Diesel Pump Rücklauftemperatur - PD

Separate topic from https://community.dieselschrauber.org/en/viewtopic.php?t=9966

goettmann wrote:

The tandem pump! There is a kind of filter there. This is supposed to capture the air bubbles and direct them to the return flow through a throttle bore. However, a small amount of (cold) supply fuel is also forced through it. That's why the return temperature is low behind the tandem pump.

Hmm, I don't think that the small amount of cold water coming from the cold water supply will lower the return temperature from approximately 150°C to 80°C.
With a supply temperature of, for example, 20°C, a mixing ratio close to 1:1 would be necessary, meaning that approximately half of the pump's flow rate would constantly be diverted to the return line, just to capture a few isolated air bubbles...?

I believe that the diesel fuel is more likely to be flushed back through the fuel cooler and the high-pressure fuel pump before it can reach the temperature of the fuel cooler.
In the steady-state partial load range, the diesel temperature sensor typically reports about 30K less than the water temperature sensor. Only at maximum power (Pmax) does the diesel temperature approach the water temperature, due to the high heat dissipation of the PDE (Positive Exothermal Device).

As a result, the fuel paths are deliberately cooled by the circulation within the ZK (presumably a component or system).
The diesel fuel is thus introduced into the PDE at the "coldest possible" temperature, which, in turn, benefits the engine's power output due to its relatively high density and short injection duration.

The transformation becomes visible, for example, when you replace a rubber hose between the filter and the ignition coil with a clear piece of material and then briefly turn on the ignition:
Despite the engine being stationary, the pre-charge pump expels the air from the clear hose faster than you can see .
Based on my gut feeling, I still estimate the return rate to be several times higher than the actual consumption, even with Pmax.

...
Hmm, I don't think that the small amount of cold water coming from the cold water supply will lower the return temperature from approximately 150°C to 80°C.

Don't be misled by the quantities being redirected there; they can be very high in some cases.

...
I believe that the diesel fuel is more likely to be flushed back through the fuel cooler and the high-pressure fuel pump before it can reach the temperature of the fuel cooler.


So, VW specifies that the diesel temperature is approximately 120-150°C at the high-pressure injectors (PDEs). I consider it a physical miracle that the return fuel, which then comes from the crankcase (without mixing at the tandem pump) at 80°C.

The high cooling of the fuel in the rotary lobe (RL) pump by fresh fuel at the tandem pump is intentional. Imagine that the fuel, coming from the fuel tank at approximately 120°C (slightly cooler than at the port injection systems), is directly routed to the fuel tank. After just a few kilometers of full-throttle highway driving, you would easily have 80-90°C (or more) in the fuel tank. The small radiator located under the passenger seat would then no longer be sufficient for cooling, and the tank (made of plastic) would be extremely at risk.
Originally, PD TDIs had an active fuel cooling system. This was controlled by the fuel temperature sensor in the return line. At the same time, it allowed for an estimation of the fuel temperature in the high-pressure system. This temperature is then used by the EDC (Engine Control Unit) for power adjustment. However, what is actually needed is a sensor on the injectors (PDEs). But that is simply too expensive. The indirect method using the return line temperature is cheaper and just as effective.
Active fuel cooling has disappeared, so this temperature sensor now only has one function.

goettmann wrote:

So, VW specifies that the diesel temperature is transmitted to the injectors (PDEs) at approximately 120-150°C. I consider it a physical miracle that the return fuel then comes from the engine (without mixing at the tandem pump) at 80°C.

*gg* Agreement regarding the miracle... provided that, for example, such temperatures are constantly present at the return inlet of the injector.
Do you have more specific information about the measurement conditions (measurement location, engine load, etc.)?

The fact that the PDE (presumably a piece of equipment) is cooled by diesel engine coolant flowing through it in parallel, and that this involves a total power output of several kilowatts, likely requires a significant coolant flow rate.

Therefore, I still suspect that...
a) The vast majority of the pump's flow is achieved through the positive displacement pump itself, and not through the air separation throttle valve in the pump.
b) the average residence time in the ZK is so short that the diesel often doesn't even reach the water temperature.
c) The 150°C local peak temperatures mentioned by VW are not average temperatures.

The pump schematic drawing on page 20 of SSP 209, which shows a comparatively tiny orifice between the inlet and outlet, confirms my suspicion – although I am aware that the image cannot be considered a realistic design drawing.

Ultimately, it's a discussion about something trivial... but I find it interesting, in my opinion.

