JP2011149388A - Exhaust emission control system - Google Patents

Abstract

An exhaust purification system capable of always producing a reducing gas with a high yield regardless of variations in fuel properties in fuel used in a fuel reformer.
An exhaust purification system generates a reducing gas by supplying NO2 in an exhaust gas in a reducing atmosphere and supplying a fuel and air to a reforming catalyst, and the generated reducing property. A fuel reformer that supplies gas to the upstream side of the second catalytic converter; and reformer control means that controls an air supply amount and a fuel supply amount to the reforming catalyst. The reformer control means modifies the reforming catalyst temperature so that the reforming catalyst temperature is included in the optimal temperature range and the O / C ratio (ratio of the air supply amount to the fuel supply rate) is included in the optimal O / C ratio range. The total amount of the air supply amount and the fuel supply amount is increased or decreased based on the estimated value of the cetane number of the fuel supplied to the catalyst.
[Selection] Figure 3

Description

  The present invention relates to an exhaust purification system. More specifically, the present invention relates to an exhaust purification system including a catalytic converter that purifies NOx in exhaust gas of an internal combustion engine in a reducing atmosphere, and a fuel reformer that generates reducing gas and supplies the reducing gas to the upstream side of the catalytic converter. .

  In recent years, various environmental pollutants discharged into the atmosphere from internal combustion engines such as generators and automobiles have been regarded as problems. Environmental pollutants cause acid rain and photochemical smog as well as damage to human health, and there is a movement to regulate their emissions worldwide. In particular, in internal combustion engines such as diesel engines and gasoline lean burn engines, since lean combustion is performed, for example, a large amount of nitrogen oxide (NOx) is discharged. Therefore, studies on purification technologies for purifying NOx are being promoted. Yes.

  For example, Patent Documents 1 and 2 are provided with a NOx purification catalyst that captures NOx in exhaust gas, and by supplying a reducing gas containing hydrogen from upstream of the NOx purification catalyst, NOx or NOx purification catalyst in exhaust gas is supplied. A technique for selectively reducing trapped NOx is shown. Among reducing gases, hydrogen is known to exhibit high reduction performance even at relatively low temperatures.

By the way, as a specific method for generating a reducing gas containing hydrogen, for example, a partial oxidation reaction using oxygen as an oxidizing agent is known as shown in the following formula (1). This partial oxidation reaction is an exothermic reaction using fuel and oxygen, and the reaction proceeds spontaneously. For this reason, once reaction starts, it can continue producing hydrogen, without supplying heat from the outside. In such a partial oxidation reaction, when the fuel and oxygen coexist at a high temperature, a combustion reaction as shown in the following formula (2) also proceeds. Further, a steam reforming reaction represented by the following formula (3) using steam as an oxidizing agent is known. This steam reforming reaction is an endothermic reaction using fuel and steam, and is not a reaction that proceeds spontaneously. For this reason, the steam reforming reaction is easily controlled with respect to the partial oxidation reaction described above.
_{C n H m + 1 / 2nO} 2 → nCO + 1 / 2mH 2 (1)
_{C n H m + (n +} 1 / 4m) O 2 → nCO 2 + 1 / 2mH 2 O (2)
_{C n H m + nH 2 O} → nCO + (n + 1 / 2m) H 2 (3)

  Patent Document 3 discloses a fuel reformer that generates a reducing gas using the above reforming reaction. By the way, in the above reforming reactions, it is known that there is a correlation between the yield of reducing gas and the catalyst temperature. Therefore, in the fuel reformer of Patent Document 3, in order to generate reducing gas with high yield, the catalyst temperature is controlled to a predetermined target temperature by adjusting the air supply amount and the fuel supply amount.

Japanese Patent No. 3642273 JP 2002-89240 A JP 2008-184942 A

By the way, it is considered that the characteristics of the reforming reaction in the reforming catalyst change when the properties of the fuel used in the fuel reformer change. However, in Patent Document 3, sufficient study is made on such changes in the properties of the fuel. Not.
In the technique of Patent Document 3, although the temperature of the reforming catalyst can be maintained at a predetermined target temperature by adjusting the air supply amount and the fuel supply amount as described above, the characteristics of the reforming reaction also change as the fuel properties change. Therefore, the supply ratio of air and fuel for maintaining the temperature of the reforming catalyst at the target temperature also changes, and as a result, the yield of reducing gas may be deteriorated due to fluctuations in fuel properties.

  An object of the present invention is to provide an exhaust purification system that can always generate a reducing gas with a high yield regardless of variations in fuel properties in fuel used in a fuel reformer.

In order to achieve the above object, the present invention is provided in an exhaust passage (for example, an exhaust pipe 4 described later) of an internal combustion engine (for example, an engine 2 described later), and purifies NOx in the exhaust of the internal combustion engine in a reducing atmosphere. A catalytic converter (for example, a second catalytic converter 47 described later) and a reducing gas provided separately from the exhaust passage and supplying fuel and air to a reforming catalyst (for example, a reforming catalyst 82 described later). And a fuel reformer (for example, a fuel reformer 8 to be described later) that supplies the generated reducing gas to the upstream side of the catalytic converter in the exhaust passage (for example, An exhaust purification system 1) described later is provided. The exhaust purification system is configured to estimate a fuel property (for example, a cetane number described later) of fuel supplied to the reforming catalyst (for example, a unit related to execution of ECU 7 described later and step S22 in FIG. 5). And reformer control means for controlling the air supply amount and the fuel supply amount to the reforming catalyst (for example, ECU 7 described later and means for executing the H ₂ enrichment control in FIG. 5). The ratio of the air supply amount to the fuel supply amount is defined as a supply ratio (for example, an O / C ratio described later), and the reformer control means is configured so that the temperature of the reforming catalyst is within a predetermined temperature range (for example, an after-mentioned And the supply ratio is between a predetermined ratio upper limit value (for example, an O / C ratio upper limit value described later) and a ratio lower limit value (for example, an O / C ratio lower limit value described later). The total amount of the air supply amount and the fuel supply amount is increased or decreased based on the estimation result of the fuel property by the fuel property estimation means.

