JP6717091B2 - Method of warming up exhaust purification catalyst of internal combustion engine - Google Patents

Description

The present invention relates to a warm-up method for early activating an exhaust purification catalyst during cold start of an internal combustion engine.

When the internal combustion engine is cold started, it is necessary to activate the exhaust purification catalyst such as a three-way catalyst provided in the exhaust system at an early stage. Therefore, as disclosed in Patent Document 1, a technique is known in which the exhaust temperature is raised by retarding the ignition timing to promote warm-up of the exhaust purification catalyst.

JP, 2007-113413, A

In the ignition timing retard as described above, the exhaust gas temperature rises, but with the ignition timing retard, the amount of HC (hydrocarbon) produced from the exhaust port of the internal combustion engine itself increases. Therefore, there is a problem that the amount of HC discharged to the outside through the exhaust purification catalyst is rather increased until the exhaust purification catalyst reaches the activation temperature.

The present invention suppresses HC during the ignition timing retard by utilizing the fuel reforming system provided in the EGR passage. That is, during idle operation after cold start of the internal combustion engine, the exhaust gas recirculation through the fuel reforming catalyst is executed, and the reforming fuel is supplied from the reforming fuel injection valve, and the fuel reforming is performed. Oxygen increasing treatment is performed to increase the oxygen concentration in the gas flowing into the catalyst.

By increasing the oxygen concentration as described above, a larger amount of hydrogen is produced in the fuel reforming catalyst. Since this hydrogen is introduced into the internal combustion engine together with fresh air, the hydrogen concentration in the combustion chamber becomes high, and the combustion speed is improved, so that HC in the exhaust gas is reduced. That is, the increase in the amount of HC produced due to the ignition timing retard is suppressed by the introduction of a large amount of hydrogen.

According to the present invention, at the time of cold start, it is possible to quickly warm up the exhaust purification catalyst while suppressing an increase in HC.

FIG. 3 is a configuration explanatory view showing a configuration of an internal combustion engine including a fuel reforming system. FIG. 6 is a characteristic diagram showing an exhaust gas recirculation region. FIG. 6 is a characteristic diagram showing the effect of oxygen concentration on the correlation between the inlet gas temperature of the fuel reforming catalyst and the concentration of hydrogen produced. The flowchart which shows the flow of a process at the time of starting. Time chart for cold start.

An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a configuration explanatory diagram showing a system configuration of an internal combustion engine 1 including a fuel reforming system that produces hydrogen from a hydrocarbon fuel. The internal combustion engine 1 is, for example, a spark ignition type internal combustion engine that uses gasoline as a fuel, and includes a fuel injection valve 3 that injects and supplies the fuel sent from the fuel tank 2 toward, for example, an intake port of each cylinder. The intake passage 4 of the internal combustion engine 1 is provided with an air flow meter 5 for measuring the amount of intake air and a throttle valve 6. In the illustrated example, a supercharger 7 including a mechanical supercharger or a compressor of a turbocharger is shown downstream of the throttle valve 6, but the supercharger 7 is not always essential. A hydrogen sensor 8 that detects the hydrogen component in the intake air is provided between the supercharger 7 and the internal combustion engine 1. Further, the internal combustion engine 1 is equipped with an ignition plug 9 in each cylinder.

An exhaust passage 11 of the internal combustion engine 1 is provided with an exhaust purification catalyst 12 using a three-way catalyst for performing exhaust purification. An air-fuel ratio sensor 13 is arranged upstream of the exhaust purification catalyst 12, and an oxygen sensor 14 is arranged downstream of the exhaust purification catalyst 12. The engine controller 15 performs known air-fuel ratio feedback control based on the detection signals of the air-fuel ratio sensor 13 and the oxygen sensor 14. The exhaust purification catalyst 12 includes a catalyst temperature sensor 16 that detects the catalyst carrier temperature. In the illustrated example, only one exhaust purification catalyst 12 is shown, but a plurality of exhaust purification catalysts may be provided.

