overview of a basic air/fuel sensor

OVERVIEW OVERVIEW OF A BASIC OF A BASIC

AIR/FUEL SENSORAIR/FUEL SENSORPRODUCED BY THE BUREAU

OF AUTOMOTIVE REPAIR Written by

Wayne Brumett

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/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 OPERATIONThe 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.

Fresh Air

O2

ION

PCM+-

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/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

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.

AIR REFERENCE

CHAMBER

A/F SENSOR OPERATION

Sensor #1

Sensor #2Ground (floating)

DiffusionChamber

A/F SENSOR OPERATION

System Operation:System Operation:Sensor #1Sensor #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 # 2Sensor # 2, is to counteract the change in voltage of Sensor #1Sensor #1, and keep it at 450 mV (stoichiometric).

AIR REFERENCE

CHAMBER

Sensor #1

Sensor #2Ground (floating)

A/F SENSOR OPERATION

To keep Sensor # 1Sensor # 1 at 450 mV, requires that Sensor # 2Sensor # 2 provide a current flow (positive or negative) that will move the O2 ions in the opposite direction that Sensor # 1Sensor # 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 # 1Sensor # 1, towards the exhaust stream. The voltage on Sensor # 1Sensor # 1 will rise.

Diffusion Chamber

AIR REFERENCE

CHAMBER

Sensor #1

Sensor #2Ground (floating)

O2 Ion

Rich Mixture

(Low O2)

A/F SENSOR OPERATION

When Sensor # 1Sensor # 1 voltage rises above 450 mV (rich mixture), the PCM reacts by providing a negative (-) current flow on Sensor # 2Sensor # 2 to move (pump) the O2 ions in the opposite direction, back towards the Diffusion Chamber, thus limiting the O2 ion flow through Sensor Sensor #1#1.

This action brings Sensor Sensor #1#1 voltage down to 450 mV.

Diffusion Chamber

AIR REFERENCE

CHAMBER

Sensor #1

Sensor #2Ground (floating)

Rich Mixture(Low O2)

O2 Ion

PCM Sends (-) amp signal to

Sensor #2

O2 Ion

When the O2 content in the exhaust is high (leanlean mixture – low O2 ion transfer), the system reacts in the opposite manner.

Sensor #1Sensor #1 sends a low voltage signal (under 450 mV) to the PCM. The PCM sends a positive (+) amperage signal to Sensor # 2Sensor # 2, to pump more O2 ions from the Air Reference Chamber through the Diffusion Chamber, to the exhaust stream (through Sensor #1’sSensor #1’s platinum plates).

This action causes more ions to flow through Sensor #1Sensor #1 and brings the voltage up to 450 mV.

A/F SENSOR OPERATION

Diffusion Chamber

AIR REFERENCE

CHAMBER

Sensor #1

Sensor #2Ground (floating)

Lean Mixture(High

O2)

O2 Ion

PCM Sends (+) amp signal to

Sensor #2

O2 Ion

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)

AIR REFERENCE

CHAMBER

Sensor #1

Sensor #2Ground (floating)

14.7 to 1

A/F Ratio

THE THE ENDEND

A/F SENSOR OPERATION