[Steve Sampson]

Having read mountains of LOP articles I remain confused. In CAFE's Ignition Dynamics III report they say, in the conclusion:

"Wide experience has shown that,
when a reasonably even mixture distribution
exists, conventional aircraft
engines can safely operate continuously
lean of peak EGT with the
following important stipulations:

1) It must be at less than 70%
p o w e r. For normally aspirated engines,
it is generally at altitudes
above 10,000 feet where w.o.t. settings,
which are best able to give
even mixture distribution, deliver
no more than 70% power.

2) It is verified by EGT m e a s u rements
that all cylinders are at least
25? lean of peak EGT. Ideally, the
m u l t i - c y l i n d e r E G T / C H T g a u g e
should have an alarm capability
such as those......."

They do not justify - to my mind/understanding - point 2; the 25F LOP.

I like to operate my O-320 RV4 at 2050rpm / 22.5" with a fuel flow well under 6 USgph frequently down at 3000' (Airspace in the UK often prevents me being higher.) This puts me well below the 70% power point, but is an extremely efficient operational regime, without loosing much speed.

I am in the carburated world, not expensively balanced injectors, so when I lean out to the point where one cylinder is about to quit, there is another one that is left up high reporting a much hotter EGT. (I should add that the running rough band is very narrow. I put this down to the P-mags. They provide smooth ignition until the cylinder goes off line. The rough band is VERY narrow.)

CHT sit at around 300F and I am blocking the cooler to get OT up to 170F. I am probably meeting CAFE's requirement in their second conclusion, but what is it they are trying to guard against? Is it valve burning, or pushing the engine close to detonation - which surely would cause CHT temps to shoot up - or what.....? For me they do not justify why this second proviso. Can anyone explain?

I notice in Mike Bush's article, as I read it, the second rule is effectively unimportant provided the first is observed.

I would really welcome a better understanding.

 

[Steve Brown]

I do it all the time

But I don't agree with either of those rules completely.

Smoothest running is at partial throttle, not full throttle. I find it very difficult to find a good LOP operating point at high altitudes, unless I pull back the throttle and inch (mp) or two.

Other VAF pilots have written that partial throttle creates turbulence in the fuel/air mixture, resulting in more even distibution. I'm not sure if that's true, but it works.

My typical LOP operation up to 6k or 7k MSL is at 22" mp, and obviously RMP wherever it falls. I lean my #3 cylinder to 1300 and everything seems to fall into place. The other EGTs are less than that and CHTs are down in the 350 range, or less, and I can't get my oil temp near 200.

The instructor I did my BFR/IPC with pointed out that I haven't been doing it long enough to be sure it wasn't hurting the engine. That's a fair point, but my view is that if it runs cool and runs smooth, I'm not hurting the engine.

Also, I have read that peak combustion pressures happen ROP. So, running peak EGT, as long as the actual EGT temperature is acceptable, seems not a big deal.

Bottom line, its my money paying for fuel and for maintenance. If I make it to anywhere near rated TBO, I'll put all these dire LOP warnings in the urban legend catagory along with "over square".

 

[David-aviator]

......Bottom line, its my money paying for fuel and for maintenance. If I make it to anywhere near rated TBO, I'll put all these dire LOP warnings in the urban legend catagory along with "over square".

Fare statement. But stuff does happen and sometimes quite early.

I sold my last airplane to a friend who operates LOP with electronic ignition. He also experienced the failure of the #4 cylinder at about 600 hours since new. I believe it was a valve failure. The engine did not quit but produced enough power to make it around the pattern and land.

I tend to view Lycoming's Flyer as the bible with regard to operating these engines. The publication is loaded with a lot of very practical information and if observed, will result in a long and prosperous relationship with the engine. Making TBO is not a big deal if one listens to what the people who invented the engine have to say about it.

At this point, operating LOP is not on their list of recommendations except for fuel injected engines producing more than 250 HP. Other than that the statement is "leaning past the peak is not recommended", and "it isn't possible to with float-type carburetors because of the fuel/air distribution problem".

The trade off of fuel saved verses unknown service issues does not seem worth the experimentation with LOP outside the engine manufacturers recommended operating box.

 

[Alan Carroll]

I am in the carburated world, not expensively balanced injectors, so when I lean out to the point where one cylinder is about to quit, there is another one that is left up high reporting a much hotter EGT. (I should add that the running rough band is very narrow. I put this down to the P-mags. They provide smooth ignition until the cylinder goes off line. The rough band is VERY narrow.)

I'm curious, is EGT still going up on the hot cylinder when you start to get roughness, or are all 4 EGTs going down?

On my O-360 I can get the EGTs to stay pretty close to each other (within 20? sometimes) on the rich side of peak, but I've never been able to lean to the point where all 4 cylinders have peaked and EGT is decreasing.

 

[erich weaver]

I notice in Mike Bush's article, as I read it, the second rule is effectively unimportant provided the first is observed.

Its clear to me that Mike doesnt beleive in the first rule. Note this quote from his excellent article:

"At 75-percent cruise power, you want to stay well away from that worst-case mixture setting, either by operating at least 100°F ROP (preferably richer) or at least 20°F LOP (preferably leaner), take your pick."

By "worst-case mixture setting", he is referring to 50 degrees ROP.

erich

 

[penguin]

I tend to view Lycoming's Flyer as the bible with regard to operating these engines. ...

At this point, operating LOP is not on their list of recommendations except for fuel injected engines producing more than 250 HP. Other than that the statement is "leaning past the peak is not recommended", and "it isn't possible to with float-type carburetors because of the fuel/air distribution problem".

