Emission gas purification system

Abstract

An emission gas purification system is provided capable of making EM clearer and further improving NOx purification capacity. An emission gas purification system 1 is provided for an exhaust system of an engine 3, and includes a purification unit 16 having a catalyst for purifying NOx according to an air-fuel ratio of an emission gas, and an air-fuel ratio control unit 2 adjusting the air-fuel ratio of the emission gas supplied to the purification unit 16. The air-fuel ratio control unit 2 purifies the emission gas by setting the emission gas A/F to be near stoichiometric or rich by reducing an intake air mass in an air-fuel mixture.

Description

This application is based on and claims the benefit of priority from Japanese Patent Application No. 2006-317629, filed on Nov. 24, 2006, and No. 2007-175980, filed on Jul. 4, 2007 the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an emission gas purification system for purifying emission gas including nitrogen oxide (NOx).

2. Related Art

Conventionally, an emission gas purification system has been known that has an oxidation catalyst (DOC, Diesel Oxidation Catalyst) in an exhaust system in order to purify an emission gas including soluble organic fractions (SOFs) exhausted from an internal combustion engine. The emission gas purification system purifies the emission gas by oxidation of SOFs with an oxidation catalyst (DOC) when the air-fuel ratio of the emission gas is lean.

Another emission gas purification system has also been known that has a NOx purification catalyst in an exhaust system of an internal combustion engine (e.g., see Japanese Unexamined Patent Application, First Publication 2004-183568). This emission gas purification system purifies soluble organic fractions (SOFs) and CO when the air-fuel ratio of the emission gas is lean or near stoichiometric, and purifies NOx when the air-fuel ratio of the emission gas is near stoichiometric or rich.

Typically, the purification of NOx with an oxidation catalyst (DOC) is carried out by controlling the air-fuel ratio of the emission gas to be equal to or below stoichiometric. When the air-fuel ratio is equal to or below stoichiometric and the temperature of the oxidation catalyst (DOC) is sufficiently high, it is possible to purify NOx included in the emission gas by supplying a reducing agent to the oxidation catalyst (DOC). However, since the temperature of an emission gas exhausted from a compression ignition internal combustion engine is relatively low, it is difficult to keep the temperature of the oxidation catalyst (DOC) high. As a result, high NOx purification capacity is difficult to obtain.

Furthermore, the purification of NOx with an oxidation catalyst (DOC) is carried out only when the air-fuel ratio of the emission gas is below stoichiometric. Consequently, the NOx purification capacity decreases when the air-fuel ratio of the emission gas becomes leaner from stoichiometric due to transient drive and such.

In contrast, according to an emission gas purification system using a NOx purification catalyst, by oxygen adsorption capacity (OSC), it is possible to adsorb oxygen to the catalyst when the air-fuel ratio of the emission gas is lean. Accordingly, when the air-fuel ratio of the emission gas is below stoichiometric for purification of NOx, an oxidation reaction for the catalyst is enhanced and the catalyst temperature can be increased. As a result, it is possible to realize higher NOx purification capacity than in the case where the oxidation catalyst (DOC) is used.

Moreover, in the emission gas purification system using a NOx purification catalyst, even when the air-fuel ratio of the emission gas becomes slightly leaner from stoichiometric, the air-fuel ratio for the catalyst can be maintained at stoichiometric due to the oxygen adsorption capacity (OSC). Thus, it is possible to prevent the NOx purification capacity from decreasing.

In view of the above, the present invention aims to provide an emission gas purification system capable of realizing high NOx purification capacity, by providing an NOx purification catalyst in an exhaust emission path and controlling the air-fuel ratio of an internal combustion engine to be near stoichiometric or rich.

SUMMARY OF THE INVENTION

In order to address to the above problems, an emission gas purification system according to the present invention is provided for the exhaust system of a compression ignition internal combustion engine, and includes: a purification unit provided with a catalyst for purifying NOx according to an air-fuel ratio of an emission gas; and an air-fuel ratio control unit for adjusting the air-fuel ratio of the emission gas supplied to the purification unit, wherein the air-fuel ratio control unit purifies the emission gas by setting the air-fuel ratio of the emission gas to be near stoichiometric or rich, by reducing a charge efficiency of an air-fuel mixture in a cylinder.

