[urq] AF meter[was What cause overboost ? ]

[Buchholz, Steven]

... the 'heart beat" is not a big surprise because that's the way the
system works.  You should not think of the OXS as much more than an
on/off switch.  What the ECU does is to flip the mixture slightly back
and forth across stoich when it is operating in closed loop mode to make
sure it is in the stoich ballpark.

As a reference, it is generally accepted that an engine produces the
most power at lambda = 0.9 and best fuel economy at lambda = 1.1 [Bosch
Automotive Handbook].  The standard "narrow band" OXS has a signal that
is "off" at 0.975 lambda and fully "on" at lambda of 1.025.  This means
that the best the "A/F" meter can tell you is whether or not the mixture
is above or below stoich.  I too have one of Orin's meters ... I planned
to install the LED display in one of the unused switch blanks on the
dash ... but by no means should anyone spend a lot of money on the 20
LED displays ... 

The fact that the OXS has such a narrow active range is why most older
engines are allowed to go open loop at WOT ... it also means that the
ECU does not have direct feedback to determine that the engine is
running at the mixture for best fuel economy when in closed loop too ...


Steve B
San José, CA (USA)
> 
> JC Whitney has a Intelletronix unit for about $39.  These meters are
really
> 0-1000mV meters in ten segments.  As a voltmeter, they're really
> accurate-I've checked them on a voltage standard.  They're all based
on a
> National Semiconductor IC.  If a A/F has ten or 20 segments, you can
bet its
> a NS IC doing the conversion.  In theory if the O2 is seeing stoich,
then it
> will output 500mV which is half on the gauge.  On my 5kcstq, its
fascinating
> to watch the ecu change mixture.  It looks almost like a heart beat
monitor.