FR3069278B1 - METHOD FOR DETERMINING THE PASSAGE OF THE ROSSE POINT AT THE EXIT OF A DEPOLLUTION MEMBER - Google Patents

Abstract

The invention relates to a method for determining the dew point passage in an exhaust line section (2) connected to an engine (1) between the exhaust gas inlet in the line (2). and the outlet of a depollution device (3, 3 ') disposed in this line (2), characterized in that the mass of water stored in the depollution device (3.3') is determined from the beginning of operation of the motor (1) in progress, the energy required to evaporate this mass of stored water, a first energy of the exhaust gas entering the exhaust line (2) since the beginning of operation of the engine (1) in progress, a second energy of the exhaust gas at the outlet of the pollution control member (3, 3 '), the difference between these two energies, and when this difference is greater than the energy required to evaporate the mass of water stored in the organ (3, 3 '), it is considered that the dew point is passed.

Description

METHOD FOR DETERMINING THE PASSAGE OF THE POINT OF

ROSEE AT THE EXIT OF A DEPOLLUTION BODY

The present invention relates to the field of internal combustion engines. The invention relates more particularly to the determination of the moment when there is no more liquid water in a part of the exhaust line between the exhaust gas inlet and the outlet of a pollution control device. exhaust gas.

Motor vehicles, equipped with an internal combustion engine, are equipped with exhaust gas pollution control devices.

Over the years, there has been a tightening of the anti-pollution standards to which all new vehicles are subjected. As a result, vehicle pollution control devices have become more sophisticated, requiring different probes, generating reliability problems, and generating significant maintenance costs for vehicle owners. In the example of document FR2986263, these depollution devices may comprise a probe for example for measuring the level of nitrogen oxide and oxygen. These probes are fragile components that, to provide accurate measurement, need to be heated to an operating temperature (around 800 ° C) and should be protected from certain elements, including water in the form of water. liquid. The combustion of an air / fuel mixture in the engine causes the creation of water vapor which, on cooling, condenses and forms water in the exhaust line. The water thus generated is expelled from the exhaust line by the exhaust gas and can, on its way, come into contact with the probe. When the probe is heated to 800 ° C, the local contact of the water with the probe causes a thermal shock by local cooling of the probe and, therefore, degradation of the probe, or even a breakage thereof. This results in a fault in the depollution device. These probes must be brought to their operating temperature after evaporation of any water in liquid form in the exhaust line.

The document FR2986263 aims at heating the probe, but is silent on the determination of the dew point, in other words the moment when there is no more liquid water in the exhaust line.

There is therefore a need to determine when there is no more liquid water in the exhaust line, between the engine output and a determined point downstream of this engine output, especially when this part of the line of Exhaust includes depollution devices likely to retain water.

To achieve this objective, it is provided according to the invention a method for determining the transition to the state without the presence of liquid water, also referred to as dew point passage, in an exhaust line portion connected to a combustion engine internal, between the inlet of the exhaust gas in the exhaust line and the outlet of a pollution control member disposed in this exhaust line, characterized in that: -on determines the mass of water stored in the depollution unit from the beginning of operation of the engine in progress, -on determines the energy required to evaporate this mass of stored water, -on determines a first energy of the exhaust gas entering the exhaust line since the start of operation of the current engine, a second energy of the exhaust gases is determined at the outlet of the pollution control device, the difference between these two energies is determined, and when this difference is greater than the energy required to evaporate the mass of water stored in the pollution control device, it is considered that the dew point has passed. The technical effect is to have a continuous monitoring of the evolution of the presence of water in the depollution device and thus to be able to precisely determine the moment when any liquid water is absent from the part of the exhaust line equipped with the depollution device.

Various additional features may be provided, alone or in combinations:

In an embodiment where the exhaust line comprises a heating probe controlled downstream of the outlet of the pollution control member, the second energy is determined at the probe.

In one embodiment, the determination of the mass of water stored in the depollution unit since the starting of the engine is a function of the water produced by the combustion during the running operation of the engine and the water supplied by the engine. ambient humidity when stopping the engine that preceded the current operation of the engine.

In one embodiment, the mass of water supplied by ambient humidity is a lump mass per unit of engine time stopped during engine shutdown that preceded the running operation of the engine.

