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IAT placing and ideal gas law


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I wondering a few things arount IAT correction.

The help file before firmware 5.2 says, that the recommendet IAT location is just before the throttle. From 5.2 on, the suggested position is after the throttle body. Especially with the use of the the charge temperature estimation equation modelle, I can't comprehend why placing the IAT sensor after the throttle should be better? As the most know, heat soak on long idle periods are the biggest problem with IAT sensor placed there. With the fuel equation calculation, we can bias the calculation more to engine side if the IAT are getting to less hot if we placed it before the throttle. In the case of fittment in the intake manifold the IAT sensor getts often to hot (heat soaked) and to much fuel is removed. Biasing the charge modell in the direction of ET will make it even worse. 

The other question is: what percentage correction does the new modelled fuel equation use?  2%, 2.5%, 3.5% per 10degree change?

I'm wondering, because in theorie (ideal gas law) the densitiy will change ~3.5% per 10 degree temperatur change. But all LINK base maps has only 2% on the IAT correction map. Are there a good explanation for this phenomen?

I personally had good results with 2 to 2.5% correction and placing the IAT Sensor just before the throttle body. 

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The idea of the air temperature sensor is to help determine the temperature (and hence density) of the air charge entering the engine.  So, if the manifold is heat soaked then the air entering the cylinder will be significantly heated by the manifold itself and so much less dense. 

The sensor does not get "heat soaked".  It is the manifold that gets hot and heats the air inside it.

It is entirely up to you where you place the sensor.  If you do not want compensation when the manifold is heating the air temperature above that before the throttle plate, then place the sensor before the throttle plate.  You will however notice that all OEM installs place the sensor in the manifold.

The fuel equation (both modelled and traditional with charge temp estimation turned on) adjusts the air density exactly as per the ideal gas law.  There is nothing stopping you carrying on with doing things exactly as you have in the past.  The traditional fuel mode can be used to tune as per older firmware.  In traditional mode charge temp estimation can be turned off and the IAT table is applied as a simple percentage correction.

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Thank you for answer. 

I agree in theorie. In practice the reason for placing the IAT sensor just before the throttle body is, that the manfold gets hot if there is only a low air flow inside (e.g idle). As longer the engine idles, as hotter it gets. The problem is that, the sensor gets "heat soaked" from the manifold and reads higher than the air is. That's because the air has to less time while it pass through the manifold to heat up to the same temeperatur as the sensor reads. 

So in practise I often have to lower the IAT correction value on idle to prevent the engine from leaning out on long idle periods. That's the reason I asked. I think the Ideal gas law correction which is ~3.5% substracts to much fuel in this situation. Anyway I have to make more testing on the new modelled fuel equation. It's good that theres is still the IAT correction table, in the case it should be needed. 

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  • 3 months later...

in the meantime I tuned some ECU's with the new charge air temp approximation. It gave me a better understanding what happens and I have to say that LINK engineers done a great job! The new modell gives more stable idle, and compensateds better for long idle periods. 

Only downside. it needs alot of time to deal in the charge approximation table. The methode descripted in the help file doesn't work for most tuners, since you don't have the time to test warm up over several days. Personally I bring the IAT's high for many times and for every load point and try to adjust the charge approximation table, that the AFR's are stable over a big temperature range. 

Are there any other tuning mehtodes you can recommend for the charge approximation table?  

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