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| Illustration 1 | g03860871 |
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(1) Clean Emissions Module (CEM)
(2) Dosing control cabinet (3) Diesel Exhaust Fluid (DEF) customer supply tank (4) Compressed air supply (5) Separate circuit aftercooler (SCAC) (6) Air cleaner (7) Turbocharger compressor (8) Turbocharger turbine (9) Mixing tube (10) Selective Catalyst Reduction (SCR) catalysts (11) Exhaust out | |
Note: Two tools are available to measure the DEF concentration. The two tools are the 431-7087 Tool Gp (Refractometer (DEF)) and the 360-0774 Refractometer (Brix). Refer to Testing and Adjusting, "Diesel Exhaust Fluid Quality - Test" for additional information.
Selective Catalyst Reduction (SCR) refers to a method of treating engine exhaust in order to reduce the undesired oxides of nitrogen compounds NOx. SCR uses a catalyst which promotes a desired chemical reactions over other possible chemical reactions. The catalyst remains unchanged. Ammonia (NH3) is mixed into the exhaust, and reacts with the NOx in the presence of the catalyst to form harmless compounds (water and nitrogen). In this system, urea ((NH2)2CO) is injected into the exhaust, which quickly decomposes into ammonia under the heated conditions. Urea is used because urea is inexpensive, and does not need the special handling that ammonia requires.
DEF is an aqueous urea solution (urea salt dissolved in water).
A 32.5 percent (by weight) solution of DEF is required for the system.
A 32.5 percent concentration has the lowest freezing point possible,
A 32.5 percent concentration of urea keeps a constant concentration through freezing and thawing.
DEF must be stored below
Note: The customer is responsible for providing primary filtering of the DEF from the customer supply tank to the dosing control cabinet. The system requires a 40 micron primary filter.
Use only DEF that meets quality properties per ISO 22241-1. Using DEF that does not meet ISO 22241-1 can result in clogging of the injection nozzle.
Contaminants can degrade the life of DEF related components. Care must be taken when refilling or performing service on the DEF system to minimize the introduction of contaminants. Initial operation should include a thorough system flushing with clean distilled water.
Primary DEF filtration is required. Filters must be compatible with DEF and should be used exclusively with DEF. Check with the filter supplier to confirm compatibility with DEF before using any filter. The DEF system primary filter must have a maximum of a 40 micron filtration level with a Beta = 1000 (99.9% Efficient). Check with your Caterpillar dealer for Caterpillar DEF filtration offerings which may meet your application specific needs
Reference: Refer to Operation and Maintenance Manual, SEBU6251, "Cat Commercial Diesel Engine Fluids Recommendations" for detailed DEF cleanliness recommendations.
DEF is a non-toxic source of ammonia.
DEF is corrosive. Do not store DEF in a tank or use supply lines that are made of the following materials: aluminum, brass and steel. Use only corrosion resistant materials such as PVC or stainless steel. Any O-rings must be Ethylene Propylene Diene Monomer (EPDM).
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| Illustration 2 | g06086348 |
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(1) Exhaust out
(2) Outlet mixer (3) Catalyst brick (4) Service door (5) Swirl mixer (6) Flapper mixer (7) Mixing tube (8) Urea injector (9) Exhaust in | |
The exhaust gas exits the engine and enters the mixing tube, where the DEF is injected into the exhaust stream.
The DEF decomposes into ammonia and water.
The mixture of exhaust and ammonia travels through the SCR.
The ammonia reacts with the NOx in the exhaust stream at the SCR catalyst to produce water vapor and nitrogen.
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| Illustration 3 | g06306395 |
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(1) SCR controller
(2) Wiring harness (3) Wiring harness connector to CEM (4) AC power in (5) Breaker (6) Air purge solenoid valve (7) Air outlet to CEM (8) Air inlet (9) DEF inlet (10) DEF outlet to CEM (11) Wiring harness connector to engine ECM (12) Service tool connector (13) DEF pump | |
The dosing control cabinet controls the following:
- Rate of DEF flow to the injector
- Compressed air
Compressed air is used for the following:
- Assist in the atomizing of the liquid DEF during injection into the exhaust stream
- Shield the liquid DEF in the injector from the exhaust heat until spraying, so that no crystallization occurs which could plug the injection nozzle.
- Purge the DEF line to the injector during shutdown in order to prevent crystallization of DEF resulting in a clogged nozzle or failed control valve.
Compressed Air Supply to Dosing Cabinet
In general the air supplied to the dosing cabinet needs to be frree of contaminates, or large quantiles of oil. Water in the compressed air can lead to component failure if exposed to freezing temperatures. An oil water separator is recommended. Air pressure must also be available even when the SCR system is not actively dosing to cool the DEF injection lance. Additional air is potentially required after the engine shuts down to complete the DEF purge process and continue to cool the DEF injection lance after engine shut down.
Oil can foul the catalyst and prevent the necessary chemical reactions from occurring. Sediment in the airline can plug the injection system.
The SCR system will use up to
To avoid clogging the air system with oil and/or sediment from piping, use a coalescing filter/separator that is 90 percent effective rated for
Air lines must be sized and routed so pressure loss across the line is no greater than
The purpose of the dosing control system is to meter DEF into the exhaust stream.
The dosing control cabinet houses an Electronic Control Module (ECM) that provides the controlling logic.
The desired rate of DEF injection is determined via a closed loop system, utilizing NOx sensors for feedback. The sensors measure the level of nitrogen oxides (NOx) entering the CEM and in the exhaust gas exiting the CEM.
If the NOx sensor indicates high NOx in the exhaust outlet, the closed loop system will request an increase of the DEF flow rate.