Do you have more specific information about the measurement conditions (measurement location, engine load, etc.)?

Yes and no...
I can't say anything about the measurement point. Engine load: partial load.

The fact that the PDE (presumably a piece of equipment) is cooled by diesel engine exhaust flowing through it in parallel, and that this involves a total power output of several kilowatts, likely requires a significant flow rate.

That is correct. The tandem pump is doing a lot of heavy work.

Therefore, I still suspect that...
a) The vast majority of the pump's flow is achieved through the positive displacement pump itself, and not through the air separation throttle valve in the pump.

I cannot confirm that. Unfortunately, I no longer remember the exact number, but the 'tandem pump loss' was very high. This was explained by the return cooling system. Again, a temperature of 150°C was mentioned.

b) the average residence time in the ZK is so short that the diesel often doesn't even reach the water temperature.

For such high flow rates, the cross-sectional area of the return line in the fuel cooler is too small. Nevertheless, the residence time of the diesel fuel in the fuel cooler must be kept as low as possible. Excessive heating of the fuel will then only result in a loss of power.
The primary focus is solely on the supply temperature. The flow rate through the ZK (presumably a component) is just sufficient to bring the fuel in the mixing chamber (VL) to acceptable temperatures. Anything beyond that is completely unnecessary energy consumption.
Why should I pay attention to the return temperatures within the ZK (zero-knowledge proof system) when I can easily manage them outside the ZK?
That's how I was explained the PDE cooling concept. I can't say anything more about it. I didn't design it.

c) The 150°C local peak temperatures mentioned by VW are not average temperatures.

The 150°C should represent 'normal' diesel temperatures within and around the PDE (Positive Displacement Engine).
Unfortunately, I didn't ask where and when exactly this temperature occurs.

The pump schematic drawing on page 20 of SSP 209, which shows a comparatively tiny orifice between the inlet and outlet, confirms my suspicion – although I am aware that the image cannot be considered a real design drawing.

Okay, but that's just a schematic diagram.

Ultimately, it's a discussion about a trivial matter... but IMO, it's interesting.}

Yes, I agree. Perhaps one of the moderators should move this branch of the discussion into its own thread. It has very little to do with the original topic.

Unfortunately, the fuel-cooling concept in PD engines is the biggest secret of this design.

goettmann wrote:

The "tandem pump loss" was very high. This was explained by the return cooling. Again, the 150°C was mentioned.

Hmm, so if the PDs are indeed operating with ~150°C diesel and the return flow is mixed down to about half that temperature before the temperature sensor (without the supply temperature being recorded!), then quite a bit of calculation must be involved to reconstruct a suitable density correction factor for the opening duration from the sensor value... the exact correction parameters will probably be another major secret of the PDs.

In addition, this creates an opportunity for performance variations (or even subtle modifications).
If the air separation/mixing throttle in the tandem pump is smaller than normal or is reduced in size, the temperature at the diesel sensor increases, and the PDE opening duration is extended .

Quote:

Quote: b) the average residence time in the ZK is so short that the diesel often doesn't even reach the water temperature For such high flow rates, the cross-section of the return line in the ZK is too small.

OK, that seems plausible.

Quote:

Perhaps one of the moderators should move this branch of the discussion into its own thread at some point.

This is now done (thanks to Dieselmartin ).

Hmm, if the PDEs are indeed operating with diesel at approximately 150°C, and the return flow is mixed down to about half that temperature before reaching the temperature sensor (without the upstream temperature being recorded!), then quite a bit of complex calculation is required to reconstruct a suitable density corrector for the opening duration from the sensor value... the exact correction parameters will likely be another major secret of the PDs .

Why is that?
The parameters are all constant.
All the mixtures in the ZK (Zylinderkopf - cylinder head) and in the tandem pump are known, and therefore, the appropriate correction value can be calculated from the return temperature.
Please note that this temperature sensor for the RL system was originally intended for active cooling. Measuring the fuel density based on this temperature was merely a byproduct. With the removal of the active cooling system, the rest of the system was not modified to avoid larger software changes (and thus, uncertainties).
Just a quick note:
Active cooling for fuel coming from the ZK (Zylinderkopf - cylinder head) at 80-90°C would be pointless anyway. Furthermore, this cooling system was also operated using the approximately 90°C warm coolant from the engine circuit (see SSP209). Therefore, the fuel coming from the ZK must have been significantly warmer.
I'm not saying that the fuel in the RL is cooled by 'flushing' it. I would estimate a cooling of around 20-30°C compared to the PDE. So, the fuel still comes out of the cylinder at over 100°C. Further mixing in the tandem pump then brings it down to around 80°C.

