Control method of an exhaust gas aftertreatment system

Abstract

Control method of an exhaust gas aftertreatment system (10) of an internal combustion engine (11), wherein the aftertreatment system is provided with: • - a first catalyst (12), oxidant, • - further reducing catalysts (13, 14, 15), • - a fuel burner device (20) which, in turn, has: • - a fuel burner (21), • - a pump (22), upstream of the burner (21), to supply the burner with an air flow (211), • - a fuel metering unit (24) for providing fuel flow to the burner (21), the control method being provided with the following steps: • - detecting the operating conditions that require the lighting of the burner (21), • - lighting the burner (21); • - detecting input parameters to determine a control strategy, • - selecting an air / fuel ratio (AFR), • - choosing a control strategy to determine thermal power and air / fuel ratio (AFR); • - requesting the determined thermal power; • - requesting the determined air / fuel ratio (AFR).

Description

[0001] CONTROL METHOD OF AN EXHAUST GAS AFTERTREATMENT

[0002] SYSTEM

[0003] Technical sector of the invention

[0004] The present invention relates to a control method of an exhaust gas aftertreatment system of an internal combustion engine. In particular, the control method is applicable to an exhaust gas aftertreatment system equipped with a fuel burner device.

[0005] Background art

[0006] An exhaust gas aftertreatment (ATS) system for internal combustion engines comprises at least one oxidizing catalyst, at least one reducing catalyst and a plurality of sensors (e.g., temperature sensors, pressure sensors, air / fuel sensors, etc.) for monitoring its operation. Its objective is to treat the exhaust gases coming from the combustion process inside the cylinders of the internal combustion engine. The aftertreatment system includes processes that mechanically, catalytically or chemically clean the exhaust gases after they leave the combustion chamber. Exhaust gas treatment is therefore carried out to convert the harmful emissions produced during combustion into non-toxic exhaust gases. Global regulations on harmful emissions, which are about to come into force, require exhaust gas after-treatment systems that guarantee high pollutant conversion performance in almost all possible driving conditions.

[0007] To meet the regulations on harmful emissions it is necessary to equip the exhaust gas aftertreatment system with an auxiliary heating device to accelerate the heating of the ATS and support it in other operating modes, for example, during the regeneration of the particulate filter.

[0008] The main advantages of an auxiliary heating device compared to classic in-cylinder heating strategies are:

[0009] - generate greater thermal power, and

[0010] - completely decouple the after-treatment system heating from the engine operating conditions.

[0011] For this purpose, it is possible to use technologies related to both a fuel burner device and an electric heater. Both technologies have several advantages and disadvantages:

[0012] - the electric heater is a simpler subsystem, with a lower number of components, but requires a 48 V electrical power supply and, therefore, a specific battery, and has greater impacts of integration into the exhaust gas after-treatment system,

[0013] - the fuel burner is a more complex system but can provide greater thermal power, does not require a 48V vehicle architecture and is easier to be integrated into the exhaust gas aftertreatment system.

[0014] The Applicant prefers this second alternative which guarantees further reduced times for heating the exhaust gas aftertreatment system.

[0015] The exhaust gas after-treatment system is, therefore, a complex system that must be controlled in real time to obtain the desired performance and for this purpose various control strategies have been developed over the years, for example, exhaust gas recirculation strategies, particulate filter regeneration strategies and numerous others. The addition of a fuel burning device, however, makes the known strategies no longer exhaustive.

[0016] There is therefore a need to define a control method for an exhaust gas aftertreatment system equipped with a fuel burner device.

[0017] Summary of the invention

[0018] An object of the present invention is a method of controlling an exhaust gas aftertreatment system for internal combustion engines, wherein the exhaust gas aftertreatment system is provided with a fuel burner device.

[0019] The control method, according to the present invention, allows optimizing the heating time of the aftertreatment system and the use of the fuel burner device in other operating conditions.

[0020] Therefore, according to the present invention there is provided a control method of an exhaust gas aftertreatment system for internal combustion engines, having the characteristics set out in the independent claim, attached to the present description.

