Seneca w/ Merlyn Fuel flow setup help
Overhauling my LTSIO360-KB throttle body assembly due to a fuel leak. I know it will need complete redo of fuel flow setup procedure per M0 manual but have some questions on this.
1- I have the Merlyn "wastegate" (no intercooler). Are there any different procedures for setting metered pressure with this? M0 calls for using differential pressure gauge to measure difference between upper deck pressure and metered fuel pressure at full power. Since the Merlyn is keeping upper deck pressure 2-4" above what is required, is this going to affect the fuel flow setup process any?
2- When we would setup the fuel system for routine checks, we used the fuel flow values from my calibrated JPI instead of a differential pressure gauge (we don't have the differential pressure gauge, just a standard single port Dwyer digital gauge that reads to 0.1psi accuracy; as my AP has always done NA Conti, this is his first turbo). Is this still reasonable for re-setting everything?
Comments
Hi Tokirbymd,
on Merlyn's website you'll find the following application note that contains all sorts of useful information about installing and troubleshooting the black magic waste gate. Your question about variance to the Continental MO metered and unmetered fuel calibration procedure is addressed on the last page: "...No changes in unmetered, or metered fuel pressures from factory specifications are required when installing the Merlyn control...."
Control.PDF (merlynproducts.com)
If you run into trouble suggest calling Merlyn. As of a year ago when you call you still get Mrs. Evans on the phone. She's the founder, an engineer and FAA DER, and knows more about engines than probably anyone you're likely to run into.
If you sent the throttle body to Mike's Fuel Metering in Tulsa for overhaul you're in good hands. After they replace all the gaskets and o-rings they will run it on their fuel flow test bench to check for leaks and set the nominal calibration. My experience is you won't need to make additional adjustments when you perform the on-engine fuel calibration, so long as the other parts in your fuel system are operating correctly. Kevin is the shop manager and lead A&P, and very helpful with fuel system problems.
I've never done the M0 calibration procedure before without a pair of Dwyer digital gauges. Most of the Continental representatives I've talked to treat that procedure with a nearly religious level of zeal. Neal George teaches the Continental A&P training class, and he's an expert on the procedure and has performed it hundreds of times. He has a shop in NW Florida (Pensacola maybe) and is active on the web. If you're considering a modification to the procedure you might call him or shoot him an email.
good luck and happy holidays,
Bob Tingley
Thanks, I found the Merlyn link after posting this. Also found some additional resources if anyone else is looking and posted the links below. Additional reading in M-0 says in note 4 that a calibrated in-line flow measuring device may be used to measure full power fuel flow. So presumably that would substitute for differential pressure measurements? It is mentioned that if correct fuel flows were obtained but incorrect pressures at full power then a separate cause should be investigated. I'm trying to think of a situation where that would be the case.
https://www.csobeech.com/files/FuelFlowSID97-3E-Turbo-Notes.pdf
I think a fuel flow meter is the best way. The other thing you can check for is mixture ratio as a function of cruise power. Your EGT's should be ~stable as you increase MP for the same mixture lever position from 55% to 75%. They might go down a bit 75% and above as the mixture can be richer. Should get decent match on FF as a function of rpm/MP and OAT as well (corrected for Merlyn table)
What you want is the system is stable across a range of power settings so you only need to make tweaks in mixture for the target EGT. Worse case is if the mixture leans out as MP is increased as this could put you at risk for detonation. It is absolutely critical you hit at least the minimum full throttle fuel flow and ideally the maximum for best cooling margin.
Eric Panning
1981 Seneca III
Hillsboro, OR (KHIO)
Tokirbymd,
using a fuel flow measurement from a gauge you DO have, as a proxy for a differential fuel pressure measurement gauge you DON’T have is a clever idea. In fact, the OEM steam powered fuel flow gauges on the panel in my Seneca do almost the same thing, just in reverse. The OEM gauges make a differential pressure measurement similar to that called for in SID97, mechanically “divide,” by the equivalent fluid impedance, and label the result in gallons per hour. I’m using the idea that fluid mechanics to first order works like an electric circuit, with voltage = pressure, current = flow, and resistance = fluid impedance. I was reminded of this analogy a few weeks ago as a cardiologist buddy extolled the virtues of Ohm’s law over lunch.
