EP2143932A1 - Method for controlling the temperature of the gases in an exhaust gas recirculation circuit - Google Patents

Abstract

The method involves realizing exothermic and endothermic reactions i.e. hydrogen producing reactions, within a catalyst of an exhaust gas recirculation line in an engine e.g. internal combustion engine, of a vehicle. Thermal balance of gas obtained from the exothermic and endothermic reactions is determined at an outlet of the catalyst. The exothermic and endothermic reactions are developed based on the determined thermal balance of the gas. An independent claim is also included for a vehicle comprising an engine.

Description

The present invention relates to a method for controlling the temperature of the gases in an exhaust gas recirculation circuit.

The combustion of fossil fuel such as oil or coal in a combustion system, particularly diesel fuel in a diesel engine, can result in the production of significant amounts of pollutants that can be emitted from the exhaust into the environment and cause damage. Among these pollutants, the emission of nitrogen oxides called NOx poses a problem since these gases are suspected to be one of the factors contributing to the formation of acid rain and deforestation.

Exhaust gas recirculation (EGR) is a system introduced in the 1970s that redirects part of the exhaust from internal combustion engines to the intake. Recirculation reduces the combustion temperature. In fact, the exhaust gases are rich in nitrogen (N ₂ ) and carbon dioxide (CO ₂ ) and dilute the fuel charge. The combustion is thus slowed down and cooled. However, the formation of NO _x in the combustion chamber is favored by the temperature. Decreasing the combustion temperature therefore leads to a reduction in the production of NO _x at the source. It is thus interesting to be able to introduce re-circulated exhaust gases of low temperature on admission.

The document JP-A-54120310 describes an apparatus for reforming the exhaust gas.

The document WO-2001/075294 discloses a method of operating an internal combustion engine comprising providing an internal combustion engine, a hydrocarbon gasoline source for supplying said engine, and a catalytic reactor in communication with said engine. The method comprises recycling a first portion of an exhaust gas from said engine to said reactor and adding said hydrocarbon gasoline to said reactor. The process also comprises autothermal reforming said hydrocarbon gasoline in said reactor using said first portion of said exhaust gas to form a gas mixture comprising unburned hydrocarbons, hydrogen, water, carbon dioxide, carbon monoxide, carbon and combining said gaseous mixture from said reactor with air to provide a combined mixture. The method further comprises supplying said combined mixture to a combustion zone in said internal combustion engine and combusting said mixture to produce a very low NO _x exhaust gas.

The document US-A-6,508,209 discloses a method of operating an internal combustion engine comprising supplying an internal combustion engine, a gasoline source for supplying said engine and a fluid reactor in communication with said engine. The process comprises adding said hydrocarbon gasoline to said reactor and reforming the vapor of said gaseous gasoline in said reactor to an oxygen level lower than the gaseous flammability limit using said first portion of said gasoline gasoline. exhaust gas as a source of heat and water to form a gaseous mixture comprising methane of hydrogen and carbon monoxide. The method further comprises combining said gas mixture from said reactor with air to provide a combined mixture and combusting said combined mixture into a combustion zone of said engine to produce the first portion of the exhaust gas and a second portion of the exhaust gas, the first part of the exhaust gas comprising up to 4% oxygen by volume and the second portion of the exhaust gas with nearly zero _NOx.

The document US-A-4,175,523 discloses a method of operating an internal combustion engine comprising a combustion chamber, an intake system having a fuel system for supplying a mixture of air and fuel to said combustion chamber, an ignition system , an exhaust system, an EGR system for recirculating a portion of engine exhaust from the exhaust system to the intake system and a fuel reforming system for the conversion of a mixture of air and fuel in a reformed fuel gas mixture rich in free hydrogen. The method includes the steps of load sensing on the motor, supplying of said combustion chamber with an air-fuel mixture while the recirculation of the engine exhaust gases is at a rate not exceeding a first level of exhaust gas recirculation rate when the engine load rate detected is less than a predetermined level of charge. The method further comprises a step of supplying the combustion chamber with the reformed fuel gas mixture in addition to the air-fuel mixture supply by substantially increasing the re-circulation of the exhaust gases at a rate higher than said first predetermined exhaust gas recirculation rate when the engine load as sensed exceeds said predetermined load level.

