Ivymike,

From an overall emmissions standpoint, it is desireable to keep your equivalence ratio as close to 1 as possible, which is what most modern engines attempt to achieve throughout the operating range (exception being WOT where richer mix is req).  By making the mixture more rich (or very lean) you will lower your NOx output by lowering flame temperatures.  This species will peak at a mix slightly leaner than stoich due to oxygen availability and elevated flame temps.  Unfortunately, by making the mixture richer to lower NOx, your HC and CO levels will increase dramatically (past stoich).  To lean out the mixture enough to lower NOx, the engine would not run well (if at all).

I think what you are referring to is the use of EGR which basically adds inert gas to the mixture.  This gives the impression of leaning out the mixture and is used in lowering flame temps (=lower NOx) and slowing the burn process.

I hope this helps.
smithsmc

hmm... it was my understanding that Honda was running their newer engines at AF ratios as high as 17:1 (very lean)?

One surprise I found was that when using EGR is that the mixture actually goes richer (displacing incoming air), thus reducing NOx.  Reducing air also lowers the combustion temperature.

With the Honda, their engines use variable valve timing along with highly advanced combustion chamber and piston design.  Their catalysts have a high oxidation rate which require a rich fuel mixture pulse every couple of seconds to keep the bed hot.

I have personally operated gaseous fuel engines (propane and natural gas) at lambda ratios exceeding 1.7, well past the lean NOx curve.  This is not possible with a liquid fuel engine and coventional induction, only with boost.

Back to your original question, most modern engines wobble about .5 point from stoich during normal operating ranges.  This keeps the cat hot for emission control and minimal NOx production.  Look at it like this. . .  Cats like it lean for CO and HC, rich for NOx.

Normal operation is Lamda 1.0, with a wobble from L=.95 to L=1.05.  Also remember that Lambda reads opposite from AF ratio.

Franz

Typical gasoline spark ignited engines today run stoichiometric MOST of the time.  The main reason is that this is where the three way catalyst (HC, CO, and NOx) operates most efficiently to reduce all 3 major regulated pollutants.  And high catalyst efficiency is REQUIRED to meet todays emissions regulations.  Leaner catalyst operation would give rise to high NOx levels, and rich operation would give rise to high HC and CO levels.  As Franzh mentions, the air/fuel ratio actually dithers around stoichiometry some small amount.  The reason is that the catalyst is more efficient this way, and it is cost effective to control the fuel this way with a cheap switching exhaust oxygen sensor.

But, regulated emissions are tested on the federal test procedure (FTP75) cycle.  This cycle only covers relatively light loads and speeds.  Since other operating points (high speed and high load) are not tested, it is not required to run stoichiometric at these conditions.  Typically at high speed and high loads, the engine runs rich.  The reason... to cool the exhaust valves and catalyst and avoid high temperature damage.  The problem is this causes poor fuel economy in addition to high HC and CO emissions.  In response to catalyst life, it likes to run rich because of the cooler temperatures.  I don't know much about catalyst life running constantly rich, but occasionally is ok as it is done on most current production cars.

Typically todays cars don't run lean.  The exceptions:
1) Yes, some car companies run lean engines.  But not many are doing it because of technical issues.  Mainly the catalyst will not get rid of the NOx under these conditions.  If you go lean enough, the NOx will drop off as combustion temperatures drop, but getting stable combustion and engine operation becomes difficult.  This can be improved with high energy ignition systems and increased gas motion (swirl) in the cylinder (ie. Honda's VTEC, which is also used on stoic. engines today).  This increases cost though.  A similar method is...

2) Direct injection spark ignited engines.  You will not find one in the U.S., but you will in Europe and Japan.  These engines run overall lean mixtures (ie. catalyst sees a lean mixture) but locally the mixture near the spark plug is rich.  There are a lot of details that could be explained, but the overall idea is to run lean in order to improve fuel economy.  The inefficiency of lean catalysts is one big problem among many with DI engines.

3) one more note is diesels run lean by design as well.  One of the biggest issues with diesel engines is NOx emissions, for the very same reason that a good lean catalyst is not available.