The philosophy behind continental’s procedure is that when the fuel calibration process was first developed decades ago, pressure gauges at a certain price point were far more accurate than series flow gauges at the same price point. So you definitely wanted pressure to be the principal observable. Today, if you can simultaneously measure flow with a series flow sensor, say on an engine computer, then you can guarantee the system impedance is correct as well, and be confident your fuel system is working as intended. Just make sure you don’t try to use the OEM panel gauge as your fuel flow reference (been there, done that) since as we said earlier it’s really making a differential pressure measurement.
One example I saw recently of a system compliant in flow, but deficient in pressure, had a leaking electric primer valve. Once the valve was bypassed and the leak mitigated, both pressure and flow fell within the table values. Lots of other things that can change the impedance too, like kinked or clogged fuel lines, a sticking spider, or one or more clogged injectors.
Bob
Bobb-
Interesting you mention the analog "flow" gauges. This plane has historically had a "split" with the right engine showing higher fuel flow on the analog gauge (higher pressure) than the left despite the flow measured by JPI flow gauge being equal. (and the JPI is very accurate based on historical fuel burn K factor). The prior owners had the plane for 10y and it was always the same with them. Merlyn does mention a possibility that when the Merlyn was installed the fuel system was calibrated to a higher critical altitude setting in the fixed wastegate bolt resulting in need to have higher fuel pressures to keep the engine from quitting when the Merlyn opened the gate and reduced upper deck pressure. Not sure if subsequent fuel calibrations would have resolved this. I can use the JPI to set fuel flow but having the analog gauge read accurately for initial setting would be helpful. So maybe I do need to go back and check flow on JPI vs full power differential pressure? Its also interesting that the pressure markings on the "flow" gauge correlate exactly with what the metered pressure should be from the M-0. Not sure if you can see on this picture but when the gauge reads 26.4 gph, the small print is 21PSI.
I see what you mean - it looks like your right analog flow meter reads about 15% higher than the left, while the JPI shows 13.1 on both channels. If you decide to chase this down and determine the error source, I'd definitely be curious to know what it is. BTW, was your JPI installed in such a manner so that it qualifies as "primary?"
Looking at the even EGT's I would trust the JPI over the differential gauge. That differential gauge brings 4 potential fuel leaks to the cockpit as well.... I think you will need it swapped/overhauled but first check for leaks at the engine for both lines as well as possible crushed lines.
Best if you could get it replaced entirely with a primary instrument. I don't think this JPI unit would qualify. There is an aerospace logic gauge that is $2k and can be primary. Overhaul of a United 6221 gauge like you have is ~ $1000. Hopefully it is an easy issue to fix not related to the gauge.
Eric Panning
1981 Seneca III
Hillsboro, OR (KHIO)
Not interested in a Primary replacement right now- both $ and space reasons. The prior owner (and I) basically use JPI to set fuel flow based on TIT. The analog gauge is just a rough estimate only. And the left side appears to work as advertised. So am wondering if it is truly a gauge problem or a problem with the fuel flow pressures in the fuel setup. I may figure out how to connect a pressure gauge in line (per service manual) to confirm.
Tokirbymd,
The analog flow meters are actually a pressure gauge labeled for flow under normal circumstances. If we treat the JPI measurements as accurate, the equal indications (L vs R) would represent equal flow. IIRC, one characteristic of a partially blocked fuel path (injector?) is a higher indication on the analog pressure gauge.
Have you analyzed the JPI engine data looking for an overly lean cylinder (if a partially blocked injector)? If it were me, I'd try this data analysis looking for a smoking gun before I start an invasive process like pulling and cleaning injectors or other components.
DJ
Those gauges do fail. Does it ever read accurate? Like at idle?
I found some bluetooth remote pressure gauges that would allow you to measure pressure in the cabin on a phone. They are alright but in the end FF is the better measure.
From the EGT readings it does not look like there is a clogged injector. I would not mess with them. The most likely clog is a red fiber from a mechanics rag...
Eric Panning
1981 Seneca III
Hillsboro, OR (KHIO)
Folks -
just wanted to pass along a suggestion from Continental for which gauges to use when setting up the fuel system on their turbo engines:
I gather a key reason these gauges are preferred is that the sensor interfaces are 316L/LSS stainless.
You will notice there are other gauges with virtually identical construction and performance numbers available from Klein and other vendors, too.