An EGR passage 17 is provided between the exhaust passage 11 and the intake passage 4 of the internal combustion engine 1 to recirculate a part of the exhaust gas of the internal combustion engine 1 from the exhaust passage 11 to the intake passage 4. The EGR passage 17 branches from the exhaust passage 11 on the upstream side of the exhaust purification catalyst 12 and joins the intake passage 4 between the throttle valve 6 and the supercharger 7.

A fuel reforming catalyst 18, which is a main part of the fuel reforming system, is interposed in the EGR passage 17, and the fuel reforming catalyst 18 is provided with a reforming fuel on the upstream side of the fuel reforming catalyst 18. A fuel injection valve 19 for reforming fuel for injecting and supplying is provided together with the mixer 20. In this embodiment, the fuel injection valve 19 for reforming fuel injects and supplies the gasoline in the fuel tank 2 as the reforming fuel, but reforms a liquid hydrocarbon fuel different from the fuel supplied to the internal combustion engine 1. It can also be used as a fuel for use. The fuel reforming catalyst 18 is formed by coating a monolith honeycomb catalyst carrier with a catalyst slurry containing, for example, a rhodium-based catalyst metal, and firing it. The fuel reforming catalyst 18 is carbonized by utilizing steam and heat contained in the reflux exhaust (EGR gas). Produces hydrogen from hydrogen fuel.

The EGR passage 17 further has a water-cooled or air-cooled EGR gas cooler 21 for cooling the EGR gas, and an exhaust gas recirculation for controlling the EGR rate along the target EGR rate, at a position downstream of the fuel reforming catalyst 18. And a control valve 22. Further, a hydrogen sensor 23 that detects a hydrogen component in the EGR gas is provided between the fuel reforming catalyst 18 and the EGR gas cooler 21, and between the EGR gas cooler 21 and the exhaust gas recirculation control valve 22. An oxygen sensor 24 is provided. The hydrogen sensor 23 and the oxygen sensor 24 are used together with the hydrogen sensor 8 in the intake passage 4 to control the amount of hydrogen supplied to the combustion chamber by fuel reforming.

An inlet gas temperature sensor 26 for detecting the temperature of the gas flowing into the fuel reforming catalyst 18 through the mixer 20 is provided between the fuel reforming catalyst 18 and the mixer 20. Are arranged. The fuel reforming catalyst 18 is provided with a catalyst temperature sensor 27 for detecting the catalyst carrier temperature.

Further, in order to increase the oxygen concentration in the gas flowing into the fuel reforming catalyst 18, in this embodiment, an air introducing passage 28 for introducing fresh air (air) is provided on the upstream side of the fuel reforming catalyst 18. .. The air introduction passage 28 has one end connected to the upstream side of the throttle valve 6 in the intake passage 4 and the other end connected to the upstream side of the reformed fuel injection valve 19 in the EGR passage 17. The air introduction passage 28 is provided with an air control valve 29 such as a butterfly valve capable of variably controlling the air introduction amount in the middle of the passage.

In the internal combustion engine 1 provided with the fuel reforming system as described above, stable combustion at a high EGR rate is achieved by adding hydrogen by fuel reforming during exhaust gas recirculation during operation. That is, in the engine controller 15, the target EGR rate is set in advance in the form of a map using the operating conditions of the internal combustion engine 1, that is, the load (torque) and the rotation speed as parameters, and the exhaust gas is set to achieve this target EGR rate. The opening degree of the reflux control valve 22 is controlled. FIG. 2 shows an exhaust gas recirculation region (indicated by reference numeral EGR) in which exhaust gas recirculation is performed after engine warm-up is completed. In the exhaust gas recirculation region, in a region where the target EGR rate is higher than a certain level, the reforming fuel is supplied from the reforming fuel fuel injection valve 19 to the fuel reforming catalyst 18, and is contained in the EGR gas. Utilizing steam and heat and the catalytic action of the fuel reforming catalyst 18, hydrogen is produced from the reforming fuel. This hydrogen merges with the fresh air in the intake passage 4 together with the EGR gas, and is introduced into the combustion chamber. Since the combustion speed in the combustion chamber is increased by introducing hydrogen, stable combustion at a high EGR rate can be realized. The amount of reforming fuel is set so that the total amount including the fuel amount from the fuel injection valve 3 of the internal combustion engine 1 can realize the stoichiometric air-fuel ratio.

