Lambda Sensors and Catalyst Temperature Measurement

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Lambda Sensors and Catalyst Temperature Measurement

Postby FioranoCars » Wed Jan 23, 2013 4:05 pm

Hi
I might be having a blond moment, but after a bit of internet research I can't find any answers!

Many of the cars we work on use a Thermocouple to determine Catalyst temperature, but new Models/Versions (facelifts) seem to be dropping the thermocouple but still getting a Catalyst temperature in the Engine ECU (Bosch Motronic). I am assuming this is from some sort of reading from the Lambda sensors (5 wire pre-cat, 4 wire post-cat), but I can't find any information of how this is calculated. All the Lambda sensors pages give wiring but none mention calculating temperature. (lots mention heating the sensor to get to a working temperature when cold etc).

Am I barking up the wrong tree for the temperature calculation (there appears to be no other sensors near the cat)?

thanks

Richard
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Re: Lambda Sensors and Catalyst Temperature Measurement

Postby Avdr » Wed Jan 23, 2013 5:34 pm

As I understand it, the Bosch LSU4 wideband sensor is 'intelligently' heated. The heating duty is varied to stabilse the sensor temperature. To the extent that if the sensor is too hot the ECU knows this (I assume from circuit resistance) and reduces duty cycle, and visa versa.

So I would imagine that by this method, sensor temperature can be worked out with reference to duty cycle of the heater circuit and heater circuit resistance.
The same may even be true for 4 wire sensors, if the heater element is constructed in similar fashion to 5 wire.

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Re: Lambda Sensors and Catalyst Temperature Measurement

Postby FioranoCars » Wed Jan 23, 2013 9:24 pm

Thanks, found some new articles searching for
Bosch LSU4 wideband sensor


Hopefully I can get some ideas of how to calculate something using the Pico, but not sure if that is possible ... any one else looked at this, feedback and pointers most welcome!

thanks
Richard
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Re: Lambda Sensors and Catalyst Temperature Measurement

Postby FioranoCars » Wed Jan 23, 2013 10:30 pm

There are 2 articles that explain the sensor very well, the first actually describes two methods of temperature calculation (there may be more!) that could be implemented, but it appears that this could be very manufacturer dependent (or ECU dependent - ie Bosch Motronic verse Other System). We are mainly interested in Bosch Motronic systems 7.1.1 and later (in case anyone has knowledge of those!).

The first site is http://www.megamanual.com/PWC/wire.htm (also http://www.megamanual.com/PWC/LSU4.htm ) where
The LSU probe does not have any form of direct temperature measurement (i.e. thermistor, etc.). However, monitoring the resistance of the reference cell yields a close representation of the probe temperature - the resistance of the reference cell varies with temperature. The Nernst reference cell has a high resistance at low temperatures (i.e. ambient temperatures) and a resistance of approximately 80-100 ohms at normal operating temperature. By monitoring the internal resistance of the reference cell, it is possible to determine an accurate UEGO probe temperature without the need of an external temperature sensor element.

There are several methods available to measure the resistance of the reference cell, including disabling the pump circuit and applying a known constant current across the reference cell and measuring the resultant voltage, finally re-enabling the pump circuit. This method requires several analog switches to apply the current and re-establish the pump servo circuit when done. Also, if a bias is applied to the Nernst cell, then an opposite polarity current with the same duration needs to be applied in order to “reset” the polarization on the cell. The one problem with this method is that it is “intrusive” to the feedback loop of the Nernst/pump.

Another method is to apply a high-frequency waveform to the pump circuit and measure the resultant deviation in EMF. The reference cell's resistance is determined by AC-coupling a square wave of known amplitude and frequency via a series resistance, and measuring the resultant AC waveform's amplitude. This waveform is always present, and since it is at a high frequency with respect to the response of the Nernst/pump feedback loop, it essentially averages out. This is the method employed in the PWB.

