EP2191125B1 - Method for diagnosing the bypass flap of an exchanger in an exhaust gas recirculation system - Google Patents

Description

FIELD OF THE INVENTION

The present invention relates to a method for diagnosing a failure of the EGR circuit of an engine, specifically the blocking of the bypass flap of the EGR exchanger.

BACKGROUND OF THE INVENTION

The bypass flap is a key element of the exhaust gas recirculation system (designated by the acronym EGR - Exhaust Gas Recirculation according to the English terminology).

Its function is to direct a desired quantity of EGR gas into a bypass circuit (or, according to the commonly used English terminology, "by-pass") of the EGR exchanger in order to benefit from hot gases for priming. catalyst.

The proper functioning of the shutter thus makes it possible to guarantee the depollution of the current diesel engines. The blocking of the shutter in bypass mode or in cooled mode has direct consequences on the pollution emitted at the output of the engine.

As the pollution control thresholds are more and more severe, it is essential, in order to meet the next standards, to diagnose such failures of the component.

The risk associated with blocking the flap is also not related solely to pollution. Indeed, a failure of the shutter can have consequences on the reliability of the surrounding components (degradation due to a too high temperature of the EGR valve and its support) and the integrity of the engine control strategies that use it (such as example that the cleaning of the valve and the exchanger, or the priming of the catalyst).

Several fault diagnosis methods have already been developed, with varying performance.

A first method, described in the document JP2006-291921 , uses a temperature sensor located at the entrance of the intake manifold and can diagnose a blockage of the flap by measuring the temperature difference between the cool mode and bypass mode. However, this method requires the actuation of the shutter to be able to perform the diagnosis. In addition, it does not detect the position in which the shutter has blocked; however, the blocking in bypass or cooled mode does not have the same impact on pollution and we want to act differently in these two cases. Moreover, this process seems relatively imprecise because the temperature sensor located at the inlet of the inlet distributor is influenced by the fresh air admitted.

Another process, described in the document JP 2003-247459 , implements a strategy based on the monitoring of the air flow before and after the activation of the bypass flap, the air intake flap and the EGR valve being completely open. The advantage of this solution is that it simply uses the flowmeter located on the fresh air intake duct. However, depending on the technical means used, this strategy can generate a significant rate of false detections, due to the EGR environment (high temperature, fouling of the connectors) and the limited reactivity of the shutter control. Indeed, pressure wave phenomena delay the vacuum control of the bypass flap. In addition, it requires an intrusion into the operation of the engine, since it requires to open the air intake flap and the EGR valve.

Finally, other applications use a contactor to know the open / closed position of the shutter. However, this strategy can also generate a high rate of erroneous detections because of the EGR environment.

The document US-2006/0042608 describes in particular a method for diagnosing the operation of an EGR circuit.
The document JP-2008144609 describes a method for diagnosing a failure of the EGR circuit of an internal combustion engine and more particularly of the flap regulating the distribution of the exhaust gases between the EGR exchanger and a bypass duct of the exchanger or bypass .

An object of the invention is therefore to define a simple and reliable, non-intrusive method for detecting any failure of the bypass flap and, where appropriate, the position in which it has blocked. This method must also make it possible to diagnose a total loss of the cooling function.

BRIEF DESCRIPTION OF THE INVENTION

• estimation de la température des gaz d'échappement à la sortie de l'échangeur EGR lorsque le circuit EGR est en mode by-pass,
• mesure de la température des gaz d'échappement à la sortie de l'échangeur EGR,
• calcul de l'écart entre ladite température estimée et ladite température mesurée,
• comparaison dudit écart avec un premier ou un deuxième seuil prédéterminé de telle sorte que :
  • si le circuit EGR est en mode by-pass et que l'écart est supérieur au premier seuil, alors on détecte un blocage du volet en mode refroidi,
  • si le circuit EGR est en mode refroidi et que l'écart est inférieur au deuxième seuil, alors on détecte un blocage du volet en mode by-pass ;

Selon d'autres caractéristiques de l'invention :

