Method and device for purifying exhaust gas of an engine

• US 6,345,496 B1
• Filed: 02/10/2000
• Issued: 02/12/2002
• Est. Priority Date: 11/09/1995
• Status: Expired due to Term

• Current Assignee: Toyota Jidosha Kabushiki Kaisha (Toyota Motor Corporation)
• Original Assignee: Toyota Jidosha Kabushiki Kaisha (Toyota Motor Corporation)
• Inventors: Suzuki, Naoto, Takeuchi, Kouichi, Igarashi, Kouhei, Yaegashi, Takehisa, Tanaka, Hiroshi, Itou, Takaaki, Fuwa, Naohide, Kinugasa, Yukio
• Primary Examiner(s): TRAN, BINH Q
• Application Number:
  US09/502,127
• Time in Patent Office: 733 Days
• Field of Search: 602/74, 602/76, 602/86, 602/85, 603/01, 602/80, 606/98, 602/97, 602/89, 603/03, 123/443
• US Class Current: 60/274
• CPC Class Codes: B01D 2255/102 Platinum group metals
• CPC Class Codes: B01D 2255/20761 Copper
• CPC Class Codes: B01D 2255/50 Zeolites
• CPC Class Codes: B01D 2255/911 NH3-storage component incorporated in the catalyst
• CPC Class Codes: B01D 53/8696 Controlling the catalytic process
• CPC Class Codes: B01D 53/9409 Nitrogen oxides
• CPC Class Codes: B01D 53/9422 for removing nitrogen oxides by NOx storage or reduction by cyclic switching between lean and rich exhaust gases (LNT, NSC, NSR)
• CPC Class Codes: B01D 53/9431 Processes characterised by a specific device
• CPC Class Codes: B01D 53/9445 Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC]
• CPC Class Codes: B01D 53/9454 characterised by a specific device
• CPC Class Codes: B01D 53/9477 with catalysts positioned on separate bricks, e.g. exhaust systems
• 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 2240/25 an ammonia generator
• CPC Class Codes: F01N 2250/12 absorption or adsorption, and catalytic conversion
• CPC Class Codes: F01N 2570/18 Ammonia
• CPC Class Codes: F01N 2610/02 the substance being ammonia or urea
• CPC Class Codes: F01N 2610/03 the substance being hydrocarbons, e.g. engine fuel
• CPC Class Codes: F01N 3/0842 : Nitrogen oxides
• CPC Class Codes: F01N 3/0871 : Regulation of absorbents or adsorbents, e.g. purging by electrically controlling the supply of combustible mixture or its constituents only F02D41/0235
• CPC Class Codes: F01N 3/20 : specially adapted for catalytic conversion F01N3/22 takes precedence; Methods of operation or control of catalytic converters
• CPC Class Codes: F01N 3/2053 : By-passing catalytic reactors, e.g. to prevent overheating
• CPC Class Codes: F01N 9/00 : Electrical control of exhaust gas treating apparatus monitoring or diagnostic devices for exhaust-gas treatment apparatus F01N11/00; conjoint electrical control of two or more combustion engine functions F02D43/00
• CPC Class Codes: F02B 1/04 : with fuel-air mixture admission into cylinder
• CPC Class Codes: F02D 41/027 : to purge or regenerate the exhaust gas treating apparatus
• CPC Class Codes: F02D 41/0275 : the exhaust gas treating apparatus being a NOx trap or adsorbent
• CPC Class Codes: F02D 41/1443 : with one sensor per cylinder or group of cylinders
• CPC Class Codes: F02D 41/1475 : Regulating the air fuel ratio at a value other than stoichiometry
• CPC Class Codes: Y02T 10/12 : Improving ICE efficiencies
• CPC Class Codes: Y02T 10/40 : Engine management systems

Abstract

An exhaust manifold (7) of an engine (1) is connected to a three way (TW) catalyst (8a), and the TW catalyst (8a) is connected to an NH₃ adsorbing and oxidizing (NH₃-AO) catalyst (10a). The engine (1) performs the lean and the rich engine operations alternately and repeatedly. When the engine (1) performs the rich operation and thereby the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst (8a) is made rich, NO_x in the inflowing exhaust gas is converted to NH₃ in the TW catalyst (8a). The NH₃ is then adsorbed in the NH₃-AO catalyst (10a). Next, when the engine (1) performs the lean operation and thereby the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst (8a) is made lean, NO_x in the exhausted gas passes through the TW catalyst (8a), and flows into the NH₃-AO catalyst (10a). At this time, NH₃ adsorbed in the catalyst (10a) is desorbed therefrom, and reduces the inflowing NO_x.