  If the total amount of the air supply amount and the fuel supply amount is increased while the supply ratio of air and fuel is kept constant, the temperature of the reforming catalyst tends to increase. Further, if the supply ratio is increased while keeping the total amount constant, the temperature of the reforming catalyst tends to increase. According to the present invention, by utilizing such a characteristic of the reforming reaction, the temperature of the reforming catalyst is included in a predetermined temperature range, and the supply ratio is between a predetermined ratio upper limit value and a ratio lower limit value. The total amount of the air supply amount and the fuel supply amount is increased or decreased based on the estimation result of the fuel property. Thereby, since the supply ratio of air and fuel supplied to the reforming catalyst can be maintained at an optimum value, the yield of the reducing gas can be kept high regardless of the fuel properties of the fuel. At this time, the temperature of the reforming catalyst can also be maintained at an optimum value, so that it is possible to prevent an excessive amount of coke from being deposited on the reforming catalyst or the reforming catalyst from deteriorating.

  In this case, it is preferable that the fuel property estimation means estimates the fuel property of the fuel based on the combustion state of the fuel injected into the combustion chamber of the internal combustion engine (for example, an ignition delay angle described later).

  According to the present invention, by estimating the fuel property based on the combustion state of the internal combustion engine, it is possible to cover the sensors necessary for estimating the fuel property with the internal combustion engine. There is no need to provide extra sensors in the mass device. Therefore, the exhaust purification system can be simplified.

  In this case, the reformer control means, based on the result of estimation of the fuel property by the fuel property estimation means so that the temperature of the reforming catalyst is included in the temperature band, the target value of the supply ratio (for example, When the target value is equal to or higher than the ratio upper limit value, the temperature of the reforming catalyst is included in the temperature zone, and the supply When the total amount of the air supply amount and the fuel supply amount is controlled to be increased so that the ratio is included between the ratio upper limit value and the ratio lower limit value, and the target value is smaller than the ratio upper limit value, the revision is performed. The total amount of the air supply amount and the fuel supply amount is controlled to be reduced so that the temperature of the catalyst is included in the temperature range and the supply ratio is included between the ratio upper limit value and the ratio lower limit value. Is preferred.

According to this invention, the target value of the supply ratio is calculated based on the estimation result of the fuel property so that the temperature of the reforming catalyst is included in the temperature range. When the calculated target value is equal to or higher than the ratio upper limit value, the total amount of the air supply amount and the fuel supply amount is controlled to be increased by using the temperature characteristics in the reforming reaction. The temperature of the catalyst may be included in the temperature zone, and the supply ratio may be included between the ratio upper limit value and the ratio lower limit value.
On the other hand, when the calculated target value is smaller than the lower limit ratio, the reforming catalyst is controlled by controlling the total amount of the air supply amount and the fuel supply amount to the decreasing side using the temperature characteristics in the reforming reaction. The temperature may be included in the temperature zone, and the supply ratio may be included between the ratio upper limit value and the ratio lower limit value. As a result, the yield of the reducing gas, particularly hydrogen, can be kept high regardless of the fuel properties of the fuel.

  In this case, the reformer control means increases the air supply amount and the fuel supply amount so as not to exceed the predetermined upper limit value, and the air supply amount and the fuel supply amount so as not to fall below the predetermined lower limit value. It is preferable to reduce the weight.

  According to the present invention, the reducing gas generated by the fuel reformer and supplied to the catalytic converter is increased or decreased by increasing or decreasing the air supply amount and the fuel supply amount under the predetermined upper limit value and lower limit value. It can be prevented from occurring.

1 is a schematic diagram showing an exhaust purification system according to an embodiment of the present invention and an engine configuration to which the exhaust purification system is applied. It is a flowchart which shows the specific procedure of the rich driving control by ECU which concerns on the said embodiment. It is a figure which shows the relationship between the temperature of the reforming catalyst which concerns on the said embodiment, and O / C ratio. It is a figure which shows the relationship between the temperature of the reforming catalyst which concerns on the said embodiment, and the total amount of air supply amount and fuel supply amount. It is a figure which shows the specific procedure which controls the fuel reformer which concerns on the said embodiment, and enriches an exhaust air fuel ratio. It is a figure which shows the relationship between the cetane number of the fuel which concerns on the said embodiment, and the ignition delay angle of the fuel injected into the combustion chamber of the engine. It is a figure which shows the relationship between the estimated value of the cetane number which concerns on the said embodiment, and the base value of the target value of O / C ratio. It is a figure which shows the relationship between the correction value with respect to the target value of O / C ratio which concerns on the said embodiment, and a temperature deviation.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing a configuration of an exhaust purification system 1 according to the present embodiment and an internal combustion engine (hereinafter referred to as “engine”) 2 to which the exhaust purification system 1 is applied. The engine 2 is a diesel engine that directly injects fuel into the combustion chamber of each cylinder 21.

  The fuel supply system that supplies fuel to the engine 2 includes a fuel pump (not shown) that pressurizes the fuel stored in the fuel tank 23, and the fuel pressurized by the fuel pump is provided for each cylinder 21 of the engine 2. And a common rail (not shown) for supplying to the injector.