• si le volet est bloqué en mode by-pass, on ferme la vanne EGR ;
• pour estimer la température des gaz d'échappement à la sortie de l'échangeur lorsque le circuit EGR est en mode by-pass, on calcule, en fonction du débit des gaz d'échappement, trois efficacités données par les formules suivantes : $${\epsilon }_1\left(Q_{\mathit{EGR}}\right)=\frac{T\mathrm{int}-\mathit{Tavt}}{\mathit{Tco}-\mathit{Tavt}}$$
  
  $${\epsilon }_2\left(Q_{\mathit{EGR}}\right)=\frac{\mathit{TeEGR}-\mathit{Tavt}}{T\mathrm{int}-\mathit{Tavt}}$$
  
  $${\epsilon }_3\left(Q_{\mathit{EGR}}\right)=\frac{\mathit{TeEGR}-\mathit{TsEGR}}{\mathit{TeEGR}-\mathit{Tco}}$$
  
  où :
  • Tint est la température des gaz d'échappement à l'entrée du circuit EGR,
  • Tco est la température du fluide de refroidissement de l'échangeur EGR,
  • Tavt est la température des gaz d'échappement en amont du circuit EGR,
  • TeEGR est la température des gaz d'échappement à l'entrée de l'échangeur EGR ;
• la température estimée des gaz d'échappement à la sortie de l'échangeur lorsque le circuit EGR est en mode by-pass est donnée par la formule : $${\mathit{TsEGR}}_{\mathit{est\_byp}}={\epsilon }_3\cdot \mathit{Tco}+\left(1-{\epsilon }_3\right)\cdot \left[\mathit{Tavt}\cdot \left(1-{\epsilon }_2\right)+{\epsilon }_2\cdot \left[{\epsilon }_1\cdot \left(\mathit{Tco}-\mathit{Tavt}\right)+\mathit{Tavt}\right]\right]\mathrm{.}$$
  

A first object of the invention is a method for diagnosing a failure of the EGR circuit of an engine comprising an EGR exchanger, an EGR valve, a bypass duct of the EGR exchanger, and a so-called bypass damper disposed upstream of the EGR exchanger and bypass duct so as to regulate the proportion of the exhaust gas passing therethrough, the EGR circuit being able to be activated in a so-called cooled mode, where the shutter is closed , and a so-called bypass mode, wherein the flap is open, this method being characterized in that it comprises the following steps:

• estimating the temperature of the exhaust gases at the outlet of the EGR exchanger when the EGR circuit is in bypass mode,
• measurement of the temperature of the exhaust gases at the outlet of the EGR exchanger,
• calculating the difference between said estimated temperature and said measured temperature,
• comparing said deviation with a first or a second predetermined threshold such that:
  • if the EGR circuit is in bypass mode and the difference is greater than the first threshold, then a blocking of the shutter in cooled mode is detected,
  • if the EGR circuit is in cooled mode and the difference is smaller than the second threshold, then a blocking of the shutter in bypass mode is detected;

According to other features of the invention:

• if the flap is blocked in bypass mode, the EGR valve is closed;
• to estimate the temperature of the exhaust gas at the outlet of the exchanger when the EGR circuit is in bypass mode, three efficiencies given by the following formulas are calculated as a function of the flow of the exhaust gas: $${\epsilon }_1\left(Q_{\mathit{EGR}}\right)=\frac{T\mathrm{int}-\mathit{Tavt}}{\mathit{Tco}-\mathit{Tavt}}$$
  
  $${\epsilon }_2\left(Q_{\mathit{EGR}}\right)=\frac{\mathit{TeEGR}-\mathit{Tavt}}{T\mathrm{int}-\mathit{Tavt}}$$
  
  $${\epsilon }_3\left(Q_{\mathit{EGR}}\right)=\frac{\mathit{TeEGR}-\mathit{TsEGR}}{\mathit{TeEGR}-\mathit{Tco}}$$
  
  or :
  • Tint is the temperature of the exhaust gas at the inlet of the EGR circuit,
  • Tco is the temperature of the cooling fluid of the EGR exchanger,
  • Tavt is the temperature of the exhaust gases upstream of the EGR circuit,
  • TeEGR is the temperature of the exhaust gas at the inlet of the EGR exchanger;
• the estimated temperature of the exhaust gases at the outlet of the exchanger when the EGR circuit is in bypass mode is given by the formula: $${\mathit{TsEGR}}_{\mathit{est\_byp}}={\epsilon }_3\cdot \mathit{Tco}+\left(1-{\epsilon }_3\right)\cdot \left[\mathit{Tavt}\cdot \left(1-{\epsilon }_2\right)+{\epsilon }_2\cdot \left[{\epsilon }_1\cdot \left(\mathit{Tco}-\mathit{Tavt}\right)+\mathit{Tavt}\right]\right]\mathrm{.}$$
  

