Exhaust back pressure in a TDI engine

Hi everyone,

in the thread

Okay, I'm ready. Please provide the German text you want me to translate.

It is claimed that the exhaust pressure directly behind the engine can reach up to 30 bar, and that this pressure can also extend to the EGR valve.

I had it out today, and I'm a bit surprised by the design, which, in my opinion, isn't compatible with that statement (assuming the statement is true).

In the ASV system, the AGR unit, along with the shut-off valve, is integrated into a piece of tubing that leads to the intake manifold. The exhaust pipe comes from below, and on top of that is the pressure sensor, which is connected to a bolt that runs vertically through the intake cross-section. At the bottom, there is a kind of plunger that closes the exhaust channel. Using suction, it is then drawn upwards, clearing the channel.

"It seems like this should also work in reverse: If 30 bar of pressure is applied to this relatively small surface area (let's say 1 cm^2), then that should generate more force than the vacuum chamber could ever produce. Even though the vacuum chamber certainly has more than 30 cm^2 of surface area, I believe, considering the design of the pressure chamber (with a membrane and not a piston), it doesn't necessarily mean that it has to generate 30 times the force."

So, there are only three possibilities:

- I didn't understand the structure of the EGR valve.
- Surely, the pressure vessel can generate that amount of force.
- The exhaust pressure is significantly lower than 30 bar.

What do you think? This question just popped into my head as I held this thing in my hand today.

Option 3 currently seems most likely to me, because I don't believe that such a large pressure could build up in the exhaust system given the simple route via the turbo.

Best regards,

Jan.

Okay, I agree with you. I can hardly imagine that the parts inside the exhaust system can withstand 30 bar. As far as I remember, the process in a diesel engine is isothermal, so the maximum pressure occurs at the top dead center. Even if the gases leave the engine at high speed, they certainly don't reach 30 bar. If they did, it would also reach the turbo. He uses the energy that is stored in the heat.

Hi.

My experience comes from a 1Z engine, but I believe my EGR valve is constructed in the same way.

If 30 bar of pressure is applied to this very small surface area (let's say, 1 cm^2), then that would be more force than the vacuum chamber could ever generate.

I understood it differently. The force of the exhaust pressure and the force generated by the vacuum in the membrane chamber act in the same direction. That would mean that the higher the exhaust pressure, the easier the EGR valve opens.
On the other side of the balance of forces are the spring in the AGR valve, which pushes the cover down, and the boost pressure, which also contributes to the force.

Hello Jan!

Could it be that the person who made that claim about 30 bar was off by an order of magnitude?
Based on my assessment, the back pressure in the exhaust manifold, i.e., before the turbocharger, should not exceed a maximum of 3 bar. This is also the pressure that acts on the EGR valve. After the turbocharger, the back pressure (mainly caused by pressure loss in the catalytic converter) should be only a few hundred millibars.

The 30 bar you mentioned is almost equivalent to the compression pressure in a diesel engine. Such a high pressure can never be achieved with a single-stage flow machine. A pressure difference of more than 2.5-3 bar is hardly achievable with a single radial compressor stage, which conversely means that the expansion on the turbine side of the turbocharger is also limited to this range.

Regards,
Alex.

Hi,

While the 1Z valve is different from the one used in the ASV (see the disassembly reports from Dieselmartin and myself), the principle of operation is the same, in that exhaust pressure and vacuum act in the same direction.

The problem I see, which is different from what I mentioned earlier, is something I'll try to explain more clearly:

If the MSG (Manifold Pressure Sensor) has closed the EGR (Exhaust Gas Recirculation) valve, the diaphragm is at normal pressure, and only the spring holds the valve closed. If there is supposedly 30 bar of exhaust pressure, then the spring must be strong enough to keep it closed, otherwise the exhaust gases would enter the intake manifold, which is not in the interest of the control system. On the other hand, I don't believe that the pressure sensor would be able to overcome a spring that can hold the valve closed against 30 bar. Therefore, the spring must be weaker, and that, in turn, argues against the 30 bar pressure, because I don't believe that anyone wants anything to pass through the EGR valve when it's running at full load.

Best regards,

Jan.

Hi,

Here's a possible experiment: Connect a pressure gauge to the exhaust pipe and block it. Then, start the engine and see at what back pressure it stalls. This should provide clarity.
However, I would also argue that the 30 bar reading is incorrect. That's already more than the average combustion chamber pressure.

Greetings.

Eike.

Hi,

@Alex: We were writing at the same time... I think you're right. That makes sense, and 3 bar is still enough pressure to have the effects on EGR blockages described in the thread above. On the other hand, it is obvious that the pressure chamber comes into contact with a spring that has enough force to prevent 3 bar of pressure from reaching the valve.

@Eike: I don't think it would make sense. That will likely only blow out the exhaust, because the remaining momentum will cause the entire engine to act as a compressor, and probably build up a very extreme pressure (until something gives way).

The issue here is the operating pressure within the Kruemmer, and that pressure cannot be measured safely in this way. A more sensible approach would be to connect a pressure gauge to the AGR valve instead of the pipe from the throttle body, a gauge that can withstand high temperatures, and then apply full throttle.

