There is a lot of black box voodoo about LoP flying, it's really simple. I think I can explain in one short email. The key is to first describe one cylinder, than introduce complexity of other cylinders.
Standard acronyms
WOT wide open throttle
FF fuel flow
MP manifold pressure
For a single cylinder , you have 2 variables to mutate: MP and FF. We will do the test at cruise altitude at WOT, so we are down to 1 variable. We cannot control engine timing, valve timing, nothing else, it is a closed system.
There are only 3 sensors to see how we are doing. EGT, CHT, RPM.
We will determine 2 parameter using an engine monitor:
- Fuel flow at peak exhaust temperature
- Temperature at peak exhaust temperature
To find these 2 numbers is easy - either start very rich fuel flow and decrease, watching EGT rise, and note the fuel flow and temp when it peaks and starts to fall.
Recall that when fuel flow is rich, excess unburned fuel cools down the combustion.
And recall that when fuel is lean, the combustion burns cooler, simply because there are fewer fuel molecules to ignite the oxygen molecules.
Peak ff and temp will depend on atmospheric conditions alone, so when you change altitude, or it gets warmer, cooler, more or less moist, it will vary slightly. Otherwise given same atmospheric conditions it is always the same (why would it not be?)
So let's say we very rich and lean the fuel flow, watching EGT rise, and it peaks at 1500 degrees and starts to fall.
Note the FF at the peak, let's say 13gph.
Now we can continue to lean until we are 50 degrees LoP, so we're looking for 1450 degrees on that cylinder.
Internally what is happening in cylinder?
Running 25' RoP is the highest cylinder pressure, or power of comustion. it is the highest stress on the engine components, and stress shows as cylinder head temperature.
When we run LoP we are simply combusting fewer particles. Less power, less fuel wasted, lower cylinder pressures, which should equate to lower CHT. YES! You will see lower CHT in LoP flying vs RoP. This is the first benefit. You are saving your engine's life.
So at this point you should have good understanding of the simple chemistry of cylinder, and how to measure peeak and how to get to 50 degrees LoP.
What's this fuss about burning out exhaust valves with LoP flying? We already know CHT is lower, which is better.
We have these great instruments - EGT - you can see the temperature of your exhaust gas as it enters the exhaust valve.
I love ti when someone tells me I am burning out my exhaust valves running LoP because i Simply ask them what their exhaust temperature is running RoP vs LoP, they give me a blank stare.They have never actually looked at the difference. I calmly tell them my exhaust temperature is almost identical, therefore, no valve damage. I also tell them my CHT runs lower in LoP flying.
Now we don't have 1 cylinder, we have 6.
When you run RoP, how do you do it?
You start super rich, you lean until the first cylinder peaks, you keep leaning until the last one peaks, then you enrichen until the average temperature is somewhere around 50 degrees RoP?
This is the absolute worst thing you can do. By using this 'average' method you have only 1 cylinder running 50 degrees rich of peak.
Since each cylinder behaves slightly differently what you've done is configured 2 cylinders to be edxtra rich (probably 75-100 degrees RoP), and the other 2 are running 0-25 degrees RoP.
So 2 of your cylinders are being operated at the highest cylinder pressure, highest CHT, they are going to be looking at a top overhaul first.
There are 3 ways you can set your temperature. You should be doing this very deliberately, not guessing and averaging.
- Set your LEANEST cylinder to be 50 degrees rich of peak, and all the others are richer. This is quite wasteful on fuel, but you won't end up with $15,000 top overhaul jobs.
- Set your RICHEST cylinder to 50 degrees lean of peak, the others will be leaner, in the danger zone. You'll get better fuel consumption and really great power output. You're destroying all but 1 cylinder by flying like this.
- Set the average, and you'll end up replacing 2 cylindres before the rest.
Now if you fly high alttiude, over 12500', none of thiis damage will occur. Your engine is only produciing about 55% power, you can run all cyolinders at peak temps, you'll see you will have low CHT because of the low power output, so you don't have to worry about any of it. One big advantage of flying high. with a non turbo.
Now if you're flying LoP at low altitude (8500) you have to follow the same rule. Set the leanest cylinder to be 50 degrees LoP, and the rest leaner.
But how much leaner will they be? this depends on the engine. In 2 minutes ou can figure out exactly how good your engine is. Grab a pen and paper and make a 3 column table:.
CYLINDER # | FUEL FLOW | EGT
You can start from lean or rich (it's better to start from lean! less damage to your engine).
Start from very lean when engine is running a little rough. Increase fuel flow. Timing is critical, you want to increase about 0.1gph every 2 seconds. At some point you will see PEAK, let's say cylinder #4 peaks first. Write down:
CYLINDER # | FUEL FLOW | EGT
4 13.7 1410
great! first cylinder done. Keep leaning and keep writing. You'll end up with 6 rows..
