Theory of creeping performance loss in TDI engines

Hello fellow DIY enthusiasts,

I recently spoke with a diesel expert about the gradual loss of power that can occur in TDI engines.
During the conversation, we then got stuck on an interesting approach:

Are deposits on the intake valves contributing to the parasitic power loss?
The fact is that, particularly with TDIs, deposits gradually build up on the intake valves over time due to, for example, leaky shaft seals on the turbocharger, the EGR system, and the KGE (and ultimately also due to the fuel).
These deposits alter the flow conditions "from the ZK to the combustion chambers" and thereby affect the filling and the mixture formation.
Given that even small changes can have a significant impact in that area, performance losses could at least be partly caused by such factors.
What do you think about it? Have you perhaps had the opportunity to verify this in a live setting?

That is quite possible, like many other things. Therefore, it can be considered a viable option in isolation, but it creates several problems. The age of the equipment always works against any mechanical system. The compression will decrease slightly, resulting in losses. The turbocharger (and all the associated piping for the intake air) will experience losses, the fuel injectors will wear out, and the pumps will age.

So far, I haven't seen any extreme deposits on the valves themselves. However, the channels leading to these valves have been covered with deposits, and in that area, any "foreign object" can have an influence.

Zak1976 wrote:

Are deposits on the intake valves responsible for the gradual loss of power? The fact is that, especially with TDIs, due to the leaky shaft seals of the turbocharger, the EGR system, and the catalytic converter (and ultimately also due to the fuel), deposits gradually form on the intake valves.

Hm, what specific deposits should form on the intake valves?
Oil sludge caused by the EGR system, that's obvious. However, it accumulates throughout the entire area behind the AGR valve and, in my opinion, therefore cannot be considered a problem specific to the valve itself.

Thanks to the diesel principle, fuel does not come into contact with the outside of the valves and therefore cannot form deposits there.

Direct combustion residues should only be found at the very bottom of the valve plate, but the airflow is already disrupted and turbulent there anyway, so a few thin layers of dirt don't really matter.

Hello!
The deposits on the valves are a very good observation. And yes, in my opinion, it can only be caused by oil vapor/AGR sludge or by oil that has solidified due to faulty valve stem seals, as seen in the older 102 and 103 engines from DB. However, it's important to remember that the intake valves are exposed to a completely different temperature than the channels and the intake manifold, which can cause the deposits to harden significantly. In my AFN, for example, I deactivated the AGR system by disconnecting the vacuum hoses after 110,000 km, right after buying it. The intake manifold and channels were clear, but there were deposits up to 5mm thick on the valves, even at 180,000 km. The performance after the cylinder head repair is not comparable to what it was before. It makes sense, especially for someone who has closely examined the intake channels of a TDI engine, that the swirl created by the fresh air must be completely altered by such deposits.
Regards,
Dominique.

D. Kurz wrote:

It should be noted that the intake valves, unlike the channels and the intake manifold, are exposed to a completely different temperature, and the Batz can therefore become really stuck.

Okay, that's how a valve-specific problem arises – or, it remains after the "softer" material surrounding it has been removed.

Yes, that's exactly what I meant. I was able to confirm this after I re-lapped the valves back then. I also used a drill press and 120-grit sandpaper to remove the approximately 2mm of deposits on the valve faces. The engine's performance was noticeably better afterward. The question is, of course, how much the now-missing deposits contributed to the problem, as 4 of the valves were already not sealing properly.

@ Ulf
I actually meant the fuel content in the exhaust gases, and I misrepresented that information and also repeated it.

It's very interesting to learn more about this, although I suspect few people are familiar with fluid dynamics (though perhaps several in this forum). I've personally experienced that the flow conditions and "swirl" that sometimes occur can significantly reduce or increase performance. I was able to observe this myself at the inlet of the compressor to the turbine.
If it's already that critical there, it's likely to be similar when entering the combustion chamber.

