Weak air-fuel mixtures supplied by the single jet carburettor will not give enough power at high speeds. Therefore some special system is required to enrich the mixture at high speeds. One such system uses a metering rod with stepped diameter end in the main jet. At ordinary cruising speeds, the larger diameter part of the metering rod is in the jet which gives less fuel flow. At higher speeds, however, the metering rod is pulled up so that now smaller diameter part is in the jet. This increases the fuel flow to provide rich mixture.
Acceleration difficulty
When sudden acceleration is desired, the throttle is opened suddenly. This causes the maximum amount of air to come at once but the fuel supply lags thereby causing what is called engine stumble or hesitation which is due to weak mixture.
To avoid this hesitation a separate pump is used which provides fuel momentarily, till the increased fuel supply from the main nozzle starts. The pump is connected through linkage to the accelerator pedal. When the pump is connected through linkage to the accelerator pedal. When the accelerator pedal is pressed, the outlet valve is opened and the fuel is forced out of the acceleration jet. When the pedal is released, however, the piston moves up thereby sucking the fuel in from the float chamber. The pump is thus ready for next discharge.
Variable venturi carburetors
A fixed venturi does not change shape and size to accommodate changing engine performance demands. Therefore, the speed of the air flowing through the venturi varies according to engine rpm and load. Because the vacuum in the venturi is the result of moving air the amount of fuel drawn from the discharge nozzle varies as air velocity in the venturi fluctuates. In some engine operating modes, the air speed, vacuum level, and fuel discharge are matched to the needs of the engine. At other times, the fuel discharge might be too little or too much. To compensate for the inadequacies of a fixed venturi, idle systems, power systems, and choke systems are needed to supplement the main metering system. These assist systems are not necessary when a variable venturi is used. A variable venturi increases in size as engine demands increase. In this way, .airflow speed through the venturi and the resulting pressure differential remain fairly constant. An example of a variable venturi carburetor. A vacuum diaphragm that receives vacuum from ports in the throttle bores between the venturi valves and the throttle plates controls the venturi valves. As the throttle plates open, vacuum in the throttle bore increases and the venturi valves open farther. As the valves open, tapered metering rods attached to the valves retract from metering jets in the sides of the throttle bores. This increases the size of the jet openings, allowing additional fuel to be drawn into the air stream so that the air/fuel ratio remains constant. By metering both the fuel and airflow simultaneously, better fuel economy and lower emissions are possible.
FEEDBACK CARBURETOR SYSTEMS
The last type of carburetor system used was the electronic feedback design, which provided better combustion by improved control of the air/fuel mixture. The feedback carburetor was introduced following the development of the three-way catalytic converter. A three-way converter not only oxidizes HC and CO but also chemically reduces oxides of nitrogen (NOx) When three-way catalytic converters were installed, engineers discovered that air/fuel ratios must be maintained very close to the stoichiometric ratio of 14.7:l in order for the converters to be effective in lowering emission levels. If the air/fuel ratio is richer than stoichiometric, HC and CO levels are high, and the converter cannot lower these emission levels to the desired limit.
However, with a rich mixture, the combustion temperature is lowered. Therefore, the production of NOx is also lower and the converter is very effective in controlling NO emissions under this condition. When the air/fuel ratio is leaner than the stoichiometric ratio, the levels of CO and HC are low, and the converter is very effective in controlling these pollutants.
However a lean air/fuel ratio bums hotter in the combustion chambers, and NO. emissions become very high. Under this condition, the converter is ineffective in controlling NO. emissions. Therefore, the air/fuel ratio must be controlled at, or very close to, the stoichiometric ratio in order for the three-way converter to reduce all three emission levels to the desired limit.
It was discovered that conventional carburetors did not provide accurate air/fuel ratio control under all conditions; therefore, the three-way converter was not effective in reducing emission levels on engines with these carburetors. Computer-controlled carburetors were designed to maintain the air/fuel ratio at, or near, the stoichiometric ratio under most operating conditions, which allows the catalytic converter to provide effective control of exhaust emissions. Monitoring the air/fuel ratio is the job of the exhaust gas oxygen sensor. An oxygen sensor senses the amount of oxygen present in the exhaust stream. A lean mixture produces a high level of oxygen in the exhaust. A rich mixture produces little oxygen in the exhaust. The oxygen sensor placed in the exhaust before the catalytic converter produces a voltage signal that varies with the amount of oxygen the sensor detects in the exhaust. If the oxygen level is high, the voltage output is low. If the oxygen level is low, the voltage output is high. An electronic control unit (the PCM) monitors the electrical output of the oxygen sensor. This microprocessor is programmed to interpret the input signals from the sensor and in turn to generate output signals to a mixture control device that meters more or less fuel into the air charge as it is needed to maintain the 14.7 to 1 ratio. Whenever these components are working to control the air/fuel ratio, the carburetor is said to be operating in closed loop. Closed loop is illustrated in the schematic. The oxygen sensor is constantly monitoring the oxygen in the exhaust, and the PCM is constantly making adjustments to the air/fuel mixture based on the fluctuations in the sensor's voltage output. However there are certain conditions under which the PCM ignores the signals from the oxygen sensor and does not regulate the ratio of fuel to air. During these times, the carburetor is functioning in a conventional manner and is said to be operating in open loop. The carburetor operates in open loop until the oxygen sensor reaches a certain temperature (approximately 600°F [316°C]). The carburetor also goes into open loop when a richer than normal air/fuel mixture is required, such as during warm-up and heavy throttle application. Several other sensors are needed to alert the PCM to these conditions. Open loop also occurs when the engine’s coolant is cold. Under this condition, the 02 sensor is too cold to produce a satisfactory signal, and the computer program controls the air/fuel ratio without the 02 sensor input. During open loop mode, the computer provides a rich air/fuel ratio. Since a computer-controlled carburetor also has a conventional choke, a richer air/fuel ratio is supplied. As the engine approaches normal operating temperature, the computer goes into closed loop and begins to use the O2, sensor signal to control the air/fuel ratio. Closed loop in many computer-controlled carburetor systems occurs when the engine coolant temperature (ECT) sensor informs the computer that the coolant temperature has reached the specified level. Therefore, the ECT sensor signal is very important, because it determines the open or closed loop status. For example, if the engine thermostat is defective and the coolant temperature never reaches normal operating temperature, the PCM never enters closed loop. When this open loop condition occurs, the air/fuel ratio is continually rich and fuel economy is reduced. Some systems go back into open loop during prolonged periods of idle operation when the 02 sensor cools down. Many systems revert to open loop at, or near wide-open throttle to provide a richer air/fuel ratio.
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