FR2986830A3 - Method for decontaminating diesel engine system devoid of nitrogen oxide trap in car, involves initiating rich mode corresponding to decreased rate of oxygen gas entering catalyst chamber to improve efficiency of oxidation catalyst - Google Patents

Abstract

The method involves initiating a rich mode corresponding to a decreased rate of oxygen gas entering a catalyst chamber (111) to improve the efficiency of an oxidation catalyst of a diesel engine system (1). A measurement value of a distance covered is measured, and the received measured value is compared with a threshold, where the initiation of the rich mode is a function of the comparison result. An estimated value of efficiency parameter of the oxidation catalyst is received, and the value of the efficiency parameter is compared with a threshold value. Independent claims are also included for the following: (1) a device for decontaminating a diesel engine system devoid of the nitrogen oxide trap (2) a diesel engine system devoid of the nitrogen oxide trap.

Description

The invention relates to the depollution of a diesel engine system without a NOx trap, particularly a diesel engine drive system of a motor vehicle. A diesel engine system may comprise a diesel engine, with an air inlet and a fuel inlet, and, downstream of the diesel engine, a catalytic chamber in which an oxidation catalyst can promote the oxidation of the incoming gases. in the chamber, in particular unburned hydrocarbons, referred to as `1-1C ', and carbon monoxide or CO. The diesel engine system may further include a catalyzed particle filter. Current oxidation catalysts include precious metals, for example platinum, palladium and / or rhodium, the cost of which is relatively high and may vary. Some diesel engine systems, complying with the EURO 6 standard, also include a trap of nitrogen oxides ("NOx trap" in English), relatively charged with precious metals.

Other diesel engine systems, including EURO 5-compliant engine systems, are free of NOx traps and include a quantity of precious metals in the lower oxidation catalyst. These engine systems are simpler in design than the EURO 6 engine systems.

It has already been attempted to improve the efficiency of a EURO 5 engine with a quantity of precious metals of the constant oxidation catalyst. It is known to promote the realization of oxidation by increasing the temperature in the catalytic chamber. For example, an engine system can be designed in which the catalytic chamber is brought closer to the combustion chamber of the engine. The document EP0808415 describes an engine system with two catalysts: a first catalyst very close to the small engine outlet to promote exothermic reactions, especially during start-up, and a second, larger catalyst to ensure the oxidation in steady state.

