LLK die 10te: Größen und Leistungsstufen | Posts 16+

Hi Ulf!

Quote:

In the case of two identical liquid-liquid cyclones connected in parallel, the theoretical internal flow velocity is reduced to half. That means the charge air remains in the intercooler for a longer time, and therefore has more time to dissipate heat. And yet, the goal is to release less heat per unit volume of air. Which (apparently unknown to me) processes are responsible for that?

Well, the connections aren't entirely clear at first glance.
The heat transfer between a fluid (in this case, a gas) and a stationary wall (the heat exchanger, also known as the cold liquid cooler) is influenced not only by the temperature gradient but also by the formation of a boundary layer and the type of flow that develops.

Here's a small practical example: In the past (with CPUs like the 386), cooling could be achieved using a heatsink and natural convection. This represents the worst heat transfer scenario because there is virtually no air velocity (relative speed between the air and the heatsink). The airflow of natural convection is laminar and has a relatively thick boundary layer, which acts as an insulator.
More powerful CPUs (starting with the 486 series) had to be equipped with a fan to improve heat dissipation. The airflow over the fins becomes highly turbulent, which significantly improves heat transfer. Furthermore, the boundary layer is much thinner.

One should imagine the interior of the liquid coolant system in a similar way. If only a gentle, laminar flow develops there (with a parabolic velocity profile across the cross-section), then the flow itself acts as an insulator and hinders heat exchange with the walls.
At high flow velocities, the flow is highly turbulent. This results in a plug-like velocity profile, meaning that the flow velocity is significantly higher even near the boundary layer. This promotes heat transfer with the wall and improves the efficiency of the liquid cooling system.
Never forget that air is a very good thermal insulator!

I hope I was able to illustrate the situation a bit.

Regards,
Alex.

donalexo wrote:

The flow of free convection is laminar and has a rather thick boundary layer that acts as an insulator... The flow around the fins becomes highly turbulent, which significantly improves heat transfer. The boundary layer is also much thinner. Bitte gib mir den deutschen Text, den du übersetzt haben möchtest. I hope I was able to illustrate the matter somewhat.

Hi Alex,

I have understood very well what you mean, thank you .

But:
Even at idle, we already have a relatively fast airflow towards the engine.
With an estimated inner pipe diameter of 6 cm (resulting in a cross-sectional area of approximately 28 cm²) and a volumetric flow rate of 450 * 1.9 liters per minute, I'm getting into the range of 5 m/s.

Since, as far as I know, the LLK (likely referring to a specific component) is the worst flow obstruction on the suction side, the flow velocities in the area of the internal cooling fins must be at least as high, and likely even higher.

A link was posted in another thread.
http://www.sigem-elektronik.de/elektro/glossar.htm#W
(Scroll down a bit to see the diagram.)
"The thermal resistance between a heatsink and the air decreases with increasing airflow velocity—but only up to about 6 m/s; beyond that, there is no further improvement."

Okay, the heat is transferring from the air to the aluminum, but I suspect that the statement also holds true in reverse.

Therefore, I speculate that parallel 2 LLKs will reach the individual thermal resistance at approximately ~2000 rpm, similar to 1 identical LLK running idle, and that the thermal resistance will not decrease further beyond that point.

This would mean that two parallel turbochargers, starting at 2000 rpm, are actually twice as effective (with half the pressure loss) compared to a single turbocharger.

What do you think about that?

Okay, so I just got back from dealing with that BMW issue...

Vehicle: Skoda Octavia Estate/Wagon 1.9 TDI ALH Automatic.
Charging pressure: 0.9 bar (1950 mbar absolute).
Outside temperature: 25°C.
driving profile: accelerator pedal stuck.
Intake air temperature, standard condition: 63°
Supply air temperature (SAT): 93°

With the side mirrors taped up, exiting the parking lot was noticeably more difficult, and it becomes significantly tougher in the area of maximum speed. I immediately drove to the next parking lot to un-tape the side mirrors. It's better that way .

CU Gremlin.

Gremlin wrote:

vehicle: Skoda Octavia Combi 1.9 TDI ALH Automatic. Charging pressure: 0.9 bar (1950 mbar absolute). Outside temperature: 25°C. driving profile: accelerator pedal stuck. Intake air temperature, standard condition: 63° intake air temperature (IAT), air intake covered: 93°

Aaah, thank you - some fresh input .

So, the LLT (likely load temperature) is approximately 38K above the ambient temperature.

Quote:

With the auxiliary lights taped over, the exit from the parking lot was noticeably slower, and in the area of maximum speed, it became significantly more sluggish.

Did you happen to log the limits (air mass / torque)?

Even with the LLK (Low-Level Knock) taped off, my soot limit was still about ~5mg above the engine's optimal speed, and I couldn't notice any smoother acceleration.

Did you happen to log the limits (air mass / torque)?


No, because the rotating part is already fixed, and the air mass reading with the diode isn't really informative.
'By the way, it seems like the maximum power (Pmax) is reached at 4250 rpm. Up to that point, it revs up normally, but then it almost stops increasing in speed... However, it also doesn't decrease in speed when going slightly uphill.'