• un moyen d'estimation de la température des gaz d'échappement à la sortie de l'échangeur EGR lorsque le circuit EGR est en mode by-pass,
• un moyen de mesure de la température des gaz d'échappement à la sortie de l'échangeur EGR,
• un moyen de calcul de l'écart entre ladite température estimée et ladite température mesurée, et
• un moyen de comparaison dudit écart avec un premier ou un deuxième seuil prédéterminé de telle sorte que :
  • si le circuit EGR est en mode by-pass et que l'écart est supérieur au premier seuil, alors le dispositif détecte un blocage du volet en mode refroidi,
  • si le circuit EGR est en mode refroidi et que l'écart est inférieur au deuxième seuil, alors le dispositif détecte un blocage du volet en mode by-pass.

Another subject of the invention relates to a device for diagnosing a failure of the EGR circuit of an engine comprising an EGR exchanger, an EGR valve, a bypass duct of the EGR exchanger, and a so-called bypass flap. disposed upstream of the EGR exchanger and bypass duct so as to regulate the proportion of the exhaust gas passing therethrough, the EGR circuit being able to be activated in a so-called cooled mode, where the shutter is closed , and a mode called bypass, where the flap is open, characterized in that it comprises:

• means for estimating the temperature of the exhaust gas at the outlet of the EGR exchanger when the EGR circuit is in bypass mode,
• means for measuring the temperature of the exhaust gas at the outlet of the EGR exchanger,
• means for calculating the difference between said estimated temperature and said measured temperature, and
• means for comparing said deviation with a first or a second predetermined threshold such that:
  • if the EGR circuit is in bypass mode and the difference is greater than the first threshold, then the device detects a blocking of the shutter in cooled mode,
  • if the EGR circuit is in cooled mode and the difference is smaller than the second threshold, then the device detects a blocking of the shutter in bypass mode.

• un moyen de mesure de la température du fluide de refroidissement de l'échangeur EGR, et
• un moyen de mesure de la température des gaz d'échappement en amont du circuit EGR.

According to another characteristic, the device according to the invention further comprises:

• means for measuring the temperature of the cooling fluid of the EGR exchanger, and
• means for measuring the temperature of the exhaust gas upstream of the EGR circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 est une vue schématique d'un compartiment moteur dans lequel on met en oeuvre le procédé conforme à l'invention,
• la figure 2 représente de manière schématique un échangeur EGR et son conduit de by-pass,
• la figure 3 illustre la décomposition théorique du circuit EGR en une pluralité d'échangeurs élémentaires,
• la figure 4 est un logigramme de la stratégie de diagnostic,
• la figure 5 illustre le cas d'un volet de by-pass fonctionnel,
• la figure 6 illustre le cas d'un volet de by-pass bloqué en mode refroidi,
• la figure 7 illustre le cas d'un volet de by-pass bloqué en mode by-pass.

Other objects, features and advantages of the invention will appear better on reading the detailed description of the invention which follows, with reference to the appended figures in which:

• the figure 1 is a schematic view of an engine compartment in which the method according to the invention is implemented,
• the figure 2 schematically represents an EGR exchanger and its bypass duct,
• the figure 3 illustrates the theoretical decomposition of the EGR circuit into a plurality of elementary exchangers,
• the figure 4 is a logic diagram of the diagnostic strategy,
• the figure 5 illustrates the case of a functional bypass panel,
• the figure 6 illustrates the case of a blocked bypass flap in cooled mode,
• the figure 7 illustrates the case of a bypass pane blocked in bypass mode.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the figure 1 , an engine compartment comprises an internal combustion engine 10, supplied with fresh air by an intake duct 11 and releasing its exhaust gas through an exhaust duct 12. Optionally, this engine compartment is also provided with a turbocharger 50 comprising a compressor 51 disposed on the intake duct 11 for compressing the air coming from the duct 53. Optionally, cooling means 40 and a shutter 30 are provided between the compressor 51 and the engine 10. The air which reaches the engine 10 is cold. The turbine 52 of the turbocharger 50 is located at the end of the duct exhaust pipe 12 and is coupled to the compressor 51. The exhaust gas is then discharged from the engine compartment via a pipe 54.

