how to sense the oxygen

1. Oxygen Sensor
Kireeti Naga Sharan Bandreddi,
Faculty of Electrical Engineering and Information Technology,
Technical University of Chemnitz,
Chemnitz, Germany
E-mail: kireeti-naga-sharan.bandreddi@s2013.tu-chemnitz.de
Abstract—Advancement of gas sensor technology over the past
few decades has led to significant progress in pollution control
and thereby, to environmental protection. An excellent
example is the control of automobile exhaust emissions, made
possible by the use of oxygen gas sensors. It was developed by
the Robert Bosch GmBH company during the late 1960s under
the supervision of Dr. Günter Bauman. Since early 1970s there
have been sustained studies on oxygen sensors and has led to
development of sensors for various applications with varying
performance characteristics. More recently, for biological and
medical applications, optical oxygen sensors are being to have
an impact. In this report, we focus on different types of oxygen
sensors, design of each sensor, their working principles and
specific applications.
Keywords—Oxygen sensor, Air to fuel ratio, lambda
I. Introduction
An oxygen sensor (or lambda sensor or exhaust
sensor) is an electronic device that measures the proportion
of oxygen in the gas or liquid being analysed. The
original sensing element is made with a thimble-shaped
zirconia ceramic coated on both the exhaust and reference
sides with a thin layer of platinum and comes in both heated
and unheated forms. The planar-style was introduced in the
year 1998 (pioneered by Bosch) and significantly reduced
the mass of ceramic content as well as incorporating the
heater within the ceramic structure. This resulted in a sensor
that started sooner and responded faster. The major
application is to measure the exhaust gas concentration of
oxygen for internal combustion engines in automobiles.
Concern over environmental pollution and health
issues has driven legislation over the past two decades and
significant research and development efforts have been
undertaken to address environmental issues. Worldwide
research in the field of gas sensors for many years has been
driven by the desire to reduce emissions from various
industrial sources. In particular, oxygen sensors have played
a key role in pollution control through automobile engine
management, optimizing industrial boilers, steel, cement
industries, biological and food processing plants and control
of chemical processes. Based on the number of sensors in
operation, the predominant use of oxygen sensors is in the
control of air-fuel mixture in the combustion engine of
automobiles and is an integral part of the ‘on board
diagnostic’ (OBD) of the exhaust emission control system.
The concentration of the partial pressure of the
oxygen in an environment can be determined using different
principles. For high temperature measurements of oxygen,
ceramic-based sensors are most practical. Equilibrium
potential measurements on solid electrolyte-electrode cells
enable oxygen measurement via the Nernst equation. The
sensor output varies logarithmically with oxygen partial
pressures. These sensors provide reproducible, stable and
accurate measurements of even low levels (ppm) of oxygen.
By imposing a diffusion barrier between the test gas flow
and the electrode, the electrolyte-electrode cell can be
operated. Then the current flowing through the cell provides
a measure of oxygen. As such, this sensor does not require
reference oxygen column and can respond linearly with
oxygen concentration. Semiconductor based sensors
measure oxygen via changes in electrical conductance
arising from alteration of defect chemistry by
chemisorptions of oxygen. All of these sensors described
above operate at high temperatures ranging from 300 ⁰C to
1000 ⁰C and can be used in harsh environment.
Monitoring oxygen under ambient conditions and
especially as dissolved oxygen is necessary in medical, food
processing and waste management industries.
A working sensor is typically characterized by
three parameters: sensitivity, selectivity and response time.
Sensitivity is the ability of the sensor to quantitatively
measure the test gas under given conditions. It is governed
by the inherent physical and chemical properties of the
materials used. Selectivity of the sensor is its ability to sense
a particular gas from interference. Response time is the
measure of how quickly the maximum signal change is
achieved with gas concentration changes. In addition,
reversibility, long term stability, size and power
consumption are other factors influencing the overall
performance of the sensor.
II. Exhaust system in an automobile
The exhaust system in an automobile consists of an
exhaust manifold, catalytic converter, resonator and a
muffler connected to a tail pipe. Inside the exhaust manifold,
hot exhaust gases along with sound waves are generated at
the end of exhaust stroke is sent to the exhaust manifold
through the exhaust valve. These sound waves and exhaust
gases pass from exhaust manifold to catalytic converter
through a pipe. Due to the partial combustion, the gases
entering inside the catalytic converter consists of a mixture
of carbon monoxides (CO), unburned hydrocarbons (HC)
and oxides of nitrogen which are harmful to
environment.
Inside the catalytic converter there are two ceramic
blocks with micro ducts consisting of platinum and rhodium
in one block while platinum and palladium in other block,
acting as catalysts. The toxic gas enter into the first ceramic
block and heat up simultaneously. This causes the catalyst to
react with the toxic gases. As the gases enter inside, the
nitrogen molecules are the first to react.

