LOP potential problem

[Shadrach]

Well, there is another cause for valves not seating properly and that can be lead deposits. One of the benefits of LOP ops is the cylinders run colder. However, if they run too cold, the additives in AVGAS that is intended to scavenge the lead in the fuel doesn't get hot enough to do a complete job and you get lead deposits. For some of us where I guess the baffling is to good in our planes and the cylinders run cold anyhow, this might be a problem.

 

I run LOP all the time and this is a possibly a problem for me. Inspection of my pistons through the spark plug hole shows lots of lead build up. In addition, I have lead fouling problems with my lower plugs. I have also experienced a sticking valve in the past. What caused my sticking valve? Nobody knows, but fortunately it was remedied by "staking the valve" in other words, beating on it with a hammer.

 

I have toyed with the idea of running some TCP in my fuel to see if I can reduce, or eliminate the lead deposits in my cylinders. I have read that some of the Cirrus drivers have to do this as running LOP in those planes is actually mandated in their POH and some have experienced lead deposit problems. Did lead build up cause my valve to stick? Don't know. Could a chunk of lead get stuck between the valve and the valve seat and cause the valve to eventually burn? Don't know, but sounds plausible, but it would seem that the lead would either melt, or pound out before there was any real damage done.

 

 

Well, I will repeat what Mark Rouch of Top Gun told me about LOP ops. [This is his theory, not mine. I am only the messanger.] He says that Lycoming cylinders and pistons are machined and designed to operate at a particular temperature and if they don't get hot enough, they don't expand enough and this causes blow by that eventually leads to glazed cylinders that will result in lower compression high oil consumption and need honing. I personally don't buy this story, but I'll throw it out there as one man's opinion on how LOP can hurt cylinders.

 

Lastly, I think it is possible (this is my theory now) that it is possible that LOP ops might cause exhaust systems to prematurely fail. Corrosion is one of the leading causes of exhaust system failures and when you run LOP, there is an excess of oxygen. Oxygen is what causes corrosion, so hot oxygen rich air would suggest a possible higher rate of corrosion. I have no proof of evidence of this and neither does anyone else. It's just a theory that no one has ever run any lab tests on and likely never will. 

 

Anecdotal evidence from owners shows that some have exhaust system failures and blame LOP and others do not and go to TBO. Is there a higher failure rate of exhaust systems LOP vs. ROP? Nobody knows because these stats are not easy to document and are not kept.

 

Anyhow, since I am rarely in a big hurry to get anywhere and I need to save money where ever I can to be able to afford this ridiculous hobby, I run LOP pretty much all the time.

 

I would not call LOP exhaust gases rich in oxygen.  I've had zero corrosion issues with my exhaust in 1200 hours and 16 years. 

[Tommy]

Thanks guys for all the info

 

You were all spot on - none of those aircrafts have engine monitor fitted and both of them had early failure of cylinders previously. It's very likely the way they run LOP puts many unmonitored cylinders right in the red box!

 

What I am more interested is the EGT spread (I don't have a FF so can't tell the GAMI spread), I guess if different cylinder peaks at different EGT then it's probably okay. ie. cylinder 1 peaks at 1550 and running LOP 1450, cylinder 2 peaks at 1500 and LOP 1380 at the same mixture setting

 

But will it cause problems if they all have the same peak but wide LOP EGT spread? For eg. slight imbalance given that cylinders of lower EGTs are outputting less power? And does it automatically follow that you have a wide GAMI spread? 

[Shadrach]

 

What I am more interested is the EGT spread (I don't have a FF so can't tell the GAMI spread), I guess if different cylinder peaks at different EGT then it's probably okay. ie. cylinder 1 peaks at 1550 and running LOP 1450, cylinder 2 peaks at 1500 and LOP 1380 at the same mixture setting

 

 

For each cylinder, peak is peak... I do not understand what "  1 peaks at 1550 and running LOP 1450" means.  

 

 

 

 eg. slight imbalance given that cylinders of lower EGTs are outputting less power? And does it automatically follow that you have a wide GAMI spread? 

