acquainted with the numerous existing types, and to understand their relative advantages and faults, in order that they may select a gas engine which is suitable to the particular requirements of each individual case.