TM 5-4240-501-14&PTHEORIES OF OPERATIONCarburetionCARBURETIONThe basic purpose of a carburetor is to produce amixture of fuel and air on which an engine will operate; todo so is relatively easy. However, producing economicalfuel consumption and smooth engine operation over awide range of speeds creates the need for a morecomplicated mechanism than a mere mixing valve.There is an additional problem in that the price of such acarburetor must be held in proportion to the price of theengine. The price of a Briggs & Stratton engine is notmuch greater than the price of the carburetor on anautomobile.Atmospheric PressureFigure 12.Keeping this in mind, we must utilize the force ofatmospheric pressure and the principles of the venturiand the airfoil.Atmospheric pressure, while it may vary slightly due toaltitude or temperature, is a constant potent force whichtends to equalize itself in any given area. It is the weightof the air in the atmosphere pushing down and outwardin all directions and is commonly figured as between 13and 15 pounds per square inch. We know that airmoves from a high pressure area to a low pressure area.To use this force of atmospheric pressure in acarburetor, we artificially create low pressure areas andthus obtain movement either of air or of intervening fuel.We will show you how a little later.The greater the difference in pressure between the twoareas the greater the velocity or the greater the distancewe can raise the fuel.In the interest of brevity we often use the terms vacuumor suction when we actually mean the difference inpressures.VenturiWhat is a venturi ? Have you ever noticed that the windblowing through a narrow space between two buildingsalways seems to be much stronger than in the open? Inother words, the velocity is greater. The same thing canbe seen in a river. The current is always faster in anarrow, shallow place than in the deep wide pools.In a fashion, these narrow places are venturi is. Thegreat bulk of air or water suddenly forced through aconstricted space has to accelerate in order to maintainthe volume of flow.This is the way a venturi is placed in a carburetor. Fig.13. The shape is carefully designed to produce certainair flow patterns.Figure 13.714
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