Drehzahlkennfeld: Why it exists?

Hello,

While pondering the functions of the quantity limitations, I got stuck on the torque limitation and its interaction with the driver's desired performance map (pedal position / rpm / desired quantity).

Why, for example, is full throttle at 2000 rpm interpreted by the ASZ (Air System Zero) as an initial fuel demand of 70 mg, which is then reduced to 56 mg by torque limitation ?

It would be simpler to program the 100% line of the driver's desired torque curve so that it corresponds to the torque limit, and then reduce the values from that 100% towards the 0% pedal position in such a way that, ultimately, the resulting driving behavior is the same as what we are familiar with.

Then, one could avoid all the complex calculations related to torque limiting and the corresponding processor load.

Since I believe the Bosch programmers are smarter than me, they must have had a good reason for doing it that way.
Sure, here's the translation:

"Just what?!"

I think it's easier for software developers to program self-contained modules.
According to your idea, any change to the key field would also require modifications to the accelerator pedal software, including all associated monitoring functions.
As it is now, each module has a clearly defined interface and is independent of the others, allowing for changes without affecting other modules.

Just my first thought.

Hi,
Perhaps it's more illustrative to consider everything in terms of torque. Newer engine control units have a torque-based structure:

The driving characteristic map (driver preference/pedal feel) corresponds to the actual torque. The frictional torque and the torque of auxiliary components (e.g., air conditioning) must now be added.
This allows us to obtain the instantaneous torque, to which the torque limitation is then applied.

Example (n=constant, climate control on/off, torque limit at 150 Nm, eta = 5).

Partial load, driver request = 70%
Pedal = 100 Nm + 20 Nm friction + 10 Nm because AC is on = 130 Nm < 150 Nm, so 130/5 = 26 mm³/stroke injection amount.

Partial load, driver request = 70% pedal = 100 Nm + 20 Nm friction + 0 Nm because AC is off = 130 Nm < 150 Nm, so 120/5 = 24 mm³/stroke injection amount.

In both cases of the TL, the same amount of torque is effectively delivered to the crankshaft (100 Nm).

Full load, driver preference = 100% pedal = 130 Nm + 20 Nm friction + 10 Nm AC assist = 160 Nm.
It is now limited to 150, so 150/5 = 30 mm³/stroke.
Currently, the effective torque is not 130 Nm, but only 120 Nm because the AC is on. If the AC is turned off, the vehicle will accelerate.


Ultimately, it depends on what comes next in the control path. From a regulatory perspective, higher desired quantities or moments than those allowed by the motor torque can sometimes make sense, for example, in an anti-lag regulation system.
Therefore, the sum of all values/quantities immediately before reaching the injectors must be limited.

I hope I was able to express myself reasonably clearly.

Isn't the torque also limited by the currently engaged gear, so that the load on the lower gears is not too high?

I think that, as it is, it's the simplest and easiest to expand.

The 160 hp version simply has a different Nm torque curve.

If adjustments were made at the very beginning of the process (e.g., the accelerator pedal), then everything behind it would need to be adjusted in other models.

m;

Abraxas wrote:

Ultimately, it depends on what comes after in the moment/quantity path. From a regulatory perspective, higher desired quantities/moments than those allowed by the torque limit can make sense, e.g., in an EGR control system. Therefore, the sum of all values/quantities immediately before reaching the injectors must be limited. I hope I was able to express myself reasonably clearly.

Thank you - I think I understood it at least partially .

dieselmartin wrote:

The 160 hp engine simply produces a different Nm torque curve. If adjustments were made at the very beginning of the calculation process (throttle), then everything behind it would need to be adjusted in other models.

Well, a new engine tuning, in my opinion, doesn't just consist of a new torque curve... with the entire software effort involved, it probably wouldn't matter much anymore if it had a different pedal mapping .

Wouldn't the torque also be limited by the currently engaged gear, so that the load on the lower gears is not so high?

Indeed, such a feature exists. However, I'm unfortunately unable to specify which V.A.G. models utilize it.

