FR3137718A1 - process for optimizing the heating of a catalyst to limit fuel consumption - Google Patents

Abstract

The invention relates to a method for optimizing the heating of a catalyst (16) after a cold start of a vehicle comprising an internal combustion engine (2), said vehicle comprising an exhaust line (4) provided with a catalyst (16) and a nitrogen oxide concentration sensor (18) at the outlet of said catalyst (16). According to the invention, said method comprises the following steps: - a step of heating the catalyst (16), - a step of measuring the concentration of nitrogen oxides by the sensor (18) at the outlet of the catalyst (16 ) as soon as said vehicle starts, - a step of determining a specific parameter which is a function of the measurement of the sensor (18), - a step of stopping the heating of the catalyst (16) as soon as said parameter has reached a value predetermined threshold. Figure for abstract: Figure 1

Description

process for optimizing the heating of a catalyst to limit fuel consumption

The present invention relates to a method for optimizing the heating of a catalyst for depolluting the exhaust gases of an internal combustion engine, to limit the fuel consumption of the engine.

A method according to the invention relates in particular to vehicles equipped with spark-ignition engines (running on gasoline) which must comply with future polluting emissions standards in Europe.

As regulatory emission thresholds are increasingly lower, particularly for nitrogen oxides NOx, the catalytic post-treatment system of a motor vehicle internal combustion engine must be able to post-treat as quickly as possible. polluting emissions leaving the engine after a cold start of the engine.

Currently, to achieve such a result, it is possible to carry out a phase of heating the catalyst by degrading the efficiency of the engine, in order to have very hot gases at the outlet of the cylinder head of said engine. The main lever used is to significantly reduce the ignition advance, typically with 35° less crankshaft angle.

The publication EP-B1-0639708 discloses a process for heating a catalyst, in which the air flow rate is increased, the mass of fuel injected is adapted and the ignition advance is shifted as far as possible in the direction late. A disadvantage of such a process is that it causes a significant increase of around 30% in fuel consumption during the entire heating phase of the catalyst.

There are other types of heating of the catalyst, in particular by electric heating, and/or by injection of air into the exhaust. However, such means will also indirectly lead to overconsumption of fuel to be able to produce the electrical energy drawn from the vehicle battery and consumed by the electric heating means and/or by the air injection means such as a electric pump.

As carbon dioxide CO ₂ emissions are also increasingly constrained, it is necessary to be able to heat the catalyst "just as necessary", therefore optimizing the heating time of said catalyst. However, the optimal heating time of the catalyst depends on numerous parameters such as:
-The temperature of the exhaust when starting the engine from cold: the lower the ambient temperature, the longer it will take to bring the catalyst to the right temperature. When stopping the vehicle which is accompanied by stopping the engine during the heating phase of the catalyst, it is also difficult to estimate the drop in temperature,
-The driving profile: if after a cold start, the customer drives at low speeds/loads, it will take longer to heat the catalyst because there is less exhaust flow,
-The state of aging of the catalyst: the more the catalyst has aged, the less effective it is, and its heating duration must therefore be increased.

All these parameters cannot be understood precisely by the engine control system, in order to optimize the heating time of the catalyst. We could use a catalyst temperature criterion beyond which we can consider that it is completely primed, that is to say that it presents an emissions post-treatment efficiency rate of 100%, but estimating this temperature remains difficult due to internal exothermic reactions. To deal with this variability, the heating duration of the catalyst is generally calibrated to the worst case, which is therefore no longer optimal in consumption for less severe cases.

With the new depollution standards, an additional nitrogen oxide concentration sensor (also known as an “NOx probe”) will be fitted to vehicle engines, downstream of the engine exhaust gas post-treatment system. , to make it possible to monitor exhaust NOx emissions in real time, and thus report a fault if the system exceeds a threshold defined by regulations.

