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Thanks to several who added comments and questions to my partially-uninformed explanation of the Continental fuel system. I'll try to respond below:
Gary,
thanks for the speed-theta explanation.
Can you explain how the electric boost works and why we meet see a FF
drop
when using it at altitude?
...also would be interested in the "air-flow" theory of operation.
Colyn
I knew you were going to ask that, Colyn. My answer is pure speculation and someone might have a better answer: If I were working at Continental many years ago and my boss said "these things are going rich when the electric pump is turned on - fix it by tomorrow, but don't change anything." Hasn't every working engineer heared those words before? What I would have said "I know - I'll add a gizmo!" I might have added a valve that changed the fuel return orifice depending on the pressure drop (or rise) across the metering pump. Only problem is that any simple gizmo would probably overcompensate under some conditions and undercompensate under others. I'll bet that whatever they added to fix the original problem overcompensates at those conditions. What happens at idle? Still goes rich, I'll bet. Just a guess, I'm afraid.
Gary ...good description,I would like to correct 1 and add 1...TCM fuel pumps use 2 carbon type blades...opps are same as gears would be.TCM manifold valves have a spring loaded poppet valve in the center of the shaft that is set at several pres. and flows from idle to full power unlike the LYC.that opens more rapid and has a total flow well beyond engine needs they mainly keep a constant head pres.and aid in idle cutoff. For LYC. there is a precision SB for a stiffer spring for the flow divider that increases head pres. and helps rough idle hot day problems a little.
not nit picken just adding to your good work.
Gene Martin
Thanks, Gene. I suspected there were some differences in the flow divider, but didn't know what it was. In a Lycoming system the functional lesson is that if there is a fuel distribution problem at idle, look at the flow divider (the injectors have nothing to do with fuel distribution at idle). If there is a problem at full power look at the injectors(the flow divider has nothing to do with fuel distribution at full flow). Somewhere in my memory there is a vision of a gear-type metering pump, but I clearly remember incorrectly. As you say, the suck-and-squish analogy still holds true. The point is that when the fuel is already at a low pressure it has to get "sucked" around sharp corners into small spaces, further lowering the pressure and causing vapor generation.
...I offer the Toyota comparison simply as an example of good engineering.
I never said my 115 HP Toyota four banger was an ideal replacement for
a Continental IO-550 operating at -60F and FL260. What I said is that
it was better engineered to do the job it was designed to do, and operates
with very low operator workload.
...
willing to improve on the status quo. The fuel system of the IO-550 is
clearly an area that is ripe for improvement. I say this based not on
my personal experience, but rather based on the experiences of those
who have written about hi boost, lo boost, surging, vapor lock, and
the sudden silence of engine stoppage. One pilot also reports that
the engine won't stop when the mixture is pulled if the boost pump is on.
......This Safety Tsar would not approve an IO-550 for
use in my aircraft without a better fuel pressure regulation system
at the least. Better yet, a fuel metering system based on mass air flow
or speed-density.
PS - thanks to Gary Casey for explaining the function, "features",
and resulting variations in the Continental Speed-Theta injection metering
system. He thought the problem through, and then chose an injection system
with Airflow-based metering (Lycoming IO-540 ?) for his aircraft.
-bob mackey
I agree with your comments, Bob. Yes, it is a parallel-valve IO540 originally 250 hp, modified with one electronic ignition(which I don't particularly like), balanced intake ports, 10:1 compression and a -10 fuel servo. Dynos at 295 to 305 hp, depending on which run you want to believe. Is it "easy" to build an electronic system that wouldn't have all these problems discussed? Yes and no (BTDT). The basic operation is very simple and overcomes most of the difficulties of the systems we typically use. Problems come with altitude performance - there is more difference than you might thing when operating up to 20,000 ft compared to a car, which has to run perfect at 5,000, but only has to keep running at 14,000. The bigger problem is redundancy. The FAA doesn't require redundancy for mechanical systems (should it?), but does for electrical systems. Backup power supplies, redundant electronics, software fault tolerance, etc. At least 80% of the effort is in satisfying those requirements. One requirement that was a big hurdle was that the primary system had to be able to fail and the changeover to the backup has to be automatic (requiring no pilot action) and result in NO power interruption of more than 20%. We can ignore those requirements for an experimental application, right? Well, I wouldn't. That's one reason I didn't put my own electronic system on my aircraft, even though I knew it inside and out.
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