This post follows on from the latest case study which can be seen here - https://www.picoauto.com/library/case-s ... blue-fault where we looked in a bit of detail to the operation of the AdBlue system.
More and more exhaust Aftertreatment systems are found on all types of off-highway machinery and this brings about more complications. We wondered if there was a test we could carry out to quickly check the operation and health of the main actuating components. The system fitted to the Komatsu is the Bosch Denoxtronic 2.2. It can also be found on some light and medium commercial vehicles. You can see a very basic layout of these actuator components in the illustration below.
Fundamentally, there are always a set number of components for an AdBlue Aftertreatment system. These consist of:
Dosing control module
DEF metering unit
SCR - Selective Catalytic Reduction unit including Ammonia filter
Heater elements for pipework, tank and supply module
AdBlue quality sensor
AdBlue level sensor
The actuating part of the circuit is easier to explain with a hydraulic diagram:
1. Dosing supply module
2. AdBlue reservoir
4. One way check for the valve and throttle. Throttle is used to generate pressure by restricting the flow
5. Pressure sensor
6. 4/2 flow control valve
7. Motor-driven single direction, fixed displacement pump
8. 2/2 solenoid valve (DEF Injector)
When the ignition is switched on, the pump will prime the system once the fluid is up to temperature. The pump will draw fluid from the tank where the pressure will begin to rise to around 9 Bar (130 psi). This pressure is created by the throttle in the return line to the reservoir as pressure is the resistance to flow. A one-way valve prevents the fluid from entering the supply lines from the reservoir on the return side.
During operation the injector will be commanded to open by the Aftertreatment ECU, allowing the solution to enter the exhaust system to begin the chemical reaction. When AdBlue is introduced to a high-temperature environment the urea breaks down to form ammonia and Isocyanic acid. This combines with the water vapour in the process of hydrolysis and creates CO2 and NH3 (ammonia). In an environment containing a catalyst and high levels of oxygen, found in lean-burn engines following combustion, the ammonia will combine with NOx present in the exhaust gas to form nitrogen, carbon dioxide and water.
When the engine is switched off, the Aftertreatment ECU will command the 4/2 flow control valve (6.), shifting its position so that the pump is now pulling the fluid from the supply line and returning it to the reservoir. Due to the one-way valve and the closed position of the injector, a vacuum will start to build in the pipework. To prevent the collapse of the pipework, the injector is pulsed at a high frequency to allow air into the pipework and control the amount of vacuum present. The pressure sensor will detect vacuum changes in the pipework, which can be useful when you are looking for blockages as shown in the case study. Below is a capture taken from a known good system using an active test function from a scan tool.
It was really interesting to see that while the injector current was flowing we could see a drop in the AdBlue pressure. This indicates that the AdBlue was indeed being delivered into the exhaust system. As this is a known good, we can use this drop in pressure to help highlight issues with blockages in the future. If there were problems with NOx levels despite this pressure drop, we would have to turn our attention to the spray pattern or the actual SCR catalyst.
I hope you agree that this is a valid test for this particular system, especially as it's becoming increasingly common. My understanding is that the wiring is the same for all units and as such below is a pin out of the plug which will hopefully help with the connection -
There is one aspect of this system that is still a slight mystery to me and despite trawling the internet, I've yet to find out what it means. It involves the PWM signal from the pump motor. When the pump is running it is a traditional PWM signal which we can graph with maths should we wish too. The unknown aspect is when the pump isn't running and the strange waveform that comes with it.
Above is the known good capture where we have the oddity at the beginning and then where the signal changes to the conventional PWM signal which is highlighted with a math channel. This pattern looks very similar to that which Steve found on the BMW fan as seen here - viewtopic.php?p=101043#p101043. I believe what we have here is something very similar. We know from the circuit diagram that temperature is also present on this signal but it is not known how this measurement is interpreted. If there is some sort of data then this certainly stops when the pump is running and we have our PWM signal. What's more is we can see the voltage levels are different between the PWM signal and the potential data which could be seen as something similar to LIN!
Here is where I ask for a little help as it would be great to find out if we can interpret this signal and convert it to something meaningful.
I hope this helps.
Ask for and share advice on using the PicoScope kit to fix Heavy Duty and Off Highway machinery here
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