  The fuel injection amount from the injector is set by an electronic control unit (hereinafter referred to as “ECU”) 7 described later. Further, the valve opening time of the injector is controlled by a drive signal from the ECU 7 so that a set fuel injection amount can be obtained.

  The engine 2 includes an intake pipe 3 through which intake air circulates, an exhaust pipe 4 through which exhaust circulates, an exhaust gas recirculation passage 5 that recirculates part of the exhaust gas in the exhaust pipe 4 to the intake pipe 3, and intake air into the intake pipe 3. And a supercharger 6 for pressure-feeding.

  The intake pipe 3 is connected to the intake port of each cylinder 21 of the engine 2 through a plurality of branch portions of the intake manifold 31. The exhaust pipe 4 is connected to the exhaust port of each cylinder 21 of the engine 2 through a plurality of branch portions of the exhaust manifold 41. The exhaust gas recirculation passage 5 branches from the exhaust manifold 41 and reaches the intake manifold 31.

  The supercharger 6 includes a turbine (not shown) provided in the exhaust pipe 4 and a compressor (not shown) provided in the intake pipe 3. The turbine is driven by the kinetic energy of the exhaust flowing through the exhaust pipe 4. The compressor is rotationally driven by the turbine, pressurizes the intake air, and pumps it into the intake pipe 3. The turbine includes a plurality of variable vanes (not shown), and is configured to change the turbine rotation speed (rotational speed) by changing the opening of the variable vanes. The vane opening of the turbine is electromagnetically controlled by the ECU 7.

  A throttle valve 32 for controlling the intake air amount of the engine 2 is provided on the upstream side of the supercharger 6 in the intake pipe 3. The throttle valve 32 is connected to the ECU 7 via an actuator, and the opening degree thereof is electromagnetically controlled by the ECU 7. Further, an intercooler 33 for cooling the intake air pressurized by the supercharger 6 is provided on the downstream side of the supercharger 6 in the intake pipe 3.

  The exhaust gas recirculation passage 5 connects the exhaust manifold 41 and the intake manifold 31 and recirculates part of the exhaust discharged from the engine 2. The exhaust gas recirculation passage 5 is provided with an EGR cooler 51 that cools the exhaust gas that is recirculated, and an EGR valve 52 that controls the recirculation amount of the exhaust gas. The EGR valve 52 is connected to the ECU 7 via an actuator (not shown), and the valve opening degree is electromagnetically controlled by the ECU 7.

A first catalytic converter 45 for purifying exhaust, a diesel particulate filter (hereinafter referred to as “DPF (Diesel Particulate Filter)”) 46, and a second catalytic converter are disposed downstream of the supercharger 6 in the exhaust pipe 4. 47 are provided in this order from the upstream side. Further, the exhaust pipe 4 generates a reformed gas containing a reducing gas such as hydrogen (H ₂ ), carbon monoxide (CO), and hydrocarbon (HC), and the generated reformed gas is supplied to the exhaust pipe 4. A fuel reformer 8 that is supplied from the upstream side of the first catalytic converter 45 is connected.

The first catalytic converter 45 includes, for example, a three-way catalyst, purifies the exhaust gas by a reaction between the catalyst and the exhaust gas, and raises the temperature of the exhaust gas with heat generated by the reaction between the exhaust gas and the catalyst. More specifically, the first catalytic converter 45 includes, for example, rhodium (NOx) that is excellent in NOx reduction ability when platinum (Pt) acting as a catalyst is supported on an alumina (Al ₂ O ₃ ) carrier. Rh) and ceria (CeO ₂ ) having excellent oxygen storage capacity in order to exhibit a stable catalytic action against a sudden change in oxygen concentration in the exhaust gas is used.
More specifically, the three-way catalyst is composed of 2.4 (g / L) platinum, 1.2 (g / L) rhodium, and 6.0 (g / L) palladium (Pd). A material composed of ceria 50 (g / L), alumina 150 (g / L), and binder 10 (g / L) is put into a ball mill together with an aqueous medium and stirred and mixed. Thus, a slurry is produced, and this slurry is coated on a Fe—Cr—Al alloy support, dried at 600 ° C. for 2 hours, and fired.

  When the exhaust gas passes through the fine holes in the filter wall, the DPF 46 deposits particulate matter (PM (Particulate Matter)) mainly containing carbon in the exhaust gas on the surface of the filter wall and the holes in the filter wall. To collect. When PM is collected to the limit of the collection capability of the DPF 46, that is, the accumulation limit, the pressure loss increases. Therefore, the DPF regeneration process for burning the PM deposited on the DPF 46 and regenerating the DPF 46 is appropriately executed.

  The second catalytic converter 47 sets the air-fuel mixture combusted in the engine 2 to be leaner than the stoichiometric ratio, and captures NOx in the exhaust under an oxidizing atmosphere in which the oxygen concentration of the inflowing exhaust is relatively high. Further, the second catalytic converter 47 reduces and purifies the trapped NOx in a reducing atmosphere in which the concentration of the reducing gas in the inflowing exhaust gas is relatively high.

In the present embodiment, the second catalytic converter 47 is formed by supporting a NOx purification catalyst having two layers on a catalyst carrier.
The lower layer of the NOx purification catalyst is 4.5 (g / L) of platinum, 60 (g / L) of ceria, 30 (g / L) of alumina, and 60 (g of Ce—Pr—La—Ox. / L) and Zr-Ox 20 (g / L) are mixed with an aqueous medium into a ball mill and stirred and mixed to produce a slurry, and this slurry is coated on the above-mentioned lower layer Formed.
Further, the upper layer of the NOx purification catalyst is 75 (g / L) obtained by ion-exchange of β-type zeolite with iron (Fe) and cerium (Ce), 7 (g / L) alumina, and 8 binders. A material composed of (g / L) is put into a ball mill together with an aqueous medium, stirred and mixed to produce a slurry, and this slurry is coated on the lower layer.

