JP2609135B2 - Oxygen sensor for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an exhaust pipe of an internal combustion engine, which measures the oxygen concentration in exhaust gas which is closely related to the air-fuel ratio of the air-fuel mixture supplied to the engine. The present invention relates to an oxygen sensor for an internal combustion engine which is measured and used for providing a feedback signal in air-fuel ratio feedback control.

<Prior Art> Conventionally, as an oxygen sensor for an internal combustion engine, for example,
There is one shown in the figure (JP-A-58-204365,
See JP-A-59-31054.

That is, a platinum Pt paste is applied to each of the inner surface and a part of the outer surface of the ceramic tube 1 containing zirconium oxide ZrO ₂ (oxygen ion conductive solid electrolyte) as a main component, and the ceramic tube 1 is closed. By firing, the electrodes 2 and 3 for taking out the electromotive force are formed. Platinum Pt is further deposited on the outer surface of the ceramic tube 1 to form a platinum catalyst layer 4,
A protective layer 5 for protecting the platinum catalyst layer 4 is formed by spraying a metal oxide such as magnesium spinel thereon.

In such a configuration, the atmosphere is guided as the reference gas to the inner cavity of the ceramic tube 1, while the outside of the ceramic tube 1 is exposed to the engine exhaust passage to come into contact with the engine exhaust gas, and the air in the atmosphere contacting the inner surface is exposed. A voltage corresponding to the ratio of the oxygen concentration to the oxygen concentration in the exhaust gas in contact with the outer surface is applied to the electrodes 2 and 3.
The oxygen concentration in the exhaust gas is detected by generating the gas in the middle.

The platinum catalyst layer 4 promotes an oxidation reaction of CO + 1/2 O ₂ → CO ₂ , HC + O ₂ → H ₂ O + CO ₂ between carbon monoxide CO or hydrocarbon HC and oxygen O _2, and increases the stoichiometric air-fuel ratio. When the mixture is burned with a rich mixture, the low-concentration O ₂ remaining in that portion is satisfactorily reacted with CO and HC to bring the O ₂ concentration close to zero, thereby generating a large electromotive force.

On the other hand, when burned in the lean relative to the theoretical air-fuel ratio, CO of O ₂ and low concentration of the high concentration in the exhaust, because of the HC, CO, yet even reacts with HC and O ₂ O ₂ , the O ₂ concentration ratio inside and outside the ceramic tube 1 is so small that almost no voltage is generated.

<Problems to be Solved by the Invention> Incidentally, nitrogen oxides generated in the exhaust gas of an internal combustion engine usually show the following reactions.

2NO + 2CO → N ₂ + 2CO 2NO + 2H ₂ → N ₂ + 2H ₂ O 2NO + 3H ₂ → 2NH ₃ + O ₂ 2NO + N ₂ O + 1/2 O ₂ Normally, the reaction becomes dominant at high temperature and low temperature The reaction in the state becomes dominant. In particular, in an engine using an alcohol blended fuel obtained by mixing an alcohol such as methanol or ethanol with another fuel such as gasoline,
Reactions for H ₂ evolution is larger than in the case of gasoline fuel becomes superior, in a low temperature state, such as during start-up or during low-speed operation a large amount of NH ₃ present in the exhaust.

Ammonia NH ₃ is oxidized in the platinum catalyst layer 4 to consume O ₂ , so that when an alcohol-mixed fuel is used, the output of the oxygen sensor shifts to the lean side as compared with gasoline fuel. Moreover, NH ₃ because the diffusion rate in the protective layer 5 compared to O ₂ is fast, reaches earlier than O ₂ in the platinum catalyst layer 4 surface on the inside of the protective layer 5, the actual exhaust gas in the protective layer 5 the outer surface Since the amount of NH ₃ on the surface of the platinum catalyst layer 4 is apparently larger than the ratio of NH ₃ and O _{2 in} the fuel, the alcohol-mixed fuel is gasoline fuel as shown in FIG. 4 (shown by a solid line in the figure). As shown by the dotted line, the electromotive force characteristic is greatly shifted toward the lean side.

