SMALL ENGINES
BERNOULLI’S PRINCIPLE AREA A
VELOCITY V
AREA a
VENTURI
LOW-PRESSURE AREA FROM VELOCITY INCREASE
VELOCITY v
EQUATION OF CONTINUITY AxV=axv PRESSURE DECREASES PROPORTIONATELY WITH VELOCITY INCREASE (NONTURBULENT FLOW)
pressure is exerted on the fuel in the fuel bowl, causing it to rise in the emulsion tube. Fuel is discharged from the emulsion tube and enters the flow of air through the carburetor. As air velocity increases, the pressure difference increases, and more fuel is forced up the emulsion tube. This process continues until there is no pressure differential between the fuel in the fuel bowl and the venturi. See Figure 5‑15.
BERNOULLI’S PRINCIPLE APPLICATIONS AIR VELOCITY CREATES LOWER PRESSURE
Figure 5‑13. Bernoulli’s principle is based on the equation of continuity, which states that if the cross-sectional area of a hollow tube is multiplied by the velocity of a fluid flowing through the tube, the product is equal at any given point along the tube.
HIGHER ATMOSPHERIC PRESSURE GREATER DISTANCE ABOVE WING INCREASES AIR VELOCITY
LOWER PRESSURE CREATES LIFT
The shape of the wing disrupts the normal air flow and makes the air on the top of the wing travel farther. The greater distance requires the air to travel faster to meet the air at the end of the wing. The faster the air travels, the greater the pressure decrease and pressure difference from air below the wing. The area of high pressure produced under the wing has enough force to lift the airplane into the air. In a carburetor, air flowing into the combustion chamber must flow through the venturi. A venturi is a narrowed portion of a tube. The venturi is shaped and functions in the same way as two airplane wings. Air velocity must increase when passing through the restricted orifice of the venturi to maintain the same volume of air passed through the carburetor to the combustion chamber. The low‑pressure area created is less than atmospheric pressure, and fuel is forced up the emulsion tube to enter the air stream. Carburetor Operation A carburetor operates by utilizing pressure differences created by features designed into the carburetor. Air is introduced into the throat of the carburetor. The throat is the main passage in the carburetor which directs air from the atmosphere and air‑fuel mixture to the com‑ bustion chamber. As air moves through the throat and is restricted by the venturi, a low‑pressure area is created at the point of greatest restriction. One opening of the emulsion tube is located directly at the low‑pressure area. As moving air passes through, ambient atmospheric 118
AIR
PRESSURE DROPS WING
AIR PRESSURE REMAINS SAME
DIRECTION OF FLIGHT
AIR FLOW
AIRPLANE WING RESTRICTION AT VENTURI INCREASES AIR VELOCITY TO CREATE LOW-PRESSURE AREA
AIR-FUEL MIXTURE FLOW
AIR STREAM AIR FLOW
ATMOSPHERIC PRESSURE
FUEL FLOWS FROM HIGHER ATMOSPHERIC PRESSURE TO LOWPRESSURE AREA
CARBURETOR Figure 5‑14. Bernoulli’s principle is used to create lift from pressure difference to fly an airplane and to supply fuel into the airstream of the carburetor.