“The unmetered side
of the fuel metering unit or the metered side? and why. And
lastly, where have you mounted the fuel pressure transducer? EI suggests
the firewall, but that leads to a long fuel line and ? does that make any
difference in the readings, dampening etc. Can you mount it closer or are
the vibrations going to cause failure of the fitting and fuel leaking over hot
cylinders?”
I have an MVP 50 engine monitor with fuel pressure
sensor mounted on the firewall and reading unmetered fuel pressure. It
does lead to a long line from throttle to firewall (and it must be fire
shielded), but does not seem to cause difficulties. I did have a sensor
go erratic at about 100 hours and replaced it with a new one. I suspect
vibration in the wore harness caused the failure, and the new installation has
better strain and vibration relief on the lines going to the pressure
sensor. The installation was exactly as outlined in the installation manual.
The sensor installation on the engine presumably causes problems due to
vibration on the engine which eventually affects the sensor which probably has
a tiny diaphragm and strain gage attached as part of a measurement bridge since
it is a four wire connection with ground and 5 volts on one pair of wires, and the
others sending the signal to the black box.
I plotted unmetered fuel pressure (before the metering
valve on the throttle body) vs. power setting and found that as you retard the
engine down toward idle (very low power settings as might occur on descent),
the fuel pressure jumps up as you described. I also got high pressure
alarms with higher RPM and lower power settings such as slowing on downwind to
get the gear down.
I believe what is happening is that the fuel metering valve
connected to the throttle shaft has some complex, precision passages machined
into it to match the fuel flow to the air flow as the throttle is closed.
Recall that the air flow past a butterfly valve (throttle valve) is highly
non-linear. The first little bit of opening causes large increases in air
flow (compared to idle position) and progressively moving the throttle to
fully open results in less and less additional air flow.
Thus to match this rapid change in air flow when opening off of
idle position the fuel valve has to have some fancy passages that open rapidly
as well as the shaft rotates. Conversely, as you approach the idle
position of the throttle, the fuel valve is going to start aggressively cutting
off the fuel flow.
I believe this is what we have seen and you are seeing. As
you approach the idle position of the throttle, but with higher RPM, the
positive displacement fuel pump starts generating higher unmetered fuel
pressures as the fuel valve closes. Momentary high pressure in flight
with throttle closure would be expected, it would seem to me, and that is
exactly what happens.
Why not measure metered fuel pressure instead? Beats
me. Seems like it would be more useful as it would correlate with flow rate.
However, I suspect that at lower fuel flows, the fuel pressure is essentially
zippo and the fuel is dribbling through the injectors and being atomized by the
high engine vacuum (low manifold pressure) that prevails when the idle is near
to idle. Engine specs for fuel pressure are also listed in terms
of unmetered fuel pressure.
Clear as mud?
Caveat: While I have studied the Continental fuel injection manuals
and procedures bulletins carefully, I am no expert.
Fred Moreno