Difference between water and diesel temperature in a PD engine

Hello,

Today, I did a data logging session for about a minute and a half, including various temperature readings.
The journey included a section on the Autobahn, with speeds reaching up to 160 km/h (with a "flying start" for the data logging).

During the analysis, I noticed that the fuel temperature is always approximately 30°C lower than the water temperature (start: 48°C / 80°C, end: 65°C / 93°C).

I'm a little surprised by that, because the diesel is being pushed by the piston, and that process should actually cause its temperature to rise?

Okay, the system is constantly "flushed" to cool the PD elements. But in doing so, the diesel absorbs even more heat, so it should actually come out hotter than the water from the cylinder head .

Or is the diesel fuel forced through the ZK (Zylinderkopf - cylinder head) so quickly and with such a high mass flow rate that, despite the heat absorption from the PD (Piezoresistive) elements surrounding the inlet and outlet channels, a "cooled zone" is created?

I also considered a faulty diesel temperature sensor, but when the engine is cold, the read diesel temperature is only 3 "resolution levels" (which equates to 2.7°C) below the water temperature... so, at the very least, they are working "synchronously" enough by about 13°C.

'I'm experiencing something similar with my AWX. However, the last time I checked, it was during the summer when the temperature was around 35°C. At that time, the diesel temperature was fluctuating quite precisely around the water temperature.' A sudden surge of gas caused the diesel temperature to rise quickly. However, erratic operation lowered the temperature to around 50-65°C.

You need to read the cooling factor; then you'll see how much cooling is currently being applied.

If your burner were defective, would it also produce soot (or have insufficient power)?

Julian wrote:

You need to read the cooling factor, then you'll see how much is currently cooling.

Hi Julian,

What process does this cooling factor describe?

In field 2 of the MWB 7 form, I have a percentage value, while the rest of the fields contain temperature readings.
Is that the cooling factor (which was constantly zero during the log)?

Ugh, I don't remember. But I had the label file, and it's described somewhere in there. Sorry, it was a long time ago. Maybe Uwe@Rosstech can come up with something on the spot.

You'll see a percentage indicating how much of the engine's cooling is currently being provided by diesel. So, from 0% (the engine control system doesn't cool the diesel at all) to 100% (the cooling system is operating at its maximum capacity).

Julian wrote:

I don't remember exactly. However, I had the label file, and it describes that somewhere. Sorry, it was a long time ago. Maybe Uwe@Rosstech can come up with something on the spot. You'll see a percentage indicating how much of the engine's cooling is currently being provided by diesel. So, 0% (the engine control system doesn't cool the diesel at all) to 100% (cooling at the maximum limit of its performance).

Hi Julian,

This requires an "active cooling system," which, as far as I know, is only installed in a few vehicles (e.g., Diesel water coolers with an integrated water pump.

The Passat with the AJM engine has this type of pump, but at least the Polo with the ATD/AXR engines doesn't (I don't have any SLP documentation for the ASZ Polo).

Could it be that my car doesn't have active diesel cooling, especially since the actuator diagnostic doesn't include anything like that?

Hm, isn't it simply stated how much percentage of the cool diesel is extracted from the tank to cool the PDEs?
For me, it's almost always at 0%, only in the summer when it's 42°C did it ever reach 60%.

I'll check my AWX system this late afternoon, and then I can tell you exactly where this is recorded, okay?

Julian wrote:

Hm, isn't it simply specified how much percentage of the cool diesel is extracted from the tank for cooling the PDEs? For me, it's almost always at 0%, only in the summer when it's 42°C did it ever reach 60%. Check my AWX system this late afternoon, and I can tell you exactly where this is recorded, okay?

Okay, sure!

I'm curious to see what comes out of this. Because for the targeted extraction of cool diesel from the tank, some kind of switching valve or similar would be required, which would be controlled by the EDC.
However, in the documentation I've seen so far for the 9N TDI, I haven't found anything like that (except for the well-known bi-metal switching valve on the filter, but that isn't controlled by the EDC).

The 9N TDI probably has a return cooler located near the fuel tank, but AFAIK it's always flowing. So, I have no idea where a "mixing unit" controlled by the EDC should be located within the fuel system.

No idea how the cooling system is handled. Gremlin would be best suited to handle INPUT... (Request on INPUT, Captain.....)

I just checked on my AWX: It's been in the garage for 2 days, and according to the onboard computer, the outside temperature is 14°C.

Analog:
Diesel temperature: 13.5°C
Coolant temperature: 14.4°C
Intake air temperature: 15.3°C

The coolant display is located in the MSG (address 01), channel 7, second field from the left. Description: Fuel Cooling State (100% ON, 0% OFF). It was at 0 the entire time for me.

I drove around a bit, but not much. After about 4 km, the diesel engine temperature was higher than the water temperature, so 65°C compared to 60°C. However, it was mostly stop-and-go traffic. And with the mild garage temperatures, the SH is not yet acting as a pre-heater... that lazy piece of equipment!

Julian wrote:

The cooling display is located in the MSG (address 01) on channel 7, in the second field from the left. Description: Fuel Cooling State (100% ON, 0% OFF). It was at 0

the whole time for me.
Hi Julian,

CLICK If the value can only be 0 or 100, then it's not a regulation, but rather something to turn on and off.
This could be the circulation pump for the auxiliary circuit of the engine coolant, which extracts heat from the diesel return - if a diesel-water heat exchanger is installed.

Then, as far as I know, you should have a relay somewhere for that pump... but in my car, I'm familiar with all the relay functions now, and there isn't one for a diesel coolant pump.
The diesel lines are clearly visible in the engine compartment; they come from the vehicle floor, pass through the filter, and end at the engine (without mentioning the supply and return lines).
"There's no branch or bend leading to a part that resembles a cooler in any way... as I said, the 9N, as far as I know, has a "passive" return cooler located just before the fuel tank."

Quote:

I drove around a bit, but not much. The diesel temperature was higher than the water temperature after about 4 km, so 65°C compared to 60°C. However, it was mostly stop-and-go traffic. And with the mild garage temperatures, the SH is not yet acting as a pre-heater... that lazy piece of equipment!

What is SH? Auxiliary heater?

It seems that the principle that diesel engines generally run at a lower temperature than water-cooled engines holds true for my engine. At the front of the engine, there are three long metal pipes: one for coolant, one for diesel fuel supply, and one for diesel fuel return.
At idle (when the engine was warm), the coolant hose was extremely hot, while the diesel return pipe was noticeably cooler.