In addition, this creates an opportunity for performance variations (or even subtle modifications).
If the air separation/mixing valve in the tandem pump is smaller than normal or is reduced in size, the temperature at the diesel sensor increases, and the PDE opening duration is extended .

All these systems are, of course, designed with tolerances in mind. However, the developers' obsession with cost-cutting often goes so far that these tolerances unintentionally accumulate, leading to performance variations that become outliers in the production run. Fortunately, manufacturing tolerances are significantly lower today.

That's precisely where the problem lies with modern HD-DI systems.
The fuel system's temperature regulation must be strictly maintained and should not be altered by external factors (e.g., KGR - likely referring to a specific component or system). This can disrupt the system's operation, and then one shouldn't be surprised by strange behavior of the engine. Jerking or stuttering is just one example of such errors. The temperature sensors are a prime source of such problems. Unfortunately, it is very difficult to prove these issues.
If the manufacturer provided more detailed documentation about the temperature profiles, it would certainly be possible to identify and eliminate some difficult-to-explain errors. However, doing so would reveal their design secrets, essentially 'giving away' their competitive advantage.

goettmann wrote:

The parameters are all constants, though. All the mixtures in the ZK (common rail) and in the tandem pump are known, and therefore, the appropriate value for the correction can be calculated from the return temperature.

Okay, let's say the diesel comes from the ZK at approximately 120°C.

The pump now mixes in diesel at -10°C (in winter) and at +40°C (in the peak of summer, when a small amount of fuel in the tank has already been heated by the constant circulation from the return line), always with a constant and known mixing ratio.

Both times, the engine was warmed up in the same way and operated under the same load and speed, and according to VW, the diesel engine with PDE injectors always runs at approximately 150°C.

When there is significant mixing with the return flow, the diesel sensor value will change considerably between -10°C and +40°C in the supply line, and the EDC will adjust the PDE opening duration accordingly, even though there is no reason for this adjustment based on the diesel temperature at the PDIs.
To avoid this, the temperature of the diesel fuel itself would also need to be factored in, but there is no sensor for that.

Quote:

Please note that this temperature sensor for the RL was originally intended for active cooling. The measurement of fuel density using this temperature was merely a byproduct.

Considering the amount of power gain that is possible through a distortion of the sensor signal, and then imagining a PD without density correction, the summer sluggishness of VP-TDIs would be a mere peanut...

...
The pump now mixes in diesel at -10°C (in winter) and at +40°C (in the peak of summer, when a small amount of fuel in the tank has already been heated by the constant circulation from the return line), always with a constant and known mixing ratio.

Both times, the engine was warmed up in the same way and operated under the same load and speed, and according to VW, the diesel engine with PDE injectors always runs at approximately 150°C.

Where do you get -10°C cold fuel for a Wamgefahrenem engine?
There is also a third mixing device between VL and RL.
I think you've overlooked the fuel preheating system in the DIFI.

There, by mixing with the (warm) return liquid, the fuel temperature is maintained at a relatively constant temperature (50-60°C).
This has been the case since the early days of Volkswagen diesel engines.

When there is significant mixing with the return flow, the diesel sensor value will change considerably between -10°C and +40°C in the supply line, and the EDC will adjust the PDE opening duration accordingly, even though there is no reason for this adjustment based on the diesel temperature at the PDIs.
To avoid this, the temperature of the diesel fuel itself would also need to be factored in, but there is no sensor for that.

As mentioned above, early attempts were made to maintain the VL temperature relatively constant in order to create consistent combustion conditions.

Considering the power gain that can be achieved by manipulating the sensor signal, and then imagining a PD without density correction, the summer performance drop of VP-TDIs would be a mere trifle.
Incorrect conclusion... See above... But fundamentally, it's correct.

goettmann wrote:

[I think you've overlooked the fuel preheating system in the DIFI. There, by mixing with the (warm) RL, the fuel temperature is maintained at a relatively constant temperature (50-60°C).

Oops, I actually missed that .
Then, the supply temperature will be -10°C, but only as long as the engine is still cold during the winter.

Only if the mixing valve in the air-fuel mixture control unit (DiFi) is defective would my scenario be possible - and in that case, we would have a potential cause for a loss of power.
If the temperature at the sensor drops below the normal level due to insufficient coolant temperature, the EDC system switches to a colder diesel supply to the injectors and reduces the injection duration.

