Water-cooled EGR: Why?

Hello,

As a justification for the water-cooled EGR system, you'll often find the general statement "to increase the EGR rate."

Since the AGR (Abgasrückführung) only regulates based on the desired air mass, it doesn't really matter whether the normally excessive amount of air being drawn in is replaced by hot exhaust gases, denser cooler exhaust gases, or even CO2 or helium, etc., right?

So, what kind of pollutants are released when, for example, instead of 100 mg of hot exhaust gases, 150 mg of cooler exhaust gases (these numbers are arbitrary) are introduced into the cylinders with each intake stroke, each with the same "amount of air replacing fresh air"?

The only thing I can imagine is that a mixture of fresh air and exhaust gases enters the cylinders at a lower temperature with cooled EGR than with uncooled EGR, which in turn would reduce the combustion temperature peaks that contribute to NOx emissions.

In this context, it would also be interesting to know why the 1.9 TDI engines with the highest power outputs (ARL and BPX) require a cooled EGR system, while the ASZ engine in the G4 and Ibiza 6L can manage with a "hot" EGR system.
Sure, the emissions tests will require something of that nature, but:

The software in the G4 ASZ / ARL versus Ibiza ASZ / BPX comparison is almost identical up to the maximum load curve of the ASZ (as far as the different chargers allow), and the EU emission cycle certainly never demands full throttle from the ASZ, because, for example, as far as I know, an SDI engine runs the same cycle.
In other words, in the EU emission cycle, ASZ, ARL, and BPX engines may only deliver a maximum of 50 horsepower, and their emission-relevant engine hardware (combustion chambers, cylinder heads, fuel injectors, valves, exhaust manifolds, etc.) is largely identical, except for the turbocharger, intercooler (SMIC/FMIC), and the resulting intake manifold design.
The AGR (Abgasrückführung) system should, in my opinion, be able to regulate the desired amount of air even with different intake manifold configurations and low-temperature cooling (which, at 50 horsepower, will only be a few degrees above ambient temperature, regardless of whether it's a SMIC or FMIC?).

Why does the standard, non-cooled EGR system work with the lower-powered versions, while the more powerful versions in the same vehicles require a cooled EGR system?

[Speculation] Perhaps because the AGR (Abgasrückführung) system partially acts like a coolant heater, warming up the engine faster and not requiring additional energy (like glow plugs)? And engines like the ARL and BPX are certainly not driven by people who modify their cars to cheat emissions regulations. [/Speculation]

And my humble speculation:
"Isn't it the case that these engines, which are almost identical in design, reach significantly higher exhaust temperatures at full load? These temperatures could potentially damage the EGR system during subsequent idling or partial load operation. I can imagine that this was a later modification, perhaps due to problems encountered during the testing phase."
Okay, I mean something like the opposite of a heated throttle body on a gasoline engine, to prevent freezing.

I don't think it's due to emission regulations. The coolant heating might just be a positive side effect, because if it were for that reason, they would have installed it in other models as well.

Nebelwerfer_TDI wrote:

[Speculation] Maybe because the AGR system, in part, acts like a coolant heater, warming up the engine faster and not requiring additional energy (like glow plug heating)? And engines like the ARL and BPX are certainly not driven by people who modify their cars illegally.

That would be nice... However, my AGR-water-cooled ASZ engine (thanks to the Tiptronic) warms up *significantly* worse than my beloved AJM engine (the thermostat is fine); and especially during the warm-up phase, the fuel consumption is also noticeably higher (but that's due to the automatic transmission, whose oil also needs to warm up).

Okay, mine doesn't have a cooled EGR system despite having an automatic transmission, but it only meets EU2 standards; however, it doesn't have a glow plug heating system.

Perhaps it makes sense to thermally relieve the engine, at least under partial load conditions, in order to compensate for the material aging caused by temperature during full load operation?

By the way, the PSA 2.0HDi model from 2002 with 66kW also has a cooled EGR system, but no intercooler; wouldn't that also indicate lower NOx emissions, thanks to cooler intake temperatures, and reduced material stress?

I believe the AGR (Abgasrückführung) doesn't cool the exhaust gases themselves, but rather keeps itself cool (that's my assumption). With such a small surface area, the cooling capacity is likely to be limited... How much gas passes through it, at what speed, and what is the size of the "cooling surface"?

Lower intake temperatures with a cooled EGR system and without a low-temperature coolant (LLK)? I can't quite understand that. It seems more likely to result in higher exhaust temperatures, which would make the EGR system more prone to issues.

