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Air Fuel Ratio Sensor

Practical Diesel Tuning

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Discussion and questions related to the course Practical Diesel Tuning

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Is the air fuel ration sensor tapped into the head and does it directly measure the exact ratio in the cylinder or is there an air volume sensor added in the intake so then the ration can be calculated with what known volume of fuel is being added?

If it is in the head, and your engine doesn't come through with one, how do you add one?

In general the front air fuel ratio sensor, usually referred to as simply the front or upstream oxygen sensor is located somewhere in the exhaust manifold and receives its information in the form of all cylinders exhaust gasses combined. This gives an average AFR of all cylinders but does not for example give an indication of an individual cylinders air fuel ratio. If one cylinder is running rich or lean that will show up in the readings of the oxygen sensor but further investigation would be required in order to determine the cause and from which cylinder the issue stems from. Typically most engines from the factory these days are using Mass Air Flow sensors mounted in the intake system to directly measure the volume and temperature of incoming air.

OK, so but for the Mass Air Flow Sensor its still on the exhaust side like the EGT.

I think there is some confusion here.

Mass flow is measured at the Airbox and no other point in the engine. Mass air flow is used to calculate an AFR based on known variables like injector calibrations fuel pressure and other things.

AFR or Lambda is usually measured at a common point in the exhaust system post turbo for example and as David said is an average of all cylinders combined. The air fuel ratio is calculated from Lambda. The stoichiometric ratio for Diesel is 14.5:1

This value is fixed so if you multiply the stoich ratio by the lambda you get your AFR. i.e. Lambda 1.2 x 14.5 = AFR 17.4:1.

If you are concerned about the AFR in each cylinder then you can add Lambda sensors at the outlet of each port on the exhaust manifold but this is a very expensive approach and generally not necessary and requires some special considerations.

I hope this helps



Air/fuel ratio is derived from the oxygen content in the exhaust gas and expressed in terms of Lambda. In typical OEM applications the sensor is placed in the exhaust manifold pre catalytic converter with a 2nd lambda sensor being placed post converter to monitor the efficiency of the catalyst.

Mass airflow is the measured quantity of air entering the engine and placed in the inlet post air filter. We can then add fuel to the desired theoretical AFR based on MAF and use the Lambda sensor to measure the actual AFR and trim the fuelling accordingly.