COUPLING 1V3546 - Caterpillar

Aftertreatment system description
Illustration 1 g06031628
Caterpillar supplied aftertreatment components are illustrated with dashed line rectangles.
Selective Catalyst Reduction (SCR) catalyst technology is used to reduce NOx emissions and particulate matter.The aftertreatment system is composed of the following components:
Air compressor
Bulk Urea Transfer Pump
DEF Dosing cabinet
DEF Filter
DEF Mixing tube
DEF Nozzle
DEF Supply tank
SCR catalysts
SCR reactor housingEngine exhaust flows into the SCR System reactor housing through the exhaust inlet. Once sufficient temperature is achieved, the DEF is injected into the exhaust. Exhaust flows through a mixer assembly in the mixing tube to ensure that exhaust is not stratified for uniformity. This mixing will ensure complete disbursement across SCR catalyst section. Precise DEF injection is monitored and controlled by an electronic controller in the dosing cabinet. The following components provide signals to the controller to control DEF injection for emission reduction:
Thermocouple located in the reactor housing.
Inlet NOx sensor located in the exhaust before the inlet to the SCR catalyst reactor housing.
Outlet NOx sensor located in the exhaust at the outlet of the SCR catalyst reactor housing.When engine operating conditions dictate, DEF fluid is injected into the exhaust stream before entering the SCR. When injected into the exhaust stream, the DEF is atomized into droplets. The atomized droplets are then sent through the mixer. The mixer disrupts the exhaust flow and allows DEF to be distributed throughout the exhaust gas. Water evaporates due to the high temperature of the exhaust. This process will cause the DEF to decompose and release ammonia (NH3) that was bound to the DEF. NH3 is free to react with the NOX and the oxygen present in the exhaust system stream. This reaction occurs on the SCR catalyst. NH3 and NOX are converted into gas particles of nitrogen and water. The inlet and outlet NOX sensors read the levels of NOX and determine how much DEF needs to be injected into the exhaust.Dosing Cabinet
The dosing control cabinet controls the following:
Rate of DEF flow to the injector
Compressed air pressure
Urea purge shutdown cycleThe aftertreatment ECM located in the dosing cabinet and the Cat ECM are designed to communicate with each other. This communication controls the complete engine/emission system to meet emission levels.DEF Tank
There are two separate DEF tanks in the dosing system.
DEF tank 1 is the customer installed DEF tank. DEF tank 1 is the bulk DEF tank. A DEF high and low-level switch in tank 1 will notify the operator when the tank requires refilling.
DEF tank 2 is the buffer tank located in the dosing cabinet. Tank 2 is a small DEF tank that is filled from DEF tank 1. Tank 2 provides DEF to the DEF pump to be injected into the exhaust.Reactor Housing
The reactor housing contains the following:
SCR catalyst substrates
DEF nozzle
DEF mixing tube
SCR catalyst temperature and pressure sensors
NOx sensorsThe aftertreatment ECM uses the sensors on the module to determine how much DEF needs to be injected into the exhaust system.Selective Catalytic Reduction Substrate
The Selective Catalyst Reduction (SCR) substrates are located in the reactor housing. These catalysts allow exhaust to flow through the