Here, in the present embodiment, when hydrogen is supplied during normal operation, the air control valve 29 is closed, and positive introduction of air through the air introduction passage 28 is not performed.

On the other hand, when the internal combustion engine 1 is cold started, the engine controller 15 sets an ignition timing retard that greatly retards the ignition timing as compared with the basic ignition timing after warming up in order to accelerate warming up of the exhaust purification catalyst 12. Execute. Then, in parallel with this ignition timing retard, the fuel reforming system is operated to supply hydrogen into the combustion chamber.

That is, during the catalyst warm-up operation by the ignition timing retard after the cold start, the exhaust gas recirculation is performed even under the idle (specifically, the fast idle in which the engine rotation speed is higher than that after warm-up) operating conditions indicated by symbol A in FIG. Exhaust gas recirculation is performed via the control valve 22, and reforming fuel is supplied to the fuel reforming catalyst 18 from the reforming fuel injection valve 19. In particular, in the fuel reforming accompanying the ignition timing retard, air is introduced upstream of the fuel reforming catalyst 18 via the air introducing passage 28 and the air control valve 29 as oxygen increasing processing, and the fuel reforming catalyst 18 is supplied with the air. The oxygen concentration in the inflowing gas is high. As the oxygen concentration increases, the hydrogen concentration in the reformed gas generated by the fuel reforming catalyst 18 increases.

As an example, FIG. 3 is a characteristic diagram showing the correlation between the inlet gas temperature of the fuel reforming catalyst 18 on the horizontal axis and the hydrogen concentration at the outlet of the fuel reforming catalyst 18 on the vertical axis. The broken line shows the characteristics when only the EGR gas was supplied and no additional oxygen (air) was supplied, and the solid line additionally supplied oxygen (air) so that the gas from the inlet contained 6% oxygen. The characteristics of the case are shown. The exhaust gas when the internal combustion engine 1 is operating at the stoichiometric air-fuel ratio generally contains about 0.5% oxygen.

As shown in FIG. 3, by increasing the oxygen concentration in the gas flowing into the fuel reforming catalyst 18, the concentration of hydrogen produced increases. Particularly, in a region where the temperature of the gas flowing into the fuel reforming catalyst 18 is relatively low, such as immediately after a cold start, the hydrogen concentration increases remarkably according to the oxygen concentration. This increase in the hydrogen concentration is mainly due to the increase in the oxygen concentration, which promotes the oxidation reaction of the fuel supplied from the fuel injection valve for reforming fuel 19 on the catalyst, and the catalyst carrier of the fuel reforming catalyst 18. This is due to the rise in temperature. Note that if the oxygen concentration is higher, the hydrogen concentration also rises due to thermal decomposition.

Further, at the time of cold start, the temperature of the catalyst carrier of the fuel reforming catalyst 18 that utilizes the oxidation reaction due to the supply of fuel and additional oxygen is increased by the temperature of the catalyst carrier of the exhaust purification catalyst 12 without addition of fuel or oxygen. It occurs in a very short time as compared with the temperature rise of.

The hydrogen having a relatively high concentration generated by the fuel reforming catalyst 18 in this way is introduced into the combustion chamber together with the fresh air as in the operation in the high EGR region after the completion of warming up of the internal combustion engine 1. .. As described above, in the combustion chamber, hydrogen contributes to the improvement of the combustion speed. Therefore, the introduction of a relatively high concentration of hydrogen suppresses the generation of unburned HC in the combustion chamber even under the ignition timing retard condition. To be done. Therefore, the exhaust purification catalyst 12 can be warmed up early by the ignition timing retard while suppressing the amount of HC discharged to the outside until the exhaust purification catalyst 12 reaches the activation temperature.

