CARBURETION AND MANIFOLDING The engine, properly lubricated and cooled, produces power by converting the chemical energy stored in the fuel into mechanical energy. A system of carburetion and manifolding functions in supplying the proper mixture of air and fuel, which is the source of the engine's power. Carburetion is the art of mixing or blending a liquid fuel with a certain amount of air to form the combustible mixture necessary to meet the required demand. Normally, this is done by a mechanical device known as a carburettor. From the outlet of the carburettor this com bustible mixture is carried through an intake pipe or conduit, commonly known as a manifold, to the various cylinders. The carburettor and manifold, together with any other device used in conveying the mixture to the cylinders, make up the induction system of the internal combustion engine. The functions of an induction system are : metering, distribution, acceleration, and starting.
Types of types of carburettors have been employed for internal combustion engines : (I) surface carburettors; (2) wick carburettors and (3) jet carburettors. In the surface carburettor the air passing over the surface or bubbling up through the liquid fuel carried along with it sufficient vapours to make a combustible mixture. This type of carburet tor was unsatisfactory as it tended to remove only the lighter fractions of the fuel, leaving the heavier ones. To eliminate this condition the wick carburettor was developed, which also pre sented a larger surface of fuel to the air stream. Both the sur face and wick carburettors have long since passed from the com mercial field. A diagrammatic sketch of the simple jet carburet tor is shown in fig. r i. Butler is credited with having originated this type of carburettor, although it was first extensively used by Maybach in connection with light oil engines. A nearly con stant fuel level is maintained in the fuel bowl A by means of the float C, which acting through the levers D permits fuel to enter through the valve at B. The suction stroke of the engine causes air to enter at F, pass through the restricted throat in which the fuel jet E is located and into the cylinders through the passage way G. The restricted throat causes a drop in pressure at that
point, and the higher pressure on the fuel chamber A causes fuel to flow through the jet E. This liquid fuel is picked up by the air stream and forms a combustible mixture.
An ideal carburettor would have the following characteristics: (I) it would correctly proportion fuel to speed and load; (2) compensate for temperature changes; (3) proportion suitable accelerating mixtures; and (4) have low resistance to air flow, to maintain high volumetric efficiency. In meeting these ideal characteristics the simple jet carburettors have been changed until they are now very complex structures. Modern carburettors can be divided into two classes; plain tube and air valve, depend ent upon the method of propor tioning fuel. The plain tube is similar to the simple jet car burettor in the respect that it depends upon a restricted throat or venturi to proportion cor rectly the fuel throughout the major portion of its range. The air valve carburettor uses the spring-loaded air valve for the same purpose.
With the development of automobile engines where operation is necessary over a wide variation of speeds and loads, it was found that numerous additions to the simple carburettor were necessary to obtain satisfactory performance. These changes included the use of auxiliary jets at idle and low part throttle engine speeds, a metering pin which varies the size of the fuel jet, and a fuel well which supplies the necessary enrichment for acceleration. Next came the development of the economizer, which provides rich mixtures for maximum power at full throttle and leaner mixture for maximum economy at part throttle. This is accomplished in one of three ways: (I) by reducing the effec tive pressure on the fuel jet at part throttle, a method which becomes inoperative at full throttle; (2) by the use of an auxiliary jet which cuts in at full throttle; or (3) by changing the jet size by means of a tapered or offset metering pin. One important development in carburettors has been the use of an accelerating pump. These pumps are operated by the throttle and provide positive enrichment during the accelerating period.