According to the present invention, by decreasing the charge efficiency of the air-fuel mixture in the cylinder, the emission gas is purified by setting the air-fuel ratio of the emission gas to be near stoichiometric or rich. Accordingly, it is possible to improve emission gas purification capacity of the purification unit, thereby making EM clearer and improving NOx purification capacity.

Furthermore, in the emission gas purification system according to the present invention, it is preferable that the air-fuel ratio control unit further performs at least one of changing of a reflux rate of the emission gas and a throttling.

According to the present invention, by performing at least one of the changing of the reflux rate of the emission gas and the throttling, it is possible to purify the emission gas by reducing the intake air mass in the air-fuel mixture and setting the air-fuel ratio of the emission gas to be near stoichiometric or rich. Thus, it is possible to select a control method accordingly by considering advantages of each method.

Moreover, in the emission gas purification system according to the present invention, it is preferable that the purification unit is provided with a catalyst portion as an active material for purifying NOx, the catalyst portion containing at least one noble metal selected from the group consisting of Pt, Pd, and Rh, as well as a material having oxygen storage capacity.

At least one noble metal selected from the group consisting of Pt, Pd, and Rh exhibits high purification capacity to the emission gas. Furthermore, the material having oxygen storage capacity serves a function of a helping catalyst that absorbs the variation in the air-fuel ratio of the emission gas.

In addition, in the emission gas purification system according to the present invention, it is preferable that the material having oxygen storage capacity is a composite oxide containing one of CeO₂ or Ce.

The composite oxide containing one of CeO₂ or Ce can serve a function of storing and releasing oxygen. Specifically, while suppressing a decrease in the purification rate of HC and CO by releasing oxygen when rich, it is possible to suppress a decrease in the purification rate of NOx by storing oxygen when lean.

Furthermore, in the emission gas purification system according to the present invention, it is preferable that the active material contains at least Rh and at least one of Pt and Pd.

Among the at least one noble metal selected from the group consisting of Pt, Pd, and Rh, a noble metal containing Rh as an essential component is particularly effective in suppressing the decrease in the purification rate of NOx when rich. Accordingly, when containing any combination of Pt and Rh, Pd and Rh; and Pt, Pd, and Rh as the active material, it is possible to exhibit particularly high emission gas purification capacity.

Furthermore, the catalyst portion of the emission gas purification system according to the present invention preferably contains, as a carrier, at least one porous oxidative product selected from the group consisting of Al₂O₃, SiO₂, ZrO₂, and zeolite.

These porous oxidative products have large surface area and are stable in structure. Accordingly, with the catalyst portion having such a porous oxidative product as the carrier, it is possible to exhibit high emission gas purification capacity.

Furthermore, in the emission gas purification system according to the present invention, it is preferable that the carrier is a composite oxide having a perovskite structure.

The composite oxide having a perovskite structure can accept metal ions at an A site and B site. Accordingly, when using this composite oxide as the catalyst carrier, it is possible to control the catalyst activity by selecting a type of the metal, etc. Therefore, by using a composite oxide having the perovskite structure as the catalyst carrier, it is possible to provide an emission gas purification system having high emission gas purification capacity.

According to the present invention, the air-fuel ratio of the emission gas is set to be near stoichiometric or rich, and the emission gas purification capacity by the purification unit is improved, thereby making EM clearer and improving the NOx purification capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an emission gas purification system according to the present invention, along with an internal combustion engine;

FIG. 2 shows a portion of an emission gas purification system;

FIG. 3 shows a simplified diagram of the emission gas purification system as shown in FIG. 1; and

FIG. 4A and FIG. 4B each show diagrams for illustrating the effects of the emission gas purification system according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment according to the present invention is described with reference to the drawings.

FIG. 1 shows an emission gas purification system 1 according to the present invention, along with an internal combustion engine 3 (hereinafter referred to as “engine”). The engine 3 (compression ignition internal combustion engine) is, for example, a four-cylinder compression ignition internal combustion engine (only one of the cylinders is shown in the figure) installed in a vehicle (not shown in the figure).

A combustion chamber 3c is formed between a piston 3a and a cylinder head 3b of the engine 3. Intake plumbing 4 and an exhaust plumbing 5 (exhaust system) are connected to the cylinder head 3b, and a fuel injector 6 (hereinafter referred to as “injector”) is attached to the cylinder head 3b so as to face the combustion chamber 3c.