Since the combustion is stabilized by the introduction of high concentration of hydrogen, the retard limit in the ignition timing retard becomes more retarded. Therefore, on the assumption that hydrogen is introduced by the reforming system, it is possible to set the ignition timing at the ignition timing retard to a more retarded side, whereby the temperature rise of the exhaust purification catalyst 12 can be further accelerated. Is.

FIG. 4 is a flowchart showing an example of processing executed by the engine controller 15 when the internal combustion engine 1 is started. In step 1, it is determined whether or not the present start is a cold start based on the engine temperature such as the cooling water temperature. If it is a warm-up restart, the routine proceeds to step 7, where the internal combustion engine 1 is operated in the normal operation mode. In this normal operation mode, as described above, hydrogen is introduced by the fuel reforming system in the predetermined high EGR region. Air introduction from the air introduction passage 28 is stopped, and the ignition timing is controlled according to the basic ignition timing corresponding to the engine operating conditions (load and engine speed).

The fuel reforming system is stopped while the internal combustion engine 1 is being started (in other words, during cranking). That is, the exhaust gas recirculation control valve 22 is closed and the reformed fuel injection valve 19 is stopped.

If the engine is cold started in step 1, the process proceeds to step 2 and ignition timing retard is executed to accelerate warming up of the exhaust purification catalyst 12. Then, in step 3, the temperature raising mode for raising the temperature of the fuel reforming catalyst 18 is executed. In this temperature raising mode, the exhaust gas recirculation control valve 22 is opened to an appropriate opening degree for exhaust gas recirculation, a relatively small amount of reforming fuel is supplied from the reforming fuel fuel injection valve 19, and air control is performed. The valve 29 is opened to a relatively small opening to provide an additional supply of a relatively small amount of air (oxygen). The supply amount of reforming fuel and the supply amount of air in the temperature raising mode are set to relatively small amounts necessary for raising the temperature of the fuel reforming catalyst 18 by the oxidation reaction on the catalyst. That is, the reforming fuel is not unnecessarily supplied before the fuel reforming catalyst 18 is sufficiently activated.

In the next step 4, it is determined whether or not the catalyst carrier temperature of the fuel reforming catalyst 18 detected by the catalyst temperature sensor 27 has reached a predetermined activation temperature. The operation in the temperature increasing mode is continued until the temperature of the catalyst carrier of the fuel reforming catalyst 18 reaches the activation temperature. The temperature rise of the fuel reforming catalyst 18 is faster than the temperature rise of the exhaust purification catalyst 12, as described above.

When the temperature of the catalyst carrier of the fuel reforming catalyst 18 reaches the activation temperature, the process proceeds to step 5 and shifts to the hydrogen starting mode. In this hydrogen start mode, the amount of reforming fuel supplied from the reforming fuel injection valve 19 is increased while the exhaust gas recirculation is continued, and the opening degree of the air control valve 29 is increased to a sufficient amount. To supply additional air (oxygen). As a result, a large amount of hydrogen is generated via the fuel reforming catalyst 18 and introduced into the combustion chamber. Therefore, the combustion speed in the internal combustion engine 1 that executes the ignition timing retard in order to accelerate the warm-up of the exhaust purification catalyst 12 becomes high and the combustion is stabilized, and as described above, the HC generation accompanying the ignition timing retard is generated. Suppressed.

In step 6, it is determined whether or not the catalyst carrier temperature of the exhaust purification catalyst 12 detected by the catalyst temperature sensor 16 has reached a predetermined activation temperature. Until the catalyst carrier temperature of the exhaust purification catalyst 12 reaches the activation temperature, the ignition timing retard and the operation in the hydrogen starting mode are continued.

