Design and operation of a simple carburetor
Device
The simplest carburetor consists of two main parts: a mixture-forming device and a float chamber. In the mixture-forming device, a flammable mixture is prepared, and the float chamber is a reservoir from which fuel is supplied to be mixed with air.
The carburetor's mixture-forming device has an air inlet, a diffuser, a mixing chamber, a throttle valve, and an outlet. The outlet pipe usually ends in a flange that secures the carburetor to the engine intake manifold.
A hose for supplying air or an air filter directly is installed on the inlet pipe. The diffuser is a local reduction in the cross-section of the mixture-forming device. Thanks to the diffuser, the conditions for fuel atomization are improved, since when the engine is running, the maximum air flow speed is created in the narrowest section of the diffuser. In this place, a sprayer is installed, which is a tube connected to a diffuser. Fuel flows out and atomizes through the atomizer.
The float chamber contains a float mechanism consisting of a float and a needle valve. The float is hinged to the wall of the float chamber. The shut-off needle of the needle valve rests on the float lever.
When fuel is supplied through the fitting into the float chamber, the float floats up and with its lever raises the shut-off needle, closing the needle valve. As soon as the fuel level in the float chamber reaches a predetermined limit, the needle valve will close completely and the flow of fuel into the chamber will stop. When fuel is consumed from the float chamber, the float lowers and opens the needle valve. Fuel begins to flow into the float chamber again until the specified level is reached. Thus, the float chamber, using a float mechanism, ensures that the specified fuel level is maintained under all engine operating modes.
The main jet is located at the bottom of the float chamber. Its main purpose is to dose fuel to obtain a combustible mixture of the desired composition. The jet is a plug with a central calibrated hole. The diameter of the calibrated nozzle hole is selected depending on the required fuel flow. The length of the calibrated hole of the nozzle, the angles of the inlet and outlet chamfers, and the diameters of the channels in the body of the nozzle are also of great importance for the formation of flammable mixtures. The main jet can be installed at the bottom or top of the sprayer.
Job
When the engine crankshaft rotates during the intake strokes and when the throttle valve is open, air passes through the carburetor's mixing chamber. Inside the diffuser, the air flow speed increases significantly, and a vacuum is created at the sprayer outlet. In this case, due to the presence of the hole, the pressure in the float chamber remains equal to atmospheric pressure. Due to the pressure difference in the float chamber and in the atomizer, fuel begins to flow through the main jet and atomizer in the form of a fountain, ending up in the neck of the diffuser. Here, a stream of incoming air crushes the escaping fuel into small droplets, which mix with air, evaporate and form a combustible mixture.
The formation of a combustible mixture in the mixing chamber of the carburetor does not occur in full. Some of the fuel in the form of droplets does not have time to evaporate and mix with air. Unevaporated fuel droplets move in the air flow and settle on the walls of the mixing chamber and inlet pipeline. Fuel deposited on the walls forms a film that moves at low speed. To evaporate the fuel film, the intake manifold is heated when the engine is running. Most often, liquid heating (from the engine cooling system) or heating with exhaust gas heat is used. Thus, we can assume that the formation of a combustible mixture ends at the end of the engine intake pipe.
The engine carburetor consists of 5 main carburetor systems:
1) main carburetor metering system designed to mix fuel with air in prescribed proportions, which is ensured using special caliber jets (fuel and air jets).
2) carburetor idle system designed to maintain engine operation at low crankshaft speeds.
3) carburetor starting system designed to supply air to the emulsion tubes through the air damper and jets.
4) carburetor economizer system designed to enrich the combustible mixture during prolonged load.
5) carburetor accelerator pump system designed for short-term enrichment of the combustible mixture during vehicle acceleration.
Preparation of a combustible mixture and operation of the main carburetor systems
The preparation of a combustible mixture is carried out by mixing two components of fuel and air in a certain proportion. Before entering the system, both components must be thoroughly cleaned of various types of contaminants and impurities. The combustible mixture is prepared in the carburetor using small-caliber jets and dampers, with the help of which the fuel is dosed and sprayed into the smallest particles, after which it is mixed with air.
The combustible mixture has its own composition, which is prepared at a certain mass ratio of fuel to air. In order for 1 kg of gasoline to burn, it is theoretically necessary to mix 14.9 kg of air with it (in calculations, 15 are taken). The truth is that nothing is perfect, and the amount of air that is consumed to prepare the combustible mixture is slightly more or less than theoretical. In this regard, the composition of the combustible mixture is characterized by the ratio of air, which participates in the process of fuel combustion, to the theoretically determined amount of air.