Publication FR-A1-3075260 discloses a method for controlling the temperature of a catalyst in the case of a hybrid vehicle. Heating of the catalyst is triggered and maintained for a predetermined duration when the exhaust temperature reaches a threshold Ts greater than the initiation temperature Ta of the catalyst, while keeping it active, and, when the instantaneous temperature, whose subsequent evolution depends on the operation of the hybrid powertrain (respective operating points of the thermal engine and the electric machine) is lower than the threshold Ts, then the heating is controlled until reaching the temperature threshold Ts.

This document does not make it possible to optimize the heating time of the catalyst.

Publication FR-A1-3081918 (RENAULT/NISSAN) discloses an improved method for managing the priming of a catalyst in which the enthalpy of the exhaust gases is calculated making it possible to determine the quantity of heat supplied to the catalyst. We determine a threshold enthalpy which characterizes the initiation, and we stop the heating when the calculated value of the enthalpy reaches the threshold. According to this publication, the threshold is a function of the value of the water temperature at start-up and the aging of the catalyst. For example, it is the product of a first factor which is a decreasing function of the water temperature, and a second factor between 0 and 1 which corresponds to aging, 0 corresponding to a new catalyst and 1 to a aged catalyst. In one embodiment, the aging is determined from the damping of a richness signal downstream of the catalyst relative to a richness signal upstream of the catalyst. The process described in this document does provide for an adaptation of the heating duration of the catalyst, but the link with the real capacity of the catalyst to clean up NOx is very indirect, and as a result, this method lacks precision. It is not easy to establish the correct values for the first and second factors.

A method according to the invention makes it possible to adapt the heating of a catalyst as necessary after a cold start of a vehicle, in particular by preventing said catalyst from continuing to be heated when it has already reached its temperature. priming.

The subject of the invention is a method for optimizing the heating of a catalyst for depolluting the exhaust gases of an internal combustion engine after a cold start of a vehicle comprising said engine, said vehicle comprising a line d exhaust equipped with said catalyst and a nitrogen oxide concentration sensor at the outlet of said catalyst.

According to the invention, said method comprises the following steps:
-a catalyst heating step,
-a step of measuring the concentration of nitrogen oxides by the sensor at the outlet of the catalyst as soon as said vehicle is started,
-a step of determining a specific parameter representative of the quantity of nitrogen oxide particles at the outlet of the catalyst, which is a function of the sensor measurement,
-a step of stopping the heating of the catalyst as soon as said specific parameter has reached a predetermined threshold value.

A method according to the invention has the particularity of relying on an element already present in the vehicle, namely the nitrogen oxide concentration sensor at the catalyst outlet, to adjust the heating of said catalyst following a cold start of the vehicle, so that said catalyst is no longer heated once it has reached its priming temperature. Indeed, in most current processes, there is no tangible information indicating that the catalyst has reached its initiation temperature and that it is therefore no longer useful to heat it. This sensor makes it possible to determine with certainty and reliability the moment when the catalyst has actually reached its initiation temperature, by means of the determination of a parameter which is representative of the quantity of nitrogen oxide particles remaining at the outlet of the catalyst. Indeed, as soon as this parameter, which evolves in the direction of reduction from the start of the engine, decreases sufficiently until reaching a predetermined threshold value, we can then conclude that the catalyst has just been primed. The probe, although serving as a basic element in a process according to the invention whose aim is to heat the catalyst as precisely as possible so that it is operational, nevertheless retains its original function of measuring the level of pollution generated by the vehicle. A process according to the invention is controlled by an on-board computer having software whose main input data is the nitrogen oxide concentration measured by the nitrogen oxide concentration sensor at the outlet of the catalyst. From this concentration of nitrogen oxides measured by the sensor, the software calculates a specific parameter, representative of the quantity of nitrogen oxide particles remaining at the outlet of the catalyst, which is compared to a predetermined threshold value. which has been previously entered into said software, said threshold value being a function of the specific parameter considered. As soon as this threshold value is reached, the computer orders an immediate stop of catalyst heating. The catalyst can, for example in the case of a spark ignition engine, be a three-way catalyst. Preferably, a particle filter is added to the catalyst to further limit the emission into the atmosphere of fine particles produced by the vehicle engine.

