In my own studying, I have learnt much about mixture and it's control. I feel I have a healthy understanding of it and I started this thread to mine more information about it from the "real world uses" of it. That said, at 40 hours in, it hasn't factored much into my training. I want to change this. (I'm inferring that the school, much like others, prefer the rich operations for aforementioned reasons)

Can you clarify what you mean by "scrub 15 traffic circles in 45 minutes"?

A PPL is a license to learn, certainly on engine management.
Keep in mind that the POH is written by the engineers first, then modified to suit the marketing department, then the lawyers get their go. The message written by the engineers is very diluted or skewed.

I am surprised no one has really talked about the fuel saving, we have pretty good engine instrumentation om our RV7 including fuel flow indication, leaning properly in the cruise brings the fuel flow down by over 10L/hour, so by leaning properly you are not only being kinder to your engine but saving considerably on fuel - even if I am hiring wet I would rather have that fuel in the tank than not!

... the book figures for fuel consumption assume appropriate leaning, and you don't want to end up with less reserve than you planned for ...

If you are hiring from most clubs and use book figures you will amost certainly end up with less fuel than planned even with leaning, those figures are not done using a 20 year old aircraft with a 2.000 hour engine flown by an average club pilot! In fact as already pointed out in this thread many who hire club aircraft do NOT lean because they do not get the cost benefit of the fuel saving, which was the point of my post.

And this is why it's important to read the POH that belongs to the aircraft, and not something copied off the internet, or a generic pilots notes book, or something else.

Here's how things should work in theory:

The aircraft, at some point in time, was manufactured and certified to run on a certain number of (then available) fuels. The POH should include any notes if there's anything specific for these fuels. If, after release of the POH, a new fuel becomes available, then the manufacturer should form an opinion as to whether and how that fuel can be used. If it can be used despite the original POH saying no, then the manufacturer should bring out a POH supplement. This is added to the original POH and legally becomes part of it. It should list all the consequences so the supplement will have the same structure as the original POH: Description, limitations, emergency procedures, normal procedures, weight and balance, ... Some of these sections can be empty: Switching from 87 to 100LL will not have any effect on W&B for instance.

If you have an aircraft whose original POH said "87 octane only", and you don't have any supplement that says "100LL is OK too", then flying it on 100LL is simply illegal and should not be done. Regardless of what the FI tells you.

But this is why it's so important to read the actual POH from the actual aircraft you're going to fly. That's the one which contains all the applicable supplements. A generic one off the internet somehow won't.

And now what happens in the real world: There are practices that deviate from both the POH and its supplements. Sometimes that's a good thing (*), sometimes it's based on OWTs, and sometimes they're downright dangerous. As in the example above: If your FI taught you specifically not to lean, and you take the aircraft out for a long cross-country basing your fuel consumption on book numbers, you're going to end up in a field. Or worse.

(*) As an example, EASA decided to do a wholesale certification of a large series of engines, both Lycoming and Continental, to run on mogas. They effectively circumvented the whole manufacturers POH supplement procedure and brought out their own STCs.

My aircraft is an Emeraude built -59 I do not have a nice shiny POH to refer to.
I have been trying to find something on the net and have only come up with a generic engine management document for the C 90.
This document only says "lean above 5000ft" pretty much by revs/ear.
I run it on mogas and have had no problems for ten years.
I too was never taught to lean and have come to the conclusion that doing it by ear/trial and error is the only way that I can learn, or get someone to teach "his" version based on his experience.
People talking about damaging the engine by getting the procedure wrong is worrying to someone like me.
Fuel consumption I have discovered by trial and error has merely given me a set of figures based on fully rich. The flying budget tells me I should do better, reductions of 30% certainly seem attractive but not at the expense of a new engine if I screw it up.
Where does the likes of me go next, safely?

You will find the answers you seek in the link to the list of articles by John Deakin, supplied above. Your C90 would have 7.8:1 compression ratio if the original pistons are fitted and if C85 pistons were fitted then this would push the compression ratio up to 8.2:1. Whatever the case, you cannot damage the engine by leaning even with forecourt fuel as the octane rating is higher than the minimum required. (Check the engine data plate for the minimum octane required.)

Please study Deakin's article No 8; "Go Ahead Abuse Your Engine." Your engine will burn cleaner, be healthier and not be damaged as some scaremongers might say.

Backpacker - that sounds interesting!
 

Your point about EASA issuing their own STC for Mogas has appeal. I run an O-360A4M in a 172. Previously, if I wanted Mogas I had to pay a Mr. Petersen in USA for an STC. Where would I find the EASA version, what cost, and do I need any mods?

