The central-generator or tube system of lighting with gasoline, differs from the other methods described, in the manner of generating and distributing the supply of gas to the lamps. In the hollow-wire system each lamp generates its own gas supply. With the central-generator system the gas for all of the lamps is generated and properly mixed with air in a central generator, and the finished gas distributed through tubes to the different burners and there burned in incandescent mantles. The gas as it leaves the generator requires no further mixing with air and therefore the burners are not of the Bunsen type.

Fig. 196. - Sectional view of the generator for the American hollow-wire gasoline lamp.

Central-generator gas machines are made in a number of different forms by different manufacturers, all of which are intended to perform the same work but differ in the mechanism employed. The machines are simple in construction and as in the hollow-wire system are capable of using lower grades of gasoline than can be used with the cold-process plants. The gas from a central generator may be used for all purposes for which gasoline gas is employed, either for lighting or heating. One difficulty in the use of the machine is the lack of flexibility when required for only a few lamps or varying number of lights. Although these plants are sometimes used for lighting and heating dwellings, their use is limited, for the reason that variation of the number of lights requires the generator to be regulated to suit the change in the gas supply. The plants cannot be conveniently cut down to one light. Their most general use is that of lighting churches, stores, halls, auditoriums, etc., where a variable amount of light is not demanded. Plants of this character are quite generally used for street lighting and for other outside illumination.

An efficient and simple plant of the central-generator type is shown in Fig. 197. The supply of gasoline is stored in a tank similar to that used with the hollow-wire system and placed in any convenient location. The gasoline is conducted to the generator G, through a hollow wire marked W. The generator is inclosed in a sheet-iron box, which is located at any convenient place in the building. From the generator the gas is conducted through the tube to the lamps L.

In Fig. 198 is shown a diagram of the generator, cut through the middle lengthwise, in which all of the working parts are shown in their relative positions. The reference figures designate the same parts of the generator in Figs. 197 and 198.

In the process of generation the tank is filled with gasoline and pressure applied with the air pump. The tanks described in Fig. 189 might be used to advantage with this plant but the one shown in Fig. 197 is so constructed that the larger tank is used for storage of gasoline. The gasoline is pumped directly into the smaller tank which alone is kept under pressure. The pump P is enclosed in the large tank; at any time it is desired to replenish the supply of gasoline, it is only necessary to open the valve V and pump the necessary supply into the small tank. This transfer may be done at any time without danger from escaping gasoline vapor.

The process of generating the gas is best understood by reference to Fig. 198, which shows the internal construction of the generator. The liquid gasoline is admitted at the bottom through the small pipe W, and then enters the space 4, where it is vaporized. The initial flow of gas is generated by heating the generator with an alcohol flame from the iron cup 1, which surrounds the generator. When the generator is heated the gasoline admitted to the generator is immediately vaporized; when, by turning the handle 6, the needle valve 5 opens a small orifice through which the heated gasoline vapor escapes into the tube 7, above.

Fig. 197. - Diagram of central-generator tube system of gasoline lighting.

The blast of vapor issuing from the orifice carries with it air of sufficient volume to render the gasoline vapor an explosive mixture that when burned in the mantle will be reduced to C02 gas.

When the initial heating by the alcohol flame is exhausted, sufficient gas has been generated so that part of it may be used as a sub-flame in the gas burner 9, to keep the generator heated. The gas is conducted to the burner from the main tube 11, through the pipe 12-14, as indicated by the arrows. The burner 9 surrounds the generator and the size of the flame is regulated by the valve 15, which is opened an amount sufficient to admit the necessary gas to the burner.

To start the generator, the cup 1 is filled with alcohol and ignited. The needle valve 2 is now opened by turning the hand-wheel 3, admitting gasoline into the generator chamber 4, where the vaporization of the gasoline takes place. The flame from the burning alcohol will heat the generator in about a minute. When the generator is hot, the needle valve 5 is opened slightly, by turning the lever 6, and the gasoline vapor under high pressure blows into the tube 7. As the gasoline vapor is blown into the tube 7, air is drawn in through the opening 8, as indicated by the arrows. The generator is practically a large Bunsen tube from which the mixture of gasoline vapor and air is conducted to the burners by a connecting pipe.

Gas machines operated on this principle are made to accommodate a definite number of lamps. After the lamps are lighted, the amount of gas is regulated to suit the number in use. If at any time it is desired to reduce the number of lamps in operation, the gas supply must be regulated to suit the lights left burning.

Fig. 198. - Cross-section of the generator for the tube system of gasoline lighting.

As an illustration, suppose that a plant of ten lamps had been burning and that it was desired to reduce the number to six; four of the lamps are extinguished by turning the levers C, which control the gas-cocks. The generator which had been supplying sufficient gas for ten lights will continue to produce the same amount until the lever 6 is turned to reduce the supply of gasoline to the required amount for six lamps. This is done by gradually closing the valve 5 until the lamps again burn brightly.

In small plants the least number of lamps that will work satisfactorily at one time is three. Automatic regulators are made for plants of considerable size but do not satisfactorily control the gas when the lamps are reduced below three in number. The gas from these plants may readily be used in kitchen ranges, water heaters and other domestic purposes. Individual plants for operating ranges in restaurants and hotels are in common use. The plants are subject to minor derangements that require correcting as they occur, but as soon as the mechanism and characteristic properties of the plant are known, the correction of any difficulty that may present itself is easily accomplished.