The patent KR-A-2004/040 764 discloses a device and method for improving combustion efficiency and reducing pollution of a diesel engine.

The document WO-2007/071848 discloses a catalyst for an exhaust line of an internal combustion engine constituted by the stacking in this order of an active phase based on precious metals forming an electrode, an electrolyte having an ionic conductivity with O ² ions greater than or equal to 0.01 mS / cm at 200 ° C and a counter-electrode. The catalytic properties of said active phase can be activated at 100 ° C. under the effect of an electric power of 0.1 μW / cm ² of active phase.

The document FR-A-2880657 discloses an exhaust gas recirculation circuit of an internal combustion engine comprising a gas flow connection adapted to extend between an exhaust manifold and an air intake manifold of a combustion chamber. combustion of the engine and a hydrogen reformer disposed on the gas flow connection.

However, the processes and devices described in the aforementioned documents are complex to implement in particular from a thermo-management point of view.

There is therefore a need for a method of controlling the temperature of the gases in an exhaust gas recirculation circuit which is simpler to implement.

For this purpose, the invention proposes a method for controlling the temperature of the gases in a line, in particular for recirculating the exhaust gas, the line comprising a catalyst, the process comprising the steps of carrying out exothermic reactions and reactions. endothermic in the catalyst), at the outlet of the catalyst, determination of the thermal balance of the gases resulting from the reactions, and promotion of exothermic reactions or endothermic reactions according to the thermal balance obtained.

The invention thus exploits the "thermal" character of the reactions, which or consume energy to take place (endothermic reactions), or provide energy (exothermic reactions) - the athermic reactions are not considered here. By playing on the endo- or exothermic nature, the temperature of a system is increased or decreased while achieving the desired reactions without resorting to an additional cumbersome and expensive system of heat exchanger type.

In one variant, the reactions produce hydrogen.

In one variant, the catalyst comprises active sites favoring exothermic reactions or endothermic reactions.

Alternatively, exothermic reactions or endothermic reactions are promoted by optimizing the activation energy of the catalyst.

In a variant in which the line is a recirculation line of the exhaust gas of an internal combustion engine, the method comprises a step of controlling the richness and composition of the exhaust gas at the inlet of the catalyst promoting the exothermic reactions or endothermic reactions. This control is preferably performed by adjusting the ratio air / fuel entering the combustion chamber. The exhaust gas recirculation line may advantageously furthermore comprise an air and fuel injecting member in the recirculation line, the control of the richness and the composition of the exhaust gases being achieved by controlling the exhaust gas. the air and fuel injection member. This injection member can operate continuously or transiently.

In a variant, the heat produced by the exothermic reactions is used to carry out the endothermic reactions.

Alternatively, the catalyst comprises a plurality of active site support bricks, the method further comprising a step of decreasing the catalyst exit gas temperature by adjusting the spacing between the bricks. Advantageously, the process then involves the use of an endothermic reaction in the last brick of the catalyst.

Alternatively, the composition of the active phase of the catalyst is adjusted to promote exothermic reactions or endothermic reactions.

The present invention also relates to an engine comprising a combustion chamber, a recirculation line of the exhaust gas from the combustion chamber and a catalyst on said line for carrying out the process as described above.

The invention also relates to a vehicle equipped with such an engine, and preferably a vehicle in which heat released by the exothermic reactions is used for the passenger compartment of the vehicle.

• § figure 1, une vue schématique d'une ligne de re-circulation des gaz d'échappement d'un moteur ;
• § figure 2, un organigramme d'un exemple de mise en oeuvre du procédé.

Other features and advantages of the invention will appear on reading the following detailed description of the embodiments of the invention, given by way of example only and with reference to the drawings which show:

• § figure 1 , a schematic view of a recirculation line of the exhaust gases of an engine;
• § figure 2 , a flowchart of an example of implementation of the method.

There is provided a method for controlling the temperature of the gases in an exhaust gas recirculation line having a catalyst. The method comprises a step of performing exothermic reactions and endothermic reactions within the catalyst which may itself be composed of one or more bricks. The method further comprises, at the outlet of the catalyst, a step of determining the thermal balance of the gases resulting from the reactions. The method also comprises a step of promoting exothermic reactions or endothermic reactions according to the thermal balance obtained.

