TDIs Sommerschlappheit - immer noch rätselhaft | Posts 32+

Yep, that's it... it was quite a challenge yesterday, running my test series with the acceleration tests and data logging.

I had considered doing a trial run with AC for comparison purposes, but I ultimately decided against it to avoid having to turn it around again (I didn't have much time).

Furthermore, I recall reading somewhere that the engine control unit disengages the AC compressor when full load is requested. Does anyone know if this is true? I can't quite imagine it, because if it were true, it would get pretty hot under prolonged full throttle on the road.

Let's see, maybe I'll repeat my logging attempts at 30 degrees Celsius, but I can't imagine that anything will come of it, because the printing times at 5 and 22 degrees are pretty much the same.

Best regards,

Jan.

Even though I might be annoying you by repeating my question:
Are we talking about a lack of airflow, or a general performance issue that would also affect the maximum speed?
I always only observe the former, while Vmax is always fine.
Could this perhaps also be a clue to the problem?
Greetings, Klaus.

Vmax, on the other hand, is always okay.
Could this perhaps also be a clue to the problem?


The density of air decreases with temperature, which in turn reduces air resistance. Without looking it up, I believe the density is approximately 1.28 kg/m³ at 0°C and only 1.18 kg/m³ at 20°C (or similar values depending on air pressure).

At the same speed, in the summer, you need approximately 5-10% less power due to air resistance. Therefore, a power deficiency in the summer might not be noticeable at the maximum speed, where air resistance is the dominant factor.

Best regards, Ernst.

@Jan6K

In many vehicle models, the compressor temporarily deactivates during kickdown (automatic transmission). I suspect this may also be the case with our TDIs. I have personally conducted tests with and without the compressor: there was no significant change in the times, i.e., within a tolerance of 1/10 of a second. When taking measurements, you only need to press the "*" button for about 6 seconds; this will prevent it from getting too hot.

Best regards,
Uwe

Ernst S. wrote:

Now, let's go back to what it might actually be: Perhaps there's an exhaust problem that VW is addressing by reducing power at high temperatures. The reduced intercooler performance and, consequently, the reduced maximum power output (or does anyone experience increased fuel consumption in the summer, excluding the use of air conditioning?) suggests nitrogen oxides. However, this is just a guess.

Hi Ernst,

Sure, that's the most likely assumption at first.
However, I want to reiterate that attempts to trick the engine control unit into thinking it's "winter" by manipulating the signals from the charge air temperature sensor and the mass airflow sensor will not work – it won't be fooled.

The engine temperature is regulated by the thermostat, so the only remaining possibility is the fuel temperature sensor.
If the broth is hotter in the summer and therefore less dense, more diesel fuel should be injected to compensate. Perhaps this compensation is too "gentle"...?

However, this could not be solved using a sensor , because a parallel resistor (to increase the apparent temperature) would have a stronger effect in winter than in summer, meaning the seasonal difference would be even more pronounced }.

'Humid air contains less oxygen than dry air, which results in a lower engine power output. This reduction is generally negligible. However, in hot and humid climates, the humidity can noticeably reduce engine power.'

In addition to that, there is the...
warmer intake air.
the smaller temperature difference for the low-pressure coolant used for cooling.
the lower efficiency of the engine,
Other thermal conditions (water temperature, oil temperature).

I can't think of anything else.

Docter wrote:

In addition to that... the lower efficiency of the engine. . .

Hi Doc,

What effects cause the engine's efficiency to decrease at higher ambient temperatures?
And by how much per degree Celsius?

Thermodynamically (Carnot process).
The efficiency is derived from the temperature difference.

Efficiency = (T_max - T_min) / T_max

Quote:

Thermodynamically (Carnot process). The efficiency is derived from the temperature difference. Efficiency = (T_max - T_min) / T_max

Hi Doc.

Aha, serious engine physics .

If you could also tell me how T_max and T_min are defined (where and at what moment they are measured), I might actually be able to understand it .

I think I remember...

T_Min is the temperature before compression and ignition, and T_max is the temperature during compression and ignition.

'Of course, these are theoretical values. However, the energy produced during ignition should, in my opinion, not change depending on temperature; the temperature itself should change beforehand. It must be the intake air temperature.'

It's been a while... if I've written anything incorrect... please correct me.

Efficiency = (T_max - T_min) / T_max

T_Min is the temperature before compression and ignition, and T_max is the temperature during compression and ignition.


I would like to know the theoretical efficiency.

(but I also had to quickly look up my thermodynamics notes, so no offense ).

What you call T_max would be the average temperature (Tm1) of the heat added (i.e., the combustion) in the Carnot process.
T_min would be the average temperature (Tm2) of the heat being discharged (i.e., the exhaust).
And this only applies in a Carnot process (i.e., without friction and with other highly idealized conditions).

And all of this is so theoretical...
Regarding the summer sluggishness, a few more degrees of temperature can fluctuate in and out, without being able to be calculated in terms of efficiency for a real machine.
Best regards.
Ernst.

Yes, that's correct...

Tmax is reached after heat absorption (state 4).

Tmin represents the initial temperature, before any heat is released to the surroundings (i.e., the starting state 1).


While this may seem overly theoretical and simplified, and may not accurately reflect the real situation, the basic principle is correct. If other factors are added, they may diminish or amplify these effects. However, the effect itself remains. It is difficult to predict, in reality, to what extent this will have an impact.

Hi everyone,

You are not alone if you are experiencing a lack of power with your TDIs.

My BMW 530d doesn't pull anything noticeable from a standstill, starting around 20°C and up to approximately 2000 RPM .

But in that regard, the performance is consistent again. I personally don't know of any highway hill that I don't take at over 210 km/h .

But having the air conditioning on in the city during stop-and-go traffic is just annoying.

Hello.
I'm looking for.

Hi,

It's possible that this is also related to the higher fuel temperature that logically occurs at higher ambient temperatures. I found an interesting study on this, but I can't determine whether the engine control unit in the TDI might be compensating for this by increasing the fuel injection amount when the fuel temperature is higher.

http://www.all4engineers.de/cgi-bin/get_articel.cgi?aktion=show_articel_infos&id=1020668374&language=de

olli

Good approach as an additional factor...

There are already quite a few people here.

Regarding fuel temperature, anyone who has a nice, cool underground garage should consider doing a test on a hot day – I'm curious to see the results.
Greetings, Klaus.