Our Stories 07 Development of Oxygen Sensors (Wide Range Oxygen Sensors)

Story 07New Type "Oxygen Sensors" ("Wide range oxygen sensors") Serve to Clean Up Exhaust Gases while Aiming For the Highest Standards.

“Oxygen sensors” are essential to achieving the strict emissions regulations throughout the world.
This sensor monitors the exhaust gas O2 concentration emitted from an engine, and transfers it into a sensor signal. Following this, a fuel injection device controls the fuel injection based on the signal and this contributes to cleaning up the exhaust gas.
There are two kinds of oxygen sensors. One is an “Oxygen sensor”, which controls the stoichiometric air-fuel ratio. The other is a “Wide range oxygen sensor”, which precisely measures and controls a wide range of air-fuel ratios from rich to lean.
NGK SPARK PLUG CO., LTD. brought oxygen sensors to the market in 1980s and developed wide range oxygen sensors in the 1990s. Since then, we have been developing technologies with car makers and in 2015 achieved mass production of the wide range oxygen sensors (ZFAS-U2). We lead the market.

Deguchi:In charge of gathering customers' needs and deciding on the development concepts, as the representative of car makers.

Igarashi:In charge of developing, producing prototypes and evaluating the elements which form a core part of oxygen sensors.

Oba:Focuses on finding optimal designs for all the oxygen sensors based on the development concepts.

Victory or Defeat in the Oxygen Sensor Market Comes Down to a Fast Start.

The technological focus in relation to oxygen sensors has been changing. What is important now is not the performance of the moving car, but the performance before it even starts to move…

Naturally, oxygen sensors have to precisely detect the exhaust O2 concentration while the car is moving, but equally important is how fast the sensors can be activated in the period between the engine being started and the car beginning to move. This is because the element responsible for carrying out the detection can not function if it is not heated to a high enough temperature. As the sensors are not activated immediately after the engine starts, there is a short period of time during which the exhaust gases can not be said to be clean.

“In terms of driving performance, existing wide range oxygen sensors are sufficient. What we wanted to do was develop oxygen sensors that can be activated as fast as possible, control fuel injection and contribute to cleaning the exhaust gases, and to develop them before our rivals could” said Deguchi, who was involved in making development concepts.

In addition to working on activating the sensor as fast as possible, Deguchi and his colleagues started a project, based on the following development concept.

  • ・ In order to improve fuel consumption, they will halve the power consumption used to heat the sensors.
  • ・When raising the elements to a high temperature, if water remains in the exhaust pipe and adheres to the elements, they cool rapidly and break. To resolve this issue, the team worked on improving water resistance.
  • ・ In order to transfer the signal to the ECU* faster, they attached the sensors near the engine and worked on improving heat resistance to a level higher than that exhibited by current products.

None of these goals were easy, and the challenge of making the elements smaller than ever before in order to reduce the power consumption of the heater was especially difficult.

*ECU…Engine control unit. This is a central device in the computer which controls the engine.

Everything Is Done for One Simple Reason : to Make the Elements As Small As Possible

In order to achieve an increased speed in engine activation and a reduction in the power consumption of heaters coupled with the elements the elements must be made as small as possible.
To accomplish this, the team firstly decided on the size of the elements and then designed the oxygen sensors to fit.

In order to reduce the power consumption of heaters, we reduced the surface area by 20% compared with current products. This element has multiple laminated thin plates of zirconia and alumina, with heaters inserted between them. Furthermore, A conductive part is inserted between the detection electrodes (which monitor the exhaust gases) and the components attached to them. All the parts involved have a very fine structure. On top of that, we have to improve the water resistance of all parts. It was a hard task.” stated Igarashi, the member in charge of developing the elements.

As an example, in order to confirm the strength of the ceramics used in the elements, the team repeatedly carried out destructive testing. Because more than one hundred elements are needed for even one trial, the number of elements used in the trials is considerable.
As well as this, in order to verify (by means of an electronic microscope) that the surface of the multi-layered ceramics was correctly designed, they used mass production techniques to manufacture large numbers of elements.
“At times I was basically attached to the microscope for an entire day” said Igarashi.

When the samples we sent to car manufacturers - samples which met our standards - came back broken, I was shocked.
“When such an unexpected result came to our attention, we put all our effort into identifying its cause. We knew that our customers expected nothing less from us, which gave us the impetus to keep moving forward” says Igarashi.
The issue of water resistance, which was one of the most challenging problems, was solved by combining two layers with different structures, which resulted in the elements being able to withstand ten times as much water as they previously could.

