Re: LMM measurement principle | Posts 32+

Hello,

Ernst S. wrote:

Does that mean that the difference is compensated in the output signal by measuring the air temperature? Or is the temperature difference already compensated within the measurement process itself, making the air temperature irrelevant and eliminating the need for a measurement? Unfortunately, I didn't understand anything from the link.

For the temperature range of -20°C to 60°C, a maximum error of +/- 2% is specified.

According to http://www.windpower.dk/de/stat/unitsw.htm#anchor138877, the air density changes by more than +/- 2% within the specified temperature range, so the temperature probably needs to be taken into account somehow.

Best regards, Rainer.

The fact that the EGR is regulated using the LMM signal has nothing to do with the calculation of cylinder filling.


old laser brain

It's not stated there...


@gremlin
The air mass (more accurately, the air mass flow rate in kg/h) is a calculated value. Once the engine is running properly, it's only affected by factors like exhaust gas recirculation (EGR), knock control (KGE), and valve overlap; nothing else changes it. Therefore, you don't need to look at cylinder pressure or temperature to determine what's happening inside the cylinder.


Okay, here's the translation:

'In a motor, there is actually never anything that is statically balanced... especially not in vehicles.'

btw: the optional temperature sensor on the HFM (High-Frequency Manifold) is used in all naturally aspirated engines for compensation .

CU Gremlin.

@ErnstS.
The temperature difference is measured before and after the heating zone, and for some reason, this eliminates the absolute temperature of the air flowing past.
And aging compresses the characteristic curve globally as a factor, because the transition coefficient enters the calculation linearly, meaning that the characteristic remains the same, but unfortunately, the display range becomes smaller.
Stay cheerful.
Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

Das Temperaturgefälle wird vor und nach der Heizzone gemessen, da kürzt sich wundersamerweise die absolute Temperatur der vorbeiströmenden Luft heraus...

Yes, if you want to know the difference between the two temperatures at the hot film, but that is only useful for determining the flow direction. This difference is not suitable for measuring heat flux. See: http://www.kraftfahrzeugtechnik-heute.de/k/de/start/product.jsp?mfacKey=ds_06_luftms_sens

It also states that the heated heat used to maintain the heating element at a constant temperature is utilized for measuring air mass flow.

To account for the air mass flow, newer mass airflow sensors also compensate for air pressure, temperature, and humidity. Therefore, the condition of the air plays a significant role in mass flow measurement. And there are many factors related to this that could be important for experiments involving summer weakness.

Best regards,
Ernst

@everyone:

Attention, this is quite a bit of text to shed some light on the matter, although the most important information was actually already contained in the various posts:

Operating principle of the hot film anemometer HFM5:
The hot-wire anemometer features a sensor element arranged within a partial flow channel. This ensures that the air flows smoothly past the sensor element without turbulence.
In the center of the sensor, a heating resistor is located on a micro-mechanical membrane and maintains it at a constant temperature (regulated!). Outside this regulated heating zone, the temperature decreases on both sides.
Zwei symmetrisch zum Heizwiderstand stromauf- und abwärts angeordnete NTC-Widerstände erfassen die Temperaturverteilung auf der Membran. Ohne Anströmung ist das Temperaturprofil symmetrisch. Bei Anströmung verschiebt sich das gleichmäßige Temperaturprofil derart, dass der Temperaturverlauf auf der Ansaugseite steiler wird, da die vorbeiströmende Luft diesen Bereich abkühlt. Motorseitig wird die Messzelle zunächst auch abgekühlt, allerdings sorgt die vom Heizelement erwärmte Luft für einen anderen Temperaturverlauf. Diese Modifying the temperature distribution to create a temperature difference between the measurement points of the NTC resistors.
The heat released into the air, and consequently the temperature profile at the sensor cell, depends on the mass of air flowing past. The temperature difference is independent of the absolute temperature of the passing air; it is a measure of the mass flow rate. In this way, both the magnitude and direction of the mass flow are determined.
The membrane is very thin, which allows the heat capacity of the sensor element to be kept low, enabling it to react quickly and dynamically to changes.
An integrated evaluation circuit converts the resistance difference into an adjusted control signal of 0...5V. The control unit has a stored characteristic curve that defines the relationship between sensor voltage and the amount of air drawn in.
In sensor versions with an integrated temperature sensor (optional), this sensor is only used for additional evaluations and not for determining the air mass. It is then positioned in front of the sensor element.

