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

dvukovic wrote:

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

Yes, but technically it’s still half cooked. CHT is mostly a function of cooling vs power. Reducing the power, as you do when leaning (LOP), will of course reduce CHT. But, you can reduce CHT also by reducing throttle, and when reducing throttle the fuel flow will naturally go down. What you really are looking for in terms of fuel burn is peak thermal efficiency, the point where the engine produce the most HP per fuel unit.

The engine doesn’t care if the reduced CHT is because of reduced throttle, leaner burn or increased cooling.

Regarding fuel flow and CHT, he should also compare the same effect with various mixture, throttle and rpm settings for the same IAS. Most of the decrease in fuel flow he gets, is in fact due to decreased power (at the risk of large imbalance in power between cylinders), not increased thermal efficiency. Besides, sometimes you want max power.

There is only one way to optimally operate an engine. Install multipoint EFI, preferably FADEC with a single lever control. If that is not an option, I would think the next best thing is to at least make sure all cylinders are balanced in terms of power. Running LOP is the best way to be sure they are not. Just my 0.02 $

Any petrol engine leaned for peak EGT (or thereabouts – the SFC curve is fairly flat around there) and with appropriate ignition timing will produce about the same power for a given fuel flow, minus mechanical losses.

It is really quite accurate and consistent. If I set 11.7 GPH at say 2000ft, 23", 2400rpm, peak EGT, I get 138kt IAS. There is nothing sloppy about the old Lyco etc engines I am sure the old steam engines were just as good.

There may be second order effects due to ambient temperature but the fuel flow determines the HP going into the propeller, in a very predictable manner.

It is true that many carburretted engines can’t go to peak EGT (or LOP) without getting too rough.

Administrator
Shoreham EGKA, United Kingdom

The main reason LOP CHTs are lower than equivalent ROP settings is this:

1. a leaner mixture burns slower than a richer mixture. it takes more time (milliseconds) for the flame front to propagate and raise the Internal Combustion Pressure .
2. If this happens later in the combustion cycle, the piston will have moved more downwards, having less compression at the moment of the burn. Compression = heat.
3. it will also help the mechanical advantage if the piston is being pushed downwards farther from the TDC.

As for fuel being less expensive than engines, I am not so sure.
I am on my third IO520 engine, halfway to TBO. The first was run before LOP was teached. The second was run LOP about 200 hours before TB0 +20%. The third is now at 1200 hours since blueprinted overhaul. All fliying except takeoff and climb is done LOP.
The amount of fuel saved by operation LOP will pay for the overhaul at a top quality shop at TBO.

EBKT

Archer-181 wrote:

To operate lean of peak don’t you need matched injectors?

It is an advantage but not totally required. John Deakin will tell you that it is worth having them and I agree. (John had the very first GAMI Injectors installed in his Bonanza)

I was led to believe this cant work on a PA28-181 which has a carb?

Not the same way it does with an injected engine. I asked the very same question to the man himself a couple of years ago and he gave me the following advice (airplane without an analyzer):

  • Lean the engine to the onset of roughness.
  • Pull carb heat until roughness stops
  • Continue leaning until roughness sets on again and then add a bit of mixture to stop the roughness.

This will reduce fuel flow dramatically (3-4 lph) with slightly lower EGT (Single probe) and lower CHT. According to John this is LOP. Not widely used and it came from one of his specialists within their seminar company but it works for me. One sign it is actually quite good is that we have much cleaner spark plugs ever since we use this.

LSZH, Switzerland

Mooney_Driver wrote:

Pull carb heat until roughness stops

Then you will operate with partial carburator heat. I’ve always read that you should not do this unless you have a carburator temperature gauge. The reason is that if the air temperature is too low for carb ice to form, you may bring it into a dangerous region by using partial carb heat.

ESKC (Uppsala/Sundbro), Sweden

If the air temperature is too low for carb ice to form, is that because saturated air at that temperature has too little water to cause carb ice as it cools more?
If so, how does raising the air temperature, with no source of water vapour, increase the risk of carb ice?

Maoraigh
EGPE, United Kingdom

Any move directly to the right appears to be a good thing.

Last Edited by Silvaire at 12 Oct 19:55

That diagram has usually been published with the axes swapped over

Administrator
Shoreham EGKA, United Kingdom

dirkdj wrote:

1. a leaner mixture burns slower than a richer mixture. it takes more time (milliseconds) for the flame front to propagate and raise the Internal Combustion Pressure .
2. If this happens later in the combustion cycle, the piston will have moved more downwards, having less compression at the moment of the burn. Compression = heat.
3. it will also help the mechanical advantage if the piston is being pushed downwards farther from the TDC.

Where have you “learned” this? As far as I can see less fuel = less heat. The power goes down, not up when running LOP.

Did I say anywhere that power increases when LOP?
Efficiency goes up and BSFC goes down when LOP, if turbocharged you can recover most of the power loss by increasing MAP and run at the same HP on less fuel.
Having a slower burning fuel/air mixture has a beneficial effect on the power output since the power pulse comes at a better point in the combustion cycle from mechanical advantage point.

EBKT
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