Claims

1. The method for purifying an exhaust gas of an engine, the method comprising the steps of:

alternately and repeatedly controlling an exhaust gas air-fuel ratio between a lean exhaust gas air-fuel ratio and a rich exhaust gas air-fuel ratio; and

introducing the exhaust gas to an NH₃ synthesizing catalyst, the NH₃ synthesizing catalyst being arranged in series with a NO_x occluding and reducing (NO_x-OR) catalyst in a common exhaust passage, the NH₃ synthesizing catalyst synthesizing NH₃ from NO_x in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio is rich, and synthesizing almost no NH₃ from NO_x in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio is lean and wherein, when the exhaust gas air-fuel ratio is lean, the NO_x-OR catalyst occludes NO_x therein, and when the exhaust gas air-fuel ratio is rich, the NOR-OR catalyst releases the occluded NO_x therefrom and reduces the NO_x.

2. The method according to claim 1, further comprising the step of controlling an engine air-fuel ratio between a lean engine air-fuel ratio to thereby form the lean exhaust gas air-fuel ratio and a rich engine air-fuel ratio to thereby form the rich exhaust gas air-fuel ratio.

3. The method according to claim 2, wherein the engine air-fuel ratio is made lean for a lean period, and the engine air-fuel ratio is made rich for a rich period, and wherein the lean and the rich periods are set in accordance with the engine operating condition, respectively.

4. The method according to claim 3, wherein the engine is provided with a plurality of cylinders or cylinder groups of which the exhaust stroke periods are different to each other, and wherein the lean period is set as the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made lean, and the rich period is set as the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made rich.

5. The method according to claim 4, wherein the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made lean and the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made rich is set to vary the cylinders or cylinder groups of which the engine air-fuel ratio is to be made rich in every cycle, the cycle being formed by one lean period and one rich period which are successive.

6. The method according to claim 4, wherein the lean and the rich period are controlled in accordance with a number ratio which is a ratio of the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made lean to the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made rich, the number ratio being set in accordance with the engine operating condition.

7. A method according to claim 2, the engine being an internal combustion engine of spark ignition type, and the method further comprising the step of controlling the ignition timing of the engine in accordance with the engine air-fuel ratio of the engine.

8. The method according to claim 1, wherein a making-rich device, different from the engine, is arranged in an exhaust passage of the engine upstream of the NH₃ synthesizing catalyst for making the exhaust gas air-fuel ratio rich, and wherein the method further comprises the steps of keeping an engine air-fuel ratio lean, and stopping a making-rich operation of the making-rich device to thereby form the lean exhaust gas air-fuel ratio, and performing the making-rich operation of the making-rich device to thereby form the rich exhaust gas air-fuel ratio.

9. The method according to claim 8, wherein the making-rich device is provided with:

a combustor of which an air-fuel ratio is controllable; and

an introducing passage for introducing the exhaust gas of the combustor into the exhaust passage upstream of the NH₃ synthesizing catalyst, and wherein the making-rich operation of the making-rich device is performed by making an air-fuel ratio of the combustor rich and adding the exhaust gas of the combustor to the exhaust gas of the engine.

10. The method according to claim 9, wherein the combustor is an auxiliary internal combustion engine having a crank shaft different from that of the engine.

11. The method according to claim 9, wherein the combustor is a burner.

12. The method according to claim 8, wherein the making-rich device is provided with a reducing agent injector for feeding a reducing agent into the exhaust passage upstream of the NH₃ synthesizing catalyst, the making-rich operation of the reducing agent injector being performed by adding the reducing agent to the exhaust gas of the engine.

13. The method according to claim 1, the step of estimating an amount of NO_x occluded in the NO_x-OR catalyst, and wherein the exhaust gas air-fuel ratio is changed from lean to rich or from rich to lean in accordance with the estimated occluded NO_x amount.

14. The method according to claim 13, wherein the occluded NO_x amount is estimated on the basis of a NO_x amount flowing into the NO_x-OR catalyst when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-OR catalyst is lean, and on the basis of the released NO_x amount when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-OR catalyst is rich.

15. The method according to claim 1, wherein the exhaust gas air-fuel ratio is changed from lean to rich or from rich to lean gradually with a changing rate.

16. The method according to claim 15, wherein the changing rate when the exhaust gas air-fuel ratio is changed from lean to rich is larger than that when the exhaust gas air-fuel ratio is changed from rich to lean.

17. The method according to claim 15, wherein the changing rate is set in accordance with the engine operating condition.

18. The method according to claim 17, wherein the changing rate becomes smaller as the engine load becomes higher.

19. The method according to claim 1, wherein the exhaust gas air-fuel ratio is set in accordance with the engine operating condition.

20. The method according to claim 1, wherein a period during which the exhaust gas air-fuel ratio is lean is longer than that during which the exhaust gas air-fuel ratio is rich is formed.

21. The method according to claim 1, wherein the exhaust gas air-fuel ratio is made equal to a target air-fuel ratio on the basis of output signals of an air-fuel ratio sensor arranged in an exhaust passage of the engine adjacent to an inlet of the NH₃ synthesizing catalyst.

