inject a quantity of the reducing agent into the exhaust gas flow in a process sometimes referred to as dosing the amount of reducing agent introduced can be dependent upon the no x load of the exhaust gasses the mixing duct 254 uniformly intermixes the reductant agent with the exhaust gasses before they enter the downstream scr catalysts disposed at the end of the mixing duct 254 proximate the rearward second wall 228 can be a diffuser 256 that redirects the exhaust gas reductant agent mixture toward the third and fourth sidewalls 230 232 of the after treatment system 220 the third and fourth sidewalls 230 232 can redirect the exhaust gas reductant agent mixture generally back towards the forward first wall 226 to perform the scr reaction process the exhaust after treatment system 220 can include a first scr module 260 disposed proximate the third sidewall 230 and a second scr module 262 disposed toward the fourth sidewall 232 the first and second scr modules 260 262 may be oriented to receive the redirected exhaust gas reducing agent mixture the first and second scr modules 260 262 can accommodate one or more scr catalysts 264 sometimes referred to as after treatment bricks the term after treatment brick however may refer to a variety of exhaust after treatment devices which scr catalysts are a subset of moreover in different aspects the scr modules 260 262 may be configured to accommodate any different number of after treatment bricks that may be in different shapes sizes and or configurations and that may operate.