The method thus makes it possible to control the temperature of the exhaust gas recirculation line. The control is done without addition of components and, in particular, without use of heat exchanger. This makes the process simple to implement. In particular, compared to the use of an additional heat exchanger type system, the solution is more flexible in the engine architecture.

The control method can be implemented in an exhaust gas recirculation line 20 of an engine 10 as illustrated in the schematic view of the figure 1 . The engine 10 can be any type of engine. The engine 10 can in particular use any fuel such as gasoline, diesel, biofuels, NGV (natural gas for vehicles) or LPG (liquefied petroleum gas). The engine 10 comprises one or more combustion chambers 12 located between an intake manifold 14 and an exhaust manifold 16. The intake manifold 14 receives air to be introduced into the combustion chamber 12. Fuel is also injected into the combustion chamber 12 generally via an injection nozzle which is not shown on the figure 1 . The exhaust manifold 16 receives the gas emissions produced by the combustion and directs them to an exhaust catalyst not shown. The catalyst processes the emissions produced by post-combustion before expulsion to the outside atmosphere.

The engine 10 further comprises a line 20 for recirculating exhaust gas. Line 20 recirculates exhaust gas to the air inlet of chamber 12 to reduce nitrogen oxide (NO _x ) emissions. The line 20 connects the exhaust manifold 16 and the intake manifold 14. The line 20 comprises a member 22 for injecting air and fuel. The injection member 22 makes it possible to introduce fuel and air into the line 20. The line 20 may further comprise a valve 24. The valve 24 makes it possible to control the flow rate of the exhaust gases reintroduced into the combustion chamber 12. A too high flow rate causes a decrease in engine efficiency 10, a loss of power and causes acceleration knocks (rattling), while too low a flow leads to nitrogen oxide over-emission and a reduction in gains in fuel consumption. Line 20 comprises a catalyst 26 for carrying out the method of controlling the temperature of the gases in line 20. A catalyst makes it possible inter alia to promote a chemical reaction with respect to another.

The figure 2 is a flowchart illustrating an example of implementation of the control method. The method comprises a step 28 of performing exothermic and endothermic reactions within catalyst 26. Exothermic reactions are chemical reactions that provide heat to the medium while endothermic reactions are chemical reactions that consume heat from the medium.

• § le reformage par oxydation partielle du carburant (POx) avec de l'air, le carburant étant pulvérisé et aussitôt transformé en vapeur :
  
           C_xH_y + x/2 O₂ → x CO + y/2 H₂     (1)
  
  
• § le vapo-reformage (STR) du carburant avec de la vapeur d'eau :
  
           C_xH_y + x H₂O → x CO + (y/2+x) H₂     (2)
  
  
• § la réaction de gaz à l'eau, dite "Water Gas Shift reaction" (WGS)
  
           CO + H₂O → CO₂ + H₂     (3)
  
  

The reactions may moreover be hydrogen-producing reactions. In an exhaust gas recirculation line, the production of hydrogen is a solution to increase the rate of re-circulated exhaust gas while maintaining the stability of the combustion in the chamber 12. Consumption fuel and NO _x emissions in chamber 12 are thereby reduced. To produce hydrogen, reforming the fuel (of composition C _x H _y ) is obtained by reacting fuel molecules with water and / or dioxygen according to the following three reactions:

• § reforming by partial oxidation of the fuel (POx) with air, the fuel being pulverized and immediately transformed into vapor:
  
  C _x H _y + x / 2 O ₂ → x CO + y / 2 H ₂ (1)
  
  
• § steam reforming (STR) of fuel with water vapor:
  
  C _x H _y + x H ₂ O → x CO + (y / 2 + x) H ₂ (2)
  
  
• § the reaction of gas with water, called "Water Gas Shift reaction" (WGS)
  
  CO + H ₂ O → CO ₂ + H ₂ (3)
  
  

The partial oxidation reforming reaction (reaction 1) is exothermic. The vapo-reforming reaction (reaction 2) is endothermic and the reaction of gas with water (reaction 3) is more weakly exothermic than reaction 1.

The heat generated by the exothermic reactions occurring in the catalyst 26 can be used for a number of applications. The heat thus obtained can be used in the field of energy recovery. The heating of industrial installations, buildings or the cabin of a vehicle are examples of application. The heat of the exothermic reactions can also be used in the field of heavy chemistry.