Everyone is entitled to their opinion, but stating that running LOP isn't possible with a carb is asking for trouble as many people have reported in these pages that they have done it. Its not as easy as with FI, but its definitely possible.

I agree that there is a lot of good stuff in Lycoming Flyer, but I don't believe just one source and try to combine information from several sources to try to figure out what is really happening. I believe John Deakin (read the Pelican's perch articles on AvWeb) and the people at GAMI when they say that operating LOP when you understand what you are doing will not hurt your engine and will probably extend the time you can run before it requires an overhaul.

From what I have read, as long as you are running under around 70% power and all the cylinders are past peak EGT (and still firing - not always possible),
then you're OK. The aim to to maintain an adequate detonation margin. Because of the less than optimum fuel distribution from one cylinder to the next with a carb, each cylinder will reach peak EGT at a different fuel flow as it reaches the required fuel/air ratio. Reducing throttle slightly and/or adding a little carb heat can help even up the fuel distribution, bringing the "GAMI spread" closer together (the difference in the fuel flow at which all the cylinders reach peak EGT reduces), which makes running LOP easier.

I don't know what the target margins lean of peak are, if I can get all 4 cylinders there and still all firing then I'm happy. I've no idea where they got 25F LOP from, but I believe the aim is to maintain detonation margin as (apparently) it is not always easy to tell your engine is detonating and it doesn't take much detonation to cause damage.

Hope this helps, Pete

 

[petehowell]

It Works!

Actual data from my carbed O-320 with dual EI. Runs cool and efficient. I can get similar results by running with carb heat or by cracking the throttle a bit. My "GAMI spread" is .2-.3 GPH with #4 being the only cyl that does not peak concurrently. I usually settle in at 150KTAS ~6.0 gph and 60-70degF LOP for cruise.

 

[Steve Sampson]

28 mpg LOP

Thanks for all the replies. They have stimulated me to do a bit more reading, and refresh my memory, and as Penguin said, he thinks the rule 2 I referred to when I started this thread, related to detonation. I am disappointed though that none of us have a really clear idea about this corner of operation of small carburated engines. (I suppose all the writing is in fact with the sale of Gami injectors in mind, so we get a little left out.)

I also think there are some very telling statements in Lyco flyer:

"Damage, to an engine from leaning does not occur at the manufacturer?s
recommended cruise power, but takes place at higher than cruise power."

"Lycoming allows leaning to peak EGT at 75% power and below on our direct-drive normally aspirated engines."

Admittedly they also say you cant run a carburated engine beyond peak and they dont reccomend it.

Since the way I want to operate in the cruise the engine is well below 60% I am going to carry on running it just as lean as I can. I must be very far from detonation.

Alan you asked what the EGT's were doing. Well here is a graph from a recent flight. I think you will be able to see that #1 and #2 have definitly dropped back, but #3 and #4, while the spread sheet says they have dropped I doubt you will see it on the graph.

I believe with enough juggling of the mixture lever I would get the fuel consumption down a bit further and you would see a clear drop on all cyls. (The red knob gearing is just too coarse.) I have been looking back through my records and I have got another flight where I got the fuel flow down another 5% and there is a clearer drop all the way around. The IAS is about 1mph less if anything. I do believe the P-mags really help with this lean operation.

The grey line is showing the miles per litre. In this case its about 7.4 or 28mpg(US) at about 160mph. I can do little better than this at height or with diligent further leaning.

What I don't see how to do is usefully use the carb heat. As soon as I apply that the MP drops because it is loosing the ram effect. Pete H, do you just reset the MP to allow for that or what?

(A separate issue you may spot. At full power the engine is a little starved of fuel. The solution for this is to increase the size of the nozzle. this will be done in due course. For now full power operations are very brief.)

 

[petehowell]

WOT

Steve,

I'm at WOT when using carb heat, so I do lose a bit of the ram pressure. If I don't use carb heat, I need to crack the throttle. On a long XC, I'll try both methods to see which yields the best MPG/speed combo. One usually is a bit better than the other.

It is interesting that I can use the throttle plate to "steer" the fuel towards the front or back cylinders to get a balanced mixture and smoother running. I do this by watching the EGT readouts on the Dynon EMS and varying the throttle position slightly. See if this works on your -4.

 

[rocketbob]

On some radial engines (I think the R-985 is one of them, on the twin beech) I remember something akin to a fan or a mesh that is there to create turbulence in the induction system.

On my list of things to do is to build a turbulator to go in-between the carb and the sump to see if fuel distribution can be improved.

My neighbor was having issues with cylinders running too hot/lean on his Jabiru powered Sonex, and the fix was to simply rotate the carb 30 degrees.

 

[49clipper]

49clipper

Guys,
I would like to jump in here with some info I learned awhile back. I work on and fly a C-177B with an O-360 and I have an RV-6 with an O-320-D1a. On the Cardinal it always ran a high egt on #1. I tried everthing to fix it to no awail and lycoming could not come up with any ideas I had not tried. On a recent flight I was leaned to cruise at 1550 egt and decided to add some carb heat to enrichen the mixture. Low and behold, not only did it lower all the egts, it evened them out on the jpi engine monitor. I then leaned again and saved 1.1 gph running at a lower egt. Wow. I then gave this a lot of thought and realized I had added turbulence to the airflow going into the carb and acquired better fuel distribution with helped dramatcially. So, when I got a chance, I tried in on my carbed O-320. It also helped, but I did not need as much carb heat and I only saved about .5gph. So, it does work, it does not harm your engine in any way, and it completely depends on the airbox, carb, engine induction system. Every one will probably be different. I am an AP/IA and have studied detonation and cylinders intensely over the last couple of years. The key is turbulence. Turbulence is good in carbs and intakes!