Slightly increasing the
mixture is not
enough to get it back as by then the RPM s have really dropped.
It usually
took a
significant increase in mixture to start the engine again. I'm
even
starting to
get used to the sensation of my engine stopping (watch out Ed,
I'm gaining on you in glider
time).
I
think, Dave, you are once again pointing out the risks and difficulty in
trying to maintain boost control through throttle
control alone.
Here is what I think what may be happening - a rapid feed back
situation. Without the boost gate and using manual throttle control
you have no "reserve" of exhaust mass flow. In other words, with
a waste gate you could be dumping say 10% of your exhaust mass through the
gate. So a slight variation in exhaust mass flow caused by
varying the aircraft attitude (and rpm) could be handled by the
wastegate system without your knowledge. As the massflow started to
slow, it simply stopped dumping as much (or any) through the gate.
This kept your boost stable and therefore the air/fuel mixture control could
be leaned (you would loose some mass flow - but hey! you were dumping 10% of
it in any case).
Now you
disable the waste gate and go to manual control of boost by the
throttle. Here YOU must adjust the throttle and mixture control
precisely and exactly to maintain X level of boost. There is no
exhaust mass flow reserve like you had with the waste gate. You are
exactly at the state where throttle and mixture produce X psi boost.
Now, the attitude of the aircraft changes causing the engine rpm
to drop slightly, this results in less mass flow - but now you have no
reserve mass flow. Mass flow drops, turbine wheel slows, boost drops,
manifold pressure decreases all of this results in even less mass flow which
causes more boost drop, etc.,etc. A feed back situation that can
quickly cascade into power decay rapidly when using manual throttle
control.
I am not
certain why the fuel mixture gets out of whack. I would think if you
waited long enough (30 sec?) the system would stabilize and the E2 catch up
with the current state and everything would start running again. I
suspect that under boost the EC2 is of course providing gobs of fuel to go
along with the high air mass flow (represented by high RPM and BOOST).
Suddenly it finds the the boost all gone or greatly decreased, in fact
without the boost the manifold pressure may plummet very sharply. The
EC2 may simply in effect shut off the fuel flow due to this rapid decrease
(same effect as if you had closed the throttle quickly).
Tracy, of
course, would be the one to ask. This is just my 0.02
worth
Ed
Anderson
David Leonard
The Rotary Roster
http://members.aol.com\_ht_a\rotaryroster\index.html
<< File:
ATT1916502.txt >>