To accurately determine the degree of enrichment or depletion of a combustible mixture, the names of the following mixtures were adopted:
1) rich mixture with excess air coefficient equal to 0.70-0.85
2) enriched mixture with excess air coefficient 0.85-0.95
3) lean mixture with excess air coefficient 1.05-1.15
4) lean mixture with excess air coefficient 1.15-1.20
The engine must operate in optimal mode. Optimal engine operation will be ensured by a normal combustible mixture. That is, the combustible mixture should not be over-rich or over-lean, since in these cases the efficiency and power of the engine are reduced.
Liquid fuel in gasoline engines cannot ensure the operation of the piston group. To create torque on the crankshaft, a series of cyclic micro-explosions in the cylinders is necessary, while liquid gasoline simply burns. When fuel is mixed with air (which contains large amounts of oxygen), a mixture is created that can create a flare that has high kinetic energy.
Automotive carburetors - history of development
At the dawn of engine building, the use of gas became unprofitable. There was a need to create a device that could, with a high degree of reliability and safety, ensure the formation of a high-quality mixture from gasoline and air. The operating principle of the first series carburetor was based on the evaporation of fuel vapor. The chamber was heated from an external heat source, gasoline vapors mixed with air due to convection.
The characteristics of such a carburetor did not allow developing more power, so this design did not take root in engine building. For the first examples of cars, it was enough that they just drove; later, the needs of customers grew, and motorsport began to develop. There was a need to create a carburetor that had no restrictions on engine power.
The next generation, invented by German engineers Daimler and Maybach, worked on the principle of fuel atomization. The size of the unit decreased (there was no need to build in a volumetric evaporation chamber with a heating tank), and productivity, on the contrary, increased significantly. In fact, a vacuum carburetor was created, the design of which is used in modern models. The main technical breakthrough - the transition of fuel into a gaseous state was forced, which gave scope for experiments with performance. Of course, the design of the Daimler-Maybach carburetor was not similar to modern designs of high-performance vacuum models with a special receiver and control of air discharge.
However, the principle of operation was the same as on any modern model.
Carburetor design (typical description for all modifications)
The diagram shows the relative position of the main components:
- Gasoline supply pipe from the fuel pump;
- A float with a needle valve that shuts off the fuel line;
- Jet for receiving fuel from the float chamber;
- Liquid fuel spray nozzle;
- Mixer chamber in which the fuel mixture is formed;
- An air damper that regulates the volume of incoming clean air flow from the filter;
- Diffuser that shapes the direction of air flow;
- A throttle valve that regulates the flow of mixture into the engine intake tract.
How does a carburetor work?
Let's consider the operation of each node.
- Gasoline under low pressure (not to be confused with high-performance nozzles of injection systems) enters the float chamber. It is important to maintain a fuel level in the carburetor that does not exceed the location of the jet. Otherwise, aerosol spraying will not occur in the mixing chamber. For each model, an upper limit for filling the chamber is set, which is mechanically “monitored” by a float with a needle valve. This design was chosen because with a small amount of force the pressure in the incoming fuel line can be maintained. When the limit is reached, the valve closes the inlet; when the level drops, it fills the chamber with gasoline;
- The disadvantage of the design (unfortunately, there is no alternative) is its high dependence on pollution. The needle valve may become stuck closed and the motor will stop running;
- Next, gasoline enters the nozzle. The diameter of this element is strictly regulated; deviations of even hundredths of a millimeter are not allowed. Otherwise, aerosol spraying will not occur at the entrance to the mixing chamber, and the air-fuel mixture will not be formed, and, as already mentioned, the internal combustion engine does not work on liquid gasoline;
- An aerosol of tiny droplets of gasoline emerges from the diffuser, ready to be mixed with air;
- The mixer chamber (actually the carburetor body) is designed to form a gaseous mixture consisting of gasoline vapor and oxygen contained in the air. Gasoline, like air, does not enter the chamber under pressure, but, on the contrary, due to vacuum. When the cylinder moves down, a difference in pressure arises, a kind of vacuum. Due to the specially designed body shape, fuel and air flows are mixed evenly, forming a high-quality mixture;
- The dampers (throttle and air), controlled by the gas pedal, measure the intensity of the air flow and the speed of fuel suction from the nozzle. The engine works more intensely, the crankshaft rotation speed changes along with power and torque.
All carburetor systems must work harmoniously: if one of the channels (nozzles) is clogged, or the position of the dampers is incorrectly adjusted, the formation of the mixture will be disrupted. Gasoline consumption will increase, power will be lost, the power unit will operate unstably, so all components must be clean, their size must correspond to factory calculations, and adjustment parameters must be adjusted. There are a number of adjustment screws on the carburetor; the correct specifications are set with their help. The illustration shows an example of an Ozone carburetor.
A well-tuned carburetor “squeezes” maximum performance out of the engine at the lowest fuel costs. Different carburetor models may have their own adjustment methods, but the general principle is the same.
