Fig. 43.—Automatic Spring
controlled Extra-air Valve.
Fig. 44.—Plan View of
Automatic Extra-air Valve.
The points of a good carburettor:—
These may be set out in the following order—
(1) Complete atomization and vaporization of the liquid fuel at all engine speeds.
(2) The supply of an adequate quantity of gas of the correct proportions with all throttle openings and at all temperatures.
(3) Sufficient mechanical strength and durability to withstand road shocks and to ensure freedom from breakdowns without undue weight or complications.
(4) Ability to continue working correctly when the car is on an incline or affected by the camber of the highway.
(5) Moderate first cost.
Pressure Feed and Gravity Feed.—In Fig. [38] we showed a gravity-fed system or one in which the petrol is fed from the tank to the float chamber of the carburettor by the action of gravity only. For this system to be successful at all times the carburettor must be placed low down to obtain a good head for the flow of petrol in the connecting pipes, as there is a practical limit to the height at which the petrol tank can be fixed. Also the pipes must have a continuous run down towards the float chamber to prevent air-locks in them, and they must be kept away from the hot exhaust system. When all these points can be secured this system is perfect. An alternative system is to force the petrol into the float chamber by maintaining an air pressure (of 2 or 3 lb. per square inch) on the surface of the liquid in the petrol tank. With this arrangement the carburettor may, if desired, be situated above the level of the petrol tank in a more accessible position, but it necessitates the fitting of a small air pump on the engine and the use of a hand air pump for starting.