The engine compartment further comprises an exhaust gas recirculation circuit (also called EGR circuit 20), the inlet 28 of which is connected to the exhaust duct 12 and whose outlet 29 is connected to the intake duct 11. This EGR circuit 20 comprises an EGR cooler or exchanger 22 connected to the inlet 28 via an upstream pipe 25 and to the outlet 29 via a downstream pipe 27, enabling the exhaust gases to be cooled before being reinjected into the engine 10 .

There is also provided a bypass pipe 24 connected, in its upstream part, to a solenoid valve 23 located upstream of the EGR exchanger 22, and in its downstream part, at the outlet of the exchanger 22. The solenoid valve 23 comprises a flap 23a which, depending on its position, allows a desired quantity of exhaust gas to pass through the bypass duct 24. Thus, by controlling the position of the flap of the solenoid valve 23, quantities of gas determined in FIG. the bypass duct 24 (where they will not be cooled) and in the EGR exchanger 22 (where they will be cooled). This makes it possible to adjust the temperature of the gases leaving the exchanger 22. It is specified that there are various technical definitions of the EGR exchanger and that the bypass circuit 24 can be either distinct from the exchanger 22 (as shown on the figure 1 ), be integrated into it (as will be seen in figure 2 ).

An EGR valve 21 is further provided at the outlet of the circuit 20 so as to regulate the amount of exhaust gas reinjected into the engine 10.

The figure 2 represents an EGR exchanger 22 with an integrated bypass duct 24. If the flap 23a is closed, all the hot exhaust gases (solid arrow) pass into the EGR exchanger where they are cooled (hatched arrows): this is called "cooled mode". If against the flap 23a is open, at least a portion of the exhaust gas pass into the bypass duct 24 and are not cooled: it is called "bypass mode". It is therefore understood that the temperature TsEGR of the exhaust gas at the outlet of the exchanger 22 is higher in bypass mode (TsEGR2) than in cooled mode (TsEGR1).

According to the invention, the diagnostic strategy is based on measuring or estimating the temperature at the outlet of the EGR exchanger 22. This Depending on the case, the temperature may be measured upstream or downstream of the EGR valve 21.

More precisely, the strategy is based on calculating the difference between the estimated TsEGR in bypass mode (denoted TsEGR _est-byp ) and the measured TsEGR (denoted TsEGR _mes ).

For this purpose, the Applicant has developed a model of the TsEGR in bypass mode which is the subject of the patent application. FR 06 10065 .

Three types of efficiencies are estimated based on the flow of EGR gases.

The EGR gas flow rate (denoted by Q _EGR ) is itself estimated as corresponding to the difference between the engine flow (denoted by the _engine Q), the filling flow rate (denoted _refilling ), and the fresh air flow rate (Q). _air ): $$Q_{\mathit{EGR}}=Q_{\mathit{engine}}\left({\eta }_{\mathit{filling}}\right)-Q_{\mathit{air}}$$

The filling efficiency is determined by means of the temperature Tcol and the pressure Pcol in the intake manifold; these values are given by sensors located in the intake manifold.

• le premier échangeur élémentaire correspond au conduit d'échappement 12 (température Tavt), jusqu'à l'entrée 28 du circuit EGR 20 (température Tint) ;
• le deuxième échangeur élémentaire correspond au conduit 25 entre l'entrée 28 du circuit EGR 20 (température Tint) et l'entrée de l'échangeur EGR 22 (température TeEGR) ;
• le troisième échangeur élémentaire correspond à la portion entre l'entrée de l'échangeur EGR 22 (température TeEGR) et la sortie de l'échangeur EGR 22 en mode by-pass (température TsEGR).

