EP0946868B1 - Method for electric diagnosis of an internal combustion engine oxygen sensor - Google Patents

Description

The present invention relates to a method electrical diagnostic of an oxygen sensor of lambda type mounted on the exhaust line of a internal combustion engine. The present invention relates more particularly to the diagnosis of lambda probe fitted to the exhaust line internal combustion engine powered with fuel mixtures of suitable wealth and especially with poor mixtures.

Internal combustion engines are conventionally equipped with an electronic command that adjusts, from strategies preprogrammed and according to the conditions of engine operation, amount of fuel injected and the instant of ignition for engines with controlled ignition.

Some engines are powered today with a fuel mixture whose richness is adapted to engine operating conditions and which are notably supplied with a lean mixture (lean burn) during the operating phases in partial load.

Among the sensors used by electronic control systems for these motors to determine the operating conditions and adapt the richness of the mixture accordingly fuel, more particularly appear the oxygen sensors arranged on the line engine exhaust. These sensors have function of informing the electronic system of control over the actual oxygen concentration in the exhaust gases.

Within the framework of future regulations emissions such as OBD2 (On Board) Diagnostic Level 2), it is expected that the systems electronics of the motors fitted to the motor or road vehicles are brought to diagnose and / or correct automatically certain failures that have a direct impact on pollutant emissions.

This is how it is planned to set up in electronic control systems for engines, means of diagnosis of any sensor malfunctions that participate in engine control and more specifically oxygen sensors.

The purpose of these diagnostic means is to cause, when an element fails acting on the level of engine pollution, i.e. activation of degraded operating modes and / or the lighting of an indicator on the dashboard warning the driver (regulations little severe), i.e. the implementation of measures corrective, or even stopping the vehicle (strict regulations).

OEMs and builders automobiles have therefore developed a number of technical devices for diagnosing the malfunctions of the sensors fitted to the engines in general and oxygen sensors in particular.

We can thus cite the document DE-A-19.534.141 which concerns more particularly the control of electrical wires ensuring connection sensors to the injection computer and which allows in particular to diagnose the existence of malfunctions such as a "short circuit" or another "open circuit" following the rupture of a thread.

The process described in document DE-A-19.534.141 is directly implemented by the injection computer. It consists of comparing regularly the tension values observed at each input of the injection computer to ranges of values memorized and to be deduced therefrom deviation, the existence of a fault in the line.

However, this process is not of general application because it does not allow diagnose the electrical connection of a probe to oxygen type lambda, which indicates only one state rich or poor compared to stoichiometry (or richness 1), when such a probe is positioned downstream of a catalytic converter of treatment accelerating reactions in particular oxidation of exhaust gases. Such downstream positioning of an oxygen sensor is used to control the operation of the catalytic converter or for operate a so-called "double loop" regulation when engine operating conditions require to operate with a mixture stoichiometric, such as idle, etc.

Indeed, during the phases of stoichiometric mixture operations, the exhaust gas at the outlet of a catalytic converter intended to accelerate oxidation reactions, below called oxidation catalytic converter (being understood that such a pot is not limited to the single accelerating oxidation reactions and that it can also accelerate reduction reactions as in the case of three-way pots (called selective or trifunctional) to one or more beds), have a relatively oxygen content stable due to chemical conversions, so that the signal from a lambda oxygen sensor arranged downstream of this pot does not switch between values voltage extremes corresponding to "rich" states or "poor" only over relatively long periods long, the simple observation of non-tilting of the probe on either side of the voltage of polarization therefore does not allow to diagnose the state of the electrical connection.

The object of the invention is therefore to remedy to the disadvantages of the prior art by proposing an electrical diagnostic process according to claim 1.

According to a characteristic of the process diagnostic of an oxygen sensor object of the invention, measuring the voltage level of the signal from the oxygen sensor in the predetermined operating conditions of the engine, is only observed after a period of adapted timing.

• la figure 1 est une vue schématique d'un moteur à combustion interne équipé d'un dispositif électronique de commande permettant la mise en oeuvre du procédé selon l'invention ;
• la figure 2 est une vue de détail du circuit électrique reliant un capteur de mesure, telle que la sonde à oxygène aval, au dispositif électronique de commande présenté à la figure 1.