Nebelwerfer_TDI wrote:

Perhaps it makes sense to thermally relieve the engine, at least under partial load, in order to compensate for the material aging caused by temperature under full load.

That would be a (initially) purely customer-friendly measure that I actually don't trust VAG to implement... unless the cooled EGR system is more prone to failure and would lead to more repairs .

And if you ever log the data (using diagnostic trouble code 3) of a functioning EGR valve and see how little throttle is required for the EGR valve to start closing, then I suspect that thermal relief in this (lower load) range won't significantly compensate for the increased full-load stress caused by the ARL (air recirculation lever) and BPX (boost pressure control) systems.

Quote:

By the way: the PSA 2.0HDi model from 2002 with 66kW also has a cooled EGR system, but no intercooler; wouldn't that also indicate lower nitrogen oxide emissions, thanks to cooler intake temperatures, and reduced material stress?!

So, another guess based on exhaust values... where are the professionals with verified answers ?

I believe the AGR (Abgas Rückführung) doesn't cool the exhaust gases, but rather keeps itself cool (my assumption). With such a small surface area, it's likely to have limited cooling capacity.

You're forgetting about the temperature difference - the exhaust gases can already reach 400-500°C at partial load, and with a coolant temperature of 80-90°C, there's already a significant heat transfer occurring.

Lower intake temperatures with cooled EGR and without a low-temperature coolant system?

When you integrate the value with respect to the combustion chamber temperature across the entire load/power range (including engine speed), the resulting area should be smaller when using cooled Exhaust Gas Recirculation (EGR) compared to when it is not used.

Higher exhaust temperatures, therefore more susceptible to EGR issues.

But you're right - without an exhaust gas recirculation system (EGR), the exhaust gas temperatures (even under partial load) will be higher.

unless the cooled EGR system were to become more prone to failure and cause more repairs

You're right - there have reportedly been cases of defective AGR coolers, although, as far as I know, those were not VAG vehicles.

So, another assumption based on exhaust values .

Another speculation: the 'larger' 1.9 TDI engines have PD injectors with larger injection holes, which results in a faster combustion process. However, to allow enough time for mixture formation, the injection must occur earlier. This leads to increased nitrogen oxide emissions, which are compensated for by the cooled exhaust gas recirculation (EGR) system.

If someone has access to AKTE (works at a dealership, etc.), they could use the part numbers to check if there's a connection between the size of the PDE (positive displacement element) and the presence of a cooled EGR (exhaust gas recirculation) system within the VAG (Volkswagen Group) vehicles.

Quote:

Water-cooled EGR - why?

Sure, here's the translation:

"Brief and concise answer:"

SSP 305, page 21.
+
SSP 316, pages 32 and 33.

Yes, I would also be interested in that.

My engine also has a water-cooled EGR system.

Hello everyone,

The decision by the engine developers to use cooled exhaust gas recirculation (EGR) was based on several reasons. To understand this, one must have a precise understanding of how EGR works at free-running intake temperatures in order to appreciate the advantages of exhaust gas cooling. I will assume this knowledge for the following explanation.

As is widely known, exhaust gas recirculation (EGR) primarily affects the process temperatures in addition to influencing the chemical composition of the cylinder charge (inert gas content). The average temperature during combustion increases proportionally with the amount of recirculated exhaust gas, while local peak temperatures are reduced (leading to NOx reduction). Therefore, it is not surprising that cooling is also used to lower the average process temperature.

Why? Because approximately 50,000 cooler intake air temperatures can result in about a 10% improvement in fuel efficiency, due to the more favorable high-pressure process (see comparison process).
The reasons for this can be broadly attributed to lower heat loss through the walls and an earlier start to combustion, which is due to a higher proportion of uniform combustion. Disadvantage: higher pressure gradients (noise).

The formation of pollutants during EGR cooling is extremely complex, but very roughly: less NOx and soot, but significantly more HC/CO due to a shortened after-oxidation phase (which isn't a major issue because of adjustable coolers and oxidation catalysts).

Conclusion:
By using cooled exhaust gas recirculation (EGR), the conflict between NOx emissions, particulate matter (soot), and fuel consumption can be significantly reduced. The NOx level can be lowered by approximately 20% (a challenge for more complex designs).
Disadvantages: Cost, higher volume in the flight path -> problems with transient behavior.
The improved NOx tolerance of the 4V engines will soon make this feature indispensable, unless active NOx reduction technologies like NOx storage or SCR are implemented.

Greetings.

Hi!

