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I'm not sure that I could scan the
diagrams successfully so I'll try my best to describe them.
Looking at the fuel system schematic
(2-15 of the TSIO-550 Maintenance & Overhaul Manual), fuel from the tank
goes through the boost pump, then the vapor separator, then into the engine
driven pump. Boost pump flow can bypass the engine driven via a check valve. The
engine-driven pump output is regulated by engine speed; also by an idle
pressure relief valve, an aneroid full power valve, and by the mixture
control. The last three items work by recirculating fuel to the inlet side of
the pump. The fuel return to the tank is located between the pump output
and the mixture control. Fuel from the boost pump is also shown as flowing
through the pump outlet, before the fuel return. The mixture control is the last
point in the system to remove excess fuel. After that it goes through the
throttle body where it is metered, then on to the spider.
I also have a small TCM manual that
describes the injection system. It states that the pump has excess capacity so
that pressure can be maintained at all speeds through the use of a calibrated
orifice and relief valve in a recirculation loop. I assume that's for
non-aneroid pumps.
The unmetered fuel pressure (between the
pump and the throttle body) for the E is 7.0-9.0 PSI at idle and goes to
32-36 full throttle. Full throttle flow is 245-255 lbs/hr (roughly 41-42
GPH).
I'm looking at a Dukes pump that's rated
11.0 PSI low and 25 PSI on high. Flow rate is 35 GPM @ 21.0 PSI. I don't
know how this compares to the pumps being used in Lancair installations, but
wouldn't seem that the Dukes could overload the system, especially if the
engine-driven pump was inoperative. If you were idling and hit the high boost,
yes. At full TO power it should be less than the engine-driven output. My
question is: could the output of the two pumps combine and overwhelm the return
line causing pressure and thus fuel flow to go out of limits?
It seems to me that the biggest concern is at
takeoff. At altitude there would be time to restart. The installations on
my planes (TCM IO-360, IO-550B) use a ceramic resistor for the low
side. Would there be a way to vary resistance to the high side to keep
output of the boost pump within tolerance even if the high setting was selected
by accident?
BTW, I don't use low boost on takeoff. Some
manuals recommend it, but to me it's one more thing to remember and I've
operated that way for 1,000 hours with no problem. The only time I had a problem
was when I forgot to turn the boost off after startup! Rough idle, low
power and a lot of smoke. -Bill Wade
----- Original Message -----
Sent: Thursday, June 14, 2007 10:50
AM
Subject: [LML] Re: Boost Pump
Is it safe to assume
then that the fuel return line isn’t able to return all the fuel that isn’t
being used? Maybe there are restrictions in the return line that
prevents fuel from retuning at a flow rate that would prevent it from also
bypassing the mechanical pump?
Or does the return
line start after the bypass value for the mechanical
pump?
There is a variable orifice as
well as the bypass valve involved in the fuel return system that will affect
fuel rate beyond the return line size. To get fuel injection to work
somewhat automatically with a pure mechanical system is very difficult.
Adding a turbo further complicates the effort. Having a mixture knob
controlled by the pilot is a big deal. A good deal if properly
controlled. My advise is to be very careful when trying to modify the
way the mechanical fuel injection system meters fuel. VERY simply put,
the pressure at the injector controls the fuel rate. Increasing the
inlet fuel pressure at the mechanical engine driven pump will increase the
fuel rate. Let me give you some food for thought. The increased
fuel rate caused by turning on the high boost pump may be acceptable to the
engine during the winter months in Minnesota but will the same fuel rate
increase be acceptable to the engine today in Arizona (110 def F). In my
opinion high boost on the TSIO550 should be used for aid in starting and as a
back-up in case of a mechanical engine driven fuel pump failure
only.
Craig
Berland
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