The injector 6 is positioned at a central portion of a top wall of the combustion chamber 3c, and connected sequentially to a high-pressure pump and a fuel tank (neither are shown in the figure) via a common line. The fuel consumption amount TOUT, which represents a valve opening time of the injector 6 is controlled by way of a driving signal from the ECU 2 (air-fuel ratio control unit) (see FIG. 2).

Furthermore, a crankshaft 3d of the engine 3 is provided with a magnetic rotor 30a, and a crank angle sensor 30 is constituted by this magnet rotor 30a and an MRE pickup 30b. The crank angle sensor 30 outputs a CRK signal and a TDC signal as pulse signals to the ECU 2, according to the rotation of the crankshaft 3d.

The CRK signal is outputted for every predetermined crank angle (30 degrees, for example). The ECU 2 derives a revolution speed NE of the engine 3 (hereinafter referred to as “engine revolutions”) based on the CRK signal. The TDC signal is a signal representing that the piston 3a of each cylinder is at a predetermined crank angle position in the vicinity of top dead center (TDC) when an intake stroke starts, and is outputted every 180 degrees of crank angle for a four-cylinder type engine of the present example.

The intake plumbing 4 is provided with a charge compressing device 7. The charge compressing device 7 is provided with the turbocharger unit 8 constituted by a turbocharger, an actuator 9 coupled to the turbocharger unit 8, and a vane opening control valve 10.

The turbocharger unit 8 includes rotatable compressor blade 8a provided for the intake plumbing 4, rotatable turbine blade 8b and a plurality of pivotal variable vane 8c (only two of these are shown in the drawing) that are provided for the exhaust plumbing 5, and a shaft 8d that integrally couples the compressor blade 8a and the turbine blade 8b. The turbocharger unit 8 performs a compressing operation, along with the turbine blade 8b being driven to rotate by an emission gas in the exhaust plumbing 5, to apply a pressure to intake air in the intake plumbing 4, by having the compressor blade 8a, which is integral with the turbine blade 8b, be driven to rotate.

The actuator 9 is of a diaphragm type that is actuated by a negative pressure, and is mechanically coupled to each of the variable vane 8c. The actuator 9 is supplied with a negative pressure via a negative pressure supply path from a negative pressure pump (neither of these are shown in the figure). In the middle of the negative pressure supply path, the vane opening control valve 10 is provided. The vane opening control valve 10 is constituted of a solenoid valve, and the negative pressure supplied to the actuator 9 varies by controlling the opening of the solenoid valve by a driving signal from the ECU 2. With this, a charge pressure is controlled by changing the opening of the variable vane 8c.

An air-to-water intercooler 11 and a throttle butterfly 12 are provided in the intake plumbing 4 on the downstream side from the turbocharger unit 8, in the stated order from the upstream side. The intercooler 11 serves to cool intake air in such a case in which the temperature of the intake air increases due to the compressing operation of the charge compressing device 7. The throttle butterfly 12 is connected to an actuator 12a constituted by a direct-current motor, for example. An opening TH of the throttle butterfly 12 (hereinafter referred to as “throttle butterfly opening”) is controlled by controlling a duty cycle of the current supplied to the actuator 12a with the ECU 2.

According to one preferred embodiment of the present invention, the charge efficiency of air-fuel mixture in the cylinder is decreased without throttling by changing the opening of the variable vane 8c without operating the throttle butterfly 12. More specifically, setting the variable vane 8c to an open state in accordance with the control by the ECU 2 according to an operation of an accelerator pedal (not shown in the drawing), without the operation of the throttle butterfly 12 decreases a pressure in the charge compressing unit and an intake air mass (decreases the charge efficiency of the air-fuel mixture in the cylinder), and an air-fuel ratio A/F of the emission gas is set so as to be near an ideal air-fuel ratio (hereinafter referred to as “stoichiometric”) or rich, thereby purifying the emission gas.

Furthermore, although it is not shown in the drawing, the engine 3 is provided with an intake (IN) valve for taking the air in an intake step and an exhaust (EX) valve for exhausting the air in an exhaust step. The IN valve and the EX valve are controlled to be opened and closed according to the control of the ECU 2.

Here, increasing an amount of overlap (a time period in which the IN valve and the EX valve are open at the same time) increases the air mass, and reducing the amount of overlap decreases the air mass. Moreover, expanding the amount of overlap decreases a pumping loss.

