Method for controlling nitrogen oxide emissions of an internal combustion engine powered by hydrogen

Abstract

A control method for an after-treatment system (10) of exhaust gases from an internal combustion engine powered by hydrogen during engine transient operation, the method comprising the following steps: - verifying whether the air / hydrogen ratio with respect to the stoichiometric metering of the main combustion in the engine cylinder is less than a limit air / hydrogen ratio with respect to the stoichiometric metering, below which there is an excess production of nitrogen oxides, - verifying whether a storage catalyst (20) of the system (10) has exhausted its accumulation capacity of nitrogen oxides, - if at least one of the two checks gives a positive result, activating at least one delayed hydrogen injection configured to obtain a value of concentration in volume of residual oxygen at the outlet of the storage catalyst (20) less than 0.1% and a temperature at the inlet of the storage catalyst (20) between 400°C and 600°C.

Description

[0001] METHOD FOR CONTROLLING NITROGEN OXIDE EMISSIONS OF AN

[0002] INTERNAL COMBUSTION ENGINE POWERED BY HYDROGEN

[0003] DESCRIPTION

[0004] Technical Sector of the Invention

[0005] The present invention relates to a method for controlling nitrogen oxide emissions during transients of an engine a internal combustion powered by hydrogen and equipped with a post-treatment system for exhaust gases. The method is particularly suitable for internal combustion engines powered with hydrogen by direct injection.

[0006] Background Art

[0007] Motor vehicles typically operate using an internal combustion engine (hereinafter also ICE) to convert the chemical energy of a fuel, such as gasoline or diesel, into mechanical energy transmitted to the wheels to move the vehicle. Unfortunately, fossil fuels contribute to pollution and the emission into the atmosphere of greenhouse gases, such as for example carbon dioxide CO2. Because of these inconveniences, it is always paying more attention to reducing consumption of fuel and pollutants emitted by vehicles for the transport of goods or people equipped with ICE.

[0008] To alleviate some of these inconveniences, they have been proposed internal combustion engine powered by hydrogen that in stationary or slow transient conditions work with lean mixtures (high air / hydrogen ratios compared to the stoichiometric dosage, in the following Lambda, i.e. > 2.2) and do not produce polluting emissions except for negligible quantities of nitrogen oxides (hereinafter also NOx). Such motors require particular measures to ensure correct functioning in the case of fast transients such as, for example, those deriving from the sudden torque request by the driver to produce the acceleration of the vehicle.

[0009] During these phases, which in the rest of the document will be more briefly indicated as "transient" or "transient maneuvers", the combustion must be adapted by reducing the Lambda to follow the torque request by the user. In this way, greater quantities of nitrogen oxides are produced. Although NOx can be stored in a NOx accumulation catalyst (hereinafter also NSC, from the English NOx Storage Catalyst) or converted by a selective reduction catalyst (hereinafter also SCR), a high production of NOx could saturate the capacity of these catalysts limiting efficiency with a consequent emission into the environment of high quantities of NOx. A known method to limit the production of NOx from combustion is to delay the time of ignition of the charge. However, in hydrogen motors, this method may not be sufficient to prevent NOx products from saturating the capacity of the catalysts and, moreover, greatly penalizes the efficiency 25 of combustion causing a significant increase in fuel consumption.

[0010] Therefore, there is a need to define a control method for an after- treatment system for exhaust gases in internal combustion engines, powered by hydrogen, which is suitable for managing transient maneuvers and which solves the inconveniences mentioned above.

[0011] Invention Summary

[0012] To substantially solve the technical problems above mentioned, one object of the present invention is a control method for an after-treatment system for exhaust gases in internal combustion engines powered by hydrogen, during transient maneuvers. The method is based on the management of such maneuvers by means of late injections of hydrogen that favor the conversion of nitrogen oxides into ammonia on the catalyst placed downstream of the engine.

[0013] Therefore, according to the present invention, a method of control is provided for an after-treatment system for exhaust gases of a internal combustion engine powered by hydrogen having the characteristics set out in the independent method claim, attached to the present description.

[0014] According to another aspect of the present invention, an after- treatment system for exhaust gases of an internal combustion engine powered by hydrogen is provided, the system being suitable for the implementation of the control method mentioned above and having the characteristics set out in the independent system claim, attached to the present description.

[0015] Further embodiments of the invention, preferred and / or particularly advantageous, are described according to the characteristics stated in the dependent claims attached.

