Engine emission control system
Systems and methods are provided for reducing exhaust NOx species using a monolith substrate having a plurality of channels configured with radially varying cell density. An injector injects reductant into the substrate unobstructedly, without use of a mixer. In some embodiments, a twist angle of the channels is based on a distance from an inlet of the substrate and varies exponentially as a function of the distance.
1. A method of operating an emission control system coupled to an exhaust passage of an engine, comprising:
injecting an amount of reductant upstream of a first monolithic cylindrical substrate having a plurality of channels of radially varying cell density, the radial variation including inner channels with a higher cell density and outer channels with a lower cell density, the injected amount of reductant flowing unobstructedly from an injector into the first substrate without use of a mixer;
flowing exhaust gas through the first substrate to mix the exhaust gas with the injected reductant; and
flowing the mixed exhaust gas over an SCR catalytic washcoat to catalytically convert a component of the mixed exhaust gas,
wherein the channels are along a length of the first substrate, and wherein the channels are rotated progressively along a lateral axis along the length of the first substrate, a twist angle of the rotation varying exponentially as a function of a distance from an inlet of the first substrate.
2. The method of claim 1 , wherein in addition to the SCR catalytic washcoat over which the mixed exhaust gas flows, another SCR catalytic washcoat is included on the first substrate.
3. The method of claim 2 , wherein flowing the mixed exhaust gas over the SCR catalytic washcoat includes flowing the mixed exhaust gas through a second substrate positioned downstream of the first substrate, the SCR catalytic washcoat included on the second substrate.
4. An emission control system of an engine exhaust passage, comprising:
a monolithic-substrate having channels of varying cell density extending from an inlet end to an outlet end of the monolithic-substrate with a channel twist angle varying exponentially as a function of monolithic-substrate length;
a downstream-substrate;
an upstream injector to inject reductant to flow unobstructedly to the monolithic-substrate without use of a mixer via an upstream reductant injector; and
an SCR catalytic washcoat on the monolithic-substrate or on the downstream-substrate.
5. The system of claim 4 , wherein the catalytic washcoat includes zeolites.
6. The system of claim 4 , wherein the monolithic-substrate is cylindrical and wherein the cell density of the channels varies radially with inner channels having a higher cell density and outer channels having a lower cell density.
7. The system of claim 6 , wherein the channels are rectangular in cross-section.
8. The system of claim 7 , wherein at least some of the channels are plugged.
9. The system of claim 8 , wherein the downstream-substrate has a plurality of channels, and wherein an SCR catalytic washcoat is positioned on the downstream-substrate, a cell density of the channels of the downstream-substrate different from the cell density of the channels of the monolithic-substrate.
10. The system of claim 4 , wherein the reductant is urea or ammonia.
11. The system of claim 4 , wherein the monolithic-substrate has a lower thermal mass than the downstream-substrate.
12. An emission control system coupled to an exhaust passage of an engine, comprising:
a monolithic upstream substrate having a plurality of channels of varying cell density, the channels formed within the upstream substrate and having a length extending between an inlet end of the upstream substrate and an outlet end of the upstream substrate, the cell density varying radially through the upstream substrate with a higher cell density at a center of the upstream substrate and a lower cell density at a periphery of the upstream substrate, the channels twisted along the length of the channels, where a twist angle of the channels is based on a distance from the inlet of the upstream substrate and varies exponentially as a function of the distance, and where a position of a selected channel at the inlet end is twisted with respect to a position of the selected channel at the outlet end;
a reductant injector positioned upstream of the upstream substrate for injecting reductant that flows unobstructedly from the injector to an inlet of the upstream substrate without use of a mixer;
a downstream substrate having a plurality of channels of uniform cell density; and
an SCR catalytic washcoat coated on the downstream substrate and the upstream substrate.
13. The system of claim 12 , wherein the upstream substrate is cylindrical and a cross section of each channel is rectangular.
14. The system of claim 13 , wherein a first end of a channel at the inlet of the upstream substrate is twisted by an angle from a second end of the channel at an outlet of the upstream substrate, the angle measured along a lateral axis of the upstream substrate.