Thus, the emission gas purification system 1 according to the present invention can keep the air-fuel ratio A/F to be rich by controlling the IN valve and the EX valve using the ECU 2, and varying the charge efficiency to expand the amount of overlap. Furthermore, the change of the charge efficiency based on the valve timing of the IN valve and the EX valve does not accompany throttling, and therefore it is possible to reduce the pumping loss, and accordingly, the fuel consumption as well.

Moreover, the intake plumbing 4 is provided with an air flow sensor 31 on the upstream side from the turbocharger unit 8. In addition, an intake pressure sensor 32 is provided between the intercooler 11 and the throttle butterfly 12. The air flow sensor 31 detects an intake air mass QA, the intake pressure sensor 32 detects a charge pressure PACT in the intake plumbing 4. These detected signals are outputted to the ECU 2.

Furthermore, an intake manifold 4a of the intake plumbing 4 is partitioned into a swirl path 4b and a bypass path 4c from an assembled portion to a branched portion. The swirl path 4b and the bypass path 4c, respectively, communicate to each of the combustion chamber 3c via an intake port.

The bypass path 4c is provided with a swirling device 13 for producing a swirl in the combustion chamber 3c. The swirling device 13 is provided with a swirling valve 13a, an actuator 13b for opening and closing the swirling valve 13a, and a swirl control valve 13c. The actuator 13b and the swirl control valve 13c are constituted in the same manner as the actuator 9 and the vane opening control valve 10 of the charge compressing device 7, respectively. The swirl control valve 13c is connected to the negative pressure pump. According to the above described configuration, an opening of the swirl control valve 13c is controlled by a driving signal from the ECU 2 to change the negative pressure supplied to the actuator 13b, and the change in the opening of the swirling valve 13a controls the degree of the swirl.

In addition, the engine 3 is provided with an EGR device 14 having an EGR tube 14a and an EGR control valve 14b. The EGR tube 14a is connected between the intake plumbing 4 and the exhaust plumbing 5. More specifically, the EGR tube 14a is connected so as to couple the swirl path 4b at the assembled portion of the intake manifold 4a and the exhaust plumbing 5 on the upstream side of the turbocharger unit 8. Through the EGR tube 14a, a portion of the emission gas from the engine 3 flows back or is refluxed to the intake plumbing 4 as an EGR gas. With this, the combustion temperature within the combustion chamber 3c decreases, thereby reducing NOx in the emission gas.

The EGR control valve 14b is constituted by a linear electromagnetic valve attached to the EGR tube 14a. An amount of the EGR gas is controlled by adjusting an amount of valve lift VLACT of the EGR control valve 14b by the driving signal from the ECU 2 that has been duty-controlled.

Furthermore, the EGR device 14 is provided with an EGR cooling device 15 for cooling the EGR gas. The EGR cooling device 15 includes the bypass path 15a, an EGR path switching valve 15b, and an EGR cooler 15c. The bypass path 15a is provided on the downstream side of the EGR control valve 14b of the EGR tube 14a, so as to bypass the EGR tube 14a. The EGR path switching valve 15b is attached to the branched portion of the bypass path 15a. The EGR cooler 15c is attached to the bypass path 15a in the middle thereof. The EGR path switching valve 15b selectively switches between the side of the EGR tube 14a and the side of the bypass path 15a at a portion on the downstream side of the EGR path switching valve 15b according to the control by the ECU 2.

According to the above configuration, when the EGR path switching valve 15b switches to the side of the bypass path 15a, the EGR gas flows to the bypass path 15a to be cooled by the EGR cooler 15c, and then flows back to the intake plumbing 4. On the other hand, when the EGR path switching valve 15b switches to the other side, the EGR gas only passes through the EGR tube 14a and flows back to the intake plumbing 4 without being cooled.

Here, according to one preferred embodiment of the present invention, controlling the EGR control valve 14b to change an emission gas reflux rate (EGR rate) without operating the throttle butterfly 12 decreases the intake air mass within the air-fuel mixture without throttling. More specifically, in accordance with the control of the ECU 2 according to the operation of the accelerator pedal (not shown in the drawing), the EGR control valve 14b is set to the open state to increase the emission gas reflux rate (EGR rate), thereby decreasing the intake air mass. By this, the air-fuel ratio A/F of the emission gas is set to be near stoichiometric or rich in order to purify the emission gas.

