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Fuel flow at idle

rv8ch

Well Known Member
Patron
I'm seeing about 1.4 to 1.5 GPH at idle, around 700-800 RPM. Aux fuel pump on increases it by about 0.1 GPH. Idle is perfect and smooth - no problems at all.

However, this seems a bit lean, based on some calculations I've done which might be wrong. I just used max FF max HP and max RPM for my io-360 and by reducing the RPM I reduce the HP, which of course should reduce the FF.

Down around 600 RPM my calcs show that I should be at about 40HP and 3.3 GPH. I'm sure there is something obvious that I don't understand, but not clear what. Perhaps since the engine is not under any load, the HP requirements are of course far lower than 40, and this would reduce the FF requirement? But the engine is pumping air...and I guess it needs the correct ratio of fuel to burn? Or perhaps the mass of air flowing is a lot lower, even though the volume could potentially support 40HP?

The original reason for looking at this was to ensure that I was leaning properly on the ground. FF at takeoff on my last flight was about 16.3 at 2500 RPM, which I think is right on target.

I have a Mattituck IO-360, 8.5:1, Superior Cold Air Sump, AFP FM-200, dual pmags, auto plugs BR8ES. Everything is working perfectly, just trying to understand this a bit better.
 
I'm seeing about 1.4 to 1.5 GPH at idle, around 700-800 RPM. Aux fuel pump on increases it by about 0.1 GPH. Idle is perfect and smooth - no problems at all.

However, this seems a bit lean, based on some calculations I've done which might be wrong. I just used max FF max HP and max RPM for my io-360 and by reducing the RPM I reduce the HP, which of course should reduce the FF.

Down around 600 RPM my calcs show that I should be at about 40HP and 3.3 GPH. I'm sure there is something obvious that I don't understand, but not clear what. Perhaps since the engine is not under any load, the HP requirements are of course far lower than 40, and this would reduce the FF requirement? But the engine is pumping air...and I guess it needs the correct ratio of fuel to burn? Or perhaps the mass of air flowing is a lot lower, even though the volume could potentially support 40HP?

The original reason for looking at this was to ensure that I was leaning properly on the ground. FF at takeoff on my last flight was about 16.3 at 2500 RPM, which I think is right on target.

I have a Mattituck IO-360, 8.5:1, Superior Cold Air Sump, AFP FM-200, dual pmags, auto plugs BR8ES. Everything is working perfectly, just trying to understand this a bit better.

Normally max EGT is the only time we can determine actual A/F ratio without an oxygen sensor, but at idle it is hard to use that indicator. Peak rpm in a mixture sweep is the best we can do to see where we are on the A/F curve. Also include your MAP with the mixture sweep plot. All this is easier if you can have EFIS record the data, also best done hot after landing and ensure you have leaned on final, and re-leaned after touchdown so no pooled fuel is affecting your test.

Results extrapolated to idle are hard to do as the volumetric efficiency changes dramatically due to exhaust pressure being much higher than intake. This affects the mass of residual gas in the chamber at intake closing.

I find SAVVY is the easiest to manipulate for limited plots. YMMV
 
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Down around 600 RPM my calcs show that I should be at about 40HP and 3.3 GPH. I'm sure there is something obvious that I don't understand, but not clear what. Perhaps since the engine is not under any load, the HP requirements are of course far lower than 40, and this would reduce the FF requirement? But the engine is pumping air...and I guess it needs the correct ratio of fuel to burn? Or perhaps the mass of air flowing is a lot lower, even though the volume could potentially support 40HP?

Research speed density. THis is how early electronic fuel injection determined fuel quantity (later moved to mass airflow in addition to RPM and MAP). It uses both RPM and MAP to determine the amount of fuel injected. It used various other means to adjust the finalamount, but the core calc was done via a combo of MAP and RPM. Many early systems also used Alpha-n, where throttle position played a large part also. Some just used TP for acceleration adjustment

RPM alone will not give reliable scaling, as MAP is a big part of fuel required. Your carb or servo is mostly working off a mass airflow concept (above idle anyways), but speed density will help you to correlate MAF to RPM/MAP. The power charts all have RPM and MAP on their axis', but few bother to go as low as idle and the x FF = y HP formulas don't work down there and also why most every carb or servo (excludng lawnmower engines) has separate idle circuitry to deal with the unique FF requirements there.

Long story short is that idle is different and cannot be compared to most other power production modes of the engine, though speed density still works for calculating FF.
 
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I'm seeing about 1.4 to 1.5 GPH at idle, around 700-800 RPM. Aux fuel pump on increases it by about 0.1 GPH. Idle is perfect and smooth - no problems at all.

However, this seems a bit lean, based on some calculations I've done which might be wrong. I just used max FF max HP and max RPM for my io-360 and by reducing the RPM I reduce the HP, which of course should reduce the FF.

Down around 600 RPM my calcs show that I should be at about 40HP and 3.3 GPH. I'm sure there is something obvious that I don't understand, but not clear what. Perhaps since the engine is not under any load, the HP requirements are of course far lower than 40, and this would reduce the FF requirement? But the engine is pumping air...and I guess it needs the correct ratio of fuel to burn? Or perhaps the mass of air flowing is a lot lower, even though the volume could potentially support 40HP?

The original reason for looking at this was to ensure that I was leaning properly on the ground. FF at takeoff on my last flight was about 16.3 at 2500 RPM, which I think is right on target.

