Basic construction of the carburetor

The simple carburetor is composed of upper, middle and lower three parts, the upper part has air inlet and floating chamber, the middle part has throat, measuring hole and nozzle, and the lower part has throttle. The float chamber is a rectangular container that stores gasoline from the gasoline pump. Inside the container is a float using the float surface (oil surface) to highly control the oil intake. The first end of the nozzle in the middle is connected with the measuring hole of the floating chamber, and the other end of the nozzle is at the throat of the throat.


The throat is wasp waist, large at both ends and small in the middle, in which the cross-sectional area at the throat is the smallest. When the engine starts, the piston downward suction, the suction airflow through the throat at the speed is the largest, the static pressure is the lowest, so the throat pressure is less than the atmospheric pressure, that is to say, the pressure difference between the throat and the float chamber, that is, with the "vacuum", the greater the pressure difference, the greater the vacuum. Gasoline under the action of vacuum from the nozzle out of the oil, because the air flow rate at the throat of the throat is 25 times that of gasoline, so the oil flow from the nozzle is scattered by the high-speed air flow, forming fog particles of different sizes, that is, "atomization". The initially atomized oil particles are mixed with the air into a "mixture" and enter the combustion chamber of the cylinder through the throttle, intake pipe and intake valve. Here, the opening size of the throttle and the rotation speed of the engine determine the vacuum at the throat, and the opening change of the throttle directly affects the proportion of the mixture, which are important reasons for the operation of the engine.


Here involves a concept of "air fuel ratio", the so-called air fuel ratio refers to the ratio of air quality and fuel quality, scientists think 1 kg gasoline completely combustion needs about 14.7 kg of air, the air fuel ratio of 14.7:1, the air fuel ratio mixture called standard mixture, because the value is difficult to achieve in practice, so also known as "theory mixture". The air-fuel ratio is greater than the standard mixture is called dilute mixture, and less than the standard mixture is called thick mixture.


Because the concentration change of the mixture is closely related to the load change of the engine under various operating conditions, the simple carburetor is far from meeting the requirements of changing at any time, so people constantly add new devices to the simple carburetor to adjust the working state of the carburetor. Today, the carburetor has been formed with a variety of auxiliary devices, mainly idle, thickening, acceleration and starting devices. At present, the common carburetor of the 4-cylinder engine is the double-cavity split carburetor, which has two tuats, working separately or simultaneously according to the different working conditions of the engine. The common carburetor of 6-cylinder engine is double cavity parallel moving carburetor, which is actually two single cavity carburetors together, each cavity is responsible for the mixed gas supply of half of the target cylinders. There are also multi-cavity carburettors, usually mounted on more powerful engines.


Among the various functional devices of the carburetor, the main oil supply device is the oil supply device required by the engine except idle speed, which is the basic oil supply structure of the carburetor. An idle device is a device that provides a small and thick mixture at idle speed to maintain a stable minimum engine speed. The thickening device is an additional oil supply device when the engine is under high load to make up for the shortage of the main oil supply. The acceleration device is an additional oil supply device when the throttle opening suddenly increases when the car accelerates, so that the engine speed and power can increase rapidly. The starting device is a device that provides a very thick mixture when the engine is running in a cold start. The common way is to install a damper in front of the throat to control the air intake.


Idle speed is especially mentioned here. Idle speed is the most commonly used engine working condition, used for the engine hot start process and no shutdown, etc. It is of great significance to the driving performance of the car, especially in the city, the idle speed often determines the fuel consumption and pollution degree of the car.


 The idle speed of the engine is generally only 600-800 RPM, the throttle is close to closing, the throat vacuum generated by this speed can not smoothly absorb the gasoline from the float chamber, but the vacuum behind the throttle is very high. Therefore, only need to set an idle oil channel on the basis of the simple carburetor, the nozzle is located after the throttle, the problem will be solved.


Because the idle speed needs less and thick mixture, it is more sensitive to the running condition of the engine. To realize the idle state of both stability and the lowest speed, it is necessary to adjust the oil control and the minimum opening of the throttle. The current carburetor idle has two adjustment screws to adjust the oil and throttle opening. At the same time, in order to prevent the phenomenon of the car off the ignition switch and the engine is still running, the idle solenoid valve in the idle oil duct, specially responsible for opening and closing the idle oil duct, to ensure that the engine can quickly stall.


 Basic construction of the carburetor