Furthermore, a purification unit 16 is provided in the exhaust plumbing 5 on the downstream side of the turbocharger unit 8.

Under the atmosphere of stoichiometric, the purification unit 16 purifies the emission gas by oxidizing HC and CO in the emission gas and reducing NOx. Moreover, the purification unit 16 stores NOx in the emission gas in an oxidizing atmosphere where a level of oxygen in the emission gas is high. The stored NOx is reduced by a reducing agent in the emission gas in an oxidizing atmosphere where the oxygen level is low, and thus purified.

The following describes a composition of the catalyst used in the purification unit 16.

The purification unit 16 includes a material, as an active material that stores and purifies NOx, that contains at least one noble metal selected from the group consisting of Pt, Pd, and Rh, and that exhibits oxygen storage capacity (OSC). Moreover, it is preferable to use a composite oxide containing CeO₂ or Ce as the material that exhibits the oxygen storage capacity. It should be noted that the material exhibiting the oxygen storage capacity is used in order to maintain a high NOx purification rate by lessening the change in the oxygen level on a surface of the catalyst when the air-fuel ratio varies from rich to lean.

Furthermore, it is preferable that the active material contains at least Rh, and contains at least one of Pt and Pd. Containing Pt and Pd, as described above, can improve purification capacity for HC and CO on the lean side. In addition, Rh and such exhibit higher reducing capacity for NOx on the rich side or near stoichiometric.

A carrier may further contain at least one porous oxidative product selected from the group consisting of Al₂O₃, SiO₂, ZrO₂, and zeolite. In addition, the carrier can be a composite oxide having a perovskite structure.

Moreover, an amount of Pt, Pd, and Rh supported in total is from 0.1 g/L to 20 g/L, and preferably from 1 g/L to 10 g/L. Here, when the amount of Pt, Pd, and Rh supported in total is less than 0.1 g/L, the number of active sites of the noble metal decreases, and may not exhibit desirable purification capacity. On the other hand, when the amount supported is greater than 20 g/L, it is not possible to achieve an improvement in activity by the amount of noble metals that have increased, resulting in an increase in the cost. Furthermore, the amount of Pt, Pd, and Rh supported in total is preferably at least 1 g/L considering the cold activation upon starting up the engine 3, and no more than 10 g/L considering the dispensability of the noble metals in a washcoat.

Moreover, an amount of the composite oxides supported containing CeO₂ or Ce in total having oxygen storage capacity is from 1 g/L to 200 g/L, and preferably from 5 g/L to 150 g/L. Here, when the total amount of the composite oxides supported containing CeO₂ or Ce is smaller than 1 g/L, the amount of oxygen to be adsorbed and discharged is too small and atmosphere buffering capacity is not significantly exhibited. On the other hand, when the total amount of the composite oxides supported is greater than 200 g/L, a catalyst coating layer becomes too thick to be supported, and a pressure drop increases. In addition, the total amount of the composite oxides supported containing CeO₂ or Ce is preferably at least 5 g/L in order to obtain sufficient atmosphere buffering capacity, and is preferably no more than 150 g/L in terms of ease of support and suppressing an increase in the pressure drop.

Furthermore, an amount of the porous oxidative products supported is from 5 g/L to 300 g/L, and preferably from 10 g/L to 200 g/L. Here, the amount of the porous oxidative products supported smaller than 5 g/L is too small to support the noble metals in a dispersed manner and cause the noble metals to sinter. On the other hand, when the supported amount of the porous oxidative products is greater than 300 g/L, the catalyst coating layer becomes too thick to be supported, and a pressure drop increases. Moreover, the amount of the porous oxidative products supported is preferably at least 10 g/L in order to support the noble metal in a highly dispersed manner, and is preferably no more than 200 g/L in terms of ease of support and suppressing an increase in the pressure drop.

Furthermore, an A/F sensor 33 is provided on the upstream side of the purification unit 16 of the exhaust plumbing 5. The A/F sensor 33 linearly detects an oxygen level A/F in the emission gas in a wide range of air-fuel ratios from a rich region to a lean region. The ECU 2 calculates an actual air-fuel ratio A/FACT representing an air-fuel ratio of an actual gas that has been burned in the combustion chamber 3c, based on the oxygen level A/F detected by the A/F sensor 33. To the ECU 2, a detection signal representing an amount of operation (hereinafter referred to as “accelerator position”) AP of the accelerator pedal (not shown in the figure) is further inputted from an accelerator position sensor 35.

