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Methods and systems are provided for determining degradation of a particulate filter in an exhaust conduit. In one example, a method may include diverting exhaust gas to a secondary soot sensor assembly downstream of a first filter, comprising a second filter and determining degradation based on tim

Methods and systems are provided for determining degradation of a particulate filter in an exhaust conduit. In one example, a method may include diverting exhaust gas to a secondary soot sensor assembly downstream of a first filter, comprising a second filter and determining degradation based on time intervals between subsequent filter regenerations of the second filter in the secondary soot sensor assembly.

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1. A method, comprising: flowing exhaust gas from downstream of a first filter into each of a first venturi coupled inside an exhaust pipe and a second venturi coupled in a passage external to the exhaust pipe, the passage including a second filter coupled to an electric circuit; and indicating degr

1. A method, comprising: flowing exhaust gas from downstream of a first filter into each of a first venturi coupled inside an exhaust pipe and a second venturi coupled in a passage external to the exhaust pipe, the passage including a second filter coupled to an electric circuit; and indicating degradation of the first filter based on an interval between successive regenerations of the second filter. 2. The method of claim 1, further comprising, in response to the indication, limiting an engine speed or load. 3. The method of claim 1, wherein the indicating includes indicating degradation responsive to the interval between successive regenerations of the second filter being lower than a threshold duration. 4. The method of claim 3, wherein the interval is measured from initiation of a first regeneration event of the second filter to initiation of a second, immediately subsequent regeneration event of the second filter. 5. The method of claim 3, further comprising, regenerating the second filter responsive to a ratio of flow rates through the first and second venturi being higher than an upper threshold. 6. The method of claim 5, wherein the first venturi is larger than the second venturi, and wherein the ratio of flow rates through the first and second venturi is based on a first pressure at a motive inlet of the first venturi relative to a second pressure at the motive inlet of the second venturi. 7. The method of claim 6, wherein the first pressure is estimated by a first pressure sensor coupled to the motive inlet of the first venturi, and wherein the second pressure is estimated by a second pressure sensor coupled to the motive inlet of the second venturi. 8. The method of claim 5, wherein regenerating the second filter includes closing a switch of the electric circuit and flowing electricity through the second filter until ratio of flow rates through the first and second venturi is lower than a lower threshold. 9. The method of claim 1, wherein the first filter is a larger diesel or gasoline particulate matter filter having a higher soot capacity, and wherein the second filter is a smaller metal filter having a lower soot capacity, and wherein the indicating includes indicating the first filter is leaking by setting a diagnostic code. 10. The method of claim 1, wherein the second filter is coupled downstream of the second venturi, and wherein flowing exhaust into the second venturi includes: flowing exhaust gas from the exhaust pipe into an inlet pipe, and from the inlet pipe into the passage, the inlet pipe converging with the passage at a location external to the exhaust pipe, and from the passage into the exhaust pipe via an outlet pipe, the passage converging into the outlet pipe at a location downstream of the second filter and external to the exhaust pipe. 11. The method of claim 10, wherein a direction of flowing exhaust gas through the inlet pipe and the outlet pipe is substantially perpendicular to a direction of exhaust flow through each of the exhaust pipe, and the first and second venturi. 12. An engine exhaust system, comprising: an exhaust pipe including a first venturi tube coupled downstream of a first particulate filter;a soot detection system including an inlet pipe and an outlet pipe coupled to the exhaust pipe, downstream of the first particulate filter, the inlet pipe merging into a second venturi tube external to the exhaust pipe, the outlet pipe merging out of the second venturi tube external to the exhaust pipe; a second particulate filter coupled between a motive outlet of the second venturi tube and the outlet pipe, the second particulate filter coupled to a source of electricity via a switch;one or more sensors for estimating a flow rate through each of the first and second venturi tubes; anda controller with computer readable instructions stored on non-transitory memory for: flowing a first portion of exhaust gas from downstream of the first filter through the first venturi tube;flowing a remaining portion of exhaust gas through the second venturi tube;regenerating the second filter based on a ratio of flow rates through the first and second venturi tubes; andadjusting engine operation based on a time elapsed between successive regenerations of the second filter. 13. The system of claim 12, wherein the inlet pipe comprises a plurality of perforations on a side of the inlet pipe within the exhaust pipe and proximate to the first filter, a diameter of the perforations adjusted to enable conglomerated particulates to impinge on a side of the inlet pipe within the exhaust pipe and distal to the first filter, the conglomerated particulates released from the inlet pipe into the exhaust pipe via a perforation on a bottom of the inlet pipe. 14. The system of claim 12, wherein the one or more sensors includes a first pressure sensor coupled between a motive inlet and neck of the first venturi tube for estimating the flow rate through the first venturi tube, and a second pressure sensor coupled between the motive inlet and neck of the second venturi tube for estimating the flow rate through the second venturi tube. 15. The system of claim 14, wherein the first venturi tube is a larger venturi tube with a higher flow rate, and the second venturi tube is a smaller venturi tube with a lower flow rate. 16. The system of claim 12, wherein regenerating the second filter based on the ratio of flow rates through the first and second venturi tubes includes initiating regeneration of the second filter when the ratio is higher than an upper threshold, and terminating regeneration of the second filter when the ratio is lower than a lower threshold. 17. The system of claim 16, wherein adjusting engine operation based on a time elapsed between successive regenerations of the second filter includes regenerating the first filter when the time elapsed is higher than a threshold interval by retarding spark timing or enriching the exhaust gas, and indicating degradation of the first filter when the time elapsed is lower than the threshold interval, regeneration of the first filter discontinued responsive to the indication of degradation. 18. A method, comprising: flowing exhaust gas from downstream of a particulate filter positioned in an engine exhaust conduit into each of a first venturi and a metal filter positioned downstream of a second venturi in an exhaust bypass, the exhaust bypass coupled across the first venturi and external to the exhaust passage, adjusting engine operation based on degradation of the particulate filter, the degradation determined based on a time interval between a first regeneration and a second regeneration of the metal filter, the first and second regeneration based on a ratio of flow rates across the first and second venturi. 19. The method of claim 18, wherein the adjusting includes: during a first condition, in response to the time interval being greater than a threshold time interval, regenerating the particulate filter in the engine exhaust conduit when particulate filter regeneration conditions are met via one or more of a retarding spark and decreasing an air/fuel ratio; andduring a second condition, in response to the time interval being less than the threshold time interval, indicating to an operator degradation of the particulate filter and adjusting an engine actuator to reduce engine torque output,wherein the threshold time interval is based on a time elapsed between completion of a regeneration event immediately preceding the first regeneration and completion of the first regeneration, and wherein the time interval between the first regeneration and the second regeneration of a metal filter includes a time elapsed between completion of the first regeneration and completion of the second regeneration. 20. The method of claim 18, wherein the first and second regeneration based on the ratio of the flow rates across the first and second venturi includes regenerating the second filter when the ratio of the flow rate through the first venturi relative to the flow rate through the second venturi is higher than an upper threshold, the flow rate through the first venturi based on an estimated pressure upstream of a neck of the first venturi, the flow rate through the second venturi based on an estimated pressure upstream of the neck of the second venturi, and maintaining the regenerating of the second filter until the ratio is lower than a lower threshold.