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Old 07-24-2003, 06:04 PM   #39
Scooby Newbie
Member#: 40416
Join Date: Jul 2003
Location: CA
68 Porsche VW Bus


Hi Jon,

I think I understand how your pressure compensation works. I think it depends, for one thing, on the assumption that when the throttle suddently closes (like during a gear upshift) then the A/F ratio and pressure of the exhaust gas will abrubtly change to that of "free air assumed at normalized sea-level pressure."
No, that's not how it works. That would compensate only after the fact. The way it works is that our circuit forms with the oxygen sensor a free running oscillator with variable duty cycle, frequency, output voltage and output waveform shape. Those four variables vary with lambda, exhaust pressure, sensor temperature and sensor age. As you can see, with the correct math we have a system of equations where the individual source variables (lambda, exhaust pressure, temperature and sensor contamination) can be extracted. This is the essence of the patent. Conventional designs use a PID loop, where the sense cell is the input and the pump current is the control output. By their nature they can measure only one variable, the pump current. Also PID controllers assume a fixed time-delay in the step response of the controlled system. Because the response time of the sensors varies with lambda and pressure (forget age and temperature for now), they typically are designed for the worst case response and are either slow or have overswing outside the design point.
After having said that, our patent lawyer will probably kill me.

I measured on our system a response of ~80ms between free air and lambda 0.8. The measurement method was to blast high pressure calibrated gas into a small measurement chamber (<1ccm) in which the sensor was mounted. I found a small overswing of about 0.08 lambda, which was due to the sensor being rapidly cooled by the measurement gas while the heater could not respond fast enough. In a real world application on a carburated 350ci V8 with dual exhausts and 1 cylinder misfiring, the misfire was visible on the lambda bar on the display(as lean peak) at up to ~900 RPM. Because of this fast response (also visible on the analog outs) it would probably be beneficial to put a low-pass filter on an analog output used with a DVM for AFR display.

The lower AFR limit for the NTK is what we measured with our circuit, where we are careful not to exceed the assumed NTK limits. This does not say the NTK cannot go richer. Same goes for the Bosch.

BTW: Quick test for the LM-1 (for those who got one or will):
After calibration with free air, breathe on the sensor. It will show the approx. oxygen content of your breath. If you breathe through a MAF sensor you should be able to calculate calories burned.

Don't breathe calibrated lambda gases (bottled smog).

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