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<< Lancair Builders' Mail List >>
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>>>Both of you gentlemen seem to have some definite opinions regarding
TCM's FADEC System. Could either or both of you spell out your concerns
for the system? <<<
Jack,
Yes, I can spell out my concerns. But understand, my reluctance, because of
our work on PRISM, which is an electronic control system, but based on a
very very different approach. So take my opinions in that light. Remember,
these are opinions based on TCM's own statements, and I could have
misunderstood what they were saying and things may get better in the final
form.
TCM has had some good people working on that system. They have done this
under difficult circumstances with the FAA, because the FAA is only now
getting their policy together with respect to electronic piston engine
controls.
However, I think their approach is fundamentally flawed in its overall
design philosophy and functionality.
1) TCM's "single lever" engine control system (FADEC is a generic term for
"Full Authority Digital Electronic Control" of anything... engine, flight
controls, etc, but most often applied to engine controls) is, so far, a two
lever "single lever" control. I.e., it still has the prop control. I
gather that an electronic governor is in the works for the future.
2) The system's wiring harness is, shall we say, "...awe inspiring."
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.
f) In 1997, I compared notes with some of the early research on the
FADEC. We were able to match the cylinder to cylinder fuel/air ratios
better than the FADEC, by simply using the GAMIjector(R) fuel injectors.
4) Operating envelope. The TCM "Full Authority" system actually has the
precise effect of PROHIBITING operation of your engine at high power ( >2300
RPM) at any mixture setting below about 75d F Rich of Peak. Thus, those
people that now operate at 2400 RPM and ANY MP (low or high MP) and 75F
LOP will NOT be able to do that with FADEC.
5) ONLY if you manually reduce the RPM to below 2300 RPM will the system
then "understand" that you want "best economy" and then lean the mixture to
some lean mixture.
6) Thus, as designed, you give up the most useful and the most efficient
part of the engine operating envelope, if you install TCM's system.
7) It has no capability to operate on unleaded fuels.
8) If you are flying from New Orleans to Key West, at 2299 RPM, with the TCM
control system leaned to best economy, and you have a CHT or EGT probe fail,
the system will force the cylinder with the failed CHT or EGT probe to go to
full rich mixture.
9) The result of that is that the engine will run very rough. If you push
the RPM to 2301, then you will go back to rich mixtures on all cylinders and
the engine will run smoother. But, then you may get to swim in the Gulf of
Mexico from fuel starvation, due to the unexpectedly high fuel burn.
10) Count the sensors. 6 EGT. 6 CHT. 2 fuel pressure. 2 induction air
tem. That is just the beginning, as you have crank position sensors in
addition, although the crank position sensors should be very reliable. In
any event, you have to have them all working in order to legally "dispatch"
the airplane. I see a failed EGT or CHT probe on at least every 4th
airplane that comes into our shop.
11) VERY long high voltage ignition harness.
12) I have not seen a weight on the system, but I suspect it will weigh
more than the magnetos it replaces.
I am doubtful that there will ever be any improvement in average fuel
consumption with that system. That system does not allow any engine to
operate at high power with lean of peak mixtures.
It uses something like 440F as a CHT trigger point for enrichment of the
fuel when a CHT gets too hot. THAT gets my attention. I *THINK* that
means in many airplanes in every normal climb, the CHTs will be hovering in
the 430 to 440F range.
At any rate, these are my impressions, so far.
Regards, George
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