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Engine management / leaning / peak / lean of peak (merged)

Pilot-H wrote:

The best way to run LOP is wide open throttle. That gives the best possible airflow. Then pull back the mixture to 60 – 65% power (there is a specific fuel flow for LOP power)

With wide open throttle, air flow through the engine will also vary roughly in proportion to RPM. So assuming a specific and fixed fuel flow the amount to which you are LOP, and therefore the efficiency, will vary using when this technique depending on RPM chosen by the pilot. This sounds to me on the face of it like a Cirrus or fixed pitch propeller technique, meaning RPM is assumed to be outside of pilot control. Or not?

This discussion seems to be slowly convincing me that when cruise power is required (e.g. ~65%) at a normal cruise altitude (say 8000 ft or above) you achieve best efficiency and lower engine temps by (1) selecting the RPM that will give you roughly the power you need at the chosen altitude (with full throttle) and (2) leaning as far as you can without excessive engine roughness. Then watch CHT to make sure it stays under your preferred limit. EGT indication apparently doesn’t have much value as long as you lean aggressively so that you’re not rich of peak.

(You could also advantageously advance the spark timing at 8000 ft or above before doing any of that, but that would be a different discussion and new hardware)

Last Edited by Silvaire at 18 Dec 03:22

So, what is the simple and advise on leaning we could give to renters in old Pipers and Cessna’s that have no engine monitoring instruments except maybe or maybe not one EGT? Most of them fly no higher than 2000 or 3000 feet and at most pull the red lever back a little without knowing what is happening as a result?

EDLE, Netherlands

The best way to run LOP is wide open throttle

That is how all high altitude flight is done in non-turbo Lyco/Conti planes, but you cannot do that below about 8000ft because you need to be below 75% power to lean to peak EGT. That rules out the vast majority of non IR holding pilots.

Welcome to EuroGA, @pilot-h

You could also advantageously advance the spark timing at 8000 ft or above before doing any of that, but that would be a different discussion and new hardware

I reckon this is why running at a low rpm when running LOP does deliver a bit more MPG, when it shouldn’t really (because power can come only from burning fuel). The lower RPM ignites the mixture further back in time before TDC.

what is the simple and advise on leaning we could give to renters in old Pipers and Cessna’s that have no engine monitoring instruments except maybe or maybe not one EGT?

Once in cruise, lean till the IAS (or RPM) suddenly drops (you can see it, feel it and hear it) or until the onset of roughness if that occurs earlier, and then enrich just enough to restore it.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

The lower RPM ignites the mixture further back in time before TDC.

I don’t think so. The spark still occurs around 21° before TDC (fixed timing), but the flame front is slowed by the leaner mixture so peak pressure takes place later after TDC and the pressure is exerted for longer.

LFPT, LFPN

Silvaire wrote:

Power is always a function of fuel flow, plus air flow. You can’t burn fuel without air. EGT is a function of the proportion of fuel flow to air flow, even with excess air (i.e. lean of peak)

Yes it is, but you do not need a table to figure out your power output when running LOP.

Also those power tables usually suppose that you lean for best power. If you are richer or leaner than that, you get less power than what the table says, and possibly quite significantly so.

Silvaire wrote:

This discussion seems to be slowly convincing me that when cruise power is required (e.g. ~65%) at a normal cruise altitude (say 8000 ft or above) you achieve best efficiency and lower engine temps by (1) selecting the RPM that will give you roughly the power you need at the chosen altitude (with full throttle) and (2) leaning as far as you can without excessive engine roughness. Then watch CHT to make sure it stays under your preferred limit. EGT indication apparently doesn’t have much value as long as you lean aggressively so that you’re not rich of peak.

On the NA SR22 at 8000’ I set 75% (WOT, 2500 RPM, leaned ROP, about 17 gph) and then lean to a FF of about 13.5 gph. That gives my target 65% BHP (the NA engine outputs 14.9 HP/gph) and produces cylinder temperatures of about 310 ℉. I save 2 gph compared to the ROP best power setting in the tables, which is not much, but I increase my range and run the engine significantly cooler and cleaner.

