RPM vs LOP

[Dan Thomas]

In a carburetor, air passes through a venturi, creating a vacuum which draws fuel into the air. The throttle is regulating this flow, hence indirectly regulating the amount of vacuum and thereby how much fuel is drawn in. Thus the mixture does change with throttle position and sorta/kinda works okay.

If that was true, the engine would fail anytime any significant changes were made in throttle setting. The combustible air:fuel mixtures range from 8:1 (rich) to 18:1 (lean). Varying fuel flow by the square would really mess that up.

The fuel has viscosity, and it has to pass through channels and the main metering jet as well as the mixture control valve. All that drag ALSO increases by the square of its velocity, and that has the effect of levelling off the fuel flow rise, making it more more parallel with the airflow. It's not exponential at all. If it was, we'd see massive EGT deviations with even small throttle movements.

In this diagram, the "power jet" is referring to the main metering jet.

 

[Rgbeard]

In a fuel injected engine, why is there even a need to adjust mixture? The injectors are controlled by a computer anyway, so why not reduce flow based on density altitude?

this is misinformation.

 

[Dan Thomas]

this is misinformation.

No lack of that around aviation. So many pilots don't understand the airplane at all. They often think that airplanes, especially their engines, are the same as cars. Big mistake.

They need to get a copy of this book and read the whole thing:



I used it when I taught Aircraft Systems in college.

 

[schmookeeg]

If all goes well, will be doing this for the first time in about 2 months. I have been putting off ordering an engine monitor. Gonna get off my wallet and do that today. Sounds like 4 cylinder CHT is extremely helpful for that process.

Probably not essential, but that break-in carries some infant mortality concerns, so it's nice to see things are progressing smoothly and as-expected, because that "auto-rough" sensation will be there for certain, and it's nice to see the CHTs trending down and not up.

 

[Ed Haywood]

Probably not essential, but that break-in carries some infant mortality concerns, so it's nice to see things are progressing smoothly and as-expected, because that "auto-rough" sensation will be there for certain, and it's nice to see the CHTs trending down and not up.

Does that change if engine has been test run by engine shop?

 

[Narwhal]

My question on LOP engine operation is how long to wait before setting the mixture when leveling off after climb to initial cruise altitude.

Especially in winter I have noticed that the EGT's absolute value will increase by 20-40 degrees after being run for about 15-20 minutes compared to an initial level off at say, 3500 feet after just being run for 5-10 minutes. If I immediately lean it back to 50 LOP at top of climb, when I scan the EGT about 10 minutes later it will now be only 10-30 LOP, i.e. my EGTs have risen 20-40 (in the "red box" theoretically, but since I usually only go LOP at 65% power or less I don't think it's doing anything detrimental except burning a little more fuel than necessary). I've started to get in the habit of running the motor 150 rich of peak for about 5 minutes after level off before going LoP.

 

[Martin Pauly]

My question on LOP engine operation is how long to wait before setting the mixture when leveling off after climb to initial cruise altitude.

Especially in winter I have noticed that the EGT's absolute value will increase by 20-40 degrees after being run for about 15-20 minutes compared to an initial level off at say, 3500 feet after just being run for 5-10 minutes. If I immediately lean it back to 50 LOP at top of climb, when I scan the EGT about 10 minutes later it will now be only 10-30 LOP, i.e. my EGTs have risen 20-40 (in the "red box" theoretically, but since I usually only go LOP at 65% power or less I don't think it's doing anything detrimental except burning a little more fuel than necessary). I've started to get in the habit of running the motor 150 rich of peak for about 5 minutes after level off before going LoP.

There is no need to wait. Unlike CHTs, EGTs respond instantly to changes in fuel/air mixture. The reason is that each combustion event is a discrete event, with no relation to the one before or the one after. Whatever fuel and air you have during combustion determines how much power is made in this stroke, and how hot the exhaust gas is. The only delay you'll see is in the EGT probe itself and the display on the engine monitor, which may need a second or so to catch up.

If your EGTs change after 15-20 minutes like you described, that can have two explanations: either your friction lock allows throttle and/or mixture to change slightly, or you are flying into air of different density. For example, when flying from warm into cold air without changing anything else, cold air has higher density, so the mixture will be leaner (more air, same fuel). And that causes an EGT change.