According to a possible characteristic of the invention, as a variant, the specific parameter is a concentration of nitrogen oxides directly determined by the sensor.

According to a possible characteristic of the invention, the specific parameter is a mass flow rate of nitrogen oxides at the outlet of the catalyst corresponding to the product of the concentration of nitrogen oxides and the flow rate of the exhaust gases at the outlet of the catalyst.

According to a possible characteristic of the invention, in another variant, the specific parameter is a level of post-treatment efficiency of the nitrogen oxides by the catalyst, calculated as the ratio between the concentration of nitrogen oxides at the catalyst outlet and the concentration of nitrogen oxides at the catalyst inlet. In other words, this is the proportion of nitrogen oxides emitted by the engine which are not treated by the catalyst.

According to a possible characteristic of the invention, the concentration of nitrogen oxides at the inlet of the catalyst is measured by an upstream sensor for the concentration of nitrogen oxides, said upstream sensor being placed on the exhaust line between the engine and the catalyst.

According to a possible characteristic of the invention, as a variant, the concentration of nitrogen oxides at the inlet of the catalyst corresponding to the concentration of nitrogen oxides at the outlet of the engine is determined from a model giving this concentration as a function of the operating point of the engine, said model being calibrated in advance by means of bench tests.

According to a possible characteristic of the invention, the operating point of the engine depends on a set of parameters comprising at least the engine speed, the engine torque, and the water temperature (i.e. coolant).

According to a possible characteristic of the invention, the heating of the catalyst is carried out using a technique to be chosen from a degradation of the combustion efficiency with modification of the value of the ignition advance, and an electric heating which can be coupled with air injection at the exhaust. These heating techniques are similar to those already existing.

According to a possible characteristic of the invention, the catalyst is a three-way catalyst.

An optimization method according to the invention makes it possible to heat a depollution catalyst as accurately as possible after a cold start of a vehicle, by relying on an element already present in said vehicle, namely the concentration sensor. nitrogen oxide at the outlet of the catalyst. It therefore does not require the insertion of added equipment, which is a source of cost, bulk and additional weight. It also has the advantage of offering a realistic and reliable method making it possible to know at what precise moment the catalyst has reached its initiation temperature, to avoid continuing to unnecessarily heat said catalyst.

A detailed description of a preferred embodiment of an optimization method according to the invention is given below, with reference to the following figures:

There is a schematic view of a powertrain of a vehicle in which an optimization process according to the invention can take place.

There is a diagram showing an example of variation in the quantity of nitrogen oxide particles as a function of time after a cold start, before and after catalyst priming,

There is a diagram showing an example of variation of vehicle speed as a function of time to obtain the diagram of the .

A method for optimizing the heating of a catalyst according to the invention is particularly suitable for a vehicle equipped with a spark ignition engine.

Referring to the , an example of a powertrain 1 of such a vehicle, schematically comprises an internal combustion engine, for example a gasoline engine 2, an air intake line 3 intended to supply air to said engine 2, and an exhaust line 4 intended to evacuate exhaust gases coming from this engine 2. The air intake line 3 is materialized by a conduit 5 comprising an air inlet 6, followed by a filter air 7 and opening into a turbocharger compressor 8. The conduit 5 extends to connect said compressor 8 to an intake manifold 9 passing through a throttle body 10, said intake manifold 9 distributing air into combustion chambers 11 of said engine 2. An exhaust manifold 12 makes it possible to evacuate the exhaust gases coming from the combustion chambers 11 of the engine 2, towards the exhaust line 4. The exhaust manifold 12 opens into a turbine 13 of the turbocharger which is coupled to the compressor 8 by a common shaft, and the exhaust line 4 extends beyond said turbine 13 to an outlet 14, passing through a depollution device 15 comprising a three-way catalyst 16 followed by a particle filter 17. The three-way catalyst 16 is inserted between the turbine 13 and the particle filter 17. A nitrogen oxide concentration sensor 18 is placed downstream of the particle filter 17 and upstream of the outlet 14 of the exhaust line 4. The sensor 18 makes it possible to measure the concentration of nitrogen oxides NOx at the outlet of the pollution control device 15 after the catalyst 16 has.