2. The catalyst causes the oxides of nitrogen to reform
into nitrogen and oxygen respectively.
The gas then flows through the micro ducts of the
second ceramic block where it reacts with the platinum and
palladium. Here, the carbon monoxide reacts with oxygen
molecules to form carbon dioxide (CO2).
The unburned hydrocarbons also react with oxygen to form
water and carbon dioxide.
The exhaust gas now becomes less toxic and comes from
catalytic converter having mixture of carbon dioxide,
nitrogen and water vapours. The exhaust gas now becomes
less toxic but consists of sound waves generated by the
engine.
To cancel the noise of these sound waves the gas is
made to flow through the muffler, which consists of
chambers of different sizes. The gases with sound waves
enter the first chamber of the muffler, which has drilled
holes around its surface. Some of the waves come out
through these holes and move back and forth against the
walls of the muffler. As more waves come out through the
holes, the space for movement reduces which causes friction
and ultimately destroys the sound waves.
Sound waves with more intensity pass through the
first chamber and try to enter into the second chamber. Here
the sound waves again collide with the walls and destroy
due to friction. The loudest sound waves make through both
the chambers and enter the Helmholtz resonator. Here the
sound waves hit the walls of the resonator and bounce back,
generating opposite sound wave of same frequency. This
phenomenon causes the sound waves to cancel each other.
Before flowing out through the tail pipe, the gas
and sound waves are made to flow through the third
chamber, where the noise is further reduced due to friction.
Finally, the exhaust gases consisting of less harmful gases
along with considerably low sound waves move out of the
tail pipe into the atmosphere.
III. Application in automotive
Oxygen sensor is located on the exhaust line, one
before the catalytic converter (upstream/pre-cat sensor) and
one after the catalytic converter (downstream/post-cat
sensor). Upstream sensor is used for regulating the fuel
supply that is it monitors the oxygen content of the vehicle’s
exhaust gases and sends a voltage signal to electronic
control unit (ECU). Where as a downstream sensor is placed
on 1996 or newer vehicles to monitor the performance of
catalytic converter and also detects whether the upstream
sensor is still working properly.
High pressure and temperature exhaust gases
leaving the engine cylinder during the exhaust stroke, travel
through the exhaust manifold and come in contact with the
oxygen sensor placed before the catalytic converter. The
sensing element at the front of the sensor consists of a
zirconium dioxide sensing element enclosed within a steel
shell. The sensing element is further connected to platinum
electrodes and wire leads down the line.
Exhaust gas consisting of oxygen molecules, come
in contact with the sensing element after flowing through the
holes on the steel shell. Outside air is made to flow through
the gaps between the connecting cables. This air is then
heated to enable the ions to produce voltage. The difference
in concentration of oxygen molecules in the exhaust gas and
the ambient air drives the oxygen ions from higher to lower
concentration.
Figure 1: Oxygen sensor in a car
Due to the movement of the oxygen ions from one
platinum layer to the other, a potential difference is
generated. A rich mixture surges the voltage approximately
up to 0.9V. On contrary, lean mixture drops the voltage
down to 0.1V.
These voltages signals are fed to electronic control
unit. Thus, the electronic control unit compares it with the
pre-stored standard data to decide whether the mixture is
lean or rich. These calculations are used to manipulate the
air-fuel ratio during the subsequent stroke.
IV. Design of two-step lambda oxygen sensor
Finger-type sensor
Sensor ceramic element with protective tube: The
solid electrolyte is a ceramic element which is impermeable
to gas. It is composed of a mixed oxide of zirconium and
yttrium in the shape of finger. The surfaces have been
provided on both sides with electrodes. The platinum
electrode inside the exhaust pipe, acts like a small catalytic
converter, and treats the exhaust gas catalytically and brings
back to stoichiometric balance (λ=1). In addition, the side
exposed to exhaust gas has a porous, ceramic multilayer to
protect it against contamination erosive damage, mechanical
impact and thermal shocks by metal tube. The sensor’s open
inner chamber facing away from the exhaust gas is
connected to the outside air, which acts as a reference gas.
Sensor with heater element and electric
connections: A ceramic support tube and a disc spring hold
and seal the active, finger-shaped sensor ceramic element in
the sensor housing. A contact element between the support
tube and the active sensor ceramic element provides the
contact between the inner electrode and the connecting

3. cable. By using a metal sealing ring, the outer electrode is
connected to the sensor housing.