The three aforementioned efficiencies correspond to the decomposition of the EGR circuit 20 into three elementary exchangers, represented in FIG. figure 3 :

• the first elementary exchanger corresponds to the exhaust duct 12 (temperature Tavt), to the inlet 28 of the EGR circuit 20 (temperature Tint);
• the second elementary heat exchanger corresponds to the conduit 25 between the inlet 28 of the EGR circuit 20 (temperature Tint) and the inlet of the EGR exchanger 22 (TeEGR temperature);
• the third elementary heat exchanger corresponds to the portion between the inlet of the EGR exchanger 22 (TeEGR temperature) and the outlet of the EGR exchanger 22 in bypass mode (TsEGR temperature).

$${\epsilon }_2\left(Q_{\mathit{EGR}}\right)=\frac{\mathit{TeEGR}-\mathit{Tavt}}{T\mathrm{int}-\mathit{Tavt}}$$

$${\epsilon }_3\left(Q_{\mathit{EGR}}\right)=\frac{\mathit{TeEGR}-\mathit{TsEGR}}{\mathit{TeEGR}-\mathit{Tco}}$$

où :

• Tint est la température des gaz d'échappement à l'entrée du circuit EGR (20), Tco est la température du fluide de refroidissement de l'échangeur EGR (22), Tavt est la température des gaz d'échappement en amont du circuit EGR (20), TeEGR est la température des gaz d'échappement à l'entrée de l'échangeur EGR (22).

Which results in the following three equations: $${\epsilon }_1\left(Q_{\mathit{EGR}}\right)=\frac{T\mathrm{int}-\mathit{Tavt}}{\mathit{Tco}-\mathit{Tavt}}$$

$${\epsilon }_2\left(Q_{\mathit{EGR}}\right)=\frac{\mathit{TeEGR}-\mathit{Tavt}}{T\mathrm{int}-\mathit{Tavt}}$$

$${\epsilon }_3\left(Q_{\mathit{EGR}}\right)=\frac{\mathit{TeEGR}-\mathit{TsEGR}}{\mathit{TeEGR}-\mathit{Tco}}$$

or :

• Tint is the temperature of the exhaust gas at the inlet of the EGR circuit (20), Tco is the temperature of the cooling fluid of the EGR exchanger (22), Tavt is the temperature of the exhaust gases upstream of the circuit EGR (20), TeEGR is the temperature of the exhaust gases at the inlet of the EGR exchanger (22).

The estimated temperature TsEGR is deduced in bypass mode: $${\mathit{TsEGR}}_{\mathit{est\_byp}}={\epsilon }_3\cdot \mathit{Tco}+\left(1-{\epsilon }_3\right)\cdot \left[\mathit{Tavt}\cdot \left(1-{\epsilon }_2\right)+{\epsilon }_2\cdot \left[{\epsilon }_1\cdot \left(\mathit{Tco}-\mathit{Tavt}\right)+\mathit{Tavt}\right]\right]$$

The estimation of the temperature TsEGR in bypass mode therefore requires the presence of three temperature sensors: Tco, Tavt, Tcol and a pressure sensor Pcol.

• si le volet est commandé en mode by-pass, la différence entre TsEGR_est-byp et TsEGRmes doit être faible ;
• si le volet est commandé en mode refroidi, la différence entre TsEGR_est-byp et TsEGRmes doit être élevée.

Having the temperature TsEGRmes measured by a probe located at the outlet of the EGR exchanger 22, and the estimated temperature TsEGR in bypass mode according to the formula described above, it is possible to continuously diagnose the functionality of the shutter. pass 23a, both in bypass mode and cooled mode, and this, without driving (that is to say without intrusion into the operation of the shutter). It is considered that if the flap 23a is functional, then:

• if the shutter is controlled in bypass mode, the difference between TsEGR _is-byp and TsEGRmes must be small;
• if the shutter is controlled in cooled mode, the difference between TsEGR _is-byp and TsEGRmes must be high.

• si la différence |TsEGR_est-byp - TsEGR_mes| est supérieure à S_bm alors que le volet est commandé en mode by-pass, on considère que le volet est bloqué en mode refroidi ;
• si la différence |TsEGR_est-byp - TsEGR_mes| est inférieure à S_cm alors que le volet est commandé en mode refroidi, on considère que le volet est bloqué en mode by-pass.

Thus, by statistical studies, a threshold S _bm and a threshold S _{cm are} defined so that:

• if the difference | TsEGR _is-byp - TsEGR _mes | is greater than S _bm while the flap is controlled in bypass mode, it is considered that the flap is blocked in cooled mode;
• if the difference | TsEGR _is-byp - TsEGR _mes | is less than S _cm while the shutter is controlled in cooled mode, it is considered that the shutter is blocked in bypass mode.

The blocking causes of the flap 23a may be a mechanical seizure, the disconnection of the hose of the bypass solenoid valve 23 or a control problem.