Nevertheless, this principle imposes certain design constraints on the engine system. In particular the exhaust face of the engine must be redesigned to incorporate two catalysts and an additional pump, and this may involve an increase in the mass of the engine system. There is therefore a need for a depollution of diesel engine systems devoid of NOx trap to reconcile cost, efficiency and simplicity. There is provided a process for the depollution of a diesel engine system without a nitrogen oxide trap and comprising a diesel engine and a catalytic chamber with a catalyst for the oxidation of unburnt hydrocarbons and carbon monoxide. The method comprises a step of triggering a rich mode in which the oxygen level of the gases entering the catalytic chamber is decreased in order to improve the efficiency of the oxidation catalyst. It has indeed been observed that the oxygen is likely to bind to the precious metals of the catalyst, and thus reduce the effectiveness of the catalyst. By providing a rich mode, in which the oxygen level in the catalytic chamber is reduced, it is possible to reduce (i.e. withdraw the fixed oxygen) at least partially the metal sites of the catalyst, due to the fact that chemical equilibrium in the catalytic chamber. The process described above can thus make it possible to improve the catalytic efficiency and / or to reduce the amount of precious metals of the catalyst in the diesel engine system, while leaving a great freedom of design of the line of exhaust system of diesel engine. Reducing the amount of precious metals can at least partially overcome the supply problems associated with fluctuations in the prices of precious metals, and / or limit the cost of the engine system. The invention is in no way limited by the triggering factor of this rich mode. For example, it is possible to trigger the rich mode at regular time intervals. In an advantageous embodiment, it is possible to receive measurement values of the mileage traveled, to regularly compare these measured values with a threshold, and, when the measurement value exceeds a threshold, to trigger the rich mode. The mileage traveled is then reset. Thus, it is forbidden any triggering of the rich combustion mode since the last trigger, as long as the vehicle has not traveled a distance greater than or equal to a predetermined kilometric interval. In another advantageous embodiment, it is proposed to determine the effectiveness of the oxidation catalyst. A criterion characterizing the effectiveness of the pollution control system is thus determined. When this efficiency falls below a threshold, the rich mode is triggered. For example, the method described in document FR 2917123 may be used. When the diesel engine system is in the rich mode, the oxygen content of the gases entering the catalytic chamber is lower than in normal mode. In other words, the value of the wealth (the ratio: mass of fuel / air mass) of these entering gases is modified. This rich mode can for example be obtained by injecting more fuel than in normal mode and / or reducing the proportion of fresh air in the combustion chamber. For example, an additional fuel injection step may be provided so that the amount of fuel in the combustion chamber of the engine is higher. In order to reduce the proportion of fresh air in the combustion chamber, it will be possible for example to reduce the flow of air entering, for example by closing, partially or completely, a fresh air intake flap in the combustion chamber and / or by partially or completely opening an EGR (Exhaust Gas Recuperation) valve in order to reintroduce more flue gas into the combustion chamber than in normal mode. There is further provided a computer program product including instructions for performing the steps of the method described above when this program is executed by a processor. In addition, it proposes a device for the depollution of a diesel engine system without a nitrogen oxide trap, this system comprising a diesel engine and a catalytic chamber with an oxidation catalyst for oxidizing unburned hydrocarbons and carbon monoxide. carbon. The depollution device comprises means for controlling the combustion mode of the diesel engine, these means being capable of triggering a switch to a rich mode for decreasing the oxygen content of the gases entering the catalytic chamber in order to improve the efficiency of the oxidation catalyst. This device can thus make it possible to implement the method described above. These control means may for example comprise processing means, for example a microcontroller, a microprocessor, or the like, with means for receiving a mileage value or a catalytic efficiency parameter, calculation means for develop a trigger control signal, and developed signal transmission means to the engine. The invention is however in no way limited by a numerical implementation of the method described above. For example, it would be possible to provide fully mechanical and / or analog control means. It is further proposed a diesel engine system without nitrogen oxide trap, the system comprising a diesel engine and, downstream of said diesel engine, a catalytic chamber with an oxidation catalyst for oxidizing hydrocarbons and carbon monoxide. carbon. This system further comprises a device as described above. There is further provided a vehicle, for example a motor vehicle, comprising the diesel engine system described above. The invention will be better understood with reference to the drawings, which are given for illustrative purposes. Figure 1 shows schematically an example of a diesel engine system according to one embodiment of the invention.

FIG. 2 is a flowchart of an exemplary method algorithm according to one embodiment of the invention. FIG. 3A is a graph with, on the abscissa, percentages corresponding to the level of oxygen (also called oxygen) in the gas entering the catalytic chamber, and in the ordinate of the values in percentage of estimated efficiency for carbon monoxide, for different temperatures. incoming gas.

FIG. 3B is a graph with the abscissa of the percentages corresponding to the oxygen level in the catalytic chamber, and the ordinate of the estimated percentage efficiency values for the unburnt hydrocarbons or HC, for different entering gas temperatures. With reference to FIG. 1, a diesel engine system 1 includes a compressor 13, a diesel engine 10 with a combustion chamber (not shown), and an inlet gas intake flap 101.

In the embodiment shown, the internal combustion engine 10 is designed to operate with partial recirculation of the exhaust gas under pressure. Such an HP EGR (Exhaust Gas Recuperation High Pressure) system, including an EGR valve 102, and can recover some of the flue gas. In an alternative embodiment, the engine system may be designed with partial recirculation of the exhaust gas under low pressure. Such an LP EGR system (of the "Exhaust Gas Recuperation Low Pressure"), here shown in dashed lines, can implement an EGR valve 102 '. The diesel engine system 1 may further comprise a sensor 15, for example a pressure sensor, a temperature sensor, a richness sensor (also called an oxygen sensor) or the like.

The system 1 further comprises a turbine 14, as well as a pollution control system 11. The pollution control system 11 comprises a catalytic chamber 111 or DOC (of the "Diesel Oxydation Catalyst"), and a particulate filter 112 or CSF (Catalysed Soot Filter).

