OVERVIEW
OF A BASIC
AIR/FUEL SENSOR
PRODUCED BY THE BUREAU
OF AUTOMOTIVE REPAIR
Written by
Wayne Brumett
A/F SENSOR OPERATION
The following is a simplistic explanation of
how a wideband (broad planar) Air/Fuel
(A/F) Sensor operates. This is a complicated
subject area, and not easily understood by
many. In an attempt to reduce the level of
confusion, and enhance understanding, this
presentation covers just the very basic
areas of this subject. To this endeavor,
some of the more technical material was
intentionally left out of the program.
A/F SENSOR OPERATION
A simple O2 sensor
is constructed with:
•Platinum
Electrodes (2); and
•Zirconium Dioxide
element (between
the electrodes)
A/F SENSOR OPERATION
As exhaust displaces the
oxygen in the exhaust
pipe:
•The platinum electrodes
react (catalytic reaction)
to the potential difference
in oxygen content
between the outside air
and the inside of the
exhaust pipe.
• This potential difference
creates an electrical
current (voltage).
Low
O2
Fresh
Air
A/F SENSOR OPERATION
The current is developed,
because the platinum plate in
the exhaust stream will draw
oxygen (O2) ions from the
fresh air side to the exhaust,
as part of the catalytic
reaction. It is the transfer of
the O2 ions through the
platinum plates that creates
the current flow. The less O2
in the exhaust, the higher the
current flow (voltage) due to
the larger potential difference
in ion concentration from the
outside air to the inside of the
exhaust.
+
- PCM
O2
ION
Fresh
Air
A/F SENSOR OPERATION
The Zirconium oxygen sensor just described
worked well for its time, but unfortunately, this type
of sensor was prone to producing wide voltage
fluctuations (above and below 450 mV), and was
limited in its range. The PCM was forced to make
fuel/timing adjustment decisions, based on an
average of these fluctuating signals. With the
government requirement for low and ultra low
vehicle emissions, a more accurate method of
determining the exhaust O2 content and a wider
range of Air/Fuel ratio (upwards of 23/1) was
needed to achieve these new low emission levels.
A/F SENSOR OPERATION
A new sensor was
developed, called the
wideband (broad planar)
Air/Fuel sensor. This sensor
is based on the operation of
the old O2 sensor just
discussed. This sensor is
actually two O2 sensors
housed in one area, with a
common chamber between
them.
A/F SENSOR OPERATION
The wideband
(broad planar)
Air/Fuel sensor adds
two chambers to the
original Zirconium
sensor:
• A Diffusion
Chamber.
•An Air Reference
Chamber.
AIR
REFERENCE
CHAMBER
DIFFUSION
CHAMBER
A/F SENSOR OPERATION
Between the Diffusion
Chamber and the Air
Reference Chamber
is the second O2
sensor:
• Platinum Electrodes
(2)
• Zirconium Dioxide
Element
(between the electrodes)
• These plates are
wired in a parallel
circuit with the plates
to the left (exhaust
stream)
Diffusion
Chamber
AIR
REFERENCE
CHAMBER
A/F SENSOR OPERATION
In this circuit, the two sensors
share a common (floating)
ground.
The sensor that is in contact
with the exhaust stream is
commonly referred to as the
“sense” (signal) cell, which we
will call Sensor #1.
The sensor in contact with the
Diffusion Chamber and the
Air Reference Chamber is
commonly referred to as the
(Ion) “Pump” cell, which we
will call Sensor #2.
Sensor #1
Ground
(floating)
Sensor #2
AIR
REFERENCE
CHAMBER
Diffusion
Chamber
A/F SENSOR OPERATION
System Operation:
Sensor #1 operates as a
traditional O2 sensor,
sending a high voltage
signal (above 450mV) to
the PCM when the A/F
ratio is rich (low O2) , and
low voltage signal (below
450 mV) when the A/F ratio
is lean (high O2).
The purpose of the
Diffusion Chamber and
Sensor # 2, is to
counteract the change in
voltage of Sensor #1, and
keep it at 450 mV
(stoichiometric).
Sensor #1
Ground
(floating)
Sensor #2
AIR
REFERENCE
CHAMBER
A/F SENSOR OPERATION
To keep Sensor # 1 at 450 mV,
requires that Sensor # 2
provide a current flow (positive
or negative) that will move the
O2 ions in the opposite direction
that Sensor # 1 has them
moving (relative to exhaust O2
content).
Example:
If a rich mixture enters the
exhaust stream (low O2), many
O2 ions will flow from the
Diffusion Chamber, through
Sensor # 1, towards the
exhaust stream. The voltage on
Sensor # 1 will rise.
Rich Mixture
(Low O2)
Sensor #1
Ground
(floating)
Sensor #2
O
Diffusion
2
Io
Chamber
n
AIR
REFERENCE
CHAMBER
A/F SENSOR OPERATION
When Sensor # 1 voltage
rises above 450 mV (rich
mixture), the PCM reacts by
providing a negative (-)
current flow on Sensor # 2
to move (pump) the O2 ions
in the opposite direction,
back towards the Diffusion
Chamber, thus limiting the
O2 ion flow through Sensor
#1.
Rich
Mixture
(Low O2)
Sensor #1
Ground
(floating)
Sensor #2
O
2
Io
n
O
Diffusion
2
Io
Chamber
n
AIR
REFERENCE
CHAMBER
This action brings Sensor
#1 voltage down to 450 mV.
PCM Sends (-)
amp signal to
Sensor #2
A/F SENSOR OPERATION
When the O2 content in the
exhaust is high (lean mixture
– low O2 ion transfer), the
system reacts in the opposite
manner.
Lean
Mixture
(High
O2)
Sensor #1
Sensor #1 sends a low voltage
signal (under 450 mV) to the
PCM. The PCM sends a
positive (+) amperage signal
to Sensor # 2, to pump more
O2 ions from the Air Reference
Chamber through the
Diffusion Chamber, to the
exhaust stream (through
Sensor #1’s platinum plates).
This action causes more ions
to flow through Sensor #1
and brings the voltage up to
450 mV.
Ground
(floating)
Sensor #2
O
2
Io
O
n
Diffusion
2
Io
Chamber
n
AIR
REFERENCE
CHAMBER
PCM Sends (+)
amp signal to
Sensor #2
A/F SENSOR OPERATION
As you can see, Sensor # 2
controls Sensor # 1’s
voltage, by applying a
positive or negative current
flow. The PCM monitors the
current flow change on
Sensor # 2, and makes fuel
and timing corrections
based on the size of these
changes.
If there is zero current flow
on Sensor # 2, the air/fuel
ratio is at equilibrium (stoichiometric)
14.7 to 1
A/F
Ratio
Sensor #1
Ground (floating)
Sensor #2
AIR
REFERENCE
CHAMBER
A/F SENSOR OPERATION
THE
END