[0021] Further embodiments of the invention, preferred and / or particularly advantageous, are described according to the characteristics set out in the attached dependent claims.

[0022] Brief description of the drawings

[0023] The invention will now be described with reference to the attached drawings, which illustrate some non-limiting implementation examples, in which:

[0024] - figure 1 illustrates a simplified diagram of an exhaust gas aftertreatment system for an internal combustion engine, - figure 2 illustrates the diagram of the aftertreatment system of figure 1 with greater detail of the components,

[0025] - figure 3 illustrates an operating model of a fuel burning device of the aftertreatment system of figure 1,

[0026] - figure 4 schematically illustrates a control method of the fuel burner device of the after-treatment system of figure 1, according to a preferred embodiment of the invention,

[0027] - figure 5 is a graph representing the air flow as a function of the required thermal power, in a first operating condition, and

[0028] - figure 6 is a graph representing the air flow as a function of the required thermal power, in a second operating condition.

[0029] Detailed description

[0030] The control method of an exhaust gas aftertreatment system of an internal combustion engine will now be described with reference to an exhaust gas aftertreatment system provided with a fuel burner device.

[0031] With reference to figure 1, the exhaust gas aftertreatment system 10, by means of one or more catalysts, processes the exhaust gases coming from combustion inside an internal combustion engine 11, equipped with a electronic control 16.

[0032] The aftertreatment system 10 is provided with a fuel burner device 20 which in turn comprises a fuel burner 21, a pump 22 upstream of the burner 21 and a control module 26 of the fuel burner device 20.

[0033] With reference to figures 2 and 3, the exhaust gas aftertreatment system 10 can comprise:

[0034] - a first catalyst 12, for example an oxidizing catalyst 12 which operates to complete oxidation reactions of polluting combustion products, for example carbon monoxide and / or unburned hydrocarbons,

[0035] - further catalysts 13, 14, 15, three in number in the example of figure 2, for example reducing catalysts which operate to complete reduction reactions of further polluting combustion products, for example nitrogen oxides. Conveniently, a reducing catalyst 13 is provided with a particulate filter. Alternatively, the particulate filter could be a component independent of the catalyst,

[0036] - injectors 17 of urea-based fluid for the operation of the reducing catalysts 13, 14,

[0037] - temperature sensors 18 for monitoring temperatures upstream and / or downstream of the catalysts,

[0038] - nitrogen oxide sensors 19 for monitoring the concentration of nitrogen oxides upstream and / or downstream of the catalysts.

[0039] The electronic control unit 16 for controlling the internal combustion engine 11 conveniently also manages the aforementioned catalysts 12, 13, 14, 15.

[0040] In greater detail, the fuel burner device 20 comprises:

[0041] - the fuel burner 21 which operates to provide a flow of hot flue gases 213. The burner 21 can preferably be located upstream of the first catalyst 12 (as in figure 2) or, alternatively, upstream of the second or third catalyst, depending on the architecture and specific needs of the application,

[0042] - the pump 22, preferably an electric pump, upstream of the burner 21 to provide the burner with a flow of air 211 at room temperature, previously filtered by an air filter 30 of the internal combustion engine 11,

[0043] - a possible controlled control valve 23, placed between the electric pump 22 and the burner 21, for regulating the air flow 211 to the burner. Alternatively, the air flow 211 can also be regulated by intervening on the "duty cycle" of the pump 22,

[0044] - a fuel metering unit 24, for example an injector, to provide a fuel flow to the burner 21,

[0045] - an ignition device 25 (for example, a spark plug), of the air-fuel mixture inside the burner 21,

[0046] - the control module 26 of the fuel burner device 20 for carrying out the control method according to the present invention. The control module 26 can also be integrated into the electronic control unit 16,

[0047] - a pressure sensor 28 for monitoring the pressure upstream of the first catalyst 12.