Therefore, when an alcohol-mixed fuel is used, there is a problem that lean combustion tends to occur, which increases the emission of nitrogen oxides and deteriorates exhaust characteristics.

The present invention has been made in view of the above problems, and when applied to an internal combustion engine using an alcohol-mixed fuel, eliminates the influence of the difference in diffusion rates between NH ₃ and O ₂ and reduces the amount of nitrogen oxide. An object of the present invention is to provide an oxygen sensor for an internal combustion engine that can perform fuel ratio control.

<Means for Solving the Problems> Therefore, in the present invention, electrodes are formed on each part of the inner and outer surfaces of the oxygen ion conductive solid electrolyte, and the inner surface side electrode contacted with the atmosphere and the outer surface contacted with the engine exhaust are formed. In an oxygen sensor for an internal combustion engine configured to detect an oxygen concentration in exhaust gas by an electromotive force generated between the electrode and a surface-side electrode, ammonia oxidation that promotes an oxidation reaction of ammonia on the outermost surface of the oxygen ion conductive solid electrolyte is provided. A catalyst layer is provided, and a nitrogen oxide reduction catalyst layer for promoting a reduction reaction of nitrogen oxide is provided inside the ammonia oxidation catalyst layer.

<Operation> In the above configuration, ammonia NH ₃ is previously oxidized by the ammonia oxidation catalyst layer on the outermost surface to be consumed by the oxidation reaction, and the influence of the diffusion rate difference between O ₂ and NH ₃ can be eliminated. Furthermore, by reacting nitrogen oxides including those generated in the ammonia oxidation catalyst layer with CO, HC, etc. in the inner nitrogen oxide reduction catalyst layer, the amount of CO, HC that reacts with O _{2 in the} platinum catalyst layer is reduced. since the density difference between the relatively atmosphere side O ₂ concentration decreases, it is lowered electromotive force is richer than the conventional lean detection is made with. Therefore, in the air-fuel ratio feedback control, the air-fuel ratio is controlled to be richer than the conventional one, so that the nitrogen oxide can be reduced.

<Example> An example of the present invention will be described below with reference to the drawings.
The same elements as those of the conventional example shown in FIG. 3 are denoted by the same reference numerals.

In FIG. 1, an inner electrode 2 made of platinum Pt is provided on a part of an inner surface and a part of an outer surface of a ceramic tube 1 having a closed tip mainly composed of zirconium oxide ZrO ₂ (oxygen ion conductive solid electrolyte). The outer electrode 3 is formed, and a platinum catalyst layer 4 formed by vapor deposition of platinum Pt and a protective layer 5 such as magnesium spinel are formed on the outer surface of the ceramic tube 1 as in the conventional case.

In this embodiment, the nitrogen oxide reduction catalyst layer 6 for promoting the reduction reaction of nitrogen oxide NOx is further provided on the surface of the protective layer 5.
And ammonia on the surface of the reduction catalyst layer 6 (on the outermost surface).
An ammonia oxidation catalyst layer 7 for accelerating the oxidation reaction of NH ₃ is laminated.

The reduction catalyst layer 6 comprises a metal oxide such as TiO ₂ , La ₂ O ₃ or Al ₂ O ₃ as a carrier and a reduction catalyst such as Rh or Ru for 0.5 to 5 minutes.
It is formed to be contained in the range of wt%.

The oxidation catalyst layer 7 is formed using Al ₂ O ₃ or the like as a carrier and containing a Pt catalyst at 0.1 to 5 wt% and a Ni catalyst at approximately 0.2 wt%.

According to such a configuration, 2NO + 3H ₂ → 2NH _{3 in a} low temperature state of the engine
Ammonia NH generated in large amount by + O ₂ reaction
₃ reacts with O ₂ in the exhaust gas in the oxidation catalyst layer 7 on the outer surface in accordance with the following reaction formula.

