jas56d@msn.com
Here is a question that comes after thinking quite a bit.
The 914 engine has two electric fuel pumps that feed fuel to the engine. The
fuel pressure is supposed to be regulated by a fuel regulator on the top of
the airbox. It maintains the fuel pressure at about 3.5 psi above the airbox
pressure. The normal range is from 2.0 to 5.0 psi delta above airbox
pressure. Should the pressure go over 5.0 delta, there is risk of flooding
the carbs.
I have been trying to figure a way to measure that parameter and have not
found a cheap way, since a delta pressure sensor to do that job is very
expensive, I think.
I had an idea that I acted on and that was to record fuel pressure vs
manifold pressure, to see if the relationship would be similar to the airbox
relationship. It turned out to be linear and I set up my monitor to compare
the fuel pressure reading to manifold pressure with a derived simple math
function. I ran the engine today, and it basically worked OK, staying within
my prescribed limits based on reason.
It worked fine, except for the fact that the fuel pressure reading jumped 2.2
psi with the second fuel pump turned on. This has always bothered me that the
fuel pressure is supposed to be regulated, yet it significantly increases
with the second fuel pump activated along with the first. I am measuring raw
fuel pressure in the line coming from the pumps, going to the engine.
The Rotax manual has the two pumps in series, yet the Europa manual calls for
them to be in parallel. Mine are as the Europa manual calls. I am now
wondering if the airplane should be operated with one pump running, rather
than 2. I have a system set up to automatically activate the second pump,
should the first one fail.
An additional factoid is that my engine spits small amounts of soot onto the
leading edge of the wing and I am wondering if I am getting that from running
the engine on the virge of flooding with 2 pumps turned on for aake off and
landing. I would love to talk to a Rotax engineer, but I find them hard to
come by. Anybody have any ideas on this? Should the fuel pressure be measured
in the line from the regulator to the carb? If I institute the policy of
taking off with one pump, that would solve this puzzle, but I am not sure
what is correct.
Thanks,
Dave
A227
Mini u2
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