I hope this detailed description has been helpful to you.

Regards,
Alex.

donalexo wrote:

The temperature difference is independent of the absolute temperature of the flowing air. It is a measure of the mass flow rate. In this way, both the magnitude and direction of the mass flow are captured.

Hello everyone again,

1. This also aligns with my current understanding of the significance of the LMM signal.
Soundsoviel kg (=Masse!) Luft fließt pro Minute (oder andesr gequantelt) durch den LMM.
And precisely that measurement of "mass" is needed for the soot map, regardless of the temperature and pressure at which the air flows through the mass airflow sensor.
(Of course, the mass flow rate would be lower with the same pressure but a higher temperature, but this does not contradict the previous statement.)

2. Let me introduce myself again:
As soon as a portion of the fresh air flow is "replaced" by other gases (from the EGR valve or crankcase ventilation system) on its way to the cylinder, the airflow measured by the mass airflow sensor (MAF) is reduced by precisely that volume.
Therefore, the current LMM signal can always be used for calculating the combustion-relevant fresh air mass flow rate (initially including scavenging losses) through the engine.
The boost pressure (and possibly the intake manifold temperature) are used to calculate and subtract scavenging losses, while the intake manifold simulation (with a storage function for boost pressure fluctuations) is used to calculate corrections for fluctuating boost pressures.

At the end (after division by the engine speed), we have the calculated net fresh air charge per cylinder, which is actually available for combustion.

Initially, neither boost pressure nor air temperature are needed anywhere in this calculation (since the mass airflow sensor already provides a correspondingly "corrected" signal).

The only part of the calculation where I could potentially see a benefit from using low-temperature coolant is in the flow losses. These increase with boost pressure, but are they also dependent on the low-temperature coolant temperature?

Therefore, the current LMM signal can always be used for calculating the combustion-relevant fresh air mass flow rate (initially including purge losses) through the engine.

That's obvious, but that wasn't the point either. The air mass flow sensor itself needs to know the temperature of the air in order to determine the mass flow rate. The temperature must be taken into account in the measurement circuit of the mass flow sensor. Heat flow always consists of a mass flow rate multiplied by a temperature difference and a specific heat capacity. And if the air is the same temperature as the heated element... then the mass flow sensor will not indicate any mass flow, no matter how large it actually is!
/viewtopic.php?t=3347 is a simple explanation of the circuit. It also explains how the temperatures are handled. You always need to know the temperature in order to increase the heating resistance by 160°C.


However, if Bosch itself refers to the two sensors and then still explains the mass flow measurement using the amount of heat dissipated, it's probably quite clear. Even though the website doesn't go into great detail.
http://www.kraftfahrzeugtechnik-heute.de/k/de/start/product.jsp?mfacKey=ds_06_luftms_sens
It is clear that the two measurement points have absolutely no correlation with the temperature profile. The principle is simply this: if one temperature is higher than the other, the current flows in one direction, and otherwise it flows in the opposite direction. However, the magnitude of the difference is irrelevant.

The two sensors are only used for determining the direction. They cannot be used to determine a heat flow. And even if it were possible to use them in that way, you would still need the air temperatures at both locations, not just the temperature of the hot film, in order to infer the mass flow rate.

Within the LMM (likely referring to a Liquid Metal Magneto-Hydrodynamic machine), the mass flow rate must always be calculated using a temperature value. The energy that the LMM needs to vaporize is the heat flux. And a small mass flow rate that is very cold will absorb the same amount of heat as a large mass flow rate that is already hot.
And modern Laser Material Processing (LMP) systems know the temperature, density, and humidity of the air and calculate the mass flow rate based on the resulting heat absorption capacity.

Whether the temperature signal is also transmitted to the control unit doesn't really matter... it would only be interesting to know if this signal reaches the control unit for experiments related to 'summer weakness.'

But no one has ever doubted that the LMM signal indicates the fresh air mass flow, Ulf.

Best regards, Ernst.

