Many turbine engines are flat rated, effectively making them naturally aspirated. Flat rating means the engine can produce more power but is limited so as to help with hot and/or high altitude conditions.

For instance, the TPE 331-10 engine used in the MU-2 is a capable of 1000 shp at sea level standard conditions, but flat rated to 665 shp in the aircraft. The engine can produce full power (665 shp) to about 16,000 ft msl.

This is a pretty common practice. So in many ways they act more like a turbocharged engine than a normally aspirated piston engine.

Many turbine engines are flat rated, effectively making them naturally aspirated. Flat rating means the engine can produce more power but is limited so as to help with hot and/or high altitude conditions.

For instance, the TPE 331-10 engine used in the MU-2 is a capable of 1000 shp at sea level standard conditions, but flat rated to 665 shp in the aircraft. The engine can produce full power (665 shp) to about 16,000 ft msl.

This is a pretty common practice. So in many ways they act more like a turbocharged engine than a normally aspirated piston engine.

Not to be pedantic, but wouldn't that be Normalized rather than charged?

Maintenance of sea-level power to critical altitude is turbo normalized. I.e. a derated turbine.

Turbo charging increases manifold pressure above that seen at sea level.

The difference is namely a function of how you control the wastegate (yes, this is a ridiculous oversimplification, but accurate enough for the discussion.)

Not so fast buckwheat.

On run up this AM.

Left engine:

#5 EGT drops on Left Mag
#4 EGT drops on Right Mag.

;(

Ignition wires replaced on both sides.

First Trip:

KORF to KGNV and return:

Norfolk to Gainesville was at 14,000, Gainesville to Norfolk was at FL210.

I recorded numbers and did a GAMI check each way.



27.2” x 2220 27.2” x 2220
14,000’ 0C
FF 13.9. 13.9
LOP

EGT/CHT
1. 1389/310 1364/304
2. 1349/297 1435/301
3. 1491/320 1462/301
4. 1455/304 1511/305
5. 1467/330 1469/308
6. 1456/298 1521/298
TIT1 1591 1618 -TIT1 is left side of engine (2, 4, 6)
TIT2 1587 1588 (1, 3, 5)

GAMI Check/EGT @ peak

1. 15.6/1480 15.3/1446
2. 15.3/1430 15.3/1513
3. 15.6/1561 15.4/1551
4. 15.5/1541 15.6/1590
5. 15.3/1521 15.5/1536
6. 15.5/1531 15.5/1597

GAMI spread:
.3 .4

Pretty good maybe down a letter for 2 & 5 on left engine and 1 & 2 on right engine.

Here’s the flight back:

FL210 -12C

27” x 2280. 24.5” x 2280
FF 13.6 13.2 (TIT limited)

1. 1403/321 1391/313
2. 1356/306 1443/309
3. 1523/335 1511/313
4. 1486/312 1535/309
5. 1492/343 1500/317
6. 1491/210 1557/314

TIT1 1608 1643
TIT2 1601 1617

GAMI Check

1. 14.4/1472 14.7/1458
2. 14.3/1421 14.4/1518
3. 14.1/1578 14.3/1560
4. 14.9/1508 14.4/1604
5. 14.7/1522 14.3/1565
6. 14.8/1543 14.7/1617

GAMI Spread
.8 .4

Two issues:

1. Right engine TIT1 runs high limiting power available.

2. Left engine has a higher GAMI spread at high altitude causing it to have a noticeable roughness LOP. (Understandable due to the .8 GAMI spread)

I looked a variation in EGT temps as they all vary a little in cruise

1. 1404-1405(1) 1385-1392(7)
2. 1353-1356(3) 1441-1445(4)
3. 1527-1533(6) 1507-1514(7)
4. 1491-1496(5) 1535-1544(9)
5. 1496-1500(4) 1508-1514(6)
6. 1491-1501(10) 1563-1574(11)
TIT1 1609-1619(10) 1640-1652(12)
TIT2 1603-1609(6) 1613-1621(8)

Left engine #6 is outlier
Right engine #3, #4 & #6 are outliers

Right engine #4 & #6 are on same bank as TIT1 (the one that runs high).

So……

What’s going on here?

Why is TIT1 on right engine running higher than the other Turbine Inlet Temperatures, (probe was replaced no change)?

Why would my GAMI spread change on one engine (and not the other) at higher altitude?

Engines are Lycoming IO-540-S1A5s with about 1000 hours @ average of 200 hrs/yr.

My best guess is that as my turbos are getting towards time for OH some difference in the upper deck pressure (L-R) feeding the index airlines to the injectors is causing a difference in how the fuel is spraying and that is causing a difference in burn between cylinders.

2nd trip:

KPNS to KORF climbed to FL250, both engines maintained 29” x 2550 all the way to FL250.

At 2200 RPM ROP left engine couldn’t maintain 27”.
LOP at 2200 RPM right engine reverse boot strapped (comp stall) to ambient.
Left engine didn’t.
Reduced MAP ROP and reverse boot strapping stopped on right engine.
@2400 both engines would run LOP at 11.1 gph(TIT limited) but IAS was around 130.

