Exhaust purification device for internal combustion engine

• US 6,237,330 B1
• Filed: 04/13/1999
• Issued: 05/29/2001
• Est. Priority Date: 04/15/1998
• Status: Active Grant

• Current Assignee: Nissan Motor Co., Ltd.
• Original Assignee: Nissan Motor Co., Ltd.
• Inventors: Takahashi, Hideaki, Nakamura, Takeshi, Nishizawa, Kimiyoshi
• Primary Examiner(s): Jeffery, John A.
• Application Number:
  US09/289,980
• Time in Patent Office: 777 Days
• Field of Search: 60/285, 60/302, 60/299, 123/443
• US Class Current: 60/285
• CPC Class Codes: B01D 53/9495 Controlling the catalytic process
• CPC Class Codes: F01N 13/009 having two or more separate purifying devices arranged in series
• CPC Class Codes: F01N 13/011 having two or more purifying devices arranged in parallel
• CPC Class Codes: F01N 13/107 More than one exhaust manifold or exhaust collector
• CPC Class Codes: F01N 2250/12 absorption or adsorption, and catalytic conversion
• CPC Class Codes: F01N 2570/04 Sulfur or sulfur oxides
• CPC Class Codes: F01N 2570/16 Oxygen
• CPC Class Codes: F01N 3/0842 Nitrogen oxides
• CPC Class Codes: F02D 2200/0818 SOx storage amount, e.g. for SOx trap or NOx trap
• CPC Class Codes: F02D 41/0055 Special engine operating conditions, e.g. for regeneration of exhaust gas treatment apparatus
• CPC Class Codes: F02D 41/0082 per groups or banks F02D41/0087 takes precedence
• CPC Class Codes: F02D 41/028 Desulfurisation of NOx traps or adsorbent
• CPC Class Codes: F02M 26/15 in relation to engine exhaust purifying apparatus
• CPC Class Codes: Y02T 10/12 Improving ICE efficiencies

Abstract

The temperature of a downstream catalyst is effectively increased while avoiding temperature increase of an upstream catalyst, so as to remove SOx from the downstream catalyst. For this purpose, a first front three-way catalyst 21 of a first exhaust gas passage 5a connected to a first cylinder group, a second front three-way catalyst 22 of a second exhaust gas passage 5b connected to a second cylinder group, and a rear three-way catalyst 23 installed in an exhaust gas passage 5c which combines the exhaust of both of these exhaust gas passages, are provided. When the conditions hold for performing temperature increase of the rear three-way catalyst 23, the control unit 11 sets the air-fuel ratio of the exhaust supplied to one front catalyst to richer than the stoichiometric air-fuel ratio, and sets the air-fuel ratio of the exhaust gas supplied to the other front catalyst to leaner than the stoichiometric air-fuel ratio. Due to this, unburnt fuel and oxygen which passed through the front catalysts flow into the rear catalyst, promote the reactions in the rear catalyst so as to raise its temperature, and thereby promote discharge and reduction of SOx.

Claims

1. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased, and

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst;

a first air-fuel ratio sensor installed in said first exhaust passage; and

a second air-fuel ratio sensor installed in said second exhaust passage,

wherein said first air-fuel ratio controller computes a first feedback correction coefficient based on the output of said first air-fuel ratio sensor, corrects a fuel amount supplied to said first cylinder group using this first feedback correction coefficient, and thereby feedback controls the air-fuel ratio of exhaust led to said first front three-way catalyst, and

wherein said second air-fuel ratio controller computes a second feedback correction coefficient based on the output of said second air-fuel ratio sensor, corrects a fuel amount supplied to said second cylinder group using this second feedback correction coefficient, and thereby feedback controls the air-fuel ratio of exhaust led to said second front three-way catalyst,

wherein said first and second sensors are so designed that their outputs vary sharply around the stoichiometric air-fuel ratio relative to variation of the air-fuel ratio of the exhaust gas,

