1) The turbo injectors do not have
screens like the NA injectors do. They have air inlets from an upper deck
reference line. Scott is using those.
Do the turbo injectors have better air
passages that allow them to do a better atomization job than the normally
aspirated injectors, even when installed in a normally aspirated engine?
Said another way, for the same delta P do the screens on the NA injectors
restrict air flow compared to the turbo injectors with their air shroud and
supply line?
OK, let’s see what might be possible
with a normally aspirated engine. The ram pressure at 200 KIAS is about 2
inches Hg, give or take. Let’s assume you get 100% of this for the
fuel injector air flow using a nice pitot tube set up like Scott’s.
And let’s assume that the engine induction system gets about 75% of this
for a little manifold pressure boost at WOT, this being a typical figure for a
good diffuser taking in induction air. Then the difference of 0.5 inch Hg
is available for atomization. This is clearly better than zero, but a lot
less than 3” Hg. So might one make further improvements in
atomization by somehow boosting the injector air flow? (The mind boggles….)
Or is 0.5 inches Hg enough at WOT cruise?
3) Mark wrote: I cobbled together a similar arrangement to supply ram air to the
injector shrouds. I can't say I see a bunch of speed improvement as I'm not one for
careful analytical testing. I can say - the improvement in engine
smoothness when running LOP in high altitude cruise is DRAMATIC.
Please tell us more about your set up,
Mark. Is your engine induction air coming from a separate scoop than the
cooling air? Does your cooling air come through small inlet ports in the
cowl? Which engine?
Here is why I ask. If you have a
good induction air inlet that is getting a high percentage of the ram
air pressure, and smaller cooling air inlet ports that do a lesser job
at recovering the ram pressure, then it is entirely possible that your manifold
pressure is higher than the cooling air pressure surrounding the normally aspirated
injectors.
This “reverse pressure delta P”
would reverse the injector air flow, and may cause the blue staining that was
mentioned around the NA injectors. It would also cause poor atomization
that becomes most evident operating LOP.
But if you now add a pitot set up like
Scott’s, you get more air pressure fed to the injectors, and you restore
the injector air flow to the way it should be – air flowing into the
intake manifold instead of out of it. Atomization improves and your
ability to lean further improves as a result. Do you think that may have been
happening on your plane?
The reason I persist on this topic is that
for those of us using IO-550’s in Legacy or ES (or rare birds like me, in
an LIV) where induction air is coming from inside the cowl, there may be
a lot to learn that could improve our cruise efficiency, particularly worthwhile
with 5 buck avgas.
I see another test program looming on the
horizon…..
Still Curious Fred