Listers---Below is the reply of Dennis Simanaitis, tech guru of Road &
Track Magazine. I wrote him looking for clarification on ignition timing
and its effect on tail pipe emissions. There has been some debate on
this over the years. I'm not sure it's any clearer after this response!
Anyway, here it is. Dick Taylor
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From: EngEd@aol.com
Message-ID: <35.180d9042.2889c56b@aol.com>
Date: Fri, 20 Jul 2001 13:33:31 EDT
Subject: Your R&T Query
To: tr6taylor@webtv.net
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Hello, Dick,
From what I've read and heard, timing does have an effect on all three
pollutants. Generally, though, emissions are more sensitive to fuel/air
mixture.
At the ideal stoichiometric mixture (i.e., the chemically correct mix of
fuel and air for complete combustion), most sources say the temperature of
combustion is highest. This implies that NOx would be highest too. Some say
that slightly lean of stoichiometric is actually a bit hotter and hence a bit
higher in NOx.
All this is of no inspectional interest because most states don't measure
NOx anyway.
HC and CO tend to increase with mixture richness and decrease with
leanness--but only until any lean misfire occurs (which makes both soar).
Thus, if only HC and CO are being measured, a lean (but not too lean)
mixture is best.
Now, on to timing and its (less profound) effects: Retarded timing tends
to raise combustion temperature. Hence it promotes NOx generation. It
increases fuel consumption as well, and this could adversely affect HC and
CO.
Advancing the timing raises NOx as well. Plus, it may cause knock, which
is uncontrolled combustion in the wrong place at the wrong time. Knock raises
combustion temperature, and hence raises NOx as well. Plus, as knock is
uncontrolled combustion, its HC and CO aren't optimal either.
Hence, in an HC/CO-measuring test, it would seem that slight retardation
might be ok; but advancing this same degree might hurt.
The Bosch Automotive Handbook says "Because various -- and in this context
sometimes mutually antagonistic -- factors such as fuel economy,
driveability, etc., are also potential optimization criteria, it is not
always possible to specify the ideal ignition timing for minimum emissions."
And if The Bosch Automotive Handbook doesn't know, I sure don't.
Thanks sincerely for your query. -- Dennis Simanaitis, engineering editor,
R&T
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