wherein said first air-fuel ratio controller computes a first feedback correction coefficient by subtracting a first lean shift proportional part when the output of said first air-fuel ratio sensor changes from lean to rich, and adding a first rich shift proportional part when it changes from rich to lean,

wherein said second air-fuel ratio controller computes a second feedback correction coefficient by subtracting a second lean shift proportional part when the output of said second air-fuel ratio sensor changes from lean to rich, and adding a second rich shift proportional part when it changes from rich to lean, and

wherein said microprocessor is further programmed to set a control midpoint value of the air-fuel ratio of exhaust gas controlled by said first and second air-fuel ratio controllers, to a predetermined air-fuel ratio by setting said first lean shift proportional part, said rich shift proportional part, said second lean shift proportional part and said second rich shift proportional part respectively to predetermined values.

2. An exhaust purification device for an internal combustion engine as defined in claim 1, further comprising

a rear air-fuel ratio sensor installed in said exhaust passage, and

a rear air-fuel ratio controller for correcting the setting of the air-fuel ratio by said microprocessor based on the output of said rear air-fuel ratio sensor, and controlling the air-fuel ratio of the exhaust led to said rear three-way catalyst to a predetermined air-fuel ratio.

3. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear thruway catalyst should be increased, and

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst;

a first air-fuel ratio sensor installed in said first exhaust passage; and

a second air-fuel ratio sensor installed in said second exhaust passage,

wherein said first air-fuel ratio controller computes a first feedback correction coefficient based on the output of said first air-fuel ratio sensor, corrects a fuel amount supplied to said first cylinder group using this first feedback correction coefficient, and thereby feedback controls the air-fuel ratio of exhaust led to said first front three-way catalyst, and

wherein said second air-fuel ratio controller computes a second feedback correction coefficient based on the output of said second air-fuel ratio sensor, corrects a fuel amount supplied to said second cylinder group using this second feedback correction coefficient, and thereby feedback controls the air-fuel ratio of exhaust led to said second front three-way catalyst,

wherein said first and second sensors are so designed that their outputs vary sharply around the stoichiometric air-fuel ratio relative to variation of the air-fuel ratio of the exhaust gas,

wherein said first air-fuel ratio controller computes a first feedback correction coefficient by subtracting a first lean shift proportional part after a first lean shift delay time from when the output of said first air-fuel ratio sensor changes from lean to rich, and adding a first rich shift proportional part after a first rich shift delay time from when it changes from rich to lean,

wherein said second air-fuel ratio controller computes a second feedback correction coefficient by likewise subtracting a second lean shift proportional part after a second lean shift delay time from when the output of said second air-fuel ratio sensor changes from lean to rich, and adding a second rich shift proportional part after a second rich shift delay time from when it changes from rich to lean, and

wherein said microprocessor is further programmed to set a control midpoint of the air-fuel ratio of the exhaust gas controlled by said first and second air-fuel ratio controllers, to a predetermined air-fuel ratio by setting said first lean shift delay time, said first rich shift delay time, said second lean shift delay time and said second rich shift delay time respectively to predetermined values.

4. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio; and

a microprocessor programmed to;

determine the conditions under which the temperature of the rear thruway catalyst should be increased, and

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst,

wherein said microprocessor is further programmed to alternate between a first setting mode wherein the air-fuel ratio of the exhaust controlled by said first air-fuel ratio controller is set leaner than the stoichiometric air-fuel ratio and the air-fuel ratio of the exhaust controlled by said second air-fuel ratio controller is set richer than the stoichiometric air-fuel ratio, and a second setting mode wherein the air-fuel ratio of the exhaust controlled by said first air-fuel ratio controller is set richer than the stoichiometric air-fuel ratio and the air-fuel ratio of the exhaust controlled by said second air-fuel ratio controller is set leaner than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of the rear three-way catalyst, and

wherein said microprocessor is further programmed to alternate between said first setting mode and said second setting mode whenever a running time in one of these setting modes reaches a predetermined time.

5. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased,

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst,

alternate between said first setting mode and said second setting mode whenever a running time in one of these setting modes reaches a predetermined time, and

determine conditions under which Sox deposited on said rear three-way catalyst should be discharged, and determine that the conditions hold for increasing the temperature of said rear three-way catalyst when the conditions hold for discharging Sox which has been absorbed and deposited on said rear three-way catalyst, and

a startup detecting sensor for detecting startup of said internal combustion engine,

wherein said microprocessor is further programmed to determine that the conditions hold for discharging SOx which has been absorbed and deposited on said rear three-way catalyst when startup of said engine is detected.

6. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio; and

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased,

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst,

alternate between said first setting mode and said second setting mode whenever a running time in one of these setting modes reaches a predetermined time, and

determine conditions under which Sox deposited on said rear three-way catalyst should be discharged, and determine that the conditions hold for increasing the temperature of said rear three-way catalyst when the conditions hold for discharging Sox which has been absorbed and deposited on said rear three-way catalyst, and

estimate an SOx amount which has been absorbed and deposited on said rear three-way catalyst, and determine conditions for discharging SOx deposited on said rear three-way catalyst based on said estimated SOx deposit amount.

7. An exhaust purification device for an internal combustion engine as defined in claim 6, wherein

said microprocessor is further programmed to determine that the conditions hold for discharging SOx deposited on said rear three-way catalyst when said estimated SOx deposition amount is equal to or greater than a first predetermined amount, and determine that SOx discharge conditions no longer hold when said estimated SOx deposition amount has fallen to or below a second predetermined amount less than said first predetermined amount.

8. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased,

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst,

alternate between said first setting mode and said second setting mode whenever a running time in one of these setting modes reaches a predetermined time, and

determine conditions under which Sox deposited on said rear three-way catalyst should be discharged, and determine that the conditions hold for increasing the temperature of said rear three-way catalyst when the conditions hold for discharging Sox which has been absorbed and deposited on said rear three-way catalyst, and

an NOx concentration sensor for detecting the NOx concentration of the exhaust flowing from said rear three-way catalyst, and

wherein said microprocessor is further programmed to estimate an NOx amount absorbed on said rear three-way catalyst, and determine that conditions hold for discharging the SOx deposited on said rear three-way catalyst based on said detected NOx concentration and said estimated NOx absorption amount.

9. An exhaust purification device for an internal combustion engine as defined in claim 8, wherein

said microprocessor is further programmed to determine that the conditions hold for discharging deposited SOx when the NOx concentration, detected at a time when the NOx amount absorbed on said rear three-way catalyst is equal to or greater than a predetermined amount, is greater than a predetermined permitted value.

10. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased,

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst,

alternate between said first setting mode and said second setting mode whenever a running time in one of these setting modes reaches a predetermined time, and

determine conditions under which Sox deposited on said rear three-way catalyst should be discharged, and determine that the conditions hold for increasing the temperature of said rear three-way catalyst when the conditions hold for discharging Sox which has been absorbed and deposited on said rear three-way catalyst, and

a running condition detecting sensor for detecting running conditions,

wherein said microprocessor is further programmed to determine that the conditions do not hold for increasing the temperature of said rear three-way catalyst when the detected running conditions are outside a predetermined SOx discharge running region, regardless of whether or not the conditions hold for discharging SOx deposited on said rear three-way catalyst.

11. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased,

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst,

alternate between said first setting mode and said second setting mode whenever a running time in one of these setting modes reaches a predetermined time, and

determine conditions under which Sox deposited on said rear three-way catalyst should be discharged, and determine that the conditions hold for increasing the temperature of said rear three-way catalyst when the conditions hold for discharging Sox which has been absorbed and deposited on said rear three-way catalyst, and

a temperature measuring sensor for detecting a temperature of said rear three-way catalyst,

wherein said microprocessor is further programmed to determine that the conditions do not hold for increasing the temperature of said rear three-way catalyst when the temperature of said three-way catalyst exceeds a permitted temperature even when the temperature of said rear three-way catalyst is equal to or greater than a predetermined SOx discharge temperature, regardless of whether or not the conditions hold for discharging SOx deposited on said rear three-way catalyst.

12. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased,

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst,

alternate between said first setting mode and said second setting mode whenever a running time in one of these setting modes reaches a predetermined time, and

determine conditions under which Sox deposited on said rear three-way catalyst should be discharged, and determine that the conditions hold for increasing the temperature of said rear three-way catalyst when the conditions hold for discharging Sox which has been absorbed and deposited on said rear three-way catalyst, and

a temperature measuring sensor for detecting a temperature of said rear three-way catalyst,

wherein said microprocessor is further programmed to set the air-fuel ratios of the exhaust controlled by said first and second air-fuel ratio controllers such that the air-fuel ratio of the exhaust led to said rear three-way catalyst is richer than the stoichiometric air-fuel ratio when the conditions hold for discharging SOx deposited on said rear three-way catalyst, and the temperature of said rear three-way catalyst is equal to or greater than an SOx discharge temperature, and

wherein said microprocessor is further programmed to set the degree of richness of the air-fuel ratio to be greater the larger the SOx amount deposited on said rear three-way catalyst.

13. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased,

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst,

alternate between said first setting mode and said second setting mode whenever a running time in one of these setting modes reaches a predetermined time, and

determine conditions under which Sox deposited on said rear three-way catalyst should be discharged, and determine that the conditions hold for increasing the temperature of said rear three-way catalyst when the conditions hold for discharging Sox which has been absorbed and deposited on said rear three-way catalyst, and

a temperature measuring sensor for detecting a temperature of said rear three-way catalyst,

wherein said microprocessor is further programmed to set the air-fuel ratios of the exhaust controlled by said first and second air-fuel ratio controllers such that the air-fuel ratio of the exhaust led to said rear three-way catalyst is richer than the stoichiometric air-fuel ratio when the conditions hold for discharging SOx deposited on said rear three-way catalyst, and the temperature of said rear three-way catalyst is equal to or greater than an SOx discharge temperature, and

wherein said microprocessor is further programmed to set the degree of richness of the air-fuel ratio to be smaller the longer the elapsed time from when air-fuel ratio rich shift control starts.

14. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased, and

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst, and

an exhaust valve opening timing control device for applying an advance correction to an opening timing of an exhaust valve which is preset according to running conditions when the conditions hold for increasing the temperature of said rear three-way catalyst.

15. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased, and

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst, and

an exhaust gas recirculation control device for applying a reduction correction to an exhaust gas recirculation amount which is preset according to running conditions when the conditions hold for increasing the temperature of said rear three-way catalyst.

16. An exhaust purification device for an internal combustion engine comprising:

a first front three-way catalyst installed in a first exhaust passage connected to a first cylinder group;

a second front three-way catalyst installed in a second exhaust passage connected to a second cylinder group;

a rear three-way catalyst installed in an exhaust gas passage which combines the exhaust of said first and second exhaust passages;

a first air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said first front three-way catalyst to a predetermined air-fuel ratio;

a second air-fuel ratio controller for controlling the air-fuel ratio of the exhaust led to said second front three-way catalyst to a predetermined air-fuel ratio;

a microprocessor programmed to;

determine the conditions under which the temperature of the rear three-way catalyst should be increased, and

set the air-fuel ratio of the exhaust controlled by one of said air-fuel ratio controllers to a richer air-fuel ratio than the stoichiometric air-fuel ratio, and set the air-fuel ratio of the exhaust controlled by the other of said air-fuel ratio controllers to a leaner air-fuel ratio than the stoichiometric air-fuel ratio, when the conditions hold for increasing the temperature of said rear three-way catalyst, and

an ignition timing control device for applying a delay correction to an ignition timing which is preset according to running conditions when the conditions hold for increasing the temperature of said rear three-way catalyst.