Back to your first question: there is no need to wait at all after you level off in cruise before leaning.

- Martin

 

[Martin Pauly]

They need to get a copy of this book and read the whole thing:

Good suggestion, but this needs to be augmented with systems knowledge specific to the airplane a pilot flies. There are so many important differences from one to the other, and the fuel delivery system is a great example for how much variety exists, as this thread shows. Carbureted or fuel injected? How do I prime before start? Altitude-compensating fuel pump or not? Where (which tank) is excess fuel returned to? Know your airplane!

- Martin

 

[jesse]

Last question, when running LoP at ~60% power in my Lycoming IO-360 (experimental version called by Lyc. YIO-360-EXP128 rated at 186 HP), in winter with temperatures around 0-20 F often my CHT's will only be around 280 during cruise. Oil temp seems normal at a consistent 185 +/- 5 degrees regardless of mixture setting. I have researched this a bit and it doesn't seem to matter, but does anyone know of any reason why this would be harmful? The engine uses a lightspeed dual electronic igntion.

Do you have a modern engine analyzer with a CHT probe on every cylinder? If so, you can fashion up something to block the amount of air entering the cowling, just make sure you don't make your hottest cylinder too hot. For inspiration, just do some googling, it is very common for airplanes to have some sort of "winter kit" that blocks air. Sometimes official ones. Usually unofficial **** some A&P hacked up at some point over the years.

If you are looking at gauges that have a single probe and are decades old, ignore it, there is no point in taking action based on a reading of one cylinder when you don't know what the others are doing.

 

[jesse]

Last question, when running LoP at ~60% power in my Lycoming IO-360 (experimental version called by Lyc. YIO-360-EXP128 rated at 186 HP), in winter with temperatures around 0-20 F often my CHT's will only be around 280 during cruise. Oil temp seems normal at a consistent 185 +/- 5 degrees regardless of mixture setting. I have researched this a bit and it doesn't seem to matter, but does anyone know of any reason why this would be harmful? The engine uses a lightspeed dual electronic igntion.

Do you have a modern engine analyzer with a CHT probe on every cylinder? If so, you can fashion up something to block the amount of air entering the cowling, just make sure you don't make your hottest cylinder too hot. For inspiration, just do some googling, it is very common for airplanes to have some sort of "winter kit" that blocks air. Sometimes official ones. Usually unofficial **** some A&P hacked up at some point over the years.

If you are looking at gauges that have a single probe and are decades old, ignore it, there is no point in taking action based on a reading of one cylinder when you don't know what the others are doing.

 

[Dan Thomas]

Good suggestion, but this needs to be augmented with systems knowledge specific to the airplane a pilot flies. There are so many important differences from one to the other, and the fuel delivery system is a great example for how much variety exists, as this thread shows. Carbureted or fuel injected? How do I prime before start? Altitude-compensating fuel pump or not? Where (which tank) is excess fuel returned to? Know your airplane!

- Martin

All of that is in the airplane's POH or AFM. We have seen dozens of questions on this forum that indicate that pilots either don't have that manual as required by law, or they're not consulting it.

 

[Ed Haywood]

All of that is in the airplane's POH or AFM. We have seen dozens of questions on this forum that indicate that pilots either don't have that manual as required by law, or they're not consulting it.

Many GA aircraft were type certificated before 1 March 1979:

 

[jesse]

Yes, it has 4 probe EGT/CHT displayed through a Garmin G3X instrument system.

Great. Real world...Probably won't matter - but getting the temperature up to at least 330 or so, would in theory, be better for things, less lead buildup, among other things.

Usually I see the air blocking plates put over the oil cooler but my oil temp is fne, no different than summer. It's just the CHT's, and I don't think they're dangerously low. I think 270 CHT is the coldest I've seen during cruise for extended periods 50 deg LoP with a fuel flow in the 5.7 GPH range). I'm just asking on here to double check since we have some pretty experienced pilots on here, and in my previous O-470L equipped C182B LoP wasn't an option due to roughness.