Circuit operation is very simple. A known square-wave source of 5 volts peak-to-peak and at a frequency of 1 to 3 KHz (generated by the DSP) is capacitively coupled to the reference cell positive terminal. Overall current is limited by a series resistance (plus Ri internal resistance) to 500 microamps peak to peak, or 250 microamps around the Vbias point (Vbias is set to 2.5 volts to allow for bi-polar pump operation) - this value meets the specification outlined in the Bosch LSU 4.2 data sheet. The alternating current signal generates a corresponding alternating voltage with value based on the internal resistance Ri. For example, if Ri = 100 ohms, then 500 microamps (P-P) multiplied by 100 ohms yields 50 millivolts p-p, or 25 mv around the Vbias point. Actually, the series current limit resistance and Ri form a resistor divider circuit driven by a voltage potential.

To measure the voltage, a capacitor is used to block the DC offset (i.e. reference cell voltage) and pass the alternating signal. A gain stage is introduced and the voltage is fed into a A/D port on a processor. Note that this signal is an AC signal, so ADC sampling needs to correlate with the polarity of applied square wave signal – this is known as synchronous rectification. An alternative method would be to use a bridge rectifier circuit to recover the positive/negative swings and then filter before application to the ADC channel.

A picture is worth a ton of words:

ri.gif
ri.gif (10.28 KiB) Viewed 2758 times


The second site is http://wbo2.com/lsu/lsuworks.htm but the subject of temperature measurement is only fleetingly mentioned.

In searching I have found lookup tables for the lambda, which I'll try and incorporate into a custom probe when I get the time and chance of hands on with a car to test it. Found lots of stuff in fact, but nothing with specific info on the temperature calculation implementation by ECU or Car manufacturers!

So has anyone come across any manufacturers data on how they used the sensor to calculate temperature?
Or can shed any light at all 9from the data mentioned here or elsewhere, please let me know ...

Any other links of course welcome

thanks
Richard
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Re: Lambda Sensors and Catalyst Temperature Measurement

Postby FioranoCars » Fri Jan 25, 2013 12:29 pm

Having done as much searching as I can, and getting nowhere, I decided to call Maserati (as they are one of our main vehicle) and see if they could help ...

They could confirm the Lambda sensor is used for the temperature measurement and that it is only used above circa 300C (below this the sensor is not at an operational temperature), and that it used a pulsed High Frequency signal, but sadly no idea of the values used or the calculations ...

Maybe the Pico technical boys have some knowledge?

Anyone else working with Bosch Motronic systems have any ideas?

I will try and capture some signals, including AC and DC (if they make any sense) with some ECU reported temperatures and see if anyone can figure out what's going on?!

thanks
Richard
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Re: Lambda Sensors and Catalyst Temperature Measurement

Postby KimAndersen » Fri Jan 25, 2013 3:34 pm

It's a rather complex subject to measure a catalyst temperature and to find some information about this subject on the web. I have found some info from different manufactor of cars but as with any source you get there will be some interpretation of these documents.

The first source of information was from http://www.motorcraftservice.com/vdirs/ ... sm1104.pdf where there is alot of good stuff regarding - how Fords OBDII system works and operate´s.

Ford uses something called a " Catalyst Temperature Model " to predict the temperature in catalyst and is a calculated method based various inputs from engine speed sensor - air mass sensor and commanded spark.

This so called " Catalyst Temperature Model " is also used in other areas of ECU - such as misfire dection and thereby also controls the emission related calculations.

This information is taken direct from the document called " 2011 MY OBD System Operation Summary for Gasoline Engines" and only a small bite of this comprehensive document.

Here is what there stood under the headline - Cold Start Emission Reduction System Monitor.

Temperatures Used

The actual catalyst temperature is the same inferred catalyst temperature that is used by other portions of the engine control system, including the CSER control system. The inputs to this actual temperature are measured engine speed, measured air mass, and commanded spark.

The expected catalyst temperature is calculated using the same algorithm as the actual catalyst temperature, but the inputs are different. Desired engine speed replaces measured engine speed, desired air mass replaces measured air mass, and desired cold start spark replaces commanded spark. The resulting temperature represents the catalyst temperature that is expected if CSER is functioning properly.


As stated - this information only covers Ford cars - what other automotive manufacturers use of technology to determine the catalyst temperature I don´t know. I have some other documents that covers Lamborghnini OBD systems, but i will rather wait with this info until later.

If you are looking after a table regarding the catalyst temp vs. inputs from other sensors as a lambda sensor then i haven't got it yet.