Here's my translation:

'In this context, I had the following idea: If we've already made the 'particle filter' – a technological workaround – practically mandatory through media manipulation, then we can at least try to extract something positive from it.'

The exhaust gas from the EGR system is tapped after the particulate filter. This means that fewer solid particles are present, which, together with the oil mist, can cause the intake passages to become clogged.

The formation of pollutants during EGR cooling is extremely complex, but very roughly: less NOx, less soot, but significantly more HC/CO

When there is 'significantly more HC/CO', it is questionable whether the target conflict regarding consumption can be mitigated, because HC/CO (like soot) represents unused energy from the fuel.

Hello.

In addition, it is worth mentioning the specific heat values of flue gas (1.122 kJ/(kg*K)) and air (1.005). This is because lower return temperatures can significantly affect the peak compression temperature.

It would be an oversimplification to conclude that the motor's efficiency (effective power/fuel input) has deteriorated based solely on increased HC and CO values. After all, we are talking about concentrations in the ppm range.

Nebelwerfer_TDI wrote:

If someone has access to the AKTE system (works at a friendly company, etc.), they could check, based on the part numbers, whether there is a connection between the large PDE (positive displacement element) and the cooled EGR (exhaust gas recirculation) system in VAG vehicles......

More likely no. Even the engines mentioned in my original post don't fit that scheme: The ASZ in the G4 and Ibiza, the ARL, and the BPX all have the same PDE (550), but the ASZ (switchable) engines have the non-cooled EGR valve.
In the automatic G4 model, the ASZ engine still features a cooled EGR system.

bafische wrote:

By using the cooled EGR system, it is possible to significantly alleviate the conflicting goals of NOx/soot/fuel consumption. The NOx level can be reduced by approximately 20% (a problem for "more complex" designs)

.
Thank you for your answer; I was hoping for something along those lines .
The question remains whether the ASZ (presumably an engine or component) is already so close to the EU3 emission limits without an EGR cooling system that a front-mounted intercooler (FMIC) along with longer intake manifolds and a different turbocharger would be absolutely necessary to avoid exceeding the EU3 limits, making the additional cost of an EGR cooling system unavoidable.

Interestingly, in this context, it's also worth noting that the EU4 successor to the 118 kW BPX engine (named BUT or something similar) does not have the throttle valve that is known from the AXR engine, while the slightly older EU4 96 kW 1.9-liter engine (BLT) still has this throttle valve.
As far as I could tell from the documentation, only the PDEs (including the MSG/software) were modified to bring the BPX up to EU4 standards.


@Bertil
Let's see if I can find the SSP somewhere...

As is widely known, hot Exhaust Gas Recirculation (EGR) primarily affects the process temperatures in addition to influencing the chemical composition of the cylinder charge (inert gas content). The average temperature level during combustion increases proportionally with the amount of recirculated exhaust gas, while the local peak temperatures are reduced (NOx reduction). Therefore, it is not surprising that cooling is also used to lower the average process temperature.


Okay, so I think the only reason to cool the EGR is to get more air into the cylinder with a cooler EGR. Or, to put it another way, with the same lambda value, you can introduce more EGR. With a cooled EGR, the boost pressure doesn't have to be as high, which reduces the work required for charge exchange (fuel consumption). If you want a higher EGR rate, it simply needs to be cooled, because that's the only way to address the conflict between soot and NOx emissions.


Why? Because approximately 50,000 cooler intake air temperatures can result in about a 10% improvement in fuel efficiency, due to the more favorable high-pressure process (see comparison process).
The reasons for this can be broadly attributed to lower heat loss through the walls and an earlier start to combustion, which is due to a higher proportion of uniform combustion. A disadvantage is the higher pressure gradients (noise).


Can you explain that to me in more detail? I'm really interested. I'm especially not quite clear about the connection between intake air temperature and fuel consumption benefits.


Here's my translation:

'In this context, I had the following idea: If we've already made the 'particle filter' – a technological crutch – practically mandatory through media manipulation, then we can at least try to extract something positive from it.'

The exhaust gas from the AGR system is tapped after the particulate filter. This means that fewer solid particles are present, which, together with the oil mist, can cause the intake passages to become clogged.


Great idea! Have you also thought about where the exhaust gas recirculation (EGR) can be reintroduced? A pressure difference is always required for EGR transport. The pressure is likely higher just before the intake manifold than it is after the diesel particulate filter (DPF).

For many years, there have been discussions about installing an AGR (Abgasrückführung) system before the compressor. However, due to the lack of a DPF (Diesel Particulate Filter) in the past, such a system has never been feasible. Let's see what the future holds.