Sorry to ask so much! Regards, CT.

Curlytips, I've been searching for the EASA document but I can't find it anywhere anymore. Sorry.

Has an STC (US) ever been needed in the UK to use mogas? There was a CAA booklet, and an EASA document, listing the conditions for using filling station fuel in aircraft.
Now the CAA have washed their hands of any responsibility.

Is this perhaps the document you are looking for: CAP 747 Mandatory Requirements for Airworthiness dated 29 June 2012? I tried to upload the document as a PDF file but it is too large. Page 407 of 468, deals with motor gasoline (MOGAS.)

I took the relevant section and saved it as .DOC

My airplanes checklist tells you to lean on the ground immediately after starting - it says an inch (its a PA28-181).

In the air I lean at 2000ft and up - in my EGT I run hottest at around 1480 - it leaned all the way to get the peak then backed off/enriched (my airplane has a carb not injection so none of that lean of peak BS for me!) - when this is done at 8000ft+ the mixture is maybe 1/2" inch off the lean cut off position, v.scary.

I also found a 240 page EASA document that debated everything about risks etc (due ethanol etc) and talking about how to mitigate. But it also contains copies of STCs that say I can use it. So with that and Chuck's info, I feel quite comfortable about legality of usage.

However, I'd prefer to stay with the aviation organisations that supply mogas (so it has the tighter specification), but airfields with pumps seem rare. Is there a current list of who has, and pricing?

Failing that, what fields do you know apart from Sywell? Thanks all! CT.

Pulling the red lever back in cruise is good, lean to peak EGT with smooth running and LEAVE IT THERE. (Lycoming's advice, not mine.) With a carb engine you CAN run LOP, so it is not BS; you just have to live with an occasional, irregular rumbling due to unequal air / fuel distribution.

At 8000' your mpg economy should be better than 2000' so as long as the engine runs smoothly, do not worry about the position of the red knob. After all, you are not at sea level. :)

You can lean at any altitude. The absolute EGT number has no value except to determine peak, at what point it starts to fall. The hottest EGT is NOT the hottest exhaust valve temp. Think of EGT as a navigation instrument, you are at peak, before or past.

With a carb, you can try to lean slightly past peak and if not smooth, use a little carb heat to help fuel vapourisation. Please read John Deakin's articles. Once you understand you will not go back to the old way and do you engine and you bank account a favour at the same time.

Crash one , not able to link in the O-200 manual , which also has C-90 stuff in .
Google it TCM O-200 overhaul manual . Lots of stuff in it inc. leaning procedures .
We lean appropriately an O-200 .

rgds condor .

Thanks. I think that's the one I got.
Operators manual for A&C engines and O 200.
Includes A65--C90 and O200.

I said earlier that I've run the C90 for ten years on Mogas with no trouble.
True it's never missed a beat, but I have replaced both rear cylinders due to exhaust valve serious wear during that time. Noticed by a lack of power slowly creeping up over months.
One of them I could see daylight between the guide and stem!
Is this mechanical wear caused by hard carbon build up on the stem/guide?
What are the bad effects of running full rich all the time?
All the plugs are black, no sign of the brown that I would prefer.

Do a search for this article: Lean of Peak: Ignorance Returns (Or Maybe It Never Left)
Well written, with links.

Black plugs mean you're running rich, you are rich, or both.;)

I am less rich than I would have been had I not run rich, so if I stop running rich I will become richer.

Great post. Thank you.

I have a Lycoming O-320 E2D with standard low compression pistons that produces 150 hp.
All I have is an exhaust temp gauge on the rear left cylinder exhaust.
I run 100% 87 octane Mogas with upper cylinder lubricant.

The articles all seem to be geared towards more complicated engines with full engine analyzers.

I would like to learn how to run 'lean of peak' for long cross country's.
From now on I will always lean aggressively on the ground.
I will also always use full power for take off.

My question is:
If I fly at a slow between 2200 and 2400 rpm.
Can I just pull the mixture back until it runs rough and the push it back in until it smooths out.
Making sure to watch the exhaust temp first rise and then fall back.
I understand that you can not hurt these older low compression smaller engines with the mixture control?
Is that correct!

Yes,yes,yes.:ok:

I agree about the articles being mostly concerned with big engines, well monitored, CHT and EGT gauges etc. However, today is try things out day with more confidence.

