FR2677123A1 - Test method for a device for recirculating exhaust gas into an internal combustion engine - Google Patents

Abstract

The average oxygen richness of the air/fuel mixture is measured during a first phase of operation of the motor in stabilised regime, during which the recirculation device (10, 11, 12) is inactive, and during a second subsequent phase, during which this device is activated, and the state of operation of the engine is diagnosed by comparing the difference (S0-S1) of the oxygen richnesses measured during these two phases, with at least one predetermined value ( DELTA S). As a variant, the richness or oxygen richness differences are substituted by the difference (ti0-ti1) of the average injection times established by a closed-loop control, during the two successive phases of the operation of the engine. Application to the detection of correct operation or defective operation of an exhaust gas recirculating device.

Description

 The present invention relates to a method for testing an exhaust gas recirculation device in an internal combustion engine and, more particularly, to such a method making it possible to verify the good mechanical condition of this device and the accuracy a member incorporated therein to regulate the amount of exhaust gas recycled in the engine.

 Increasingly stringent anti-pollution standards make it necessary to reduce the quantities of harmful gases released into the atmosphere by internal combustion engines. The standards target in particular nitrogen oxides, considered to be particularly harmful. It is known that the quantity of nitrogen oxides contained in the exhaust gases of an internal combustion engine is a function of the temperature reached in the combustion chambers during the compression and explosion phases of the air / fuel mixture. introduced into these rooms. The higher this temperature (especially the maximum temperature), the greater the amount of nitrogen oxide produced.

 To reduce the temperatures involved, it has been thought to inject inert gases into the engine, and in particular the exhaust gases themselves, of very low oxygen content (only 1 to 2%). This artificially increases the mass of gas involved during combustion, which decreases the temperature reached at the end of it. This temperature reduction process is used exceptionally, in certain engine operating phases where such temperature reduction is necessary to comply with emission standards.

 There is shown in Figure 1 of the accompanying drawing a known device for implementing this method. This device is associated with an engine M supplied with air by an intake manifold 1, through an air flow adjustment butterfly valve and a filter 3. The exhaust gases are discharged into an exhaust line 4 possibly provided with a pot 5 containing a redox catalyst for exhaust gases. Still conventionally, a computer 6 supplied by signals from a sensor 7 of intake pressure P which makes it possible to define the engine load and an engine speed sensor N (not shown) ensures the calculation and the control of the opening time ti of fuel injectors 8 placed in the intake manifold 1 of the engine, to ensure the adjustment of the air / fuel mixture supplying this engine. This control can be carried out in a closed loop at using a signal supplied to the computer by an oxygen sensor 9, as is well known.

A known recirculation device, suitable for an engine thus equipped, comprises for example conduits 10 and 11 connecting the outlet 4 of the exhaust gases to the intake manifold 1, through a valve 12 enabling the quantity of gas to be adjusted. recycled by adjusting the valve opening rate, according to a "mapped" law in the computer, of the form
valve opening rate = kf (N, P) (I)
As shown in Figure 1, this valve opening rate 12 is a function of the position of a needle 13 integral with a membrane 14 separating two chambers 15 and 17, the chamber 15 being connected to the intake manifold 1 through a solenoid valve 16. The computer controls the position of the needle 13 according to a map (P, N) stored in memory, by a modulation of the depression established in the chamber 15, controlled by a PWM modulation of the solenoid valve 16 to two ways and two positions.

 The proper functioning of such an exhaust gas recirculation device obviously conditions compliance with antipollution standards in terms of nitrogen oxide rejection. However, this operation can be altered by various causes, such as an accidental break or detachment of the conduits 11 and 12, a failure of the valve 12 or of the solenoid valve 16 or a drift in the rate of opening of said valve, in particular.

 The object of the present invention is therefore to provide a method for testing an exhaust gas recirculation device in an internal combustion engine, making it possible to detect the appearance of a possible malfunction of this device so that 'there is remedy.

 This object of the invention is achieved, as well as others which will appear on reading this description, with a method for testing an exhaust gas recirculation device in an internal combustion engine with controlled closed loop of the engine air / fuel mixture, according to which a variation in the quantity of gas recycled by this device is controlled, the concomitant variation in the oxygen richness of the engine air / fuel mixture is evaluated and the operating state of the device is diagnosed on the basis of this evaluation.

