Simple Carburetor

The process of preparing a combustible mixture of a certain composition from finely atomized fuel and air that occurs outside the engine cylinders is called carburetion, and the device in which this process occurs is a carburetor.

Simplest Carburetor working principle

The principle of operation of the simplest carburetor is similar to the principle of operation of the atomizer.

Carburetor works this way: liquid under the action of discharge flows from the atomizer, and then mixes with air to form a combustible mixture.

Simple Carburetor parts (carburetor construction)

Simple Carburetor parts

Simple Carburetor Diagram: intake system diagram with the simplest carburetor; 1 – pipeline; 2 – hole in the float chamber; 3 – diffuser; 4 – spray; 5 – throttle valve; 6 – mixing chamber; 7 – nozzle; 8 – float chamber; 9 – float; 10 – needle valve; Y – simple carburetor, X – ideal carburetor. A nozzle is a metal plug with a small calibration hole through which a certain portion of fuel per unit time passes.

The simple carburetor consists of a float chamber 8, a diffuser 3, an atomizer 4 with a jet 7, a mixing chamber 6 and a throttle valve 5. In the float chamber there is a hollow float 9 pivotally connected to the axis and acting on the needle valve 10. Fuel is pumped into the float chamber through a pipeline 1. The hole 2 connects the float chamber to the surrounding air, so atmospheric pressure is constantly maintained in the chamber. The carburetor’s float chamber is connected to the mixing chamber 6 by a spray gun 4 in which the nozzle 7 is mounted.

How a simple carburetor works

During filling of the float chamber 8, the float 9 gradually floats up. At a certain level of fuel, the needle valve 10 closes the hole in the inlet pipe and the flow of fuel into the float chamber is stopped. During the intake stroke, the piston in the engine moves to the BDC and a vacuum is created in the cylinder, which is transmitted to the mixing chamber of the carburetor.

The discharge in this chamber depends on the position of the throttle: with the covering of the damper, the consumption decreases, and with the opening increases. While the engine is not run, in the float chamber and in the sprayer, the fuel is at the same level, with the upper end of the sprayer being slightly higher than the fuel level (2-3 mm).

During the engine operation, the air entering the carburetor passes through a narrow section of the diffuser, as a result of which the air velocity in it, and therefore the vacuum are increase. A pressure differential is created between the float chamber and the diffuser, so that the fuel begins to gush from the atomizer.

Fuel is spraying, mixes with air and partially evaporates and enters to the engine cylinders as a combustible mixture. As the throttle position changes, the composition of the combustible mixture prepared in the carburetor is changing significantly.

Carburetor features schematic diagram

Schematic diagram shows the carburetor features of a simple Y and an ideal X carburetor.

Carburetor features schematic diagram
Carburetor Features

They show the change in the composition of the combustible mixture of the carburetor depending on the load (on the position of the throttle valve). As the throttle valve  opens in the simplest carburetor, the combustible mixture becomes more enriched, and in only two cases (points C and D) does the composition of the mixture coincide with the composition of the combustible mixture prepared by the ideal carburetor (with the throttle fully open and at some intermediate position). Thus, the main disadvantage of the simplest carburetor is the inability to prepare a combustible mixture of the desired composition.

Author: delfi

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