CU Gremlin.

@ Ulf:

Regarding our problem with heat transfer from the charge air to the intercooler coolant.

Inside the intercooler, a flow is generated. The heat transfer from the charge air to the aluminum body of the intercooler is more intense depending on the higher the flow velocity and the greater the temperature difference between the charge air and the surface of the intercooler.
Considering only the internal heat transfer between the charge air and the coolant, a higher flow velocity leads to a higher degree of turbulence in the flow. The laminar boundary layer, which acts as an insulator, becomes thinner with increasing Reynolds number, meaning that heat transfer improves.
Imagine it this way: At low flow rates, many air molecules do not come into contact with the walls because they can move along a path that remains relatively far from the walls. Despite the longer residence time of the molecules in the liquid coolant (LLK), these molecules have only a small chance of transferring heat to the LLK.
When the turbulence level is higher, this laminar flow is disrupted by so-called "eddies" (turbulent vortices). These small "swirls" occur chaotically and cause a significant exchange of energy between the core zone of the flow and the edge regions, both in terms of kinetic energy and heat transfer.

In order for this "advantage" regarding internal heat transfer to be realized, the external heat transfer between the coolant and the airflow must be able to dissipate the heat flow.

Regards,
Alex.

donalexo wrote:

Imagine it this way: At low flow rates, many air molecules do not come into contact with the walls because they can move along a path that remains relatively far from the walls. Despite the longer residence time of the molecules in the liquid coolant (LLK), these molecules have only a small chance of transferring heat to the LLK. When the turbulence level is higher, this laminar flow is disrupted by so-called "eddies" (turbulent vortices). These small "swirls" occur chaotically and cause a significant exchange of energy between the core zone of the flow and the edge regions, both in terms of kinetic energy and heat transfer.

Hi Alex,

These are some really good explanations from "niche areas" that very few people know about, but which can sometimes help us move forward -> thank you.

One question though:
The faster the flow, the more turbulence there is in the liquid cooling system (LCS), and the more air molecules come into "very close" contact with the heat-dissipating metal. I now understand that .

But at the same time, the residence time of individual molecules in the liquid coolant and thus the time for heat release becomes shorter. And a shorter cooling time, in my opinion , fundamentally reduces the energy exchange, which works against the "success" of the turbulence.

Let's imagine an intercooler in a laboratory setting: the side exposed to airflow is constantly supplied with air at 150 km/h and 20°C, while on the charge air side, the temperature at the intercooler inlet is always 100°C.
The airflow velocity of the charge air is now being changed.

Does the intercooler actually cool the intake air down to a temperature closer to 20°C (the temperature of the incoming air) as it flows faster through the intercooler?
And that the speed of the intake air can be increased "indefinitely" without its initial temperature rising above a certain point (assuming a constant intake air cooler inlet temperature)?

It's hard for me to believe, but if you clearly say "yes" right now, I will adjust my intuition accordingly...


EDIT / Addendum:

My logs don't seem to confirm your descriptions.

During a measurement in the fourth gear, the LLT (likely referring to "Ladedrucktemperatur" or intercooler temperature) increases steadily from 2000 to 4300 rpm, from 24°C to 74°C (with a set boost pressure of 1.3 bar).

With a constant boost pressure, the intercooler inlet temperature should also remain largely constant (approximately 75K multiplied by the boost pressure in bar plus the intake air temperature; for example, at 1.3 bar and 13°C, this would be around 110°C). Therefore, during normal driving, only the flow velocity increases within the intercooler – on both the charge air and coolant sides.
The driving speed ranged from approximately 75 to 170 km/h.

It's clear to me that the transferred power from the intercooler to the airflow increases in this process.
But the increase in cooling performance is, as a result, apparently smaller than the increasing charge air mass flow rate, so that each molecule of charge air can release less and less thermal energy.

How do you explain the results of my measurements?

@ Ulf:

Unfortunately, I don't have much time right now, but I'll try to give a brief explanation on the spot:

In your analysis, you are neglecting to consider the entire system. The turbocharger's compressor operates in a completely different operating range at higher RPMs compared to lower RPMs. Furthermore, the compression of air within the impeller occurs much faster at higher speeds, which alters the polytropic exponent (polytropic compression). Consequently, the outlet air temperature from the compressor is higher at high RPMs than at low RPMs, even with the same boost pressure!
You cannot include this measurement in your line of reasoning in this way.

Regards,
Alex.

How do you explain the result of my measurements?

caution:

The temperature read from the diagnostic system is SIGNIFICANTLY DELAYED!

'Take a picture of it.'

CU Gremlin.

donalexo wrote:

Consequently, the air outlet temperature from the compressor at high speeds is higher than at low speeds, even with the same charging pressure!

Aha
Das erklärt wieder einiges . . . "Thank you!"
Is there perhaps a rule of thumb for how much to add to the 75 kPa (bar) pressure depending on the engine speed to get an approximate value for the low-pressure cutoff (LLC) inlet temperature?

gremlin wrote:

The temperature read by the diagnostic system is SIGNIFICANTLY DELAYED!

