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<< Lancair Builders' Mail List >>
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>>3) The fuel rail-mixture system is a direct imitation from the automotive
world.
a) Unlike a car, an aircraft spends about 98% of its time at some
RPM at or above 2300 RPM.
b) A fast acting solenoid valve pulses ON/OFF the fuel to each
injector.
c) At low RPM, this pulsed action is "sequential", that is, it is
"timed" to begin pulsing somewhat before the corresponding intake valve
opens and to continue pulsing until sometime before the intake valve closes.
d) Typically, in all of these solenoid fuel rail systems, at higher
RPM, i.e., 2300, the solenoid is not fast enough to do this effectively,
and the system forgets about the "sequential" concept, and just uses the
pulsing valve to control the total amount of fuel, becoming a very complex
imitation of the existing system.... i.e., a continuous flow port injection
system, that we are all already flying with.
e) The consequences of c & d are that the only time this very
complex system does you much good with respect to "smoothness" of operation,
is when the engine is starting and at taxi speeds.<<
I agree, George, with most of the comments above. However, imitating
Automotive technology isn't necessarily bad - it just might not apply. The
sequential injection scheme primarily improves emissions and air/fuel
distribution at light loads and low speeds as you have suggested. At higher
loads the pulse width gets longer and at higher speeds the open time of the
injector starts to overlap the valve event (injecting into an open valve is
"bad" in that a greater portion of the fuel will not have time to evaporate
and mix with the charge). In fact, most systems are designed to run at
about 80% open time at full speed/load. Therefore you can expect the
operation to be essentially the same as for a continuous flow system, just
as George said. The injectors don't "pulse" during the opening, however -
they are opened once per engine cycle and remain open for the length of time
necessary to deliver the desired quantity of fuel. This is an exact copy of
an automotive fuel system, and it works quite well. As to the 2300 rpm
"decision point" that is a function of the calibration, not the concept of
the system. A FADEC takes the engine management away from the operator and
puts it into the controller - you don't have a choice about LOP, ROP and all
the other things we talk about all the time. How well it works depends on
the people that decide for you how the engine will be set up. They could do
it right and everyone will be happy or they could make a compromise some of
us won't be happy with. I can't comment on the decisions they made - George
might have better insight into the calibration decisions they made.
Gary Casey
ES project
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