Mixture guage (long)

[Mark Gibson <Mark@domain.elided>]

  James Seabolt asked a question about the response from an O2 sensor.   
 Although you all probably know this, I'm going to share it with you   
anyway because it took me so *$@% long to figure it out myself, and I'd   
like to think that someone would benefit.

  Despite differences in mounting and heated/unheated configuration, all   
O2 sensors are basically the same.  The oxygen sensor is a wonderful   
thing, but it has an almost purely "on-off" output.  That is, it can tell   
the computer "too lean" or "too rich", but it can't say "just right".  So   
you get 0.1 V at "lean" OR 0.9 V at "rich" (or is it vice versa?), but   
never, say, 0.45 V ("just right").

  It actually works the same way as the thermostat in your house - the   
furnace can only be either "on" or "off", but the thermostat can still   
control the temperature in the room by deciding when "on" and "off"   
occur.

So the O2 sensor says "I'm rich!", and the engine leans the mixture a   
smidgen.  After a few milliseconds of this, the computer overshoots and   
the sensor now says "I'm lean!" and the engine adds a little more fuel.   
 Then the cycle repeats.  Thus, if your fuel injection system is working   
properly, the mixture will ping-pong between slightly too lean and too   
rich.  The O2 sensor is sensitive enough (and fast! Reaction time is   
measured in milliseconds.) that the mixture doesn't have a chance to get   
too far off before the FI computer corrects it.  In fact, it should cycle   
between rich and lean about once every 2 seconds (roughly) at idle.

  Now, this is all well and good for warm-engine steady-speed cruising,   
but the computer ignores the O2 sensor under several circumstances, which   
James noticed.  When the engine is cold, it needs a richer mixture (e.g.   
a choke), so the computer is programmed to give a constant (richer)   
mixture at those times.  This is called "open-loop".  Once warm, the car   
enters "closed-loop", which gives the "ping-pong" effect described above.   
 When you accelerate, the engine needs a richer mixture (hence the   
accelerator pump in a carb).  So the computer ignores the O2 sensor at   
this time and adds more fuel.  When you decel, the engine doesn't need   
any fuel at all, so the engine will often shut off the fuel injectors   
completely.

  This is all based on my study of the FI system in my Maser 228   
(Weber-Marelli injection/ignition).  However, I know now that most O2   
sensor-equipped cars work the same way.


P.S. I've always wondered how those mixture meters could possibly work,   
given the response of an O2 sensor.  Guess they don't!