Summer Weakness: Causes & Solutions

Hello everyone,

Was it also so hot with you guys today?

Left Bielefeld at midnight the day before yesterday... 6 degrees Celsius, the TDI engine had power like nothing, and today on the way home from work... 33 degrees Celsius, air conditioning on full blast... THAT is a really stark difference and a good demonstration of the TDI's summer weakness.

Did you notice that as clearly today?

Best regards,

Jan.

Jan6K wrote:

Did you notice that as clearly today?

Hi,

Yes, I noticed ! Today, I had to replace my TDI with a 1.4-liter 75 horsepower gasoline "monster." I felt like I couldn't get up every hill!

My AXR is lost again...

'So, my 130 horsepower in the AWX didn't seem significantly more tired than usual. And yesterday, on the way home with the camper, I also didn't notice any significant differences.'

However, with my 1Z, I also experienced this 'summer slump' to a very extreme degree.

Hello!

Yes, yes, this summer slump. I've noticed the typical weaknesses at higher temperatures in both my AHU (Air Handling Unit) and my ACV (Air Conditioning Valve). Furthermore, when driving in the city, the refrigerant gets so hot that you have to drive a considerable distance before it cools down enough to provide effective cooling again.

I had some ideas, like a cooling fan controlled by a thermostat, or a water spray system controlled by the kickdown switch on the accelerator pedal. On the T4 (ACV), the radiator isn't really exposed to much airflow, so what else can be done without immediately replacing it with the larger radiator from the 111kW 2.5 TDI? (Plans for that are available).

Thoughts and plans. I was also told that a low air-to-fuel ratio can lead to higher thermal stress on the engine, because there is more oxygen available for combustion?

Please correct me if I'm wrong.

Greetings, Peter.

My AAT (presumably referring to a vehicle) handles in the opposite way. It's particularly nimble when it's warm.
However, it's not necessarily producing less smoke than when it's cold. I know that, theoretically, the intercooler should make it produce less smoke, but there must be other factors at play.

joergs wrote:

So, my 130 horsepower in the AWX didn't seem significantly more tired than usual...

However, with my 1Z, I also experienced that summer slump extremely.

Hello everyone.

This confirms, and possibly supports, a suspicion I've had for some time that the temperature compensation of the RWG (rolling wave generator) is not 100% accurate.
PDs don't have a RWG.

I wanted to log the group's data, including the quantity limits (throttle, torque, and soot), during a dynamic regeneration (DZR) test while it was cold, and then repeat the same process when it was hot.

If the soot limit doesn't shift significantly, it would, in my opinion, mean that the EDC believes it's injecting the same amount of fuel all the time, which is probably not the case.

Unfortunately, I missed the cold weather period. Does anyone else happen to have a CSV file for a reproducible full-load test performed in very cold weather? Perhaps someone could run a comparison test when it's "nicely" hot (ugh, sweat!).

Hi,

Regarding my AAT (presumably referring to an 'Aptitude Assessment Test' or similar), the results are quite ambiguous.
I notice a less responsive feel in the city, or generally when accelerating quickly from a standstill or at low RPMs, especially in hot weather (as I also experienced with the 1Z engine).
The maximum speed, on the other hand, remains quite impressive.
I finally got to drive at full speed on the highway today, and according to the speedometer, I reached almost 220 km/h. That's equivalent to my winter top speed, and it's even a bit higher.

Hello!

I can fully confirm the behavior observed by Markus H. for my Golf III (AFN) (although his number 220 needs to be replaced with 205).

Greetings, Klaus.

Hello.

There's a conversion formula for power.

P = PNorm * pLuft / 1013 * sqrt(293 / (273 + tLuft))
PNorm ... Standard performance.
pLuft ... Pressure in mbar.
Air... Temperature in degrees Celsius.

So, a vehicle with an 81 kW engine actually has around 76-84 kW, depending on the season and weather conditions. This should actually affect naturally aspirated engines much more severely than turbocharged engines, which would need to compensate for it. I have no idea why VW doesn't do that.

The constant top speed makes sense because the air resistance decreases by the same factor as the power.


Best regards,
Ernst.

Deleted upon request.

We already discussed this topic during the last hot summer. In our household, we have two TDIs: an older AHF with VE injection and an AJM with PD injection. Both of them exhibit this summer weakness.
The last theory, which has not been disproven, was that this effect occurs due to the altered viscosity of the fuel, and that this is not compensated for by the engine control unit.
"Due to the higher viscosity of colder diesel fuel, there are fewer leakage losses in the fuel injection pump, resulting in more fuel being injected. My own observations confirm this. My vehicles are parked in a temperature-controlled garage and their performance is affected by the temperature in the garage. This 'summer weakness' effect only occurs after a longer period of exposure to higher ambient temperatures, which suggests that it is not caused by the hot-running intercooler, but rather by components or the fuel itself gradually adapting to the outside temperature."
My theory is also supported by my experience with biodiesel: even with a small addition, the driving performance improves significantly, and fuel consumption increases, even though biodiesel has a lower calorific value. When running solely on biodiesel, the driving performance also drops dramatically. Biodiesel has a higher viscosity than conventional diesel, and when added, the contents of the fuel tank become more viscous.
I think it sounds quite obvious, doesn't it?