  Specifically, the second catalytic converter 47 configured to carry the NOx purification catalyst as described above operates as follows to purify NOx in the exhaust gas.

[Lean state 1]
First, when the so-called lean burn operation is performed in which the air-fuel mixture of the engine 2 is set leaner than the stoichiometric ratio, and the exhaust gas flowing into the second catalytic converter 47 is in an oxidizing atmosphere, NOx in the exhaust gas passes through the upper layer. Then, it reaches the lower layer and is oxidized (for example, NO → NO ₂ ) by platinum. The oxidized NOx (for example, NO ₂ ) is once adsorbed and stored in the lower layer. At this time, platinum functions as a NO oxidation catalyst, and ceria and Ce—Pr—La—Ox function as a NOx adsorbent.

[Rich state]
Next, for example, the air-fuel mixture of the engine 2 is set near the stoichiometric ratio or richer than the stoichiometric ratio, or the reducing gas generated by the fuel reformer 8 is introduced into the exhaust pipe 4. When the exhaust gas flowing into the second catalytic converter 47 is made a reducing atmosphere by performing rich operation control, carbon dioxide and hydrogen are generated by a shift reaction of carbon monoxide and water in the exhaust gas (see the following formula (4)). ). Further, the NOx stored in the lean state 1 and the NOx in the exhaust gas react with hydrogen to generate ammonia (see the following formula (5)). The ammonia produced here moves to the upper layer and is adsorbed and stored in the zeolite. Note that the exhaust air flowing into the second catalytic converter 47 in the reducing atmosphere by performing the rich operation control as described above is also referred to as enriching the exhaust air-fuel ratio below.
CO + H ₂ O → H ₂ + CO ₂ (4)
NOx + H ₂ → NH ₃ (5)

[Lean state 2]
Next, when the lean burn operation is performed again and the exhaust gas flowing into the second catalytic converter 47 is made into an oxidizing atmosphere, ammonia stored in the upper layer and NOx in the exhaust gas are converted into an ammonia selective catalytic reduction method (NH ₃ -SCR). ) Is converted into nitrogen (see the following formula (6)), and the nitrogen is released from the upper layer. At this time, the iron and cerium ion exchange β zeolite functions as an NH ₃ -SCR catalyst.
NOx + NH ₃ + O ₂ → N ₂ + H ₂ O (6)

  Thus, according to the second catalytic converter 47, ammonia generated in a reducing atmosphere is adsorbed by the zeolite, and the adsorbed ammonia reacts with NOx during the lean burn operation, so that the NOx can be efficiently purified. Can do.

  The fuel reformer 8 includes a gas passage 81 having one end connected to the exhaust pipe 4, a reforming catalyst 82 provided in the gas passage 81, and a raw material from the other end of the gas passage 81. And a raw material supply device 83 for supplying to 82.

  The raw material supply device 83 mixes fuel stored in the fuel tank 23 and air supplied by the compressor 84 at a predetermined ratio to produce a raw material, and this raw material is supplied to the reforming catalyst 82 in the gas passage 81. Supply. The raw material supply device 83 includes an air valve 85 that controls the amount of air supplied to the reforming catalyst 82 (hereinafter referred to as “air supply amount”) and the amount of fuel supplied to the reforming catalyst 82 (hereinafter referred to as “air supply amount”). A fuel valve 86 for controlling the fuel supply amount), and an air valve 85 and an injector (not shown) that mixes the air and fuel supplied via the fuel valve 86 and injects them into the reforming catalyst 82. . The air valve 85 and the fuel valve 86 are connected to the ECU 7 through actuators (not shown), respectively, and the air supply amount, the fuel supply amount, and the ratio of the air supply amount to the fuel supply amount (air supply amount / fuel supply). Amount) is controlled by the ECU 7. In the present embodiment, the ratio of the air supply amount to the fuel supply amount is referred to as an O / C ratio.

  The reforming catalyst 82 includes at least one metal catalyst component selected from the group consisting of rhodium, platinum, palladium, nickel, and cobalt, and at least one oxidation selected from the group consisting of ceria, zirconia, alumina, and titania. Or a composite oxide based on these. The reforming catalyst 82 reforms the raw material supplied from the raw material supply device 83 to generate a reformed gas containing a reducing gas such as hydrogen, carbon monoxide, and hydrocarbon. More specifically, the reforming catalyst 82 generates a reformed gas containing hydrogen and carbon monoxide by a partial oxidation reaction between a hydrocarbon constituting the raw material and air.

  In this embodiment, as the reforming catalyst 82, ceria and rhodium powder are weighed so that the mass ratio of rhodium to ceria is 1%, and this powder is put into a ball mill together with an aqueous medium and stirred and mixed. Thus, a slurry prepared by coating a slurry on a metal carrier, and drying and firing at 600 ° C. for 2 hours is used.

  The fuel reformer 8 includes a heater (not shown) that includes a glow plug, a spark plug, and the like, and can heat the reforming catalyst 82 when the fuel reformer 8 is started. It has become. The fuel reformer 8 is provided separately from the exhaust pipe 4. That is, the raw material supply device 83 and the reforming catalyst 82 of the fuel reformer 8 are not provided in the exhaust pipe 4.