 According to a preferred implementation of this method, the average oxygen richness of the air / fuel mixture is measured during a first phase of engine operation in stabilized regime during which the recirculation device is inactive and during a second phase during which this device is activated and the operating state of the engine is diagnosed by comparing the difference in the oxygen richness measured during these two phases, with at least a predetermined value. The oxygen richness can be measured using a "linear" or "proportional" oxygen sensor placed at the outlet of the engine exhaust gases and forming part of a closed-loop control device for the air mixture. / engine supply fuel. This makes additional use of a probe intended for another use, a particularly economical and rational solution.

 Alternatively, the difference in oxygen richness can be substituted for the difference in the average injection times established by the closed-loop control of the air / fuel mixture and observed during the two successive phases of engine operation. These injection times are also conveniently available in the computer.

Other characteristics and advantages of the present invention will appear on reading the description which follows and on examining the appended drawing in which
FIG. 1 diagrammatically represents an engine equipped with means for closed-loop control of the air / fuel mixture and an exhaust gas recirculation device of a known type, and
FIG. 2 represents timing diagrams of signals useful for explaining the test method according to the present invention, of the recirculation device shown in FIG. 1.

 As seen above, the computer 6 normally controls the duty cycle of the switching of the solenoid valve 16, to thereby adjust the vacuum in the chamber 15 of the valve 12 and therefore the opening rate of this valve. Thus, a predetermined quantity of exhaust gas can be injected into the intake manifold of the engine and corrected at any time according to parameters representative of the instantaneous operating state of the engine, such as the pressure d P and the engine rotation speed N.

 According to the present invention, the computer is duly programmed to test, periodically for example, the operating state of the exhaust gas recirculation device. To do this, the computer controls a sudden, predetermined variation in the rate of recirculation of these gases, and observes the consequences of this variation either on the output signal of the oxygen sensor 9, or on the value of the injection time. ti that it calculates, in particular from the signal supplied by this probe, to correct the richness in fuel of the mixture which feeds the engine.

 The timing diagram A of FIG. 2 represents, by way of example, a PWM control of the solenoid valve 16 making it possible to implement the invention. The engine operating in stabilized regime, preferably in an operating phase of this engine which uses recirculation of exhaust gases to contain the temperature of these gases (and therefore the production of nitrogen oxides), we proceed , according to the invention, to a verification of the operation of the exhaust gas recirculation device. To do this, the computer controls by PWM the solenoid valve 16, according to the timing diagram in FIG. 2, A. In stabilized engine speed, in a first phase which ends at time T ,, the gas recirculation is cut by a zero setting of the duty cycle of the PWM control of the solenoid valve 16.

Then, during a second phase of predetermined duration ending at time T1, the computer controls a predetermined opening of the valve 13, by an appropriate adjustment of the duty cycle of the control of the solenoid valve 16 which establishes a proportional vacuum in the chamber 15 of the valve 12 and therefore a predetermined opening of the needle 13. Normal operation is restored after T1.

 According to a first implementation of the method according to the invention, there is used in the device of FIG. 1, an oxygen probe 9 "linear" or "proportional". Such a probe is known capable of providing a signal which is substantially linearly proportional to the oxygen content of the exhaust gases of an internal combustion engine. As an example of such a probe, mention may be made of the probe sold under the name NTK UEGO by the Japanese company NGK SPARK PLUG Co. Ltd.

 With such a probe in the device of FIG. 1, according to the test method according to the present invention, the average value SO (see FIG. 2, B) of the output signal of the linear oxygen probe 9 is evaluated during the first phase which ends at To and we put this value in memory.

 In stabilized regime, the inlet pressure P of the gases entering the engine remains the same after the instant To and the engine then sucks in an air / fuel mixture depleted in oxygen because part of the volume of the gases entering consists of gases exhaust much poorer in oxygen (1 to 2%) than the air sucked outside through filter 3 (20%). Therefore, the calculation of the amount of fuel injected is distorted because the engine load measured by the pressure sensor 7 is the same for air depleted in oxygen or not. This oxygen depletion is immediately perceived by the linear oxygen sensor 9 (see timing diagram B figure 2). The probe output signal shifts, during the second phase, to lower values of average value S1.

 It should be noted that, during this phase of recirculation of the exhaust gases, care will be taken to neutralize the closed loop correction carried out by the computer on the basis of the information delivered by the oxygen sensor, so as not to modify the calculation of the quantity of fuel injected.

 Three conclusions can be drawn from the comparison of the difference (SO-S1) with respect to a predetermined difference value AS established by the computer from the requested opening rate of the valve 12, which it controlled during the time interval (Tl-To).

