Automobile Fire-engines.
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A Description of the Pumps Employed on the Principal German Engines, Compared to the Gwynne, as Fitted to a Dennis.
By an Engineer Contributor.
At various times in the course of the present world war, we of Great Britain have been strongly exhorted by members of the Government to capture the trade of the Central Empires of Europe, wherever and whenever possible.
A long experience of the automobile industry induces me to believe that the automobile fire-engine business is one in which we as a nation of engineers can and should excel.
It appears that this branch of engineering is one which offers ample scope for development, and from a commercial point of view-presents great possibilities, particularly in relation to business with Russia, Southern Europe, and the States of South America. The trade of these countries has in the past been almost monopolized by Germany. In what follows I have attempted to outline the various fire-fighting machines constructed and produced in Germany in comparison with the best of British manufacture, as a guide to the lines of future development.
German concerns subsidizedlby their Government have for some years been developing and improving the motor fire-engine with characteristic thoroughness and organization, while in this country the development has been left to the initiative of quite a few engineers and manufacturers.
To their credit be it said, however, that their productions compare favourably with those of Germany in regard to efficiency in design, construction and service. In all probability that very British machine the Dennis-Gwynne represents the finest automobile fireengine yet produced in any country.
To produce a successful fire-engine, it is essential that the engineering skill and knowledge of the pump designer should be added to those of the chassis builder. Only by a combination of these two can the best result be obtained.
For the automobile maker to attempt to build a pump to replace the results of years of experiment and research on the part of the expert pump manufacturers would be as futile as the efforts of the pump specialists to produce an automobile. As the automobile has superseded the horse for most commercial purposes, so will it ultimately do the same in the way of transporting fire-fighting apparatus; probably to a greater extent, if only by reason of its speed and consequent elimination of time, the great factor in fighting fire. • It follows, therefore, that a successful fire-engine must possess speed, as obviously the finest pump made is of no value to any firedepartment if it arrives on the scene of action long past the time when it should have been combating the flames. It is, then, fairly. apparent that the manufacturer who can produce a fire-fighting .machine evolved from the combined efforts of the automobile engineer and the pump specialist, at a moderate price, has a great opportunity within his grasp. To produce such a machine successfully, a. thorough underctanding of the attendant problems is necessary, not merely the ability to manufacture the article, but also knowledge of the policy to pursue to secure trials with prospects of adoption should the machine perform satisfactorily, while an accurate conception of the facilities at hand of various fire departments is essential.
A question of equipment which is still unanswered and round which controversy has raged, and will continue to rage, is that concerning the type of pump to use.
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German makers also are still divided en the same , question, as to the relative merits of centrifugal' pumps, reciprocating pumps and rotary-piston pumps, but they have managed to develop each of these vane: ties to such a point that those fire departments which ' are friendly to the automobile movement are willing to accept the idea that fire-fighting tactics should be modified to agree with the capacity and range of the best and most modern machine. It remains for British manufacturers to meet thp situation in a similar manner.
The automobile fire-engine question resolves itself into two parts entirely independent of each other. One part of the matter deals with the transportation of the apparatus, and the other part has to do with its operation on the scene of activities ; the chief interest centres on the fire-engine pump.
The Fire-engine Pump.
Formerly the fire-engine pump was invariably a slowstroke, stream pump having reciprocating pistons. Of recent years, however, the tendency has been toward a general adoption of centrifugal or turbine pumps, where their operation has been through the medium of high-speed internal-combustion engines. Each system, of course, has its attendant defects and advantages. Piston ,pumps have the advantage that they are adapted for raising water from a low level where no hydrant pressure is available, and the disadvantage that the intermittent pressure stroke of reciprocating pistons subjects the hose tc .a constant oscillating action in contact with the ground, which causes the hose to wear out prematurely, and can only be partly obviated by the installation of au air chamber to act as an equalizer. This type of pump also requires provision against sudden changes in the water pressure, and against the wear to which pistons are exposed when sand is taken in with the water.
Regarding. the centrifugal pump, the weakest point has !been found in the special provisions which have
to be made in order to make it act as a suction pump; so as to draw from a low level where no hydrant presa sure is available at the commencement of operations. If the supply hose leaked or joints were not screwed tight, dependence had to be placed on these previsions not only at the start but after every intermission in the play of the stream.
The continued operation of the pump was not in itself sufficient to prevent the column of water in the supply hose from breaking under these conditions and the flow of water from the nozzle was not kept constant.
The provisions adopted to meet this limitation of the centrifugal pump consist usually in fitting into the water circuit a small positive pump capable of
acting i . either on water or air, or n equipping the vehicle with an auxiliary water tank holding anything up to 100 gallons, from which the supply pipe can be filled at. the commencement of operations or after interruptions.
