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  #1  
Old 01-15-2021, 05:18 PM
Greg Arehart's Avatar
Greg Arehart Greg Arehart is offline
 
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Location: Delta, CO/Atlin, BC
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Default Basic fuel injection question

I've been changing out my injector nozzles (this is a Silverhawk system on a Superior IO360 if that makes any difference) in an attempt to better balance my engine for LOP operations. At this point I have 4 different nozzles to get the EGTs to go over peak at the same time/fuel flow: 0.028 (cyl 1); 0.027 (cyl 2); 0.026 (cyl 4) and 0.024 (cyl 3). Comparison of the area of the injector nozzles by simple math says that the orifice on cyl 1 is ~25% larger than that on cyl 3.

So my question is basically what is controlling the absolute amount of power in each cylinder? As I see it, there are two possibilities:

1) the air volumes going to each cylinder are different, and therefore one has to compensate for lesser air in some cylinders by reducing the orifice size. If this is the case, then I would expect to get less power from cyl 3 than cyl 1 (because less overall fuel is burned). This might ?? result in a rougher-running/misbalanced engine. In summary, both less air and less fuel in some cylinders, but still all cylinders are stoichiometric at peak EGT.

2) the air volumes going to each cylinder are (approximately) the same but the fuel pressure/flow through the smaller nozzle on cyl 3 is higher than the larger nozzle on cyl 1. This would result in approximately the same amount of fuel in each cylinder, and the same amount of power coming from each cylinder. In summary, nearly the same amount of air and fuel in all cylinders, and all cylinders are stoichiometric at peak EGT.

I'm hoping that option 2 is what really happens, but I don't know and haven't been able to sleuth out the answer on the internet.
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  #2  
Old 01-15-2021, 05:38 PM
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airguy airguy is offline
 
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Option #1 is what is really going on. Almost all engines, unless they been ported/polished and flow matched on a bench, will have differences in the amount of air they inhale and exhale, requiring different orifice sizes to exactly match the mixtures.

I haven't seen an imbalance as large as the one you are describing, though.
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  #3  
Old 01-15-2021, 05:48 PM
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DanH DanH is offline
 
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Location: 08A
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Quote:
Originally Posted by Greg Arehart View Post
I've been changing out my injector nozzles (this is a Silverhawk system on a Superior IO360 if that makes any difference) in an attempt to better balance my engine for LOP operations. At this point I have 4 different nozzles to get the EGTs to go over peak at the same time/fuel flow: 0.028 (cyl 1); 0.027 (cyl 2); 0.026 (cyl 4) and 0.024 (cyl 3). Comparison of the area of the injector nozzles by simple math says that the orifice on cyl 1 is ~25% larger than that on cyl 3.

So my question is basically what is controlling the absolute amount of power in each cylinder? As I see it, there are two possibilities:

1) the air volumes going to each cylinder are different, and therefore one has to compensate for lesser air in some cylinders by reducing the orifice size. If this is the case, then I would expect to get less power from cyl 3 than cyl 1 (because less overall fuel is burned). This might ?? result in a rougher-running/misbalanced engine. In summary, both less air and less fuel in some cylinders, but still all cylinders are stoichiometric at peak EGT.

2) the air volumes going to each cylinder are (approximately) the same but the fuel pressure/flow through the smaller nozzle on cyl 3 is higher than the larger nozzle on cyl 1. This would result in approximately the same amount of fuel in each cylinder, and the same amount of power coming from each cylinder. In summary, nearly the same amount of air and fuel in all cylinders, and all cylinders are stoichiometric at peak EGT.

I'm hoping that option 2 is what really happens, but I don't know and haven't been able to sleuth out the answer on the internet.
Greg, a interesting case.

#1 is the classic explanation. In general, cooling demand is proportional to mass flow. Is cyl #3 running cooler than its brothers?

As for option 2, perhaps a bottle check to see if all nozzles do or don't flow the same?
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  #4  
Old 01-15-2021, 05:59 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by airguy View Post
I haven't seen an imbalance as large as the one you are describing, though.
Yes. BTW, bad math. #1 is 16% higher than #3 in diameter, or 32% in area. Id look for an induction leak on #3. Lean to peak egt during run up; pull throttle to idle. Does #3 egt go up more than others? If yes youve got a leak. (This works even better in flight, letting the air drive the prop.)
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  #5  
Old 01-15-2021, 06:20 PM
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Carl Froehlich Carl Froehlich is offline
 
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Do not assume a perfect, direct relationship between injector nozzle size and fuel flow. This is a common distribution system, so other components effect fuel flow. As example, changing out one nozzle to a smaller orifice will cause slightly more fuel flow to the others. That is why balancing injectors tends to be an iterative process.

While there may be a slight difference in air flow to each cylinder, you would have other indication of an air leak. Here a good check it to compare the GAMI spread at a lower altitude (throttle not fully open) and GAMI spread up high (throttle wide open). If you have a lean cylinder low but not high I would investigate that cylinder for an air leak.

Continue to strive for a 0.0 gph GAMI spread.

Carl
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  #6  
Old 01-15-2021, 07:10 PM
Greg Arehart's Avatar
Greg Arehart Greg Arehart is offline
 
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Pardon my math, actually the #3 cyl on my last test was 0.025 (I have an 0.024 I'm going to try, so accidentally put in the wrong size in the post) and that does give me a 25% difference in area.

I am aware of the interrelationship of changing sizes on the fuel flow in other lines, but not sure how to compensate. I would think it is a second-order effect.

Will do some checks for induction leaks, and a bottle flow test. I looked for induction leaks a while back (before starting the current experiments), but haven't done so recently.

#3 cylinder (coolest) is about 20-25 degrees cooler than #1 (hottest) until quite lean by which point #3 ends up the warmest by ~10 degrees.

#4 cylinder tracks #3 reasonably closely and #2 is similar to #1.

Currently the GAMI spread ranges from near zero to ~0.5 gph over a few cycles.

Thanks for the ideas. I'll check back in after some tests.

Cheers,
Greg
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  #7  
Old 01-15-2021, 09:55 PM
scsmith scsmith is offline
 
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Cylinder temps are complex to interpret because they are a combination of mass flow (power) and cooling variation. It is hard to sort out which is which. For example, my cylinder #2 runs the warmest - go figure - it is because the cooling-fin bypass installed on the back of #3 is not present on the front of #2.

In general, the Lycoming (and clone) vertical induction sump does not flow nearly as evenly as the horizontal induction sump used on the angle-valve engines. So the spread in orifice diameter is most likely correcting for poor charge distribution primarily, and but may also have some variation in distribution in the injector spider as a secondary effect.
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