FIELD OF THE INVENTION

The present invention relates to a method of diagnosing an oxygen sensor.

BACKGROUND OF THE INVENTION

The invention concerns generally the area of exhaust gas after treatment especially in regard to motor vehicles driven by internal combustion engine and particularly a procedure to diagnose an oxygen sensor. The oxygen sensors are used in an exhaust pipe of the engine to determine the exact air/fuel ratio of the engine. The oxygen sensors are vulnerable to contamination and aging. They can become sluggish and slow to respond to changes in the air/fuel mixture as contaminants build up on the sensor element.

Contaminants include phosphorus from motor oil and even sulfur in the coolant oil and other additives in the fuel. Further, the health of the sensor depends on dirt accumulated on the platinum electrodes of a pump cell and Nernst cell, reduction in ionic conductivity of the ceramic of the sensor and capacitance between the platinum electrode and the ceramic material in the pump cell and the Nernst cell. There are known techniques available for determining health of the oxygen sensor such as monitoring the response of oxygen sensor while fuel cutoff event and etc.

ADVANTAGES OF THE INVENTION

The present invention is advantageous in that it measures a response time of a oxygen sensor in a particular sequence of operation of the sensor and determines the health of the sensor. This response time can be used by a control unit to calculate the response delay and compensate the same for better control of the combustion in the engine.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Different embodiments of the invention are disclosed in detail in the description and illustrated in the accompanying drawing:

Figure. 1 illustrates a schematic of a oxygen sensor and control of the sensor.

Figure. 2 illustrates a method to determine a response time of the oxygen sensor.

Figure. 3 shows a timing diagram of various signals of the oxygen sensor.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure. 1 illustrates schematic of an oxygen sensor 10 and control of the same using a device 100. The oxygen sensor 10 is positioned in an exhaust pipe of an internal combustion engine to measure the proportion of oxygen in the remaining exhaust gas 12 of the combustion engine. The oxygen sensor 10 made of solid electrolyte such as stabilized zirconium dioxide (Zr02) ceramics, which is an oxygen ion (O^) conductor.

The oxygen sensor 10 comprises a pump cell 20, Nernst cell 30, a diffusion barrier 22, an orifice 24 and a heating element 50. The oxygen sensor 10 has a reference chamber 14 with oxygen concentration that of ambient air and a measuring chamber 40, which is connected to the exhaust gas 12 by a diffusion barrier 22. The exhaust gas 12 enters the diffusion barrier 22 through the orifice 24, which later diffuses into the measuring chamber 40. The oxygen sensor 10 is a ceramic cylinder plated with porous platinum electrodes: an external pump electrode 16, an internal pump electrode 17, a Nernst electrode 18 and a reference electrode 19. The oxygen sensor 10 is connected to a device 100. The device 100 is a microprocessor or a microcontroller or an electronic control unit. The device 100 comprises a controlling means 70, a measuring means 80 and a determining means 90. The measuring means 80 measure response time of the oxygen sensor 10 to determine the health of the oxygen sensor 10 by the determining means 90.