'In my opinion, this is also unnecessary: Since there should be no air loss or gain between the mass airflow sensor (MAF) and the engine, the actual intake manifold temperature is actually irrelevant.'


Hello Ulf,
That's almost correct, except for the part about the mixture of gases that comes from the crankcase and is located behind the mass airflow sensor (MAF). However, that shouldn't make much difference.

What about the compressibility of air at different temperatures?
Does it perhaps thicken significantly better at 10°C than at 30°C? Does an increase in pressure of, for example, 20% also mean an increase in mass of 20%?

I've made the following observation with my ALH: When driving at full throttle (maximum speed of 188-190 km/h on the highway in flat terrain), the boost pressure is 0.9 bar according to the LDA, and the accelerator pedal is fully depressed. If I then slightly lift off the gas pedal (enough to reduce the boost pressure minimally), the car actually becomes a little faster, or I can maintain the speed with a slightly lower boost pressure (0.75-0.8 bar).
According to the MFA (Multifunction Display), the fuel injection amount or consumption does not decrease.
How can this be explained? Is the boost pressure not that crucial? Or does the efficiency increase again with decreasing excess air?
Greetings.

Eike.

Hi,
We have used hot-wire anemometers extensively at the university, opting for hot wires instead of hot film sensors. To achieve the shortest possible response times, the temperature was kept CONSTANT. The current required to maintain this temperature was measured.
If the hot-film sensors used in our TDI systems operate with a constant temperature difference, it would be interesting to know whether this difference is adjusted just as quickly, or whether measurement errors that could lead to summer weakness might arise through this process.

Ernst S. wrote:

Aber daran, daß das LMM Signal den Frischluft- massenstrom angibt ... daran hat nie jemand gezweifelt Ulf.

Hi Ernst,

I understood it differently because other posts essentially said the same thing.
"Without air temperature and intake manifold pressure input into the engine computer , the mass of air per stroke cannot be calculated."
Specifically, this applies in principle even before the wash loss correction is applied.

Or maybe I misinterpreted those posts...

I understood it differently because other posts essentially said the same thing.
'Without air temperature and intake manifold pressure input into the engine computer , the mass of air per stroke cannot be calculated.'


Yes, that's right, there were some posts like that. This is going to be another long article about the 'summer slump' because so many fundamental things need to be clarified first. Unfortunately, there are just so many possibilities.
With a simple gasoline injection engine with lambda control, the air tightness would definitely be the issue.
But what about the regulations regarding TDI?

Best regards from Ernst, who is at a loss when it comes to summer weakness.

@ErnstS
According to my information, the mass flow rate (LM) is determined using a characteristic curve based on the temperature difference between measurement points located upstream and downstream of the heating zone, not by measuring the heat input. I'm only familiar with this approach from hot-wire anemometers.
According to my information, it is not known that the HFM5 uses humidity control for compensation (since the specific heat capacity and the heat transfer coefficient depend on relative humidity, which shifts the characteristic curve mentioned above).
Stay cheerful.
OlBe

P.S.: To clarify (or resolve the confusion) regarding the actual topic, we are still waiting for the injection amounts. Has anyone already logged them?

P.S.: Regarding the influence of low-temperature temperatures on summer performance, I would like to add that, regardless of the mass flow rate, when looking at the T-s diagram, the isobars are more spread out at higher temperatures. This means that with the same mass flow rate and the same charging pressure, but higher low-temperature temperatures, more compressor work must be performed during the compression stroke, resulting in less usable medium pressure (power) being available. However, since the mass flow rate will be lower at higher low-temperature temperatures and the same charging pressure, even less power is available...

@olbetec & donalexo

First, olbetec posted an excerpt from an article that donalexo then posted in its entirety (?). Since some of the sentences are identical, I suspect you found it on the internet. Could one of you please post the URL?

Thank you and best regards.
Certainly! Please provide the German text you would like me to translate into English. I will only provide the translation.

@Ernst S.:
Hello Ernst S., unfortunately, my source of information is not electronic. Therefore, I am unable to provide a link here.
However, the information I provided is absolutely reliable because it comes directly from Bosch.

Quote:

The air mass flow meter itself must know the temperature of the air in order to determine the mass flow rate. The temperature must be taken into account in the measurement circuit of the LMM. A heat flow always consists of a mass flow and a temperature difference multiplied by a specific heat.