[0048] With reference to figure 3, a model of the fuel burner device 20 is organized in this way: the burner 21 has as inputs the air flow 211 at room temperature and a thermal power in the form of a fuel flow 212 equipped with its own calorific value. Furthermore, the burner 21 has at its outlet the flow of burnt gases 213 which joins with a flow of exhaust gases 214 coming from the internal combustion engine 11. The union of these two flows is modeled by means of a mixing model 215 which, therefore, it has as its output a mixed stream 216 of burnt gases sent to a catalyst 12, 13, 14, 15, for example to the first oxidizing catalyst 12.

[0049] The model of the device 20 depends on the characteristics of the burner (thermal power / air ratio), on the temperature of the air flow 211 at the inlet of the burner 21, on the flow of the exhaust gases 214 and its temperature and takes into account the thermal inertia of the system.

[0050] According to the present invention, the control method of the fuel burner device 20 allows to optimize:

[0051] - the heating time of the aftertreatment system 10,

[0052] - the use of burner 21 in other operating conditions, for example, during regeneration of the particulate filter or during high temperature cycles,

[0053] - the protection of the catalysts from excessively high temperatures.

[0054] This is achieved by adjusting the burner thermal power and air / fuel ratio (AFR) via the burner device 20 model (Figure 3), setting the optimal combustion mode and aftertreatment system strategy.

[0055] Referring to Figure 4, the control method of the fuel burner device 20 of the aftertreatment system 10 includes the following steps:

[0056] - detecting SI 10 operating conditions that require the ignition of the burner 21. These operating conditions are linked to the temperatures of the aftertreatment system 10 depending on the operating mode and the required thermal power;

[0057] - turning S120 on burner 21;

[0058] - detecting S130 the input parameters to determine a control strategy. These parameters are: pressure of the burnt gases 213 downstream of the burner 21, temperature of the exhaust gases 214 upstream of the mixing with the burnt gases 213 of the burner 21, temperature of the mixed stream 216 of burnt gases upstream of the first catalyst 12, for example the oxidation catalyst; - selecting S140 an air / fuel ratio;

[0059] - with the data obtained from the two previous phases, choosing S150 the control strategy to determine the thermal power and the air / fuel ratio;

[0060] - requesting S160 the determined thermal power;

[0061] - requesting S170 the determined air / fuel ratio.

[0062] These last two data will then be managed and set by the control module 26 of the fuel burner device 20.

[0063] Advantageously, the operating conditions that require the ignition of the burner 21 include: a) heating of the exhaust gas aftertreatment system 10. This occurs when the temperatures of the after-treatment system 10, for example, the temperatures of the oxidizing catalyst 12 or of a reducing catalyst 14 are not high enough to allow the required reduction of harmful emissions.

[0064] However, the heating of the after-treatment system is limited, according to the present method, by the temperatures permitted for the different catalysts. In particular, the control method acts by limiting the maximum temperature at the outlet of the burner 21 and to do this it can use both the control of the air / fuel ratio (by setting higher values of the air / fuel ratio) and the limitation of thermal power in function of the pressure of the gas leaving the burner. The flow rate of the pump 22 is used to change the air / fuel ratio, while the flow of fuel 212 to the fuel meter 24 is used to change the thermal power.

[0065] The heating of the aftertreatment system is further limited by a maximum temperature gradient threshold at the first catalyst 12. This is accomplished by limiting the thermal power based on the flow of exhaust gases 214 at the exit of the engine 11, the temperature, the burner 21 and the modeling of the mixed stream 216 of burnt gases sent to the first catalyst. More specifically, the thermal power limitation is achieved by the control method through a predictive action aimed at not exceeding the constraints of the components, a predictive action based on the inversion of the operating model of the fuel burner device 20, b) support for the regeneration of the particulate filter present on the reducing catalyst 13. The method allows the oxygen concentration to be increased to improve the regeneration efficiency of the particulate filter. This is achieved by using the fuel burner device 20 with low thermal power and high air / fuel ratio. By doing this, the oxygen concentration upstream of the particulate filter is increased and the regeneration efficiency is improved, especially during the last part (30-50%) of the regeneration. In this way, both the frequency and duration of regeneration of the particulate filter are reduced, c) cooling of the after-treatment system and the burner. Burner 21 is used in pump mode for cooling the burner device itself and the components of the after-treatment system. The pump 22 of the fuel burner device 20 can be used to cool the aftertreatment system 10 when the temperature is quite high (e.g., T>400°C) to improve the conversion efficiency of nitrogen oxides. Additionally, this mode can be used to cool the device itself when heating the aftertreatment system is not required.

