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The catalyst and particulate filter assembly OC is a ceramic catalytic converter which oxidizes hydrocarbons in the exhaust and generates heat for DPF regeneration. The DPF is a highly engineered silicon carbide wall-flow catalyst that traps particulates. As soot gathers in the system it begins to restrict the filter and the filter needs to be periodically cleaned. The soot can be cleaned in 2 different ways: Passive regeneration and active regeneration. Both methods occur automatically and require no action from the driver/operator.
The selective reduction catalyst improves the exhaust emissions and fuel efficiency by injecting a reductant into the exhaust system. The reductant, also referred to as diesel exhaust fluid, is a 32.5% solution of urea in deionized water. At the inlet of the catalyst, there is a port for the reductant injector which is followed by a grate diffuser and a twist mixer. When the reductant is introduced into the system, it atomizes in the grate diffuser and mixes evenly with exhaust gases in the twist mixer. During this time, the heat of the exhaust gases causes the urea in the reductant to split into Carbon Dioxide (CO2) and ammonia. As the ammonia and NOx pass over the catalyst, a reduction reaction takes place and the ammonia and NOx are converted to nitrogen and water.
Passive regeneration occurs naturally as a result of normal engine operating conditions. During passive regeneration, the exhaust constituents/temperature are at an appropriate level where some soot can be reduced or oxidized (burned) thus cleaning the filter.
Active regeneration, which is initiated by the PCM, will occur when there is not enough passive regeneration occurring due to vehicle drive patterns. In an active regeneration, the DPF is cleaned by raising the exhaust temperature to a point where the soot is burned away. After the soot is burned off, the exhaust temperature and back pressure (restriction) fall back to normal levels.
The reductant tank stores the reductant.
The reductant tank filler hose is a 2-piece design consisting of a filler hose and a vent hose.
The reductant pressure line supplies the reductant from the reductant pump assembly to the reductant injector. There are different reductant pressure lines for different vehicle wheelbases. The reductant pressure line is heated to prevent freezing.
The reductant pump assembly pumps reductant to the reductant injector. It contains a diaphragm pressure pump, pressure sensor, purge valve, outlet filter, and internal heating element.
The reductant heater and sender assembly contains the pickup tube for the reductant pump module, electric heating element, temperature sensor, and electrode-type level sensor.
The reductant injector is a Pulse Width Modulated (PWM) solenoid controlled directly by the PCM. The injector receives reductant from the reductant pressure line and sprays it into the exhaust stream, where it is mixed into the exhaust gases before entering the catalyst.
The NOx sensor detects levels of NOx in exhaust gases and sends input to the NOx sensor module. For additional information, refer to Section 303-14B.
The NOx sensor module receives input from the NOx sensor and sends it to the PCM. For additional information, refer to Section 303-14B.
The NOx sensor detects the level of NOx in the exhaust gas and sends that input to the NOx sensor module. The NOx sensor module sends that input to the PCM which commands a reductant injection. The reductant injector opens and the reductant pump operates, filling the reductant pressure line and the reductant injector to purge the air out of the system. When all of the air is purged, the reductant injector closes, allowing the reductant pump pressure to build to 500 kPa (73 psi). With the system fully primed, the reductant injector provides the reductant to the catalyst as commanded by the PCM. The catalyst contains a copper catalyst washcoated on a zeolite substrate. At the inlet of the catalyst is a port for the reductant injector, followed by a grate diffuser and a twist mixer. When the reductant is introduced into the system, it atomizes in the grate diffuser and mixes evenly with exhaust gases in the twist mixer. During this time, the heat of the exhaust gases causes the urea in the reductant to split into Carbon Dioxide (CO2) and ammonia. As the ammonia and NOx pass over the catalyst, a reduction reaction takes place and the ammonia and NOx are converted to nitrogen and water. This reaction takes place at up to 95% efficiency, allowing the engine to run leaner and more efficiently. The high levels of NOx that are produced under lean conditions are now eliminated.
The PCM commands the Glow Plug Control Module (GPCM) to provide voltage to the reductant pump assembly internal heating element, reductant pressure line heater and the reductant heater and sender assembly when the reductant temperature approaches its freezing point of -11°C (12°F). The reductant heater and sender assembly heating element is located directly above the pickup tube inlet filter. When the reductant heater and sender assembly temperature sensor detects the reductant temperature dropping to its freezing point, the reductant heater and sender assembly heating element thaws and maintains a pool of liquid reductant within the reductant heater and sender assembly reservoir.
The reductant heater and sender assembly level sensor incorporates four stainless steel electrodes. Three electrodes arranged vertically provide a high, middle, and low level signal and the fourth electrode runs the length of the level sensor and acts as a ground. The reductant is a good conductor of electricity. When the reductant tank is full, the reductant closes a circuit between all three level electrodes and the ground electrode, indicating the tank is full. As the reductant is consumed, the level drops and uncovers each electrode in sequence. The PCM calculates the reductant level based on these signals.
When the vehicle is shut down, the PCM closes the reductant injector and actuates the reductant pump purge valve, causing the reductant pump to reverse the pump flow and bleed down the pressure from the reductant pressure line. The PCM then opens the reductant injector to allow air to enter the reductant pressure line, which in turn allows the reductant pump to purge all remaining reductant from the system and return it to the reductant tank. The PCM then closes the reductant injector and returns the reductant pump purge valve to the forward pump position.
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• Servicing procedures
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