Yes. Richen only enough for smooth running, no more. At the low power setting you mention, no matter what you do with the red knob, you cannot cause damage. A lean engine is a clean engine, keep it that way. :)

Gave it a go today. First time I've used the red knob. On the ground I killed it once or twice, not quick enough pushing back in as it faltered.
Airborne pretty much a non event as the speed keeps it turning. I noticed the lack of throttle response when lean but a quick push back to rich was no problem, ref the "mixture rich first when increasing throttle". Getting that back to front did no harm except to run rough till I richend it.
All in all not a problem.

I flew my O-320E2D for four and a half hours today, playing with the red knob all day.
Not having much experience in general aviation other than the standard pilots licenses some forty years ago.
Apparently I was rather unfamiliar with the mixture control.
I can't remember it's use even being taught.

I'm glad I installed a vernier mixture control cable during the aircraft's major overhaul.
I also installed a monster throttle cable, all from McFarlane Aviation.
I wish I had installed a full engine analyzer instead of an single exhaust probe.
But it is just a primary/intermediate trainer for my children to use.

No problems with leaning on the ground.
But once airborne it wouldn't go past peak temp without running very rough.
When I got back, I changed the oil, which at 25 hours was due.
Now the cowlings are off, I will clean and rotate the spark plugs.
Then I'll see if I can get it to run lean of peak.

Then I will install THIS, which arrived last week.

If you are concerned about high cylinder head temperatures ruining your engine.
May I suggest a exhaust pipe shroud cover from Maple Leaf Aviation in Canada.
This cover stops air entering the lower engine cowl around the exhaust pipe.
The more efficient venture effect increases the airflow of the engine compartment exhaust, allegedly lowering cylinder head temperatures.
And it;s pretty cool looking too.

This shouldn't be an issue with running 'lean of peak' as this is supposed to lower cylinder head temperatures from peak anyway.
But from my limited experience.
Peak exhaust temperature which is at the beginning of the economy zone is more attainable than the lower end of the spectrum.
I think that your magneto, harness and spark plugs must be in tip-top condition to get the full drop from peak, and still run smoooooth.
There is no fuel saving from peak temperature over full lean of peak, just higher cylinder head temperatures.

I may have to stop thermal shock in the winter time if it is too effective by keeping the power on during approach.

Anyway, thank you for your help and input in educating me.

If a carburettor engine runs rough when leaned, it means all cylinders are not developing same power due to uneven fuel/air distribution. You may help this by applying a little carb heat. Also changing throttle position may help. Maybe your spark plugs are not performing well enough.
To test the ignition system do a LOP may test in flight. At safe altitude, check one mag, take hand off switch and wait a few seconds. If the engine would run very rough or quit, pull the mixture to cut-off, put may switch back to both and slowly reopen mixture. I improved my lop running dramatically by switching brands of spark plugs to Tempest.

It's great to see folks putting these practices into their flying.

If these engines are running richer than necessary because of the engine/carb design, I'm thinking that if the throttle is set to max static revs on the ground with mixture fully rich, then leaned to best revs, this would not be " fully rich" but would depend on elevation, pressure, density, temp etc? and should give best take off power on that particular day/place?
I'm not trying to rock the boat suggesting oddball ideas and I doubt if this would save anything worthwhile in fuel but, if the theory is correct then it would help me get my head completely round understanding this.

Crash One, the engines are not just running rich because of engine/carb design. The most important reason for running overly rich (at full mixture forward) is due to heating. The engines we're talking about are air cooled, but they do not have sufficient cooling capability in their cooling fins to keep the engine cool when it's producing 100% power. So we run the mixture extra rich at that power setting. The fuel that's not burned helps cool the engine through evaporation.

What you're suggesting is entirely correct (for a fixed pitch prop - for a CS prop you'd use a slightly different method as you can't use RPMs as your indicator). You can run up the engine to full throttle, then lean for best power (max RPM) to get the best performance out of your engine. However, in that situation the engine is developing 100% power so it'll heat up very quickly to beyond the maximum allowable values (CHT mostly) which may lead to detonation and other sorts of engine damage. So don't do this when you care about your engine.

Having said that, that procedure is exactly what you would do in a high density altitude (approximately > 5000' DA) situation, such as when you're taking off from an airfield in the mountains. In that case, the density altitude is such that the aircraft cannot produce more than about 70% of rated power anyway, and you need all the power you can get from the engine in order to fly before you run out of runway.

