Many types of carburetors have been built to accommodate different load conditions, engine designs, and air/fuel requirements. Different carburetors feature different drafts, different numbers of barrels, different types of venturi, and different flow rates.

Carburetor Draft

Draft is defined as the act of pulling or drawing air. A carburetor's direction of draft is one way in which carburetors are classified. Most engines have a downdraft carburetor that has air flowing vertically down into the engine. In the sidedraft carburetor, air flows through the carburetor in a horizontal direction. Many early sports cars used a sidedraft carburetor. An updraft carburetor brings the air and fuel into the engine in an upward direction. Not many automobiles use this type, but they are used in forklifts and other industrial engine applications.

Carburetor Barrels

A carburetor barrel is a passageway or bore used to mix the air and fuel. It consists of the throttle plate, venturi, and air horn. A one-barrel carburetor is used on small engines that do not require large quantities of air and fuel.

A two-barrel carburetor has two throttle plates and two venturis. The area where the air comes into the carburetor is common on both barrels. A two-barrel carburetor may have one barrel that is smaller in diameter than the other one.

A four-barrel carburetor has four barrels to mix the air and fuel. The engine operates on two barrels during most driving conditions. When more power is needed, the other two barrels add fuel to increase the amount of horsepower and torque produced by the engine.

Venturi Types

Carburetors are also categorized according to the type of venturi they use. Simple carburetors have a single venturi. The double (dual) venturi has an additional secondary or boost venturi. The bottom of the center (boost) venturi is located at the greatest restriction area of the next larger venturi. This arrangement multiplies the vacuum developed in the venturi. The result is better vaporization and atomization and more control of fuel entering into the air stream. Thus, increasing the venturi effect increases the efficiency of the carburetor.

Even more control and atomization occur with a triple venturi design. The discharge tube feeds fuel into the smallest venturi for maximum control and atomization. Some carburetors have a variable or changing venturi. As the throttle pedal is depressed, the venturi increases in size. The venturi decreases in size when the throttle pedal is released.

VARIABLE VENTURI CARBURETOR

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 (and vacuum) 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 increase in size as engine demands increase. In this way, airflow speed through the venturi and the resulting pressure differential remains fairly constant. Thus, a variable venturi carburetor is also known as a constant velocity carburetor or a constant depression (vacuum) carburetor.

The venturi valves are controlled by a vacuum diaphragm that receives vacuum from ports in the throttle bores between the venturi valves and the throttle plates. As the throttle plates open, vacuum in the throttle bore increases and the vanturi valves open farther. As the valve 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 airstream so the air/fuel ratio remains constant. By metering both the fuel and airflow simultaneously, better fuel economy and lower emissions are possible.

FEEDBACK CARBURETOR SYSTEM

The latest type of carburetor system is the electronic feedback design, which provides 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).

However, for the three-way catalyst to work efficiently, the air/fuel mixture must be maintained very close to a 14.7 to 1 ratio. If the air/fuel mixture is too lean, NOX is not converted efficiently. If the mixture is too rich, HC and CO does not oxidize efficiently. 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. 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 (a lean mixture), the voltage output is low. If the oxygen level is low (a rich mixture), the voltage output is high.

The electrical output of the oxygen sensor is monitored by an electronic control unit (ECU). This microprocessor is programmed to interpret the input signals from the sensor and in turn 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. The oxygen sensor is constantly monitoring the oxygen in the exhaust, and the control module 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 control module 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 conventional manner and is said to be operating in open loop. (The control cycle has been broken.)

The carburetor operates in open loop until the oxygen sensor reaches a certain temperature (approximately 600F). 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 electronic sensor provides input relating to engine temperature. A vacuum sensor and a throttle position sensor indicate wide open throttle.

Early feedback systems used a vacuum switch to control metering devices on the carburetor. Closed loop signals from the electronic control module are sent to a vacuum solenoid regulator, which in turn controls vacuum to a piston and diaphragm assembly in the carburetor. The vacuum diaphragm and a spring above the diaphragm work together to lift and lower a tapered fuel metering rod that moves in and out of an auxiliary fuel jet in the bottom of the fuel bowl. The position of the metering rod in the jet controls the amount of fuel allowed to flow into the main fuel well.

The more advanced feedback systems use electrical solenoids on the carburetor to control the metering rods. These solenoids are generally referred to as duty-cycle solenoids or mixture control (M/C) solenoids. The solenoid is normally wired through the ignition switch and grounded through the electronic control module. The solenoid is energized when the electronic control module completes the ground. The control module is programmed to cycle (turn on and off) the solenoid ten times per second. Each cycle lasts 100 milliseconds. The amount of fuel metered into the main fuel well is determined by how many milliseconds the solenoid is on during each cycle. The solenoid can be on almost 100 percent of the cycle or it can be off nearly 100 percent of time. The M/C solenoid can control a fuel metering rod, an air bleed, or both.

