Particulate matter sensor

1. A particulate matter sensor for sensing particulate matter present in exhaust gasses flowing from an upstream end to a downstream end of an exhaust conduit for an internal combustion engine, said particulate matter sensor comprising:

a sensing element with a sensing face, said sensing face configured to extend into said exhaust conduit, and further configured to provide a signal indicative of an amount of particulate matter detected in said exhaust conduit;

an inner shield surrounding said sensing face and having an inner shield inlet passage for admitting exhaust gases into said inner shield to be sensed by said sensing face, said inner shield also having an exit passage to allow exhaust gases to exit said inner shield; and

an outer shield surrounding a portion of said inner shield to define an outer shield chamber extending axially beyond said inner shield, said outer shield having an outer shield inlet passage configured for communicating exhaust gases into said outer shield chamber from said upstream end of said exhaust conduit, wherein exhaust gases enter said inner shield from said outer shield chamber.

2. A particulate matter sensor as in claim 1 , wherein exhaust gases enter said inner shield only from said outer shield chamber.

3. A particulate matter sensor as in claim 1 , wherein exhaust gases flow out of said outer shield chamber only through said inner shield inlet passage.

4. A particulate matter sensor as in claim 1 , wherein said sensing face is configured to face toward said downstream end of said exhaust conduit.

5. A particulate matter sensor as in claim 4 , wherein said inner shield inlet passage is configured to face toward said downstream end of said exhaust conduit.

6. A particulate matter sensor as in claim 1 , wherein said exit passage is configured to face toward said downstream end of said exhaust conduit.

7. A particulate matter sensor as in claim 1 , wherein said exit passage is configured to communicate exhaust gases directly to said exhaust conduit from said inner shield.

8. A particulate matter sensor as in claim 1 , wherein said outer shield inlet passage is configured to face toward said upstream end of said exhaust conduit.

9. A particulate matter sensor as in claim 1 , wherein said outer shield inlet passage is sized to be substantially larger than said inner shield inlet passage.

10. A particulate matter sensor as in claim 1 further comprising a locating feature to radially orient said particulate matter sensor in a mounting boss of said exhaust conduit.

11. A particulate matter sensor as in claim 1 , wherein said inner shield inlet passage includes a flow director extending therefrom into said inner shield for directing exhaust gases to impinge on said sensing face.

12. A particulate matter sensor as in claim 1 , wherein said inner shield inlet passage is axially offset from said exit passage.

13. A particulate matter sensor as in claim 12 , wherein said outer shield is attached to the perimeter of said inner shield at a location axially between said inner shield inlet passage and said exit passage.

14. A particulate matter sensor as in claim 1 , wherein said sensing element is configured to extend to the exterior of said exhaust conduit.

15. A particulate matter sensor as in claim 1 , wherein said inner shield has an inner shield orientation feature that cooperates with an outer shield orientation feature of said outer shield to radially orient said inner shield with said outer shield.

16. An exhaust system for an internal combustion engine, said exhaust system having an exhaust conduit with an upstream end, a downstream end, and a particulate matter sensor for sensing particulate matter present in exhaust gasses flowing from said upstream end to said downstream, wherein said particulate matter sensor comprises:

a sensing element with a sensing face extending into said exhaust conduit and configured to provide a signal indicative of an amount of particulate matter detected in said exhaust conduit;

an inner shield surrounding said sensing face and having an inner shield inlet passage for admitting exhaust gases into said inner shield to be sensed by said sensing face, said inner shield also having an exit passage to allow exhaust gases to exit said inner shield;

an outer shield surrounding a portion of said inner shield to define an outer shield chamber extending axially beyond said inner shield, said outer shield having an outer shield inlet passage for communicating exhaust gases into said outer shield chamber from said upstream end of said exhaust conduit, wherein exhaust gases enter said inner shield from said outer shield chamber.

17. An exhaust system as in claim 16 , wherein exhaust gases enter said inner shield only from said outer shield chamber.

18. An exhaust system as in claim 16 , wherein exhaust gases flow out of said outer shield chamber only through said inner shield inlet passage.

19. An exhaust system as in claim 16 , wherein said sensing face faces toward said downstream end of said exhaust conduit.

20. An exhaust system as in claim 19 , wherein said inner shield inlet passage faces toward said downstream end of said exhaust conduit.

21. An exhaust system as in claim 16 , wherein said exit passage faces toward said downstream end of said exhaust conduit.

22. An exhaust system as in claim 16 , wherein said exit passage communicates exhaust gases directly to said exhaust conduit from said inner shield.

23. An exhaust system as in claim 16 , wherein said outer shield inlet passage faces toward said upstream end of said exhaust conduit.

24. An exhaust system as in claim 16 , wherein said outer shield inlet passage is sized to be substantially larger than said inner shield inlet passage.

25. An exhaust system as in claim 16 , wherein said particulate matter sensor further comprises a locating feature to radially orient said particulate matter sensor in a mounting boss of said exhaust conduit.

26. An exhaust system as in claim 16 , wherein said inner shield inlet passage includes a flow director extending therefrom into said inner shield for directing exhaust gases to impinge on said sensing face.

27. An exhaust system as in claim 16 , wherein said inner shield inlet passage is axially offset from said exit passage.

28. An exhaust system as in claim 27 , wherein said outer shield is attached to the perimeter of said inner shield at a location axially between said inner shield inlet passage and said exit passage.

29. An exhaust system as in claim 16 , wherein said sensing element extends to the exterior of said exhaust conduit.

30. An exhaust system as in claim 16 wherein said inner shield has an inner shield orientation feature that cooperates with an outer shield orientation feature of said outer shield to radially orient said inner shield with said outer shield.

31. An exhaust system as in claim 16 , wherein the axial center of said outer shield inlet passage is offset from the diametric center of said exhaust conduit by no more than 20 percent of the diameter of the exhaust conduit.

32. An exhaust system as in claim 31 , wherein the axial center of said outer shield inlet passage is offset from the diametric center of said exhaust conduit by no more than 10 percent of the diameter of the exhaust conduit.

33. An exhaust system as in claim 16 , wherein the diametric center of said exhaust conduit falls within said outer shield inlet passage.