engines / fuel

[Ralph Baker]

There are other allied concerns with mogas than just the lead. Remember that mogas is formulated both by region and season and its distribution is not tracked as is 100LL. For one, the mogas vapor pressure is higher and increasingly so in winter formulations. The STCs all apply to certified aircraft with consistent construction (at least we hope so) and before an STC is issued an extensive flight test program is done to ensure the mogas operates in that specific aircraft safely. An example wmight be that fuel lines do not boil and vapor lock due to exhaust proximity.

On another subject Ben Visser (ex Shell ) noted that mogas and 100LL can be mixed in any proportion and that the octane rating change is not linear. In other words that initial addition of 100LL gives more octane number boost than later additions. An example of diminishing returns. There is no formula to determine what percentage of added 100LL gives what percentage increase. It's related to the many combinations of petroleum molecules and additives that can be used to make fuel. As I have stated before, Visser said some time back that a conventional Lyc or Continental run on 100LL for the first 300 hours after rebuild would make TBO on auto fuel if operated correctly on adequate octane for that engine.
Ralph Baker





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[eric.r]

Yep, the STC's are for the airframe with a particular engine. Fuel flow tests
on some airframes were inadequate (They wouldn't pass on avgas either, but
common sense never prevails!) I once new a guy who new a guy who new
about somebody with an IO-470 Bonanza that ran auto fuel in cruise, using
100LL only for take-off and landing. Cooler CHT, clean plugs, no problems
whatsoever with bladders, vapor lock etc.
5/11/2006 7:28 PM, rebaker@sc.rr.com wrote to rebel-builders:

-> There are other allied concerns with mogas than just the lead. Remember
that mogas is formulated both by region and season and its distribution is not
tracked as is 100LL. For one, the mogas vapor pressure is higher and
increasingly so in winter formulations. The STCs all apply to certified aircraft
with consistent construction (at least we hope so) and before an STC is
issued an extensive flight test program is done to ensure the mogas operates
in that specific aircraft safely. An example wmight be that fuel lines do not
boil and vapor lock due to exhaust proximity.
->
-> On another subject Ben Visser (ex Shell ) noted that mogas and 100LL can
be mixed in any proportion and that the octane rating change is not linear.
In other words that initial addition of 100LL gives more octane number boost
than later additions. An example of diminishing returns. There is no formula
to determine what percentage of added 100LL gives what percentage
increase. It's related to the many combinations of petroleum molecules and
additives that can be used to make fuel. As I have stated before, Visser said
some time back that a conventional Lyc or Continental run on 100LL for the
first 300 hours after rebuild would make TBO on auto fuel if operated
correctly on adequate octane for that engine.
-> Ralph Baker
->
->





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