 If SO-Sl is little different from AS, it must be concluded that the mechanical integrity of the exhaust gas recirculation device is complete and that the precision of the control of valve 12 is correct since this device has properly transmitted to the oxygen sensor 9 the disturbance introduced into the composition of the gases entering the engine. No intervention aimed at repairing the device or correcting the response of the valve 12 to its command by the computer 6, via the solenoid valve 16, is then to be expected.

If SO-Sl belongs to the domain defined by the expression: # S # x. # S,
x representing a predetermined precision as a percentage, it must be concluded that the mechanical integrity of the device is not affected, that the conduits 10, 11 are correctly connected and that the valve 12 reacts to control orders. It is the precision of the reaction of this valve to these orders which explains the error detected. The computer 6 can improve this precision by appropriately correcting the gain k of the control law (I) mentioned above.

 Finally if SO-Sl leaves the domain defined by the expression: AS + x.AS, it must be concluded that the exhaust gas recirculation device suffers from a mechanical breakdown or from an adjustment of the valve 12 impossible to correct by adjusting the gain k. It is then necessary to plan an intervention to check and possibly repair the conduits 10,11, the valve 12 or the solenoid valve 16.

 As an alternative to the test method described above, which operates on the basis of the signal delivered by the linear oxygen sensor 9, it is also possible to use the injection time ti as determined by the closed-loop control established by the computer 6. There is shown in C in Figure 2 the sampled and digitized signal which appears on the output ti of this computer, which controls the duration of opening of the injector (s) 8. As in the previous case, at l 'instant T0, the computer controls the opening of the valve 12, with a predetermined opening rate. The resulting oxygen depletion of the gas entering the engine is perceived by the computer through the oxygen sensor 9 which can then be of the conventional type or of the "linear" type. The closed-loop control of the richness of the mixture air / fuel then has the effect of reducing the injection time, from an average value tio, prior to T0, to an average value ti1 during the interval (T1-To). We then compare (tio-til) to a predetermined value Ati to highlight, as we did above with (S1-SO)) and AS the three situations corresponding respectively to correct operation, to demanding operation an adjustment of the valve opening law, and to a faulty operation requiring intervention on the exhaust gas recirculation device.

 Of course, the invention is not limited to the embodiments described and shown which have been given only by way of example. Thus one could reverse the order of the phases during which the exhaust gas circulates and does not circulate, respectively. Also, rather than comparing two situations in which there is and there is not, respectively, exhaust gas recirculation, one could successively control two rates of recirculation of these gases, of different levels. More generally, the invention is based on the observation of the reaction of the entire system (engine, engine control, exhaust gas recirculation device) to a controlled disturbance in its supply of exhaust gas.

Claims (7)

 1. Method for testing a device (10,11,12) for recirculating exhaust gas in an internal combustion engine with closed-loop control of an air / fuel mixture supplying the engine, characterized in what we order a variation in the quantity of gas recycled by this device, we evaluate the concomitant variation in the oxygen richness of the air / fuel mixture supplying the engine and we diagnose the operating state of the device from this assessment.  2. Method according to claim 1, characterized in that the average oxygen richness of the air / fuel mixture is measured during a first engine operating phase in steady state during which the recirculation device (10,11, 12) is inactive and during a second consecutive phase during which this device is activated and the operating state of the engine is diagnosed by comparing the difference (SO-S1) of the oxygen richness measured during these two phases, to at least a predetermined value (AS).  3. Method according to claim 2, characterized in that the oxygen richness of the air / fuel mixture supplying the engine is measured from a signal delivered by a linear oxygen sensor (9) placed in a line engine exhaust gas outlet.  4. Method according to claim 2, applied to an engine in which the composition of the air / fuel mixture is adjusted by adjusting the opening time (sorting) of at least one fuel injector (8), characterized in that that, for the diagnosis of the operating state of the exhaust gas recirculation device, the difference in oxygen richness is replaced by the difference (asti) in the average injection times established by the closed loop control and observed during the two successive phases of engine operation.  5. Method according to any one of claims 2 to 4, characterized in that one compares the noted difference (SO-Sl; tio-til) to its estimated value (#S; #ti) in the event of correct operation of the recirculation device and there is such an operation when this difference is substantially equal to the estimated value.  6. Method according to claim 5, applied to a recirculation device comprising a regulating valve (12) for the recirculation of the exhaust gases, the response of which to a control signal can be adjusted, characterized in that one corrects this response as a function of a deviation of the difference noted (S0-S1; tio-til) from the estimated value (#S; #ti), when this deviation remains less than a predetermined deviation.  7. Method according to claim 6, characterized in that a failure of the exhaust gas recirculation device is diagnosed when said deviation exceeds said predetermined deviation.