Subject: [FlyRotary] Re: [FlyRotary]
Dave,
It sounds like you have a lean surge region. Engine will drop down in rpm
to where the fuel Map provides ample fuel, the engine produces more power
and starts to accelerate and at some higher rpm enters a region of
insufficient fuel delivery where the engine can not maintain that rpm and
starts to decrease. As the engine drops in rpm it again encounters a region
where sufficient fuel exists and starts to accelerate again, repeats, etc.
OK, this I understand
Here is what I believe is happening. In my case, When the engine is both
low in rpm and low in manifold pressure (15" or below) the idle is fine.
However, if I lower the engine rpm even more the manifold pressure actually
starts to increase (may go over 17").
This I don’t understand.
Tracy's EC2 actually computes the
fuel injector pulse duration based on manifold pressure sensed (RPM simply
determines the rate that the injectors are triggered). So as the engine
rpm is lowered (it should require less fuel) But, past a certain point
lowering the rpm causes the manifold pressure to start to increase (engine
speed is simply not sufficient to maintain the lower manifold pressure)
Normally one would decrease rpm by further lowering the MAP by further closing of the throttle plate. Do you mean you reduce the rpm by some means other than decreasing manifold pressure? Like changing the mixture?
when
that happens the EC2 senses that INCREASE in manifold pressure and treats
the increase as a demand for more fuel when in reality the new lower rpm
requires proportionately less fuel. This gives you an overly rich condition
so you reduce the air/fuel ratio with you mixture knob. When you do that
you reduce the mixture sufficiently from the overly rich side to cause the
engine to increase in rpm this may cause the manifold pressure to actually
decrease (goes say from 17" to 15" as the rpm picks up a bit). Manifold
pressure decrease causes less fuel to be injected causing a lean region and
engine dies back in rpm, etc..
After sufficient leaning, I was able to get my engine to run OK on the dyno at low rpm with the 550cc/min injectors (primary in the housing); with fixed 40psi fuel pressure. The issue I couldn’t resolve was the ‘overshoot’ – decelerating from a higher rpm and MAP going to a level below where I could make an adjustment – resulting in the engine dying. The EM-2 is going to allow me to resolve that; right?
Al
Can't say for sure but my guess is that this situation will probably never occur, given the load characteristics on an operational airplane. Or it might. When you suddenly decelerate, the manifold pressure and required injector pulse width may go below the point where those big injectors can accurately meter the proper fuel batch. If you lower the fuel pressure and the problem gets better or goes away, that is a good indication that the injectors are operating below their minimum pulse width.
Tracy