  Various sensors such as an in-cylinder pressure sensor 91, an air flow meter 94, a UEGO sensor 95, an exhaust gas temperature sensor 96, and a reforming catalyst temperature sensor 97 are connected to the ECU 7. The in-cylinder pressure sensor 91, the air flow meter 94, the UEGO sensor 95, the exhaust gas temperature sensor 96, and the reforming catalyst temperature sensor 97 are respectively the in-cylinder pressure of the engine 2, the intake air amount of the engine 2, and the exhaust air in the exhaust pipe 4. The fuel ratio, the exhaust temperature downstream of the second catalytic converter 47, and the temperature of the reforming catalyst 82 are detected, and a signal substantially proportional to the detected value is transmitted to the ECU 7.

  The ECU 7 shapes input signal waveforms from various sensors, corrects a voltage level to a predetermined level, converts an analog signal value into a digital signal value, and a central processing unit (hereinafter referred to as “a processing unit”). CPU ”). In addition, the ECU 7 includes a storage circuit that stores various calculation programs and calculation results executed by the CPU, and an output circuit that outputs a control signal to the fuel injection valve of the engine 2.

FIG. 2 is a flowchart showing a specific procedure of rich operation control, which is repeatedly executed by the ECU. Hereinafter, as described in detail, in the rich operation control of the present embodiment, a normal enrichment control in which the air-fuel mixture of the engine 2 is set to a richer side than the stoichiometric ratio or the stoichiometric ratio, and a fuel reformer The H ₂ enrichment control for supplying the reformed gas generated in step 1 to the exhaust pipe can be selectively executed according to predetermined conditions.

  In step S1, it is determined whether or not it is time to enrich the exhaust air-fuel ratio. More specifically, the NOx trapping amount integrated value of the second catalytic converter is acquired, and when this integrated value exceeds a predetermined threshold value, it is determined that the timing for enriching the exhaust air-fuel ratio has been reached. Note that the amount of NOx trapped by the second catalytic converter can be estimated sequentially based on the intake air amount, the fuel injection amount of the engine, and the like. Therefore, the NOx trapped amount integrated value can be calculated by integrating the NOx amount trapped by the second catalytic converter. If the determination in step S1 is YES, the process proceeds to step S2, and if the determination in step S1 is NO, this process is immediately terminated.

  In step S2, the temperature of the second catalytic converter is calculated based on the detection value of the exhaust temperature sensor, and it is determined whether or not the second catalytic converter temperature is lower than a predetermined first determination temperature TA. If this determination is NO and the second catalytic converter temperature is equal to or higher than the first determination temperature TA, it is determined that the execution of the normal enrichment control is suitable, and the process proceeds to step S3. In step S3, the target value of the exhaust air-fuel ratio in the normal enrichment control is calculated, and the process proceeds to step S4. The target value of the exhaust air / fuel ratio is calculated based on the second catalytic converter temperature, the NOx trapping amount integrated value, the engine speed, the required torque, and the like.

  In step S4, execution of normal enrichment control is started, and the process proceeds to step S5. In this normal enrichment control, at least one of the main injection amount, the post injection amount, and the intake air amount of the engine is adjusted so that the exhaust air-fuel ratio matches the target value calculated in step S3. In step S5, it is determined whether or not the normal rich timer is “0”. If this determination is NO, the process proceeds to step S7. If this determination is YES, the process proceeds to step S6, and after starting the normal rich timer, the process proceeds to step S7. In step S7, it is determined whether or not the measured value of the normal rich timer is smaller than a predetermined end determination value. If this determination is YES, this processing is immediately terminated. On the other hand, if this determination is NO, the process proceeds to step S8. In step S8, the normal rich control is terminated, the normal rich timer is reset, and this process is terminated.

On the other hand, if the determination in step S2 is YES and the second catalytic converter temperature is lower than the first determination temperature TA, it is determined that execution of the H ₂ enrichment control is suitable, and the process proceeds to step S9. In step S9, the target value of the exhaust air-fuel ratio in the H ₂ enrichment control is calculated, and the process proceeds to step S10. In step S10, execution of H ₂ enrichment control is started, and the process proceeds to step S11. The detailed procedure of the H ₂ enrichment control will be described later with reference to FIGS.

In step S11, the start of _{H 2} rich timer and normal rich timer, moves to step S12. In step S12, the temperature of the second catalytic converter is calculated based on the detected value of the exhaust temperature sensor, and it is determined whether or not the second catalytic converter temperature is equal to or higher than a predetermined second determination temperature TB. If this determination is YES, the process proceeds to step S14, and if NO, the process proceeds to step S13. Here, the second determination temperature TB is set higher than the first determination temperature TA.

In step S13, the value of H ₂ rich timer is determined whether or not smaller than a predetermined end determination value. If this determination is YES, the process proceeds to step S7, and if NO, the process proceeds to step S14. The end determination value for the H ₂ rich timer is set shorter than the end determination value for the normal rich timer. In step S14, the supply of the reformed gas by the fuel reformer is stopped, the H ₂ rich timer is reset, and the process proceeds to step S7.

Next, with reference to FIG. 3 to FIG. 8, the procedure of H ₂ enrichment control by the fuel reformer in step S10 will be described. First, before describing the specific procedure, the characteristics of the reforming reaction in the fuel reformer will be described in detail.