Advantageously, the heat produced by the exothermic reactions of the catalyst 26 can be used to carry out the endothermic reactions. This avoids using an additional energy source to trigger those reactions that require heat input. Thus, by way of example, the partial oxidation reforming and steam reforming reactions can be carried out in the catalyst 26. The heat generated by the partial oxidation reforming reaction makes the endothermic steam reforming reaction possible. In such a situation, the line 20 has a positive energy balance since the molecules which make it possible to increase the temperature are well adapted for the combustion in the chamber 12. Thus, even if the molecules are not transformed in the catalyst 26, they are consumed in the combustion chamber. That means that the molecules that increase the temperature serve to reduce NO _x emissions in chamber 12 or fuel consumption.

The method further comprises a step 30 for determining the thermal balance of the gases resulting from the reactions at the outlet of the catalyst 26. The step 30 makes it possible to know the thermal effect of the various reactions and in particular whether the gases obtained at the outlet of the Catalyst 26 are hotter or colder than the input gases. The measurement of the temperature of the gas output can be achieved by a temperature sensor such as a thermocouple. Such a sensor may for example be placed just after the catalyst 26. When a temperature of the gases obtained at the outlet of the catalyst 26 is desired at a precise location on the line 20, it may also be advantageous to place the sensor in the place on line 20.

The control method also comprises a step 32 for promoting exothermic reactions or endothermic reactions according to the thermal balance obtained. When the measured temperature is too low relative to the desired temperature, the exothermic reactions are favored and if the measured temperature is too high, it is this time the endothermic reactions that will have to be favored. The method thus makes it possible to control the temperature of the exhaust gas recirculation line. There is no need to use an additional component, such as a heat exchanger, and to increase or reduce existing pipes in the recirculation line. The method is therefore simple to implement.

To achieve step 32, several means are possible. For example, the catalyst 26 may comprise active sites favoring rather the exothermic reactions or rather the endothermic reactions. An active site is the place where the chemical reaction takes place. In the case of catalyst 26, the active site can be likened to the metal particle that catalyzes the chemical reaction. The use of active sites thus makes it possible to promote the exothermic or endothermic reactions without adding additional components to the line 20, which facilitates the implementation of the control method.

The exothermic or endothermic reactions may also be promoted by optimizing the activation energy of the catalyst 26. The activation energy of a reaction is the energy barrier (or more precisely the free enthalpy barrier) at to go through the reagents so that the reaction, that is to say the transformation into products takes place. The activation energy depends in particular on the reaction under consideration but also on the catalyst used (i.e. the active site and its direct environment). The optimization of the activation energy of the catalyst 26 makes it possible to use the components already present on the line 20 without using an additional component.

It is possible to optimize the activation energy and consequently to modify the catalytic activity by acting on the composition of the active phase (also known as the "washcoat") of the catalyst 26. As a result, the The composition of the active phase of the catalyst 26 may further be adjusted to promote exothermic reactions or endothermic reactions. The interest is that it is simple to adapt the catalyst 26. The nature and the quantity of precious metals in the active phase can in particular change the activity of the catalyst 26. Similarly, the nature of the dopants (cerium (Ce), zirconium (Zr), praseodymium (Pr), yttrium (Y), etc.) makes it possible to favor more or less this or that reaction. It is recognized, for example, that cerium favors the so-called "gas-to-water" reaction, whereas the rhodium which is not the most active in WGS is the main actor of the vapo-reforming of HC. The amount of rhodium is an important element for the partial oxidation reaction (PO _x ) to be performed with significant efficiency.

The control method may further comprise a step 34 for controlling the richness and the composition of the exhaust gas at the inlet of the catalyst 26. The richness and the composition of the gases determine the characteristics of the medium of the chemical reaction. For example, the medium can be more or less reducing. Similarly, the O ₂ content of the medium may vary. Now, the properties of the medium favor more or less the reactions. Step 34 thus makes it possible to promote endothermic or exothermic reactions. Step 34 takes place before step 28 of carrying out the reactions as illustrated in the flowchart of the figure 2 .

In the particular case of the hydrogen-producing reactions studied above, the reactions (1), (2) and (3) are not favored by the same media. The partial oxidation reaction uses a large supply of oxygen, while the reactions involving water such as "steam reforming" and "water gas" require conversely rather reducing conditions. Thus, in step 34, the O ₂ content of the medium is increased to promote the exothermic reactions while the medium is rendered reductive by fuel injection in the case where the endothermic reactions are to be preferred.