 

The first sentence is not a given, it's likely not true at all.  This is difficult for many pilots to get past.   Probe placement and angle affect raw EGT numbers. You could probably change the reading by 25df or more just by changing/moving the probe location on the header.  EGT numbers are of no use in calculating GAMI spread.  The example I gave you had a GAMI spread of 0 (as in perfect) but the EGT spread was 135df. 

 

Do try to read all of these...twice!  Once you've done your homework, you will have the foundation to get into details. 

 

Pelican's Perch #63:

Where Should I Run My Engine? (Part 1)

Pelican's Perch #64:

Where Should I Run My Engine? (Part 2 -- The Climb)

Pelican's Perch #65:

Where Should I Run My Engine? (Part 3 -- Cruise)

Pelican's Perch #66:

Where Should I Run My Engine? (Part 4 -- Descent)

[carusoam]

There is a right way, then about 50 shades of not right way...

1) To measure a Gami spread, it requires knowing FF.

2) A tight Gami spread, allows going deep LOP.

3) Having EGTs on all cylinders, allows you to determine peak for each cylinder.

4) Knowing peak on each cylinder allows you to know how deep LOP your engine can go.

5) If you set up your JPI, it will display °LOP for each cylinder. This removes the challenges of probe location.

6) Deep LOP (near 100°F) requires lower altitudes...

7) Shallow LOP (10°F or peak) can be obtained at higher altitudes...

8) Running LOP is recommended for <65% BHP.

9) NA planes usually go below 65% BHP around or above 8K'.

10) A bad Gami spread will make it difficult to run LOP.

11) A lousy Gami spread will make it difficult to run deep LOP.

12) A good Gami spread will make it easy to run LOP.

13) An excellent Gami spread will make it easy to run deep LOP.

14) Knowing your Gami spread will make maintenance decisions easier.

15) Get FF at the nearest opportunity, you will probably like it.

16) Many modern engine monitors use FF and OAT to calculate BHP. See if that is available for your monitor. The POH has these numbers as well. MAPA has printed 'key' numbers as a means to make it easier to remember/calculate...

How did I do?

Best regards,

-a-

Ross is a faster types than I am, and more knowledgable on the subject...

[PaulB]

Before the days of LOP what did mechanics blame burnt exhaust valves and ruined cylinders on?

[Shadrach]

There is a right way, then about 50 shades of not right way...

1) To measure a Gami spread, it requires knowing FF.

2) A tight Gami spread, allows going deep LOP.

3) Having EGTs on all cylinders, allows you to determine peak for each cylinder.

4) Knowing peek on each cylinder allows you to know how deep LOP your engine can go.

5) If you set up your JPI, it will display °LOP for each cylinder. This removes the challenges of probe location.

6) Deep LOP (near 100°F) requires lower altitudes...

7) Shallow LOP (10°F or peak) can be obtained at higher altitudes...

8) Running LOP is recommended for <65% BHP.

9) NA planes usually go below 65% BHP around or above 8K'.

10) A bad Gami spread will make it difficult to run LOP.

11) A lousy Gami spread will make it difficult to run deep LOP.

12) A good Gami spread will make it easy to run LOP.

13) An excellent Gami spread will make it easy to run deep LOP.

14) Knowing your Gami spread will make maintenance decisions easier.

15) Get FF at the nearest opportunity, you will probably like it.

16) Many modern engine monitors use FF and OAT to calculate BHP. See if that is available for your monitor. The POH has these numbers as well. MAPA has printed 'key' numbers as a means to make it easier to remember/calculate...

How did I do?

Best regards,

-a-

Ross is a faster types than I am, and more knowledgable on the subject...

 

 

Great post -a-!

 

I'm not faster than you, I've just written these exact posts so many times, I have canned cut and paste answers (I should actually do that)...

 

The <65% thing is great for folks just figuring things out.  I am betting that many of us run way more power than that both LOP or ROP (Aaron has offered testimony to that fact). 

 

My POH says that I can lean to 25 or 100ROP below 75% power. 

 

M20J POH say the same...