  FIG. 3 is a graph showing the relationship between the O / C ratio and the reforming catalyst temperature. In FIG. 3, the horizontal axis represents the O / C ratio, and the vertical axis represents the reforming catalyst temperature. In FIG. 3, the alternate long and short dash line shows the relationship between the O / C ratio and the reforming catalyst temperature when the fuel has a low cetane number, and the broken line shows the O / C ratio and the reforming catalyst when the fuel has a high cetane number. The relationship with temperature is shown. Further, in FIG. 3, a hydrogen yield curve showing the relationship between the O / C ratio and the hydrogen yield is shown by a solid line superimposed on the above graph showing the relationship between the O / C ratio and the reforming catalyst temperature. The graph and the hydrogen yield curve showing the relationship between the O / C ratio and the reforming catalyst temperature in FIG. 3 show the O / C ratio while maintaining the total amount of air supply and fuel supply at predetermined values, respectively. It shows what was obtained by changing.

As shown by hatching in FIG. 3, when the reforming catalyst temperature is low and the O / C ratio is small, coke is deposited on the reforming catalyst, and when the reforming catalyst temperature is high and the O / C ratio is large, the reforming catalyst is It will deteriorate. For this reason, in order to prevent such coke accumulation and catalyst deterioration, it is desired to maintain the reforming catalyst temperature within the optimum temperature range. On the other hand, as shown in FIG. 3, the hydrogen yield curve has an upwardly convex shape with respect to the change in the O / C ratio. Therefore, in order to maintain a high hydrogen yield and efficiently generate hydrogen, it is desired to maintain an O / C ratio within a predetermined optimum O / C ratio band including a reaction stoichiometric ratio.
From the above, the air supply amount and the fuel supply amount can be adjusted so that the reforming catalyst temperature and the O / C ratio are included in the optimum reaction region where the optimum temperature zone and the optimum O / C ratio zone overlap. desired.

On the other hand, as shown in FIG. 3, the reforming catalyst temperature tends to increase as the O / C ratio increases, that is, the proportion of air in the raw material supplied to the reforming catalyst increases. Further, when the cetane number of the fuel is increased, the reforming catalyst temperature tends to be lowered even with the same O / C ratio.
Therefore, in order to maintain the reforming catalyst temperature within the above optimum temperature zone under a constant total amount of air and fuel, it is necessary to adjust the O / C ratio according to the cetane number. For this reason, for example, when a high cetane number fuel is used, as shown by an asterisk in FIG. 3, unless the O / C ratio is set larger than the optimum O / C ratio band, the reforming catalyst temperature is set within the optimum temperature band. It may become impossible to maintain. For example, when a low cetane number fuel is used, the reforming catalyst temperature is maintained within the optimum temperature zone unless the O / C ratio is made smaller than the optimum O / C ratio zone, as indicated by a circle in FIG. It may not be possible.

  FIG. 4 is a diagram showing the relationship between the reforming catalyst temperature, the air supply amount, and the total amount of fuel supply. FIG. 4 shows what is obtained when the total amount of air and fuel is changed while keeping the O / C ratio constant. As shown in FIG. 4, the reforming catalyst temperature tends to increase according to the total amount of air and fuel. This indicates that the reforming catalyst temperature can be increased or decreased by increasing or decreasing the total amount of air and fuel while keeping the O / C ratio constant.

Considering the relationship between the total amount of air and fuel and the reforming catalyst temperature, the case of using a high cetane number fuel or a low cetane number fuel will be discussed again with reference to FIG.
As described above, since the fuel has a high cetane number, the O / C ratio when the reforming catalyst temperature is controlled within the optimum temperature range becomes larger than the optimum O / C ratio range as shown by the star. May end up. At this time, if the O / C ratio is controlled within the optimum O / C ratio zone, the reforming catalyst temperature becomes lower than the optimum temperature zone. When such a high cetane fuel is used, both the O / C ratio and the reforming catalyst temperature can be included in the optimum reaction region by controlling the total amount of air and fuel to the increase side. .
Further, as described above, since the fuel has a low cetane number, the O / C ratio when the reforming catalyst temperature is controlled within the optimum temperature range is smaller than the optimum O / C ratio range as indicated by a circle. It may become. At this time, if the O / C ratio is controlled within the optimum O / C ratio zone, the reforming catalyst temperature becomes higher than the optimum temperature zone. When such a low cetane number fuel is used, the O / C ratio and the reforming catalyst temperature can both be included in the optimum reaction region by controlling the total amount of air and fuel to the reduction side. .

FIG. 5 is a diagram showing a specific procedure of H ₂ enrichment control using the above-described reforming reaction characteristics, that is, control for enriching the exhaust air-fuel ratio by the fuel reformer. As will be described in detail below, in the H ₂ enrichment control, the air supply amount and the fuel supply amount are controlled based on the estimation result of the cetane number of the fuel using the characteristics of the reforming reaction as described above. By doing so, both the reforming catalyst temperature and the O / C ratio are included in the optimum reaction region.

  In S21, supply of air to the reforming catalyst is started. Note that the target value of the air supply amount at this time is Qair_normal set according to the target value of the exhaust air-fuel ratio determined in step S9 of FIG. Here, the target value Qair_normal of the air supply amount set in accordance with the target value of the exhaust air / fuel ratio is operated under the condition that the hydrogen yield is maximized, and further the reducing property of the amount corresponding to the target value of the exhaust air / fuel ratio is set. The air supply amount is assumed when the O / C ratio and the total amount of air and fuel are adjusted to generate gas.