• au démarrage du véhicule, le dispositif s'initialise (case 101);
• tant que les conditions ne sont pas stabilisées (case 102), le diagnostic est inactif; en effet, pour augmenter la fiabilité de détection, on effectue le diagnostic sur des points de fonctionnement où le régime et le couple sont stables, en s'affranchissant des modes transitoires qui génèrent des estimations de TsEGR et de Q_EGR très dispersées ;
• lorsque les conditions sont stabilisées (case 103), on détecte le mode d'activation du circuit EGR : mode by-pass ou mode refroidi (case 104) ;
• si le circuit EGR est en mode by-pass :
  • on mesure la température TsEGR et on calcule la température TsEGR estimée en mode by-pass, puis on calcule l'écart de température : $$\mathrm{\Delta bm}=\left|{\mathrm{TsEGR}}_{\mathrm{est\_by}}-{\mathrm{TsEGR}}_{\mathrm{mes}}\right|\left(\mathrm{<figure-callout\; id="105"\; label="case"\; filenames="imgf0003.png"\; state="\{\{state\}\}">case</figure-callout>}105\right);$$
    
  • on compare l'écart de température Δbm avec le seuil S_bm déterminé au préalable (case 106),
  • si Δbm est inférieur au seuil S_bm, le volet est considéré comme fonctionnel et le diagnostic est désactivé (case 102),
  • si Δbm est inférieur au seuil S_bm, on détecte une défaillance, attribuée au blocage du volet en mode refroidi (case 107) ;
• si le circuit EGR est en mode refroidi :
  • on mesure la température TsEGR et on calcule la température TsEGR estimée en mode by-pass, puis on calcule l'écart de température : $$\mathrm{\Delta cm}=\left|{\mathrm{TsEGR}}_{\mathrm{est\_byp}}-{\mathrm{TsEGR}}_{\mathrm{mes}}\right|\left(\mathrm{<figure-callout\; id="108"\; label="case"\; filenames="imgf0003.png"\; state="\{\{state\}\}">case</figure-callout>}108\right);$$
    
  • on compare l'écart de température Δcm avec le seuil S_cm déterminé au préalable (case 106),
  • si Δcm est supérieur au seuil S_cm, le volet est considéré comme fonctionnel et le diagnostic est désactivé (case 102),
  • si Δcm est inférieur au seuil S_cm, on détecte une défaillance, attribuée au blocage du volet en mode by-pass (case 110).

The logigram of the figure 4 illustrates more precisely the logical approach to diagnosis:

• when the vehicle is started, the device initializes (box 101);
• as long as the conditions are not stabilized (box 102), the diagnosis is inactive; indeed, to increase the reliability of detection, it is diagnosed on operating points where the regime and the torque are stable, avoiding the transient modes that generate estimates of TsEGR and Q _EGR widely dispersed;
• when the conditions are stabilized (box 103), the activation mode of the EGR circuit is detected: bypass mode or cooled mode (box 104);
• if the EGR circuit is in bypass mode:
  • the temperature TsEGR is measured and the estimated temperature TsEGR is calculated in bypass mode, then the temperature difference is calculated: $$\mathrm{\Delta bm}=\left|{\mathrm{TsEGR}}_{\mathrm{est\_by}}-{\mathrm{TsEGR}}_{\mathrm{my}}\right|\left(\mathrm{<figure-callout\; id="105"\; label="hut"\; filenames="imgf0003.png"\; state="\{\{state\}\}">hut</figure-callout>}105\right);$$
    
  • comparing the temperature difference Δbm with the threshold S _bm determined beforehand (box 106),
  • if Δbm is below the threshold S _bm , the flap is considered functional and the diagnosis is deactivated (box 102),
  • if Δbm is below the threshold S _bm , a fault is detected, attributed to the blocking of the shutter in cooled mode (box 107);
• if the EGR circuit is in cooling mode:
  • the temperature TsEGR is measured and the estimated temperature TsEGR is calculated in bypass mode, then the temperature difference is calculated: $$\mathrm{\Delta cm}=\left|{\mathrm{TsEGR}}_{\mathrm{est\_byp}}-{\mathrm{TsEGR}}_{\mathrm{my}}\right|\left(\mathrm{<figure-callout\; id="108"\; label="hut"\; filenames="imgf0003.png"\; state="\{\{state\}\}">hut</figure-callout>}108\right);$$
    
  • the temperature difference Δcm is compared with the threshold S _cm determined beforehand (box 106),
  • if Δcm is greater than threshold S _cm , the shutter is considered functional and the diagnosis is deactivated (box 102),
  • if Δcm is less than the threshold S _cm , a fault is detected, attributed to the blocking of the shutter in bypass mode (box 110).

