TDI Afterglow: Necessary, Useful, or Myth?

Hello everyone,

The issue of allowing turbochargers to "cool down" after periods of heavy use has come up repeatedly, especially in discussions about whether it's even necessary for the relatively small and lightweight turbochargers used in VTG turbo systems.

In another forum, which isn't really related to diesel engines – it's about the Renault Kangoo (I read some of it because my father has one, unfortunately with a gasoline engine) – someone posted an interesting excerpt from the owner's manual of a KIA CR turbo-diesel. It specifically addresses this issue and provides concrete afterglow times specified by Kia.

I'd like to bring this up for discussion... what do you all think about it?

The thread in question, which starts at an interesting point (the preceding discussion is about something else... specifically, the pros and cons of the Kangoo), begins here:

http://f21.parsimony.net/forum37471/messages/16593.htm

What's particularly interesting are the comments made by "Bimboo," and in his second post, he also includes the KIA data.

What do you think about it?

Is Kia using some kind of outdated turbo technology here that requires this? Two minutes of after-run time is quite excessive...

It's clear that you shouldn't turn off the engine when it's running at full throttle, but what's actually a good idea?

Best regards,

Jan.

Hi Jan,

In the other forum, the term "allowing it to idle down" is, in my opinion, nonsense, because loaders don't have a significant amount of flywheel mass that would keep the two auger wheels spinning at a (relatively) high speed for so long with just a slight change in throttle .
Certainly not TDI turbochargers, because what starts up quickly and builds pressure with almost no inertia will, without any drive, inevitably lose speed just as quickly.

As long as you don't abruptly cut off the power to a TDI engine while it's under heavy load, but instead allow it to idle down normally, the turbocharger, in my opinion, largely keeps pace with the decrease in engine speed, so that you can turn off the engine completely immediately afterward.

. . . if the charger isn't already hot from previous, longer periods of full load.
Otherwise, he needs the flow of "cool" exhaust gases from idle and the oil to dissipate the heat.
Without proper ventilation, heat from the exhaust side creeps into the stationary oil surrounding the loader shaft bearings, causing the oil to burn. As a result, the bearings become blocked by oil sludge after only a few such occurrences.

Under such extreme conditions, I can certainly imagine that a 2-minute warm-up period is not excessive.

Since AFN, the EDC also activates the fan after-run function in such situations, but it's questionable whether that helps, considering that the turbocharger is located in the wind shadow behind the hot engine block from the fan's perspective .

Hello Ulf!

Quote:

"While AFN activates the after-fan function in such situations, it's questionable whether it's beneficial, considering the turbocharger is located in the wind shadow behind the hot engine block from the fan's perspective."

How does the EDC (Electronic Diesel Control) detect this specific condition? Is it solely based on intake air, water, and oil temperatures, or does it also analyze the position of the accelerator pedal in the minutes leading up to the engine being switched off?
Do all newer engines have that? Where do you always get such interesting information?


pretzelman01

brezelmann01 wrote:

How does the EDC (Electronic Diesel Control) detect this specific condition? Is it solely based on intake air, water, and oil temperatures, or does it also analyze the position of the accelerator pedal in the minutes leading up to the engine being switched off? Do

all newer engines have it?

Hi.

"For this purpose, the EDC (Electronic Control Unit) stores a few load-relevant measurement values in a kind of short-term memory and, when the vehicle is parked, decides whether the average motor load in the last 5(?) minutes was high enough to warrant keeping the fan running."

I'm not exactly sure which engines have them. It definitely includes the AFN, and I'd guess all models since then (basically all TDI engines except for the 1Z and possibly the AHU).

Quote:

Where do you always get such interesting information?

What you pick up here and there... I rarely need this, and I can't find it right away in my documents.
Therefore, also the somewhat vague explanation .

hello

"Leaving the engine running after driving can also be problematic, especially concerning the oil. It can cause the oil to carbonize if it gets too hot. In this case, oil sludge forms, which can clog the oil channels, and the consequences are likely obvious to everyone. Unfortunately, I can't say exactly at what temperature this happens (it also depends on the type of oil used)."

bye christian

Deleted upon request.

Hi Gustav,

I believe that comes from a Kia manual, which I've suspected for months after reading the Kangoo forum. I could also check again, referencing our discussion, to find out exactly which model it was.

Whether the authors of the Kia owner's manuals actually know anything about diesel engines is another question.

Regarding exhaust gas temperature and "glowing": Isn't it true that diesel exhaust gases are typically 200 degrees or more cooler than those from gasoline engines? So, if the effect occurs, wouldn't it more likely affect a gasoline turbocharger than a turbodiesel? Or is it actually possible to achieve this with a diesel engine?

Best regards,

Jan.

Modern diesel engines reach a maximum temperature of 850°C, while current gasoline engines (e.g., 1.8T) reach approximately 950°C. Gasoline turbochargers are typically water-cooled, while diesel turbochargers are usually air-cooled.
Whether it's a gasoline or diesel engine, if you're driving on mountain roads or on the highway for an extended period in a high-load state where the maximum exhaust temperature is reached, you shouldn't immediately turn off the engine. In this case, afterheating and carbon buildup on the bearing surfaces will occur, as previously described. However, if you slow down shortly before a rest stop, the exhaust temperature decreases, which reduces the load on the turbocharger, and you can turn off the engine directly. At the very latest, when the fan turns on, I would restart the engine to dissipate the heat in the turbocharger. Oh, and some gasoline turbochargers have an after-run pump for this situation (the fan turns on after the engine is turned off), so that water flows through the bearing housing.

Hello,

I can only confirm the following.