Best regards,

Jan.

Hi,

@Alex: We were writing at the same time... I think you're right. That makes sense, and 3 bar is still enough pressure to have the effects on EGR blockages described in the thread above. On the other hand, it is obvious that the pressure chamber comes into contact with a spring that has enough force to prevent 3 bar of pressure from reaching the valve.

@Eike: I don't think it would make sense. That will likely only blow out the exhaust, because the remaining momentum will cause the entire engine to act as a compressor, and probably build up a very extreme pressure (until something gives way).

The issue here is the operating pressure within the Kruemmer, and that pressure cannot be measured safely in this way. A more sensible approach would be to connect a pressure gauge to the AGR valve instead of the pipe from the throttle body, a gauge that can withstand high temperatures, and then apply full throttle.

Best regards,

Jan

Hello Jan,

Would a portion of the exhaust pressure at the shaft seal of the turbo escape towards the oil circuit if the pressure is significantly higher than the oil pressure? I believe that alone suggests lower exhaust pressures. How else would one achieve a tight and durable seal for the shaft seal at those rotational speeds?

Greetings.

Eike.

PS: You're probably right that the exhaust will burst if you block it and give it full throttle. But shouldn't it simply stall when idling?

Hi,

Of course, the oil seals are another factor to consider, you're right about that.

I'm not so sure what would break if I tried running it in neutral... I definitely wouldn't attempt it.

Best regards,

Jan.

Hi again.

Another consideration:

I don't think the pressure inside the Kruemmer exceeds 3 bar continuously, but that doesn't say anything about pressure spikes caused by resonance or something like that.

In the cited post, the discussion focused on the stress on the AGR pipe's shut-off mechanism. I've been thinking about the AGR valve itself. If, due to resonances in the manifold, there's a very brief period of extremely high pressure, it probably wouldn't affect the AGR valve, because the valve's inertia, combined with the spring force, would likely prevent any movement. However, a different rule applies to metal detectors, as they could be damaged.

What do you think of this idea? Perhaps everything fits together this way, or do you think resonance-induced pressure peaks are nonsense?

Best regards,

Jan.

'I have no idea how much pressure is in the exhaust manifold, but I just tried to calculate how much pressure the EGR valve can withstand. In doing so, I neglected to account for the fact that the internal pressure from the turbocharger also contributes to the sealing. I estimated the diameter of the EGR valve to be 1.5 cm.' The forces are derived from the assumption that the valve can be fully opened using a gravitational force of at most 10 kg.

AGR valve.
Diameter Area
d A
It is hardly possible to achieve a pressure difference of more than 2.5-3 bar with a single radial compressor stage...
I thought this only applied to the medium being compressed, and therefore it's not a relationship between the pressure at the compressor outlet and the exhaust manifold, but rather between the compressor outlet and the ambient pressure.

Jan6K wrote:

If, as a result of resonances in the manifold, there is a very short period of extremely high pressure, then it probably doesn't affect the EGR valve, because the inertia of the valve, together with the spring force, likely prevents its movement. However, a different rule applies to metal detectors, as they could be damaged. What do you think of this consideration?

Hi Jan,

If the 30 bar reading isn't a typo, then it's my opinion that these pressures could actually only occur as short-term pressure spikes, for example, when the exhaust pressure wave from the cylinder hits the turbine and is partially reflected by it.

If then standing waves occur (with a suitably sized manifold length and engine speed), and if the vibration is poorly damped (which I personally find unlikely, given the rotating and, as is well known, sound-absorbing turbocharger), it could potentially escalate to a pressure of around 30 bar...

Hi,

@habib: I would estimate it to be a little smaller than 1.5 cm, but I didn't measure it exactly yesterday. I believe the force required to push it down is greater than 50 N (or 5 kilograms of weight), but I haven't actually tried it. I just pressed down on it firmly with my thumb, but nothing moved.

@Ulf: My thinking is also going in that direction. In the "AGR blocking" thread, the discussion was about what force acts on a lock of the AGR path, and that's where the peak pressures are relevant because a small plate will either deform or tear, while the AGR valve itself would simply close again because a spring pushes it and it also has some inertia.

Best regards,

Jan.

Thomas once wrote about his knowledge of back pressure in exhaust systems. I don't know how he obtained the data, but it sounded like it was based on solid measurements. See:

/viewtopic.php?t=6501&postdays=0&postorder=asc&highlight=gegendruck&start=24

Hi Jan,

You could, as a test, "sacrifice" an LDA (likely referring to a specific electronic component) and connect it to the crumb (likely referring to a specific part of a device or circuit).

If it bursts immediately, that indicates a pressure of 30 bar.

If it's showing something and possibly still within its 0-3 Bar range, you have the correct value .

Or you can use a normal pressure gauge, but in that case, the needle will likely fluctuate wildly (I once had that happen with the boost pressure, it looked funny ).

m;

Hi Martin,

I don't think it's possible to measure pressure peaks using that method because the display is simply too slow.

The observations made by Thomas in the other thread, in my opinion, only refer to the average pressure, not the peak values.

But, as I mentioned before, the AGR valve likely smooths out those peaks due to its inertia, so it won't open.

Best regards,

Jan.