CYLINDER # | FUEL FLOW | EGT
4 13.7 1410
2 | 13.9 1420
1 14.2 1430
3 14.4 1430
6 14.6 1430
5 14.8 1400
Looking at this chart I can now infer a few interesting details:
The fuel flow spread of my engine is 14.8-13.7=0.9gph
Note At 13.7gph we have an average of 2.26gph flowing into each cylinder, but all is not equal, the pressure into the injection nozzles are slightly different. So if you run LoP, and you set cylinder #4 to be 50 LoP, you know all the other cylindres will be colder. Probably ranging from 55 to 100 degrees LoP. So your last cylinder to peak, #5, which is getting the least fuel, will be running coldest. It won't be producing as much power as it could be in ideal conditions. (imagine if you could have a mixture knob for each cylinder! You could have perfect balance).
What is the cause of the imbalance? Probably to do with the pressure inside the fuel injector lines, distance the fluid has to flow, loss to friction, and the injector nozzle itself can have bigger or smaller opening. So easy thing to do is swap injector #5 and #4.
In practice < 1gph spread is perfectly fine, all cylinders are happy. Let's say you see this;
CYLINDER # | FUEL FLOW | EGT
4 13.7 1410
2 | 13.9 1420
1 14.2 1430
3 14.4 1430
6 14.6 1430
5 16.8 1400
WOW! 16.8-13.7 = 2.1gph spread. I can easily see cylinder #5 has a clogged injector. It needs much higher pressure to get the same fuel flowing into the cylinder as the other barrels. Clean the injector or replace it.
I do this on a regular basis to analyze the health of my engine. I not only do a fuel flow test at altitude, I also do a mag check at LoP flying to test the spark plugs. If a spark plug is going bad, running on one mag or the other, I will see EGT go up. Why? Because the mixture isn't igniting properly, so bu the time the exhuast valve opens, the mixture is still burning inside the exhaust pipe.
If you have a timing issue with mags, running on one bag you will see ALL cylinders behavae incorrectly (either all too hot or all too cold). Then you know it's not a plug, it's the mag itself.
Now this business of RoP flying should only be used as a owrkaround for a poorly tuned engine. Let's say you have the scenario above with #5 cylinder injector clogged. If yo run LoP that cylinder will really be starved for fuel. You will see low CHT, and low EGT and you might feel a slight vibration because it's not a healthy ignition to move the rod, the other cylindrs are dragging it along.
if you run RoP you can mask this problem quite easily because that cylinder will be running peak temperature (or slightly LoP). I would treat this as a temporary solution to get into a maintenance shop immediately! I would not fly like this.
If you try to fly LoP and you have rough running engine - you can now figure out why. It's probably because some cylinders are too lean, and it's good motivation to tune your engine.
There is nothing fundamentally dangerous about running LoP, it's just chemistry, and old ignorance from before we had engine monitors. Imagine trying to run LoP without engine monitor.. you have rough running engine, or you're burning out cylinders, because it was impossible to figure out how to set things up. RoP is easier - you just crank so much fuel in there, that you won't be doing much damage.
On my engine I run about 13gph LoP at 11000', or 15.5gph RoP. That's a 20% increase in fuel flow.
My airspeed will increase from 170kts to about 175kts. That's about a 3% increase in TAS. So you're looking at a 17% decrease in range and a 3% savings in time. Is it worth it? Unless you're running from enemy forces probably not.
Range is also safety IMHO, more fuel left when landing is always a GOOD THING.
Let me know if this make sense, happy to talk through it on the phone.
Next time I"m in NYC we can go together, I grab note pad and you lean and we can map out both your engines to see where they're at.
There are some angry people in the aviation community who cuss at lycoming for not certifying for LoP because its' completel nonsense and destroying a lot of engines and even causing some accidents with people operating improperly. Continental not only authorizes LoP, they require it on some engines like the malibu. That engine runs such high pressures it cannot operate RoP without burning up. It's the same design as lycoming engines, so makes you wonder, why does one company authorize and one not?
My engine health is quite good, I can run LoP all day long, I am very deliberate about how I set my temperatures and what % power I run or cylinder pressures.
I am at about 1400 hours on my engine, perfect compressions, perfect fuel flow and CHT, mechanics always tell me "Oh you fly a cirrus! xpect a top overhaul on your cirrus at 1000 hours, it's gonna be expensive!! <grin>"
I tell them I'm at 1400 and they are really amazed. They don't understand this stuff at all. They just know how to pull things off and replace them with shinier ones.
After you consume all that, look at this diagram:
This is my plane LoP. My EGT has a lean finder mode so you can see the little blue lines are where peak occurred. The numbers indicate the current EGT relative to peak. For example cylinder #1 is now running 44 degrees lean of peak.
Try to answer these questions about that diagram:
- Which cylinder was first and last to peak?
- What is the EGT spread between hottest and coldest?
- What is the fuel flow spread between first and last to peak?
- Which injector nozzle is possibly clogged and should be checked and cleaned?
- Look at cylinder #6, what is different about it, and what could cause this?
(hint, one of those questions cannot be answered with that diagram alone)
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