In step S22, an estimated value of the cetane number of the fuel supplied to the reforming catalyst is calculated, and the process proceeds to step S23. More specifically, as shown below, the cetane number of the fuel is estimated based on the combustion state of the fuel injected into the combustion chamber of the engine.
FIG. 6 is a diagram showing the relationship between the cetane number of the fuel and the ignition delay angle of the fuel injected into the combustion chamber of the engine. Here, the ignition delay angle indicates a difference between the target ignition timing and the actual ignition timing. As shown in FIG. 6, even if the fuel injection timing is the same, the actual ignition timing varies depending on the cetane number of the fuel used. In step S22, an ignition delay angle is calculated based on the detection value of the in-cylinder pressure sensor of the engine, and a map as shown in FIG. 7 is searched based on the ignition delay angle, thereby estimating the cetane number of the fuel. Calculate the value. As described above, a specific method for estimating the cetane number of fuel based on the ignition delay angle is described in detail in Japanese Patent Application Laid-Open No. 2007-56776 filed by the applicant of the present application. The detailed description above is omitted.

Returning to FIG. 5, in step S23, the base value of the target value of the O / C ratio is determined by searching a predetermined map based on the obtained estimated value of cetane number, and the process proceeds to step S24. In this step S23, the target value of the O / C ratio is set according to the estimated value of the cetane number so that the reforming catalyst temperature is maintained at the target temperature set in the optimum temperature range regardless of the cetane number of the fuel. Determine the base value.
FIG. 7 is a diagram showing the relationship between the estimated value of the cetane number and the base value of the target value of the O / C ratio, and shows a specific example of the map.
As described above, when the cetane number of the fuel is high, the reforming catalyst temperature tends to be low even at the same O / C ratio, and when the O / C ratio is large, the reforming catalyst temperature tends to be high. is there. Therefore, in order to maintain the reforming catalyst temperature at the target temperature within the optimum temperature range, as shown in FIG. 6, the base value of the target value of the O / C ratio is increased as the cetane number of the fuel increases. Set.

In step S24, whether the base value of the target value of the O / C ratio determined according to the cetane number is smaller than the upper limit value of the optimum O / C ratio band (hereinafter referred to as “O / C ratio upper limit value”). Is determined. If this determination is YES, the process proceeds to step S25.
In step S25, whether or not the base value of the target value of the O / C ratio determined according to the cetane number is larger than the lower limit value of the optimum O / C ratio band (hereinafter referred to as “O / C ratio lower limit value”). Is determined. If this determination is YES, the process proceeds to step S26.

  In step S26, when it is determined that the base value of the target value of the O / C ratio determined according to the cetane number is included in the optimum O / C ratio band, the fuel is determined according to the base value. The target value Qfuel_normal of the supply amount is set, and the process proceeds to step S27. More specifically, the target value Qfuel_normal of the fuel supply amount is set so that the ratio of the air supply amount set in step S21 to the target value Qair_normal is the base value of the target value of the O / C ratio. Next, in step S27, supply of an amount of fuel corresponding to the set target value Qfuel_normal is started, and the process proceeds to step S32.

On the other hand, when the determination in step S24 is NO and the base value of the target value of the O / C ratio determined according to the cetane number is equal to or greater than the O / C ratio upper limit value, the process proceeds to step S28. Here, the base value of the target value of the O / C ratio being equal to or greater than the O / C ratio upper limit value means that the reforming catalyst temperature and the O / C ratio are required unless the total amount of air and fuel is increased. Cannot be within the optimum reaction region (see star in FIG. 3).
Therefore, in this step S28, in order to control the total amount of air and fuel to the increase side so that the reforming catalyst temperature and the O / C ratio are included in the optimum reaction region, the target value of the O / C ratio is set to the above step. The base value determined in S23 is changed to a predetermined value within the optimum O / C ratio band. Then, the target value of the air supply amount is increased from the value Qair_normal set in step S21 to Qair_high, and the fuel supply amount is set so that the ratio of the air supply amount to the target value Qair_high becomes the target value of the O / C ratio. Target value Qair_high is set, and the process proceeds to step S29.
In order to prevent the amount of reducing gas generated from becoming excessive with respect to the target value of the exhaust air / fuel ratio, the target value Qair_high of the air supply amount and the target value Qfuel_high of the fuel supply amount are set in advance, respectively. It is set on the increase side so as not to exceed a predetermined upper limit.
In step S29, the target value of the air supply amount is changed from Qair_normal to Qair_high set in the above step, and supply of fuel in an amount corresponding to the target value Qfuel_high is started, and the process proceeds to step S32.

On the other hand, when the determination in step S25 is NO and the base value of the target value of the O / C ratio determined according to the cetane number is smaller than the O / C ratio lower limit value, the process proceeds to step S30. Here, the base value of the target value of the O / C ratio is smaller than the lower limit value of the O / C ratio means that the reforming catalyst temperature and the O / C ratio are not changed unless the total amount of air and fuel is controlled to be reduced. This means that it cannot be within the optimum reaction region (see the circle in FIG. 3).
Therefore, in this step S30, the target value of the O / C ratio is set to the above-mentioned step so as to control the total amount of air and fuel to the decreasing side so that the reforming catalyst temperature and the O / C ratio are included in the optimum reaction region. The base value determined in S23 is changed to a predetermined value within the optimum O / C ratio band. Then, the target value of the air supply amount is reduced from the value Qair_normal set in step S21 to Qair_low, and the fuel is supplied so that the ratio of the air supply amount to the target value Qair_low becomes a predetermined value within the optimum O / C ratio band. The target value Qair_low of the supply amount is set, and the process proceeds to step S30.
In order to prevent the generation amount of the reducing gas from becoming insufficient with respect to the target value of the exhaust air / fuel ratio, the target value Qair_low of the air supply amount and the target value Qfuel_low of the fuel supply amount are respectively set to preset lower limits. Set to the weight loss side so as not to fall below the value.
In step S30, the target value of the air supply amount is changed from Qair_normal to Qair_low set in the above step, and supply of an amount of fuel corresponding to the target value Qfuel_low is started, and the process proceeds to step S32.

