The Latest Gillett-Lehmann Carburetter.
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A Simple and Ingenious Device which Ensures Economy of Fuel.
The three principal requirements of a carburetter are te(a), the supply, at all speeds of the engine, of a sufficient quantity of gas of the proper mixture to enable the engine to give out its utmost power for any particular speed; 413), that it shall be economical in use; and tr.), that the working of its parts shall be sLent. Many engineers try to meet the first and second of these requirements In; making the carburetter " automats' " in its action, hut in their endeavours to do this SOnTh designers complicate its construction by the addition of numerous air valves, springs, or other equally delicate parts, the noise from the working of which utterly precludes the attainment of the third requirement. However much the results that are obtained by the use of such devices may, in the opinion of their inventors, justify the employment ef these complications, they are at 4`,'tit only makeshifts.
The action of a carbureihl. is but imperfectly understood by most motor engineers. The flow of a liquid through a tube is proportional to the difference of pressure that exists at the two ends of the tube; the flow of air, however, follows a very different law, due to the fart that air, when flowing from a chamber at one pressure into another chamber at a lower pressure, expands, and a portion of the energy which produced the flow is used in producing the expansion of the air. The design of the carburetter should, therefore, be such that two curves, representing the flow of the petrol and of the air, should, as nearly as possible coincide, and the device which we describe and illustrate herewith has been designed with that etbject in view
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" THE COMMERCIAL MOTOR .' of the 7th .March, 1907 (Vol. IV), we gave some particulars of a controller which had been introduced by Carburation, Limited, of Byron House, 85, Fleet Street, EC., and which was known as the Gillett-Lehmann equalising device; this consisted of a vertical branch piece, from which three small tubes were led to different parts of any
ordinary eretbureiter, One of these tubes led to the float chamber, and the other two led into the induction pipe— one of them to a point below the throttle valve, and one to a point on the engine side of the throttle valve. The object of this arrangranent was the regulation of the flow of the petrol through the jet as well as of the flow of air past the jet, by the degree of vacuum which was induced in the induction pipe. The company has since introduced a complete carburetter which is constructed on this principle, but which also embodies a very practical throttle, and one that is scientifically correct. representative of this journal has had a good opportunity to test this carburetter on the road, and the results then noted, coupled with those obtained from 01 iii of the company's older devices that was fitted to a heavy commercial vehicle, mid \vhich he tested some IS months ago, convinced him that the principle is a good one, and that its designers have taken us a step nearer to the evolution of the perfect carburette; which will ultimately lead to the production of more sikdt and flexible engines.
The construction of the Gillett-Lehmann carburetter is clearly shown in the accompanying illustrations. The float (B) is of double-ccin ica 1 formation, but only the upper cone is made
; the lower part is merely a trumpet-like, extension, which rests on the needle-valve balance weights. The two halves of the float are screwed together, so that, by altering the overall height of the two cones, the level of the petrol in the float chamber may be adjusted. The vibration of the vehicle has much less effect on a float of this shape than upon one of the more usual type, and the chylous result of this is that the float's action is more certain, and more delicate, From the float chamber (A), the petrol finds its way to the jet (C), through channels that are drilled in the carcase of the carburetter; the jet may easily be removed for the porpoises of cleaning, by slackening back the set screw (17), which, in order to allow it to take up a position central with a countersink in the bottom of the jet (C), is made a slack lit in the screwed plug (E). 'Me tip. of the jet is made separate from the stem, in order to allow of the use of jets of different diameters in conjunction with the sante stein and its conical seating.
One of thy most interesting features in the design of this carburetter is the throttle valve (J), the formation of which is clearly shown in the largest of our illustrations. The jet (C) protrudes
CI-ix-01_101 a tapering slot in the periphery of the Ihrottle valve, and, as the latter is rotated, the space above the jet, and below the face of the cam-like inner wall of the valve, is constantly varied ; at the some time, the port in the top of the throttle-valve chamber (which leads to the admission valves of the engine.) is being opened or closed. The main air supply is admitted through the swinging branch pipe (G) which is provided with a butterfly valve (111; the latter is very tightly fitted, so that it cannot be altered except by the aid of a pair of pincers.
The valve is rotated by means of a small pinion IK1 and a straight rack (1..), and, as shown in our illustration, the valve is in the mid position of its travel. The tapering slot gives an infinitely-variable area of opening for the passetge of air around the protruding jet, and it is to this fact, together with the pressure control of the float, that the economical and silent running of engines which are fitted with this carburetter is attributed. When the throttle valve has been closed, the valve may still be rotated a few degrees, and this further move, ment uncovers a port (P) in the back wall of the valve chamber, and air is admitted through this port, and a corresponding-, screened port in the valve, to the cylinders of the engine, at such times as when coasting down a hill, or when applying the brakes.
The degre,t.s of vacuum above and below the float are regulated in the same manner as was employed in the early device, a description of the functions of the various parts of which may be found in the issue that contained our original reference. The small air passages (which in the first device took the form of three tubes) are drilled in the body of the casting, and the areas of their openings are adjusted by means of the regulating screw (0). One of these air passages leads into the top of the float chamber; another one leads into the jet chamber at the point 11{, through a slot (S.) in the back wall of the throttle valve (J); and the third one takes an upward course through a short tube (N) which leads into the induction pipe. The spindle of tile throttle valve (J) is grooved, as shown at T, in order to allow of the passage of the air to the pipe N throughout the whole range of the valve's movement ; whenever the engine is running, therefore, there is always a certain amount
" suction " on the top of the float, and this obviates any wastage of petrol through the jet when the engine is running with a closed throttle. The immediate result of this checking of the flow of the petrol is that the jet chamber does not heroine flooded, and there is an absence of the usual firing in the silencer when the throttle valve is again opened to allow the engine to resume its work, The absence of any of the usual complications that :ire to be found in some carburetters is a point worth the serious consideration of designers and manufacturers who are preparing for the introduction of new models for the next Olympia Show of commercial vehicles. The maker of the Gillett-Lehmann carburetter is only marketing one size at the present time, but we have no doubt that satisfactory arrangements for the manufacture of other sizes will shortly be made.