When the fault is confirmed, information (called DTC or "Diagnostic Trouble Code" according to the English terminology) is stored in the manufacturer memory; a service indicator (called OBD or On Board Diagnosis according to the English terminology) lights up if the emissions exceed the predicted thresholds.

Finally, if the shutter is blocked in bypass mode (box 110), a degraded mode is activated, consisting in closing the EGR valve 21 in order to reduce the temperature at its terminals.

This strategy is implemented in the engine control unit (ECU).

The figure 5 illustrates the case of a functional component.

The curve C1 in the form of a slot corresponds to the state of the control of the bypass flap: the high value corresponds to the bypass mode, the low value corresponds to the cooled mode.

The curve C2 in the form of a slot corresponds to the diagnostic condition: the high values correspond to the diagnostic phases.

In this figure, it can be seen that during the first phase, the EGR circuit is in bypass mode and the temperature difference Δbm is lower than the detection threshold in bypass mode S _bm : the flap is therefore considered to be functional. Similarly, during the second diagnostic phase, the EGR circuit is in cooled mode, and the temperature difference Δcm is greater than the detection threshold in cooled mode S _cm : the flap is thus detected as functional.

With reference to the figure 6 , we examine the case of a blocked shutter in cooled mode.

Curves C1 and C2 are defined in the same way as in the figure 5 .

During the first diagnostic phase, the EGR circuit is in bypass mode. However, the temperature difference Δbm remains higher than the detection threshold in bypass mode S _bm during a duration Tbm: the shutter is thus considered as blocked in cooled mode.

Assuming that the first diagnostic phase was carried out while the EGR circuit was in cooled mode (cf second slot of curve C2), the shutter would have been judged as functional (since Δcm is greater than S _cm ), but it would have detected as a failure in bypass mode during the next diagnostic cycle.

With reference to the figure 7 , we now examine the case of a block blocked in bypass mode.

Curves C1 and C2 are defined in the same way as in figures 5 and 6 .

During the first diagnostic phase, the EGR circuit is in bypass mode. Since the temperature difference Δbm is less than S _bm , the flap is considered functional.

The next phase of diagnosis, in cooled mode, makes it possible to highlight the failure of the shutter. Indeed, the temperature difference Δcm remains below S _bm for a duration Tcm: it is deduced that the flap is blocked in bypass mode.

Compared to other technical solutions that use a flowmeter or a contactor, the use for the diagnosis of a temperature sensor at the outlet of the EGR exchanger (measurement of TsEGRmes) improves the reliability of detection of the process. In addition, this temperature sensor can advantageously be used, as needed, for other diagnostic purposes. Thus, the method according to the invention makes it possible to detect a total loss of the cooling function; failures leading to this loss - for example, a water leak - are however rarer.

Compared to other known methods, the method of the invention also has the advantage of not being intrusive, that is to say that it does not require actuating the bypass flap for check its functionality. The implementation of this method does not cause additional pollution.

Finally, this strategy makes it possible to know the position in which the bypass pane is blocked: this information is necessary for the proper operation of the degraded mode (ie only if the pane is blocked in bypass mode), which represents an additional gain in terms of depollution.

Claims (4)

1. Method for diagnosing a failure of the EGR circuit (20) of an engine comprising an EGR exchanger (22), an EGR valve (21), a bypass duct (24) of the EGR exchanger, and a so-called bypass flap (23a), arranged upstream of the EGR exchanger (22) and of the bypass duct (24) so as to control the proportion of exhaust gases passing through the latter, wherein the EGR circuit (20) can be activated according to a so-called cooled mode, in which the flap (23a) is closed, and a so-called bypass mode, in which the flap (23a) is open, characterized in that it comprises the following steps:

   - estimation of the temperature of the exhaust gases (TsEGR_est□byp) at the outlet of the EGR exchanger (22) when the EGR circuit (20) is in bypass mode,

   - measurement of the temperature of the exhaust gases (TsEGR_mes) at the outlet of the EGR exchanger (22),