The ECU 2 is constituted by a microcomputer including an I/O interface, a CPU, RAM, ROM, etc. The detection signals from the various sensors 30 to 35 as described above are inputted into the CPU after going through A/D conversion or trimming at the I/O interface, respectively.

In response to the inputted signals, the CPU determines an drive state of the engine 3 according to a control program stored in the ROM, etc. Then, the CPU controls the engine 3 including the controls for fuel consumption amount and the intake air mass, according to the determined drive state.

Furthermore, the emission gas purification system 1 purifies the emission gas by setting an emission gas A/F to be near stoichiometric or rich by reducing the charge efficiency of the air-fuel mixture in the cylinder. Moreover, the emission gas purification system 1 purifies the emission gas by setting the emission gas A/F to be near stoichiometric or rich by further performing at least one of changing the reflux rate of the emission gas by the EGR control valve 14b and throttling by way of the throttle butterfly 12. More preferably, the emission gas purification system 1 can improve the purification capacity for NOx by the purification unit 16, while improving fuel consumption by way of controlling the intake air mass to the engine 3 without throttling, and setting the air-fuel ratio A/F of the emission gas either to be near stoichiometric or to be on the rich side.

The following describes a method of controlling the intake air mass to the engine 3, and setting the air-fuel ratio A/F of the emission gas either to be stoichiometric or to be on the rich side, with reference to FIG. 3, which is a simplified diagram of what is shown in FIG. 1. Below, the like components of FIG. 1 are indicated by the same names and numerals.

The emission gas purification system 1, as shown in FIG. 3, is provided with the intake plumbing 4 that introduces air into the engine 3, and the exhaust plumbing 5 that discharges the emission gas exhausted from the engine 3. The intake plumbing 4 is provided with the air flow sensor 31, the throttle butterfly 12, and the swirling valve 13a. The exhaust plumbing 5 is provided with the A/F sensor 33, the variable vane 8c, and the purification unit 16. Furthermore, the emission gas purification system 1 is provided with the EGR control valve 14b that refluxes a portion of the emission gas from the engine 3 to the intake plumbing 4 as the EGR gas, as well as the turbocharger unit 8 that compresses the intake air in the intake plumbing 4.

Furthermore, the EGR control valve 14b, the variable vane 8c, and the throttle butterfly 12 are opened and closed by the corresponding actuators. The corresponding actuators operate according to the control of the ECU 2. In addition, the ECU 2 performs the control of the air-fuel ratio of the emission gas supplied to the purification unit 16, by controlling any of the EGR control valve 14b, the variable vane 8c, and the throttle butterfly 12.

The following describes a relation between an increase and decrease of the air mass, the air-fuel ratio A/F, and the pumping loss according to the opening and closing operation of each actuator.

When the EGR control valve 14b is closed in response to the control of the ECU 2, the EGR rate decreases and the air mass increases. When the EGR control valve 14b is opened in response to the control of the ECU 2, the EGR rate increases and the air mass decreases. Accordingly, when the EGR control valve 14b is set to the open state, the air-fuel ratio A/F can be set to the rich side. Moreover, since the opening and closing operation of the EGR control valve 14b does not involve throttling, it is possible to reduce the pumping loss and the fuel consumption.

Furthermore, when the variable vane 8c of the turbocharger unit 8 is closed in response to the control of the ECU 2, the charge pressure increases and the air mass increases. When the variable vane 8c of the turbocharger unit 8 is opened in response to the control of the ECU 2, the charge pressure decreases and the air mass decreases. Accordingly, when the variable vane 8c are set to the open state, the air-fuel ratio A/F can be set to the rich side by changing the charge efficiency. Moreover, since the change in the charge efficiency by the opening and closing operation of the variable vane 8c does not involve throttling, it is possible to reduce the pumping loss and the fuel consumption.

Furthermore, the air mass decreases, when the throttle butterfly 12 is closed in response to the control of the ECU 2. The air mass increases, when the throttle butterfly 12 is opened in response to the control of the ECU 2. The throttle butterfly 12 is advantageous in that it is has excellent responsiveness because it throttles the intake air directly, and for is able to throttle the air in any drive state.