 

These recommendations would likely put some of the cylinders in the dreaded red box, why would leaning more be dangerous?  The answer is, it wouldn't...but there's no harm in being conservative.  I'm betting APS is being conservative when they say you can set your mixture anywhere at 65% or less.  

 

I am comfortable setting LOP at any MP.  In the winter I have run balls to the wall at negative DAs. It can be a struggle to keep CHT's above 300 at 50 LOP. CHTs were actually higher at full rich.

[ryoder]

Were those Cessnas injected or carbureted? I think the lower horsepower carbureted engines are less likely to suffer from early cylinder destruction due to not being able to run smoothly at peak or lean of peak and the lower cylinder pressure due to lower compression ratios.

[Bob - S50]

Thanks guys for all the info

 

You were all spot on - none of those aircrafts have engine monitor fitted and both of them had early failure of cylinders previously. It's very likely the way they run LOP puts many unmonitored cylinders right in the red box!

 

What I am more interested is the EGT spread (I don't have a FF so can't tell the GAMI spread), I guess if different cylinder peaks at different EGT then it's probably okay. ie. cylinder 1 peaks at 1550 and running LOP 1450, cylinder 2 peaks at 1500 and LOP 1380 at the same mixture setting

 

But will it cause problems if they all have the same peak but wide LOP EGT spread? For eg. slight imbalance given that cylinders of lower EGTs are outputting less power? And does it automatically follow that you have a wide GAMI spread? 

 

Quit worrying about the actual EGT's.  You don't need fuel flow.  If you lean until the last cylinder is LOP and the engine is still running smoothly, you are good to go.  If the engine starts to run roughly before the last cylinder goes LOP, either don't run LOP or get GAMI injectors.

 

The most fuel efficient power setting is about 30F LOP.  I typically look for 10F LOP on the richest cylinder which gives me about 30F LOP on the leanest.

 

Bob

[Bob - S50]

I'm not a big fan of LOP operations on underpowered aircraft. The speed loss and additional mechanical expense don't make much sense to me. I could, for example, add nearly an hour to my typical 700 mile trip by using reduced power/LOP to save fuel. My time and engine/prop/airframe expense do count against the fuel savings. It's not my goal to build hours. My time is precious, I'd like to arrive prior to lunch! 

 

As a professional A+P mechanic, I cannot provide any mechanical reason for avoiding proper LOP operations with healthy equipment. It's simply that my 200HP engine, operated at 100-125HP, is not enough to make me happy. 

 

LOP does not necessarily mean slower.  65% power LOP results in the same speed as 65% ROP.  However, you are correct in that if you are looking for the maximum speed, you will have to operate 50 to 100 ROP.  Might make the difference between being able to get 75% power vs 65% power.  Then again, maximum speed also means you will be running WOT, full forward on the prop control, and leaned to ideal ROP.

 

As for a mechanical reason to avoid ROP.  How about operations at lower altitudes?  Cruising at say 3500' you probably don't run firewalled and 50 ROP.  I'm guessing you either roll the RPM back, pull the throttle back, or both.  Otherwise you are probably running 85+% power (which can be OK as long as the CHT's stay low enough).  If you are pulling the power back to keep the output down to 75% or so, why not do it with a slightly higher throttle/RPM combination and LOP to get the same power/speed and save money?

 

Time is precious.  I agree there are times when speed is more important than money.  There are no absolutes.  Life is full of choices.

 

Bob

[ArtVandelay]

And of course a 10% lower HP doesn't translate into a 10% reduction of speed.

A 5 hour trip at 75% is a 5:19 trip at 65%...how important is that 19 minutes?

[dtoelke]

LOP does not necessarily mean slower.  65% power LOP results in the same speed as 65% ROP.  However, you are correct in that if you are looking for the maximum speed, you will have to operate 50 to 100 ROP.  Might make the difference between being able to get 75% power vs 65% power.  Then again, maximum speed also means you will be running WOT, full forward on the prop control, and leaned to ideal ROP.

