Martin Gregorie wrote:
> Indeed, and NOx seems to be the prime concern of those interested in
> cleaning up urban air

Exactly, but this problem only came up in the middle of the seventies
and is was the *result* of trying to clean up the exhaust. The problem
of mixture control is, it's not symmetric. Starting from a rich mixture
the efficiency (and thus the power at constant petrol supply) rises to a
maximum and then sudenly breaks off. The optimum is not somewehre an the
middly of the usable range but very near the lean limit and very far
from the rich one. So for good running and reliablility engines tended
to be set in the rich range to accomodate production spread and lack of
close control. NOx was not a problem then, hydrocarbons were (and in the
cities short lived but highly concentrated carbon monoxide). The first
clean air acts made engines much cleaner, more efficient and to higher
manufaturing standrds, but the new setting tended to coincide right with
the maximum of nitrous oxide production.

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