In one embodiment, the body of water stored in the depollution device is obtained from the relation:

With: w0: the maximum filling rate per volume of the pollution control organ, βΗζο: the affinity of the pollution control organ with water, <PH2ouq: the density of liquid water, PH2oin: Partial pressure water entering the pollution control device,

PsatH2o: The saturation vapor pressure of water, which is a function of the temperature of the exhaust gas, R: the constant of the perfect gases,

Tcata (t): the internal temperature of the depollution device,

Apore: a constant characteristic of the pore size of the active phase of the depollution device.

In one embodiment, the step of determining the mass of water stored in the depollution unit since the start of operation of the current engine is replaced by a step of determining the maximum water mass storable in the engine. depollution device and in that it is then considered that the dew point is passed when the energy difference is greater than the energy required to evaporate the maximum water body storable in the pollution control member. The invention also relates to a computer, characterized in that it comprises the acquisition means, processing by software instructions stored in a memory and

that the control means required to implement the method according to any one of the previously described variants. The invention also relates to an engine assembly comprising an internal combustion engine connected to an exhaust line, this exhaust line comprising a depollution device, characterized in that it comprises such a computer. The invention also relates to a motor vehicle, characterized in that it comprises such a motor assembly for its displacement. Other particularities and advantages will appear on reading the following description of a particular embodiment, not limiting of the invention, with reference to the figures in which: FIG. 1 is a schematic representation of a internal combustion engine equipped with an exhaust line for the depollution of the exhaust gas at the output of the engine associated with a computer for implementing the method of the invention. FIGS. 2 to 4 illustrate the evolution as a function of time respectively of the quantities of water, the temperatures and the energies of the gases in the case of a cold start, for a sensor situated after two catalysts at the output of the engine, on a stabilized operating point.

FIG. 1 shows an internal combustion engine 1, for example a compression ignition engine, operating for example with diesel fuel. This engine 1 is connected to a line 2 for exhausting the flue gases to form a motor assembly. Such an engine assembly can equip a motor vehicle for moving.

The exhaust line 2 has a first depollution member 3, in this embodiment, for example an oxidation catalyst for the oxidation of carbon monoxide and the oxidation of unburned hydrocarbons. The exhaust line 2 may comprise a second 3 'depollution member in this embodiment, immediately downstream of the first depollution member 3, for example a catalyst for selective catalytic reduction of nitrogen oxides.

The exhaust line 2 also has a first temperature sensor 4 upstream of the first oxidation catalyst 3 and a second temperature sensor 5 downstream of the second catalyst 3 '. Upstream and downstream are here defined relative to the direction of flow of the exhaust gas in the exhaust line 2.

At the second temperature sensor 5, the exhaust line 2 further comprises a probe for example an oxygen sensor.

The temperature sensors 4 and 5 as well as the probe 6 are connected to a computer 7. This computer 7 comprising the acquisition means, software instructions processing stored in a memory and the control means required to implement the method of the invention detailed after.

As already explained, in order to obtain an accurate measurement, certain sensors such as the oxygen sensor 6 are based on an analysis cell whose heating is controlled so as to start the cell heating at the appropriate moment and bring it to its temperature. optimal operation. However, this rise in temperature must be made after evaporation of any water in liquid form in the exhaust line, upstream of this sensor, because the contact of the liquid water and this cell can cause deterioration or even destruction of this one because of the thermal shock and make the sensor inoperative.

Indeed, water is likely to be stored in the exhaust line when the exhaust line is cold, or in the following life situations: - during the start-up phase: the combustion of fuel in the engine generates large quantities of water which are sent to the exhaust, during the phases of stopping the vehicle: the humidity of the air causes a storage of water by diffusion from the downstream of the exhaust line .

Two types of phenomena are at the origin of water storage:

Chemisorption: chemical interaction with the porous substrate present in the depollution device,

Physisorption: physical interaction that leads to the condensation of water on the cold walls of the metal tubes of the exhaust line and on the surface of the substrates.

Thus, the determination of the passage of this moment when there is no more liquid water in the exhaust line upstream of the sensor, also called dew point passage, is required to know if the heating of the sensor can then to be launched. For this purpose, it must be ensured of the absence of water in the depollution unit upstream of the sensor. If the depollution device contains water, and even if the gases arriving at the sensor no longer contain, it is known that eventually the water of the pollution control member will be released and will arrive on the sensor.

