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An exhaust system comprises a conical shell having an indentation traversing the exhaust passage in a coiled path so as to impart mixing on an engine exhaust gas flowing through an inner passage defined by the conical shell. The indentation may be formed into the conical shell via a stamping process

An exhaust system comprises a conical shell having an indentation traversing the exhaust passage in a coiled path so as to impart mixing on an engine exhaust gas flowing through an inner passage defined by the conical shell. The indentation may be formed into the conical shell via a stamping process.

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1. An exhaust passage for receiving engine exhaust gas, the exhaust passage having a mixing region comprising a first portion upstream of a second portion, the second portion comprising an exterior wall having a continuous helical trough depressed onto the exterior wall and wrapping around the secon

1. An exhaust passage for receiving engine exhaust gas, the exhaust passage having a mixing region comprising a first portion upstream of a second portion, the second portion comprising an exterior wall having a continuous helical trough depressed onto the exterior wall and wrapping around the second portion, the trough formed as an inwardly-protruding indentation traversing at least once around the exhaust passage in a coiled path angled with respect to a cross-section of the exhaust passage, the exterior wall defining an interior passage configured to receive the engine exhaust gas and redirect the engine exhaust gas via the inwardly-protruding indentation, the inwardly-protruding indentation comprising an inner surface facing the interior passage and an outer surface opposite the inner surface, the inwardly-protruding indentation further comprising a first terminal end where exhaust gas first impinges on the inner surface and a second terminal end where exhaust gas lastly impinges on the inner surface, each of the first terminal end and the second terminal end located on a same side of the exhaust passage and aligned along a same axis of the mixing region, and the mixing region further comprising a rounded transition between a surface of the exterior wall upstream of the trough and the first terminal end of the inwardly-protruding indentation, the rounded transition extending along an entirety of the trough from the first terminal end to the second terminal end. 2. The exhaust passage of claim 1, wherein the inwardly-protruding indentation traverses at least once around the exhaust passage in a continuous manner and has a depth into an interior of the exhaust passage of less than 20% of a diameter of the exhaust passage. 3. The exhaust passage of claim 2, wherein the rounded transition is also between the surface of the exterior wall and the second terminal end, wherein the coiled path traverses at least once around the exhaust passage in a left-handed direction. 4. The exhaust passage of claim 3, wherein the second portion of the exhaust passage is tapered inward in an upstream direction, wherein the inwardly-protruding indentation is positioned at a portion of the exhaust passage comprising a conical shell. 5. The exhaust passage of claim 4, wherein the first portion of the exhaust passage has a constant cross-sectional diameter and the second portion of the exhaust passage increases in cross-sectional diameter in a direction of exhaust flow, and wherein the inwardly-protruding indentation has a depth of protrusion into an interior of the exhaust passage that is closer to a central axis of exhaust flow than an interior of the first portion of the exhaust passage for at least one traversal of the inwardly-protruding indentation around the exhaust passage. 6. The exhaust passage of claim 1, further comprising an atomizer positioned in the first portion of the exhaust passage upstream of the inwardly-protruding indentation, the atomizer comprising exactly three louvers configured to redirect the engine exhaust gas and catch a fluid spray for atomization. 7. The exhaust passage of claim 6, wherein the atomizer is positioned within an interior passage of the exhaust passage so as to define a free region above the atomizer and a free region below the atomizer through which the engine exhaust gas can flow unobstructedly. 8. The exhaust passage of claim 6, wherein each of the three louvers has a different depth and is positioned at a different angle with respect to one another. 9. The exhaust passage of claim 6, wherein a first louver of the three louvers has a smaller depth than an adjacent second louver of the three louvers, and wherein the second louver has a smaller depth than an adjacent third louver of the three louvers. 10. The exhaust passage of claim 1, further comprising an injector positioned on a top of the exhaust passage upstream of the second portion and angled towards a bottom of the exhaust passage such that liquid injected by the injector is directed towards the bottom of the exhaust passage. 11. The exhaust passage of claim 1, wherein the inwardly-protruding indentation traverses exactly twice around the exhaust passage. 12. An exhaust system for an engine, comprising: an exhaust passage tapered inward in an upstream direction, a portion of an exterior wall of the exhaust passage having an inwardly-protruding indentation traversing at least once around the exhaust passage in a coiled path angled with respect to a cross-section of the exhaust passage, the exterior wall defining an interior passage configured to receive engine exhaust gas and direct the engine exhaust gas via the inwardly-protruding indentation, the inwardly-protruding indentation creating a thread having a width parallel to a longitudinal axis of the exhaust passage and a depth perpendicular to the longitudinal axis, the width greater than the depth, the inwardly-protruding indentation including a cross-section defined by a first transition between a surface of the exterior wall and the inwardly-protruding indentation and a second transition between the surface of the exterior wall and the inwardly-protruding indentation, an entirety of the cross-section from the first transition to the second transition shaped as a semi-circle; andan atomizer having a non-circular perimeter positioned in the exhaust passage upstream of the portion comprising the inwardly-protruding indentation so as to define a free region of the exhaust passage above the atomizer and a free region of the exhaust passage below the atomizer through which the engine exhaust gas can flow unobstructedly. 13. The exhaust system of claim 12, wherein the inwardly-protruding indentation traverses at least once around the exhaust passage in a continuous manner. 14. The exhaust system of claim 12, wherein the portion of the exterior wall of the exhaust passage having the inwardly-protruding indentation is a conical shell. 15. The exhaust system of claim 12, wherein the atomizer comprises exactly three louvers configured to redirect the engine exhaust gas and catch a fluid spray for atomization. 16. The exhaust system of claim 15, wherein each of the three louvers has a different depth and is positioned at a different angle. 17. An exhaust system for an engine, comprising: an exhaust passage comprising a twist mixing device, the twist mixing device comprising an entry passage having a constant cross-sectional diameter immediately upstream of a conical shell, an entirety of the conical shell tapered inward in an upstream direction and having a wall comprising a trough depressed into the wall, the trough having a semicircular cross-section and wrapping around the conical shell at least twice, the wall of the conical shell defining an interior passage for receiving engine exhaust gas and directing the engine exhaust gas via the trough, the exhaust passage further comprising rounded transitions between the wall and the trough, the trough having a depth of protrusion into an interior of the conical shell that is closer to a central axis of exhaust flow than an interior of the entry passage for at least one traversal of the trough around the conical shell; andan atomizer having a non-circular perimeter positioned in the exhaust passage upstream of the twist mixing device comprising the trough so as to create a free region of the exhaust passage above the atomizer and a free region of the exhaust passage below the atomizer through which the engine exhaust gas can flow unobstructedly, the atomizer further comprising exactly three louvers configured to redirect the engine exhaust gas and catch a fluid spray for atomization, each of the three louvers having a different depth. 18. The exhaust system of claim 17, wherein the wrapping of the trough is angled in a flow direction of the engine exhaust gas. 19. The exhaust system of claim 17, wherein the three louvers are nonuniformly spaced apart. 20. The exhaust system of claim 17, wherein the twist mixing device comprises a first portion and a second portion assembled to create the conical shell.