Then, the supply temperature will be -10°C, but only as long as the engine is still cold during the winter.

Yes, it would be interesting to see how quickly the RL heats up.
I think you should be able to have 40-50°C at the return line after just 1-2 minutes.
That's something we should log.

However, I'm not sure if the temperature-dependent power management of the EDC even starts working at such low coolant temperatures.


Only if the mixing valve in the air-fuel mixture control unit (DiFi) is defective would my scenario be possible - and in that case, we would have a potential cause for a loss of power.
If the temperature at the sensor drops below normal due to insufficient coolant temperature, the EDC switches to a colder diesel supply to the PDEs and reduces the opening times.
A quite conceivable error scenario.
The problem is probably that it's unlikely anyone will find it quickly. The mixture in the DIFI (however primitive it may be) cannot be diagnosed. And can you detect a mixing error in the DIFI based on the RL temperature? I have my doubts about that.

The defect can occur in either direction.

If the thermostat switch is in the 'continuous mixing' position, the supply line temperature will eventually become too high (e.g., 80°C instead of 50°C). The temperature at the fuel distributor will eventually become so high that it causes fuel vapor bubbles (I believe this happens with diesel around 200°C). The engine will misfire. The fuel injectors may report a lack of fuel supply. The return line temperature will also increase (and thus also the supply line temperature), and the engine control unit (ECU) will attempt to compensate with longer injection times until the limit is reached -> error entry.
The engine should also produce some smoke during an intermediate stage.

If the thermostat is in the 'return to tank' position, the supply temperature will be too low (at least until the tank reaches operating temperature). The return line (RL) will also be cooler. The EDC (Engine Control Unit) will then adjust the injection times accordingly, resulting in a loss of power.
No other malfunctions are likely to occur here.


The first documented case involved KGR in Pomeranian dogs. I hope this now makes it clear to everyone why this is absolutely toxic for PD (presumably referring to a specific breed or condition).

For those who think that common rail (CR) is better... they are mistaken. The temperature balance here is just as sensitive, because the fuel is also heated up by the compression (in a pump-injector system, this happens in the cylinder head). Cooling through mixing is also very important in this case.

goettmann wrote:

However, I'm not sure if the temperature-dependent performance adjustment of the EDC is already working at such low coolant temperatures.

Hmm, I've already simulated a diesel temperature of -37°C using a potentiometer, without any error being set.
If the active measurement range extends that far, why shouldn't a density correction be implemented there as well?

Quote:

And can you detect a mixing error in DIFI related to the RL temperature?

I have my doubts about that.
Assuming a normal operating temperature range of approximately 50 to 90°C for the engine (my approximate log values from idle to maximum power), this range could potentially serve as a reference point for checking, perhaps after comparing it with the logs of other vehicles...

Quote:

The defect can occur in both directions.

Nice explanation.

However, I'm not sure if the temperature-dependent performance adjustment of the EDC is already working at such low KW temperatures. Hmm, I've already simulated a diesel temperature of -37°C using a potentiometer, without any error being set.
If the active measurement range extends that far, why shouldn't a density correction be implemented there as well?

Coolant temperature... I was talking about coolant temperature. I thought the correction only becomes effective above a certain coolant temperature, but I'm not entirely sure.

It's great that you don't get an implausible signal with a coolant temperature of 90°C and a fuel temperature of -37°C. -> Software error

And can you detect a mixing error in DIFI related to the RL temperature? I have my doubts about that.
Assuming a normal operating temperature range of approximately 50 to 90°C for the cooling system on a warm engine (my approximate log values from idle to maximum power), this range could potentially serve as a reference point for checking the system, perhaps after comparing it to the logs of other vehicles...

Yes, but would you have noticed that before this discussion?
I claim: No!

The defect can occur in both directions. Nice explanation

Do you have a better wording?

goettmann wrote:

It's great that you don't get an implausible signal with a coolant temperature of 90°C and a fuel temperature of -37°C. -> Software error

I performed this test while the engine was stationary and cold, if I recall correctly -> so there's only about a 60K difference between the water and diesel without an error message .

Quote:

Quote: Nice explanation Do you have a better wording?