22. The method according to claim 1, wherein the exhaust gas purifying catalyst is comprised of at least the NO_x-OR catalyst, and wherein the NH₃ synthesizing catalyst and the NO_x-OR catalyst are carried on a common substrate.

23. The method according to claim 1 further comprising the step of introducing the exhaust gas to both of the NO_x-OR catalyst and an NH₃ adsorbing and oxidizing (NH₃-AO) catalyst, after introducing the exhaust gas to the NH₃ synthesizing catalyst, the NH₃-AO catalyst adsorbing NH₃ in the exhaust gas therein, and causing a reaction of NH₃ and NO_x in the NH₃-AO catalyst to purify the NH₃ and the NO_x and to decrease an amount of NH₃ adsorbed in the NH₃-AO catalyst when the exhaust gas includes NO_x therein and the exhaust gas air-fuel ratio is lean.

24. The method according to claim 23, wherein the NH₃-AO catalyst is arranged in the exhaust passage downstream of the NO_x-OR catalyst.

25. The method according to claim 24, the engine further having a muffler arranged in the exhaust passage, wherein the NH₃-AO catalyst is housed in the muffler.

26. The method according to claim 23, further comprising the step of estimating an amount of NH₃ adsorbed in the NH₃-AO catalyst, wherein the exhaust gas air-fuel ratio is changed from lean to rich or from rich to lean in accordance with the estimated adsorbed NH₃ amount.

27. The method according to claim 26, wherein the adsorbed NH₃ amount is estimated on the basis of an NH₃ amount flowing into the NH₃-AO catalyst when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst is rich, and on the basis of the decreased NH₃ amount when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst is lean.

28. The method according to claim 23, wherein the NH₃-AO catalyst comprises a solid acid carrying one of a transition metal and a precious metal.

29. The method according to claim 1, further comprising the step of introducing the exhaust gas to both of the NO_x-OR catalyst and an NH₃ adsorbing and oxidizing (NH₃-AO) catalyst, after introducing the exhaust gas to the NH₃ synthesizing catalyst, the NH₃-AO catalyst adsorbing NH₃ in the exhaust gas therein and causing a reaction of NH₃ and NO_x in the NH₃-AO catalyst to purify the NH₃ and the NO_x and to decrease an amount of NH₃ adsorbed in the NH₃-AO catalyst when the exhaust gas includes NO_x therein and the exhaust gas air-fuel ratio is lean, and wherein the NH₃-AO catalyst and the NO_x-OR catalyst are laminated to each other on a common substrate.

30. The method according to claim 1, wherein the NH₃ synthesizing catalyst is a three-way catalyst including at least one precious metal.

31. The method according to claim 1, wherein the NO_x-OR catalyst includes:

at least one substance selected from alkali metals, alkali earth metals, rare earth metals such as lanthanum and yttrium, and transition metals and precious metals.

32. The method according to claim 1, further comprising the step of introducing the exhaust gas to an NH₃ purifying catalyst for purifying NH₃ in the exhaust gas, after introducing the exhaust gas to the NO_x-OR catalyst.

33. The method according to claim 32, wherein the NH₃ purifying catalyst includes at least one substance selected from precious metals, and transition metals.

34. The method according to claim 32, the NH₃ purifying catalyst purifying NH₃ under the oxidizing atmosphere, wherein the method further the step of comprises keeping the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ purifying catalyst lean.

35. The method according to claim 1, wherein the NH₃ synthesizing catalyst is a NO_x occluding and NH₃ synthesizing (NO_x-NH₃) catalyst, the NO_x-NH₃ catalyst occluding NO_x in the exhaust gas therein when the exhaust gas air-fuel ratio is lean, the NO_x-NH₃ catalyst releasing occluded NO_x therefrom, reducing NO_x in the NO_x-NH₃ catalyst, and synthesizing NH₃ from NO_x in the NO_x-NH₃ catalyst when the exhaust gas air-fuel ratio is rich.

36. The method according to claim 35, further comprising the step of estimating an amount of NO_x occluded in the NO_x-NH₃ catalyst, and wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-NH₃ catalyst is made lean and rich alternately and repeatedly in accordance with the estimated occluded NO_x amount.

37. The method according to claim 35, further comprising the step of estimating an amount of NO_x occluded in the NO_x-OR catalyst, and wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-NH₃ catalyst is made lean and rich alternately and repeatedly in accordance with the estimated occluded NO_x amount.

38. The method according to claim 35, wherein the NO_x-NH₃ catalyst includes:

at least one substance selected from alkali metals, alkali earth metals, rare earth metals, and transition metals; and

precious metals.