[0066] Conveniently, a first control strategy to determine thermal power and air / fuel ratio consists in providing the maximum thermal power without exceeding the maximum temperature at the outlet of the burner 21 considering the effect of the back pressure of the mixed stream 216 of burnt gases on the pump 22 of the air.

[0067] In other words, the required thermal power determines the air flow 211 required to ensure combustion stability and improve the heating of the aftertreatment system 10: a high value of the air / fuel ratio improves the heating of the after-treatment system 10.

[0068] At the same time, the strategy must determine the minimum air flow to ensure combustion stability and not exceed the temperature limits, imposed by the components of the aftertreatment system 10, at the outlet of the burner 21.

[0069] Finally, considering the back pressure of the mixed stream 216 of burnt gases downstream of the burner 21, the strategy must determine the maximum air flow that can be delivered by the pump 22.

[0070] Figure 5 is a graph representing the air flow [g / s] as a function of the required thermal power [kW]. Three curves are shown in the graph:

[0071] - the required air flow 211,

[0072] - the minimum air flow 211min which allows not to exceed the temperature limits imposed by the components of the aftertreatment system 10,

[0073] - the maximum air flow 211max that can be delivered by the pump 22 as a function of the back pressure of the mixed stream 216 of burnt gases downstream of the burner 21.

[0074] A first operating condition occurs when the maximum air flow 211max is greater than the minimum air flow 211min. In this case, if the required airflow 211 is less than the maximum airflow 211max, no limitation of the airflow 211 is necessary. Otherwise, the required airflow 211 (point Pl in the graph) must be limited to the corresponding Pl* value of the maximum air flow curve 211 max.

[0075] A second operating condition is illustrated in figure 6. Figure 6 is also a graph representing the air flow [g / s] as a function of the required thermal power [kW]. Three curves are also shown in this graph:

[0076] - the required air flow 211,

[0077] - minimum airflow 211min,

[0078] - the maximum air flow 211 max.

[0079] The second operating condition occurs when the maximum air flow 21 1max is less than the minimum air flow 211min. In this condition the required air flow 211 is not less than the maximum air flow 211max. Consequently, the use of the required thermal power will lead to exceeding the temperature limit at the outlet of burner 21 (point P2 in the graph). Therefore, the thermal power must be limited to have an air flow 211 coinciding with the minimum air flow 211min and with the maximum air flow 211max (point P2* in the graph).

[0080] Conveniently, a second control strategy for determining thermal power and air / fuel ratio consists in providing the maximum thermal power not exceeding the maximum allowable temperature for the aftertreatment system components and, furthermore, the temperature gradient upstream of a catalyst, for example the first catalyst 12.

[0081] The limitation of the temperature gradient is carried out by inverting the existing model between the thermal power of the burner 21 and the conditions upstream of the first catalyst 12, using the following relationships:

[0082] AT = far=Tpredlcted (Pburn) ~ Told(1)

[0083] The temperature gradient AT upstream of the catalyst 12 is a function of the thermal power Pbum of the burner and is represented by the temperature variation between the temperature at the previous step Toid and the temperature at the current step Tpredicted . It is therefore a function of the burner temperature Tburn .n, the air flows mburn, d the burnt gases mexh, the exhaust gases mE0, the temperature at the engine outlet TEO and the upstream temperature of catalyst 12 in the previous step Toid. Furthermore, in formula (2) T is the time constant of the system, ts the sampling time, i.e. the calculation step, of the algorithm.

