Every Engine Sensor Explained

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JMR

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you know that 's me right ?

( when i get started explaining a sensor's role in a car that i diagnose and the owner is just a carpenter/teacher/whatever no engine knowledge person :))
 
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4RingWagons

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Reading about the correct procedure in testing a few of these currently. Great info for anyone new to the electrical side of these cars!
 
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alexnoe

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EGT: Are you sure that the DPF uses the exhaust gas temperature sensor? Normally, there is a pressure difference sensor (I think you didn't mention this type of sensor) which measures exhaust gas pressure upstream and downstream the DPF. If the difference is large, the DPF is full and goes into regeneration.

NOx sensors: My car has 3 of them: Before SCR1, between SCR1 and SCR2, and after SCR2. Using 3 sensors, it can tell which of the two DEF injectors (before SCR1 or before SCR2) is faulty if one of them is faulty.

There is one more sensor you missed and which has been in use for years already: There are ultrasound sensors which measure the speed of ultrasound waves inside the DEF tank. The speed of ultra sound waves inside the DEF tank basicly allow calculating the urea concentration. This allows the car to see if someone has put something else than DEF into the DEF tank.
 
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Uwe

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EGT: Are you sure that the DPF uses the exhaust gas temperature sensor?
Yes, the ECU has to know the temperature to determine that the conditions for DPF regeneration have been achieved. In fact, most every DPF equipped engine has more than one EGT sensor, and problems with those sensors are a common cause of DPFs getting clogged.

NOx sensors: My car has 3 of them: Before SCR1, between SCR1 and SCR2, and after SCR2. Using 3 sensors, it can tell which of the two DEF injectors (before SCR1 or before SCR2) is faulty if one of them is faulty.
What car is this? I've not heard of one that has two SCRs, but then we've gotten no diesels from VAG at all on this side of the pond since MY 2016. I suppose with two SCRs, VAG wanted to be really sure that they weren't over the NOx limits again?

-Uwe-
 
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alexnoe

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Yes, the ECU has to know the temperature to determine that the conditions for DPF regeneration have been achieved. In fact, most every DPF equipped engine has more than one EGT sensor, and problems with those sensors are a common cause of DPFs getting clogged.
Ah ok. In the video, @JMR said that if this sensor was faulty, it could trigger DPF regenerations more often, but I don't see how: The pressure difference sensor should be used to make the decision to start a regeneration cycle. On the other hand, it makes sense to use the exhaust gas temperature sensor to confirm that a sufficiently high temperature has been achieved.

What car is this? I've not heard of one that has two SCRs, but then we've gotten no diesels from VAG at all on this side of the pond since MY 2016. I suppose with two SCRs, VAG wanted to be really sure that they weren't over the NOx limits again?

-Uwe-
Škoda Superb, engine type 2.0 TDI, EA288 evo, engine code DTUA. The EA288 evo engine was introduced with the VW Golf Mk8 in late 2019. The technology is from Bosch, they call it a "double injection SCR" (Link to Bosch) system.

If you want an SCR catalyst to be useful in cities, it must be installed right next to the engine, leading to fast warm-up and avoiding cool-down. At this place, your space is somewhat limited, and you cannot make it as big as you might want to. Also, making it bigger increases warm-up time. Solution: Install two of them. Using VCDS, I determined a warm-up time of about 2 minutes for SCR1 even at speeds around 20 mph. Besides that, engines that use this NOx aftertreatment layout were planned to be almost compliant with the next step of emission regulation ("Euro 7"). They only want to make minor modifications, like SCR size, or equipping SCR1 with an electric heating powered by the MHEV battery, but they don't want to develop another new engine.

Bosch/VW also say that at very high engine load, an SCR catalyst in the underfloor works better because the one close to the engine gets too warm, but NOx sensor readings don't really confirm this claim. SCR1 seems to work almost always, and I don't see a correlation between high engine load and loss of SCR1 efficiency. Sometimes, it is higher, sometimes it is lower, but I don't get when or why.

Nowadays, all Volkswagen and Mercedes 4 cylinder diesel cars use such a layout. I'm not sure about BMW, but I think they use LNT+SCR in some cars, 2x SCR in others.

Another point after all the discussions about NOx polution from diesel cars is that they didn't just want to be barely compliant, exactly hitting the legal limit. With real driving emission tests not only being performed, but results being actually published, they wanted to get NOx emission results as low as what their petrol cars typically have.
 
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VWT6

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JMR

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Errr..the video isn t mine..i was just saying that as a joke :)
 
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NEtech

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Also some info in SSP 595
 
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VWT6

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VW Self-study Programme 514 - The New EA288 Diesel Engine Family, Design and Function (published 05/2013)
states the following:

The soot load of the particulate filter is calculated by two pre-programmed load models in the engine control unit.
One load model is determined from the user's driving style and the exhaust gas temperature sensor and lambda probe signals.
Another soot load model is the flow resistance of the particulate filter. It is calculated from the signals from the exhaust gas pressure sensor 1 G450, the exhaust gas temperature sensors and the calculated exhaust mass flow from the engine control unit.
 
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alexnoe

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