Ended up setting 25.5 x 2400 ROP = 238KTAS.
@18.5 gph

With tailwind my GS was over 300KTs for most of flight.

Lots of data, but I don’t know which is related to my complaints and which is just normal variability.

Thoughts, suggestions appreciated.

Forrest, I often times wonder if my turbos are really sharing the load. Just a thought.

If a turbo locks up or physically comes apart, it’s pretty obvious.
Wastegates start to stick or a butterfly that is worn or damaged can be visually identified.

A dead plug (or bad ignition wire) shows up on the JPI.

A bit of rubber floating around in an injector can be harder because the problem can be intermittent or only occur in flight, change in the EGT for the affected cylinder tells were to look and the biggest pain is getting the piece of rubber out of the injector (this is the biggest disadvantage to one piece GAMI injectors).

Back to the turbos:

It would be an interesting experiment to misadjust the wastegates so one turbo gets less exhaust and then see what happens to EGTs and GAMI fuel flows on each side.

I hope someone who had twin turbos AND misadjusted wastegates will show up here and relate what happened when they were adjusted properly.

This is the sort of thing George could sort out on his test rig.

I think Trey is on the right track.

I did some reading on how fuel injectors work on turbocharged engines.

The index air from the upper deck mixes with the injected fuel and the combination flows into the intake to mix with the air from the lower deck.

A weak turbo would reduce the pressure (and volume) of the index air which LOP means those cylinders would run a bit warmer. At higher altitudes the lower performance of the worn turbo would be more apparent.

Starting with the right engine, 2, 4, & 6 (left side of engine) all have higher EGTs than cylinders on the right side.

Is an indication that the left Turbo on that engine is weak.

Am I on the right track, too?

The upper deck pressure is going to be the same on the RH and LH side and doesn’t care if one turbo is doing more work than the other, both banks are fed off the filter AirBox.

What will be affected by one turbo doing more work than the other will be exhaust back pressure on the cylinder bank that’s feeding it (246 or 135)

You could check by measuring exhaust back pressure on the LH wastegate and the RH wastegate. In a perfect world they would be equal, but probably not the case. I am not really sure this would show up very significantly, more just a theory I’ve wondered about. Originally the exhaust manifolds had that interconnect “balance tube” but it’s function was to keep the pipes from damage when you had a compressor stall.

Trey,
I couldn’t find the right diagram and I’m not with the plane, but where is the index air sourced?

Forrest,
Your symptoms suggest you have one wastegate closing slightly more than the other wastegate. This can be caused by worn bearings on the ends of the cross-shaft or worn mounts for those bearings connected to the engine mount. There should be no play in that cross-shaft. Do you have the maintenance manual?

I have access to one on the A-F website.

If I have a high EGT/Low CHT, I think:

1. mag timing is low
2. Cylinder is not firing correctly, high resistance plug, carbon tracing in the magneto, ignition lead leaking voltage somewhere, plug gap too large
3. Poor fuel atomization due to clogged air bleed

Basically anything that makes combustion less efficient is going to turn into wasted heat energy in your exhaust pipes where the combustion process is completing.


I know there’s ways to do pilot tricks to find ignition/plug issues in flight. But I’ve never found anything in flight that I couldn’t also find on the ground.

Also, I’m jealous that you have had so much success running lean of peak. I want to do it so bad in my 601P, but I keep getting chicken that I’ll miss a run away egt alarm or something while I’m cruising distracted. I’m sure I’ll get the compulsion to figure the LOP thing out sometime, I know my first step is I need to wire my Fuel flow into my JPI790 so I can record fuel flow with the egt and review the data log, then sort out injectors etc.

I pretty much just burn the gas and take the speed like the junkie I am.

I get it, knowing means one has the responsibility for understanding what is going on.

But that said:

In a high compression Aerostar LOP is pretty easy, and in some ways less workload than ROP.

IMO you need the ability to monitor all 4 Turbine Inlet Temps.

And you need to accept that you will use 1650 as your max TIT.

There is a school that says to run ROP leaned to 1550.

I’m not a fan.

Rough numbers that puts you a bit richer than peak.

And that makes for maximum peak cylinder pressure.

The motor isn’t going to blow up and at 75% you will go fast.

But your fuel burn will be significant and my plane used to throw oil and some soot run like that.

And those extra 15 kts are going to cost an additional 10gph.

Do a GAMI check and see what your difference is, if it’s more than .3-.5gph per cylinder on each engine, order some GAMIs and then do another check, then swap injectors with GAMI to get down to .2-.3 gph.

The nice thing about operating LOP is MAP matters less.

If you are descending LOP, and a wastegate sticks and you don’t notice it and MAP spikes, it’s no big deal.

ROP it could be.

I normally target 13-13.5 gph.

Maybe 14.0 gph below 10K.

Low fuel flows mean I can (and do) carry less gas, and performance is better.

CHTs in cruise are around 300 deg.

Last thought:

Running 1650 max TIT LOP may be hard(er) on turbos than limiting TIT to 1550 ROP, or it may be that 1550 ROP is just better at hiding the symptoms of worn turbos.