Moreover, the throttling by the throttle butterfly 12 is not a function commonly employed in a compression ignition internal combustion engine. By using the throttling function by the throttle butterfly 12, the emission gas purification system 1 according to the present invention can expand the maximum amount of the EGR gas that can be introduced (refer to Reason 1 below), and can improve the response (refer to Reason 2 below).

Reason 1

A differential pressure increases between before and after the EGR control valve 14b by decreasing an internal pressure in the intake manifold 4a by throttling (not negative). With this, it is possible to introduce a greater amount of EGR gas. This exhibits a particular effect in a low load region. In particular, this is an essential function in order to reduce PM and NOx at the same time by low temperature combustion.

Reason 2

The method of changing the charge pressure of the turbocharger unit 8 involves a time lag after the variable vane 8c are opened and closed until the revolution speed of the turbine blade 8b changes. Accordingly, the throttling by the throttle butterfly 12 is superior in responsiveness.

Furthermore, the method of introducing the EGR gas by the EGR control valve 14b employs lean combustion in which the engine 3 drives on the lean side. In this case, the EGR gas includes air, and time is required before settling at the intended air mass. Accordingly, the throttling by the throttle butterfly 12 is superior in responsiveness. Moreover, although it is preferable to decrease the charge pressure in terms with an amount of PM generation in a case in which the intake air mass is reduced in a high load region, closing an intake valve of the throttle butterfly 12 is faster in response, than opening the variable vane 8c of the turbocharger unit 8, in order to reduce the charging pressure, and thus advantageous.

Furthermore, examples of a timing at which the air-fuel ratio A/F of the emission gas is set to be stoichiometric or rich (i.e., reducing conditions) to purify the emission gas include, for example, a timing at which the temperature of the purification unit 16 and the load of the engine 3 are detected, and the detected temperature of the purification unit 16 becomes lower than a predetermined temperature, and the detected load of the engine 3 becomes higher than a predetermined load.

Thus, it is possible to improve the emission gas characteristics by setting the emission gas A/F to be near stoichiometric or rich (i.e., reduction condition) when the temperature of the purification unit 16 is lower than the predetermined temperature and the load of the engine 3 is higher than the predetermined load.

As described above, the emission gas purification system 1 purifies the emission gas by reducing the charge efficiency of the air-fuel mixture in the cylinder to set the emission gas A/F to be near stoichiometric or rich. Furthermore, the emission gas purification system 1 can set the emission gas A/F to be near stoichiometric or rich, and improve the purification capacity of the emission gas by the purification unit 16 by way of performing at least one of the change of the reflux rate of the emission gas by the EGR control valve 14b and the throttling by the throttle butterfly 12. Therefore, the emission gas purification system 1 can make the EM clearer and further improve the purification of NOx. In particular, when the configuration in which the charge efficiency of the air-fuel mixture in the cylinder is changed, and the reflux rate (EGR rate) of the emission gas is changed as needed, it is possible to reduce the pumping loss, as throttling is not involved. Accordingly, it is possible to improve the emission gas purification capacity while improving the fuel consumption. Moreover, emission gas purification capacity can be further improved in a case in which a configuration that exhibits superior purification capacity as described above is applied to a catalyst portion of the purification unit 16.

The present invention is not limited to the embodiment as described above, and can be implemented in various ways. Moreover, the present invention can be applied to various industrial internal combustion engines other than engines mounted to vehicles such as an engine for a ship propulsion apparatus such as an outboard engine in which a crankshaft is provided in the vertical direction. In addition, details of the present invention can be modified accordingly within the scope of the invention.

WORKING EXAMPLE

FIGS. 4A and 4B show the results of a comparison of NOx purification rates and catalyst temperatures when a rich spike (fuel rich combustion) is performed and measured for the purification unit 16 of the emission gas purification system 1 according to the present invention and a conventional purification unit (constituted by an oxidation catalyst (DOC, Diesel Oxidation Catalyst)). For the purification unit 16, a catalyst having an active material of about 10 g/L containing Pt, Pd, and Rh in a ratio of 2:5:1, a complex compound (Ce/Zr) of about 100 g/L, and a porous oxidative product (Al₂O₃) of about 100 g/L was used. Furthermore, for the conventional purification unit (DOC), a catalyst having about 10 g/L of an active material containing Pt and Pd in a ratio of 2:1, and about 100 g/L of a porous oxidative product (Al₂O₃) was used.

