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Originally Posted by tasdrouille
More and more recent cars have slow wideband O2 sensors. Can't you get a better estimate of fuel usage by simply combining MAF and AFR. I'd think that would be pretty close to reality. With my Elantra I have set up X-Gauges for average trip MAF and Lambda. So far it's been pretty accurate. I just wish I could have an X-Gauge displaying a result based on 2 PIDs...
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If you are looking at a wideband, MAF, and your odometer, you should be very close to reality. But there are some inherent errors.
First, the gauge might be reporting AFR, but the sensor is actually measuring 'equivalency ratio'. That is, it is measuring how much O2 must be added or removed to reach stoich in relationship to partial pressure.
This number is inverted and reported as lambda, with 1.0 being stoich. To report 'AFR', lambda is multiplied by a fixed factor for the fuel, say 14.7 for gasoline, but that is really just an approximation, the exact ratio will vary on blend, and even fuel temp.
Eq. Ratio is actually better for engine control. You don't care about AFR, you care about combustion result. When the engine needs stoich for max EGT, you get stoich. Ideally, the ECU could run closed loop on the wideband, then when you hit .84 lambda, you'd know you are at best power.
But the way the sensors are generally measured is slow. The chamber is pumped to stoich through a PID loop. Also, for durability the sensors themselves are often slow. So, typically, the wideband is used to help the ECU make decisions, but not tightly coupled, in a stock auto.
There are a few other cases of error. For example, the sensors are typically calibrated at sea level, standard temp. But the PP of O2 changes as we deviate from this, which translates into error. Also, your AFR reading probably pegs at a value when you are super lean (injectors closed).
But, all in all, you've hit on a pretty accurate method. MAF is a great way to monitor air, and lambda gets you very close to true AFR when running pretty much everywhere but very lean (when only small amounts of fuel are involved anyway).
In general, I think that measuring IPW pulse by pulse is probably the tightest monitor only measurement, but you still have variables like injector on/off behavior.
As far as who I work for, the simple answer is I report directly to the president here:
EmiSense Technologies, LLC: Smart Sensors - Clean Emissions
But it is slightly more complicated. But I'll put an introduction in the proper area later today or tonight.
-jjf