[0016] Brief Description of the Drawings

[0017] The invention will now be described with reference to the attached drawings, which illustrate some non-limiting implementation examples, in which:

[0018] - Figure 1 is a simplified diagram of an after-treatment system of exhaust gases from an internal combustion engine powered by hydrogen, according to an embodiment of the present invention, and - Figure 2 is a flow diagram of a method of control for the exhaust gas after-treatment system of Figure 1.

[0019] Detailed Description

[0020] By way of purely illustrative and non-limiting example, the method of control for an after-treatment system for exhaust gases in internal combustion engines powered by hydrogen will now be described with reference to the aforementioned figures. It should be noted that the method is specifically described for an application of a hydrogen-powered engine with direct injection.

[0021] With reference to Figure 1, an after-treatment system 10 of exhaust gases, for the purposes of the present invention, comprises at least:

[0022] - a storage catalyst 20 of nitrogen oxides (NSC), upstream of the internal combustion engine (of known type and therefore not illustrated in the figures). The NSC is a catalyst composed at least of platinum, barium oxide and alumina - but may also include palladium, rhodium, cerium oxide, etc. - in which the nitrogen oxides originating from engine are stored during lean combustion operation until the catalyst saturation is reached,

[0023] - a selective reduction catalyst 30 downstream of the storage catalyst. The selective reduction catalyst 30 can be "stand-alone" (SCR) or mounted on a particulate filter (SCRoF, from the English Selective Catalytic Reduction on Filter). In the following, for purely illustrative purposes, reference will be made to the latter. As will be better explained below, the SCRoF can be of the passive or active type, depending on whether or not it uses urea from an external tank (typically Diesel Exhaust Fluid, hereinafter also DEF). The method described below will refer to a passive SCRoF, but as will be clear, the same method can also be applied to selective reduction catalysts of the active type.

[0024] The system 10 conveniently has suitable sensors 40 for acquiring the temperatures, the content of nitrogen oxides and / or the concentration of oxygen, quantities that will be managed by a control unit for the engine management. The sensors 40 are positioned downstream of the storage catalyst 20 or downstream of the selective reduction catalyst 30, along a path 50 of the engine exhaust gases of the internal combustion engine.

[0025] The present invention is a method that is applied to an exhaust gas after-treatment system such as the one described above and that manages the conversion of the high NOx products from an ICE powered by hydrogen during transient maneuvers, i.e. during transitions from lean mixture to rich mixture.

[0026] The method is based on a specific control of hydrogen injections, a control based on multiple injections of which the last are delayed with respect to optimal combustion times (late injections) of hydrogen in the cylinder, during transient maneuvers or whenever it is necessary to empty the NSC. This control aims, on the one hand, to consume the residual oxygen from combustion and to obtain a hydrogen release in the outlet from the engine sufficiently high to promote the reduction of most of the NOx on the NSC; on the other hand, to avoid delaying the combustion times during transients to limit the fuel consumption.

[0027] The reduction of NOx through the NSC in the described conditions mainly releases ammonia (NH3). The ammonia produced by the storage catalyst 20, when the aforementioned hydrogen retarded injection strategy is activated, is stored in the selective reduction catalyst 30 (SCRoF or SCR) positioned downstream and used during lean engine operation to reduce nitrogen oxides when the storage catalyst is full and / or has reduced its efficiency. By doing so, the duration of lean engine operation is maximized and the consumption of DEF is minimized, or even the need for an external DEF system is avoided.

[0028] Therefore, in dynamic transient conditions or whenever it is deemed necessary to empty the storage catalyst (NSC), the activation of one or more retarded injections with predetermined amounts of hydrogen serves to:

[0029] - consume the residual oxygen from the main combustion,

[0030] - increase the temperature at the inlet of the NSC, and

[0031] - provide enough hydrogen to reduce all the nitrogen oxides stored in addition to those produced during combustion.

[0032] In this way, the emptying of the storage catalyst 20 is obtained.

[0033] In fact, with the increase in temperature, the NSC releases all the stored nitrogen oxides and with hydrogen in the exhaust significantly in excess of oxygen, most of the NOx is converted into water and ammonia.

[0034] The ammonia produced and released under these conditions is then stored in the selective reduction catalyst 30 (SCRoF / SCR) placed as a second catalyst and used to convert the NOx released through the NSC during lean operation and / or during the lean mixture / rich mixture transition.

[0035] Furthermore, compared to strategies based on ignition delay for the management of transients with highly dynamic load requirements, the present invention improves the transient performance of the vehicle with a minimum penalty on fuel consumption.

[0036] The method according to the present invention will now be described with the aid of the flow chart illustrated in Figure 2.