(And to help your understanding even further: I was flying as a passenger in a kitplane with very sophisticated engine controls. We took off from a sealevel airport and even during the early stages of the departure the pilot started leaning. However, he did not lean for peak EGT, max RPM or anything like that. He only leaned just a bit and used the CHT temperature as his most important indicator. For every twist of the vernier mixture control his CHTs would rise a few degrees, and he leaned so that his CHTs were just below the yellow bar. Through this, he achieved a few percent more power, and a bit of fuel saving, without the risk of engine damage. So yes, you can even lean when the engine is operating at 100% power, but you really have to be careful and know what you're looking for. And it would be foolish to attempt this in a carbureted engine with just one CHT probe.)

I have Tempest spark plugs.
They were not new with the engine rebuild.
I put them into the engine after I first bought the plane.
I was trying to figure out why it ran rougher than I remember.

It was an estate sale.
The owner bought a new motorcycle the day before.

It was sold as 1350 since major overhaul.
But it wasn't.
It was a field overhaul, in 1981.
Then I realized the engine was a worn out piece of junk.
Sintered iron oil pump gears, AD'd in 1979.
Slick 4000 mags last overhauled in 1985.
They went in the dumpster and new Bendix's replaced them.

Engine, carburetor, magnetos, harness were all replaced/rebuilt.
I'm trying to get these plugs to 200 hours.
I have a new set ready on standby.

I'll try a LOP test when I go again.
But first some maintenance and upgrades.

Backpacker.
Thank you for that. It confirms my beliefs.
Years ago in the days of SU carburettors where you could adjust the mixture by screwing the needle valve up or down, not getting full power or running too hot if it was too lean etc.
Though too rich seemed to equate to too cold, lean ran too hot. This was of course in water cooled engines so the heat/cooling issues were solved otherwise.

"The fuel that's not burned helps cool the engine through evaporation." Not very convincing explanation:

The reason a lean or very rich mixture cools the CHT/EGT is that they slow down the flame front. The fuel/air mixture takes longer to burn, placing the power pulse later in the cycle, where the piston is moving more downwards already, expanding more, cooling down more.

Peak EGT and peak CHT are not coincident, they do not occur at the same mixture. The important one is the CHT since it reflects the long term heat dissipation of the head. Peak EGT is already past peak CHT and is a good place to run at 65% or below.

Trying hard to remember the sixties and referring to the BSA 650 twin I had as a "fuel cooled oil burner". Leaning the mixture too much made it run hot. If the ignition was too far retarded or too far advanced it ran hot, though retarded was worse, I think?
The Hilman Imp boiled dry every ten miles on the way from Edinburgh to Cornwall so we stopped in the middle of the Borders and re set the timing, using fence wire and the interior light bulb. The radiator cap could be removed after 100 miles engine still running!
Happy days.

Here is another excellent starting page:https://www.avweb.com/news/pelican/182179-1.html

Pay attention to the graph that is near the bottom of this page. There is a lot of information there how EGT/CHT/FF and HP relate to each other. Hint: you can only see EGT (maybe) and CHT in the cockpit, you have to navigate by those.

Hi Crash One, I don't think any of this carburettor tuning procedure is relevant to car or Motor-cycle engines.. This is because the aero engines operate for long periods of time at a fixed high throttle opening.


Motor Bike riders are constantly varying the throttle to maintain cornering stability etc, and require instant response to a snap throttle opening. This requires a richer mixture to be already present, to ensure correct power delivery.


Also I have an almost Concours vintage motor cycle that has always been run rich. It suffered a blocked slow running jet one day, and caused the chrome exhaust pipe to become coloured because of the increased EGT... A replacement exhaust pipe will cost me over £50.


There are other IC Engines that use much different tuning techniques to ensure safe rpm operation.
.

Quite right carb tuning is irrelevant. I was just remembering that it could be done in days of yore and by ear, no computers.
By the way, ref to blue exhausts, I thought we used to polish it out with Sovol Autosol? Toothpaste was just as good. Depends how bad it is I think.

Thanks for the Toothpaste tip, it did remove most of the colouration.


Just thinking on about the other engines which will not tolerate lean mixtures... Model aero engines are always tuned for Maximum revs, then Richened half a turn to loose about 1000 rpm... say from 15000 to 14000. If this is not done, the first time the throttle is quickly advanced, the engine will suffer a 'lean cut' and will stop.


I think you need to pay very close attention to running a GA engine lean, especially if you have set it in the cruise at FL100, and then descended a few thousand feet.


Most cars these days have the economy mode that runs the mixture very lean, with the aid of the exhaust lambda probe, but this adversely affects the car's acceleration compared with the normal power mode.