Each carburetor has instructions for setting parameters. Adjustment can be done independently, or at a specialized service. When operating conditions change (the amount of oxygen in the air, regular load on the car, turning on the air conditioner in the summer, etc.), the settings should be re-adjusted.
What is the difference between a classic carburetor and an electronically controlled device?
The principles of operation of a mechanical carburetor were described above. All settings are set using screws and cannot be changed dynamically during operation. The carburetor circuit is constantly being improved, and new models (some of which are still in production today) have quite a lot of electronics. For example, almost all mechanical models are equipped with a solenoid valve.
Let's take a closer look at this device:
The fact is that when the gas pedal is fully released, the throttle valve is closed, and the engine should, in theory, stall. To operate the internal combustion engine without load (just so as not to start it every time after stopping), an idle system has been introduced. With its help, even with the dampers closed, a minimum volume of gasoline and air enters the housing. The formed fuel mixture is sufficient to maintain the operation of the power unit without load on the crankshaft.
This parameter requires precise adjustment: if the idle speed is too high, gasoline consumption will increase, and if it is too low, the engine will stall when stopping. When operating conditions change (temperature, the presence of an air conditioner with air conditioning, additional equipment that puts a load on the generator), the idle speed changes, so an idle speed valve (electric) was installed, which controls the process linearly, depending on the load.
There is no control program; only the power wire goes into the valve. Depending on certain operating conditions, the position of the valve changes.
These are not all electronic systems that can be introduced into the mechanics of the process. For example, all adjustments are made to a control unit, such as an ECU for injection engines. Such a microcomputer constantly monitors the load parameters on the power unit and can change carburetor settings in real time. Asking yourself the question: “which carburetor is better to install?”, You can consider introducing a modern design into the car. Unlike traditional carburetors, electronic systems do not require periodic adjustments, but are more expensive and more difficult to maintain and repair. To provide the electronics with initial data, various sensors are installed on the engine that monitor the motor parameters. Based on the information received, the carburetor actuators are activated.
Types of carburetors by manufacturer - which one to choose?
Everyone has heard the difference of the so-called. Chinese products, and carburetors of famous brands (the list of which includes DAAZ, Solex, and Ozon...). In fact, this is nothing more than prejudice. A product produced at the factory, in compliance with the technology, and with a quality certificate, will work well regardless of the geography of production. Only the so-called “no-name” products, collected by peasants from the Middle Kingdom literally with a file on their knees, are of low quality, so when selecting a new carburetor, first of all, focus on the reputation of the manufacturer and the availability of accompanying documentation. Of course, warranty obligations must also be provided by service centers within accessibility. That is, if you live in Kaliningrad, and the nearest manufacturer’s service center is in Dimitrovgrad, it makes sense to find another copy.
Bottom line
You should not be afraid of this seemingly complex device. The operation scheme is simple and reliable; the key to normal functioning is the cleanliness of all internal elements and correct settings.
If you have any questions, leave them in the comments below the article. We or our visitors will be happy to answer them
Dear friends, in this manual we will try to explain the basic principles of operation of any carburetor, its structure, with illustrations and fairly detailed comments. This article will be especially useful for beginners who want to understand the topic. In this article we will consider the following points:
Engine operating modes and the composition of the combustible mixture, the idle system and the transition system, the design of the float chamber and the principles of its operation, the main carburetor metering system, the starting system, the operating principle of the econostat and much more. After all, the appetite of your car directly depends on the correct operation of all these components. It can be either higher or lower than that indicated in the technical specifications of your car. Eg expenses VAZ - 2114, 2110, 2112 You can find out by following the link, you can look at the passport costs of the seven VAZ-2107 , etc. In general, be patient, grab some popcorn and get ready for some interesting reading.