What do you think about this info !

I have other stuff that i could publish if needed.

Kim
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Re: Lambda Sensors and Catalyst Temperature Measurement

Postby FioranoCars » Fri Jan 25, 2013 3:49 pm

What do I think?
... Amazing! but it'll take a while to read and digest 251 pages!

It will be interesting to see how Ford and other Manufacturers (anyone help with info Please?) calculate and implement temperature, as well as other areas of engine management/emissions control, as the whole arena of Engine management has and continues to evolve at a tremendous pace.

Thanks for posting this, anything else please go ahead!

cheers
Richard
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Re: Lambda Sensors and Catalyst Temperature Measurement

Postby Robski » Fri Jan 25, 2013 4:38 pm

@ Fiorano cars

Why don't you contact Tom Roberts of the Autonerdz & ask if he can accommodate you for what you require to know & pay the annual fee because i don't think you're going to get the answers that you seek in the public arena........i stand to be corrected.
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Re: Lambda Sensors and Catalyst Temperature Measurement

Postby FioranoCars » Sat Jan 26, 2013 6:12 pm

Hi Robski
Thanks for the tip, but I'm really interested in helping and sharing gratis where I can, and while I'd like to understand the equation, i think each manufacturer (or OE ECU supplier) has it's own strategy to calculate the temp, so I think it's more than one answer.

Equally it's not got a time limit on it, it's a pet project so when we have an issue one day in the future, we can do the accurate diagnosis to prove the ECU reported error is correct (and not a wiring fault or indeed the exhaust running hot due to cat breaking down, etc etc), so I'm happy to try and engage the rest of the forum ... some of whom I'm sure must have had a training session where the subject has come up, or where they might raise it as a question !!! ???

but thanks, if all else fails then I might give Tom a go

Richard
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Re: Lambda Sensors and Catalyst Temperature Measurement

Postby FioranoCars » Sat Jan 26, 2013 6:27 pm

Kim
Your are a master of technical detail!

from the documentation I have found for Lamborghini their look up table, see below. Sadly the Lambo's use their own ECU (called the LIE) and are wierd compared to most other ECU's, but at least a start, and when we have some time, I'll do some basic tests to see if the table works!

It looks like (unless I buy a differential probe) it will need 3 channels to calculate (OK 2 if I wanted to take silly risks, but I don't), as the table uses resistance, so I'd need:
ChA - Volts In
ChB - Volts Out
ChC - Amp clamp (hope it's within range/resolution!)
Maths channel to calculate volt drop (ChA - ChB) then Divided by Amps = Resistance

Unless anyone has a better method for calculating resistance?

here is the excerpt from the paperwork with the temperature calculation:
7.2.4.2. Open Loop Operation Of UEGO Heater
When Tcat reaches 70°C, the UEGO sensor heater may be driven to high power. Due to the fact
that RpVs ( Uego Temperature calculation) reading is not possible until Vs value is below 1.3V, a
fixed heater feed of 12V is supplied to the sensor (that is the reason why this step is called ‘open loop
heater management’). In this step, Vs signal must drop below 1.3V within 6 seconds, otherwise a
P0135 fault code is stored.
When Vs < 1.3V, RpVs reading is possible. RpVs is the electric resistance of a ceramic
component (one of the cells of the sensor, as described in section 7.2.1) which can be used, for this
purpose, as a thermoresistor to calculate UEGO Temperature.
Please find following the link among RpVs and UEGO Temperature (tab. 7.2.4 – 2).

Code: Select all
RpVs (Ohms)   10  40  60  70  85  100  110  130  150  180  210  240 1000
UegoTemp(°C)  947 872 818 795 756  700  660  615  587  566  556  545  0

Tab. 7.2.4 – 2 Link among RpVs Value and UEGO Temperature


Sadly can't use a custom_probe lookup table feature to create the scaling, as Maths channels don't support lookup tables (any chance of adding that to the feature request list?!) ...

So many thanks to Kim, you're a real hero!

Still after a Bosch Motronic strategy for this, oddly thought that would be the one I'd get first and never ever find the Lamborghini one, but I'm happy!

thanks
Richard
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