Abstract: An after-treatment system for an engine is disclosed herein. The engine has a horizontal axis and a vertical axis. The after-treatment system includes a housing assembly, a coolant circuit with a coolant tank, and at least one injector unit. The injector unit includes a first port and a second port. The second port is in fluid communication with the first port. The first port is fluidly connected to the coolant tank. Further, the injector unit is mounted on the housing assembly, such that the first port is positioned substantially vertically above the second port, along the vertical axis. In addition, the injector unit includes a datum line that passes through the first port and the second port. An included angle defined by the datum line relative to the horizontal axis is generally about 60 degrees, when a width of the housing assembly is parallel to the horizontal axis.