39. The method according to claim 35, wherein the NO_x-OR catalyst includes:

at least one substance selected from alkali metals, alkali earth metals, rare earth metals, and transition metals; and

precious metals.

40. The device for purifying an exhaust gas of an engine having an exhaust passage, comprising:

exhaust gas air-fuel ratio controlling means arranged in one of the engine and the exhaust passage for alternately and repeatedly controlling an exhaust gas air-fuel ratio between a lean exhaust gas air-fuel ratio and a rich exhaust gas air-fuel ratio;

an NH₃ synthesizing catalyst arranged in the exhaust passage downstream of the exhaust gas air-fuel ratio controlling means, the NH₃ synthesizing catalyst synthesizing NH₃ from NO_x in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio is rich, and synthesizing almost no NH₃ from NO_x in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio is lean; and

a NO_x occluding and reducing (NO_x-OR) catalyst arranged in the exhaust passage downstream of the NH₃ synthesizing catalyst, the NO_x-OR catalyst occluding NO_x in the exhaust gas therein when the exhaust gas air-fuel ratio is lean, and releasing the occluded NO_x therefrom and reducing the NO_x when the exhaust gas air-fuel ratio is rich.

41. The device according to claim 40, wherein the exhaust gas air-fuel ratio controlling means comprises an engine air-fuel ratio control means for controlling an engine air-fuel ratio between a lean engine air-fuel ratio to thereby form the lean exhaust gas air-fuel ratio and a rich engine air-fuel ratio to thereby form the rich exhaust gas air-fuel ratio.

42. The device according to claim 41, wherein the engine air-fuel ratio control means makes the engine air-fuel ratio lean for a lean period, and makes the engine air-fuel ratio rich for a rich period, and wherein the lean and the rich periods are set in accordance with the engine operating condition, respectively.

43. The device according to claim 42, wherein the engine is provided with a plurality of cylinders or cylinder groups of which the exhaust stroke periods are different to each other, and wherein the lean period is set as the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made lean, and the rich period is set as the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made rich.

44. The device according to claim 43, wherein the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made lean and the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made rich are set to vary the cylinders or cylinder groups of which the engine air-fuel ratio is to be made rich in every cycle, the cycle being formed by one lean period and one rich period which are successive.

45. The device according to claim 43, wherein the lean and the rich period are controlled in accordance with a number ratio which is a ratio of the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made lean to the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made rich, the number ratio being set in accordance with the engine operating condition.

46. The device according to claim 41, the engine being an internal combustion engine of spark ignition type, and the device further comprising ignition timing control means for controlling the ignition timing of the engine in accordance with the engine air-fuel ratio.

47. The device according to claim 40, wherein the exhaust gas air-fuel ratio controlling means comprises making-rich means, different from the engine, arranged in the exhaust passage upstream of the NH₃ synthesizing catalyst for making the exhaust gas air-fuel ratio rich, and keeping means for keeping the engine air-fuel ratio lean, and wherein the making-rich means stops a making-rich operation thereof to thereby form the lean exhaust gas air-fuel ratio, and performs the making-rich operation thereof to thereby form the rich exhaust gas air-fuel ratio.

48. The device according to claim 47, wherein the making-rich means comprises:

a combustor of which an air-fuel ratio is controllable; and

an introducing passage for introducing the exhaust gas of the combustor into the exhaust passage upstream of the NH₃ synthesizing catalyst, and wherein the making-rich means performs the making-rich operation thereof by making an air-fuel ratio of the combustor rich and adding the exhaust gas of the combustor to the exhaust gas of the engine.

49. The device according to claim 48, wherein the combustor is an auxiliary internal combustion engine having a crank shaft different from that of the engine.

50. The device according to claim 48 wherein the combustor is a burner.

51. The device according to claim 47, wherein the making-rich means comprises a reducing agent injector for feeding a reducing agent into the exhaust passage upstream of the NH₃ synthesizing catalyst, the reducing agent injector performing the making-rich operation thereof by adding the reducing agent to the exhaust gas of the engine.

52. The device according to claim 40, wherein the device further comprises occluded NO_x amount estimating means for estimating an amount of NO_x occluded in the NO_x-OR catalyst, and wherein the exhaust gas air fuel ratio controlling means changes the exhaust gas air-fuel ratio from lean to rich or from rich to lean in accordance with the estimated occluded NO_x amount.

53. The device according to claim 52, wherein the occluded NO_x amount estimating means estimates the occluded NO_x amount on the basis of a NO_x amount flowing into the NO_x-OR catalyst when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-OR catalyst is lean, and on the basis of the released NO_x amount when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-OR catalyst is rich.

54. The device according to claim 40, wherein the exhaust gas air fuel ratio controlling means changes the exhaust gas air-fuel ratio from lean to rich or from rich to lean gradually with a changing rate.