In the Carter thermo-quad carburetor, variable air bleeds control the air/fuel ratio. This carburetor contains two fuel supply subsystems: the high-speed system and the low-speed system. The high-speed system meters fuel with a tapered metering rod positioned in the jet by the throttle. Fuel is metered into the main nozzle well where air from the feedback-controlled variable air bleed is introduced. Since this air is delivered above the fuel level, it reduces the vacuum signal on the fuel, thereby reducing the amount of fuel delivered from the nozzle.

The idle system is needed at times of low airflow through the venturi because there is insufficient vacuum at the nozzle to draw fuel into the airstream. After leaving the main jet, fuel is supplied to the idle system by the low-speed jet. It is then mixed with air from the idle by-pass, then accelerated through the economizer and mixed with additional air from the idle bleed before being discharge from the idle ports below the throttle. Air from the variable air bleed is introduced between the idle air bleed and idle port. This air reduces the vacuum signal on the low-speed jet and, consequently, the amount of fuel delivered to the idle system.

The thermo-quad uses a mixture control or pulse solenoid to control the variable air bleeds. The solenoid has only two positions of operation: opened when energized to bleed air to both the high speed and low-speed circuits or closed when de-energized, cutting off the air bleeds.

A less common method to control the air/fuel mixture is with a back suction system feedback. The back suction system consists of an electric stepper motor, a metering pintle valve, an internal vent restrictor, and a metering orifice. The stepper motor regulates the pintle movement in the metering orifice, thereby varying the area of the opening communicating control vacuum to the fuel bowl. The larger this area, the leaner the air/fuel mixture. Some of the control vacuum is bled off through the internal vent restrictor. The internal vent restrictor also serves to vent the fuel bowl when the back suction control pintle is in the closed position.

The 7200 VV carburetor was also produces with a feedback stepper motor that controls the main air bleed. The stepper motor controls the pintle movement in the air metering orifice thereby varying the amount of air being metered into the main system discharge area. The greater the amount of air. the leaner the air/fuel mixture. A hole in the upper body casting of the carburetor allows air from beneath the air cleaner to be channeled into the main system discharge area. The metered air lowers the metering signal at the main fuel metering jets.

Electronic Idle-Speed Control

To maintain federally mandated emission levels, it is necessary to control the idle speed. Most feedback systems operate in open loop when the engine is idling. To reduce emissions during idle, most feedback carburetors idle faster and leaner than nonfeedback carburetors.

To adjust idle speed, many feedback carburetors have an idle speed control (ISC) motor controlled by an electronic control module. The ISC motor is a small, reversible, electric motor. It is part of an assembly that includes the motor, gear drive, and a plunger. When the motor turns in one direction, the gear drive extends the plunger. When the motor turns in the opposite direction, the gear drive retracts the plunger. The ISC motor is mounted so the plunger can contact the throttle level. The ECU controls the ISC motor and can change the polarity applied to the motor's armature to control the direction in which it turns. When the idle tracking switch is open (throttle closed), the ECU commands the ISC motor to control idle speed. The ISC provides the correct throttle opening for cold or warm engine idle.

The electronic control module receives input from various switches and sensors to determine the best idle speed. Some of the possible inputs follow.

- Engine coolant temperature sensor

- Air charge temperature (ACT) sensor

- Manifold absolute pressure (MAP) sensor

- Barometric pressure (BP) sensor

- Park/neutral or neutral gear switch

- Clutch engaged switch

- Power steering pressure switch

- A/C clutch compressor switch

- Idle tracking switch (ITS)

Based on the input signals from the system's sensors, the ECU increases the curb idle speed if the coolant is below a specific temperature, if a load (such as air-conditioning. transmission, power steering) is placed on the engine, or when the vehicle is operated above a specific altitude.

During closed choke idle, the fast-idle cam holds the throttle blade open enough to lift the throttle linkage off the ISC plunger. This allows the ISC switch to open so the ECU does not monitor idle speed. As the choke spring allow the fast-idle cam to fall away and the throttle return to the warm idle position, the ECU notes the still low coolant temperature and commands a slightly higher idle speed.

As the engine warms up, the plunger is retracted by the electronic control module. If the A/C compressor is turned on, the ECU extends the plunger a certain distance to increase engine idle speed to compensate for the added load. When the throttle is opened and the lever leaves contact with the plunger, an idle tracking switch (ITS) in the end of the plunger signals the ECU. The electronic control module then fully extends the plunger where, upon contact with the lever (during acceleration), it acts as a dashpot, slowing the return of the throttle lever. When the engine is shut down, the plunger retracts, preventing the engine from dieseling. It then extends for the next engine startup.

In some systems, if the engine starts to overheat, the ECU commands a higher idle speed to increase coolant flow. Also, if system voltage falls below a predetermined value, the ECU commands a higher idle speed to increase alternator speed and output.

Normally, idle speed adjustments are not possible on carburetors with electronic idle speed control. Attempting to adjust idle speed by adjusting the ISC plunger screw results in the ECU moving the plunger to compensate for the adjustment. Idle speed does not change until the ISC motor uses up all of its plunger travel trying to compensate for the adjustment, at which point the  system is completely out of calibration. When idle speed driveability problems occur, the ISC system is usually responding to or being affected by the problem, not causing it.