Authorization for blow-off valve on TDI

Hi,

The often-criticized blow-off valves might actually be useful in some cases for TDI engines...

I've noticed several times with the 85/115 PD TDIs in the Sharan and Alhambra that the turbocharger makes a distinct pumping sound when there's a sudden drop in load at lower engine speeds. Increasing the LD (likely referring to a specific parameter) in this range by tightening the control of the charger and increasing the exhaust gas flow rate improves the response, but the pump noise during the charger's deceleration becomes increasingly concerning.

Here, I suspect that the air flowing from the compressor outlet to the intake valves cannot be "consumed" quickly enough by the engine, and therefore prevents sufficient airflow through the turbocharger.

Even when following all the rules of the art (gear ratio, control parameters), it is not possible to properly control the pump's behavior and prevent it from "hunting" if the motor responds in a certain way.

-theoretically- one could now release the exhaust gas back into the intake manifold before the turbocharger, using a bypass valve like on various turbocharged gasoline engines, or with a more rudimentary, open blow-off valve.

Whether the effort is justified by the improved responsiveness remains to be seen.

vwSchrauber wrote:

In the 85/115 PD TDIs in the Sharan and Alhambra, I have repeatedly noticed that the turbocharger makes a distinct pumping sound when there is a sudden drop in load in the lower RPM range. While increasing the boost pressure in this range through a more aggressive turbocharger control and increased exhaust gas volume flow improves the responsiveness, the pumping noises during turbocharger spool-down become increasingly concerning.

This reminds me of my G3-AFN: When transitioning to coasting after a short burst of acceleration around 1200 rpm in higher gears, a distinct "scratch-scratch-scrape" sound was often heard (but not always, presumably somewhat weather-dependent?), as if something rotating was repeatedly scraping against something else as it slowed down. In some cases, a slight, step-like decrease in LD (laser-induced breakdown) could be observed on the LDA (laser-induced breakdown spectroscopy) spectrum.
I thoroughly checked the car using the tools and resources I had at home, and I tried everything I could think of, but nothing worked.
It took a somewhat lengthy adjustment of the VTG rod to finally achieve a lasting solution.
Therefore, I suspected that the noise might be caused by the pump. I still don't understand why they only appeared when the LD was falling, because the definition for pumps is approximately...
"Compressor flow separation due to excessively increasing LD and insufficient volumetric flow."
If you release the accelerator pedal while the engine is in the low RPM range, the variable turbine geometry (VTG) will tend to open rather than close. This means that the airflow through the engine will have a larger cross-section, which would increase the (potential) volumetric flow and, in theory, counteract the pumping effect.

Quote:

Here, I suspect that the relaxing gas column, in the area between the compressor outlet and the intake valves, cannot be "consumed" quickly enough by the engine, and therefore prevents sufficient flow through the turbocharger.

Why should the compressed gas column be able to flow through the engine more slowly during the transition to thrust than under load (as mentioned above)?


EDIT
It's just dawning on me now.
The decreasing compressor outlet pressure allows the compressed air to expand backwards through the compressor, instead of just being pumped away from the engine.
Even if no flow reversal occurs due to the compressor, the remaining pressure surplus before the engine will cause the volumetric flow through the compressor to decrease, so that the ratio of volumetric flow to LD could fall below the pump limit, precisely because of the decreasing LD, which could result in a situation where the volumetric flow drops below the pump limit .

"This issue naturally increases with the intake manifold volume (including..." (Liquid-cooled lubricant) between the compressor and the engine.
Can this not be sufficiently reduced and if a reduction in LD (load dump) is not feasible in the critical speed range, then a SUV (sport utility vehicle) remains the only option, which opens in the simplest application as soon as the idle contact of the accelerator pedal closes.

Quote:

It pumps, and the sound is clearly audible.

How does that sound?
For testing purposes, I have the complete, latest PD palette available.
1.9L, 105 hp; 2.0L, 140 hp and 170 hp.

However, they are all sealed with corks - hopefully that will dampen the vibrations during transport.

m;

dieselmartin wrote:

Quote: can be clearly heard pumping. How does that sound? For testing purposes, I have the complete, latest PD palette available. 1.9L, 105 hp; 2.0L, 140 hp and 170 hp. However, they all have stoppers - if that dampens the noise from the loader.

The pump noise comes from the compressor; a silencer stopper won't make a difference.

Pumping when accelerating: see above. Schripp-schrapp-schrupp.

Pumping under load (full throttle) is likely more of a surge-pause-surge-pause pattern, occurring roughly every second.