A pump current Ip in the pump cell 20 is controlled by the device 100 by the controlling means 70. The controlling means 70 is a hardware circuit dedicated for controlling the pump current Ip. The controlling means 70 controls the oxygen concentration within the measuring chamber 40 at a constant oxygen concentration such that air/fuel ratio maintained at a stoichiometric mixture corresponding to lambda = 1 by varying the pump current Ip through the pump cell 20. The constant concentration is achieved by removing the excess oxygen or adding the deficient oxygen in the measurement chamber 40 using the pump current Ip. The current applied to maintain the constant oxygen concentration corresponding to lambda = 1 with in the measuring chamber 40 is the measure of the oxygen present in the exhaust gas 12. The Nernst electrode 18 and the reference electrode 19 of the Nernst cell 30 provide an output voltage Nv corresponding to the quantity of oxygen in the exhaust gas relative to that of the oxygen in the reference chamber 14. The Nernst cell 30 is an electrochemical cell that produces the output voltage Nv that is proportional to the difference in partial pressure of oxygen concentration between measuring chamber 40 and the reference chamber 14. The output voltage Nv is created by oxygen ions (0) migrating through the solid electrolyte (Zr02) material of the cell. At a high concentration of oxygen within the measuring chamber 40, the Nernst cell 30 provides a low voltage level and at a low concentration of oxygen, the Nernst cell 30 provides a high voltage level. At a set point of oxygen concentration lambda = 1, the voltage across the Nernst cell 30 is at a reference voltage level. An output voltage of 0.05 V to 0.35V (50mv to 350mV) represents a "lean mixture" of fuel and oxygen. In the lean mixture, oxygen concentration in the exhaust gas is higher than the oxygen concentration corresponds to lambda = 1. If the presence of the lean mixture is indicated by the Nernst cell 30 voltage, a positive current is applied by controlling means 70 through the pump cell 20 to pump the excess oxygen ions out of the measuring chamber 40. Thus excess oxygen concentration is removed from the measuring chamber 40 and lambda = 1 is achieved in the measuring chamber 40. The current required to pump out the excess amount of oxygen ions to maintain the oxygen concentration lambda =1 in the measurement chamber is the measure of oxygen concentration in the exhaust gas. An output voltage of 0.5 to 0.9V {500mv to 900 mV) represents a "rich mixture", one which is high in unburned fuel and low in remaining oxygen. In rich mixture presence of oxygen molecule in the exhaust gas 12 is less than that corresponds to lambda = 1. If the presence of the rich mixture is indicated by the Nernst cell 30 voltage, a negative current is applied by controlling means 70 through the pump cell 20 to pump the oxygen concentration into the measuring chamber 40. Thus oxygen concentration deficiency is compensated in the measuring chamber 40 and the constant value lambda = 1 is achieved in the measuring chamber 40. The current required to pump in the amount of oxygen ions to achieve lambda =1 is the measure of oxygen concentration in the exhaust gas. The ideal set point is approximately 0.45 V (450 mV) which is a reference voltage level.

Figure.2 illustrates a method of determining response time of the oxygen sensor 10. The oxygen sensor 10 is diagnosed on board a vehicle in accordance with this invention. The oxygen sensor 10 becomes active after the engine is started and a temperature of the sensor 10 reaches an operating temperature by heating the heating element 50 of the oxygen sensor 10. The oxygen sensor 10 is diagnosed to determine a response behavior of the oxygen sensor 10 in a particular sequence of operation. The particular sequence of operation is executed by the device 100 in a predefined time and is calibrated in the device 100. In the first step 501, the oxygen sensor 10 is switched ON, Initially the oxygen sensor 10 is working in a normal working condition such that air/fuel ratio maintained at a stoichiometric mixture (lambda =1) in the measuring chamber 40 using the pump current Ip which results in a first voltage level VI across the Nernst cell 30.

During this step SOI, the pump current Ip is maintained by the controlling means 70 such that the voltage across Nernst cell 30 provides the first voltage level VI which is the reference voltage level. In the next step 502, the pump current Ip is switched OFF for a predetermined time Tl. Switching OFF of the pump current Ip will generates a second voltage level V2 across the Nernst cell 30 after the new exhaust gas 12 diffuses into the measurement chamber 40. During the switching OFF of the pump current Ip, the new exhaust gas 12 diffuses through the diffusion barrier 22 into the measuring chamber 40.

In case of lean mixture, the oxygen concentration in the measuring chamber 40 is relatively more compared to the concentration at lambda = 1 and the Nernst cell 30 provides the low voltage. In case of rich mixture, oxygen concentration in the measuring chamber 40 is less compared to the concentration at lambda = 1 and the voltage across the Nernst cell 30 is of high voltage. For example the second voltage level V2 is around 50 mV to 350mv in the lean mixture and 500 to 900mv in the rich mixture of the air/fuel mixture. In the step 503, the pump current Ip is switched ON after the predetermined time Tl. Now the pump current Ip through the pump cell 20 is maintained by the controlling means 70 such that oxygen concentration in the measuring chamber 40 is equal to lambda = 1. The voltage across the Nernst cell 30 now reaches the first voltage level VI after switching ON the pump current Ip. The time duration Td taken to reach the first voltage level VI from the second voltage level V2 is the measure of the response behavior of the oxygen sensor 10. A measuring means 80 measures the time duration Td to determine the response behavior of the oxygen sensor 10. in this time duration Td the oxygen concentration in the measuring chamber 40 is below lambda = 1. For example the oxygen concentration may vary from oxygen concentration corresponding to the lambda = 0.7 to oxygen concentration up-to lambda 0.95 for a rich mixture. Similarly for a lean mixture, the oxygen concentration in the measuring chamber 40 is above lambda = 1. For example the oxygen concentration may vary from oxygen concentration corresponding to the lamda = 1.05 to oxygen concentration up-to lambda = 1.6. The health of the oxygen sensor 10 is good if lesser the time duration Td to reach the first voltage level VI and sensor response is slow if the time duration Td is more when compared to a reference threshold value. The slower behavior of the oxygen sensor is either due to ageing or contaminants build up on the oxygen sensor. Based on the time duration Td driver of the vehicle is informed about behavior of the oxygen sensor 10 whether the oxygen sensor 10 needs a replacement or not. In the step S04, the time duration Td is used by the device 100 to calculate a response delay of the oxygen sensor 10 and compensate the same for the better control of the combustion. This time duration Td is stored in a non volatile memory of the device 100 and can be used to calibrate the response behavior of the oxygen sensor 10.

