TDIs Summer Slump: Still a Mystery

Hi everyone,

am I the only one who hasn't yet understood why TDIs perform worse in the hot summer than in freezing temperatures (2000 - 4000 rpm test)?

Or, to put it another way: How does the engine computer distinguish between summer and winter?

The assumption of increased intake air mass seems plausible at first. Then, it should be possible to achieve at least approximately the usual ventilation times in winter, simply by increasing the boost pressure, which would result in a higher mass airflow rate (LMM).
"It doesn't work; I've tried it. Even increasing the pressure by about 0.1 bar (which is essentially switchable on my car) doesn't result in any (clear) change in the recorded time, according to several tests."

Next approach: Charge air temperature. Rainer already checked this (the signal was manipulated), but without any results.

What remains now? The fuel temperature signal is supposed to ensure a constant "fuel injection quantity," so it should be excluded if it's functioning correctly.

Engine temperature --> Should the engine control unit, as a precaution, reduce the fuel injection amount if the engine temperature exceeds the opening temperature of the thermostat?

Or does winter diesel, in the end, have a higher calorific value than summer diesel ?

Who knows what and can enlighten me?

Hi,

ulf wrote:

Does winter diesel ultimately have a higher calorific value than summer diesel? Who knows what and can enlighten me?

Maybe I will.

Here are the calorific values for summer and winter diesel:
http://www.aral.de/corporate/_download/kraftstoffe/cont_4_3_6/25a.pdf

Best regards,
Munch.

P.S. I haven't noticed any summer sluggishness in my TDI.

ulf wrote:

Next approach: Intake air temperature. Rainer already checked this (signal was faked), but with no result.

Hello Ulf,

I've forgotten the exact resistance value. Maybe it was simply too cold at the time for the heating element to have any effect?
The intake air temperature reported to the engine control unit at that time was approximately 17°C, while the outside temperatures were usually around 10°C or even colder.
Since it didn't achieve anything back then, I removed the resistor again.
It really feels much hotter than the current 26°C. However, without a stopwatch (or phone) it's difficult to take accurate measurements right now...

Best regards, Rainer.

Hi Ulf,

At least with the ASV, the values don't seem to differ between 8 degrees and 22 degrees (yesterday's measurement). Subjectively, it feels slightly heavier, but that might be due to other reasons, because it's not a direct before/after comparison; there are always weeks in between.

Let's see what the next measurement brings, and how warm it is there (I don't know yet when I'll have time to do it) .

Best regards,

Jan.

According to my calculations, Ulf, the filling in the cylinder should increase by approximately 10% when the temperature is reduced. However, this only considers the ambient air temperature. How does the intake air temperature behave? Does it change proportionally to the ambient temperature?

It should also be considered that if you burn a smaller amount of fuel, you will also have less power available to drive the turbocharger.
Have you ever recorded the boost pressure? Could it be that you're reaching the maximum boost pressure, but just more slowly?

What do you think?

Johannes.


I found something else about IVECO, maybe Audi will do something similar.
Even without common rail technology.

Due to the high fuel pressures generated in the common rail system, fuel temperature is of great importance because, above a certain temperature, the diesel fuel no longer provides sufficient lubrication for the high-pressure pump. Therefore, this temperature is continuously monitored by the control unit. If the fuel temperature is too high (which can occur in high ambient temperatures, with the engine under full load, and when the fuel tank is nearly empty), and proper lubrication of the high-pressure pump is no longer guaranteed, the control unit determines the fuel density and volume based on the received signals, adjusts the fuel supply, and limits engine power to reduce the fuel temperature.

Hello,

It might not be helpful to anyone, but here it is:

My Polo is faster in the summer than in the winter. On a specific section of the A3 highway, it reaches approximately 140-142 km/h according to the speedometer in the winter. In the summer, I can reach 155-160 km/h.
It's registered with a top speed of 142 km/h (45 racing horsepower, gasoline engine). The speedometer consistently shows a speed 8 km/h higher than the actual speed, starting from 30 km/h. I measured this using the 'You are driving XYZ km/h' display.

Can you explain this phenomenon? I've been experiencing the same thing for years.

best regards

Adrian.

Hi,

'The TDI engine maintains consistent power output and emissions performance under all typical driving conditions.'
Source: Dieter Korp, 'Jetzt helfe ich mir selbst' (Now I'll help myself), Volkswagen Golf Diesel/SDI/TDI models from '91 onwards.