Referring to Figures 2a and 2b, the catalyst 16 is effective when it is at its starting temperature, which it reaches several tens of seconds after a cold start. Indeed, following a cold start of the engine corresponding to time 0s on the diagrams, for approximately the following 50 seconds, the quantity of nitrogen oxides emitted by engine 2 and materialized by curve 19, is equivalent to the quantity of nitrogen oxides determined at the outlet of catalyst 16 and materialized by curve 20. By “quantity”, we mean in the example of the a mass flow. These results tend to prove that over the first 50 seconds of this first phase, the catalyst 16 has not reached its priming temperature, because it hardly treats the nitrogen oxides coming from the engine. On the other hand, in a second phase extending this first phase, the quantity of nitrogen oxides at the outlet of the catalyst 16 tends towards 0 as illustrated by the corresponding curve 20, while curve 19 materializing the quantity of oxides d nitrogen at the outlet of engine 2 and upstream of catalyst 16, shows regular peaks corresponding to high emissions of nitrogen oxide particles when the speed of the vehicle increases (see ). During this second phase, the catalyst 16 is effective with respect to nitrogen oxide particles since it allows almost none to pass, suggesting that it has reached its initiation temperature at the start of this second phase.

Currently, it very often happens that the moment from which the catalyst 16 was initiated is not precisely known, and that the catalyst 16 continues to be heated even though it has already reached its initiation temperature. However, such a situation can cause certain disadvantages such as, for example, premature wear of the catalyst 16 which risks no longer being able to effectively fulfill its role of treating nitrogen oxides.

An optimization method according to the invention makes it possible to know precisely the moment when a catalyst 16 in the heating phase after a cold start of the vehicle has reached its starting temperature, meaning that this heating must be interrupted instantly.

A method according to the invention, making it possible to optimize the heating of the catalyst 16 after a cold start of the vehicle, comprises the following steps:
-a step of heating the catalyst 16,
-a step of measuring the concentration of nitrogen oxides by the sensor 18 at the outlet of the catalyst 16 as soon as said vehicle is started,
-a step of determining a specific parameter representative of the quantity of nitrogen oxide particles at the outlet of the catalyst, which is a function of the measurement of the sensor 18,
-a step of stopping the heating of the catalyst 16 as soon as said parameter has reached a predetermined threshold value.

The catalyst heating stage 16 begins just after a cold start of the vehicle. This step is carried out using a technique to be chosen from a degradation of the combustion efficiency with modification of the value of the ignition advance, and an electric heating which can be coupled with air injection at the exhaust. These heating techniques are similar to those already existing.

The step of measuring the concentration of nitrogen oxides by the sensor 18 at the outlet of the catalyst 16 as soon as said vehicle is started, is carried out without making the slightest adjustment to said sensor 18 and said catalyst 16. A method according to invention relies on the presence of the nitrogen oxide concentration sensor 18 already present in the vehicle, to know the priming state of the catalyst 16.

