Well, down at lower RPM the air is going to take longer to get throught the intake. Either way, there is at least enough volume of air in the intake to account for 1 intake cycle for each rotor. That is 2 consecutive misfires and definately noticable as an engine skip. After adjusting the MAP table, I don't get a staging bog, so much as a skip.
My engine runs about the same on either pair of injectors. At idle, switching from primaries to secondaries (ie. during run up) will cause a skip, but not switching from secondaries back to primaries. In cruise flight I get a little skip going either direction, I assume because there are a couple of cycles of over-rich running when heading down.
Al's method of putting the primaries at the end of the runners obviously works fine. I have tested that configuration. There are some downsides especially for my set-up. Starting the engine took an extra second or 2 of cranking. My throttle body is not very air tight around the axle, so using the secondaries creates some fuel leakage there whenever the pressure in the intake is higher than ambient (this is the main reason why I currently use the primaries almost exclusively - too much hassle to replace the TB). Even without forced air, the throttle body is a little inverted and a lot of fuel drips or otherwise goes backward out of the intake system even when running at high power. My turbo plumbing catches it, but for a while I was n.a. and it made quite a mess of things.
On Thu, Feb 19, 2009 at 1:01 PM, Al Gietzen <ALVentures@cox.net> wrote:
I hate to bring math into thisJ, but at the speed the air is going in the runner at the stage point, we're talking a few milliseconds even if the injector is 2 ft away. Not likely to be a noticeable contributor.
But speaking of swapping which set gets staged, that's what I do – I stage the primary (the ones in the ports). I found that the engine ran smoother at idle running on the secondary only – which on mine are about a foot from the port. It was an easy swap on the 20B version of the EC2 with the external staging relay.
However; there was a time when I also occasionally experienced a mild 'Bog'. It was when I had the stage point set at about 17", and occasionally crossed that point during high altitude cruise. For a long time couldn't figure out what it was (even posed the question here and didn't get an answer). Finally realized I was operating near the stage point, and that was it. Lowered the stage point, retuned; and now have no clue when it is crossing that point during operation.
Al G
Subject: [FlyRotary] Re: Staging Bog and Lean mixture
With the time lag at the staging point (as Tracy described), do you think it would help to switch the locations of the injectors and use the injectors that are at the front-end of runners as the primaries and the ones in the ports as the secondaries? I'm thinking that the secondaries would then have a little extra time to start flowing before the mixture provided by the primary injectors is cut in half. I'm guessing that some of the bog may be caused by the secondaries being located at the other end of the runners? Switching the injectors around may minimize the bog effect?
On Thu, Feb 19, 2009 at 1:43 PM, David Leonard <wdleonard@gmail.com> wrote:
For me (and probably many others), there is the additional factor that my secondaries are at the end of the runners while the primaries are in on the engine block. That means there is a slight delay in the getting the fuel from the secondaries. I do compensate a little by turning up the mixture a little on the first couple of bins above the staging point. Helps, but cant fix the problem all together.
On Thu, Feb 19, 2009 at 8:59 AM, Ed Anderson <eanderson@carolina.rr.com> wrote:
Ok, thanks for the explanation, Tracy. Makes sense to me.
So the fuel increase with the secondaries coming on line during staging
trails the shut-off of fuel on the primary injectors by some X amount. So
if I visualize it correctly I would see the fuel flow coming out of the
secondaries increase as a ramp function compared to the step function shut
off of the primaries. All happening in milliseconds or less of course.
Well, heck, all you have to do is calculate the cross over point where the
ramp function of the secondary meets the primary step function just as it
closes and then start the secondary injectors to opening before the
primaries are closed. How, you do that - given all the interacting
variables, I haven't any ideal - but sounds simple to me (in concept). I AM
kidding {:>) {:>)
I think I'll just stick with my current technique as it seems to do the job.
By the way, here is an interesting patient perhaps related to the topic
United States Patent 7475671
A method for compensating fuel injection timing when a torque signal or
throttle command indicates that a transient operation is about to transpire.