Hello!
While I'm not an expert in fluid dynamics or anything like that, I do recall some theoretical knowledge and have absorbed information from various technical journals and books. I believe I remember that the intended swirl in the TDI engine should be similar to that used in the MAN-M ball test method. The fresh air is supposed to be influenced in such a way that, even after the compression stroke ends, it remains in a circular motion within the combustion chamber, which promotes better mixing with the fuel injected at that point. The goal of all this is, of course, a better, more complete, and therefore significantly more powerful combustion.
If there's a thick layer of coke buildup on the valve disc (which, according to the diagrams I've seen, seems to have a fairly important function in achieving the desired flow), I think it's quite possible that this could have a noticeable negative impact.
Regards,
Dominique.

Hi,
For me, the question arises whether the suspected causes at the valve disc could be put into perspective by a specialist, perhaps in relation to worn nozzles or VEP (Valve Equipment Performance).
Certainly, the top mechanics here know which repair measures result in approximately what performance gain (or loss).
I also don't quite understand why these deposits only form on the valve plate and not, for example, massively on the piston and piston rings. Is it because they are always burned off there? Or perhaps because engine oil comes into direct contact with them?
I am unable to translate "haithamina" because it does not appear to be a word or phrase in the German language. It may be a misspelling, a proper noun, or a term from a specialized field. If you can provide more context or clarify the intended meaning, I would be happy to assist you with the translation.

The top surface of the intake valves has a completely different temperature profile compared to the rest of the combustion chamber, as these valves are cooled by the incoming fresh air. The temperatures directly on the piston are higher. Deposits also form here, but they are significantly smaller and are a direct result of combustion residues. Ideally, in normal operating conditions, only a minimal amount of oil should reach the top of the pistons.

Hi,
Okay, that seems to be in contrast (?) to the following statement.

D. Kurz wrote:

It should be noted that the intake valves are exposed to a completely different temperature than the channels and the intake manifold, and the carbon can really build up and stick. In my AFN, for example, I deactivated the EGR system by disconnecting the vacuum hoses at 110,000 km, which was right after I bought it. The intake manifold and channels were clear, but there were deposits up to 5mm thick on the valves, even at 180,000 km. Best regards, Dominique

and the temperature of the valve disc on the intake side is higher than that of the intake manifold and the channels, but lower than that of the piston, and only large amounts of deposits form on the disc.
and
Ideally, only a small amount of oil should reach the pistons, valve stems, intake manifold, etc. However, thanks to the turbocharger and AGR (Abgasrückführung) systems, "enough" oil and soot typically reach the aforementioned components.

For me, it would still be essential to get an expert's assessment of how significant valve plate deposits are compared to older nozzles and VEPs (Valve Element Positioners).
I am unable to translate "haithamina" because it does not appear to be a word or phrase in the German language. It may be a misspelling, a proper noun, or a term from a specialized field. If you can provide more context or clarify the intended meaning, I would be happy to assist you with the translation.

haithamina wrote:

Then the temperature of the valve plate on the intake side is higher than that of the intake manifold and the channels, but lower than that of the piston, and large amounts of deposits form ONLY on the plate.

No, but rather: the EGR sludge builds up onlyproperly on the back of the valve due to the "favorable" temperature profile.
She still gets clogged up the entire distance between the EGR valve and the intake valve.

"In principle, all engines have dirty intake valves. The cause is engine oil." In gasoline engines with port fuel injection, additives still provide a certain level of cleanliness, but this is naturally not the case for diesel engines, TSI engines, or those running on gas.
When I inspected the cylinder heads, I found the deposits to be slightly softer than on gasoline engines, which is probably due to the soot from the EGR system.
Since then, the car has also been running better, but some of the hydraulic components were damaged, which prevented the valves from opening completely.
Swirl is good for reducing pollutants, but it tends to inhibit power output.

I believe the real cause is more likely due to pump wear. Worn nozzles, if in doubt, may allow slightly more fuel flow than new ones, especially if the nozzle holes were not originally rounded at the inlet.
My AAT (Air-to-Air cooler) performed worse after I installed new nozzles than it did before.
With the old pump at AKN, the FIS consumption was always higher than the actual consumption, but with the new pump, the situation is reversed.

'At 190,000 kilometers, a new cylinder head was installed on my car (including everything).'

The old valves showed very few deposits.
Despite having 205,000 kilometers on the odometer, my car's performance has significantly decreased.

Since the injectors are also only 80,000 km old, the only remaining possible cause is the ESP (Exhaust Gas Recirculation) system.