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Teknikimobil.com – Loyal friends of mobil technicians, this time we will continue the discussion about carburetors vs electronic control injection while running. This is the eighth part of the article on the differences between carburetor systems and injection control systems (ECI). We will compare the two fuel delivery systems to the combustion chamber when the car is running (after warming up).
Throttle valve is slightly more open than idle (Source : M Step I MPI).
Carburetor System
When the primary throttle valve is opened slightly wider than idle, the throttle valve side will be higher than the slow port as shown in the picture above.
The negative pressure below the throttle valve, shown in the image below, will act on the slow port which previously provided air to the fuel coming out of the slow port. And is an addition to the fuel that is sucked out of the idle port. In this way, the levels of incoming air and fuel flow increase so that the engine will spin faster.
How the fuel comes out of the slow port (left image); How the primary side provides fuel (right image).
The work of the slow system up to this point is a functional step (connection function) of the slow system which has been achieved before sending fuel from the main system.
After that the main system takes over, and the negative pressure acting on the idle port and slow port is reduced. Fuel from the slow system is reduced as shown in the picture above. If this functional step does not take place smoothly, problems will occur such as the machine idling (“kincat” in regional language).
The way the fuel comes out is through the main air bleed (left); How to regulate fuel by main air bleed.
“ The picture above (left) shows how the fuel comes out through the main air bleed. The greater the level of incoming air flow, the greater the negative pressure acting on the main nozzle. And a large pressure difference with the inside of the emulsion tube will cause bubbles to appear in the fuel (Emulsion state). Meanwhile, the picture (right) shows how to regulate fuel by main air bleed. If the level of incoming air flow is even higher, the bubbles produced will reach their peak. ”
When the primary throttle valve is wide open, more fuel will come out of the main system. The air is mixed with the fuel in the main air duct, as shown in the figure above (left). Fuel with air bubbles will be supplied to the engine.
The main air passage at this point will create air bubbles in the fuel (emultion state). Bubbly fuel will flow more easily. And besides that, increasing the level of air flow in the venturi will suck in more fuel. In the situation shown in the figure above (right), the flow rate of the fuel sucked out is controlled by the position of the holes in the air ventilation tube which mixes more air with the fuel.
How the secondary side works.
When the primary throttle valve opens $50^o$ to $60^o$, the secondary side lock will be released until the secondary throttle valve can open.
The fuel delivery system determines various things when the engine is operating
With a negative pressure type carburetor, as shown in the picture above, if the negative pressure acting on the vacuum port located on the venturi is low, even though the negative pressure is acting on the secondary diaphragm side, the throttle valve cannot open because of the return spring. Therefore, the throttle valve will remain closed until the negative pressure that occurs in the vacuum port is large enough. When the negative pressure increases until it is able to open the secondary throttle valve, the negative pressure acting on the step port, as shown in the image below, will suck fuel out of the step port. The completion of the connection function (step function) is marked by the sucking of fuel out of the secondary main nozzle. (Some carburetors are not equipped with this mechanism, a mechanism that will open the secondary side when the primary side is also open).
Step function from the secondary side.
When the throttle valve opens wider and the engine speed increases, a stronger negative pressure acts on the vacuum port so that the secondary side throttle valve will open to suck fuel from the primary and secondary side main systems, allowing the vehicle to travel at high speed. This is the main function performed by the carburetor under general operating conditions.
How the damper valve works at maximum load. [table id=1 /]
ECI system
Under general working conditions, with reference to the basic spray amount, the ECI system determines a fuel spray amount. Namely what is required by the engine based on information on the amount of incoming air from the air flow sensor. And engine speed information is sent from the crank angle sensor.
When the throttle valve is slightly opened, the combination of the amount of air passing through it and the amount of air passing through the bypass channel will be reduced in the form of voltage from the air flow sensor. The counter circuit will determine the amount of fuel sprayed based on voltage information and engine speed information sent from the crank angle sensor.
When the throttle valve opens wider, the air that previously flowed through the bypass channel of the throttle body stops flowing. But more will flow through the throttle valve, and the amount of fuel sprayed will increase.
Air-Flow ratio feedback system (Air and fuel ratio feedback system)
The computer of the ECI system monitors the amount of oxygen contained in exhaust emissions in the form of a $O_2$ sensor signal ($O_2$ sensor excited voltage). Apart from that, it also compares the sensor voltage $O_2$ with a reference voltage value stored in the computer. And determines whether the air and fuel ratio of the mixture given to the engine is rich or lean by referring to the stoiciometric air and fuel ratio.
Carburetor feedback control
Based on the calculation results, the computer will slightly reduce or increase the amount of continuous spraying. Thus, the ratio of air and fuel fed to the engine is always adjusted to always approach the stoiciometric ratio of air and fuel. This setting is called a feedback compensation setting.
With the feedback compensation setting, the amount of spraying and the components in the exhaust emission only have very small variations. The ratio of fuel to air supplied to the engine also varies around the stoichiometric air to fuel ratio.
Same goes for the carburetor system. Because the air and fuel ratio is controlled by the computer, the main jet compensation opening of the main system will be changed. Or the amount of fuel sucked in will be increased or decreased. As shown in the picture above, to slightly change the fuel and air ratio.
We can see together that the carburetor system is more complicated than the ECI system, but the results obtained by the ECI system are more accurate. This is what makes the difference that the electric control system is more efficient compared to conventional (mechanical) systems, in this case the carburetor. In the next article we will discuss carburetor vs electronic control injection at maximum acceleration. []