[0037] The method includes seven controls and three actions. The actions are inherent to the management of retarded hydrogen injections, the activation of which depends on the controls carried out.

[0038] After activation S100, the method sequentially executes the first three controls:

[0039] - a first control SI 10 verifies whether the Lambda of the main combustion in the engine cylinder (Lambda_main) is less than a ratio Lambda limit (Lambda_minNOx) below which there would be an excess of production of nitrogen oxides. This is a fundamental control that verifies whether there is a passage from a lean mixture, typical of a constant regime trend, to a rich mixture, typical of transient maneuvers, for example, a sudden torque request to obtain a quick acceleration of the vehicle. According to the applications, this limit value (Lambda_minNOx) is between 1.8 and 2.2;

[0040] - a second control S120 verifies whether the storage catalyst 20 has exhausted its capacity to accumulate nitrogen oxides (NSC full). This is also a fundamental control, aimed at verifying the need for regeneration, i.e. emptying, of the storage catalyst 20;

[0041] - conveniently, a third control S130 verifies whether the accumulation of ammonia in the selective reduction catalyst 30 is low. This control aims to verify whether there is a need to introduce further ammonia into the selective reduction catalyst 30. This operation, in the case of active SCRoF, can be managed independently of the present control method, vice versa, in the case of passive SCRoF, it requires a compensation action in the present method.

[0042] In the event that all three of the above described controls give a negative response, the method according to the invention does not require any action and concludes S200.

[0043] Instead, in the event that at least one of the first three controls gives a positive response, the subsequent controls are bypassed (for example, with the first control SI 10 positive, the two subsequent controls S120 and S130 are bypassed), and it is necessary to carry out further controls and / or implement one or more actions.

[0044] A fourth control S140 verifies whether the accumulation of ammonia in the selective reduction catalyst 30 is high. This control is activated only in the case in which the second control - NSC full S120 - has given a positive response. In this case, a high accumulation of ammonia in the selective reduction catalyst 30 means that the SCRoF is in a position to manage, i.e. to reduce the excess nitrogen oxides that are no longer stored in the storage catalyst 20. Therefore, in this case, the method does not require the execution of any action.

[0045] In the case in which the accumulation of ammonia is not high, a fifth control S145 verifies whether, as a result of previous situations, the retarded hydrogen injections are already active.

[0046] Obtaining a negative response to the fifth control S145, the method requires the implementation of a first action and, conveniently, also of a second action:

[0047] - a first action S150 activates at least one delayed hydrogen injection (Latejnjection) having as its objective to have in the combustion chamber a quantity of hydrogen sufficient to consume almost all the residual oxygen, convert all the nitrogen oxides present inside the storage catalyst 20 and those generated during combustion. In particular, the delayed hydrogen injection must be such as to obtain a concentration value in volume of residual oxygen at the outlet of the storage catalyst 20 less than 0.1% and a temperature at the outlet of the storage catalyst 20 between 400°C and 600°C. The objective can be conveniently achieved by estimating the Lambda of the main combustion (Lambda_main) and calculating the amount of hydrogen needed according to the following inequality: where the amount of oxygen (O2, combustion NO Late) and nitrogen oxides ( NOX,EO) are those coming from the main combustion, while N Ox, stored, NSC corresponds to the concentration of NOx that would occur if all the NOx trapped by the NSC were released under the current exhaust flow conditions. The above inequality is a stoichiometric calculation of the reactions that serve as a target for the calibration during development, when the quantities described can be measured and / or calculated starting from experimental measurements. In vehicle operation, the first actuation of the late injection / s will be in "open loop" according to the map-calibrated values;

[0048] - successively, a second action S160 measures, by means of the sensors 40, the oxygen and the temperature at the outlet of the storage catalyst 20 (02 & Temperature @ NSC_outlet).

[0049] In the case in which the fifth control S145 has detected that the delayed hydrogen injections are already active, evidently the first action (activation of the delayed injections) is bypassed and the method only performs the second action S160.

[0050] After the implementation of the second action S160, the method continues with two further controls:

[0051] - a sixth control S170 verifies that the oxygen at the outlet of the storage catalyst 20 is less than 0.1% (O2 @NSC out < 0.1%) in order to ensure that all the residual oxygen from the combustion has been consumed and that the conditions necessary for the reduction of nitrogen oxides are present inside the catalyst;

[0052] - a seventh control S180 verifies that the temperature at the outlet of the storage catalyst 20 is within a predetermined interval. Conveniently, this interval is between 400°C and 600°C, as it is the one that allows obtaining the most effective transformation of hydrogen, oxygen and nitrogen oxides into water and ammonia.