The diesel engine system 1 meets the Euro 5 standard and does not include a NOx trap. The amount of oxidation catalyst (the mass of precious metals) present in the catalytic chamber 111 is therefore relatively small. The system 1 further comprises a depollution device 12, for example a digital signal processing processor. This processor 12 is in communication with the catalytic chamber 111, and receives isothermal response parameter values of this catalytic chamber 111. The processor 12 is arranged to estimate by implementing the method described in the document FR 2917123, values of Efficiency parameter of the oxidation catalyst present in the chamber 111. This estimation method will not be described here further. The eff parameter efficiency parameter values are received by a portion of the processor 12 during a step referenced in FIG. 2. Each eff param value is compared with a threshold during a step 21. The result of this comparison 21 determines the combustion mode of the diesel engine 10. As the efficiency value is greater than the threshold THR, the device 12 imposes a poor combustion mode. A MODE mode variable value is assigned a predetermined value POOR VALUE corresponding to the lean mode, for example 0. On the contrary, if the value of the efficiency parameter Eff param is less than or equal to this threshold THR, then the device 12 triggers a transition to rich mode. During a step 23, the MODE variable is assigned a RICH VALUE value corresponding to the rich mode, for example 1. To return to FIG. 1, following this passage in rich mode, the device 12 commands to close further the intake flap fresh air 101 upstream of the engine 10. This limits the rate of fresh air in the combustion chamber, and therefore, ultimately, the oxygen content of the gases entering the catalytic chamber 111 The device 12 also controls the EGR valve 102. In the case of a changeover to rich mode, the device 12 commands a greater opening of this EGR valve 102 in order to recover more flue gases, and thus to reduce the oxygen level in the process. the gases entering the catalytic chamber 111. In addition, the device 12 is in communication with a not shown processor, or possibly coincides with this processor, the processor managing the fuel injection into the combustion chamber of the engine 10.

When the rich mode is triggered, an additional injection is imposed after the main injection. Thus, in normal injection mode, also called lean mode, several injections are made during a motor cycle, including one or two optional pilot injections intended to minimize engine noise, preceding a main injection intended to create torque to drive. the vehicle. In addition, it is possible to provide an additional "after injection" injection in English, which is also optional.

In rich mode, the pilot injection (s) can be maintained. The main injection intended to create torque is of course maintained, but after a waiting time, we can provide an additional addition, also called "post injection" in English, to increase the amount of fuel in the chamber. combustion of the engine 10 and thus helping to lower the oxygen content of the gases leaving the combustion chamber. This additional injection or "post injection" thus allows to introduce more fuel into the combustion chamber, and thus ultimately generate more flue gas.

Because of this additional injection, the rich mode may be more fuel efficient than the lean mode. The rich mode can generate an overconsumption of fuel, being able for example to reach 4 `) / 0. In addition, this mode of combustion leads to a dilution of fuel in the engine oil, which degrades the lubricating properties of the oil. It is therefore advantageous that this mode of combustion is triggered as little as possible. It is also advantageous to limit in time the periods during which the motor 10 operates in rich mode. The process illustrated in FIG. 2 makes it possible, for example, to trigger the rich mode only when it is necessary, in view of the estimated efficiency of the oxidation catalyst in the catalytic chamber. In lean mode, that is to say with an excess of oxygen relative to the quantity of fuel entering the combustion chamber, the oxygen levels, by mass, at the exhaust are generally between 5 and 18 `) / 0 according to the conditions of use. On the other hand, in rich mode, the oxygen level may be less than 4%, and possibly zero.

Figures 3A and 3B show the impact of the oxygen level during a thermal purge. On each of these figures are represented on the abscissa the oxygen content by mass in the purge, this rate varying between zero and 25 °) / 0. The ordinates are shown efficiency values estimated for Figure 3A, and for unburned hydrocarbons for Figure 3B. These efficiency values were measured on standardized pollution control cycles by subtracting the mass of this pollutant downstream of the catalyst from the mass of the pollutant (carbon monoxide or unburned hydrocarbon) emitted upstream of the catalyst, and dividing the difference obtained. by the mass of this pollutant emitted upstream of the catalyst. Various measurements were made at different temperature values. It thus emerges that relatively high temperatures make it possible to improve the efficiency Thus, with reference to FIG. 3A, for a dioxygen level in the purge of 10%, the efficiency at 250 ° C. is close to 58 ° C. / 0, while the efficiency at 400 ° C is close to 65 °) / 0. Nevertheless, if the temperature has an influence on the efficiency, it also appears that the oxygen level in the purge also plays a role Thus, for a zero oxygen level, the efficiency exceeds 80% for carbon monoxide, whereas it is below 70% when the oxygen level in the purge is greater than or equal to 10%. With regard to unburned hydrocarbons, for a zero oxygen content, the efficiency exceeds 65%, whereas it remains below 60% for an oxygen level in the 10 'purge. ) / 0. Reducing the oxygen level in the purge to zero thus makes it possible to gain almost 15 points in the case of unburned hydrocarbons, e t almost 25 points in the case of carbon monoxide.