In order to determine when the heating of the sensor 6 can be started, it must be ensured that there is no water in the set of pollution control devices 3, 3 'upstream of the sensor 6. In fact, if The depollution unit contains water, and even if the gases arriving at the probe contain no more, it is known that eventually the water of the pollution control member will be released and will arrive on the probe. For this purpose, the energy that has been consumed to vaporize the water stored in the exhaust line 2 is calculated as the difference between the energy entering the line and that exiting at the probe 6, the heating of which is to launch. When the energy consumed for the vaporization is greater than a threshold determined according to the amount of water stored in the exhaust line, the heating of the probe can be initiated without risk of damaging it.

The vaporization reaction of the stored water is endothermic. Thus, as long as water is stored in the depollution device, the energy downstream thereof is less than that upstream.

Energy entering and leaving the exhaust line can be calculated by thermal balance based on temperature measurements and gas flow. The thermal power of a circulating gas is:

Pgaz QmCpTgaz

With

Pgaz: the thermal power of the gas (W)

Qm: the gas flow (kg / s)

Cp: the thermal capacity of the gas (J / kg / K)

Tgaz: the gas temperature (K)

Thus, the thermal powers of the gas entering and leaving the exhaust line are respectively:

Qm Pp Pentrance

Psortie Qm Cp Psortie According to the equation of the heat transfer to a body, at each instant, the power transferred by the gases to the exhaust line is ΔΡ, with: ÙF - Pentrée ~ Psortie Qm Pp (Center ~ Psortie ' )

Assuming that during a cold start the thermal power of the incoming gas is transferred entirely to the line, then during the heating phase of the line Psortie Pambiante The total energy transferred to the exhaust line at a given instant t since a startup is worth then:

Thus, in our case, with reference to FIG. 1, the total energy of the gases transferred in the exhaust line portion 2 comprising the depollution elements 3 and 3 'is given by the energy deviation ΔΕ: ΔΕ = El (t) - E2 (t)

With El (t), the total energy of the gases transferred in the part of the exhaust line comprised between the temperature measurement point T1 of the gases entering the line obtained by the temperature sensor 4 and the output of the exhaust line 2, since the beginning of the running operation of the engine 1:

And E2 (t), the total energy of the gases transferred in the part of the exhaust line between the measurement point of T2 by the temperature sensor 5, therefore at the level of the sensor 6, and the exit of the line 2, since the beginning of the current operation of the engine 1:

This energy deviation ΔΕ will be used to heat the catalysts 3, 3 ', to heat and then vaporize the water stored in these catalysts. The water is stored in the bricks of the catalysts 3, 3 'of the exhaust line 2 by physisorption phenomenon on the filters and chemisorption on the catalysts, and can have two sources: combustion and ambient humidity.

The combustion reaction in the engine: the combustion of fuel in a heat engine can be approximated by the equation:

Thus, the mass of water contributed by the combustion reaction from the beginning of the path can be calculated by means of the formula:

With: MH2o: molar mass of water (g.mol · 1) MCxHy: molar mass of fuel (g.mol · 1) QCarb: fuel flow (g.s-1)

It is also possible to take into account the water brought by the ambient humidity during the shutdown preceding the operation of the current engine. The ambient humidity when the engine is stopped is given by the relation:

With:

Pvap: the pressure of the water vapor present in the air,

Psat (T): the vapor pressure saturation of the water at a given temperature T.

Since the information necessary for this estimation of the water mass provided by the ambient humidity is not always available, it is possible to use a fixed value of the mass of water stored via the ambient humidity per unit of time. engine stopped during shutdown before engine running.

The water mass stored instantaneously can be obtained from the maximum instantaneous storage rate, V <t), of a catalyst, which rate can itself be determined from the Dubinin-Radushkevich equation:

With: w0: the maximum filling ratio by volume of the active phase of the depollution unit, βΗζ0: the affinity of the active phase of the depollution device with water, <PH2ouq: the density of the liquid water, PH2oin: The partial pressure of water entering the depollution device,

PsatH2o: The saturation vapor pressure of water, which is a function of the temperature of the exhaust gas, R: the constant of the perfect gases,

Tcata (t): the internal temperature of the depollution device,

Apore: a constant characteristic of the pore size of the active phase of the depollution device.

This model has the advantage of being a simple model based on data specific to each depollution device technology, but simple to obtain.