Cylinders are cooled by air. Reduce the airflow to those cylinders and the temperature will go up. Be mindful of the airflow for the oil cooler and its temperature while using the kit. For inspiration, some random pictures from google images:

Every airplane that I've owned/operated in the midwest has had some sort of winter kit. Sometimes those winter kits came with the airplane or were bought from the OEM, more often than not, some A&P or owner fabricated something at some point that has been with the airplane ever since.

Anyhow. Be safe. If you modify something to change one temperature, make sure it is not changing a temperature elsewhere for the worse. I would stay over the airport while testing such things. If temperatures do something you don't like, you can reduce power and land.

You'd be better off getting advice on this subject from a forum dedicated to your experimental aircraft type. I have no idea what airplane you are flying, what the cowling looks like, how the air is flowing, etc. This is something other aircraft owners flying the same type have likely experimented with already extensively.

Disclaimer: Given how little I know specifically about the variables of your airplane, don't use any of the above advice outside of a flight simulator.

 

[Dan Thomas]

Cylinders are cooled by air. Reduce the airflow to those cylinders and the temperature will go up.

You can have lots of airflow and still have overheated cylinders. The condition of the baffling, and its seals, is of utmost importance. The engineers designed all of it, cowling and baffling, to create a positive pressure above the engine and a negative one below it, to get air flowing through the cooling fins. It doesn't take much of a leak through broken or missing baffles or the seals to lower that pressure differential, which lowers cooling flow.

I have found seals rotten or hardened and curled over so they don't seal anymore. I have found big holes in baffling, where cables used to go through or where an alternator cooling hose used to attach. I have found pieces of baffle missing. I have found intercylinder baffles missing, left out at the last engine change. Did the mechanic think the engine would get better cooling with more flow between the cylinders? Those baffles are to force the air through the fins and not let it escape through the gap between adjacent cylinders and heads. There are no unnecessary parts in that system. I have found big gaps in 172 baffling where the big Prestolite starter was replaced with a compact permanent-magnet starter, much smaller diameter, but the gap in the baffle around the starter left wide open. I have found the front and rear cylinder bafffles unsecured, their wires and springs left off, and the air being allowed to flow past the fins instead of through them.

It's amazing that some engines don't just seize up solid. As it is, the cylinder life must be pretty poor on some airplanes.

 

[SSR]

I wonder why they did that. Do they think today's pilot's don't understand percent power?

I really have no idea, especially since break in procedures still call for percent of power operations. However, in 30 years of flying, this is the first time I've ever considered looking it up. I've broken in several fresh engines and more cylinders than I can count. Until now, I just flew a fairly precise profile dictated by my mechanic.

Perhaps it's because text book answers like that are not accurate over time, as the engine wears. I suppose few of the performance charts are.

 

[SSR]

The problem is that the amount of vacuum created by the venturi (and thus fuel flow) is proportional to the square of the speed of the flow through the venturi

That makes sense. Wouldn't fuel injected work differently? Flow speed still changes, but I'd think it would mechanically adjust fuel too? At least within reason?

 

[Pinecone]

Yes it does. But such adjustments are fairly crude and you need to fine tune with the mixture control

 

[Dan Thomas]

That makes sense. Wouldn't fuel injected work differently? Flow speed still changes, but I'd think it would mechanically adjust fuel too? At least within reason?

A look at the Continental fuel controller is instructive.



The fuel comes in the bottom and is delivered to the cylinders out the top. Unused fuel is returned to the system. The throttle plate control in the throttle body is linked to the fuel control valve by a rod, and is adjustable to get the idle mixture correct. As the throttle opens, the fuel valve also opens and sends fuel proportionately to the cylinders, to match the airflow. On the right-hand end of the fuel control there's another lever, and that's connected to the pilot's mixture control, to fine-tune the mix for DA changes, climb power, and so on.

In the airplane, it looks like this:



Throttle control on the right, mixture on the left.

Lycoming's RSA system does it differently. It takes airflow measurements at the air inlet, both ram (dynamic pressure) and venturi (static pressure) and uses them to operate the fuel control valve, Fuel pressures also figure into it, as varying pressure would otherwise upset the air:fuel ratios.