Figure 2: Finger type sensor in exhaust pipe
A protective metal sleeve, which at the same time
serves as a support for the disc spring, locates and fixes the
sensor’s complete inner structure. It also protects the sensor
interior against contamination. The connecting cable is
grooved to the contact element which protrudes from the
sensor, and is protected against moisture and mechanical
damage by a temperature resistant cap. It is also equipped
with an electrical heater element. This ensures that the
ceramic element temperature remains sufficiently high, even
at low engine load and thus low exhaust-gas temperature.
This external heating is so quick that the sensor reaches the
operating temperature (350 ⁰C) within a response time of
20...30s and thus ensures low and stable exhaust-gas
emissions.
Planar lambda sensor
In terms of its function, the planar lambda sensor
corresponds to the heated finger-type sensors with a voltage-jump
curve at λ=1. However, on the planar sensor, the solid
electrolyte is comprised of a number of individual laminated
foils stacked one on top of the other. The sensor is protected
against thermal and mechanical influences by a double-walled
protective tube.
Figure 3: Planar lambda sensor
Its shape is like a long stretched-out wafer with
rectangular cross section. The surfaces of the measuring
cells are designed to protect against the erosive effects of the
exhaust. The heater is a wave shaped element containing
noble metal, integrated and insulated in the ceramic wafer
and ensures that the sensor heats up quickly even in the
event of low power input. The reference air passage inside
the sensor operating as a reference gas sensor, has access to
the ambient air. It can therefore compare the residual oxygen
in the reference atmosphere. Thus, the planar sensor voltage
also demonstrates an abrupt change in the area of the
stoichometric composition of air/fuel mixture (λ=1).
V. Method of operation of two-step lambda
sensor
Two-step lambda oxygen sensors operated on the
principle of a galvanic oxygen concentration cell with a
solid electrolyte (Nernst principle). The Nernst equation is
given as follows:
’
Where Us = Sensor voltage, R = Gas constant, T = Absolute
temperature, F = Faraday’s constant, = Partial pressure
of oxygen in the exhaust side and ’ = Partial pressure of
oxygen in the reference side
The ceramic element is conductive for oxygen ions
from a temperature of approximately 350 ⁰C (safe, reliable
operation at > 350 ⁰C). Due to the abrupt change in the
residual-oxygen content on the exhaust-gas side in the range
of λ=1 (e.g., 9*10-15 % vol. for λ = 0.99 and 0.2% vol. for
λ=1.01), the different oxygen content on both sides of the
sensor generates an electrical voltage between the two
boundary layers. This means that the oxygen content in the
exhaust gas can be used as a measure of the air/fuel ratio.
The integrated heater ensures that the sensor functions even
at extremely low exhaust-gas temperatures.
The voltage output by the sensor Us is dependent
on the oxygen content in the exhaust gas. In the case of a
rich mixture (λ < 1), it reaches 800...1000mV, and, in the
case of a lean mixture (λ > 1), it reaches only about 100mV.
The transition from rich to lean occurs at =
450...500mV.
Figure 4: Voltage curve of a two step lambda sensor for
different operating temperatures
The temperature of the ceramic element influences
its ability to conduct the oxygen ions, and thus the shape of
the output-voltage curve as a function of the excess-air
factor λ. In addition, the response time for a voltage change

4. when the mixture composition changes is also strongly
dependent on temperature.
Whereas these response times at ceramic-element
temperatures of below 350 ⁰C are in the seconds range, the
sensor responds at optimum operating temperatures of
around 600 ⁰C in less than 50ms. When an engine is started,
therefore, lambda closed-loop control is deactivated until the
minimum operating temperature of about 350 ⁰C is reached.
During this period the engine is open-loop-controlled.
VI. Design of broad-band lambda sensor
The broad-band lambda sensor is a planar dual-cell
limit-current sensor. It features a measuring cell made of
zirconium-dioxide ceramic , and is a combination of
a Nernst concentration cell (sensor cell which functions in
the same way as a two-step lambda sensor) and an oxygen
pump cell for transporting the oxygen ions. The oxygen
pump cell (Fig. 5, Pos. 8) is arranged in relation to the
Nernst concentration cell (7) in such a way that there is a
10...50μm diffusion gap (6). Here, there are two porous
platinum electrodes: one pump electrode and one Nernst
measuring electrode. The diffusion gap is connected to the
exhaust gas by way of a gas-access passage (10). A porous
diffusion barrier (11) serves to limit the flow of oxygen
molecules from the exhaust gas.
Figure 5: Planar broad band lambda sensor
On the one side, the Nernst concentration cell is
connected to the surrounding atmosphere by a reference-air
passage (5), and on the other, it is connected to the exhaust
gas in the diffusion gap.
The sensor requires control-electronics circuitry to
generate the sensor signal and to regulator the sensor
temperature.
