Re: Multi-Species Saha Equation
- From: af250@xxxxxxxxxxxxxxxxxxx (John Park)
- Date: 25 Jun 2007 05:03:19 GMT
John Schutkeker (jschutkeker@xxxxxxxxxxxxxxxxxxxx) writes:
"N:dlzc D:aol T:com \(dlzc\)" <dlzc@xxxxxxx> wrote in news:gEkfi.390219[...]
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Dear John Schutkeker:
"John Schutkeker" <jschutkeker@xxxxxxxxxxxxxxxxxxxx> wrote in
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"N:dlzc D:aol T:com \(dlzc\)" <dlzc@xxxxxxx> wrote in
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Dear Agent Smith:
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...
And why should a lean mixture burn hotter than
a stoichiometric one, anyhow? Shouldn't
stoichiometry maximized thermal energy output.
I don't know where that is coming from.
Common sense. Complete reaction of all species liberates all the
energy. Anything non-stiochiometric is pushing the reaction uphill
again.
Except that what happens in an IC engine's cylinder isn't one reaction.
It's a horribly messy series of parallel reactions, some competing with others
and all going at different rates. Just for starters, both the fuel (a
mixture of hydrocarbons) and the nitrogen in the air can react with oxygen.
You have a mixture of nitrogen oxides, some excited oxygen species, both the
major oxides of carbon, hydroxyl radicals and a whole raft of radicals
drived from the hydrocarbons, all interacting. And the individual reaction
rates and equilibria are also functions of the temperature, which depends
on the amount of energy liberated at any instant . . . .
There's no guarantee that every reaction will go to completion in the time
available. (Traces of benzene can be formed in hydrocarbon combustion and
survive to be emitted in the exhaust; the oxidation of CO is relatively slow
and quite temperature dependent, &c &c.)
(2) Since both nitrogen and hydrocarbons are competing for the oxygen, one
*might* (subject to the uncertainties outlined above) expect that a rich
mixture will produce energy most quickly and so lead to the highest
temperatures.
Empirically, the stoichiometric mixture is close to the composition that
produces the most oxides of nitrogen. And the maximum engine power does
come from a rich mixture, but maximum fuel economy comes from a lean.
It's complicated.
--John Park
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