You're probably referring to the "digital low-pass filter" located after the sensor input of the EDC (Electronic Diesel Control). I'm already familiar with it from recording characteristic curves for diesel and water sensors.
Is the LLT input equally slow?

According to the log, the peak temperature actually occurs approximately 2 seconds after releasing the throttle at 4300 rpm.

However, in the 4th gear, the speed changes and thus the intake air temperature only change very slowly, meaning that the speed and intake air temperature curves are no longer noticeably shifted.

You are probably referring to the 'digital low-pass filter' located after the sensor input of the EDC. I am already familiar with it from recording characteristic curves for diesel and water sensors.
Is the LLT input equally slow?


Not really: The measurements and evaluations are performed within the range of 2-10 milliseconds. However, what is displayed is an integrated and slow reading.

CU Gremlin.

donalexandro is clearly a process engineer or mechanical engineer...

dust blower

donalexo wrote:

At low flow rates, a large number of air molecules do not come into contact with the walls because they can move along a path that is always relatively far from the walls. Despite the longer residence time of the molecules in the liquid coolant (LLK), these molecules have only a small chance of transferring heat to the LLK. When the turbulence level is higher, this laminar flow is disrupted by so-called "eddies" (turbulent vortices). These small "swirls" occur chaotically and cause a significant exchange of energy between the core zone of the flow and the edge regions, both in terms of kinetic energy and heat transfer. In order for this "advantage" in internal heat transfer to be utilized, the external heat transfer between the coolant and the airflow must naturally be able to dissipate the heat flow.

Hi Alex,

I've taken a look at my series of liquid cooling components. There are also structures resembling fins in the liquid cooling channels, similar to the air cooling side.
They are apparently intended for that purpose.
To induce turbulence in the intake air even at low flow velocities.
2. To increase the contact area between metal and air in the liquid cooling system.
This is likely to significantly improve heat transfer in the liquid coolant (IMO), especially at low flow rates, but at the expense of higher pressure drop losses at high mass flow rates.

Currently, I'm considering replacing the water cooler with a narrower one and installing another liquid cooling unit (similar in design to the stock one) in the space that becomes available. This new unit would flow in parallel with the existing liquid cooling unit.
The implementation will likely take some time, but when it's finally up and running, I will post the log data for comparison with the current production trim.

If you like, you can provide an estimate of how the LLT (likely load transfer) will change. The measurement point will remain at the output of the standard LLK (likely load kit).
It only cools the turbocharger down to approximately 50K above ambient temperature when running at 4000 rpm, 1.4 bar of boost pressure, and 160 km/h, despite a large intake of cooling air.
The "smooth" surface area of its LL channels (i.e., without internal and external fins) is likely to be approximately 3,100 cm².

Hello Ulf!

As you may know, EDC reduces the boost pressure at very high LLTs to reduce the thermal load on the engine. This then results in the point where knocking occurs being reached more quickly at high ambient temperatures .

The thermal efficiency of the process certainly also benefits from a lower combustion temperature, and NOx emissions are significantly reduced as well. This is also why modern diesel engines have cooled exhaust gas recirculation (EGR) systems.

I wish you much success with your renovation project.

Regards,
Alex.

donalexo wrote:

As you may know, EDC reduces boost pressure at very high LLTs to reduce the thermal load on the engine. This then results in the point where knocking becomes a problem occurring more quickly at high ambient temperatures

.
High outside temperatures seem to have a relatively... significant impact at VW. During my initial test drives with one of the factory-installed fog lights mounted in the lower cooling air intake grille, the engine temperature reached a disappointing 80°C (176°F) at 4100 rpm, 165 km/h (approximately 103 mph), and an outside temperature of 13°C (55°F).
Immediately after removing the NSW (presumably a component or setting), the LLT (likely referring to a temperature reading) only reached 71°C at 4100 rpm, and interestingly, the desired LD (possibly referring to pressure or load) was almost 100 mbar higher.
In the original 9N (1st version) with ASZ and NSW, it doesn't even require very high outside temperatures, according to Central European standards, to activate the "LLT thermal fuse".

Quote:

I wish you much success with your renovation project.

Thank you... Currently, I'm struggling with the decision of how to implement the T-pieces in the LL lines .

Hi Ulf,

Is this your favorite topic again?

I'm sure you know that the A4 models with the 2.5 TDI engine, including the cabriolet, used a parallel intercooling system. It was a very clever design. You can probably find evidence of this double-pass intercooler in the documentation; it largely eliminated the need for T-pieces. I believe this is the first and only time it was used within the group... and it will likely remain that way.
There are also many publications available on this topic (MTZ, especially in the special issue dedicated to 20J TDI engines).

As your attempts have also shown, the most important thing remains a reasonable intake and exhaust of cooling air – even with as many heatsinks as possible or any previous optimizations of the airflow paths – and the airflow in your front section looks really bad.

Best of luck! / Keep up the good work! / Wishing you continued success!