Taken altogether these disadvantages do not amount to much, while the great simplicity of the centrifugal pump and the wide range of fire conditions which it can meet by mere throttle control, so far as the reach and volume of the stream thrown by it are concerned, have acted as powerful incentives to designers of this type of pump; and to automobile spe
cialists in its adoption.
Hence a manufacturer of automobiles may take up the production of fire-engines without interfering with the system of routine of his other productions by fitting centrifugal pumps and thereby at once attaining simplicity, cheapness and efficiency.
A Pump which is Popular in Berlin.
In Germany the high-pressure centrifugal pump has been greatly developed, and in Fig. 1 are shown views of a rotary pump known as the Rundlaufpumpe, for which the Berlin Fire Service has a preference.
• It is in reality a piston pump with most of the advantages and disadvantages of this type.
Its special merit lies in the possibility of using. it at high speeds and 'consequently being able to drive it from a high-speed petrol engine without the use of a cumbersome reduction gear.
It consists of rotary elements operating in a housing and its action depends upon alternating enlargement and reduction of the spaces formed between the rotary and the fixed parts. The pump consists of a shaft (A) mounted in the extended bearing-members noticed to right and left in the longitudinal views This shaft carries a cylindrical body (B) in which are cut four radial slots, lengthwise of the body, forming pockets, in each of which is lodged a hard rectangular rubber block subject to axial displacement. When the shaft i£3 turned displacement is effected, because the ends of the blocks are guided by contact with the interior end walls (D) of the large bearing members through which the shaft passes, these walls being cut on an oblique curve, and at all corresponding points placed just so far apart that the distance beween them is equal to the length of each of the blocks.
The spaces (E) are so disposed that those leading to the exit communicate at the right periods with the water which the rotation of the rubber blocks bring under pressure, while those connecting with the entrance are brought into communication With water under suction and practically continuous flow from the suction channels is thus effected.
By having four movable plates, eight working spaces are produced, and this arrangement, in conjunction with high speed, gives a degree of continuity in the pressure almost equal to that obtained in a centrifugal pump.
Fig. 2 represents longitudinal and cross-section of the latest type of this pump, from which it will be seen that the movable blocks are guided by means of obliquely-mounted ball bearings (K), and the number of the plates has been increased to six.
Another.pump which has ,gained considerable notoriety and many 'adherents in Germany is the (lentil pump,as fitted to the smallest type of Adler fire-engine. The feature which calls for attention in this pump is its capability of operating on air, and, therefore, being able to draw water from a lower level, thus representing an advance on the very principle of centrifugal pumps. No check valve at the lower end of the suction hose is needed in order to make the suction effective.
The construction, therefore, has the advantage of the centrifugal system in being capable of using water in which there is sand and mud. It is also shorter than the ordinary high-pressure centrifugal pump with four stages.
In Fig. 3 are shown longitudinal and transverse sections of the (lentil pump, the numerals sepresentins the passage of the water in the conduits of the pump. It flows in through one of the two suction pipes, the one not used being plugged, and reaches the introductory channel (L). This channel does not take it to the entire area of the central entrance to the first centrifugal disc (A), but only to upper half of this area. Here its speed is increased, and it enters the channel 2, and is by-passed through 3 back to the lower half of the disc (A), where its speed and pressure are again raised. Passing from 4 to 5, and from 5 through 6 to the lower entrance to the second disc (B), and thence through 7 to 8 to the upper half of the same wheel and out through channel 9 into the pressure channel 10, the water again reaches higher speed and pressure twice, so thatin all four increases of pressure are accomplished by means of two centrifugal wheels.
As the water enters the suction chambers from opposite sides, end thrust of the pump shaft is avoided, which means considerable simplification in the design of the bearings. The suction and pressure pipes are located so high in relation to the pump body that the water does not -run out of the latter when the supply hose or pressure hose is uncoupled. The pump body thus retains the water after an interruption of service.
The Adler chassis is fitted with an auxiliary water tank of nine gallons capacity, to which' the pump is connected by two pipes, one opening into the pressure chamber 10, and the other into the suction chainher 1. When the engine is in operation both these conduits are closed. At the start, provisled no hydrant pressure is available, they are both opened, and when the tank is to be refilled only the conduit leading to the pressure channel is open. In order to start, the suction hose is connected to the water supply, while the slide valves of the suction pipe and the different pressure hose couplings remain closed. The pump shaft is then coupled to that of the motor, and circulation between the tank and pump is effected. The water from the tank now runs into the sucticn chamber of the pump through pipe shown at the tcp in Fig. 3. (To be continued.)