[0084] The flow of air mburnis a function of the thermal power Pbum. The function is then approximated (Taylor approximation truncated at the first term) by a linearization of the burner characteristics (power relative to air) at the current burner power Pburn-

[0085] Then AT is set up on the maximum temperature gradient allowed at the first catalyst 12:

[0086] AT = ATMax(4) where ATMaxfor the first catalyst 12 it is usually equal to 50°C / s.

[0087] Finally, the function to obtain the maximum allowed thermal power is set and reversed using an iterative calculation that updates the thermal power Pburnat each iteration.

[0088] The iteration procedure is stopped when convergence is detected:

[0089] This second strategy also serves to limit the power and therefore the temperature TMax to the first catalyst 12, temperature which is usually around 850°C.

[0090] Ultimately, the innovative characteristics of the proposed control method are:

[0091] - control of burner 21 using thermal power and air / fuel ratio to optimize heating performance while ensuring hardware protection,

[0092] - use of burner 21 to support the regeneration of the particulate filter, increasing the concentration of O2 in the line.

[0093] - use of the burner in pump mode to cool the line and improve nitrogen oxide conversion performance.

[0094] In addition to the ways of implementing the invention, as described above, it should be understood that numerous further variations exist. It must also be understood that said ways of implementation are only exemplary and do not limit neither the object of the invention, nor its applications, nor its possible configurations. On the contrary, although the above description makes it possible for the skilled man to implement the present invention at least according to one of its exemplary configurations, it must be understood that numerous variations of the described components are conceivable, without thereby departing from the object of the invention, as defined in the attached claims.

Claims

CLAIMS1. Control method of an exhaust gas aftertreatment system (10) of an internal combustion engine (11), wherein the aftertreatment system comprises:- a first catalyst (12), oxidant,- further reducing catalysts (13, 14, 15) and a particle filter,- a fuel burner device (20) which, in turn, comprises:- a fuel burner (21),- a pump (22), upstream of the burner (21), to supply the burner with an air flow (211),- a fuel metering unit (24) for providing fuel flow to the burner (21), the control method comprising the following steps:- detecting the operating conditions that require the lighting of the burner (21),- lighting the burner (21);- detecting input parameters to determine a control strategy,- selecting an air / fuel ratio (AFR);- choosing a control strategy to determine thermal power and air / fuel ratio (AFR) among the followings: a) providing the maximum thermal power by not exceeding the maximum temperature at the burner (21) outlet and considering the effect of the back pressure of the mixed stream (216) on the air pump (22), or b) providing the maximum thermal power by not exceeding the maximum allowable temperature for the components of aftertreatmentsystem and, moreover, the temperature gradient upstream of a catalyst (12, 13, 14, 15);- requesting the determined thermal power;- requesting the determined air / fuel ratio (AFR).

2. Control method according to claim 1, wherein the operating conditions requiring lighting of the burner (21) comprise: a) heating the exhaust gas aftertreatment system (10), b) supporting the regeneration of the particulate filter, c) cooling the aftertreatment system (10) and the burner (21).

3. Control method according to claim 1 or 2, wherein the input parameters for determining a control strategy are pressure of a flue gas stream (213) downstream of the burner (21), temperature of an exhaust gas stream (214) from the internal combustion engine (11), temperature of a mixed stream (216) of burnt gases upstream of a catalyst (12, 13, 14, 15).

4. Control method according to claim 1, wherein an air flow (211) must be between a minimum air flow (211min) which allows not to exceed the allowable temperatures of the aftertreatment system (10) and a maximum air flow (211max) that can be delivered by the pump (22) as a function of the back pressure of the mixed stream (216) of burnt gases downstream of the burner (21).

5. Control method according to claim 4, wherein, if the maximum air flow (211max) is smaller than the minimum air flow (211min), it is necessary to limit the thermal power to have an air flow (211) coinciding with the minimum air flow (211min) and with the maximum air flow (211max).

6. Control method according to claim 1, wherein the limitation of the temperature gradient is performed by inverting a mathematical model between the thermal power of the burner (21) and the flow rate and temperature conditions upstream of the catalyst (12, 13, 14, 15).

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