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.
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 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.
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.
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.
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
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
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
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.
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
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,
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
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
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
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)
- Manifold absolute pressure (MAP)
- Barometric pressure (BP) sensor
- Park/neutral or neutral gear
- 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
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.