The red line is the incoming fuel. The fuel out is on the right. The two diaphragms vary the fuel rate. The mechanical fuel controller has a link rod like the Continentals, but it controls the idle and low-power mix only, when the ram and static pressures are mostly absent. There is a mixture control lever on the near side of that valve, for the pilot's input.

 

[Ed Haywood]

A look at the Continental fuel controller is instructive.

View attachment 114740
Lycoming's RSA system does it differently. It takes airflow measurements at the air inlet, both ram (dynamic pressure) and venturi (static pressure) and uses them to operate the fuel control valve, Fuel pressures also figure into it, as varying pressure would otherwise upset the air:fuel ratios.

The red line is the incoming fuel. The fuel out is on the right. The two diaphragms vary the fuel rate. The mechanical fuel controller has a link rod like the Continentals, but it controls the idle and low-power mix only, when the ram and static pressures are mostly absent. There is a mixture control lever on the near side of that valve, for the pilot's input.

What is the source of the metered fuel pressure - the pink line?

 

[Dan Thomas]

What is the source of the metered fuel pressure - the pink line?

It comes from the output section of the fuel control valve. The metered and unmetered fuel pressures operate one of the diaphragms. The dynamic and static air pressures operate the other. Between the two, they get the fuel delivery accurate for the airflow. The metered fuel is the stuff that goes through the ball valve that is controlled by the diaphragms, out through the flow divider to the injectors. The metered fuel flow will be affected by the unmetered fuel pressure, hence the need to have it pushing the valve closed when its pressure rises, to keep the metered flow accurate.

 

[Narwhal]

Great. Real world...Probably won't matter - but getting the temperature up to at least 330 or so, would in theory, be better for things, less lead buildup, among other things.


Cylinders are cooled by air. Reduce the airflow to those cylinders and the temperature will go up. Be mindful of the airflow for the oil cooler and its temperature while using the kit. For inspiration, some random pictures from google images:

Every airplane that I've owned/operated in the midwest has had some sort of winter kit. Sometimes those winter kits came with the airplane or were bought from the OEM, more often than not, some A&P or owner fabricated something at some point that has been with the airplane ever since.

Anyhow. Be safe. If you modify something to change one temperature, make sure it is not changing a temperature elsewhere for the worse. I would stay over the airport while testing such things. If temperatures do something you don't like, you can reduce power and land.

You'd be better off getting advice on this subject from a forum dedicated to your experimental aircraft type. I have no idea what airplane you are flying, what the cowling looks like, how the air is flowing, etc. This is something other aircraft owners flying the same type have likely experimented with already extensively.

Disclaimer: Given how little I know specifically about the variables of your airplane, don't use any of the above advice outside of a flight simulator.

My airplane is a 2022 Cub Crafters Carbon Cub FX3 (CCX-2000 is the type designation) with a cc363i engine which is a 186 HP Lycoming YIO-360-EXP128.

On my previous airplane, a 1959 C182B with the O-470L, I did put speed tape over half of the air-oil heat exchanger (oil cooler) in the winter months after consulting my A&P. I did this to get the oil temps high enough so that the water would boil out of the fuel and stave off corrosion since we don't get to fly too much up here in the winter due to weather and lack of sunlight. CHT never dropped below 300 in the 182 during cruise that I recollect because I couldn't run it LoP, granted it only had a single probe CHT/EGT. I don't think the tape actually did a whole lot in the Cessna (perhaps the vernatherm was already closed and causing oil to bypass the oil cooler anyway), but I was advised against putting the factory blocking plates on due to risk of excessive CHT, even though the cowling had the mounting holes for it.

Anyway, this is what the cowling looks like on my FX3. There is no engine winterization kit available that I know of, but at least the oil stays at 185+ all year running Aeroshell 15W50. I will scrutinize my engine data logs to see what the sustained CHT's have been on the last few flights before I go too far down that rabbit hole I guess. Or I could just accept the fuel burn and cruise around at 65%+ power and RoP to keep things warmer. Thing is most of my missions don't involve going anywhere, just sight seeing and landing in fun spots for sport, so I'm usually cruising at 55% and LoP at low level, which is enough to get me 90 MPH IAS at about 5.5 gph.