That was meant as a genuine compliment for the subsequent descriptions

But let's go back to the topic of the "regulated" diesel supply temperature, which you mentioned is around 50-60°C.
Isn't the whole thing achieved by the small plastic part in the return line, which is clipped onto the DiFi?
So, the bimetallic valve (or whatever actuator is used) must be located there, and in my opinion, it can only measure the temperature of the return flow coming from the engine as the "control parameter," but not the actual supply water temperature.
[edit]At least, the RL (radio line) antenna at my former AFN (Armed Forces Network) station was probably designed to look like this.[/edit]

Based on an assumed switching threshold of 50°C, the following states result:

"Return temperature below 50°C -> directed to the filter -> during the warm-up phase, the forward temperature increases approximately in parallel with the return temperature, reaching almost 50°C (reduced by the proportion of cold diesel from the tank and cooling effects on the filter and the forward line)."

2. If the return temperature exceeds 50°C, it is directed to the tank, passing through the diesel cooler. The supply is drawn exclusively from the cold tank, so it is not regulated and, in winter, may reach the aforementioned -10°C at the inlet of the tandem pump (once the engine coolant and the supply line to the engine have cooled down sufficiently).
Then, the mixing-related mismatches in diesel temperature and PDE (Piezo Direct Injector) opening duration mentioned above could become a reality.

The fact that the diesel fuel is slightly warmed from the tank to the tandem pump within the warm engine compartment can be disregarded for this calculation. For example, if the pump temperature is -10°C, the fuel in the tank might be -15°C (which, of course, will also be warmed up again due to the return flow, more slowly or quickly depending on the fuel level).

3. Return flow at 50°C -> Mixture state between conditions 1 and 2.

What do you say now?
Where is my flaw in reasoning or my misinformation?
[edit] Do RL-based control algorithms in distributed frequency inverters (DiFi) for permanent magnet (PM) motors potentially use a temperature sensor inside the filter housing instead of switching based on the RL temperature?
I don't want to unnecessarily disassemble mine, because the engine starts incredibly well and runs smoothly right now - I don't want to risk ruining that by potentially introducing air leaks.

It's great that you don't get an implausible signal with a coolant temperature of 90°C and a fuel temperature of -37°C. -> Software error I performed this test while the engine was stationary and cold, if I recall correctly -> so we can be 'only' about 60,000 km without a fault entry

Suuuper software

...
So, the bimetallic valve (or whatever actuator is used) must be located there, and in my opinion, it can only measure the temperature of the return flow coming from the engine as the 'control parameter,' but not the actual supply water temperature.

Bimetal is correct.
Basically, this bimetallic switch measures the housing temperature. This should correspond to the VL temperature within a few degrees.
Function and structure, see:
http://f23.parsimony.net/forum49387/messages/112895.htm



...

What do you say now?
Where is my flaw in reasoning or my misinformation?

Hmmmm....

Okay, I could get used to that. However, that would mean the entire fuel cooling concept wouldn't work anymore. Then we'd be dealing with constant compatibility issues.
'We should investigate at what temperatures this valve switches. I've always assumed it's based on the DIFI housing temperature.'

Here's a quick thought:
I only remember that even with the older VP TDI injectors, the fuel temperature was regulated to a nearly constant 50-60°C. In the VP ESP injectors, it was further heated to about 70-80°C due to the compression, and then to about 100-120°C (depending on the load) due to contact heat in the ESDs (Electronic Spray Devices). This temperature is considered the ideal atomization temperature for diesel. The PD (Piezo) injectors aren't that far off from that.



That's how I see it.

Unfortunately, I cannot confirm either of those things (at least, not based on your description above ).

HI,

As a small piece of data, I can confirm the 50-60 degree range, as I observed a diesel temperature of 63 degrees in my VP37 after 1.5 hours of highway driving at a moderate speed (120-140 km/h) with an outside temperature above 30 degrees.

The Temp exhibition particularly interested me in relation to POEL mixtures.

The same route (as you can see, with GRA) only required 43 degrees Celsius of diesel in the winter.
http://www.luxus-wg.de/~martin/auto_bilder/ladelufttemp.gif

m;

goettmann wrote:

We should investigate at what temperatures this valve switches.

Perhaps a simple check of the VL line by feeling it after a short drive with a warm engine and a full diesel tank might be sufficient.

According to my theory, the return liquid (RL) should then completely enter the tank, and the vapor-liquid line (VL) leading to the central collection point (ZK) should be almost as cool as the VL line arriving at the filter.

According to your assumption, the voltage level (VL) cable leading to the main distribution board (ZK) should be approximately 50-60°C warm.

The difference should be noticeable, even if it's not as pronounced given the current weather.
I'll try it later. The metal pipe sections at the front of the PD-ZK are perfect for that.