The mass of water that can be stored on the brick of a catalyst is then:

With: 7oZcata: the volume of the pollution control organ PH2ouq: the density of liquid water

Thus the amount of water stored instantaneously on the bricks of the catalysts 3, 3 'is at most this mass of water storable "ieaustocfcaWe (0 ·

In order to simplify the calculation and to ensure the safety of the sensor 6, it is possible to define the maximum water mass that the bricks of the catalysts 3 and 3 'can store. This maximum water mass can be maximized by using brick temperature and gas pressure values that maximize the maximum instantaneous storage rate, ip (t). This provides a margin of safety for the activation of the heating of the sensor 6. The energy required to heat the bricks of the catalysts 3, 3 'of the exhaust line 2. For each brick, this energy can be calculated by the formula :

Pchau manufactures J. ~ mbrique_i- CPbriquej. · (373 - 7ref_t> rique_i)

With:

Echauf_briquej: energy required to carry the brick i to 100 ° C (J) mbriquej: mass of the brick (kg)

Cpbriquej: heat capacity of the brick (J.kg-1.K-1)

Pref_briquej 'initial temperature of the brick (J) The energy needed to heat the water on the bricks, which can be calculated by the formula:

Water / Water - Water- (373 - Tref_eau)

With:

Echauf_eau: energy needed to bring the brick i to 100 ° C (J) meau: mass of water stored on the brick (kg)

CpeaU: heat capacity of water (J.kg-1.K-1)

Pref_water: initial temperature of the water stored on the brick (J) The energy needed to vaporize the water, which can be calculated by the formula:

Evap_water water-F water

With:

Evapeau: energy needed to vaporize water (J) water: mass of water stored on the brick (kg)

Heau: mass enthalpy of water vaporization (J.kg-1)

When the thermal energy supplied to the line by the gas is greater than the energy required to vaporize the stored water, it is possible to start the heating of the piloted sensor without risk of damaging it because there is no more water in it. upstream of the sensor in the exhaust line.

In a simplified variant, it is possible not to calculate the mass of water stored in the exhaust line at any time but instead a maximum amount of water can be stored by the catalysts 3, 3 'and consider whereas the dew point is passed if the energy difference between the catalysts 3, 3 'is greater than the energy required to vaporize the maximum amount of water that can be stored by the catalysts 3, 3' of the exhaust line 2. It is then possible to trigger the heating of the sensor 6 without risk of deterioration.

With reference to FIGS. 2, 3 and 4 is illustrated the evolution as a function of time, t, respectively of the quantities of water, Qm_eau, of the temperatures, T, and of the thermal energies of the gases, Eth g, in the case of a cold start, for a sensor 6 located after two catalysts 3, 3 'at the output of the engine 1, on a stabilized operating point.

The start is divided into six phases noted from I to VI. In Fig. 2, curve 20 indicates the amount of water produced by the engine, curve 21 indicates the amount of water in the first catalyst (reference 3 in Fig. 1) and curve 22 indicates the amount of water in the second catalyst (reference 3 'in Figure 1). In FIG. 3, the curve 30 indicates the temperature of the gases leaving the engine, the curve 31 indicates the internal temperature of the first catalyst and the curve 32 the internal temperature of the second catalyst. In FIG. 4, the curve 40 indicates the thermal energy of the gases leaving the engine, the curve 41 indicates the thermal energy of the gases leaving the first catalyst and the curve 42 the thermal energy of the gases leaving the second catalyst. . The difference between the curve 40 and the curve 41 (for example at the brace referenced 43) represents the energy absorbed by the vaporization of the water contained in the first catalyst, as long as it contains therein. The difference between the curve 40 and the curve 42 (for example at the brace referenced 44) represents the energy absorbed by the vaporization of the water contained in the first and the second catalyst, as long as they contain them.

Phase I: The exhaust line is cold and both catalysts initially do not contain water. The water leaving the engine is stored on the first catalyst, the induced exotherm and the hot air from the engine raise its internal temperature. The second catalyst does not receive water, everything is stored by the first.

Phase II: The first catalyst has reached its maximum storage in water, the engine output water passes through and will now store in the second catalyst, whose internal temperature begins to rise.

Phase III: The first catalyst continues to heat thanks to the calories provided by the engine, the storage capacity decreases with temperature, it can not keep all the water it contains and begins to empty: the endotherm induced by the removal of water slows the rise in temperature of the first catalyst. The second catalyst continues to fill with water from the engine and water removed by the first catalyst.

Phase IV: The first catalyst continues to destock its water and the second catalyst is now saturated, that is, it has reached its maximum water storage.

