CUMMINS SCR Air assisted system

 

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CUMMINS SCR Air assisted system

 

 

Air assist system
Dosing Pump Connections
1. DEF in from tank (Urea)
2. Air supply in (vehicle)
6
2
3. DEF out to Injector nozzle (Urea &
Air mix)
5
4. DEF return to tank (only used during
4
prime)
3
5. 37 PIN ITT Cannon connector
1
broadcasting on the J1939 OEM
supplied Harness
6. Power supply to Solenoid 2 pin
connector (from Doser)
DEFDU
¾ The DEFDU (doser) is designed to deliver a flow of DEF (urea) according to
the message sent by the engine ECM.
¾ The system uses air supplied by the compressor to bring the dosed volume of
DEF (urea) to the injection nozzle.
¾ The DEFDU has a built in heating device that allows the doser to operate with
outside temperatures lower than -11C, if the vehicle also has heated DEF lines
and DEF tank.
It will also function in a working temp of up to 80 C, but DEF above 50deg C for
extended periods will damage the doser Unit.
¾ The DEFDU has an integrated its self-diagnostic procedure, informing the
Engine ECM about its status by sending messages via the J1939 CAN link.
¾ The Air assisted DEFDU can accommodate both 12 VDC & 24 VDC supply
voltages.
The electronic control arrangement makes the DEFDU very fast and accurate. The
desired flow rate is delivered with a repeatability of 1%, at the steady state
conditions at which the pump is calibrated. (20°C ambient and liquid temperature)
System Principle and Operation
The SCR system operates on the principle of selective catalytic
reduction (SCR). The SCR system converts the Nitrogen Oxides
produced by the engine (exhaust) into Nitrogen and Water.
Liquid reagent is injected upstream of a catalyst in the exhaust
system using a very accurate dosing pump. The amount of DEF
injected by the pump is controlled by the Engine ECM.
Under normal operation the Dosing unit has separate control
Phases:-
Initialisation
Priming
Dosing mode - standby / DEF injection
Purging
Release
System Priming
Initialisation. At Key on the Doser begins initialisation, the motor is operated
to position the pump and self test internal components such as the pressure
switch and temperature sensor.
When the Engine is started an instruction is sent to the dosing pump, from the
Engine ECM, to go into the PRIMING phase. DEF is circulated from the tank,
through the pump and back to the tank to purge air from the system. The
pump runs at 100% flow for 30 seconds. If the PRIMING phase is successful
you will then hear the air solenoid operate and air will flow continuously
through the doser and into the nozzle.
If the PRIMING phase is not successful first time, the air solenoid will de-
energize and the priming sequence as described above will repeat. This
sequence can be repeated up to 20 times before ERROR 1682 SCR Pump
State Control Doser Error is displayed. (10 -12 mins)
NOTE: The 1682 fault code has been expanded to help pinpoint the problem.
UREA DOSER RETURN LINE ERROR = FC3575
ƒAftertreatment Diesel Exhaust Fluid Pressure Sensor - Data Valid
But Above Normal Operating Range - Moderately Severe Level
UREA DOSER LOW AIR UREA ERROR = FC3738
ƒAir Supply Actuator - Data Valid But Below Normal Operating Range
- Moderately Severe Level
UREA DOSER MOTOR ERROR = FC3569
ƒAftertreatment Diesel Exhaust Fluid Dosing Unit Input Lines -
Mechanical system not responding or out of adjustment
UREA DOSER PRIME TIMEOUT ERROR = FC3548
ƒAftertreatment Diesel Exhaust Fluid Dosing Unit Loss of Prime -
Condition Exists
PRIMING PHASE
Initialisation
Engine run
Signal - Prime
Dosing Mode,
“Standby”
A: Air Tank
B: Air filter
C: Doser unit
D: Engine ECM
E: Wema Sensor
F: Urea Tank
G: Nozzle
H: EGP Inlet Temp
I:EGP Outlet Temp
J: EGP
Workshop task.
Doser Priming
System Dosing
At the end of the PRIMING phase the Air Solenoid Valve opens and the pump
motor stops. The pump is now in “standby”, ready to dose urea as requested by
the Engine ECM. The quantity and timing of urea being dosed is controlled by the
engine ECM depending on the NOx produced by the engine and the exhaust
temperature.
The pump will not start dosing DEF unless both exhaust temperature sensors
reach 200 degrees, however air will be continuously passed through the Dosing
Pump, Injector Supply line & Injector while the engine is running to prevent the
nozzle from becoming blocked and to provide a cooling effect.
AIR DOSING / STAND BY PHASE
Engine running
A: Air Tank
B: Air filter
C: Doser unit
D: Engine ECM
E: Wema Sensor
F: Urea Tank
G: Nozzle
H: EGP Inlet Temp
I:EGP Outlet Temp
J: EGP
6 Required Conditions for Dosing DEF
1.
200 degrees C @ both Catalyst Inlet and Outlet
(For Euro 5, inlet above 200 deg C, average of both above
200 deg C)
2. Certain SCR System Fault codes NOT active*
3. DEF Tank Level above 6%
4. Air pressure above 4 bar, and urea pressure 3 bar
(Primed!)
5. Above -3 degrees C (DEF temp)
6. Cummins NOx Algorithm (estimator algorithm) calculates
NOx production is above a predetermined value
* 1668, 1669, 1673, 1681, 1682, 1697, 1698, 1699, possibly 1712,
2976, 3163, 3231, 3235, 3236,
DOSING PHASE
Engine running
A: Air Tank
B: Air filter
C: Doser unit
D: Engine ECM
N2 &
E: Wema Sensor
F: Urea Tank
H2O
G: Nozzle
H: EGP Inlet Temp
I:EGP Outlet Temp
J: EGP
Workshop task.
“Dosing” phases
System Purging
When the Key Switch is turned to the OFF position the system enters the
PURGING phase. Compressed air flows through the injector and the pump.
The air removes any DEF droplets from the system that could otherwise
crystallise and block the pump valves, urea lines or the injection nozzle. The
phase lasts for 30 seconds then the system switches off, the solenoid can be
heard “clicking” off.
Purging is performed with every key off cycle. “Incomplete purge” counts can be
read within Insite
If a high count of incomplete purge counts are found, power connections to the
doser must be investigated. The OEM installation must provide power for the
Doser to purge after key off for sufficient time. This can be an issue with
hazardous cargo vehicles - petrol tankers, also some bus fleets.
This is followed by the air pressure release phase.
If certain SCR system Faults becomes active, then the SCR system will switch
off, but first purge the system of DEF.
PURGING PHASE
Clear nozzle line
Engine off / 30sec
power supply!
A: Air Tank
B: Air filter
C: Doser unit
D: Engine ECM
E: Wema Sensor
F: Urea Tank
G: Nozzle
H: EGP Inlet Temp
I:EGP Outlet Temp
J: EGP

 

 

 

 

 

 

 

 

 

 

 

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