Best regards,
Uwe.

Deleted upon request.

With my 5 questions, I don't want to 'put you on the spot' right now, but I was simply interested in what you base your statement on. Perhaps I asked the question incorrectly, so please forgive me, and I understand if you don't respond.

Yes, I think you're overthinking it... it's simply the standard formula to account for the variable density of the air (and not a special TDI formula that I developed after extensive testing).
I copied it from the textbook 'Design of High-Speed Internal Combustion Engines' (by the way, the author is from AVL). However, you can probably figure it out without the textbook, because I think everyone learns about the factors that influence air density in physics class.

And then it's also logical to think that with a turbocharger already present, all of this could be compensated for. Perhaps you could tell me why VW doesn't do that, or what other factors cause the summer weakness (or winter strength).

Best regards,
Ernst.

Commentary from a non-expert!

Hello!

I hope you won't take this the wrong way, but my thought process was as follows.

The density of diesel fuel decreases with higher temperatures, so it expands significantly, which means I need more fuel and more air to achieve the same performance. Therefore, I can only explain this intended "summer weakness" by the fact that the VW people would never be able to meet their stated fuel consumption figures otherwise. There's a new regulation that requires manufacturers to adhere to their stated fuel consumption figures, with a deviation of only a few percent.

Hello Uwe!

I don't quite understand this whole thing about "bio"diesel. Biodiesel has a lower calorific value. Okay, so if I add a "small" amount, it increases performance, but fuel consumption increases? Does that mean that I'm improving the viscosity of the diesel to the point where I have more power, but the EDC (Engine Control Unit) is regulating it in such a way that I have higher fuel consumption? By improving the viscosity, I'm actually achieving the same thing as with colder fuel; the correct amount of fuel is entering the combustion chamber, just like in colder seasons. And with such a small addition of biodiesel, does the increased consumption really become noticeable? How much biodiesel did you add?

If the EDC system were so effective at compensating for high air temperatures, why does the soot content increase so significantly that all TDI engines start producing excessive smoke?

Greetings, Peter.

Um...

I'm also familiar with the summer weakness, especially when starting from a traffic light... There are no problems with that when driving on the highway.

But I don't think it's the diesel engine's fault.

The composition of the combustion chamber is essentially determined by the charge pressure and the temperature of the intake air.
While the former cannot exceed a maximum value, the latter depends on the temperature of the intake air and the coolant temperature. If the car only receives hot air just above the road in the city, or even no air at all (due to traffic lights or stop-and-go conditions), how can it cool the intake air when suddenly subjected to full load?

The EDC (Electronic Diesel Control) only registers 'oops, 70° charge air temperature, need to inject a little less fuel otherwise it will smoke,' and even then, the TDI engine will starve for fuel if it's running lean.

It also perfectly matches my observations on country roads.

On occasion, I used to lie, but today, it's about bicycles.

CU Gremlin.

Mc_Givertechnik wrote:

I haven't quite understood the whole thing about "bio"diesel. Biodiesel has a lower calorific value. Okay, if I add a "small" amount, it increases the performance, but does the consumption increase? Does that mean that I improve the viscosity of the diesel to the point where I have more power, but the EDC (Engine Control Unit) adjusts it so that I have higher consumption? By improving the viscosity, I actually achieve exactly the same thing as with colder fuel; the correct amount of fuel actually enters the combustion chamber, just like in colder seasons. And with such a small addition of biodiesel, does the increased consumption really become noticeable? How much biodiesel did you add?

Hi Peter,

I added about 10 to 20% Biofuel to the tank. However, I want to point out the uncertain compatibility of the biofuel. My car has run for 100,000 km on it and is still running, but I'm not using it anymore.
With my mixture, the viscosity doesn't improve; it increases. This means the juice becomes thicker.
The pump builds up pressure. In the decreasing volume, there are inevitably gaps through which a portion of the fuel leaks back into the circuit. This portion then does not enter the combustion chamber during the injection process. If the viscosity is higher, the leakage is smaller, and the actual amount of fuel entering the cylinder is greater. This increases the power output, but due to the reduced energy content and the increased distance traveled, fuel consumption increases. In my case, this amounted to up to a liter when running solely on biodiesel, and about 0.3 liters in mixed operation.
Ich gehe bei dieser Theorie allerdings davon aus, dass die Steuerung zumindest die Temperaturänderung nicht kompensiert. In der ESP sitzt ein Kraftstofftemperaturgeber, das Steuergerät weiß also bescheid. Es wäre ein leichtes für das Steuergerät, die Einspritzmenge bei Kälte zu reduzieren. Scheinbar tut sie das nicht. Die Änderung der Viskosität (evtl. auch die Oberflächenspannung?) allein kann das Steuergerät nicht erkennen. Also ein 0-cent tuning??

Best regards,
Uwe.