THE WILD AUTOMATIC CARBURETTER.
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Adjustable Multi-jet. Rotary Throttle to Give Correct Mixture at all Speeds.
THE petrol engine is nothing if not flexible ; carburetters have been brought to such perfection of design that it has become possible more or less cOrrectly to adjust air and fuel proportions over a very wide range of engine speed. The effort to attain perfection has involved the provision of devices of varying complexity, especially in the arrangements made for stow* running, although, of coarse until temperature and velocity can be made absolutely interchangeable the perfect carburetter is not to be expected.
Briefly, it may be said that the two-fold function of any carburetter (viz., vaporizing the petrol and effecting the mixture of the petrol vapour with air), is achieved by causing the petrol to debouch through a small orifice into a current of air, Generally, the current of air is arranged on streamline principles, through a Venturi tube, the cross-sectional area being attenuated at the point where the petrol issues from the jet. The effect of this arrangement. (otherwise known as a "choke tube ") is to increase the velocity and:diminish the pressure of the air-at that point, whereby the suction on the jet is augmented and the resistance to the issue of petrol is decreased. In all engines it is the fact that, with increase of piston speed, the amount of petrol issuing from the jet increases in greater proportion than the induced air, whereby at high engine speeds the mixture tends B36 to become too rich. For this reason, extra air intakes are provided in many engines. Reference has been made to this circumstance, in order to indicate the difficulty confronting the designer, a difficulty which arises from the fact that the relationship between the two factors comprising the mixture, petrol and air, does not remain constant at all engine speeds. Hence arises the complexity of carburetter design.
In the Wild carburetter, the usual butterfly throttle is replaced by a sleeve, denoted by the letter (F) in the accompanying diagram. This sleeve fits into the upper part of the carburetter, and, formed at the lower end of it, is a spindle which passes through a chamber in the well and protrudes at the bottom so as to admit of connection with the control handle or pedal. It will be seen that a portion of the sleeve is cut away to form a port, and through this must pass ,the air induced by the descending pistons. By operating the control, the sleeve is given a rotary motion, whereby the area of the open port is diminished. In order that the sleeve may also regulate the petrol, it is made -to rest on the plate (E), which in turn is superimposed on the jet plate (D). The latter is accommodated in a recess bored in the well casting, and has a number of jet holes -drilled on one sector of its periphery ; these exactly coincide with similar holes drilled in the jet-adjusting plate (E).
Petrol from the well patses through these holes and issues into the stream of air at the point where the port is formed in the sleeve. If the sleeve be rotated in such a way as to close the air port, equally will it cover some or all of the jets on which its base rests ; thus, to throttle the air decreases also the petrol supply. Although there is no choke-tube in the streamline sense, it will be seen that the adjustable shutter (G) may be set so as to vary at will the maximum opening of the pert. et Another adjustment is that provided for the jets, effected by the external screw which dis*places the plate (E), whereby the coincidence ofthe "holes in E and those in the jet plate (D) is upset, and the effective jet area reduced. It is claimed that this type of carburetter will give a correct mixture at all engine speeds, and that the carburetter, once set, will need no adjustment by the driver, except that the latter may close the shutter in order to start the engine. Certainly a rich mixture could be .obtained for starting purposes, by thus closing the shutter, while opening the control wide enough to uncover sufficient jets. It is a pity that the actual emission of petrol into the air stream does not take place in the centre of flow, where the suction is greater and the danger from eddies less-than at the sides of the stream. Eddies would, no doubt, be set up by the sudden alteration of cross-section due to the shutter.
Evidently the success of the carburetter will depend upon its correct adjustment in the shops. It represents a distinct step, in that it offers opportunities for proper correlation of air and petrol supply through the possibility of varying the ,diameters of the jet holes around the sector of the jet plate. With proper adjustment it should give excellent results. •