In step S32, the target value of the O / C ratio is corrected based on the deviation between the actual temperature of the reforming catalyst detected by the catalyst temperature sensor and the target temperature set in the optimum temperature range, and this processing is performed. Exit. More specifically, a correction value for the target value of the O / C ratio is calculated by searching a predetermined map based on the deviation of the reforming catalyst temperature, and this correction value is the target set in step S23. The target value of the O / C ratio is corrected by subtracting it from the base value of the value or the target value changed in steps S28 and S30.
FIG. 8 is a diagram showing the relationship between the correction value for the target value of the O / C ratio and the temperature deviation, and is a diagram showing a specific example of the map.
As shown in FIG. 8, the correction value when the temperature deviation is “0” is set to “0”, and the correction value is set to a larger value as the temperature deviation increases. Thus, as the actual temperature becomes higher than the target temperature, the target value of the O / C ratio is corrected to a smaller value, that is, the ratio of the fuel supply amount to the total amount is increased.

According to this embodiment, the following effects can be obtained.
(1) In the present embodiment, the air based on the estimation result of the cetane number of the fuel so that the reforming catalyst temperature is included in the optimal temperature range and the O / C ratio is included in the optimal O / C ratio range. Increase or decrease supply and fuel supply. Thereby, since the O / C ratio can be maintained within the optimum O / C ratio band, the yield of reducing gas, particularly hydrogen, can be kept high regardless of the cetane number of the fuel. At this time, the reforming catalyst temperature can also be maintained within the optimum temperature range, so that excessive amounts of coke can be prevented from being deposited on the reforming catalyst and the reforming catalyst can be prevented from deteriorating.

  (2) In this embodiment, the engine is equipped with sensors necessary for estimating the cetane number by estimating the cetane number of the fuel based on the ignition delay angle of the fuel injected into the combustion chamber of the engine. Therefore, it is not necessary to provide extra sensors in the fuel reformer 8. Therefore, the exhaust purification system 1 can be simplified.

(3) In this embodiment, the base value of the target value of the O / C ratio is calculated based on the estimation result of the cetane number so that the reforming catalyst temperature is included in the optimum temperature range. When the calculated target value base value is equal to or greater than the O / C ratio upper limit value, the reforming catalyst temperature is included in the optimum temperature range by controlling the total amount of air and fuel to the increase side, and , The O / C ratio can be included in the optimum O / C ratio band.
On the other hand, when the base value is smaller than the O / C ratio lower limit value, the reforming catalyst temperature is included in the optimum temperature range by controlling the total amount of air and fuel to the reduction side, and the O / C ratio is It can be included in the optimum O / C ratio band. As a result, the yield of the reducing gas, particularly hydrogen, can be kept high regardless of the cetane number of the fuel.

  (4) In this embodiment, the reducing gas supplied to the second catalytic converter 47 is excessively or deficient by increasing or decreasing the air supply amount and the fuel supply amount under the predetermined upper limit value and lower limit value. Can be prevented.

  The present invention is not limited to the embodiment described above, and various modifications can be made.

DESCRIPTION OF SYMBOLS 1 ... Exhaust purification system 2 ... Engine (internal combustion engine)
47 ... Second catalytic converter (catalytic converter)
7. ECU (fuel property estimation means, reformer control means)
8 ... Fuel reformer 82 ... Reforming catalyst

Claims (4)

A catalytic converter provided in an exhaust passage of the internal combustion engine for purifying NOx in the exhaust of the internal combustion engine in a reducing atmosphere;
Provided separately from the exhaust passage, supply reducing fuel by supplying fuel and air to the reforming catalyst, and supply the generated reducing gas to the upstream side of the catalytic converter in the exhaust passage. An exhaust purification system comprising a fuel reformer,
Fuel property estimation means for estimating the fuel property of the fuel supplied to the reforming catalyst;
Reformer control means for controlling an air supply amount and a fuel supply amount for the reforming catalyst,
The ratio of air supply to fuel supply is defined as the supply ratio,
The reformer control means includes:
Fuel property estimation result by the fuel property estimating means so that the temperature of the reforming catalyst is included in a predetermined temperature range and the supply ratio is included between a predetermined ratio upper limit value and a ratio lower limit value. The exhaust purification system is characterized in that the total amount of the air supply amount and the fuel supply amount is increased or decreased based on the above.   The exhaust gas purification system according to claim 1, wherein the fuel property estimating means estimates the fuel property of the fuel based on the combustion state of the fuel injected into the combustion chamber of the internal combustion engine. The reformer control means includes:
Calculating the target value of the supply ratio based on the estimation result of the fuel property by the fuel property estimating means so that the temperature of the reforming catalyst is included in the temperature range;
When the target value is equal to or higher than the ratio upper limit value, the temperature of the reforming catalyst is included in the temperature zone, and the supply ratio is included between the ratio upper limit value and the ratio lower limit value. And control the total amount of air supply and fuel supply to the increase side,
When the target value is smaller than the ratio upper limit value, the temperature of the reforming catalyst is included in the temperature zone, and the supply ratio is included between the ratio upper limit value and the ratio lower limit value. The exhaust purification system according to claim 1 or 2, wherein the total amount of the air supply amount and the fuel supply amount is controlled to the reduction side. The reformer control means includes:
4. The air supply amount and the fuel supply amount are increased so as not to exceed a predetermined upper limit value, and the air supply amount and the fuel supply amount are decreased so as not to fall below a predetermined lower limit value. The described exhaust purification system.

Images