[0053] If both these last two controls give a positive response, the method does not require the execution of further actions and restarts with the first control S110.

[0054] Instead, in the case in which at least one of the last two controls gives a negative response, the successive control, if any, is bypassed, and it is necessary to perform a further action.

[0055] Specifically, a third action S190 further regulates the quantity and timing of the delayed hydrogen injections having as objective that the oxygen at the outlet of the storage catalyst 20 is less than 0.1% and / or the temperature at the outlet of the storage catalyst 20 is within the predetermined interval.

[0056] Subsequently, the method restarts with the first three controls S110, S120, S130.

[0057] The invention is applicable to various applications for vehicles and can be used in exhaust gas after-treatment systems with or without active urea dosing.

[0058] Ultimately, the method according to the present invention solves the technical problem of excessive production of nitrogen oxides during transient maneuvers by activating delayed hydrogen injections to convert nitrogen oxides into ammonia.

[0059] A further advantage is given by the fact that the ammonia produced can also be used in a selective reduction catalyst, which may also not have a dedicated urea supply.

[0060] In addition to the embodiment of the invention, as described above, it must be understood that there are numerous other variants. It must also be understood that such embodiments are only exemplificative and do not limit either the scope of the invention, nor its applications, nor its possible configurations. On the contrary, although the description above allows the skilled technician to implement the present invention at least according to one of its embodiments exemplificative, it must be understood that many variants of the described components are possible, without thereby departing from the scope of the invention, as defined in the attached claims.

Claims

C LA I M S1. Method of control for an exhaust gas aftertreatment system (10) of a hydrogen fueled internal combustion engine during transient operation of the engine, the method comprising the following steps:- checking whether the air / hydrogen ratio compared to the stoichiometric dosage of the main combustion in the engine cylinder is less than a limit air / hydrogen ratio compared to the stoichiometric dosage, below which there is an excess of nitrogen oxides production,- checking whether a storage catalyst (20) of the system (10) has exhausted its capacity to accumulate nitrogen oxides,- if at least one of the two checks gives a positive result, activating at least one hydrogen late injection configured to obtain a concentration value in volume of residual oxygen at the outlet of the storage catalyst (20) lower than 0.1% and a temperature at the outlet of the storage catalyst (20) between 400°C and 600°C.

2. Method according to claim 1, wherein the calibration of the times and quantities of hydrogen in the delayed injections satisfies the following inequality:where the quantities of oxygen (O2, combustion no Late) and nitrogen oxides (NOX,EO) are those coming from the main combustion, while the quantity (NOx, stored, NSC) corresponds to the concentration of nitrogen oxides that would occur if all the nitrogen oxides trapped in the storage catalyst (20) were released under the current exhaust gas flow conditions.

3. Method according to claim 1 or 2, wherein after the activation step of at least one delayed hydrogen injection, the method comprises the following further steps:- measuring the oxygen and temperature of the exhaust gases at the outlet of the storage catalyst (20),- checking whether the oxygen at the outlet of the storage catalyst (20) is less than 0.1%,- checking whether the temperature at the outlet of the storage catalyst (20) is within a predetermined range,- if at least one of the two checks gives a negative result, adjusting the quantity and timing of the at least one hydrogen late injection so that the volume concentration of oxygen at the outlet of the storage catalyst (20) is less than 0.1 % and / or the temperature at the outlet of the storage catalyst (20) is within the predetermined range.

4. The method of claim 3, wherein the predetermined temperature range is 400°C to 600°C.

5. Method according to any of the previous claims, wherein the activation step of the at least one hydrogen late injection is executed if the accumulation of ammonia in a selective reduction catalyst (30) of the system (10) is low.

6. Method according to one of claims 1 to 4, wherein the activation step of at the least one hydrogen late injection is not executed if the accumulation of ammonia in the selective reduction catalyst (30) of the system (10) is high.

7. System (10) for the aftertreatment of exhaust gases of an internal combustion engine, configured to execute the method according to any of the previous claims, the system comprising:- a nitrogen oxide storage catalyst (20), - a selective reduction catalyst (30) downstream of the storage catalyst (20),- a plurality of sensors (40) for acquiring the nitrogen oxide and / or oxygen content and / or the temperature of the exhaust gases, which are positioned downstream of the storage catalyst (20) or downstream of the selective reduction catalyst (30).

8. System (10) according to claim 7, wherein the selective reduction catalyst (30) is mounted on a particulate filter.

9. System (10) according to claim 7 or 8, in which the selective reduction catalyst (30) is of the passive type, as it does not have an external supply of reducing agent.

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