> Carbon ( actually lead build up ) isn't the issue as in days
> past, unleaded
> fuel leaves no lead deposits and a cleaner combustion chamber.
Actually, even unleaded fuel leaves some deposits, due to the various
additives. Unfortunately, unlike lead and carbon, these deposits are much
harder and hence more permanent. This results in "octane creep", where
engines require higher octane gasoline as they age.
OTOH, I just found a study where a research group took 28 cars and drove
them 15,000 miles on a test track while comparing combustion chamber
deposits, and monitoring their effect on emissions. 3 different fuels were
used, to provide low, medium and high rates of deposit accumulation. What
they found was interesting, higher CCD resulted in higher NOx (no surprise
there, NOx production is related to compression ratio), and better fuel
mileage ! Other emissions were not affected.
Randall
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