The objects, aspects and advantages of the present invention will be better understood from the description given below of embodiments of the invention, presented by way of non-limiting examples, with particular reference to the accompanying drawings, in which :

• Figure 1 is a schematic view of an internal combustion engine equipped with an electronic control device for implementing the method according to the invention;
• FIG. 2 is a detailed view of the electrical circuit connecting a measurement sensor, such as the downstream oxygen sensor, to the electronic control device presented in FIG. 1.

According to the drawings, only internal combustion engine components required to the understanding of the invention were represented and to facilitate reading, same elements have the same references of a figure to another.

Referring to Figure 1, we see a internal combustion engine in particular intended to equip a motor vehicle. This engine is a four-stroke multi-cylinder engine, of the type direct injection and controlled ignition operating according to several wealth values and especially in a poor mixture. Obviously the present invention is not limited to an engine with direct injection and can also be applied to an indirect injection engine provided that last works according to several values of richness and in particular in poor mixture.

The illustrated motor conventionally comprises an air intake circuit 3 provided with a flap 4 of butterfly type to adjust the quantity intake air and an idle bypass circuit cooperating with a control valve 5. Fuel injection takes place directly in each combustion chamber 12 thanks to a electro-injector 2 housed in the cylinder head of the engine and opening into the roof of bedroom 12. A spark plug ignition system 6 ensures initiation of combustion of the fuel mixture previously compressed in the cylinders of the engine.

The burnt gases are released to the atmosphere through an exhaust line 10 fitted with a catalytic converter 13 intended in particular to treat with oxidation polluting gases and more particularly unburnt HC and carbon monoxide CO and, from a catalytic converter 19 of treatment of polluting gases and more particularly nitrogen oxides NOx by reduction.

The catalytic converter 19 also called NOx trap has more particularly the function of store the nitrogen oxides produced, especially during lean mixture operating phases and to regularly operate their catalytic conversion in the presence of hydrocarbon during phases of rich mixture operation.

The opening of each electro-injector 2 as well as the advance when the candles are ignited 6 or still opening the control valve idle 5 are directly controlled by a electronic control system or computer injection 7.

In the injection computer 7 are memorized formulas and parameters fundamental for optimal engine tuning, these parameters being obtained before the bench engine test. These are in particular the parameters concerning the injection start time or phase fuel injection time, the opening time of injectors 2 which corresponds to an amount of fuel injected and therefore a wealth of fuel mixture filling the chambers with combustion 12, or even the ignition phase candles 6, etc.

The injection computer 7 consists of basically a microprocessor or unit central CPU, RAM, memory dead ROMs, analog-to-digital converters A / D, and different input interfaces and outings.

The computer microprocessor 7 has electronic circuits and appropriate software to handle signals from different sensors adapted, deduce the states of the engine and set implement predefined operations in order to generate the appropriate control signals for destination in particular of injectors and coils ignition.

Among the computer input signals 7 are shown: "load" information given by a throttle position sensor gas, the "manifold pressure" information given by a pressure sensor 9 arranged in the circuit intake 3 downstream of the throttle body 4, "speed" information given by a angular position 8 cooperating with a target toothed by the crankshaft and the information "richness" given by the output signals of two oxygen sensors or probes 14 and 15 arranged on the exhaust line 10 of the engine on the part and on the other side of the catalytic converter 13.

The opening time Ti of the injectors determining the dosage of the quantity of fuel injected and the richness of the fuel mixture filling each combustion chamber 12 is adapted according to the operating conditions of the motor, for example from values predefined stored as maps pressure / speed in the device memories engine control electronics 7. Values theoretical of the duration of injections read are then modulated by corrective parameters depending in particular on the air temperature, the water temperature, battery voltage, accelerations, clicking, etc., as well as by the output signal from the oxygen probes 14 and 15.

Oxygen sensors 14 and 15 are connected via a connecting wire at a specific input / output stage of the injection computer 7.

