Searching for something else, I stumbled across old tech articles on
mogas/avgas/vapor-lock, and I think the following is very useful.
They cite research by the University of Michigan, FAA's Tech Research
Center, et. al, and appeared in a pub here, "Light Plane Maintenance,"
back in the days of auto fuel tests and the FAA approval process.
In general, while Reid vapor Pressure, vs. temperature/altitude, for
avgas/mogas is a big factor (7 psi vs. up to 14 psi), it only loosely
correlates with real world vapor lock. Also, RVP in mogas can vary
among fuel brands, but not in avgas. Real world vapor pressure for
either fuel is highly affected by other factors:
-- The presence of even small amounts of water significantly lowers
the temp at which vapor lock can occur, because the vapor pressures of
immiscible fluids are additive. The means any gascolator should be
heat-shielded if located near the engine, or better yet in a cooler
area of the airframe. Even with a gascolator in a cool location,
they say that provision should be made for draining water from the
fuel tank also, more so with mogas than avgas.
-- Agitation of the fuel lowers the temp at which vapor lock occurs,
with several implications:
a) A boost pump is best placed as near the tank as possible. Temps
are lower, and flow has a chance to smooth out (re below).
b) Once vapor in the fuel begins to cause engine roughness, it gets
worse.
c) Normal engine vibration causes agitation, worst at a resonant
frequency, suggesting flexible fuel line routing and means of security
are important. While provision for engine movement is necessary,
transmission of either low or high frequency vibrations directly to
the hose should be minimized.
d) An engine-driven diaphragm pump is the worst offender, aggravated
by heat. Tests showed that it always pumped a greater volume of 100LL
than mogas, whether the system was on the verge of vapor lock or not.
-- Vapor lock tendency was found to be highest in flow rates of 2.5 to
5.0 gal/hour, on a test setup using a Lyc. O-320 (c. 8 GPH),
presumably with 3/8" O.D. fuel lines. This may account for accident
cases where vapor lock was strongly suspected, and it occurred shortly
after a power reduction in the landing pattern. Since up to 5.0 GPH is
within the flow rate of engines appropriate for the Europa, this is
very interesting. Cited are friction effects, but if this means
hydrodynamics in general, then fuel line ID makes a difference, since
while flow rate doesn't change, velocity does, with a power in the
formula to so compute. However, 5/16" flexible hose has a similar ID
to 3/8" aluminum of typical wall thickness, and smaller/larger lines
are undesirable for other reasons. But a possible further advantage
of a fuel return line may be keeping flow rate above 5.0 GPH, at least
at higher power settings.
-- Similarly, 90-degree fuel line elbows, especially forward of the
firewall, are bad. 45-degree is more tolerable.
-- A series fuel system (engine-driven with upstream boost) is always
effective in curing vapor lock, provided the boost pump is itself
installed so as to be immune from vapor lock. Plumbing these pumps in
parallel may not.
-- Altitude has the most effect of all, with avgas immune at altitudes
appropriate for normally-aspirated engines. While lower outside temps
are encountered with altitude, engine room temps control. Though
lacking injection, perhaps the prime reason the 914 has no mech pump?
-- This all suggests an advantage to early warning of vapor lock. In
the O-320 tests, it was found that EGT rises sharply before the engine
roughness begins. At high flow rates, probe response time is a
limiting factor; but at 2.5 GPH, as much as a minute of warning is
available. A digital EGT is needed, due both to scaling issues on the
face of an analog instrument and the need for prompt triggering of a
warning device. Not discussed was sensing fuel temperature, but it
would seem that where to place the probe, and setting it's redline,
are subject too many variables. The largest are altitude and greatly
varying RVP in mogas.
-- The presence of aromatics and detergents in mogas were found to
have little effect on vapor pressure in the fuels tested. Similar as
to ethanol or alcohol, with some argument as to the latter, but either
of these fuels are undesirable for other reasons. Where "MTBE" as an
oxygenator has no effect re vapor lock, MTBE is being outlawed here
due to environmental effects. The effect of replacement oxygenators or
any future additive is always unknown, but automotive fuel injectors
can't tolerate much vapor in the fuel, for some solace?
-- A mixture of 100LL and unleaded mogas helps in the vapor lock
problem, but also the resulting small amount of lead can be beneficial
for the valves in Lycomings and the like. But for the Rotax??
Regards,
Fred F.
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