Bob
Damn- $1000 for a differential gauge? I think I'll buy two 0-30psi Dwyer gauges and subtract the difference myself.
GAMI spread check in flight is good- no injector problem. So that isn't it. Plane is apart for annual but once I get the engines put back together (OH throttle body) I may put a gauge in between the manifold and the panel gauge to compare the two. At least I would know if its a real pressure error or gauge error.
Finished the annual today with the M-0 fuel flow setup. Both engines setup nicely with good unmetered and metered flow rates as measured by a gauge for unmetered and JPI flow rate metered (I had been in touch with Neal George who recommended pulling the Merlyns off and restablishing with the bolt bypass during setup but this seemed a bit extreme). I'm happy with fuel flow overall.
But I still have a problem with #1 on both engines going "cold" with EGT of 400-600 during any idle lower than 900 RPM. Seems to want to live around 1100-1200 RPM to make the whole engine happy. We searched for any intake leak, the nozzles are giving a good GAMI spread so they aren't the issue- at least in cruise. Its annoying and we ended up setting idle to 800-900 RPM just to avoid potential for the engine dying at idle on flare. Is this due to the Merlyn not performing? Or just the turbos aren't spooled up until over 1000-1200? My A&P says don't worry about it if it flies well. Just want to know if this is a TSIO360 thing or not.
The airflow balance among cylinders in the TSIO360 is not the best and may be a factor in your low EGTs. The GAMIs compensate by providing a different fuel flow to help match the different airflow. This works well as long as the relative airflow between cylinders is similar. At low rpm you may be seeing a much different airflow in #1 cylinder (furthest downstream along with #2) vs the others. Unfortunately I’m at a loss as to further diagnosis other than the obvious (induction air leak, fuel flow restriction). Have you tried Continental’s tech support?
DJ
That's kind of what my A&P and I figured. And since its on both engines makes sense. The manual does say to set idle RPM "to the desired setting that provided smooth operation with minimal control adjustment for ground operation". So maybe I just set idle RPM a bit higher and don't worry about it.
What do you have for mixture rise on cutoffs? And why concerned on egt at idles? Carl
48 yrs A/P IA DAL aircraft inspector. 172N
Bendix or slick mags, time on each, time on engines? Pressurized mags? carl
48 yrs A/P IA DAL aircraft inspector. 172N
Carl- pressurized bendix mags. Engines are ~900 SMOH, Mags ~350h. Mixture rise is 60-75. EGT dropping at idle to 500-650deg would be consistent with misfiring or not firing at all when at idle. Not a concern for ground work other than potential for plug fouling. But if this happened on landing rollout and need for go around, am concerned one engine would spool up faster than the other resulting in adverse yaw issues.
T
Assuming your fuel flows are set up to spec and having a slightly richer mixture. Hasn't helped I would tend to be looking at weak magnetos when they are hot. If you have pressurized slicks, I would be more suspect than if they were Bendix mags. Either way, being pressurized, they're getting exposed to a lot of additional heat. As mentioned, I had that issue of when we had the pressurized sky master. And after doing a complete fuel system set up. It was perfect so I eliminated the fuel issue.
If you don't have it, even adding additional blast cooling to the mags might buy you some more time and usability. Be worth a try first.
48 yrs A/P IA DAL aircraft inspector. 172N
I see you have the bendix magS which are pretty good. But they could still be suffering from heat. Make sure to do a good timing. Check and if you don't have some fine wire plugs, those would be worth a try a fire better in general, and you could cheat by even going to a smaller gap on your massive electroplugs, which might help out a little bit too. The goal is to not have the mag work so hard when it's hot.
Obviously the fine wire plugs are expensive try in one bank at a time and see if it makes a difference.
48 yrs A/P IA DAL aircraft inspector. 172N
Planewrench- we just dialed in the timing perfectly on both sides at annual ( I know how perfect as I was helping). And have fine wires on both sides both top and bottom. I don't think hot mags are the issue as it happens immediately after startup even when the cylinders and engine are still relatively cool; as well as after a long flight.
T
Just want to confirm that you set up the fuel system per the continental guidelines. Which would have you set the idle in the low 600 RPM range to check the unmetered fuel pressure. Did you use a good calibrated gauge on the low pressure as well as a calibrated gauge on the metered pressure . using the JP. I while we think it is very accurate. Probably would not be my first choice.