4NH ₃ + 5O ₂ → 4NO + 6H ₂ O For this reason, ammonia NH ₃ hardly reaches the surface of the platinum catalyst 4 inside the protective layer 5 and the same oxidation reaction of O ₂ and CO, HC occurs as in the conventional case. The oxygen concentration on the surface of the oxidation catalyst layer 7 is substantially equal to the oxygen concentration on the surface of the oxidation catalyst layer 7.

Therefore, the influence of the diffusion speed difference between O ₂ and NH ₃ in the protective layer 5 can be eliminated, and the electromotive force characteristics of the oxygen sensor can be prevented from shifting to the lean side based on the diffusion speed difference.

Further, when the NO generated by the oxidation reaction in the oxidation catalyst layer 7 and the NO contained in the exhaust gas reach the reduction catalyst layer 6 inside the oxidation catalyst layer 7, CO and HC, which are unburned components in the exhaust gas, are removed. The following reaction is promoted.

2NO + 2CO → 2CO ₂ + N ₂ 2NO + 2H ₂ → 2H ₂ O + N ₂ As a result, the platinum catalyst layer 4 inside the reduction catalyst layer 6
Since the unburned components CO and HC that react with the O ₂ that has reached the above-mentioned level have been reduced by the reaction in the reduction catalyst layer 6, the O ₂ concentration increases accordingly.

Therefore, the oxygen sensor in the ceramic tube 1 concentration difference between the inside of the O ₂ concentration and the O ₂ concentration of the exhaust side is decreased, the stoichiometric air-fuel ratio as shown in FIG. 2 (lambda = 1) from the rich side to contact with the atmosphere Is reduced to the slice level, and lean detection is performed.

As the concentration of nitrogen oxides in the exhaust gas is higher, the unburned components CO and HC that react with the nitrogen oxides increase and the reaction with O ₂ decreases, so that lean detection is performed on the richer side.

Therefore, when the air-fuel ratio feedback control is performed based on the detection result of the oxygen sensor, the air-fuel ratio is controlled to be richer as the nitrogen oxide concentration is higher.

In this embodiment, the present invention is applied to a tube-type oxygen sensor. However, it is apparent that the present invention can be applied to a plate-type oxygen sensor using zirconia.

Further, the protective layer 4 may be provided between the oxidation catalyst layer 7 and the reduction catalyst layer 6 on the outermost surface side.

<Effects of the Invention> As described above, according to the present invention, it is possible to correct the shift of the electromotive force characteristic to the lean side based on the difference in the diffusion rate between O ₂ and NH _3, and to achieve a richer effect by the action of the reduction catalyst layer. Lean detection is performed on the side. Therefore, when the present invention is applied to the use amount of the alcohol-mixed fuel, it is possible to prevent the air-fuel ratio from being controlled to the lean side and to reduce the nitrogen oxides.

[Brief description of the drawings]

FIG. 1 is an enlarged view of a main part showing an embodiment of the present invention, FIG. 2 is an output characteristic diagram of the oxygen sensor of the above embodiment, FIG. 3 is a sectional view showing the configuration of a conventional oxygen sensor, and FIG. FIG. 5 is a diagram showing output characteristics in the case of a conventional gasoline fuel and alcohol mixed fuel. DESCRIPTION OF SYMBOLS 1 ... Ceramic tube, 2 ... Inner electrode, 3 ... Outer electrode, 4 ... Platinum catalyst layer, 5 ... Protective layer, 6 ... Reduction catalyst layer, 7 ... Oxidation catalyst layer

Claims (1)

(57) [Claims] An electrode is formed on each part of the inner and outer surfaces of an oxygen ion conductive solid electrolyte, and is generated between an inner surface electrode in contact with the atmosphere and an outer surface electrode in contact with engine exhaust. An oxygen sensor for an internal combustion engine configured to detect an oxygen concentration in exhaust gas by an electromotive force, wherein an ammonia oxidation catalyst layer that promotes an oxidation reaction of ammonia is provided on an outermost surface of the oxygen ion conductive solid electrolyte, and the ammonia oxidation catalyst layer is provided. An oxygen sensor for an internal combustion engine, wherein a nitrogen oxide reduction catalyst layer for promoting a reduction reaction of nitrogen oxides is provided inside the catalyst layer.