Engine operating modes and composition of the combustible mixture
COMPOSITION OF COMBUSTIBLE MIXTURE To operate an internal combustion engine, a mixture of fuel and air is required. In carburetor engines, fuel (gasoline) is mixed with air in a certain proportion outside the cylinders and, having partially evaporated, forms a combustible mixture. This process is called carburetion, and the device that prepares such a mixture is called a carburetor. The mixture, passing through the intake pipe, enters the engine cylinders, where it mixes with the remaining hot exhaust gases, forming a working mixture. Particles of atomized fuel evaporate. To start the engine and operate it in different modes, a different composition of the combustible mixture is required. Therefore, the carburetor is designed in such a way that it allows you to change the quantitative ratio of atomized fuel and air in the mixture entering the engine cylinders. For complete combustion of 1 kg of fuel, about 15 kg of air is required. A fuel-air mixture in this proportion is called normal. The engine operating mode with this mixture has satisfactory indicators in terms of efficiency and developed power. A slight increase in the amount of air in the air-fuel mixture compared to its normal content (but not more than 17 kg) leads to a lean mixture. With a lean mixture, the engine operates in the most economical mode, i.e. Fuel consumption per unit of developed power is minimal. The engine will not develop full power with such a mixture. If there is excess air (17 kg or more), a lean mixture is formed. The engine runs unstably on such a mixture, and fuel consumption per unit of power output increases. With an over-lean mixture containing more than 19 kg of air per 1 kg of fuel, engine operation is impossible, since the mixture does not ignite from a spark. A slight lack of air in the air-fuel mixture compared to normal (from 15 to 13 kg) contributes to the formation of an enriched mixture. This mixture allows the engine to develop maximum power with slightly increased fuel consumption. If the air in the mixture is less than 13 kg per 1 kg of fuel, the mixture is rich. Due to lack of oxygen, the fuel does not burn completely. An engine with a rich mixture operates in an uneconomical mode, intermittently and does not develop full power. An over-enriched mixture containing less than 5 kg of air per 1 kg of fuel does not ignite - engine operation on it is impossible. STARTING THE ENGINE When starting a cold engine, part of the sprayed fuel settles on the walls of the intake manifold, and part of the evaporated fuel, entering the cylinders, condenses on the walls. In addition, at low air temperatures, mixture formation worsens, since the evaporation of gasoline slows down. Therefore, to start a cold engine, it is necessary for the carburetor to prepare a re-enriched air-fuel mixture. IDLING At idle, the engine speed is low and the carburetor throttle valves are almost completely closed. Because of this, cylinder ventilation is not as effective as compared to operation at medium and high crankshaft speeds and the amount of combustible mixture entering the engine is small. The working mixture contains a large amount of exhaust (residual) gases. Therefore, for stable engine idling, a rich mixture is necessary. PARTIAL LOAD MODE At partial loads, the engine does not require full power. The throttle valves are not fully open, but the ventilation of the cylinders is good. Therefore, in this mode, a lean combustible mixture is sufficient. The ratio of the power developed by the engine to the amount of fuel consumed allows us to consider the partial load mode the most economical. FULL LOAD MODE At full load, the engine requires maximum or close to maximum power. At the same time, the engine operates at high speeds, and the throttle valves are fully (or almost completely) open. This mode requires an enriched mixture with an increased combustion rate. MODE OF SHARP INCREASE IN LOAD When the engine operates under a sharp increase in load, for example when accelerating a car, an enriched mixture is required. But since the mixture formation process has some inertia, in order to prevent the occurrence of a “failure” when accelerating, additional short-term enrichment of the combustible mixture is required. To do this, additional fuel is injected directly into the carburetor mixing chamber.
BASIC CARBURETTOR SYSTEMS
Modern carburetors are equipped with a dozen different systems and devices that have an extensive network of channels, numerous calibrated holes, complex lever transmissions and pneumatic chambers. It’s not easy to understand this intricacy right away. Therefore, it is useful to consider all the main systems separately using simplified diagrams as examples. And you should start with the operating principle and design of a simple carburetor.
To operate a gasoline engine, it is necessary to add fuel to the intake air, which then burns in the cylinder during the working stroke of the piston. In order for the fuel to reliably ignite and burn completely, it is necessary to thoroughly mix it with air and at the same time maintain the optimal composition of the combustible mixture in all engine operating modes. These functions are performed by a carburetor connected by an inlet pipe to the engine cylinders. The simplest carburetor consists of two chambers: a float chamber and a mixing chamber. The process of preparing a combustible mixture continues along the entire path of movement of fuel and air along the intake tract, right up to the cylinders, but begins with atomization of fuel in the carburetor mixing chamber. For this purpose, a tube-shaped sprayer is installed in the mixing chamber. The tube section is brought to the center of the chamber diffuser. A diffuser is a narrowing section of the mixing chamber. The air flow speed in the diffuser increases, and a vacuum occurs at the atomizer. Under the influence of this vacuum, the fuel flows out of the atomizer and is intensively mixed with air. Fuel enters the atomizer from the float chamber, with which it is connected by a channel. A nozzle is installed in the channel - a plug with a through hole of a certain size and shape. The jet restricts the flow of fuel into the atomizer. One of the conditions for normal operation of the carburetor is the correct setting of the fuel level in the float chamber. The fuel level in the chamber is maintained using a float mechanism with a needle valve. Fuel is supplied to the float chamber through the fuel line. As the chamber fills, the float rises and the needle closes the valve hole, while the air displaced by the fuel is discharged out through a special hole. The float chamber and the sprayer are communicating vessels. The fuel level in the float chamber is set so that it is just below the nozzle exit. At an increased level, the fuel will leave the atomizer, over-enriching the mixture; at a low level, the supply of fuel to the atomizer is insufficient, resulting in the formation of a very lean combustible mixture. In order to change the composition of the mixture, an air damper is installed in the mixing chamber above the diffuser. As the choke valve closes, the mixture will become richer. Excessive closing of the throttle will lead to over-richness of the mixture and engine shutdown. To regulate the amount of air-fuel mixture entering the cylinders, a throttle valve is installed in the lower part of the mixing chamber. When the air and throttle valves are fully open, there is minimal resistance to air flow. The simplest carburetor prepares a combustible mixture of optimal composition only in a certain range of crankshaft speeds. The range depends on the jet capacity, diffuser cross-section, fuel level and throttle position. An automobile engine must operate over a wide range of crankshaft speeds and under constantly changing loads. To prepare a mixture of optimal composition in all possible operating modes, automobile carburetors are equipped with additional systems.
