what _IS_ a good cruise A:F ??

[Gregory R. Travis]

On Tue, 25 Mar 1997, Fred Miranda wrote:

> At light load cruise, you will probably run into lean misfire before detonation.
> 
> I run one of my cars with 0-.02v from the O2 sensor(heated) at cruise, no
> problem.
> 15-15.5/1
> 
> And once ran it at 16-17/1 with no apparent problems. (my UEGO box was
> overloaded)
> 
> Fred
> 
> At 09:45 AM 3/25/97 -0600, you wrote:
> >>In a message dated 97-03-25 09:16:40 EST, you write:
> >>
> >>> question .... what _IS_ a good cruise A:F ??  I recently lost
> >>
> >>All the documentation I've seen on "family production cars" generally
> >>indicate a 14:1 ration between air/fuel.
> >
> >okay, I guess I asked for it .... obviously stoich (14.7:1) is
> >"best".  Let me re-phrase the question.  What is the leanest
> >that is recommended to run and still avoid detonation ??
> >
> >Tom Cloud <cloud at peaches.ph.utexas.edu>

Just some anecdotal gas to throw on the fire.

In normally-aspirated light aircraft piston engines, it's extremely
common to run the mixture right at stochiometric* in cruise.  This gives
very good fuel specifics and helps keep the combustion chambers clean.

This is generally done at or below 75% (65% for some engines) of the absolute
power ratings for the engines (i.e. at MUCH higher power ratings than
the average automobile will see in any steady-state cruise) and it's done
for hours on end with no damage to the engines.

In fact, with refined injector manufacturing and balanced inductions, it's
now becoming fashionable to run the same engines LEAN of stochiometric for
even better fuel specifics.  One of the major manufacturers actually specifies
that a specific model of their engines be run lean of stochiometric during
cruise.  This keeps cylinder head temperatures down while giving good fuel
economy.

I guess what I'm saying is that I find it hard to believe that you can
damage an auto engine, running at 15-30% of its rated power in cruise, by
running it lean of stoch. where the same operation doesn't hurt an ancient
air-cooled aircraft engine running at 65-75% of its rated power.

At least not by detonation.  I do certainly remember tales of warped and
burned exhaust valves in Volvos and Volkswagens that were attributed to
running with a very lean mixture (which continued to burn as the ex. valve
opened).  I'm not sure what the mechanism is that makes aircraft engines
immune to that.

greg

* p.s.  I prefer "Equivalence Ratio" to Stochiometric since the values
expressed in the latter can actually change as a function of fuel type,
grade, environmental conditions, etc.

** p.s.  Measurement of mixture, in aircraft engines, is done relative to
         exhaust gas temperature - not with an oxygen sensor.  Exhaust
         gas temperature peaks at an equivalence ratio of 1 ("stochiometric")
         and drops on either side (lean or rich) of that mixture setting.


greg		greg at indiana.edu	http://gtravis.ucs.indiana.edu/