Other important limitations would include water temperature (trivial! Usually implemented in the same control unit as the dynamometer), smoke limitation (controlled via air mass), and fuel temperature (especially with Bosch CR systems). Based on these limitations, a minimum selection can then be determined.

From a system structure perspective, limiting the input size of the 'pedal characteristic curve' would actually be more complicated.

Abraxas wrote:

Other important limitations would include water temperature (trivial! Usually implemented in the same control cabinet as the torque sensor)

.
Hm, can this intervention be detected during diagnosis? For example, if the engine is "running hot," would the torque limit value still be 55 mg at 1900 rpm, or would it be something lower, like 40 mg (just an example)?

Hi Ulf,

As is the case almost everywhere (exceptions, of course, always prove the rule), the billing process is strictly separated from all other functions.
You can see the derating through the diagnostic information, possibly via WTF or a similar system, because the amount of power output is not limited by this.
This means: within the defined limits, whether it's water, oil, speed, pressure, smoke, or fuel, the torque remains constant. However, the amount that is allowed is the minimum value among all these limits.
There is no speed-dependent torque limiter, but there are other speed-dependent limitations in place, although they are not actively engaged. (Otherwise, VW TDIs wouldn't behave the way they do.)

I wouldn't underestimate the pedal mapping; it's a sacred cow for manufacturers, and it's not something they'll just discard easily, even when there's a new car or engine. This mapping is the key to the driving feel.

Abraxas is absolutely correct in his explanations regarding the rotary motor, but unfortunately, this correct, theoretical approach is only implemented with hesitation.
The Drehmo system has evolved more historically because the power reduction (power limitation) is implemented at the motor's power supply unit (PSU), not at the vehicle itself (where it wouldn't work with pedal-based systems). Therefore, an agreement was reached to use a control unit (CU) into which the stationary voltage limiter (VL) can be integrated – this is the Drehmo CU. Because this CU was one of the first to be integrated into the electronic motor control system, it has remained in use.

bafische wrote:

Regarding the diagnosis, you can see the derating through WTF or similar methods, as the amount derived from the torque is not limited by this. That means: within the limitations, whether it's water, oil, speed, velocity, pressure, smoke, or fuel, the torque is always the same. However, the released amount is the minimum of all.

Hi Bavarian,

Using VAGCOM, I only see the following limits for the driver's preference, torque, and smoke emissions (for a manual transmission).

If a "WTF limit" (emergency mode?) is triggered, what signs might indicate this?
Is it possible that the injection quantity (e.g., in the MWB 1) is even lower than the smallest of the three limitations mentioned above?

ulf wrote:

bafische wrote: Regarding the diagnosis, you can see the derating through WTF or similar methods, as the amount derived from the torque is not limited by this. That means: within the limitations, whether it's water, oil, speed, velocity, pressure, smoke, or fuel, the torque is always the same. The released amount, however, is the minimum of all. Hi Bavarian, Using VAGCOM, I only see the following limits for the driver's preference, torque, and smoke emissions (for a manual transmission). If a "WTF limit" (emergency mode?) is triggered, what signs might indicate this? Regarding the fact that the injection quantity (e.g., in the MWB 1) is even lower than the smallest of the 3 limitations mentioned above?

Hi Ulf,

Unfortunately, I don't know the specific EDC measurement values that are hidden behind the measurement blocks in the diagnostic data, but based on the logic, it should be something like that.
If the measured values for smoke reduction, torque limitation, and driver demand are displayed individually, you definitely need another measurement channel for the so-called "actual" value, which is the real amount that is ultimately passed on to the pump after all calculations (e.g., quantity limitation). (Usually, none of these limitations are active, and the system operates at the desired quantity, meaning the actual quantity is approximately equal to the desired quantity.) This actual value should also be included in the MWB1 data.
If you graphically represent these 4 sets of data after a measurement, and adjust the WTF (likely referring to a temperature sensor) using resistance to 119°C, you could verify this yourself.