Later in this document, various outdoor temperatures will also be listed.
I'm not entirely sure that's completely correct.

Furthermore, it is mentioned that, for example, in high altitudes, the thinner air is compensated for by increased boost pressure.
Wouldn't that lead to very rapid wear and tear on the loader if it were used constantly in mountainous terrain?

Markus.

Hello,

Can you explain this phenomenon? I've been experiencing the same thing for years.

best regards

Adrian

Smaller winter tires?

Hi Markus,

due to the high altitudes:

Somewhere, I believe in the documentation for the industrial engine, it also stated that above a certain altitude (probably 1500 meters), the boost pressure is reduced to prevent the turbocharger from over-spinning. For this purpose, the air pressure is monitored and adjusted accordingly.

Best regards,

Jan.

In my opinion, the amount of fuel injected also depends on the coolant temperature. When I warm up my TDI, it has a good flow rate until the oil temperature reaches its normal level. However, after a longer drive, when the engine gets even warmer (oil temperature greater than 100°C) due to a lot of acceleration, the flow behavior leaves much to be desired.

The lubrication problem of the fuel injection pump can also be a possible cause.

Diesel-Dirk wrote:

In my opinion, the amount or mass of fuel injected is also determined depending on the coolant temperature.

Hi Dirk,

"My coolant temperature fluctuates between 88°C (when driving downhill for a long time) and 95°C (when driving at full throttle at speeds of 200+ km/h). This is relatively independent of whether it's summer or winter. Under normal driving conditions, it's around 90-91°C. Given the temperature regulation via the thermostat, I consider it unlikely that the fuel injection quantity depends on the coolant temperature under normal operating conditions."
However, if the coolant temperature is excessively high, something like that could be conceivable.

Best regards, Rainer.

Hello,

Smaller winter tires?

I drive with summer tires all year round.


Best regards,

Adrian.

Rainer K. wrote:

It really feels much tougher at the current 26°C. However, without a stopwatch (phone), it's difficult to measure accurately at the moment...

Hello, this morning I stopped at a temperature of 24°C.
Unfortunately, it takes 5.65 seconds to accelerate from 55 km/h to 110 km/h.
I was alone in the car, with a curb weight of 1255 kg, my own weight being approximately 76 kg, and the tank was about half full.
It was already moving faster.

Supplement 10.7. 18:52
In the 'flotter' measurement, different tires were used, which I had forgotten. Therefore, it is not comparable .
I guess I'll have to figure out how to create some logging functionality that can be calibrated using the rolling circumference of the tire...

Best regards, Rainer.

Hello,

I've been thinking about your diesel problem. In my opinion, the summer weakness of diesel engines might be related to the humidity in the air. The more I think about it, the more logical it seems.
Here's my thought process:

Everyone has probably made a bonfire at some point. What burns better: a wet piece of wood or a dry one? My grandmother used to have an old coal-burning stove. It was similar there. You couldn't heat the apartment with damp coal. The calorific value of the coal or wood is reduced by the amount of energy needed to bring the water (the moisture in the wood) to a boil. Additional energy is required to bring the water, which is 100 degrees hot, to a gaseous state.
For diesel engines, the principle would be similar: The gas-diesel mixture ignites more easily and works more effectively, the less water vapor needs to be vaporized for the explosion to occur.

The colder it is, the less moisture the air can hold (in absolute terms). Therefore, a humidity level of 50% means that the absolute amount of water in the air is much greater in the summer than it is with a relative humidity of 50% in the winter. Does that make sense?

I would need to look up the exact formulas.

best regards

Adrian.

Hello!
I have a G3 TDI, 81 kW, with the AFN engine.
"Even with mine, I've been annoyed for a long time by the really noticeable lack of power starting around 23 degrees Celsius."
However, I always achieve the maximum speed.
Is it different for you? Doesn't that also give a clue about the possible reasons for the poor performance?

Greetings, Klaus.

The colder it is, the less moisture the air can hold (absolute amount). Therefore, a humidity of 50% in the summer means that the absolute amount of water in the air is significantly greater than a relative humidity of 50% in the winter. Doesn't that sound logical, or?


Not really... because even then, there would still be a bigger difference between a rainy day and a beautiful, dry summer day than between summer and winter...
I'm familiar with the summer/winter tire difference from my TDI... I haven't noticed any difference between rain and dry conditions yet.
Sure, I will. Please provide the text you would like me to translate from German to English.