55. The device according to claim 54, wherein the changing rate when the exhaust gas air-fuel ratio controlling means changes the exhaust gas air-fuel ratio from lean to rich is larger than that when the exhaust gas air-fuel ratio controlling means changes the exhaust gas air-fuel ratio from rich to lean.

56. The device according to claim 54, wherein the changing rate is set in accordance with the engine operating condition.

57. The device according to claim 56, wherein the changing rate becomes smaller as the engine load becomes higher.

58. The device according to claim 40, wherein the exhaust gas air-fuel ratio is set in accordance with the engine operating condition.

59. The device according to claim 40, wherein a period during which the exhaust gas air-fuel ratio controlling means forms the lean exhaust gas air-fuel ratio is longer than that during which the exhaust gas air-fuel ratio controlling means forms the rich exhaust gas air-fuel ratio.

60. The device according to claim 40, further comprising an air-fuel ratio sensor arranged in the exhaust passage between the exhaust gas air-fuel ratio controlling means and the NH₃ synthesizing catalyst, and wherein the exhaust gas air-fuel ratio controlling means makes the exhaust gas air-fuel ratio equal to a target air-fuel ratio on the basis of the output signals of the air-fuel ratio sensor.

61. The device according to claim 40, wherein the NH₃ synthesizing catalyst and the NO_x-OR catalyst are carried on a common substrate.

62. The device according to claim 40, further comprising an NH₃ adsorbing and oxidizing (NH₃-AO) catalyst arranged in the exhaust passage downstream of the NH₃ synthesizing catalyst, the NH₃-AO catalyst adsorbing NH₃ in the exhaust gas therein, and causing a reaction of NH₃ and NO_x in the NH₃-AO catalyst to purify the NH₃ and the NO_x and to decrease an amount of NH₃ adsorbed in the NH₃-AO catalyst when the exhaust gas includes NO_x therein and the exhaust gas air-fuel ratio is lean.

63. The device according to claim 62, wherein the NH₃-AO catalyst is arranged in the exhaust passage downstream of the NO_x-OR catalyst.

64. The device according to claim 63, the engine further having a muffler arranged in the exhaust passage, wherein the NH₃-AO catalyst is housed in the muffler.

65. The device according to claim 62, further comprising adsorbed NH₃ amount estimating means for estimating an amount of NH₃ adsorbed in the NH₃-AO catalyst, and wherein the exhaust gas air-fuel ratio controlling means changes the exhaust gas air-fuel ratio from lean to rich or from rich to lean in accordance with the estimated adsorbed NH₃ amount.

66. The device according to claim 65, wherein the adsorbed NH₃ amount is estimated on the basis of an NH₃ amount flowing into the NH₃-AO catalyst when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst is rich, and on the basis of the decreased NH₃ amount when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst is lean.

67. The device according to claim 62, wherein the NH₃-AO catalyst comprises a solid acid carrying one of a transition metal and a precious metal.

68. The device according to claim 40, further comprising an NH₃ adsorbing and oxidizing (NH₃-AO) catalyst arranged in the exhaust passage downstream of the NH₃ synthesizing catalyst, the NH₃-AO catalyst adsorbing NH₃ in the exhaust gas therein, and causing a reaction of NH₃ and NO_x in the NH₃-AO catalyst to purify the NH₃ and the NO_x and to decrease an amount of NH₃ adsorbed in the NH₃-AO catalyst when the exhaust gas includes NO_x therein and the exhaust gas air-fuel ratio is lean, and wherein the NH₃-AO catalyst and the NO_x-OR catalyst are laminated to each other on a common substrate.

69. The device according to claim 40, wherein the NH₃ synthesizing catalyst is a three-way catalyst including at least one precious metal.

70. The device according to claim 40, wherein the NO_x OR catalyst includes:

at least one substance selected from alkali metals, alkali earth metals, rare earth metals, and transition metals; and

precious metals.

71. The device according to claim 40, further comprising an NH₃ purifying catalyst arranged downstream of the exhaust gas purifying catalyst for purifying NH₃ in the inflowing exhaust gas.

72. The device according to claim 71, wherein the NH₃ purifying catalyst includes at least one substance selected from precious metals and transition metals.

73. The device according to claim 71, the NH₃ purifying catalyst purifying NH₃ under the oxidizing atmosphere, wherein the device further comprises keeping-lean means arranged in the exhaust passage between the exhaust gas purifying catalyst and the NH₃ purifying catalyst for keeping the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ purifying catalyst lean.

74. The device according to claim 40, wherein the NH₃ synthesizing catalyst is a NO_x occluding and NH₃ synthesizing (NO_x-NH₃) catalyst, the NO_x-NH₃ catalyst occluding NO_x in the exhaust gas therein when the exhaust gas air-fuel ratio is lean, the NO_x-NH₃ catalyst releasing the occluded NO_x therefrom, reducing the NO_x in the NO_x-NH₃ catalyst and synthesizing NH₃ from NO_x in the NO_x-NH₃ catalyst when the exhaust gas air-fuel ratio is rich.

