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Leaning - will a correctly leaned engine always end up with the same fuel consumption?

I have a question. This is for a piston, non turbo, carburetor engine leaned to rich of peak only between 2,000 – 5,000 feet [I am not asking about 20,000 feet]. Fixed prop

In the cruise between say 2,000 – 5,000 feet when I am correctly leaned (rich of peak) at my normal cruise of 2450 RPM, I always seem to end up burning 31- 32 liters per hour (The EDM is accurate to about 2%). That is still air/ good conditions. Sometimes it might be a bit more but I’m not sure this is because I can’t be bothered to lean properly.

So the questions are as below. To summarise, would it make sense to always have the same fuel consumption at the same RPM in this narrow range of heights?

Question 1 – If you are higher (only in this narrow range up to 5,000 ft) the engine should be slightly less efficient due to thinner air. But then there is less air friction on the Aircraft so perhaps these two things cancel out?

Question 2 – Could Tail wind/ Head wind make any difference? I don’t seem to notice much difference and I can’t immediately think why it should.

Question 3 – Temperature – but perhaps this is similar to question 1 as we are talking about the spacing of air molecules.

Any thoughts?

United Kingdom

I recently saw a graph of EGT and other measurements for different temperatures. I cannot say to which engine it applied or if it was engine/airframe specific. However does any one know if such a graph may exist for a Continetal IO360 or am I “barking up the wrong tree” here. I am interested in knowing what is the peak EGT for example and therefore staying rich of this.
Thanks

UK, United Kingdom

Archer-181 wrote:

Question 2 – Could Tail wind/ Head wind make any difference? I don’t seem to notice much difference and I can’t immediately think why it should.

No, because the plane is only moving through the air. So long as the airspeed is the same, the ground speed doesn’t matter.

Archer-181 wrote:

Question 1 – If you are higher (only in this narrow range up to 5,000 ft) the engine should be slightly less efficient due to thinner air. But then there is less air friction on the Aircraft so perhaps these two things cancel out?

You might see some variation in fuel efficiency in terms of l/nm, but in terms of l/hr, it should be the same or perhaps decrease ever so slightly.

At the precision level that Lycoming type engines operate, leaning is just setting the fuel air mixture, so the more air you move through the carb, the more fuel you burn.

At 2000 ft, air has a density of 1.155kg/m3, versus 1.056kg/m3 at 5000ft altitude, which is an 8.5% difference. Assuming you don’t have too many other changes, (better cooling at low altitude, higher temperatures at low altitude, non linear combustion issues, etc), you should see no more than an 8.5% difference between flying at 2kft vs at 5kft. However, this assumes full throttle and manifold pressure=ambient pressure.

In reality, because your engine is throttled, the manifold pressure should be more or less constant, and thus the fuel burn more or less constant.

If you look at the 172 POH, you can see that above 8000ft the engine can produce no more than 75% power, with a similar decrease in fuel burn. So, you would need to look at the POH for your plane, but my guess is that the reason you see constant fuel burn is that the manifold pressure is constant at the altitudes you’re looking at.

United States

Thanks redRover – I think I actually understood all of this a couple of years ago – apart from being in the POH, in the Archer’s it is all written on the sun visor!

I think I’ve half forgotten it all as for my day to day flying it’s not much use! Thanks for reminding me and also for the explanation of the manifold pressure. I hadn’t thought about that and it seems to make a lot of sense.

United Kingdom
when I am correctly leaned (rich of peak) at my normal cruise

By the way, below 75% power no amount of leaning will damage the engine, so you can safely operate lean of peak and save fuel.

I recently saw a graph of EGT and other measurements for different temperatures. I cannot say to which engine it applied or if it was engine/airframe specific. However does any one know if such a graph may exist for a Continetal IO360 or am I “barking up the wrong tree” here. I am interested in knowing what is the peak EGT for example and therefore staying rich of this.

These graphs are relative. EGT may vary from aircraft to aircraft, there is no universal peak value.

LKBU near Prague, Czech Republic

Yes, it should be the same, on fixed pitch propeller, for a given rpm setting = power %, a correctly max leaned engine will give the same gallons or litres per hour

The intuition on question 1 is correct, for fixed pitch (FP) as energy/h = horse power = rpm, so fuel consumption should be the same but you need to keep that right molecular mix as air density goes down (this is what is leaning is about, not to save fuel but to use more air !)

For variable pitch (VP), with a given power % = RPM & manifold pressure (MP) and max lean, you should get the same fuel burn

What you can’t acheive is max power = 29’ in MP for VPs or max RPM for FP as you climb for the reason you mentioned…

Not sure if someone will actually do leaning using a target fuel burn for given power % =
rpm setting (PoH says: 10 gal/h or 8.8g/h) and a fuel flow meter? In your PA28, I think I would stop leaning at when fuel flow is at 33l/h irrespective of hight

Last Edited by Ibra at 11 Oct 22:41
ESSEX, United Kingdom

I came across a very good video that explains leaning of the engine:


Last Edited by dvukovic at 11 Oct 22:40
Belgrade LYBE, Serbia

To operate lean of peak don’t you need matched injectors? I was led to believe this cant work on a PA28-181 which has a carb?

United Kingdom

dvukovic wrote:

I came across a very good video that explains leaning of the engine

Thanks, interesting. Additionally the Bonanza looks like a really nice plane.

I like this video for leaning info:


has a Beagle...
LOWG Graz Austria

The RPM of a carburetted engine will affect the mixture distribution. Generally, if you find an RPM for which the mixture distribution is equal between the cylinders, that will work each time, so operate at that RPM as much as you can. I choose only to operate LOP when fuel consumption is critical, which, of course, I try to prevent with good flight planning! Fuel is the cheapest, and most reliable cooling possible to reduce the risk of engine overheat damage.

Home runway, in central Ontario, Canada
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