Fuel is introduced into the air stream through small holes (the main jets) at the narrowest part of the venturi, where the air is at its highest speed.ĭownstream of the venturi is a throttle (usually in the form of a butterfly valve) which is used to control the amount of air entering the carburettor. The main metering circuit consists of a pipe which temporarily narrows, forming a venturi. From the late 1930s, downdraft carburetors become more commonly used (especially in the United States), along with sidedraft carburetors (especially in Europe).įuel circuits Main metering circuit Ĭross-sectional schematic of a downdraft carburetor Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the top. The orientation of the carburetor is a key design consideration. In SU and other variable jet carburetors, it was corrected by varying the jet size. Most commonly this has been corrected by using multiple jets. So jets sized for full power tend to starve the engine at lower speed and part throttle.
The fuel jets are much smaller and fuel flow is limited mainly by the fuel's viscosity so that the fuel flow tends to be proportional to the pressure difference. The main disadvantage of basing a carburetor's operation on Bernoulli's Principle is that being a fluid dynamic device, the pressure reduction in a venturi tends to be proportional to the square of the intake airspeed.
Instead, the airflow through the carburetor increases, which in turn increases the amount of fuel drawn into the intake mixture. In most cases (except for the accelerator pump), the driver pressing the throttle pedal does not directly increase the fuel entering the engine. The carburetor works on Bernoulli's principle: the static pressure of the intake air reduces at higher speeds, drawing more fuel into the airstream. Most engines use a single carburetor shared between all of the cylinders, however some high-performance engines have used multiple carburetors. Air from the atmosphere enters the carburetor (usually via an air cleaner), has fuel added within the carburetor, passes through the inlet valve(s) and then enters the combustion chamber. The carburetor is located upstream of the inlet manifold.
0 kommentar(er)