The signal supplied by the probe 14 arranged at the outlet of the exhaust manifold upstream of the catalytic converter 13, probe which is preferably proportional or UEGO type, used to correct the amount of fuel that is injected upstream of the engine cylinders through a adapted feedback loop, so that adjust the richness value of the fuel mixture to the chosen value taking into account the conditions of engine operation.

The second probe 15 disposed downstream of the pot catalytic 13 is a nonlinear type probe or lambda, set to the corresponding wealth 1 to the stoichiometric ratio. Probe 15 is shown in Figure 1 also downstream of the pot 19 but this is not limitative of the invention and it is perfectly possible to arrange the probe 15 upstream of the pot 19, this position upstream of the pot 19 being preferable in particular for operating a rapid diagnosis during rich operation in particular to purge the pot 19.

Such a lambda 15 probe has several utilities, it is used to measure the performance of catalytic converter 13 as well as to adapt the report air / fuel supplied by the first loop during richness 1 operating phases, in changing for example its operating point or the transfer function used to to alleviate in particular the aging of the probe upstream 14 or even to operate the regulation in real time fuel / air ratio.

Such a probe 15 is conventionally formed by a ceramic body (such as dioxide zirconium) part of which is in the flux exhaust and the other is connected to air atmospheric, both sides of the body coated with permeable platinum electrodes gas. The electrodes of the lambda 15 probe powered by a suitable electric current called bias, supply voltage in return characteristic of the richness of the fuel mixture.

The voltage between the terminals of the electrodes happens to be modified significantly on either side of the bias voltage corresponding to wealth 1, by the differences of oxygen concentration between the two sides of the probe, due to the special properties of the materials used. Monitoring this tension therefore makes it possible to determine the evolution of the richness of gases compared to stoichiometry.

Referring to Figure 2, we can see schematically the stage 17 input / output of the injection computer 7 dedicated to the lambda probe 15. This stage 17 intended to supply a current of adapted polarization includes a circuit electronic including two resistors in series R1 and R2 at the terminals of which a tension goes substantially constant.

The lambda 15 sensor is connected to this circuit through its wire link 16 in parallel with resistor R1. The voltage Vs which is read across the resistor R1 by a resistive circuit R3 and capacitive C1 which is connected to a converter analog / digital not shown, is the voltage of output Vs of the lambda probe 15.

The choice of the different components of the interface 17 electronic circuit and in particular resistors R1 and R2 are adapted so that the output voltage fluctuates for example between "low" states of approximately 100 mV and "high" states of approximately 700 mV, for respectively "poor" wealth or "rich" of the fuel mixture, that is to say when the exhaust gas content is respectively below or above the setpoint of the richness, the bias voltage substantially corresponding to the output voltage when the richness of the fuel mixture is at the setpoint being approximately 400 mV. The sensitivity of sensor 15 is adapted so that any variation in wealth around value setpoint causes the voltage to switch over output to its upper limit values or bass.

In accordance with the above, the process diagnostic of the connecting wire 16 between the injection computer and downstream lambda probe 15 implementation of the injection computer 7 is then the next one.

A short circuit of the connecting wire 16 to the ground is identified by a Vs probe output voltage lower than an adapted threshold value Vccm such than for example 50 mV. The comparison between Vs and Vccm is therefore operated at regular intervals during engine operation and condition link 16 is considered satisfactory if the voltage of output Vs is greater than Vccm, on the other hand if Vs becomes less than Vccm then probe 15 is declared as a short circuit to ground and adapted strategies such as operating in degraded mode or the emission of a signal alert intended to warn the driver by in particular through an indicator light or again storing a diagnostic code correspondent in order to orient the convenience store to the defective item, are triggered by the injection computer 7.

A short circuit of the connecting wire 16 to the battery voltage is identified by a voltage of probe output Vs greater than a second value of adapted threshold Vccb such as for example 1.5 V. The comparison between Vs and Vccb is therefore made at regular intervals while the engine and condition of link 16 is judged satisfactory if the output voltage Vs is lower Vccb, on the other hand if Vs becomes greater than Vccb then probe 15 is declared in battery short circuit and appropriate strategies of the aforementioned type are then triggered by the injection computer 7.

