How does the carburetor work?
Carburetor - prepares and dispenses the fuel mixture, which then goes into the cylinder. Its design is quite complex and includes several fuel metering systems. It consists of two chambers: a mixing chamber and a float chamber. The mixing chamber is formed by a diffuser - a so-called channel, the cross-section of which is smaller in the center than at the ends. As air passes through the diffuser, its velocity in the center increases and its pressure drops below atmospheric pressure. The mixing and float chambers communicate with each other and with the fuel channel (atomizer), in which a part with a calibrated hole - the nozzle - is installed. The vacuum created in the diffuser causes the fuel to flow out of the atomizer in the form of small droplets, which are then crushed and evaporate in the air stream, forming a combustible mixture.
What are the types of carburetors?
The simplest carburetor is capable of preparing an optimal fuel-air mixture in only one engine mode, at a constant load and rpm. With any vehicle engine, including a scooter, both of these parameters are constantly changing, and there is also the problem of cold starts. Therefore, the carburetors used in scooters are much more complicated.
Modern carburetors have a main metering system, starting, idling and transitional systems, as well as a mixture correction system to maximize power. To control the engine, the carburetor has a throttle connected by a cable to the throttle lever on the steering wheel. It is used to regulate the amount of fuel-air mixture delivered to the cylinder.
Two types of carburetors are used in modern scooters:
- slider - The throttle on the carburetor changes the diameter of the outlet by moving the spool. You can adjust the amount of air passing through the carburetor, and thus the engine speed. The cone-shaped needle tapers downward and enters the atomizer and is rigidly connected to the spool. The main fuel injector is mounted on the other side of the atomizer. All these parts form the main metering system of the carburetor. When the throttle knob is turned, the needle moves with the spool, and since its diameter at the bottom is smaller than at the top, the cross-section of the fuel channel increases when the spool and needle are raised. By changing the position of the conical needle in relation to the spool, the composition of the mixture can be adjusted slightly: enrich by raising the needle or, conversely, deplete by lowering it.
- Constant-volume (CV) - In constant-volume carburetors, the movement of the "throttle" knob is not transmitted to the spool associated with the metering needle, but to a rotary throttle closer to the carburetor outlet. This throttle, like the spool in the carburetor above, regulates the amount of air flowing through the carburetor. The diaphragm chamber above the spool communicates with the mixing chamber of the carburetor. In this way, the movement of the spool (and with it the fuel metering needle) is controlled by the vacuum in the intake line. At low loads, when the throttle is closed, the vacuum in the mixing chamber (and therefore in the cavity above the diaphragm) is low and the spool and needle are lowered by the spring. At high loads, with the throttle open, the increased vacuum is transferred to the cavity above the diaphragm and raises the spool with the metering needle. The advantage of this type of carburetor is that the spool maintains a constant vacuum in the atomizer area, ensuring an optimal fuel-air mixture ratio. When the throttle opens (the slider is raised), the main metering system engages to provide the desired mixture throughout the engine's operating range.
The latter are mainly used on scooters with four-stroke engines.