The main metering system of the carburetor is designed to supply the main amount of fuel in all engine operating modes, except idle mode. At the same time, at medium loads it should ensure the preparation of the required amount of lean mixture of approximately constant composition. In the simplest carburetor, as the throttle valve opens, the increase in air flow passing through the diffuser is slower than the increase in fuel flow flowing from the atomizer. The combustible mixture becomes rich. To prevent over-enrichment of the mixture, it is necessary to compensate its composition with air, depending on the degree of opening of the throttle valve. In a carburetor, such compensation is carried out by the main metering system. In Solex carburetors, compensation is carried out by pneumatic braking: fuel enters the atomizer not directly from the float chamber, but through the emulsion well - a vertical channel in which the emulsion tube is installed. The walls of the tube have holes for the exit of air entering it from above through an air nozzle. The flow of fuel into the emulsion well is determined by the fuel nozzle. In the emulsion well, the fuel is mixed with air coming out of the holes in the emulsion tube. As a result, fuel emulsion enters the atomizer rather than pure fuel. As the throttle valve opens, the vacuum in the diffuser increases and the flow of emulsion from the atomizer increases. At the same time, the flow of air into the emulsion well through the air nozzle increases, which reduces the flow of fuel from the float chamber through the fuel nozzle. The amount of fuel passing through the nozzle corresponds to the amount of air entering the diffuser, which ensures compensation of the mixture composition. The required composition of the combustible mixture is determined by the selection of the flow sections of the fuel and air nozzles, as well as the type of emulsion tube.
BALANCED FLOAT CHAMBER
In the simplest carburetor, the float chamber is connected to the atmosphere through a hole in the cover. During operation, as the air filter becomes dirty, the vacuum in the diffuser of such a carburetor will increase and, consequently, the mixture will begin to become richer. To eliminate the influence of air filter contamination on the composition of the combustible mixture, the internal cavity of the float chamber is connected by a channel to the carburetor neck.
For. running the engine at idle speed with a minimum crankshaft speed requires a small amount of combustible mixture. Therefore, the throttle valve should be almost completely closed. In this case, the vacuum in the diffuser is not enough for the main dosing system to start operating. Therefore, the carburetor is additionally equipped with an idle system, which prepares the air-fuel mixture in an amount that ensures stable engine operation with the throttle valve closed. The channels of the idle system connect the throttle space (the cavity of the intake pipe) with the emulsion part of the mixing chamber. When the engine is idling, a high vacuum forms under the throttle valve. Under the influence of vacuum, fuel from the emulsion well passes into the idle fuel channel, where it is mixed with air entering through the air channel from the upper part of the mixing chamber. The ratio of fuel and air in the emulsion is determined by the throughput of the fuel and air jets, which are installed in the idle channels. Next, the emulsion enters the throttle space, where it mixes with air passing through the gap between the chamber wall and the damper. The gap is adjusted with the “quantity” stop screw (SOLEX). The amount of fuel emulsion passing through the channel into the throttle space is regulated by a screw with a cone-shaped tip (a “quality” screw). When tightening the screw, the flow area of the channel decreases. And vice versa. When the throttle valve is gradually opened, the air flow through the mixing chamber increases, but the amount of incoming emulsion remains at the same level. The vacuum in the diffuser is still not enough for the main dosing system to start working. As a result, the mixture becomes leaner and a “failure” is observed in the engine’s operation. To ensure a smooth transition from idle to medium load mode, a transition system is used, which is combined with the idle system. The transition system channel connects the emulsion channel of the idle system with the over-throttle space of the mixing chamber. The outlet of the channel is located in such a way that, after opening the throttle valve, it appears in the vacuum zone; through it, an additional amount of emulsion enters the mixing chamber, smoothing the transition from one engine operating mode to another. At idle, when the throttle valve is closed, some of the air is mixed into the fuel emulsion through the transition system channel. Changes in the composition of the mixture are compensated by the selection of jets. When turning the “quantity” screw, the throttle valve opens slightly. As a result, the air flow through the channel of the transition system decreases, and through the gap between the walls of the mixing chamber and the damper increases. The amount of combustible mixture entering the engine increases, and the crankshaft speed increases. When the screw is unscrewed, the damper closes and the crankshaft speed decreases.