The language model definitely does not require temperature information.

Quote:

Und wenn die Luft gleich heiß ist wie der Heißfilm .... nun dann zeigt das LMM keinen Massenstrom an auch wenn er noch so groß ist !!

This statement is fundamentally correct, however, the temperature of the heating element is *so* much higher than the maximum possible intake air temperature that this effect is practically negligible. In theory, you are absolutely right.

Here's a small example illustrating the temperature independence of the measurement signal: In winter, if the engine draws in air with a very low temperature, the absolute temperature at both NTC measurement points will be significantly lower than in mid-summer, when much warmer air is drawn in.
However, the temperature difference at the measurement points is the same when the same mass flow rate passes through. This naturally means a higher volumetric flow rate in summer than in winter, with an identical mass flow rate. The density difference of the air, which depends on temperature, is automatically captured by the temperature profile (the steepness of the temperature curve).

Regards,
Alex.

The language model definitely does not require temperature information.


That sounds like it's contradicting the statement that 'the LMM signal and the temperature signal together lead to the air mass flow.' However, we didn't mean that, I just wanted to mention it for others.

Even if the circuit is designed internally in such a way that the temperature of the air does not affect it anywhere, there may still be slight deviations.
http://www.kraftfahrzeugtechnik-heute.de/k/de/start/product.jsp?mfacKey=GS_16_HFM
This is the homepage of Bosch. It states: 'Integrated temperature measurement of the intake air, temperature compensation using a characteristic map.'

If you add up all the information, you could perhaps look at it this way:
LMM signal to the outside = mass flow rate (temperature irrelevant, no question).
The LMM signal is determined by measuring the temperature at the two sensors, which also provides information about the mass flow rate. However, there are small errors due to various factors, and the influence of temperature is compensated for using a characteristic curve as specified by Bosch.

If anyone knows the formula?
The heat transfer should be equal to the heat transfer coefficient multiplied by the area and the temperature difference between the air and the HFM (High-Frequency Material).
The air mass flow rate is calculated by dividing the heat flux by the specific heat and the temperature difference of the heated air.
I can eliminate or keep constant many potential factors through two measurements, but ultimately, there's always some influence from temperature remaining. Even if I can keep that influence very small, it doesn't necessarily have to be related to the density difference (which I never actually mentioned), but rather to the heat transfer formula.

To clarify, I never intended to explain summer weakness solely through the inaccuracy of the Large Margin Method (LMM). Rather, it was about an attempt to circumvent summer weakness, in which case the LMM might have had an influence.
That's why I don't want to bore you any further with this, because this whole story about temperature compensation probably doesn't matter to you.

Best regards, Ernst.

Here's the translation:

'First of all: Unfortunately, I still don't have a VAG-COM device, so I'm just talking generally.'
always based on subjective empirical values.

The maximum boost pressure that can be achieved on my 1998 vehicle with an AFN engine is 1.4 bar.
The 'tuning' box, which can also be activated, is practically set to its most aggressive setting, and...
It also produces no soot emissions at full load.

The noticeable acceleration from 1700 to approximately 2200-2300 rpm increases steadily and powerfully.
However, from this point onwards, it appears that the engine is no longer producing power.
can.

'Lately, while browsing through the seemingly endless number of threads...'
I'm encountering the term 'torque limitation' again (according to ulf, it's a fixed value stored in the AFN).
I'm now wondering if, at the specified speed, ... (followed by a table with RPM and mg/hub).
simply exceeded or reached its maximum performance potential, or whether...
Torque limiting is activated.

There was another article where Ulf wrote that at AFN, the driver with the...
The desired pedal position can inject more fuel than the torque limiter allows.
would allow.

I've been thinking for quite some time about whether to switch from the original .205 nozzles in my AFN...
should switch to version .216.
However, this purchase would likely be pointless if the performance wasn't...
'...but would be limited by the electronics, rather than this component.'

If a torque limitation cannot be ruled out, my question is:
whether it is possible to bypass or increase this limitation without using VAG-COM or chip tuning.
can completely disable it.
I've already read that it's not possible to do this with a box, and that makes perfect sense.

I'm looking forward to the answers!
Best regards, Peter.