 

As for a mechanical reason to avoid ROP.  How about operations at lower altitudes?  Cruising at say 3500' you probably don't run firewalled and 50 ROP.  I'm guessing you either roll the RPM back, pull the throttle back, or both.  Otherwise you are probably running 85+% power (which can be OK as long as the CHT's stay low enough).  If you are pulling the power back to keep the output down to 75% or so, why not do it with a slightly higher throttle/RPM combination and LOP to get the same power/speed and save money?

 

Time is precious.  I agree there are times when speed is more important than money.  There are no absolutes.  Life is full of choices.

 

Bob

Beat me to it.  LOP is not inherently slower! (really wish people would stop saying this, it's just plain false)  BUT, for any given power setting the throttle will have to be more open (higher MP in turbo'd a/c) for LOP operations than the equivalent power setting ROP.  This is because running LOP you are below the stoichiometric air-fuel ratio (i.e. there is unusued oxygen present) whereas running ROP you are above the ratio (unused fuel present).

 

Think about it this way.  The power produced is roughly equal to the amount of fuel burned in the combustion process (not exact due to cooling effects of the cylinder and unburned fuel, but close enough).  So, in the LOP situation you are 'fuel-limited', meaning any change in fuel flow causes a direct change in the power generated.  On the other hand, ROP you are 'oxygen-limited', meaning that changing the fuel flow has little effect on power generation (but does affect cooling).  Thus, as you lean from the ROP to LOP you WILL experience a loss in power (this is why some people incorrectly conclude that LOP is slower).  In reality you are comparing two different power settings (say 75% ROP to 60% LOP).  To return to the original ROP power setting you need to add throttle/MP after the pull.

 

As a sidenote, at high altitude this may not be possible with non-turbo'd aircraft if your throttle is already full open (which is where the above statement has some truth).

[Browncbr1]

The previous owner of my plane got 3500hours out of the previous motor (without ever changing cylinders) running 25-50LOP down low and 25LOP-PEAK up high... he said he decided to overhaul the motor because of all the oil leaks and figured he'd gotten his money out of it by then.   This is how he flew the current motor for 400 hours also without problem, so i'm sticking with how he ran it...   i think the key is to be mindful of %HP before leaning.. 

[carusoam]

Dtoe...,

Are you a scientist, engineer or dentist?

You have a nice explanation for the LOP vs. ROP...

Best regards,

-a-

[dtoelke]

Dtoe...,

Are you a scientist, engineer or dentist?

You have a nice explanation for the LOP vs. ROP...

Best regards,

-a-

 

Engineer 

[Hondo]

Most probable causes of burnt valves are:

Irregular valve sealing with cylinder head valve seat. Carbon residues generated by incomplete combustion (rich mixture) will appear at the seat region and will jeopardize the sealing between the valve and its seat  

This allows combustion gases to escape past the valve concentrated at only one point

 

Incorrect valve clearance can jeopardize the valve sealing and also cause this type of failure  Not likely.

 

Carbon buildup is much more likely running ROP than LOP.  The EGT is the same at 50 ROP as at 50 LOP.   Running LOP will completely burn the fuel without leaving carbon behind to compromise

the exhaust valve seal and CHTs will be 30+ degrees cooler.

 

Running LOP at cruise power settings (65% - 55%) will reduce the likelihood of exhaust valve failure, the engine will run cleaner, cooler and more efficiently.

 

Deakin and Busch like it for good reasons.

[Hondo]

Deakin explains  exhaust valve failure.

http://www.avweb.com/news/pelican/182155-1.html

He says poor valve seat to valve fit became problematic for Continental engines after 1991.

The key elements in good valve cooling are:

1. Good valve face to valve seat contact (it needs to be nearly perfect); and

2. Good valve stem to valve guide fit; and

3. Cool cylinder head temperatures.

These VASTLY outweigh the effect of EGT.

Deakin flatly asserts that virtually all valve problems originate with the factory or the overhaul shop.

 

Evidently, Monti Barrett agrees. Barrett Precision Engines uses Serdi to get a precise valve seat to valve fit.

 

http://www.autoheadperformance.com/main/page_our_shop_serdi_seat_cutter.html