Phase V: The first catalyst is empty, so it continues to warm up more quickly. At time t1, the energy difference between the output of the engine and the outlet of the first catalyst has reached the threshold corresponding to the amount of water stored up to the first catalyst. This gap no longer increases because the vaporization of the stored water is complete.

The temperature of the second catalyst also increases, it can not retain the water it contains and empties progressively. As before, the endotherm of water destocking slows its rise in temperature. Spraying water stored on the second catalyst amplifies the energy gap between the motor output and the outlet of the second catalyst.

Phases VI: The two catalysts are now empty, and the second catalyst quickly rises in temperature in turn. At time t2, the energy difference between the motor output and the outlet of the second catalyst has reached the threshold corresponding to the amount of water stored up to the second catalyst. This gap no longer increases because the vaporization of the stored water is complete.

The line is now heated enough and can no longer store water. It is then possible to start the heating of the sensor 6 downstream without risk. The invention is not limited to the embodiment described. Alternatively, the engine may be a spark ignition internal combustion engine, operating for example with gasoline. The pollution control member may be different from an oxidation catalyst. The pollution control member may be for example a three-way catalyst, a selective catalytic reduction device for nitrogen oxides, a particulate filter, etc.

Alternatively, the probe may be any type of measuring probe having a sensitive cell to be heated to an operating temperature. The invention makes it possible to optimize the availability of the sensor by guaranteeing its integrity. The invention further reduces the risk of degradation of the sensors and a saving of a temperature sensor at each sensor to be heated to an operating temperature.

Claims (3)

1. Method for determining the passage in the state of absence of liquid water, also referred to as dew point passage, in an exhaust line section (2) connected to an internal combustion engine (1), included between the inlet of the exhaust gases in the exhaust line (2) and the outlet of a pollution control device (3, 3j disposed in this exhaust line (2), characterized in that: the mass of water stored in the depollution unit (3,3 ') since the beginning of operation of the engine (1) in progress, -on determines the energy required to evaporate this mass of stored water, -on determines a first energy (E1 (t)) of the exhaust gas entering the exhaust line (2) since the beginning of operation of the current engine (1), -on determines a second energy (E2 (t)) exhaust gas at the outlet of the pollution control device (3, 3 j, -on determines the difference (ΔΕ) between these two energy, and when this difference is greater than the energy required to evaporate the mass of water stored in the depollution device (3, 3 j, it is considered that the dew point is passed. 2. Method according to claim 1, characterized in that the exhaust line (2) comprising a probe (6) controlled heating downstream of the outlet of the pollution control member (3, 3 j, the second energy (E2 (t)) is determined at the level of the probe (6) 3. Method according to claim 1 or claim 2, characterized in that the determination of the mass of water stored in the depollution device (3 3j since the start of the engine (1) is a function of the water produced by the combustion during the running operation of the engine (1) and the water supplied by the ambient humidity when the engine stopped which has preceded by the current operation of the engine (1) 4. The method as claimed in claim 3, characterized in that the mass of water supplied by ambient humidity is a fixed mass per engine time unit stopped when the engine is stopped. which preceded the running operation of the engine (1). according to any one of the preceding claims, characterized in that the mass of water stored in the depollution device (3, 3 j is obtained from the relation: With: w0: the maximum filling rate per volume of the pollution control organ, βΗζ0: the affinity of the pollution control organ with water, <PH2ouq: the density of liquid water, pH2oin: Partial pressure of water entering the pollution control unit, psatH2o: The saturation vapor pressure of the water, which is a function of the temperature of the exhaust gases, R: the constant of the perfect gases, Tcata (t): the temperature internal of the depollution device, Apore: a constant characteristic of the pore size of the active phase of the depollution device. 6. Method according to any one of the preceding claims, characterized in that the step of determining the mass of water stored in the depollution device (3, 3 ') since the beginning of operation of the engine (1) current is replaced by a step of determining the maximum water mass storable in the pollution control device (3, 3 ') and in that it is considered that the dew point is passed when the difference energy (ΔΕ) is greater than the energy required to evaporate the maximum water body storable in the pollution control element (3, 3 '). 7. Computer (7), characterized in that it comprises the acquisition means, processing by software instructions stored in a memory and the control means required to implement the method according to any one of the preceding claims. . 8. Engine assembly comprising an internal combustion engine (1) connected to an exhaust line (2), this exhaust line (2) comprising a depollution member (3), characterized in that it comprises a calculator (7) according to claim 7. 9. Motor vehicle, characterized in that it comprises a motor assembly according to the preceding claim for its displacement.