   - calculation of the difference between said estimated temperature (TsEGR_est□byp) and said measured temperature (TsEGR_mes),

   - comparison of said difference with a first (S_bm) or a second (S_cm) predetermined threshold so that:

   • if the EGR circuit (20) is in bypass mode and the difference is greater than the first threshold (S_bm), then a blocking of the flap (23a) in cooled mode is detected,

   • if the EGR circuit (20) is in cooled mode and the difference is less than the second threshold (S_cm), then a blocking of the flap (23a) in bypass mode is detected,

   and in that, to estimate the temperature (TsEGR_est□byp) of the exhaust gases at the outlet of the exchanger (22) when the EGR circuit (20) is in bypass mode, three efficiencies are calculated, as a function of the flow rate of the exhaust gases (Q_EGR), given by the following formulae: $$•{\mathrm{\epsilon }}_1\left(Q_{\mathit{EGR}}\right)=\frac{T\mathrm{int}-\mathit{Tavt}}{\mathit{Tco}-\mathit{Tavt}}$$

   

   $$•{\mathrm{\epsilon }}_2\left(Q_{\mathit{EGR}}\right)=\frac{\mathit{TeEGR}-\mathit{Tavt}}{T\mathrm{int}-\mathit{Tavt}}$$

   

   $$•{\mathrm{\epsilon }}_3\left(Q_{\mathit{EGR}}\right)=\frac{\mathit{TeEGR}-\mathit{TsEGR}}{\mathit{TeEGR}-\mathit{Tco}}$$

   

   • in which:

   • Tint is the temperature of the exhaust gases at the inlet of the EGR circuit (20),

   • Tco is the temperature of the coolant of the EGR exchanger (22),

   • Tavt is the temperature of the exhaust gases upstream of the EGR circuit (20),

   • TeEGR is the temperature of the exhaust gases at the inlet of the EGR exchanger (22).
2. Method according to Claim 1, characterized in that, if the flap (23a) is blocked in bypass mode, the EGR valve (21) is closed.
3. Method according to Claim 1, characterized in that the estimated temperature (TsEGR_est□byp) of the exhaust gases at the outlet of the exchanger (22) when the EGR circuit (20) is in bypass mode is given by the formula: $${\mathit{TsEGR}}_{\mathit{est\_byp}}={\mathrm{\epsilon }}_3\cdot \mathit{Tco}+\left(1-{\mathrm{\epsilon }}_3\right)\cdot \left[\mathit{Tavt}\cdot \left(1-{\mathrm{\epsilon }}_2\right)+{\mathrm{\epsilon }}_2\cdot \left[{\mathrm{\epsilon }}_1\cdot \left(\mathit{Tco}-\mathit{Tavt}\right)+\mathit{Tavt}\right]\right]$$

   
4. Device for diagnosing a failure of the EGR circuit (20) of an engine comprising an EGR exchanger (22), an EGR valve (21), a bypass duct (24) of the EGR exchanger, and a so-called bypass flap (23a), arranged upstream of the EGR exchanger (22) and of the bypass duct (24) in order to control the proportion of exhaust gases passing through the latter, wherein the EGR circuit (20) can be activated according to a so-called cooled mode, in which the flap (23a) is closed, and a so-called bypass mode, in which the flap (23a) is open, characterized in that it comprises:

   - a means of estimating the temperature of the exhaust gases (TsEGR_est□byp) at the outlet of the EGR exchanger (22) when the EGR circuit (20) is in bypass mode,

   - a means of measuring the temperature of the exhaust gases (TsEGR_mes) at the outlet of the EGR exchanger (22),

   - a means of calculating the difference between said estimated temperature (TsEGR_est□byp) and said measured temperature (TsEGR_mes), and

   - a means of comparing said difference with a first (S_bm) or a second (S_cm) predetermined threshold so that:

   - if the EGR circuit (20) is in bypass mode and the difference is greater than the first threshold (S_bm), then the device detects a blocking of the flap (23a) in cooled mode,

   - if the EGR circuit (20) is in cooled mode and the difference is less than the second threshold (S_cm), then the device detects a blocking of the flap (23a) in bypass mode,

   and in that it also comprises:

   - a means of measuring the temperature (Tco) of the coolant of the EGR exchanger (22), and

   - a means of measuring the temperature (Tavt) of the exhaust gases upstream of the EGR circuit (20).

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