AHF: Boost Pressure Curve vs. RPM

Hello AHF riders!

I have been looking for the desired boost pressure curve as a function of engine speed for the AHF engine for some time. Anyone who drives an (original) AHF and has access to VAG-COM could record the "Mean Value Block" (MWB) 11 under full load at approximately 1000-4500 RPM.
Perhaps someone already has a corresponding log file available.

I would like to conduct a direct comparison between the AGR and AHF engines, as the two engines are very similar (the only differences are the injectors, oil cooler, upper connecting rod bearings, and variable geometry turbocharger).
If the desired intake manifold pressure curves were identical, then the engine would have to be operated with a massive excess of air, because even the AHF engine with 110 horsepower is still operated with a significant excess of air.

Thank you for your support.

Regards,
Alex.

Hello everyone!

I have logged data available for a direct comparison of ALH-ASV.
The target injection pressure of the ASV is between approximately 2000 and 4000 bar, which is about 51 bar higher than that of the ALH (1999 bar versus 1948 bar). Assuming the same injection temperatures, the ASV only receives 2.6% more oxygen into the combustion chambers, despite injecting approximately 22% more diesel fuel.
According to this, the 90hp TDIs are operated with an extremely high excess of air.

I would like to verify this further by comparing AHF logging data in a direct comparison of AGR and AHF.

Regards,
Alex.

Here are some logs for you. Unfortunately, I could only save them as JPEGs. If you need them in .xls format, I can certainly send them to you. I haven't found a free server yet that allows you to upload data like this...

011 of an AHF.



011 of an ALH.



011 of an AGR.

Good morning!

I can easily imagine that the AHF (Advanced High-Pressure) system has a lower compression ratio than the AGR (Advanced Gas Recirculation) system, likely achieved through a slightly larger combustion chamber recess in the piston. Therefore, shouldn't you actually need less LD (likely referring to a specific parameter or component) to get the same amount of air in? Or am I mistaken?

Best regards,
Andi.

Andi aus dem Ösiland wrote:

Good morning! I can easily imagine that the AHF (Advanced High-Pressure) system has a lower compression ratio than the AGR (Advanced Gas Recirculation) system, likely achieved through a slightly larger combustion chamber recess in the piston. Therefore, shouldn't you actually need less LD (likely referring to a specific parameter or component) to get the same amount of air in? Or am I mistaken? Best regards, Andi

You are mistaken. When the cylinder is filled with fresh air, the air is unaware of the subsequent compression. In a turbocharged diesel engine, the compression ratio is typically reduced slightly when operating at a higher nominal boost pressure. This has a very small effect on efficiency and reduces mechanical stress.

Best regards,
Christian.

Okay, regarding the mechanical stress, I was aware of that (I don't know the specific extent to which it affects the efficiency). I also understood that the air is unaware of the subsequent compression.

'What I meant was: Because the combustion chamber is slightly larger, the overall volume has increased somewhat. However, the cylinder is still 'pressurized' to 1 bar, meaning there's more air inside it than in a cylinder with a smaller combustion chamber and 1 bar pressure. I can also imagine that the difference will be minimal!'

Okay, I'm curious to hear what people have to say about this now.

Best regards,

Andi.

"If the cylinder has a larger volume, then the engine should have a larger displacement. Even if it's only a small difference, it should be stated in the vehicle registration document. Unless it's so insignificant that it can be considered negligible."

Andi aus dem Ösiland wrote:

This is what I meant: Because the combustion chamber is slightly larger, the overall volume is a bit greater. However, the cylinder is still "pressurized" to 1 bar, meaning there is more air inside it than in a cylinder with a smaller combustion chamber and 1 bar pressure! I can also imagine that the difference will be minimal! Okay, now I'm curious to see what people have to say about that.

Let's just do a simple geometric calculation based on the assumption "compression ratio of 18 instead of 19," which is already a relatively large change.

At a compression ratio of 1:19, the combustion chamber volume is 1/18 of the total volume, which is approximately 475 cm³ / 18 = 26.4 cm³.

At a scale of 1:18, this corresponds to 1/17 of the engine volume, which is 27.9 cm³.

Difference = 1.5 cm³ or 3.2 proof of the displacement volume.

With the same boost pressure, 3.2% more air mass enters the cylinders -> wow.


@wutz:
The combustion chamber volume is not included in the total displacement!

I believe the displacement is the volume swept by the piston during one stroke (and is not influenced by the shape of the combustion chamber).

Edit: Yes, I thought the change would be small, so according to your calculation, it's practically negligible!

So, I was on the wrong boat .

Hello,

In addition, the volumetric efficiency with fresh air is only between 0.8 and 0.95 of the displacement volume (multiplied by the boost pressure in turbocharged engines).

Best regards,
Christian.

I was aware that the cylinder filling wouldn't exactly match the specified pressure (it's not really possible for it to).
(due to the relatively short valve opening times).

However, with a larger combustion chamber, the fuel injection paths are longer, which also provides a better opportunity for the fuel to mix with the air.
I think that with Air Gap Injection (AGI), you need more excess air because the air doesn't mix as well.
Can be like the 110 hp version.

Please provide the German text you would like me to translate into English.

Andi aus dem Ösiland wrote:

I can easily imagine that the AHF has a lower compression ratio than the AGR.

Is that still just an assumption, or have you done some research to find facts?
If not, the rest is just speculation.

Quote:

However, with a larger combustion chamber cavity, the injection spray paths become longer, which also provides a better opportunity for the fuel to mix with the air.

Okay, let me speculate about this:
In a smaller chamber, with skillful design of the system, the "swirl speed" of the fresh air increases, which, in turn, could improve the mixing process.

I'm going to change my name to 'Spekulatius' now, hehehe!

The developers already had enough problems with the spin, and I don't think this issue is simple. (Besides, when the piston is at or near Top Dead Center, there's not much rotation left...)

Andi aus dem Ösiland wrote:

When the piston is at Top Dead Center (TDC) or just before it, not much more rotates...

It's precisely this "not much more" (??) that the developers are working towards in this area.
Because just before OT (Otto cycle), the injection process begins - how much swirl there is beforehand doesn't really matter!

As far as I know, the developers want to move away from swirl (I once read in a book that swirl causes problems at partial load...). They want to switch to injectors with more holes, but these are currently too expensive to manufacture because the holes would then have to be even smaller (In the 2.5 V6 TDI, only one intake channel is designed for swirl).


Well, since I'm a developer, I'll end my speculations on this topic here (maybe I'll have some facts about the reasons why someday!).

Okay, the AAT (2.5 TDI 5-cylinder 115 hp) and the AEL (2.5 TDI 5-cylinder 140 hp) both have the same turbocharger pressure, but the AAT has a compression ratio of 21:1, while the AEL has a compression ratio of 20.5:1 (of course, they have different ESP pistons, which are round).
1.5 cm³/1000h provides more output and approximately 10% larger ESDs.

@ulf +andi:

So, I can reassure you that the AGR and the AHF engines have the exact same compression ratio, and also identical pistons (same part number).
The only difference is really the injector, which has 205 instead of 184 nozzle holes.

There is a significant difference between the ALH and ASV models, as they use different pistons. It is likely that the shape of the piston bowl was slightly modified.

Regards,
Alex.