Did you have this problem before the fuel system repair and did you send both systems out for overhaul or just one?
48 yrs A/P IA DAL aircraft inspector. 172N
Set up per M-0 with unmetered pressure set at 6.5 at 700RPM using Dwyer digital gauge and RPM rise to 60 on each side (M-0 says 25-50, Piper says 75-100- so we split the difference). We did not have the equipment for a full power run with both metered and upper deck pressure (for the differential metered pressure) which is why we went with the calibrated JPI fuel flow instead for metered fuel flow. Its interesting that Piper makes no mention of a differential pressure test setup in their service manual but instead references fuel flow at 40"/2600RPM. Problem with cylinders #1/2 on both engines going "cold" has been present both before and after overhauling the throttle body assembly.
At this point, I'm going to assume that the issue in the "cold" cylinders is simply lack of airflow at idle due to lack of turbo being spooled up. As long as I keep idle RPM high enough that the engines don't hesitate with advancing the throttle and inflight behavior is normal I figure keep flying and watch it.
As for the analog gauge reading incorrectly; I'm going to assume its a gauge error but eventually I'm going to hook up a T to measure the actual pressure in the line to compare to the gauge.
T
The information that you have cyuldered 1 and 2 on each engine. Showing different readings from the rest is intriguing..also important to know that this was going on. Before you had the units worked on Since opposing cylinders share a cam lobe, i would check valve lift on those cylinderss and see if they compare Equally with the the other cylinders. have you mechanic do a dry tapet clearance to make sure they are within limits. Also I would verify the flow dividers. Are the correct part numbers for the mod of your eng. I Believe they make different Spring and diaphragm part numbers. So you want to make sure the correct ones are installed. Sounds like you have the right numbers on the fuel system for setup.
You mentioned it runs kind of crappy. Even when cold does it run lean or rich? After start up, can you tell?
Carl
48 yrs A/P IA DAL aircraft inspector. 172N
Carl- good thoughts regarding the tappet clearance and valve lift. That should be easy to do. I have heard of the flow divider spring having problems partially occluding the flow at low power states. But is intriguing that it is the exact same for both engines which is why I thought it was a TSIO360 characteristic. Once RPM are above 1000, it runs perfect and all performance numbers are book value.
Basically, the flow divided purpose is to shut off fuel flow when pressure drops below a set point so it would give you a good idle cut off. Since EGT is an indicator of fuel mixture and mixture is predicated on air flow and fuel. That's why the potential exists for possible Val ve issues. If the same group did, both motors just adds a adfitionall factor for troubleshooting.
While possibly far-fetched in your case. In my massive reading library, I did come across a case of lousy running lthat eventually was traced to a poor casting with slag in the induction cast spyder on top of the engine. Caused crappy flow to one cyl. That was some very involved, troubled shooting.
Since it looks like your fuel flow and your timing are dead nuts. I think we almost have to look for a mechanical reason. One more deep thought is if you have good records on the overhaulverify exactly what cam was put in and which lifters were installed. I know on the 520 series. They make different versions and can be easily intermix, with some interesting operational problems.
Carl
48 yrs A/P IA DAL aircraft inspector. 172N
Those engines are definitely known to have a little bit of a lope at idle speeds. But it's pretty safe to say they shouldn't quit landing.
48 yrs A/P IA DAL aircraft inspector. 172N
Tokirbymd,
If I missed it in this thread, apologies. What is your engine monitor reporting for fuel flow at the low RPM just as the engine starts to die?
DJ
Mine is the same both engines. At 1000 it comes alive and I attribute that to the turbo really kicking in. It has a bit of a "lope" as Carl pointed out below 1000 rpm.
You expressed a concern on go-arounds and adverse yaw if one engine is slow. My experience on go-arounds/touch and go's is you will always have enough power and rpm that you will be above this transition rpm.
On take off I hold brakes to ~ 35MP and confirm balanced fuel flow, instruments, etc then start my roll advancing throttle slowing to ~ 40 MP.
I think there are some scenarios of low level single engine operation training where it would be a concern. No one should be practicing single engine go arounds at low altitude but I had a twin instructor insist on it in my youth... Felt sketchy to say the least! This is part of why it is important to declare an emergency when operating with a single engine. The tower and traffic won't know if you don't tell them and short final is too late to mention it....
Eric Panning
1981 Seneca III
Hillsboro, OR (KHIO)