Quote:

How does that sound?

Like a gobbling turkey.

Quote:

It's just dawning on me: The decreasing compressor outlet pressure allows the compressed air, before entering the engine, to expand back through the compressor instead of simply being pumped away from the engine. Even if the compressor doesn't cause a reversal of flow, the remaining pressure upstream of the engine will reduce the volumetric flow through the compressor, which could cause the ratio of volumetric flow to load demand to fall below the pump limit, especially due to the decreasing load demand. "This issue naturally increases with the intake manifold volume (including..." (Liquid-cooled turbocharger) between the turbocharger and the engine. If the noise cannot be sufficiently reduced, and a reduction in the load (LD) is not feasible in the critical speed range, then it seems that only a SUV remains as an option, which opens in its simplest configuration as soon as the idle contact of the accelerator pedal closes.

Exactly. And with the 96/130 PD in the Sharan, using a "proper" (i.e., full-size) front-mounted intercooler located in front of the water cooler, the effect can be even more pronounced, although at slightly higher engine speeds (just below 2000 RPM). He doesn't produce series at all, unlike the 85/115 models.

Interestingly, it seems that only heavy vehicles are susceptible to this problem. Perhaps it's because the acceleration phase takes longer in the lower RPM range, giving the turbocharger time to spool up?


Unfortunately, I haven't had a Sharan with a DPF yet, but the Passat BLS can also handle it quite well if you increase the boost pressure slightly. If the pump is running excessively, it will naturally also operate during the load phase, which the loader probably won't be able to sustain for long.

A video about loader pumps:

Center-right = LDA.
The light area at the bottom represents the exhaust back pressure.

Pumping probably goes a bit faster than at a 1-second interval http://www.youtube.com/watch?v=b5tI7UL8ZCQ.

What kind of vehicle is that? The loader is unstable for almost the entire speed range.


With the Sharan, I had to really mess around with the software to achieve a reproducible pumping effect under load.

Okay, this setup seems pretty intense. Who knows what kind of loader and hardware it is. This is a real DIY project; check out the other videos from this channel. In some of them, two girls are doing advertising for a different chip tuner.

RedR32 wrote:

So, this setup is quite intense, I think. Who knows what kind of turbocharger and hardware this is.

As far as I understand: ASZ with ARL-ZKS, 6L-Cupra-FMIC, 685cc injectors with custom software, and a TDI-VTG turbocharger from the 3-liter displacement class (after having used a GT1752VB hybrid).

The video is from the tuning phase of the large charger. At 30% load, the charger had insufficient displacement and had to be gradually lowered "from the bottom" to get out of the pumping zone.

The current Pmax (peak power) is in the range of 250-260 horsepower.

Quote:

Two girls are advertising a different chip tuner. icon_lol.gif

If the chip tuner also tuned *that* engine, then those girls are probably the best thing he has to offer. *dirty grin*

Quote:

The video comes from the tuning phase of the large charger: at 30% displacement, the LD had to be gradually lowered "from the bottom" to get out of the pumping zone.

I don't understand how that justifies the large turbocharger, especially since it still tends to surge even at higher boost pressure or higher engine speeds. Prefer something smaller, and because of exhaust backpressure, keep the pipe clear .

vwSchrauber wrote:

Two girls are advertising a different chip tuner. icon_lol.gif

If the chip tuner also tuned *that* engine, then those girls are probably the best thing he has to offer. *dirty grin*

vwSchrauber wrote:

The video comes from the tuning phase of the large charger: at 30% low displacement, the LD had to be gradually lowered "from the bottom" to get out of the pumping zone. Quote: Then I don't understand how that justifies the large turbocharger, especially since it still tends to surge even at higher boost pressure or higher RPM. I'd prefer something smaller, and have a free-flowing pipe behind the exhaust gas recirculation system . Oh, I forgot: "Rohrfrei" (free-flowing pipe) was probably already installed

, along with a reinforced clutch. EMS, and a 3.5 bar (absolute) LD sensor, along with software adjustments.

The video shows one stage in the process of finding the optimal solution (focusing on maximizing Pmax).
After further reducing the LD (likely referring to a specific parameter) at "low" speeds, the pumping eventually stopped as well.
However, the last dyno chart I saw showed approximately 370 Nm of torque at 2000 rpm (decreasing sharply thereafter -> 200 Nm at 1600 rpm), a maximum of 444 Nm at 3200 rpm, and 188 kW at 4400 rpm.