O2 sensor Primer

["Graydon D. Stuckey" <graydon@apollo.gmi.edu>]

On Sun, 1 Dec 1996, Angela Dupin wrote:

> What difference is there between the various 3 wire Bosch Audi sensors, 
> besides the connectors?  is the signal different?

Phil, & everyone else that is interested...

	This has been discussed in previous threads, but I guess there 
have been some problems with the archives, so for the benefit of those 
that weren't here, or can't get to the archives, I'll recap briefly.

	All oxygen sensors are alike basically.  They compare the oxygen
content of the exhaust gas to a reference gas housed in a small chamber
inside the sensor.  The difference between the two oxygen levels will
cause a small voltage to develop between the two gases.  The reference gas
and the general construction is designed so that the voltage is 0.5v when
the engine is burning at stoichiometry(approx 14.7:1.) A richer mixture is
indicated by a greater voltage while a leaner mixture is indicated by a
lower voltage.  O2 sensors are extremely accurate at or near stoich, but
they loose their accuracy as the mixture deviates from stoich by more than
10%.  There are many factors that affect their accuracy, but that is a
discussion that is far more advanced than I can lead. 

	The variations in the commercially available sensors are mostly 
centrered around heaters, and ground locations.  The single wire sensors 
have a signal output, and the sensor case is the ground.  The 3 wire 
sensors have an internal heater which bringhs the sensor up to operating 
temperature mroe quickly, and will keep it at that temp if you idle the 
engine for too long.  The 4-wire sensors add an additional wire for a 
separate ground, in case you are using an exhaust manifold material which 
does not conduct electricity.

	In case some of you are saying, "But there _are_ other types of 
sensors!" I would add that there is a new sensor called a wide band 
air/fuel ratio sensor which stays accurate from about 12:1 up to over 
22:1 air/fuel ratios.  These are very expensive, but nonetheless, a few 
manufacturers are using them, particularly in the newer "lean-burn" engines.

	That's the short form.  HTH

Later, ---------------------------------------------------------- 
Graydon D. Stuckey 	'85 Mazda RX7 GS, no toys 
graydon@apollo.gmi.edu 	'86 Audi 5000 CS Turbo Quattro, has toys
Flint, Michigan USA	'89 Thunderbird SC, lotsa toys