Effect of water injection on UEGO sensor 1

[turbomotor]

We are making plans to set up an air cooled opposed 6 cylinder engine with digital electronic fuel control. As the target bmep is about 25 bar, the heat load on the engine exceeds the typical air cooling function, and we plan to use water injection to help cool the cylinder head and prevent detonation. (Some racers call the fluid ADI, but it does more than just prevent detonation.)

I am looking for advice from someone with experience using O2 sensor to help calibrate injector drive signal, and specifically the impact of water injection (upstream of intake valve) on the operation of the O2 sensor. Maximum water injection (at full power) is expected to be about 0.04 lbm water / lbm air.

Will this injection have an impact on the O2 sensor signal?

Once the fuel injectors are calibrated, we will switch the ADI from water to 50MW (50/50 water methanol (by weight)). But we will do the calibration work using just water, so that we do not introduce additional combustibles into the mix when considering O2 sensor feedback.

Thanks for your comments.

 

[BrianPetersen]

It shouldn't matter unless the amount of water is excessive to the extent that EGT drops so far that the sensor doesn't produce a proper signal, and there is no reason why you would want to do that. Will you be monitoring EGT? You should. Just make sure it is in a good range.

 

[Lou Scannon]

I no longer have access to the datasheets and equations, but it seems to me from my past experience as a calibration engineer at a major OEM, that the raw calculated O2% from UEGO output requires a slight correction for the inferred or measured humidity in the combustion air. Since water injection is the same as increased humidity as far as the UEGO sensor is concerned, I assume this correction would be applicable to your case. It is a fairly small correction, and for the work I'm speaking of, it was necessary in order to normalize NOx, BMEP and BSFC for varying (absolute) humidity. If your requirements are less exacting, you can probably ignore the correction.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[turbomotor]

Both posts very helpful. Thanks.

 

[gruntguru]

Hemi was it just the O2% that required correction or lambda as well?

je suis charlie

 

[Lou Scannon]

My response was narrowly addressing the OP question regarding the O2 sensor signal.
Of course the combustion itself is also affected by humidity, like any inert diluent. My experience is specifically with Otto cycle engines, so I can't speak for diesel; but we had to correct our combustion recipe for absolute humidity. So after correcting the UEGO signal, there were lookup tables for compensating spark timing, to normalize the start of combustion, and compensating target lambda, to normalize rate of heat release. The objective of all this was to normalize BSNOx, BMEP, and BSFC. It wasn't perfect, but a whole lot better than no correction.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

Well AFAIK the sensor output correlates to AFR not O2%. My gut response to the OP would be "humidity will not affect the sensor's ability to indicate the AFR".

je suis charlie

 

[Lou Scannon]

In fact, the UEGO sensor is inferring the O2% (and can even infer "missing O2" in rich situations) through a complex electrochemical system. At the engine calibrator level, this information about O2 is converted to some other more useful parameter such as lambda or AFR.
Regarding humidity effects, what I can further tell you is that well qualified engineers at Cummins, using information from well qualified engineers at NGK, devised and implemented an ambient humidity correction algorithm for the UEGO sensor raw signal, as part of the air-fuel ratio control for the B-, C-, and L-Gas Plus on-highway products of the 2000's.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

I repeat - the UEGO sensor correlates to AFR not O2%. It is not the case that a UEGO (or a narrow band lambda sensor) responds primarily to exhaust oxygen content - it responds to the ratio of oxygen vs unburned fuel compounds (H2, HC, CO etc) in the exhaust - ie lambda. From your link:

When the air/fuel mixture is rich, the reference cell produces a high Vs voltage (above 0.450 volts). The Precision Wideband Controller reacts to produce a pump current (Ipump) in one direction to consume the free fuel. The pump cell requires a "negative" current that goes from zero to about 2.0 milliamps when the air/fuel ratio is near 11:1.

When the air/fuel mixture is lean, the reference cell produces a low Vs (lower than 0.450 volts). The Precision Wideband Controller sends the pump current in the opposite direction to consume free oxygen. The pump cell requires a "positive" current that goes from zero up to 1.5 milliamps as the mixture becomes "free air".

When the air/fuel mixture is 14.7:1 (the stoichiometric ratio for gasoline), the pump cell requires no output current. Since the free oxygen or free fuel has been neutralized by the pump current, the Vs feedback signal goes to about 0.450 volts (the same as the Vref value).

Regarding humidity. It is absolutely true that humidity levels have an effect on ambient oxygen content and fuel rates will need to be adjusted if the desired AFR must be achieved. Water vapour in the combustion air is however a diluent - AFAIK lambda and UEGO sensors respond only to reactants (oxidation and reduction components).

If this is not correct and humidity does affect the sensor output, I would be interested to hear the mechanism. Perhaps the oxygen concentration in the "air-side" of the sensor has an effect?

je suis charlie

 

[Lou Scannon]

Regrettably, I do not have access to the source information from NGK and Cummins, and I do not recall the physical mechanism, so I'm not able to expand on my previous remarks.
But you could be onto something with the air side idea. At any rate, the effect is quite small, and unless you're splitting EPA emission hairs, probably can be ignored.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz