Re: Oxygen Sensors and The ARC2

[mamarand@xxxxxxxxxxxxxxxxxxxxxxxxxx (mamarand)]

     Regarding the recent posting on oxygen sensors and the ARC2 
     calibrator:
     
     <...I guess you are saying that a stock O2 sensor has too much drift, 
     noise, error etc. to accurately use it as a calibration sensor for 
     precise performance tuning.  Then how can it be used for real-time AF 
     mixture control in an engine?  Said another way, if I were to buy a 
     very accurate calibrated O2 sensor and plug it in, would I see a lot 
     better performance out of my car?  Or does the chip sort of do most of 
     the work open-loop and then the O2 sensor only makes minor corrections 
     for gross out-of-range conditions?  This would mean the stock O2 
     sensor doesn't really do much under normal operating conditions, and 
     your remarks about unsuitablity for tuning would make good sense...>
     
     Oxygen sensors are extremely accurate around the stoichiometric 
     air/fuel ratio of 14.7:1.  They are very repeatable and stable over 
     temperature at that point.  Oxygen sensors are an integral part of the 
     closed loop system that is used to continuously tune engines.  This 
     tuning process is used to extract maximum power while optimizing fuel 
     economy and minimizing emissions.
     
     While oxygen sensors exhibit a loss of accuracy and increased 
     temperature dependence for air/fuel ratios significantly different 
     from 14.7:1, they may still be used to make highly accurate air/fuel 
     ratio measurements.  In closed loop operation which occurs at most 
     engine loads aside from wide open throttle, The ECU (DME) continuously 
     adjusts the air/fuel ratio toward stoichiometric.  It continuously 
     overshoots and swings back the other way.  On an air/fuel ratio meter 
     this appears as dithering or a back and forth sweep on the display.  
     The rate of dithering is related to RPM and is primarily dependent on 
     the delay time from the ignition event and the arrival of the 
     resultant exhaust wave front at the oxygen sensor.  The duty cycle of 
     this dithering may be used to accurately measure non-stoichiometric 
     values. A method called cross counting is used to gate a high speed 
     clock to measure the duty cycle.
     
     This effect is readily observed on good quality, stock oxygen sensor 
     based air/fuel ratio meters like the ARM1.  At stoichiometric in 
     closed loop mode, the meter will sweep back and forth in a balanced 
     fashion.  At non-stoichiometric values the dithering will be 
     unbalanced to one side or the other.
     
     <So ignore the O2 sensor for a while.  These add-on boxes use three 
     knobs and these knobs control some scalar quantity maybe over roughly 
     1/3 the rpm or 1/3 HFM output signal range each, or are these the gain 
     parameters of a PID or similar controller?  If it's gain parameters 
     for a closed loop controller, I don't see how the shape of the ideal 
     curve matters, because you are only at one point on it at any given 
     time.  However, if the O2 sensor truly sucks as a real-time feedback 
     control sensor, then obviously 3 points is not enough to keep the chip 
     operating in mostly open-loop mode.>
     
     The Split Second ARC2 is an air/fuel ratio calibrator that is used to 
     alter the transfer function of the air metering device.  It can be 
     used in air flow meter, MAF, MAP and Karman Vortex applications.  The 
     low, mid and high settings do far more that adjust three points.  
     These controls vary the offset, gain and linearity of the signal.  The 
     effect of these controls could also be described as shift, bend and 
     slant.  The ARC2 does an algorithmic calculation in real time which 
     results in a smooth, seamless transformation of output data values.  
     The ARC2 is capable of dialing-in a balanced dithering condition over 
     the full closed loop operating range.   
     
     <I don't think anybody in their right mind would claim that such a 
     system would outperform a purpose-designed chip which had used a 
     high-priced O2 sensor to calibrate for a fixed set of mods.  But is it 
     utterly unreasonable to expect improved performance, or is this also 
     total BS?...>  
     
     One of the features of the ARC2 is its ability to replace an air flow 
     meter with a mass air flow sensor.  In addition to the calibration 
     function already described, the ARC2 compensates for elevation and 
     temperature. Most importantly it performs critical filtering which 
     makes the output of the fast responding, electronic mass air flow 
     sensor look like the output of the slow, mechanical air flow meter.  
     This is how the ARC2 preserves good drivability.  The filter function 
     in the ARC2 would be extremely difficult to implement through a code 
     change in an existing ECU.
     
     Mark Amarandos
     '88 M5

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