75. The device according to claim 74, further comprising occluded NO_x amount estimating means for estimating an amount of NO_x occluded in the NO_x-NH₃ catalyst, and wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-NH₃ catalyst is made lean and rich alternately and repeatedly in accordance with the estimated occluded NO_x amount.

76. The device according to claim 74, further comprising occluded NO_x amount estimating means for estimating an amount of NO_x occluded in the NO_x-OR catalyst, and wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-NH₃ catalyst is made lean and rich alternately and repeatedly in accordance with the estimated occluded NO_x amount.

77. The device according to claim 74, wherein the NO_x-NH₃ catalyst includes:

at least one substance selected from alkali metals, alkali earth metals, rare earth metals, and transition metals; and

precious metals.

78. The device according to claim 74, wherein the NO_x-OR catalyst includes:

at least one substance selected from alkali metals, alkali earth metals, rare earth metals, and transition metals; and

precious metals.

79. The method for purifying an exhaust gas of an engine, the method comprising the steps of:

alternately and repeatedly controlling an exhaust gas air-fuel ratio between a lean exhaust gas air-fuel ratio and a rich exhaust gas air-fuel ratio; and

introducing the exhaust gas to a NH₃ synthesizing catalyst, the NH₃ synthesizing catalyst synthesizing NH₃ from NO_x in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio is rich and synthesizing almost no NH₃ from NO_x in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio is lean and wherein a NH₃ adsorbing and oxidizing (NH₃-AO) catalyst is arranged downstream of the NH₃ synthesizing catalyst in a common exhaust passage therewith, and wherein, when the exhaust gas air-fuel ratio is lean, the NH₃-AO catalyst adsorbs NH₃ in the exhaust gas therein, causes a reaction of NH₃ and NO_x in the NH₃-AO catalyst to purify the NH₃ and the NO_x and decreases an amount of NH₃ adsorbed in the NH₃-AO catalyst when the exhaust gas includes NO_x therein.

80. The method according to claim 79, further comprising the step of controlling an engine air-fuel ratio between a lean engine air-fuel ratio to thereby form the lean exhaust gas air-fuel ratio and a rich engine air-fuel ratio to thereby form the rich exhaust gas air-fuel ratio.

81. The method according to claim 80, wherein the engine air-fuel ratio is made lean for a lean period, and the engine air-fuel ratio is made rich for a rich period, and wherein the lean and the rich periods are set in accordance with the engine operating condition.

82. The method according to claim 81, wherein the engine is provided with a plurality of cylinders or cylinder groups of which the exhaust stroke periods are different to each other, and wherein the lean period is set as the number of the cylinders or cylinder groups of which the engine air-fuel ratio is made to be lean, and the rich period is set as the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made rich.

83. The method according to claim 80, the engine being an internal combustion engine of spark ignition type, and the method further comprising the step of controlling the ignition timing of the engine in accordance with the engine air-fuel ratio.

84. The method according to claim 79, wherein a making-rich device, different from the engine, is arranged in the exhaust passage of an engine upstream of the NH₃ synthesizing catalyst for making the exhaust gas air-fuel ratio rich, and wherein the method further comprises the steps of keeping an engine air-fuel ratio lean, and stopping a making-rich operation of the making-rich device to thereby form the lean exhaust gas air-fuel ratio, and performing the making-rich operation of the making-rich device to thereby form the rich exhaust gas air-fuel ratio.

85. The method according to claim 84, wherein the making rich device is provided with:

a combustor of which an air-fuel ratio is controllable; and

an introducing passage for introducing the exhaust gas of the combustor into the exhaust passage upstream of the NH₃ synthesizing catalyst, and wherein the making-rich operation of the making-rich device is performed by making an air-fuel ratio of the combustor rich and adding the exhaust gas of the combustor to the exhaust gas of the engine.

86. The method according to claim 84, wherein the making-rich device is provided with a reducing agent injector for feeding a reducing agent into the exhaust passage upstream of the NH₃ synthesizing catalyst, the making-rich operation of the reducing agent injector being performed by adding the reducing agent to the exhaust gas of the engine.

87. The method according to claim 79, further comprising the step of estimating an amount of NH₃ adsorbed in the NH₃-AO catalyst, wherein the exhaust gas air-fuel ratio is changed from lean to rich or from rich to lean in accordance with the estimated adsorbed NH₃ amount.

88. The method according to claim 87, wherein the adsorbed NH₃ amount is estimated on the basis of an NH₃ amount flowing into the NH₃-AO catalyst when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst is rich, and on the basis of the decreased NH₃ amount when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst is lean.

89. The method according to claim 79, wherein the exhaust gas air-fuel ratio is changed from lean to rich or from rich to lean gradually with a changing rate.

90. The method according to claim 79, wherein the exhaust gas air-fuel ratio is set in accordance with the engine operating condition.

91. The method according to claim 79, wherein the exhaust gas air-fuel ratio is made equal to a target air-fuel ratio on the basis on output signals of an air-fuel ratio sensor arranged in an exhaust passage of the engine adjacent to an inlet of the NH₃ synthesizing catalyst.

92. The method according to claim 79, the engine further having a muffler arranged in an exhaust passage thereof, wherein the NH₃-AO catalyst is housed in the muffler.

93. The method according to claim 79, wherein the NH₃ synthesizing catalyst is a three-way catalyst including at least one precious metal.

94. The method according to claim 79, further comprising the step of introducing the exhaust gas to a NH₃ purifying catalyst for purifying NH₃ in the exhaust gas, after introducing the exhaust gas to the NH₃-AO catalyst.

95. The method according to claim 94, wherein the NH₃ purifying catalyst includes at least one substance selected from precious metals and transition metals.

96. The method according to claim 79, wherein the NH₃-AO catalyst comprises a solid acid carrying one of a transition metal and a precious metal.

97. The method according to claim 79, wherein the NH₃ synthesizing catalyst is a NO_x occluding and NH₃ synthesizing (NO_x-NH₃) catalyst, the NO_x-NH₃ catalyst occluding NO_x in the exhaust gas therein when the exhaust gas air-fuel ratio is lean, the NO_x-NH₃ catalyst releasing the occluded NO_x therefrom, reducing the NO_x in the NO_x-NH₃ catalyst and synthesizing NH₃ from NO_x in the NO_x-NH₃ catalyst when the exhaust gas air-fuel ratio is rich.

98. The method according to claim 97, further comprising the step of estimating an amount of NO_x occluded in the NO_x-NH₃ catalyst, and wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-NH₃ catalyst is made lean and rich alternately and repeatedly in accordance with the estimated occluded NO_x amount.

99. The method according to claim 97, further comprising the step of estimating an amount of NH₃ adsorbed in the NH₃-AO catalyst, and wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-NH₃ catalyst is made lean and rich alternately and repeatedly in accordance with the estimated adsorbed NH₃ amount.

100. The method according to claim 97, wherein the NO_x-NH₃ catalyst includes:

at least one substance selected from alkali metals, alkali earth metals, rare earth metals, and transition metals; and

precious metals.

101. The method according to claim 97, wherein the NH₃-AO catalyst comprises solid acid carrying one of transition metals and precious metals.

102. The device for purifying an exhaust gas of an engine having an exhaust passage, comprising:

exhaust gas air-fuel ratio controlling means arranged in one of the engine and the exhaust passage for alternately and repeatedly controlling an exhaust gas air-fuel ratio between a lean exhaust gas air-fuel ratio and a rich exhaust gas air-fuel ratio;

an NH₃ synthesizing catalyst arranged in the exhaust passage downstream of the exhaust gas air-fuel ratio controlling means, the NH₃ synthesizing catalyst synthesizing NH₃ from NO_x in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio is rich, and synthesizing almost no NH₃ from NO_x in the NH₃ synthesizing catalyst when the exhaust gas air-fuel ratio is lean; and

an NH₃ adsorbing and oxidizing (NH₃-AO) catalyst arranged in the exhaust passage downstream of the NH₃ synthesizing catalyst, the NH₃-AO catalyst adsorbing NH₃ in the exhaust gas therein, and causing a reaction of NH₃ and NO_x in the NH₃-AO catalyst to purify the NH₃ and the NO_x and to decrease an amount of NH₃ adsorbed in the NH₃-AO catalyst when the exhaust gas includes NO_x therein and the exhaust gas air-fuel ratio is lean.

103. The device according to claim 102, wherein the exhaust gas air-fuel ratio controlling means comprises an engine air-fuel ratio control mean for controlling an engine air-fuel ratio to make the engine air-fuel ratio lean to thereby form the lean exhaust gas air-fuel ratio and to make the engine air-fuel ratio rich to thereby form the rich exhaust gas air-fuel ratio.

104. The device according to claim 103, wherein the engine air-fuel ratio control means makes the engine air-fuel ratio lean for a lean period, and makes the engine air-fuel ratio rich for a rich period, and wherein the lean and the rich periods are set in accordance with the engine operating condition.

105. The device according to claim 104, wherein the engine is provided with a plurality of cylinders or cylinder groups of which the exhaust stroke periods are different to each other, and wherein the lean period is set as the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made lean, and the rich period is set as the number of the cylinders or cylinder groups of which the engine air-fuel ratio is to be made rich.

106. The device according to claim 103, the engine being an internal combustion engine of spark ignition type, and the device further comprising ignition timing control means for controlling the ignition timing of the engine in accordance with the engine air-fuel ratio.