The main metering system ensures smooth engine operation only when the throttle valve is opened very smoothly. When the damper is opened abruptly (for example, for intense acceleration of a car), the mixture formation process is initially disrupted. To eliminate “failure” in engine operation in this mode, the carburetor is equipped with a special device - an accelerator pump. It is designed to briefly enrich the combustible mixture when the throttle valve is opened sharply. On carburetors, a diaphragm-type accelerator pump driven by the throttle axis is widely used. When the damper opens, a cam mechanically connected to its axis rotates and presses the diaphragm pusher. When the throttle valve closes, the cam stops acting on the pushrod. The diaphragm, under the action of the return spring, moves to its original position, creating a vacuum in the pump cavity. The discharge valve ball closes the hole in the well under the atomizer, and the suction valve ball allows fuel to flow into the pump. Gasoline from the float chamber passes through the suction valve, filling the pump cavity. When you sharply press the gas pedal, the cam presses on the telescopic pusher, compressing its spring. In this case, the discharge valve ball rises under fuel pressure, opening the way for fuel from the pump cavity to the atomizer. There is no sudden movement of the diaphragm, because fuel cannot pass quickly through the small nozzle outlet. Since the pusher spring is stiffer than the return spring of the diaphragm, the former, overcoming the resistance of the latter, moves the diaphragm, displacing a portion of fuel through the injection valve and nozzle into the carburetor mixing chamber. The injection process takes up to several seconds. This ensures stable operation of the engine when accelerating the car, and, in addition, the diaphragm is protected from rupture under the influence of fuel pressure.
When starting the engine, the crankshaft speed is low, the vacuum in the intake system is low, and gasoline does not evaporate well. In addition, as noted earlier, on a cold engine, especially at low ambient temperatures, most of the resulting fuel vapor condenses in the intake tract. Therefore, for a stable engine start, it is necessary to prepare a knowingly over-enriched air-fuel mixture in the carburetor. To do this, close the air damper and open the throttle slightly. Then a vacuum is created in the diffuser, sufficient for the required amount of fuel to flow out of the nozzle even when the crankshaft rotates slowly. A working mixture suitable for starting the engine is formed. But as soon as the first flashes appear in the cylinders, so that the engine does not stall from over-enrichment, it is necessary to open the air damper slightly, opening the way for air to enter the diffuser. To perform these operations, the carburetor is supplemented with a special starting device. On carburetors of domestic car engines, a manually controlled starting device is widely used. It consists of an air damper, an automatic device for opening it slightly and drive elements. The driver closes the air damper from inside the car using a handle, which is connected by a rod to the damper drive. The actuator allows the damper to open slightly, and the return spring tends to keep it in the closed position. The carburetor is equipped with a device that automatically opens the air damper to the required amount, which prevents the fuel mixture from becoming over-rich immediately after start-up. The device consists of a chamber with a diaphragm, a spring and a rod. The chamber is connected through a channel to the rear throttle space of the carburetor. With the beginning of stable engine operation, a sharp increase in vacuum occurs behind the throttle valve, from where it is transmitted through the channel to the chamber. The diaphragm, overcoming the resistance of the spring, moves and through the rod opens the air damper, leaning the mixture. Due to the fact that the damper is mounted asymmetrically on the axis, under the influence of vacuum, it tends to open in the mixing chamber, “helping” the starting device. The air damper is connected to the throttle valve by a mechanism that ensures that the throttle valve opens slightly when the air valve is completely closed. The amount of throttle valve opening should ensure stable operation of a cold engine when warming up. As the engine warms up, the driver manually opens the air damper and closes the throttle, reducing the crankshaft speed to the minimum stable speed.
To obtain maximum power from the engine, a rich fuel mixture is required. To prepare it, the carburetor is equipped with a special system called a power mode economizer. The system ensures that additional fuel enters the atomizer, bypassing the main fuel jet. To turn on the power mode economizer, a pneumatic or mechanical drive is used. The pneumatic actuator is activated when the vacuum in the mixing chamber drops, and not as the throttle valve opens. This makes it possible to enrich the mixture to the required extent when accelerating the car, ensuring good throttle response, and maintaining a lean mixture during uniform movement, ensuring efficiency. When the throttle valve is closed, the vacuum from the throttle space flows through the channel to the economizer diaphragm. In this case, the diaphragm compresses the return spring, and its pusher does not touch the economizer valve ball, and the valve is closed. When the throttle valve is opened, the vacuum under it (and therefore at the diaphragm) decreases. Under the action of the spring, the diaphragm moves, and its pusher, recessing the valve ball, opens the economizer channel. Additional fuel from the float chamber enters the nozzle of the main metering system, enriching the mixture.