The step 34 of controlling the richness and composition of the exhaust gas can be achieved by adjusting the air / fuel ratio at the inlet of the combustion chamber 12. This is simple to achieve.

The control of the richness and composition of the exhaust gas can also be achieved by controlling the air injection and fuel injection member 22. This allows easy and precise control. In the case of hydrogen-producing reactions, injecting compounds such as O ₂ through the member 22 also makes it possible to avoid carrying out the following direct oxidation reactions:

C _x H _y + (x + y / 4) O ₂ → xCO ₂ + y / 2 H ₂ O

CO + 1/2 O ₂ → CO ₂

Direct oxidation reactions convert unburned HC and CO to CO ₂ and H ₂ O to prevent H ₂ production.

As needed, the member 22 may operate continuously or transiently. The continuous operation can be interesting to stay in so-called "self-therm" mode. In a "self-heat" mode, the temperature of the gases re-circulated in the intake manifold 14 can in particular be maintained at a precise temperature. A transient mode operation makes it possible to adapt the thermal level according to the need in terms of energy savings.

In the case where the catalyst 26 is in fact composed of several catalytic bricks, the control method may further comprise a step of reducing the temperature of the gases leaving the catalyst (by adding a brick of STR which is a reaction endothermic). The decrease in temperature is interesting because the formation of NO _x is favored by the temperature (Zeldovich mechanism). The decrease in gas temperature can be adjusted by adjusting the spacing between the bricks. Compared with the use of an additional heat exchanger type system, the solution is more flexible in the engine architecture. Indeed, the dimensions of the catalyst 26 do not impose particular constraints and it is not useful to increase or reduce the pipes in the line 20 recirculation. In addition, the absence of heat exchanger reduces the cost of the line.

Claims (15)

A method for controlling the temperature of the gases in a line (20), in particular of exhaust gas recirculation, the line (20) comprising a catalyst (26), the process comprising the steps of - realization (28) of exothermic reactions and endothermic reactions within the catalyst (26), at the outlet of the catalyst (26), determining (30) the thermal balance of the gases resulting from the reactions, promoting (32) exothermic reactions or endothermic reactions according to the thermal balance obtained. The process of claim 1 wherein the reactions are hydrogen producing. The process of claim 1 or 2 wherein the catalyst (26) has active sites promoting exothermic reactions or endothermic reactions. The process according to one of claims 1 to 3, wherein the exothermic reactions or endothermic reactions are promoted by optimizing the activation energy of the catalyst (26). The method according to one of claims 1 to 4, wherein the line is a line (20) for recirculating the exhaust gas of a heat engine, comprising a step of controlling (34) the richness and the composition exhaust gas at the catalyst inlet (26) promoting exothermic reactions or endothermic reactions. The method of claim 5, wherein the control (34) of the richness and composition of the exhaust gas is achieved by adjusting the air / fuel ratio at the inlet of the combustion chamber (12). The method of claim 5 or 6, wherein the exhaust gas recirculation line (20) further comprises an air and fuel injection member (22) in the recirculation line (20). ,
the control (34) of the richness and composition of the exhaust gas being achieved by controlling the member (22) for injecting air and fuel. The method of claim 7, wherein the injection member (22) operates continuously or transiently. The process according to one of claims 1 to 8, wherein the heat produced by the exothermic reactions is used to carry out the endothermic reactions. The process according to one of claims 1 to 9, wherein the catalyst (26) comprises a plurality of active site support bricks, the method further comprising a step of decreasing the temperature of the catalyst outlet gases (26). ) by adjusting the spacing between the bricks. The process of claim 10 further comprising using an endothermic reaction in the last catalyst brick (26). The process according to one of claims 1 to 11, wherein the composition of the active phase of the catalyst (26) is adjusted to promote exothermic reactions or endothermic reactions. An engine (10) comprising a combustion chamber (12), a line (20) for re-circulating the exhaust gases of the combustion chamber (12), a catalyst (26) on the line (20) for recirculating the exhaust gases for carrying out the process according to one of claims 1 to 12. A vehicle comprising the engine (10) according to claim 13. The vehicle of claim 13, wherein the heat generated by the exothermic reactions is used for the passenger compartment of the vehicle.

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