When the temperature of the catalyst carrier of the exhaust purification catalyst 12 reaches the activation temperature, the process proceeds to step 7 and shifts to the normal operation mode. That is, the ignition timing retard is ended, the ignition timing is set to match the basic ignition timing, and the air introduction via the air introduction passage 28 is stopped. At this time, if the engine is idle, exhaust gas recirculation and fuel reforming are stopped.

FIG. 5 is a time chart showing the state of the warm-up operation after the cold start executed according to the above-mentioned flowchart. The column (a) shows the characteristics of the engine exhaust HC discharged to the outside through the exhaust purification catalyst 12 of the internal combustion engine 1, the column (b) shows the state of the ignition timing retard flag indicating the execution of the ignition timing retard, and the column (c). Shows the change of the catalyst carrier temperature of each of the exhaust purification catalyst 12 and the fuel reforming catalyst 18 and the corresponding operation mode.

When the cold start is performed at time t0, the ignition timing retard is started substantially at the same time. Since the exhaust temperature rises due to this ignition timing retard, the catalyst carrier temperature of the exhaust purification catalyst 12 rises relatively quickly. Further, the fuel reforming system starts to operate in the heating mode. By this operation in the temperature raising mode, a relatively small amount of reforming fuel and additional oxygen are supplied to the fuel reforming catalyst 18, so that the catalyst carrier temperature of the fuel reforming catalyst 18 rapidly rises. In particular, the temperature rise of the fuel reforming catalyst 18 is obtained more rapidly than the temperature rise of the exhaust purification catalyst 12. Then, at time t1, the catalyst carrier temperature of the fuel reforming catalyst 18 reaches a predetermined activation temperature T1.

When the activation temperature T1 is reached at time t1, the fuel reforming system shifts to the hydrogen starting mode. Providing a sufficient amount of reforming fuel and additional oxygen in the hydrogen start mode produces a large amount of hydrogen. Therefore, the hydrogen concentration in the reformed gas, and thus the hydrogen concentration in the intake air in the combustion chamber, becomes high. This stabilizes the combustion under the ignition timing retard and reduces the amount of HC produced.

In the column (a), the characteristic of the HC emission amount when hydrogen is not introduced by the fuel reforming is shown by a broken line as a “comparative example”. Has a large amount of generated HC, and particularly the amount of generated HC further increases with ignition timing retard. The HC thus produced in large quantity is discharged without being purified before the exhaust purification catalyst 12 is activated.

On the other hand, as shown by the solid line in the column (a), according to the above-described embodiment, by supplying hydrogen that contributes to the improvement of the combustion speed at a high concentration as the reformed gas, the generation of HC itself is suppressed, The amount of HC emission released before the activation of the exhaust purification catalyst 12 is reduced.

Then, at time t2, the exhaust purification catalyst 12 reaches the activation temperature T2, and shifts to the normal operation mode. This ends the ignition timing retard. The activation temperature T2 of the exhaust purification catalyst 12 and the activation temperature T1 of the fuel reforming catalyst 18 are usually temperatures that are not significantly different from each other.

The retard amount of the ignition timing retard does not have to be constant until the exhaust purification catalyst 12 reaches the activation temperature T2, and the retard amount gradually increases as the catalyst carrier temperature rises (or time elapses after starting). May be reduced.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications can be made.

For example, the temperature increasing mode in the above embodiment may be omitted and the hydrogen starting mode may be started immediately after the cold starting. That is, in the temperature raising mode of the above-described embodiment, the fuel reforming catalyst 18 becomes unnecessary by supplying a relatively small amount of reforming fuel and additional oxygen necessary for raising the temperature of the fuel reforming catalyst 18. Although the fuel is not supplied to suppress the catalyst deterioration and the like, even if the temperature increasing mode is omitted and the hydrogen starting mode is immediately started, there is no great difference in the temperature rise of the fuel reforming catalyst 18. As in the embodiment, it is possible to suppress HC immediately after the cold start.

