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241: Wideband lambda sensor deviation?

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One thing that was not talked about during this webinar, is the pressure dependency of a wideband sensor.

How are you working around this?


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This is actually an issue irrespective of altitude since you're likely to see a measurable variation in pressure in the exhaust system even at sea level die to the restriction in the system. If you want to ensure the accuracy of the lambda readings then there are some wideband controllers that incorporate an exhaust pressure sensor for internal compensation.

So lets think of a example where this could cause a huge problem, that i`m not sure many consider.

Lets say you take a NA engine and race up pikes peak with it ( or aero engine ) , Exhaust pressure will be close or slightly above atmospheric pressure.

At the top of the peak, this should be something like 57kpa abs. Looking at the deviation table in the link, it indicates that the lambda sensor will be about -15-20% off the correct value.

If this is not taken into consideration during the calibration of the engine, it can make a huge impact on the fueling of the engine. ( Either by the calibrator calibration the fuel for the engine based on wrong data, or if closed loop fuel is working the ecu will adjust fuel into the wrong actual lambda. )

So guess my question is: Are there any ecu based systems that will adjust for this? ( like motec m1, since this was used in the example ) Or any other experience you could share?

Even at the likes of Pikes Peak this is probably less of an issue than you'd think. Last time I stood at the top of Pikes Peak the barometric pressure was about 65-67 kPa although obviously this will vary with atmospheric conditions. On its own this would potentially result in an error of approximately 12% when running richer than lambda 1.00 (WOT operation which we're most concerned about). While in itself I'd say that yes, this is worrying, the reality is that even on a 'race' exhaust system on an N/A engine your exhaust pressure won't be zero. Even with a conservative amount of back pressure of 10-15 kpa which would be considered low, the error is now reduced to something like 6-7% - Still potentially concerning but much less dramatic.

The reality is that this is only an issue for a very limited market but if you're there then there are some solutions. One would be to use a wideband controller with built in back pressure compensation. The Link CAn-Lambda does this, as does the HV Electronics controller just to name two off the top of my head. You could alternatively set up a fuel comp or lambda target comp table based off back pressure to skew the targets so that your actual readings are where you want to be. Alternatively you may want to consider looking at the newer Bosch ADV sensor that is designed for pre-turbo installation and has a much lower sensitivity to back pressure. Right now the Emtron is the onbly ECU I'm aware of that natively supports this sensor but we can expect the list to grow.

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