In terms of controlling a possible breakage of the connecting wire 16 (or circuit open), the strategy implemented consists of wait for the occurrence of predetermined operation of motor 1 which involves switching the output voltage Vs the lambda 15 probe to a low value (or high) and to observe if indeed we have such a value.

The first step of the process therefore consists to follow engine operating conditions and to identify the occurrence of a phase predetermined engine 1 operation during which the richness of the fuel mixture remains different from stoichiometry for a period sufficient such that the operating phases under partial load during which the engine is supplied with a lean mixture (richness 0.7).

Partial loads during which fuel injection is depleted, are characterized by maintaining the pedal throttle in an intermediate position between the raised foot and full foot position (information provided by a position sensor linked to the accelerator pedal or the throttle valve gas) and on the other hand, by a rotation regime of the motor between threshold values predetermined.

According to the invention, the strategy of diagnosis then consists in comparing with each phase operating in lean mixture (or according to all other periodicity, for example the first phase of lean operation of each cycle of motor operation) the output voltage Vs of probe 15 (which must then be at its level low 100 mV characteristic of a fuel mixture poor) at a third threshold value Vco such than for example 200 mV.

This comparison of Vs to Vco is not however operated only after a suitable period T during which the catalytic converter was able to saturate all of its oxygen receptor sites, so that we are sure after this period T observe a flow of oxygen-rich gas at the exit from the catalytic converter 13 and involves the switching of the voltage Vs of the probe 15 in its "low" state if everything works correctly.

Indeed, the oxidation catalytic converters generally used have for first function complete the combustion of the fuel mixture which is only incomplete inside the engine. he it is then a question of regrouping on sites catalytics of oxidizing molecules and reducing molecules present in gases exhaust so that they combine to produce water and carbon dioxide. These sites are formed on a monolith which is a porous structure with a large area of contact with the exhaust gases passing through the pot, and which is coated with various substances chemicals with catalytic properties.

This function overcomes the incomplete combustion inside the cylinders of the engine but does not eliminate the polluting substances only when the mixture initial fuel is a stoichiometric mixture.

Also, the catalytic converters have also the ability to "store" atoms oxygen by oxidation of chemicals present in the catalyst, such as cerium.

So when the exhaust that enter the catalytic converter come from the combustion of a lean mixture, they contain excess oxygen atoms and these are stored in the catalytic converter. When the mixture burned in the engine is rich, the exhaust gases which arrive in the jar contain molecules reducing but these can be combined with oxygen atoms previously stored in the catalytic converter during a phase of operating with a lean mixture to produce water and carbon dioxide.

The catalytic converter therefore has the function not only to promote chemical reactions between substances contained in gases exhaust, but it also has the function to be a buffer stock of oxygen molecules which regulates the composition of the gases emitted in the atmosphere at the exit of the pot.

The adapted delay period T is therefore determined according to the conditions of engine operation and in particular the evolution operating speed, engine displacement and the oxygen adsorption capacity of the pot catalytic used, so the air mass exhausted after this period exceeds the oxygen adsorption capacity of the pot catalytic and therefore that the gases leaving the pot catalytic 13 are poor.

If therefore the operating phase in poor mixing continues after period T, we then, and only then, does the comparison between Vs and Vco and if Vs remains greater than Vco then the probe 15 is declared in open circuit and corresponding strategies are triggered by the injection computer. If the phase of lean mixture operation stops before the end of period T, the diagnostic strategy is then stopped and restarted during the next lean mixture operating phase.

The conditions for implementing the process diagnostic of the open circuit are therefore twofold, namely the operation of the engine 1 in mixture poor and maintaining such functioning beyond of a given duration necessary for saturation in air from the catalytic converter 13. When these conditions are met, the injection computer 7 then performs the comparison of the voltage of output Vs of probe 15 at the threshold value Vco.

This effective control of the link state 16 is operated as far as possible at least once during each engine operation.