As shown in FIG. 4A, the purification unit 16 shows an improvement in the NOx purification rate compared to the conventional purification unit when the rich spike is performed (purification rate of almost 100% in FIG. 4A), and maintains a higher NOx purification rate that than that of a conventional purification unit during a certain period after the condition shifted from rich to lean (about 50 to 60 sec in FIG. 4A).

As shown in FIG. 4B, this is because the purification unit 16 can maintain a high temperature state during a certain period after the condition shifted from rich to lean (about 20 sec in FIG. 4B) compared to the conventional purification unit.

As described above, the purification unit 16 of the emission gas purification system 1 is constituted by the material having oxygen storage capacity, and therefore the purification unit 16 during the lean drive is in a state in which a certain amount of oxygen is always stored. Therefore, by setting the air-fuel ratio A/F of the emission gas to be rich, the oxidation reaction occurs between the reducing agent that flows into the purification unit 16 and the oxygen that is stored in the purification unit 16, thereby increasing the temperature of the purification unit 16 (see FIG. 4B). With this, it is possible to increase the temperature of the catalyst of the purification unit 16 up to a high temperature state in which high NOx purification capacity can be obtained, even from an drive state in which the temperature of the emission gas is low (lean state). In other words, it is possible to expand an drive region in which the NOx purification rate is improved by setting the air-fuel ratio A/F to be rich.

Moreover, since the purification unit 16 is constituted of a material having oxygen storage capacity, it is possible to maintain the high NOx purification rate for a certain period of time, even after the air-fuel ratio A/F of the emission gas is shifted from rich to lean.

While preferred embodiments of the present invention have been described and illustrated above, it is to be understood that they are exemplary of the invention and are not to be considered to be limiting. Additions, omissions, substitutions, and other modifications can be made thereto without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered to be limited by the foregoing description and is only limited by the scope of the appended claims.

Claims

1. An emission gas purification system for an exhaust system of a compression ignition internal combustion engine, the system comprising:

a purification unit provided with a catalyst for purifying NOx according to an air-fuel ratio of an emission gas; and

an air-fuel ratio control unit for adjusting the air-fuel ratio of the emission gas supplied to the purification unit, wherein

the air-fuel ratio control unit purifies the emission gas by setting the air-fuel ratio of the emission gas to be near stoichiometric or rich, by reducing a charge efficiency of an air-fuel mixture in a cylinder.

2. The emission gas purification system according to claim 1, wherein

the air-fuel ratio control unit further performs at least one of changing of a reflux rate of the emission gas and a throttling.

3. The emission gas purification system according to claim 1, wherein

the purification unit is provided with a catalyst portion as an active material for purifying NOx, the catalyst portion containing at least one noble metal selected from the group consisting of Pt, Pd, and Rh, as well as a material having oxygen storage capacity.

4. The emission gas purification system according to claim 2, wherein

the purification unit is provided with a catalyst portion as an active material for purifying NOx, the catalyst portion containing at least one noble metal selected from the group consisting of Pt, Pd, and Rh, as well as a material having oxygen storage capacity.

5. The emission gas purification system according to claim 3, wherein

the material having oxygen storage capacity is a composite oxide containing one of CeO2 or Ce.

6. The emission gas purification system according to claim 3, wherein

the active material contains at least Rh and at least one of Pt and Pd.

7. The emission gas purification system according to claim 4, wherein

the active material contains at least Rh and at least one of Pt and Pd.

8. The emission gas purification system according to claim 5, wherein

the active material contains at least Rh and at least one of Pt and Pd.

9. The emission gas purification system according to claim 3, wherein

the catalyst portion comprises at least one porous oxidative product selected from the group consisting of Al2O3, SiO2, ZrO2, and zeolite, as a carrier.

10. The emission gas purification system according to claim 5, wherein

the catalyst portion comprises at least one porous oxidative product selected from the group consisting of Al2O3, SiO2, ZrO2, and zeolite, as a carrier.

11. The emission gas purification system according to claim 6, wherein

the catalyst portion comprises at least one porous oxidative product selected from the group consisting of Al2O3, SiO2, ZrO2, and zeolite, as a carrier.

12. The emission gas purification system according to claim 9, wherein

the carrier is a composite oxide having a perovskite structure.

13. The emission gas purification system according to claim 4, wherein

the carrier is a composite oxide having a perovskite structure.

14. The emission gas purification system according to claim 6, wherein

the carrier is a composite oxide having a perovskite structure.