I have a Mattituck IO-360, 8.5:1, Superior Cold Air Sump, AFP FM-200, dual pmags, auto plugs BR8ES. Everything is working perfectly, just trying to understand this a bit better.

How much RPM rise do you get at idle when you lean the mixture? You should be getting 50 RPM rise before it cuts out and dies. Anything more and it means you are idling too rich. Not enough rise means you’re idling too lean.
 
How much RPM rise do you get at idle when you lean the mixture? You should be getting 50 RPM rise before it cuts out and dies. Anything more and it means you are idling too rich. Not enough rise means you’re idling too lean.

I don't get any rise in RPM at all. As I keep leaning, the RPM drops until the engine starts struggling to run. Perhaps I'm too lean, or perhaps just right - not sure. I doubt being too lean at idle will cause many problems, as long as the engine keeps running. I might see trouble on a very cold day at sea level, but have not on cold (0c) days at 1400 ft, which is my field elevation. I've taken off at sea level multiple times, but never on a very cold day.
 
Who Cares?

Well, OK, subject line is a bit too testy, but yeah, exact fuel flow at idle is not a Big Thing. Sounds like you’re lean though. I look for about a 25 RPM rise when pulling the mixture smoothly back to idle cutoff on a warm engine. Some say 25-50 RPM, yah sure, that’s fine. Just so you’re a bit fat of peak, I think. No, don’t set it while it’s running. :p
 
I'm seeing about 1.4 to 1.5 GPH at idle, around 700-800 RPM. Aux fuel pump on increases it by about 0.1 GPH. Idle is perfect and smooth - no problems at all.

However, this seems a bit lean, based on some calculations I've done which might be wrong. I just used max FF max HP and max RPM for my io-360 and by reducing the RPM I reduce the HP, which of course should reduce the FF.

Down around 600 RPM my calcs show that I should be at about 40HP and 3.3 GPH. I'm sure there is something obvious that I don't understand, but not clear what. Perhaps since the engine is not under any load, the HP requirements are of course far lower than 40, and this would reduce the FF requirement? But the engine is pumping air...and I guess it needs the correct ratio of fuel to burn? Or perhaps the mass of air flowing is a lot lower, even though the volume could potentially support 40HP?

The original reason for looking at this was to ensure that I was leaning properly on the ground. FF at takeoff on my last flight was about 16.3 at 2500 RPM, which I think is right on target.

I have a Mattituck IO-360, 8.5:1, Superior Cold Air Sump, AFP FM-200, dual pmags, auto plugs BR8ES. Everything is working perfectly, just trying to understand this a bit better.

Calculating the fuel flow for idle is a fools errand. When you slowly lean it at idle, does the RPM increase a little. If so it is running rich of peak at idle. That will normally be the case. My aircraft is based at 2,200 msl with density altitudes well above that most of the year. I always lean the engine right after startup. I keep it as lean as I can and still have it run reasonably smooth until I pull on to the runway for takeoff.
 
I don't get any rise in RPM at all. As I keep leaning, the RPM drops until the engine starts struggling to run. Perhaps I'm too lean, or perhaps just right - not sure. I doubt being too lean at idle will cause many problems, as long as the engine keeps running. I might see trouble on a very cold day at sea level, but have not on cold (0c) days at 1400 ft, which is my field elevation. I've taken off at sea level multiple times, but never on a very cold day.

If you’re not getting any rise, then you are too lean at idle. I’m at 2200 feet elevation and I get about 60 RPM rise. If I were you, I would richen your idle mixture and take it for a warmup flight around the pattern. Then recheck your idle speed around 550 to 650, and lean your mixture to verify a 50 rpm rise. I’ve done this running, but you have to be careful and have someone you trust at the controls!
 
I don't get any rise in RPM at all. As I keep leaning, the RPM drops until the engine starts struggling to run. Perhaps I'm too lean, or perhaps just right - not sure. I doubt being too lean at idle will cause many problems, as long as the engine keeps running. I might see trouble on a very cold day at sea level, but have not on cold (0c) days at 1400 ft, which is my field elevation. I've taken off at sea level multiple times, but never on a very cold day.

I'm running a Superior cold sump as well and I dont get any rise either at cutoff no matter what I do with the idle mix (IO-360 B1E). I think you should hear from other Superior intakers, but my guess is this is normal because (surprise) the intake tubes have cooled. I havent yet noodled through what I think might be happening at ICO w this sump, or if its a red herring..
 
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My idle on my Mattituck IO360 must be way rich. At sea level, throttled all the way back I idle around 650. If I lean to max RPM, I get nearly 900.
 
My idle on my Mattituck IO360 must be way rich. At sea level, throttled all the way back I idle around 650. If I lean to max RPM, I get nearly 900.

Oh yeah.. too rich. Lean out the idle, lower the idle back to 650, it’s a little back and fourth. You can do it running if you are careful and keep your elbows in!
 
Down around 600 RPM my calcs show that I should be at about 40HP and 3.3 GPH. .

Mickey,

Just think about how many brake fires you would have had while taxiing if you had to restrain 40HP with the brakes to maintain a safe taxi speed.

Humor aside, my Light Speed ignition manual has the following info:
Lycoming 360 engines should idle at 0.8 –1.0 gph. After Engine Warm-up.

My O-360 shows similar idle fuel flows to your IO-360. I doubt the accuracy of my readout at very low fuel flows.
 
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