The diagnostic method according to the invention, is implemented during the phases of rich mixture operation (richness included between 1.1 and 1.7) required for treatment NOx nitrogen oxides catalytic.

Indeed, the pot 19 only stores the nitrogen oxides during the operating phases in poor mixture. The catalytic conversion of nitrogen oxides requires a reducing medium, this which is the case of hydrocarbons. It is therefore necessary to operate regularly, before the storage sites are not saturated, phases of destocking with a rich fuel mixture adapted.

The transient phases of stock purging nitrogen oxides in a rich mixture therefore constitute engine operating phases during which the richness of the fuel mixture remains clearly distinct from wealth 1 corresponding to stoichiometry and therefore during which it is possible to observe or not the tilting of the probe 15 output voltage. phases, the diagnostic process is substantially identical to that described for lean mixture operating phases, if this it is only necessary to plan a period T 'adapted and reverse the comparison since the probe 15 should if everything works well, switch towards its high threshold and not towards its low threshold.

This required time period T ' to operate the diagnosis is determined so as to ensure that the catalytic converter 13 a, at this deadline, completely emptied of its buffer stock of oxygen molecules and therefore gases exhaust at the outlet of the pot are rich in hydrocarbons and normally involve, if any works properly, the tilting of the probe output voltage at its rich level.

Furthermore, it is necessary to plan a new threshold value Vco '(for example 600 mV), so that, the phase of rich mixing continues after period T ', if Vs remains less than Vco' then the probe 15 is declared in open circuit and corresponding strategies are triggered by the injection computer.

Of course, the invention is by no means limited to the embodiment described and illustrated which was only given as an example.

On the contrary, the invention includes all technical equivalents of the means described as well as their combinations if these are performed following his spirit.

Thus, the engine may not be equipped with the catalytic converter 19 intended for the treatment of oxides of nitrogen. Similarly, the electronic system of order can order the amount of gasoline injected regardless of the probe signal at upstream oxygen 14, using other criteria of regulation that the richness of the fuel mixture as for example the stability of combustions.

Claims (4)

1. An electric diagnostic method for an oxygen probe (15) of an internal combustion engine (1) operating according to a plurality of richness values and in particular with a lean mixture, this oxygen probe (15) of lambda type being disposed on the exhaust line downstream of a catalytic converter processing pollutant gases by oxidation and transmitting an electrical signal whose voltage level is representative of the deviation of the richness from the stoichiometric ratio to the electronic control system (7) of the engine by means of an electrical connection (16) in response to an appropriate polarisation current, characterised in that it comprises the following stages:

   monitoring of the operation of the engine;

   when the predetermined operating conditions are observed or the richness of the air-fuel mixture continues differ from the stoichiometry for a sufficient period, measuring the voltage level of the signal (Vs) transmitted by the oxygen probe (15);

   comparing this voltage value (Vs) with a first threshold voltage value (Vco);

   deducing therefrom, if the voltage value is greater than the first threshold voltage, the existence of a first type of defect with respect to the electrical connection (16) of the probe (15), the predetermined operating conditions comprising phases of rich mixture operation of the engine adapted for the processing of the nitrogen oxides generated during lean mixture operating phases and which are stored in an appropriate catalytic converter (19).
2. A diagnostic method for an oxygen probe as claimed in claim 1, characterised in that the measurement of the voltage level of the signal transmitted by the oxygen probe (15) in the predetermined operating conditions of the engine (1) is observed only after an appropriate timing period (T).
3. An electric diagnostic method for an oxygen probe as claimed in one of the preceding claims, characterised in that it comprises the following stages:

   comparing the output voltage with a second threshold voltage value;

   deducing therefrom, if the output voltage value is greater than the second threshold voltage, the existence of a second type of defect with respect to the electrical connection (16) of the probe (15).
4. An electric diagnostic method for an oxygen probe as claimed in one of the preceding claims, characterised in that it comprises the following stages:

   comparing the output voltage with a third threshold voltage value;

   deducing therefrom, if the output voltage value is lower than the third threshold voltage, the existence of a third type of defect with respect to the electrical connection (16) of the probe (15).

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