Further, as the temperature raising mode, the exhaust gas recirculation control valve 22 may be opened to an appropriate opening degree and only the exhaust gas recirculation may be performed without supplying the reforming fuel or the additional oxygen.

Further, in the above embodiment, the activity of the fuel reforming catalyst 18 is determined from the temperature detected by the catalyst temperature sensor 27 in step 4 of FIG. 4, but it is determined whether or not a predetermined time has elapsed from the start, and the predetermined When the time has passed, the fuel reforming catalyst 18 may be considered to be active and the hydrogen starting mode may be entered.

Further, in the above embodiment, air is introduced through the air introduction passage 28 and the air control valve 29 as the oxygen increasing process for increasing the oxygen concentration in the gas flowing into the fuel reforming catalyst 18. The oxygen concentration may be increased by making the air-fuel ratio of the internal combustion engine 1 lean. That is, the air-fuel ratio of the internal combustion engine 1 is made leaner than the stoichiometric air-fuel ratio along with the execution of the ignition timing retard after the cold start. As a result, the concentration of oxygen remaining in the exhaust gas increases, and the oxygen concentration in the gas supplied to the fuel reforming catalyst 18 through the EGR passage 17 increases. Therefore, as in the above embodiment, a high hydrogen concentration in the reformed gas can be obtained.

Such a lean method does not require an air supply mechanism such as the air introduction passage 28 and the air control valve 29 of the above-described embodiment, and only needs to control the air-fuel ratio, which is advantageous in that the configuration is simplified. .. Also in this case, it is possible to change the oxygen concentration between the temperature rising mode and the hydrogen starting mode by changing the degree of leaning.

DESCRIPTION OF SYMBOLS 1... Internal combustion engine 2... Fuel tank 3... Fuel injection valve 4... Intake passage 5... Air flow meter 6... Throttle valve 11... Exhaust passage 12... Exhaust purification catalyst 16... Catalyst temperature sensor 15... Engine controller 17... EGR passage 18... Fuel Reforming catalyst 19... Fuel injection valve for reforming fuel 21... EGR gas cooler 22... Exhaust gas recirculation control valve 27... Catalyst temperature sensor 28... Air introduction passage 29... Air control valve

Claims (6)

An exhaust system is provided with at least one exhaust purification catalyst, and a fuel reforming catalyst and a reforming fuel for supplying fuel from the upstream side to the fuel reforming catalyst in an EGR passage for recirculating a part of exhaust gas to the intake system. In an internal combustion engine including a fuel injection valve,
During idle operation after cold start of the internal combustion engine,
Perform ignition timing retard,
Oxygen for performing exhaust gas recirculation through the fuel reforming catalyst, supplying reforming fuel from the reforming fuel injection valve, and increasing the oxygen concentration in the gas flowing into the fuel reforming catalyst. A method for warming up an exhaust gas purification catalyst for an internal combustion engine, characterized by performing an increasing process. The method for warming up an exhaust purification catalyst of an internal combustion engine according to claim 1, wherein air is introduced from the upstream side of the fuel reforming catalyst in the EGR passage as the oxygen increasing process. The method for warming up an exhaust purification catalyst of an internal combustion engine according to claim 1, wherein the air-fuel ratio of the internal combustion engine is made lean as the oxygen increasing process. The fuel supply amount for reforming is limited to a relatively small amount as compared with after activation until the fuel reforming catalyst is activated, according to any one of claims 1 to 3. A method for warming up an exhaust gas purification catalyst for an internal combustion engine as described above. The warm-up of the exhaust purification catalyst of an internal combustion engine according to claim 4, wherein the oxygen concentration is limited to a relatively low level until the fuel reforming catalyst is activated, as compared with after activation. Method. The exhaust gas purification catalyst for an internal combustion engine according to any one of claims 1 to 5, characterized in that when the exhaust gas purification catalyst reaches a predetermined activation temperature, the exhaust gas recirculation accompanied by fuel reforming is terminated. How to warm up.

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