These results thus show significant gains in pre-conditioning an oxygen-free catalyst. Thus, it is possible to increase the efficiency of a catalyst by (chemically) reducing metal sites regularly. This can be done by operating the engine in rich mode, that is to say in a combustion mode where there is excess fuel and lack of oxygen.

The oxidation catalysts used in the catalytic chambers further include precious metals, especially platinum and / or palladium in the case of a diesel engine system. The oxygen in the incoming gases has the particularity of oxidizing these metal sites, thus reducing the catalytic efficiency of the catalyst. Thus, by regularly imposing operation in rich mode, it is possible to reduce these metal sites again and improve the efficiency of the depollution. In an embodiment variant not shown, the transition to rich mode is not triggered from an estimate of an efficiency parameter, but from a measurement of a mileage traveled. A mileage interval is determined beforehand, during which the rich combustion mode makes it possible to provide an increase in the efficiency of the oxidation catalyst. Any triggering of the rich combustion mode since the last trip is prohibited, as long as the vehicle has not traveled a distance greater than or equal to this kilometric interval. Thus the mileage values received are compared to the mileage interval and as long as the mileage value is less than the mileage interval, the engine remains in lean mode. When the mileage value received reaches or exceeds the mileage interval, the rich mode is triggered, and the mileage value is reset to zero upstream of the device 12. In addition, it is possible to combine these two strategies, for example by adding after the test 21 of Fig. 2, a test step of comparing the mileage value received with the predetermined mileage interval value, and allowing the step 23 to pass only if this test is positive. If, on the other hand, the mileage value received is less than the mileage interval, then step 22 is executed. The method and the device described above thus allow an increase in efficiency of the order of 20 to 25% for carbon monoxide, and of the order of 10 to 15% for unburned hydrocarbons. This strategy can increase efficiency and / or design engine systems with a precious metal mass in the lower catalytic chamber, in order to maintain the level of efficiency required to meet the Euro 5 abatement standard. The diesel engine system may further include an oxygen sensor, not shown, to enable better management of the rich mode.

Claims (9)

REVENDICATIONS1. A method of depolluting a diesel engine system without a nitrogen oxide trap, said system comprising a diesel engine and a catalytic chamber with an oxidation catalyst for oxidizing unburned hydrocarbons and carbon monoxide, the process comprising: a step of triggering (23) a rich mode corresponding to an oxygen level of the gases entering the decreased catalytic chamber, with a view to improving the efficiency of the oxidation catalyst. The method of claim 1, further comprising: receiving a measurement value of the mileage traveled, comparing the received measurement value with a threshold, and wherein the triggering of the rich mode is a function of the result of the comparison. 3. Method according to one of claims 1 or 2, further comprising: receiving (20) an estimated value of efficiency parameter of the oxidation catalyst, comparing (21) the estimated value to a threshold, and wherein the triggering rich mode is depending on the result of the comparison. 4. Method according to one of claims 1 to 3, comprising, in case of rich mode trigger: control an additional injection of fuel in a combustion chamber of the engine. 5. Method according to one of claims 1 to 4, comprising, in case of rich mode trigger: control the at least partial closure of a fresh air intake flap (101) of the engine (10). 6. Method according to one of claims 1 to 5, comprising, in case of triggering rich mode: 35commander at least partial opening of an EGR valve (102). 7. Device for the depollution (12) of a diesel engine system (1) devoid of a nitrogen oxide trap, said system comprising a diesel engine (10) and a catalytic chamber (111) with an oxidation catalyst for oxidizing unburned hydrocarbons and carbon monoxide, the depollution device comprising: control means (12) for the combustion mode of the diesel engine, said control means being able to trigger a switch to a rich mode to reduce the rate of oxygen gas entering the catalytic chamber to improve the efficiency of the oxidation catalyst. A diesel engine system (1) without a nitrogen oxide trap, said system comprising: a diesel engine (10), downstream of said diesel engine, a catalytic chamber (111) with an oxidation catalyst for oxidizing unburned hydrocarbons and carbon monoxide, and a depollution device (12) according to claim 7. Motor vehicle comprising a diesel engine system (1) according to claim 8.30