The econostat is designed to further enrich the combustible mixture at maximum load conditions at high crankshaft speeds. An econostat is a sprayer installed at the very top of the mixing chamber, above the diffuser. Fuel is supplied into it directly from the float chamber through a channel in which a fuel nozzle is installed, which prevents over-enrichment of the combustible mixture. Sometimes, to fine-tune the economizer, an air jet is additionally installed in the upper part of the channel. Air is supplied through it, which is mixed with fuel in the channel. Since the nozzle outlet is located in a low-pressure zone, the economizer only comes into operation when the throttle valve is fully opened. In this case, the crankshaft rotation speed must be high enough to create a vacuum in the area of the nozzle outlet hole sufficient to raise the fuel in the channel to the level of the nozzle. The fuel entering through the atomizer is mixed with the flow of the fuel-air mixture, further enriching it.
Double chamber carburetor
To improve mixture formation and distribution of the combustible mixture among the cylinders, it is necessary to ensure low resistance to air movement through the carburetor diffuser at high loads and maintain sufficient vacuum in it at low loads. These requirements are best met by the design of a two-chamber carburetor with sequential chambers. The first chamber - the main one - ensures engine operation at idle speed, as well as at low and medium loads. The second - additional - is activated under heavy loads. The throttle valve drive of the second chamber can be mechanical or pneumatic. In the first case, the beginning of opening of the second chamber damper occurs at a certain opening angle of the first chamber throttle valve. In the second case, the opening moment depends on the magnitude of the vacuum in the mixing chambers.
Now all modern gasoline engines are equipped with an injection power system. Due to the fact that the injector is more advanced, it has practically replaced the carburetor in vehicles. But there are still a large number of cars on the roads, the engines of which are equipped with a carburetor system.
The carburetor is the main component of such a system, and its main task is to prepare the air-fuel mixture in the required proportion for its subsequent supply to the combustion chambers of the engine.
There are three types of carburetor systems in total, one of which, the bubbling type, is not used at all, and the other two, which include needle-membrane and float carburetors in the design, are still quite applicable and can be found on a wide variety of equipment.
Of the last two, only a float-type carburetor was used in vehicles. The needle-membrane type can be found on chainsaws, lawn mowers and even on aircraft.
The design and principle of operation of the carburetor
The float-type carburetor is a single unit included in the power system. During the use of such a system on cars, a large number of carburetors have been developed, with different design features, but they all function using the same principle.
What is a carburetor? The simplest float carburetor consists of two chambers:
- float chamber;
- and mixing.
The first task is to dose the fuel and maintain it at a certain level. Thanks to this chamber, a stable supply of gasoline is ensured under different engine operating conditions.
Structurally, it is very simple. Inside the device there is a float chamber with a float placed in it, connected to a needle-type valve, which is located in the gasoline supply channel from the gasoline pump. As fuel is consumed, the float lowers, and with it the valve, as a result the channel opens and gasoline is pumped into the cavity. When the required level is pumped, the float together with the valve rises up and completely blocks the channel.
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The second chamber ensures mixing of fuel into the passing air stream. For this purpose, a diffuser is installed in it - a specially narrowed section of the chamber. Thanks to this diffuser, the air passing through it is significantly accelerated.
These two chambers are connected to each other by a sprayer. The side that is installed in the float chamber is additionally equipped with a fuel nozzle - a special insert with a through hole of a certain diameter. Its task is to ensure the supply of a strictly defined amount of gasoline. The second end of the sprayer is led into the diffuser.
It all works like this: during the intake stroke in the engine cylinder, the piston moves downward, creating a vacuum. Because of this, air is sucked through the air intake with a filter installed in it. This intake is located on the carburetor so that the flow passes through the mixing chamber.
The movement of air during acceleration in the diffuser ensures the formation of a vacuum in the spray tube, due to which fuel begins to flow out of it and mix into the passing flow.
Regulation of the mixture supplied to the cylinders is ensured by a throttle valve, which is installed behind the diffuser. By blocking the channel through which the air-fuel mixture moves, the speed of air movement is regulated. It is this valve that the driver acts upon by pressing the accelerator.
The carburetor design involves another damper - an air damper. If the throttle regulates the supplied amount of the ready-made mixture, then the second damper shuts off the air supply. And since a vacuum is still created in the cylinders when the engine is running, the mixture turns out enriched, which is characterized by an increased fuel content.
