Recent Improvements in Carburettors.—Another defect of this simple type of carburettor becomes apparent in the larger sizes required for multi-cylinder engines. To pass the requisite quantity of petrol to keep the engine running at high speeds without creating too great a suction effort and thereby hampering the engine, necessitates the use of a jet of larger calibre, so that the liquid is no longer sprayed but issues in the form of a fine stream which is not readily vaporized. This has been overcome by the use of multiple-jet carburettors which have several jets each surrounded by its own choke tube, but all controlled by one throttle valve and supplied from one common float chamber. In this case the total cross-sectional area of all the jet orifices together could be made sufficient to pass the necessary quantity of fuel, but the bore of each individual jet orifice would be comparatively small and spraying would result as before. Another very successful device is shown in Fig. [41], in which A is the petrol jet which, in this case, has no special orifice and is surrounded by a larger tube B containing small holes for the inlet of air and outflow of petrol. As the petrol issues from the jet it strikes against the pointed cone on the end of the screw C, and is thus very successfully atomized and broken into small particles which can be readily vaporized.
Fig. 41.—Petrol Jet,
specially arranged for
Atomising the Petrol.
Fig. 42.—Compensated Petrol Jet.
A is the Main Jet and B the
Compensating Jet supplied
hrough the Orifice C.
There are several devices for keeping the strength of the mixture constant at all engine speeds irrespective of the amount of vacuum in the choke tube. One of the best of these is illustrated in Fig. [42], and consists in the use of a compensating jet. The main petrol jet A is of sufficient size to supply the requirements of the engine under full speed and with the resulting high vacuum; it is fed directly from the float chamber in the usual manner. The compensating jet B surrounds the main jet and is supplied with petrol through an orifice C, so arranged that it offers a greater resistance to flow than the passage up the centre of the main jet. At all engine speeds up to a certain predetermined maximum the compensating jet will supply most of the petrol, but as the demand increases the main jet will also begin to supply, and simultaneously the compensating jet will commence to go out of action owing to its supply of petrol becoming partly or wholly exhausted due to the restriction of the orifice C.
The simple jet-in-tube carburettor has been greatly improved by the addition of an automatic extra-air valve, of which a simple form is shown in Figs. [43] and [44]. It consists of a small mushroom type valve A, with its seating B so arranged that it can be screwed into the induction pipe of the engine. The valve is held up against its seating by a light spring C, so that at high engine speeds when there is a good vacuum in the induction pipe the pressure of the atmosphere will open the valve against the tension of the spring and allow air to pass into the induction pipe, thus reducing the amount of vacuum and simultaneously weakening the mixture.