The control adjusts the fuel injection timing in advance of the transient
operation to prevent loss of torque due to change in combustion phase
position. During engine operation in pre-mix mode of combustion, rapid
changes in fueling quantity can affect the combustion angle as well as the
temperature in the combustion chamber, causing late or early combustion
phasing. The present invention anticipates a change in combustion phasing by
sensing a change in demanded fueling rate. The Engine Control Module (ECM)
then applies a nest of algorithms to advance fuel injection timing during
acceleration or to shorten ignition delay for deceleration by retarding fuel
injection timing. As the engine returns to steady state operation, the
compensation in injection timing is progressively reduced to zero.
http://www.freepatentsonline.com/7475671.html
The phrase I like best in the abstract is ...applies a nest of algorithms ..
(sounds like a snake's nest to me {:>)).
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Tracy Crook
Sent: Thursday, February 19, 2009 10:48 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Staging Bog and Lean mixture
Hi Ed, The method you used will work fine for eliminating the staging bog.
There have been enough comments on this subject so I might as well get into
some of the underlying causes.
I mentioned before that it was related to the difference in turn-on and
turn-off times of the injectors. The turn-on time is slower than the turn
off time, especially with the snubber mod which speeds up turn-off. So,
at staging point, the injector pulse width ( IPW ) is cut in half at the
same time as the controller enables the secondary injectors. If the t-on
and t-off times were equal, the injected fuel would be almost the same as
just before staging. BUT, the actual time the injectors are turned on is
less than 1/2 of what it was because a higher percentage of the IPW is used
up in the t-on injector delay , result is a lean condition. When the
throttle is advanced more, the IPW gets longer and so the percentage of IPW
"wasted" in the t-on delay is less and the mixture returns to normal.
If the staging point is made at a higher manifold pressure (and longer IPW),
the effect is not as great and that's why tuning out the "bog" is easier.
This describes the results with the same flow rate injectors in both primary
and secondary injectors. When different flow rates are used, the picture
gets more complicated. Mode 6 (staging flow rate differential) is even
more important in these cases. That's why I now ask for what injectors are
used so I can roughly pre-set Mode 6 to make tuning it easier.
Also a factor in how difficult the bog is to tune out is the setting of Mode
2 (Injection Dynamic Range) but that is a longer subject. Suffice it to
say that setting Mode 6 and 2 up prior to MAP tuning in Mode 1 will make
your life easier.
Hope this helps in understanding what is going on.
Tracy
On Tue, Feb 17, 2009 at 5:45 PM, Ed Anderson <eanderson@carolina.rr.com>
wrote:
Tracy, for what it's worth, I also see the leaning of the mixture at the
staging point which happens to also coincide with the staging "bog".
I never knew what was causing the bog and since I don't fly there nor
encounter if during flight, it's never really bothered other than wondering
about what was causing it.
Once I had a screen display to watch the bin pointer, the staging point and
the air/fuel mixture on the same screen, all at the same time, it became
very clear that leaning of the mixture (or at least that is the indication
of air/fuel ratio indicator) is happening.
As my bin pointer moves from the hump at the idle (low rpm) region, it jumps
from the low rpm map to the high power/manifold pressure chart above bin 64
- not staged yet. Still no bog, but as it moves a few bins higher the
air/fuel indicator dives from rich side to off the bottom of the scale lean
and the bog begins. At this point my staging sign goes from 2 injectors to 4
injectors indicating that the EC2 has signaled staging. It last for approx
3-5 bins (memory's a bit vague here) before the A/F indicator comes back up
the scale. So something is causing a lean condition. I always speculated
that it had something to do with the air/fuel mixture in the secondaries
making the transition from "dry" to wet - but couldn't quite convince
myself.
The only way I have been able to eliminate the bog is to increase the
richness of that region (3-4 bins) to approx 80-90 (out of 255) which is not
really a rich setting, but it works and has eliminated the bog. I enrich
each bin's bar watching the A/F indicator as soon as it stops going
completely off the lean end of the scale, I stop and that seems to work for
me.
Ed
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
http://www.andersonee.com
http://www.dmack.net/mazda/index.html
http://www.flyrotary.com/
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
<http://members.cox.net/rogersda/rotary/configs.htm>
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