The Satisfaction of Resolving Contradicting Specifications

In addition to the development of elements, they developed all parts of the oxygen sensors.

“When it comes to the design of oxygen sensors, we try to think of structures which allow the elements to reach their maximum performance potential. One particular issue was how precisely we can get the signal out from the smallest elements in wide range oxygen sensors and how the elements fit in with the other parts.”, stated Oba, who is a specialist of in the development of oxygen sensors.

“The width, thickness and length of elements that we dealt with were smaller than existing ones, so getting them to fit in becomes more and more difficult. If the length is reduced, then the vertical area for electrical contact points is reduced and accordingly we have to arrange a large number of terminals in a narrower space. Furthermore, the issue of insulation means that the level of precision demanded of the ceramic parts increases. During this development process we encountered a significant number of new issues.” said Oba.

In particular, when developing a method for attaching the terminals which pick up electrical signals, the team brought all their experience and knowledge to bear.

The method of construction involved inserting the elements in between the parts. As this method did not put any load on the elements, it was an effective technique. However, as the number of parts involved was high, we also foresaw problems arising with mass production. Consequently we used a cassette method by which we inserted pre-made terminal units into the elements, a useful technique from our sensor business.
As the elements are both small and thin, the process was difficult but as it reduced the number of parts needed and also simplified the construction process, it led to improvements in cost and reliability in mass production. As we were tasked with creating a sensor able to withstand the water and heat generated by exhaust gases and the movement of the vehicle, we were able to produce the first "dew point free"* sensors in the world. By assuming the attitude that we would reach this difficult goal at any cost, we were able to do exactly that.

Dew point free : If the temperature of the exhaust pipe is sufficiently high, then water can not remain. In order to prevent cracks caused by water, we would use sensors once they reached this temperature. The use of dew point free sensors, however, eliminates the need for this process.

The Designs of Products and Mass Production Have the Same Goal Yet Oppose Each Other.

Each success brings up another challenge. Nevertheless, this is the path to our goal.

The team successfully made small, highly efficient elements as well as the whole structure of the oxygen sensors which make use of the elements. This requires that the parts be inserted quickly but also with great accuracy during mass production. Oba and his colleagues, striving to tighten the tolerances of the parts and reduce the clearance as far as possible, had several disputes with the mass production team in relation to the need for the high speed insertion of parts.

Remembering the arguments, Oba said, “The team in charge of mass production design is different from the team in charge of product design, so it is almost impossible to go back to the product design stage. However, in order to create a next-generation product, we also have to take into account the ideas of the mass production design team. This is because we all have the same goal - creating superb products to satisfy our customers’ needs.

Accordingly, to try and satisfy the viewpoints of both teams, Oba and his colleagues took a new look at the product design. Instead of revising the clearance level in order to improve the ease of inserting the parts, they changed the shape of the parts in order to be able to insert them with the same clearance level. Staff in charge of manufacturing technology and facility design worked hard to develop a new crafting technique. In order to be able to insert parts with great accuracy, they took another look at how to grasp the parts, and redesigned the production facilities accordingly. In the end they were able to balance high quality with productivity.

More than a year after mass production began, the new oxygen sensors are meeting customers' expectations both in Japan and abroad, allowing us to make a worldwide contribution to the purification of exhaust gases.

The Project Never Ends.

While meeting the strict emissions regulations held throughout the world, we also have to satisfy our customers’ demands, and this is not an easy task. However, we hope that NGK SPARK PLUG CO., LTD. will always be up to the challenge.

For example, wide range oxygen sensors don’t work if not controlled by an electronic circuit. It is very important to support customers when it comes to optimizing the circuit in order for the sensors to deliver the best performance possible. “We always support our customers directly and honestly. Even after development finishes, mass production starts and orders are completed, the project is not over. Car makers are always looking ahead to the next product, or even further ahead than that. As our customers support our present products, so we are already starting on our next challenge”, said Deguchi, who is familiar with the needs of the market.

The evolution of oxygen sensors never ends.
We hope that this evolution will continue at a rate which exceeds our customers' expectations.

Inspired by the positive response from their customers, the oxygen sensor development team continues to work passionately towards new goals.