An integrated heater (3) heats the sensor so that it
quickly reaches the operating temperature of 650...900 ⁰C
which is required for a signal that can be evaluated. This
function drastically reduces the influence of the exhaust-gas
temperature on the sensor signal.
VII. Method of operation of broad-band
lambda sensor
The exhaust gas enters the actual measuring chamber
(diffusion gap) of the Nernst concentration cell through the
pump cell’s small gas-access passage. In order that the
excess-air factor λ can be adjusted in the diffusion gap, the
Nernst concentration cell compares the gas in the diffusion
gap with the ambient air in the reference-air passage.
Figure 6: Pump current Ip of a broad band
lambda sensor as a function of the exhaust-gas
2
1
0
-1
-2
-3
excess-air factor λ
0 1 2 3 4 5
Excess-air factor, λ
The complete process proceeds as follows: By
Pump current, Ip
applying the pump voltage UP across the pump cell’s
platinum electrodes, oxygen from the exhaust gas can be
pumped through the diffusion barrier and into or out of the
diffusion gap. With the aid of the Nernst concentration cell,
an electronic circuit in the ECU controls the voltage
across the pump cell in order that the composition of the gas
in the diffusion gap remains constant at λ = 1. If the exhaust
gas is lean, the pump cell pumps the oxygen to the outside
(positive pump current). On the other hand, if the exhaust
gas is rich, the oxygen (due to the decomposition of and
H2O at the exhaust-gas electrode) is pumped from the
surrounding exhaust gas and into the diffusion gap (negative
pump current). At λ = 1, no oxygen needs to be transported,
and the pump current is zero. The pump current is
proportional to the oxygen concentration in the exhaust gas
and is thus a (non-linear) measure of the excess-air factor λ.
VIII. Pros and cons
A new or good working oxygen sensor can have
many advantages, over an automobile with ‘damaged or no’
oxygen sensor they can be as follows:
 High signal resolution and low pressure sensitivity
help provide precise engine control.
 Integral heater enables faster light-off for early
closed loop operations.
 Unique planar element design enhances thermal
shock resistance.
 Fast response helps improve fuel economy.
 Industry-leading poison-resistant coating helps
achieve better durability.
 Pumped air reference prevents air reference
contamination.
 Low power consumption reduces the vehicle’s
electrical energy requirements.
 Robust construction resists thermal shock without
cracking.
On the other hand a worn-out oxygen sensor or an
automobile with no oxygen sensor can have the
following disadvantages like fuel gets wasted, can cause
engine performance problems like, surging and
hesitating. It is the number one cause of excessive

5. harmful exhaust emissions and lastly, can cause a major
damage to catalytic converter.
IX. Future enhancements
One of the fastest growing industries in the present
world is automotive, which requires a huge demand for
precision, accuracy, fuel economy, safety issues, eco
friendly, best in design, long lasting etc. These are some of
the issues to be considered for developing an automobile to
full fill above requirements in the future using various
available technologies. One of the major issues being,
pollution which has to be reduced, from car to car in this
automotive world. Sensors play a key role in this aspect,
more importantly oxygen sensors, which need further
development day to day. Major issues like less operating
voltage, much lesser response time, reducing the weight of
heater element further, also by implementing various other
sensors in the exhaust path to monitor various other issues
so as to reduce the harmful gases, increasing the life time of
the sensor, reducing the cost, reducing the damage issues of
the sensor that is exposed out in the reference side that is
making it strong enough to with stand heat, water, etc.
X. Conclusion
As per Union of Concerned Scientists (UCS), a
gallon of gas is equal to 24 pounds of global warming
emissions. On the other hand as of United States
Environmental Protection Agency (USEPA), it is 28% of
greenhouse gas emissions are only from automobiles and a
whopping 75% of carbon dioxide emissions come from
automobiles. Previously, the use of planar type sensors has
changed an automobile to more eco-friendly and fuel
efficient type but things changed. Broad band oxygen
sensors revolutionised the entire exhaust system making it
more fuel efficient and less toxic gases. So, at the bottom
line taking the statistics from various organisations
regarding the pollution and the response of using oxygen
sensors among the automobile owners and making it a
compulsory module in the latest automobiles from the
manufacturer seems good in the days ahead.
XI. Acknowledgement
I would like to thank many people at the Chair of
Measurement and Sensor Technologies in Technical
University of Chemnitz who helped me with presentation
and report on oxygen sensors. Many people took time to
share their experience and expertise. Other people helped
me with the presentation on “Oxygen Sensor . I would like
to thank Prof. Dr.-Ing. Olfa Kanoun, Dr.-Ing Thomas
Keutel, Frank Ebert and Dr. Christian Müller for giving me
this opportunity.
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