The specific parameter can take different forms, and can for example correspond to:
- at an absolute level of emission of nitrogen oxide particles at the outlet of catalyst 16, that is to say, at a mass flow rate (expressed for example in mg/s), which is equal to the product of flow rate of exhaust gases and the concentration of nitrogen oxides measured by sensor 18.
- to the concentration (measured in ppm) of nitrogen oxides directly determined by the sensor 18.
- a level of post-treatment efficiency of nitrogen oxides corresponding to the calculation of the ratio between the concentration of nitrogen oxides at the outlet of catalyst 16, divided by the concentration of nitrogen oxides at the inlet of the catalyst 16. According to a variant embodiment of the invention, the concentration of nitrogen oxides at the inlet of the catalyst 16 corresponds to the concentration of nitrogen oxides at the outlet of the engine 2, and it is measured by an upstream nitrogen oxide concentration sensor, said upstream sensor being placed on the exhaust line between the engine 2 and the catalyst 16. According to another alternative embodiment of the invention, the nitrogen oxide concentration nitrogen at the inlet of the catalyst, corresponding to the concentration of nitrogen oxides at the outlet of the engine 2 is determined from a model giving this concentration as a function of the operating point of the engine 2, said model being calibrated in advance by means of bench tests. The operating point of the engine 2 depends on a set of parameters including at least the engine speed, the engine torque and the water temperature.

Depending on the specific parameter considered, a threshold value is predetermined, from which it can be estimated that the catalyst 16 has been initiated. As soon as said specific parameter has reached this threshold value, the heating of the catalyst stops instantly so as to no longer unnecessarily heat said catalyst 16.

A method according to the invention is managed by an on-board computer having software whose input data is the concentration of nitrogen oxides measured by the sensor 18. From this concentration of nitrogen oxides measured by the sensor 18, the software calculates the specific parameter considered, which is compared to a predetermined threshold value which has been previously entered into said software, and which corresponds to the specific parameter considered. As soon as this threshold value is reached, the computer orders an immediate shutdown of the heating of catalyst 16.

Claims (9)

Method for optimizing the heating of a catalyst (16) for depolluting the exhaust gases of an internal combustion engine (2) after a cold start of a vehicle comprising said engine (2), said vehicle comprising a exhaust line (4) provided with said catalyst (16) and a sensor (18) for the concentration of nitrogen oxides at the outlet of said catalyst (16), characterized in that said method comprises the following steps:
-a step of heating the catalyst (16),
-a step of measuring the concentration of nitrogen oxides by the sensor (18) at the outlet of the catalyst (16) as soon as said vehicle is started,
-a step of determining a specific parameter representative of the quantity of nitrogen oxide particles at the outlet of the catalyst, which is a function of the measurement of the sensor (18),
-a step of stopping the heating of the catalyst (16) as soon as said specific parameter has reached a predetermined threshold value. Optimization method according to claim 1, characterized in that the specific parameter is a concentration of nitrogen oxides directly determined by the sensor (18). Optimization method according to claim 1, characterized in that the specific parameter is a mass flow rate of nitrogen oxides at the outlet of the catalyst (16), corresponding to the product of the concentration of nitrogen oxides and the flow rate exhaust gases at the outlet of the catalyst (16). Optimization method according to claim 1, characterized in that the specific parameter is a level of post-treatment efficiency of nitrogen oxides by the catalyst (16), calculated as the ratio between the concentration of nitrogen oxides nitrogen at the catalyst outlet and the concentration of nitrogen oxides at the catalyst inlet. Optimization method according to claim 4, characterized in that the concentration of nitrogen oxides at the inlet of the catalyst (16) is measured by an upstream nitrogen oxide concentration sensor, said upstream sensor being placed on the exhaust line (4) between the engine (2) and the catalyst (16). Optimization method according to claim 4, characterized in that the concentration of nitrogen oxides at the inlet of the catalyst (16), corresponding to the concentration of nitrogen oxides at the engine outlet (2) is determined from a model giving this concentration as a function of the operating point of the engine (2), said model being calibrated in advance by means of bench tests. Optimization method according to claim 6, characterized in that the operating point of the engine (2) depends on a set of parameters comprising at least the engine speed, the engine torque, and the water temperature. Optimization method according to any one of claims 1 to 7, characterized in that the heating of the catalyst (16) is carried out using a technique to be chosen from a degradation of the combustion efficiency with modification of the value of l ignition advance, and electric heating which can be coupled with air injection at the exhaust. Optimization method according to any one of claims 1 to 8, characterized in that the catalyst (16) is a three-way catalyst.