The principles of carburetion are presented so you may better understand the inner workings of a carburetor and how the other components of the fuel system function to provide a combustible mixture or air and fuel to the engine cylinders. PRINCIPLES OF CARBURETION.
The principles of carburetion are presented so you may better understand the inner workings of a carburetor and how the other components of the fuel system function to provide a combustible mixture or air and fuel to the engine cylinders.
PRINCIPLES OF CARBURETION
The factor that affect on the mixture process
is the changing of a liquid to a vapor.
*The rate of evaporation is dependent on the following
2- ATMOSPHERIC PRESSURE
The rate of movement of the molecules increase with temperature. Because of this, the amount of molecules leaving the liquid for a given time will increase, as the temperature increases.
As atmospheric pressure increases, the amount of air molecules present over the liquid also increases. The increased presence of air molecules will slow the rate of evaporation. This is because the molecules of liquid will have more air molecules to collide with. In many cases, they will fall back into the liquid after the collision
Basic carburetor consists of the following parts
5-Main discharge tube
The parts which often fasten to the air horn body are as follows: the choke, the hot idle compensator, the fast idle linkage rod, the choke vacuum break, and sometimes the float and pump mechanisms.
The float system keeps the fuel pump from forcing too much gasoline into the carburetor bowl
An excessively high float levelwill cause fuel to flow too freely from the discharge tube, causing an overly rich mixture
whereas an excessively low float level will cause an overly lean mixture
The basic parts of the float system are the fuel bowl, the float, the needle valve, the needle seat, the bowl vent
* The needle valve is usually made of brass
The carburetor acceleration system, like the off idle system, provides extra fuel when changing from the idle system to the high-speed system. The acceleration system squirts a stream of fuel into the air horn when the fuel pedal is pressed and the throttle plates swing open.
Without the acceleration system, too much fuel would rush into the engine, as the throttle quickly opened. The mixture would become too lean for combustion and the engine would stall or hesitate. The acceleration system prevents a lean air-fuel mixture from upsetting a smooth increase in engine speed.
The, high-speed system, also called the main metering system, supplies the engine air-fuel mixture at normal cruising speeds.
This system begins to function when the throttle plate is opened wide enough for the venturi action. Air flow through the carburetor must be relatively high for venturi vacuum to draw fuel out of the main discharge tube.
The high-speed system provides the leanest, most fuel efficient air-fuel ratio. It functions from about 20 to 55 mph or 2,000 to 3,000 rpm.
The full-power system provides a means of enriching the fuel mixture for high-speed, high-power conditions.
This system operates, for example, when the driver presses the fuel pedal to pass another vehicle or to climb a steep hill.
The full-power system is an addition to the high-speed system. Either a metering rod or a power valve (jet) can be used to provide variable, high-speed air-fuel ratio.
When the engine is cold, the fuel tends to condense into large drops in the manifold, rather than vaporizing. By supplying a richer mixture (8:1 to 9:1), there will be enough vapor to assure complete combustion.
The carburetor is fitted with a choke system to provide this richer mixture.
The choke system provides a very rich mixture to start the engine and to make the mixture less rich gradually, as the engine reaches operating temperature. The two types of choke systems are the manual and automatic
A computer-controlled carburetor uses a solenoid-operated valve to respond to commands from the microcomputer (electronic control unit). The system uses various sensors to send information to the computer that calculates how rich or lean to set the carburetor air-fuel mixture.
The system is also known as a computer controlled emission system which consists of the following: oxygen sensor, temperature sensor, pressure sensor, electromechanical carburetor, mixture control solenoid, computer, and idle speed actuator
1-The oxygen sensor, or exhaust gas sensor, monitors the oxygen content in the engine exhaust
2-The temperature sensor detects the operating temperature of the engine
3-The manifold pressure sensor (MAP) measures intake manifold vacuum and engine load.
4-The computer, also called the electronic control unit (ECU), uses sensor information to operate the mixture control solenoid of the carburetor
1-EXCESSIVE FUEL CONSUMPTION
2-A SLUGGISH ENGINE
4-FAILURE OF THE ENGINE TO START
5-HARD STARTING OF A WARM ENGINE
6-SLOW ENGINE WARM-UP
7-A BACKFIRING ENGINE
There are several devices used on carburetors to improve drivability and economy.
These devices are as follows:
The fast idle solenoid, the throttle return dashpot, the hot idle compensator, and the altitude compensator. Their applications vary from vehicle to vehicle