why is rich better for power ??

[James Boughton]

Tom,
	There are a couple of reasons why richer is better if
it is power output you are interested in.  Let me start with the
torque output.  According to Heywood (everyone familiar with him?),
the indicated mean effective pressure (imep) of an engine is 
proportional to the fuel conversion efficiency multiplied by the
equivalence ratio.  Since fuel conversion efficiency falls off
with increasing equivalence ratio you end up with a peak between
the equivalence ratios of 1.0 to 1.1.  If you are unfamiliar with
equivalence ratio it is the ratio of fuel/air to stoichiometric fuel/air,
thus an equivalence ration of 1.1 is 10% rich.  The imep turns out
to be proportional to torque output (approx.)  and since at any
rpm an increase in torque is a subsequent increase in power
you end up with more power on the rich side of stoichiometric.
	Please note that combustion efficiency is not an argument
of the above discussion.  However, if we think a little about the
combustion process we will also find some improvements in power
due to statistics.  Think about 50 men and 50 women blindfolded in a
room together.  Now if we tell them to all find a partner of the opposite
sex and give them only a minute to do it you will find a few people left
without partners (How do you like this game so far?).  So lets say, for
arguments sake that 4 people were left over.  This would be an efficiency
of 96%.  This is similar to what goes on in the combustion chamber as
fuel molecules are wandering around (blindfolded) looking for oxygen
molecules.
	So we ask ourselves, how can we make more couples in the
same amount of time?  We could add more males and females.   This
would be the equivalent of turbocharging.  But in our case we are going
to be stuck with the same number of females (50) regardless:-(  So let's
try increasing the number of males (more fuel!)  Now if we give everyone
a minute we are more likely to find that the females are all used up.  This
is the statistics portion of the power improvement.  It also explains why
even at stoichiometric air-fuel ratios we have oxygen left over in the exhaust,
albeit very little.
	Here is another cool benefit.  By adding more males we also
get more females!  This would be due to the charge cooling effect of
fuel as seen in the case of methanol fueled CART cars.
	I hope this helps answer your question.

Jim Boughton
boughton at bignet.net

----------
From: 	Tom Cloud[SMTP:cloud at peaches.ph.utexas.edu]
Sent: 	Friday, September 05, 1997 10:57 AM
To: 	diy_efi at coulomb.eng.ohio-state.edu
Subject: 	why is rich better for power ??

can anyone out there tell me why stoich doesn't make the
best power ??

Tom Cloud

       it IS as bad as you think and they ARE out to get you !!