DEF flow rate is increased by increasing the frequency of the Pulse Width Module (PWM) signal to the dosing pump.
The aftertreatment ECM controls the frequency of the command signal to the dosing pump.
A communication adapter plug is provided on the side of the dosing control cabinet for easy connection.
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| Illustration 4 | g03773466 |
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(1) Nozzle
(2) Injector | |
The injector (1) is a structural member and holds the nozzle assembly (2) in the central axis of the mixing tube. The nozzle delivers the mixture of DEF and air.
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| Illustration 5 | g03773469 |
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(1) Nozzle
(3) Air (4) DEF (5) Fluid cap (6) Air cap | |
The DEF and air are mixed between the fluid cap (5) and a multi-port air nozzle cap (6). The mixed fluid is forced through the multi-port air nozzle cap into the exhaust stream. The compressed air causes the DEF to atomize (separate into tiny particles).
The DEF nozzle can become clogged with debris or crystallized DEF. DEF may be cleaned by soaking the component in warm clean water. If inspecting the nozzle, ensure that none of the spray holes are plugged. Plugging can cause poor dosing performance possibly causing DEF deposit formation with in the mixing pipe. Be sure to use the special torque when re-assembling the nozzle to the lance after cleaning. Refer to the Disassembly and Assembly for the appropriate procedure and specification.
A customer supplied tank supplies Diesel Exhaust Fluid (DEF).
Note: The customer is responsible for providing primary filtering of the DEF from the DEF tank to the dosing control cabinet. The system requires a 40 micron primary filter.
DEF must be provided from the DEF tank to the dosing cabinet between
DEF flow will vary with engine operating conditions, specifically with load. The DEF flow requirement is calculated by the Dosing Control ECM based on the closed loop NOx control.
CEM Identification Number Module
The identification number module outputs a 10 character alphanumeric serial number for the CEM to the SCR system ECM.
The SCR system ECM transmits this data to the engine ECM over the local CAN data link.
The engine ECM compares the first 3 letters of the serial number (the prefix) to a value programmed into the engine software.
Using this method, the engine ECM identifies that it is connected to the proper CEM and dosing system maintain emissions performance per the certification found on the engine emissions label. There is no requirement that engine be matched to a particular serial number CEM or dosing cabinet just the correct part number CEM and dosing cabinet part number. This check is only active during the first 100 hours of operation. If the incorrect CEM or dosing cabinet is installed, the engine will show a fault code and derate.
The NOx sensor communicates to the SCR system ECM over the CAN data link. The sensor has built-in diagnostics to indicate sensor failure modes.
The SCR system ECM controls the power supply to the NOx sensors.
Aftertreatment #1 Intake NOx Sensor
The aftertreatment #1 intake NOx sensor is monitored to control the DEF dosing rate based on engine out emissions levels.
Aftertreatment #1 Outlet NOx Sensor
The aftertreatment #1 outlet NOx sensor is monitored to control the DEF dosing rate based on catalyst out emissions. The SCR system ECM monitors the sensor to calculate the necessary DEF flow rate to meet emission targets.
Aftertreatment SCR Catalyst Intake Gas Temperature Sensor
The aftertreatment #1 SCR catalyst intake gas temperature sensor is monitored for dosing control and system protection.
Catalyst inlet temperature is important to the catalyst ability to convert NOx. The dosing control ECM monitors CEM inlet temperature and adjusts the dosing rate accordingly to maximize system performance.
Aftertreatment SCR Catalyst Outlet Gas Temperature Sensor
The aftertreatment #1 SCR catalyst outlet gas temperature sensor is also monitored for dosing control and system protection.
The SCR system ECM monitors the sensor for system protection. The pressure is monitored to indicate pressures that are too low or too high.
The pressure is measured at the inlet to the CEM. A pressure that is too high can indicate a restriction in the CEM exhaust stream. A pressure that is too low could indicate missing SCR catalyst bricks, or a sensor that has been removed from the exhaust stream.
Sensors (Dosing Control Cabinet)
The SCR system ECM monitors the sensor for system protection. Pressures that are too low or too high indicate a failure in the dosing system.
The SCR system ECM monitors the sensor for system protection. Pressures that are too low or too high indicate a failure in either the dosing system or the air supply to the dosing cabinet. Too high of a pressure can also indicate a clogged DEF nozzle.
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| Illustration 6 | g03777698 |
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Air manifold (1) Air Pressure Sensor (2) Air Assist Solenoid Valve (3) Air Pressure Regulator | |
These valves are electrically actuated and control the air supply and the DEF supply.
- Air Assist Solenoid Valve
- DEF Line/Manifold Purge Solenoid Valve
- Pump Purge Solenoid Valve
The air assist valve (solenoid) is controlled by the SCR system ECM. This valve is used to control the flow of air into the DEF nozzle.
This solenoid is normally closed. When the dosing cabinet needs to use the air supply to either inject DEF into the exhaust stream, or to purge the system, the air supply valve is opened to allow air to flow into the DEF dosing cabinet.
DEF Line/Manifold Purge Solenoid Valve
The DEF Line/Manifold Purge Solenoid Valve is controlled by the SCR system ECM. This solenoid is normally open to allow DEF flow to return to the tank. When the solenoid is energized, the valve closes, forcing the DEF to flow to the DEF injector.
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| Illustration 7 | g06306412 |
| (1) Pump purge solenoid valve | |
The Pump Purge Solenoid Valve is normally open. The Pump Purge Solenoid Valve is used to direct air pressure from the air supply manifold to the DEF pump during the purge process. Normally during dosing the valve is energized and therefore closed. Only during system check and the purge process is the valve allowed to provide air pressure to the inlet of the pump.