In order to obtain WOT on the SR22 you need to pull back the power lever some (to get it below 2500) and then push forward until you no longer see any increase in MP. That should give you 2500 RPM.

LFPT, LFPN

Yes it is, but you do not need a table to figure out your power output when running LOP.

No, you have given the answer just below for operating the SR22 LOP. There:

% Power = 14.9 x fuel flow in gph / 310 (BHP)

It works because LOP, power is only a function of fuel flow, since all fuel is burned.

If you are richer or leaner than that, you get less power than what the table says, and possibly quite significantly so.

If for a given fuel flow, you run leaner (i.e. adding more air to the same amount of fuel), you’ll get the exact same amount of power. Your engine will just run cooler and eventually rougher.

Last Edited by Rwy20 at 18 Dec 12:19

I don’t think so. The spark still occurs around 21° before TDC (fixed timing), but the flame front is slowed by the leaner mixture so peak pressure takes place later after TDC and the pressure is exerted for longer.

I think what I said is the same thing

A leaner mixture burns slower, so it needs an earlier (in time) spark, which is what the lower rpm achieves.

The effect is small – only a few %.

Administrator
Shoreham EGKA, United Kingdom

Rwy20 wrote:

It works because LOP, power is only a function of fuel flow, since all fuel is burned

Excess air (lean of stoichiometric mixture) when lean of peak does mean that all the fuel gets burned… except that when you go too lean, it doesn’t all get burned any more. In that case I think the fire can no longer spread smoothly across the combustion chamber through pockets of extra lean mixture. As a result, and regardless of detailed cause, specific fuel consumption starts to rise again when you go far lean of peak – the data in Post #5 shows that SFC is not constant when lean of peak. I would therefore maintain that you need to set the right MP and RPM before setting fuel flow to maximize the efficiency benefits of running LOP, particularly at a given fuel flow, and to prevent misfire.

Having cogitated on that for a while I will go the airport and go flying

Last Edited by Silvaire at 18 Dec 14:19

The main reason for specific fuel consumption increasing when going further lean of peak is the delay in achieving full burn and hence lower peak cylinder pressure, which means slightly less work gets done. When you reach the point where parts of the mixture won’t burn at all, the engine will run very rough.

But simply put – for any given air density and RPM there is exactly one fuel flow that will give you peak fuel efficiency on the lean side, and exactly one fuel flow that will be best power on the rich side, which will deliver MORE power.

The best way to get to that point is to simply set the desired RPM and manifold pressure, and then pull the mixture lever to exactly that fuel flow. Job done. No need to fiddle with EGTs at all. Just make sure you take temperature into account, either by adjusting the manifold pressure for temperature, or by adjusting the fuel flow.

In a modern installation for LOP, such as in the SR22TN and T, the G1000 calculates a target fuel flow and you just pull the mixture to that point.

If you have to cook your own method, because the POH or engine manual was written in the dark ages of mixture management, you will probably want to spend some time establishing these fuel flows using EGT, and write them down for future reference.

Biggin Hill

AeroPlus wrote:

Also, the rough running engine when you lean is just an indication of one cylinder running different from the others and not directly a sign that the engine is being “starved” of fuel.

No matter how you look at it, if the engine starts to run rough due to leaning, it is indeed because it is being starved of fuel. Then richening until it runs fine again, and it will operate as lean as it is possible to run without stumbling. This will normally be as “LOP” as it is possible. There may be uneven distribution for sure, but at least one cylinder has too little fuel to run properly. In a fixed pitch prop, you can see the RPM drop rapidly also. The engine is quitting, not just getting unbalanced or “rough”.

There is no way of knowing exactly where that is happening without proper instrumentation, but who cares? the engines isn’t capable of running leaner and still produce power. So you know you are around the best efficiency point somewhere, as good as the engine is capable of doing. You are in cruise anyway, 50-80% power or something, no chance of overheating.

In a modern engine with multipoint electronic injection and tuned inlet manifolds, you can theoretically go way leaner.

The elephant is the circulation
ENVA ENOP ENMO, Norway
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