107. The device according to claim 102, wherein the exhaust gas air-fuel ratio controlling means comprises making-rich means, different from the engine, arranged in the exhaust passage upstream of the NH₃ synthesizing catalyst for making the exhaust gas air-fuel ratio rich, and keeping means for keeping the engine air-fuel ratio lean, and wherein the making-rich means stops a making-rich operation thereof to thereby form the lean exhaust gas air-fuel ratio, and performs the making-rich operation thereof to thereby form the rich exhaust gas air-fuel ratio.

108. The device according to claim 107, wherein the making-rich means comprises:

a combustor of which an air-fuel ratio is controllable; and

an introducing passage for introducing the exhaust gas of the combustor into the exhaust passage upstream of the NH₃ synthesizing catalyst, and wherein the making-rich means performs the making-rich operation thereof by making an air-fuel ratio of the combustor rich and adding the exhaust gas of the combustor to the exhaust gas of the engine.

109. The device according to claim 107, wherein the making-rich means comprise a reducing agent injector for feeding a reducing agent into the exhaust passage upstream of the NH₃ synthesizing catalyst, the reducing agent injector performing the making-rich operation thereof by adding the reducing agent to the exhaust gas of the engine.

110. The device according to claim 102, further comprising adsorbed NH₃ amount estimating means for estimating an amount of NH₃ adsorbed in the NH₃-AO catalyst, and wherein the exhaust gas air-fuel ratio controlling means changes the exhaust gas air-fuel ratio from lean to rich or from rich to lean in accordance with the estimated adsorbed NH₃ amount.

111. The device according to claim 110, wherein the adsorbed NH₃ amount estimating means estimates the adsorbed NH₃ amount on the basis of an NH₃ amount flowing into the NH₃-AO catalyst when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst is rich, and on the basis of the decreased NH₃ amount when the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH₃ synthesizing catalyst is lean.

112. The device according to claim 102, wherein the exhaust gas air-fuel ratio controlling means changes the exhaust gas air-fuel ratio from lean to rich or from rich to lean gradually with a changing rate.

113. The device according to claim 102, wherein the exhaust gas air-fuel ratio is set in accordance with the engine operating condition.

114. The device according to claim 102, further comprising an air-fuel ratio sensor arranged in the exhaust passage between the exhaust gas air-fuel ratio controlling means and the NH₃ synthesizing catalyst, and wherein the exhaust gas air-fuel ratio controlling means makes the exhaust gas air-fuel ratio equal to a target air-fuel ratio on the basis of the output signals of the air-fuel ratio sensor.

115. The device according to claim 102, engine further having a muffler arranged in the exhaust passage, wherein the NH₃-AO catalyst is housed in the muffler.

116. The device according to claim 102, wherein the NH₃ synthesizing catalyst is a three-way catalyst including at least one precious metal.

117. The device according to claim 102, further comprising an NH₃ purifying catalyst arranged downstream of the exhaust gas purifying catalyst for purifying NH₃ in the exhaust gas.

118. The device according to claim 117, wherein the NH₃ purifying catalyst includes at least one substance selected from precious metals and transition metals.

119. The device according to claim 102, wherein the NH₃-AO catalyst comprises a solid acid carrying a one of a transition metal and a precious metal.

120. The device according to claim 102, wherein the NH₃ synthesizing catalyst is a NO_x occluding and NH₃ synthesizing (NO_x-NH₃) catalyst, the NO_x-NH₃ catalyst occluding NO_x in the exhaust gas when the exhaust gas air-fuel ratio is lean, the NO_x-NH₃ catalyst releasing the occluded NO_x therefrom, reducing the NO_x in the NO_x-NH₃ catalyst and synthesizing NH₃ from NO_x in the NO_x-NH₃ catalyst when the exhaust gas air-fuel ratio is rich.

121. The device according to claim 120, further comprising occluded NO_x amount estimating means for estimating an amount of NO_x occluded in the NO_x-NH₃ catalyst, and wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-NH₃ catalyst is made lean and rich alternately and repeatedly in accordance with the estimated occluded NO_x amount.

122. The device according to claim 120, further comprising adsorbed NH₃ amount estimating means for estimating an amount of NH₃ adsorbed in the NH₃-AO catalyst, and wherein the exhaust gas air-fuel ratio of the exhaust gas flowing into the NO_x-NH₃ catalyst is made lean and rich alternately and repeatedly in accordance with the estimated adsorbed NH₃ amount.

123. The device according to claim 120, wherein the NO_x-NH₃ catalyst includes:

at least one substance selected from alkali metals, alkali earth metals, rare earth metals, and transition metals; and

precious metals.

124. The device according to claim 120, wherein the NH₃-AO catalyst comprises solid acid carrying one of transition metals and precious metals.