What else is included in the design?
But this is a simplified carburetor diagram. In fact, it turns out that the carburetor consists of a large number of parts and everything is much more complicated, because the engine operates in different modes during operation, and each of them requires a mixture of the appropriate composition.
Therefore, a modern float-type carburetor has a complex structure with a significant number of channels, auxiliary systems and additional equipment. All this allows the carburetor to provide mixture formation in any operating mode.
Therefore, in the design of the carburetor, in addition to two chambers, there is:
- starting system;
- main dosing system;
- idle system;
- accelerator pump;
- economizer;
- econostat;
Each of these components has its own purpose in the carburetor design and ensures the supply of a mixture that is optimal in quantity and quality in any mode of operation of the power unit.
1. Starting system
The starting system ensures that a rich mixture is supplied to the engine cylinders when the engine is started. The main element of this system is the air damper. In domestic carburetors, it has manual control (a choke handle located in the cabin). In foreign analogues, an automatic start system is often found, which independently regulates the degree of opening of the air damper.
At the same time, the starting system is structurally designed to prevent the supply of an over-enriched mixture to the cylinders immediately after starting the engine. For this purpose, the damper drive is made so that it can open slightly on its own, ensuring a lean mixture. In addition, it is connected through a rod system to the throttle valve, which allows the carburetor to regulate the degree of opening of these valves during startup and warm-up.
2. Main dosing system
The main dosage system ensures the main supply of the mixture to the cylinder in all engine operating modes. The only thing is that it is not activated when the engine is idling. Its main task is to supply the required amount of mixture (somewhat lean) to the engine cylinders. In order to prevent over-enrichment of the mixture in transient conditions, this system compensates for the missing amount of air by supplying from the atomizer not pure gasoline, but an emulsion into which some of the air is already mixed. To do this, on most carburetors, the fuel, before entering the atomizer, passes through specially made emulsion wells, where pre-mixing is carried out.
3. System XX
The idle system ensures stable operation of the power plant at low speeds when the throttle valve is fully closed. It is a system of channels through which air and fuel are supplied under the throttle valve. That is, the mixing chamber is not used in this mode, since the XX system produces the required amount of mixture and supplies it to the intake manifold, bypassing it. Additionally, this system includes another channel - a transition channel, the task of which is to ensure the maintenance of stable engine operation during mode changes from idle to medium speed.
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4. Acceleration pump
The accelerator pump provides the required amount of mixture during sharp acceleration, when the main metering system does not have time to provide this, since it provides normal supply only when the throttle valve is opened smoothly. The task of this pump is to briefly enrich the mixture, which avoids “failure” during acceleration. For this purpose, there is a special channel, covered with ball valves and equipped with a membrane, the drive of which is carried out from the throttle. When you sharply press the accelerator, the balls slightly open the channel, and the membrane squeezes out a portion of the emulsion into a special sprayer installed in front of the diffuser.
Economizer and econostat
The economizer ensures maximum power output from the engine when needed. This is achieved by supplying an enriched mixture by feeding an additional portion of the emulsion into the main sprayer, bypassing the main dosage system.
The ecostat allows the engine to produce maximum power at high speeds. To do this, this element supplies gasoline directly from the float cavity and sprays it in front of the diffuser.
These are the main elements and systems of the carburetor. It also uses a balanced float chamber in its design. In order for the gasoline in it to be maintained at a given level, a vacuum should not form in the chamber and for this it is connected to the atmosphere. A balanced chamber involves combining it with the carburetor neck, which prevents contaminants from entering it along with air.
Carburetor Maintenance
With its complex design, the carburetor does not have many adjustments, and they only concern the idle system and the fuel level in the chamber with the float.
To establish stable operation of the engine at idle, there are two special screws - quantity (air) and quality (fuel). The first is a thrust element that regulates the degree of opening of the throttle valve to allow air to flow through the gap between it and the wall to create a mixture.
The second screw is a needle screw, installed in the channel through which the emulsion enters the throttle channel. By screwing in and out, the cross-section of this channel changes, and as a result, the amount of emulsion supplied.
The disadvantage of a carburetor is that it has a large number of channels and jets of small cross-section. Therefore, during operation, pollutants that enter along with air and gasoline settle in them and clog the channels and jets.
Therefore, it is important to periodically clean the unit. This can be done manually, with complete disassembly of the unit, washing and purging of the channels.
But recently, special cleaning products have appeared. Such cleaners are a special mixture that, when entering the channels, ensures the detachment and dissolution of deposits and resins in the channels, after which they enter the cylinders along with the fuel and burn. But it is worth noting that this product can only remove small blockages. If there is a large amount of deposits, they can only be removed manually.