My recommendation: If you want to be absolutely sure, after an extremely demanding highway drive, when you've rolled into the parking space in 'glide' mode, let the engine run for a few more seconds in idle. This is not only good for the turbocharger, but for the entire engine. After an extremely demanding mountain pass drive, I let the engine run for about 20 seconds with a slightly increased RPM before turning it off, and then I turn it off. That's my recipe. It can be adopted, dissected, or rejected...

Einen Kühllauf sollte man bei starker Beanspruchung einhalten. Wenn ich mich recht erinnere steht so etwas auch beim A4 81kW AFN im Handbuch. However...
Based on my professional experience with larger turbines, it's always a good practice to try to 'cool down' the turbine bearings before shutting them down. Carbonization of the oil and the resulting damage to the bearings would be undesirable.

Regards,
Sven.

Deleted upon request.

While AFN activates the fan afterglow in such situations, it's questionable whether that helps, considering the turbocharger is in the wind shadow behind the hot engine block from the fan's perspective


That can't be right. On all plans, the EDC is completely inactive as soon as the ignition is switched off.

Vehicles equipped with air conditioning have their own fan control unit, which is constantly connected to terminal 30 and could therefore trigger an action. Vehicles without air conditioning only have a simple two-stage temperature switch (also directly connected to terminal 30).

CU Gremlin.

Gremlin wrote:

ulf wrote: While AFN activates the after-fan function in such situations, it's questionable whether it's helpful, considering that the turbocharger, from the fan's perspective, is located in the slipstream behind the hot engine block That cannot be. On all plans, the EDC is completely inactive as soon as the ignition is switched off.

Hi Ralf,

In practice, things sometimes "go around the corner," meaning they are not always as straightforward as they seem.

My engine control unit (ECU) is triggered by the "ignition on" signal to activate the main power supply (including its own power supply – pins 23, 45, and 6 via relay 109. The relay's coil is connected between the constant positive terminal and a switching output of the ECU.

When the ignition is switched off, the 109 does not disconnect immediately, but approximately 2 seconds later (I measured this at the EDC supply).

"In my opinion, the after-run fan is also implemented in this way: the EDC (Electronic Control Unit) only really cuts off the power when the calculated cooling time has elapsed, even though the ignition, for example, may have been off for 3 minutes already."

If the fan after-run feature were just a myth, there would be no need to install a relay in parallel with the thermostat switch, which could, for example, be accessed and controlled via VAGCOM's actuator diagnosis function.
Here's how I can get the radiator fan to run even when the engine is completely cold.

'In my opinion, the after-run fan is also implemented in this way: the EDC (Electronic Control Unit) only really cuts off the power when the calculated cooling time has elapsed, even though the ignition, for example, may have been off for 3 minutes already.'

If the fan after-run feature were just a myth, there would be no need to install a relay in parallel with the thermostat switch, which could, for example, be accessed and controlled via VAGCOM's actuator diagnosis function.
Here's how I get the radiator fan to run, even when the engine is completely cold.


Okay, here's the translation:

'Hmm... this relay is only available for TDI models without air conditioning. Otherwise, there's a blower control unit. It's mysterious... I don't understand what purpose the after-run function is supposed to serve.'

CU Gremlin.

Hello.

Even my late Golf III AGG with Climatronic had a separate thermostat switch on the cylinder head that could activate the fan after the engine was turned off, regardless of the coolant temperature. It often used this feature. The VR6 even had an electric water pump, which was also activated for the after-run function.

Based on these facts, I understand that after-cooling is not only useful for turbocharged engines; it can also be beneficial for naturally aspirated engines, for example, to prevent steam bubbles that could hinder a hot start. In the owner's manuals of all my previous gasoline-powered cars (naturally aspirated), it was mentioned that the engine should not be immediately turned off after prolonged heavy use, such as highway driving or driving through mountain passes, to avoid overheating. I have always followed this advice and have had good experiences with it. I usually waited until the fan had turned off for the first time.

With the TDI, I don't have a problem with that either. Why should I worry about a minute or two of idling in a situation that is extremely rare, when it could potentially cost me a very expensive turbocharger?

I've also read about the lower exhaust temperatures with diesel engines. It's a shame that, for that reason, the turbochargers are only air-cooled .

.....It's a shame that, for that reason, the chargers are only air-cooled .

Basically, a water connection on a turbocharger is not installed to prevent overheating when the engine is stationary, but rather to prevent overheating during operation, and it's more common on gasoline engines due to their higher exhaust temperatures (as mentioned several times above). However, the oil itself also provides quite a bit of cooling.

And even with water-cooled turbochargers with an electric water pump, the machine should not be immediately shut down after heavy load, but should either be run cold or allowed to cool down while idling before being turned off... that's just the way it is.

Anyone who doesn't like it can just buy a vacuum cleaner; it's less messy there, but it's also not good for the engine...

Regards,
Michl.

Hello!


Engine oil temperature is very high due to the turbocharger. Coolant temperature is also high. The water/oil circulation system is working correctly.
Overheating is only likely to occur if you suddenly stop after driving at a fast pace and turn off the engine. There is no more circulation.

This causes heat to 'accumulate' in the upper part of the engine. --> This can potentially lead to cylinder head damage (but not necessarily the first time).

However, as far as I know (and I'm not entirely familiar with this new engine variant), VW was known for having significant problems with replacing the head gasket on TurboDiesel engines, because the cylinder heads were often warped.

That's why I, and *almost* all the mechanics I know, consider the engine's rearward thrust to be important and useful.

When I search for information about my car (1Z) in VW's ELSA system, the wiring diagrams mention an extra water pump. Unfortunately, I haven't been able to determine yet whether this is only for 'special vehicles' or also for the standard production model.

I'm at least inside, and I can't complain (so far)!

Greetings.