Figure.3 illustrates a response behavior of the oxygen sensor 10 in a timing diagram. The upper graph shows a voltage Nv verses time and lower graph shows pump current Ip verses time. As shown in the Figure.3 the output voltage of the sensor 10 is at first voltage level VI when the pump current Ip is switched ON. During which lambda = 1 is maintained in the measurement chamber 40 by varying the pump current ip through the pump cell 20 by the controlling means 70. The pump current ip is switched OFF for a predetermined time Tl and now the voltage across Nernst cell 30 is at second voltage level V2. As soon as the pump current Ip is switched OFF, there is no feedback current in the pump cell 20. The exhaust gas 12 with new oxygen concentration diffuses through the diffusion barrier 22 into the measuring chamber 40. Due to the new exhaust gas the second voltage level V2 is created across the Nernst cell 30. The second voltage level V2 indicates that the oxygen concentration of the exhaust gas in the measuring chamber 40 is not corresponding to lambda = 1. The example timing diagram shown in the Figure,3 Is for a lean mixture. After the predetermined time Tl the pump current is switched ON. The pump current Ip through the pump cell 20 Is varied by the controlling means 70 such that oxygen concentration in the measuring chamber 40 is corresponding to lambda = 1. The voltage across the Nernst cell 30 reaches the first voltage level VI. The time taken for Nernst cell voltage to vary from second voltage level V2 to the first voltage level is represented as Td. This time is shown with Td and the voltage is reached to first voltage level vl from the second voltage level V2. This measured time duration Td is stored in the non volatile memory of the device and can be used to calibrate the response behavior of the oxygen sensor 10.

Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art.

WE CLAIM:

1. A method of diagnosing an oxygen sensor (10), said method comprising the steps:

- switching ON a pump current (Ip) through a pump cell (20) of said oxygen sensor (10) to maintain the voltage across a Nernst cell (30) of said oxygen sensor (10) at a first voltage level (VI);

- switching OFF the pump current (Ip) for a predetermined time Tl through said pump cell (20) to obtain a second voltage level across said Nernst cell (30) of said oxygen sensor (10) from said first voltage level (VI);

- switching ON the pump current (Ip) after said predetermined time Tl;

- measuring a time duration (Td) in which the voltage (Nv) across said Nernst cell (30) reaches the first voltage level (VI) from the second voltage level (V2) when the pump current (Ip) is switched ON; and

- determining health of the oxygen sensor (10) from said measured time duration (Td).

2. The method according to claim 1, wherein the pump current (Ip) is switched
OFF for a predetermined time (Tl) to obtain the second voltage level (V2) across the Nernst cell (30) that represents oxygen concentration in the measurement chamber (40) other than lambda = 1.

3. A device (100) to operate an oxygen sensor (10), said sensor has a pump cell (20) and a Nernst cell (30), said device (100) characterized by:

- a controlling means (70) to switching ON and OFF a pump current (Ip) through the pump cell (20);

- a measuring means (80) to measure a time duration (Td) in which the voltage (Nv) across said Nernst cell (30) reaches a first voltage level (VI) from a second voltage level (V2) when the pump current (Ip) is switched ON from the switched OFF state; and

- a determining means (90) to determine health of the oxygen sensor (10) from said measured time duration (Td).

4. The device according to claim 3, wherein said measured time duration (Td) is stored in a non volatile memory of said device (100).