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Computing FF using desired AFR, MAP, T, RPM

rv8ch

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I'm sure this has been done, but I can't seem to find it. Perhaps it does not make sense.

It seems like our EFIS or an app should be able to tell us the exact FF we should use for a desired AFR, given the engine size, RPM, OAT, and MAP.

I did some (probably bad) math to calculate air mass and came up with something like this for 360 in3 engine like my io-360. Standard temps, AFR of 12:1, volumetric efficiency of .88.

FF io360 standard temps.png

I have in mind that considering the BSFC the FF at max power was about 15GPH, that was how I chose .88 for the VE.

Seems like any EFIS would have all the information needed to provide a hint if the aircraft is too rich or too lean.

Is this a reasonable thing to do, have a calculation for 12:1 for best power, and 14.7:1 for stoichiometric?
 
You could also put in 16 to 17 for LOP.

My SDS setup shows this while flying, kinda nice to just dial in mixture via the AFR readout.

Here is a shot of LOP at 16.4.
 

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You could also put in 16 to 17 for LOP.

My SDS setup shows this while flying, kinda nice to just dial in mixture via the AFR readout.

Here is a shot of LOP at 16.4.

That's true Mike - you could select in the EFIS what AFR you are after and it could tell you the FF needed. I believe that the SDS has an oxygen sensor to determine the AFR, is that correct?
 
I would expect a ve of .8 or less for a lycoming with its known poor intake arrangement. Lycon produces gobs of extra HP just by cleaning it up.
 
That's true Mike - you could select in the EFIS what AFR you are after and it could tell you the FF needed. I believe that the SDS has an oxygen sensor to determine the AFR, is that correct?

Correct.

It is only used to read/display the AFR at this time. I only use it to adjust the mixture manually at this time.

I seem to remember hearing the long term plan is for a closed loop setup for fully automatic fuel flow when avgas goes unleaded. As things are now, 100LL will kill the O2 sensor eventually and that is not a desirable factor in a closed loop setup.
 
I would expect a ve of .8 or less for a lycoming with its known poor intake arrangement. Lycon produces gobs of extra HP just by cleaning it up.
Thanks - any idea how to get to a definitive number? Here's the numbers dropping VE to 0.8:

AFR12 VE.8 io360.png
 
One sure point is peak EGT.

One key variable in a reasonable prediction based on the variables you mention is to know some point on the curve of A/F. We can get that from exhaust temps, peak EGT is one point we can pretty precisely obtain. Then knowing the theoretical AF for peak EGT a back calculation for AF can be done using the fuel flow while other variables remain constant as FF goes either rich or lean. It would not be prudent to extend very far in any variable though.

This is the basis of the ancient performance curves from Lycoming. I use airspeed as to determine "best power" simply by adjusting until speed peaks. It is how I test for max cruise speed at 8000 DA. This enables one to create a chart for airspeed FF for ambient Temp, density and GW. I used Kevin Hortons calculation methods to get about 4 points and create a full chart like a good POH for an old 182. It is a lot of work, and you have to know the gross weight at every test data point.

What is your goal in having this "chart"?
 
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AFR SENSOR

You could also put in 16 to 17 for LOP.

My SDS setup shows this while flying, kinda nice to just dial in mixture via the AFR readout.

Here is a shot of LOP at 16.4.

Mike, are you running unleaded autofuel? the O2 sensor mas a pretty short life running on leaded avgas and I wonder how it is holding up if you are running avgas?

I helped another builder install and set up an SDS system on his Onex and it works beautifully, but he runs only autofuel because of the O2 sensor.
Ed
 
Mike, are you running unleaded autofuel? the O2 sensor mas a pretty short life running on leaded avgas and I wonder how it is holding up if you are running avgas?

I helped another builder install and set up an SDS system on his Onex and it works beautifully, but he runs only autofuel because of the O2 sensor.
Ed

We flew the first 20 or so hours without the sensor, break in fuel flows and possible metallic debris being my reason to wait.

So far, the sensor is working just fine with the time sitting at 33.6. If the sensor does **** out, at least I will have gotten good data from it. Some folks actually run them for pretty long times.
 
I have in mind that considering the BSFC the FF at max power was about 15GPH, that was how I chose .88 for the VE.

0.875 is true for a stock angle valve. I have no clue about the parallel valve. Either will show some variation between individual engines.
 
One key variable in a reasonable prediction based on the variables you mention is to know some point on the curve of A/F. We can get that from exhaust temps, peak EGT is one point we can pretty precisely obtain. Then knowing the theoretical AF for peak EGT a back calculation for AF can be done using the fuel flow while other variables remain constant as FF goes either rich or lean. It would not be prudent to extend very far in any variable though.

This is the basis of the ancient performance curves from Lycoming. I use airspeed as to determine "best power" simply by adjusting until speed peaks. It is how I test for max cruise speed at 8000 DA. This enables one to create a chart for airspeed FF for ambient Temp, density and GW. I used Kevin Hortons calculation methods to get about 4 points and create a full chart like a good POH for an old 182. It is a lot of work, and you have to know the gross weight at every test data point.

What is your goal in having this "chart"?
Good idea, thanks for the suggestions. I will start capturing this info on upcoming flights.

The goal of the chart is just to better understand my engine, and see if the more advanced tools we have available can help operate it more efficiently.

The stability of cruise flight makes it easy to use EGT to set the FF properly, and that's the regime where it has the most benefit. I do, however, find myself adjusting FF during shorter local flights to avoid wasting fuel, better performance, cleaner plugs, etc. and I kind of know the numbers that should be in there based on rough calculations done by my GRT of "% power". For example, if I'm bopping around at 80% power my FF should not be 16 or 17 - probably 12 would be closer to best power. Just like my GRT AP tells me when I need to adjust the trim, I think it could tell me I'm wasting fuel, with the right algorithm.
 
Nice and interesting exercise Mickey.
I have a carbed engine and basic EFIS only, but was also toying with an AFR display idea. Seeing no direct practical value for me, it has been put on the back burner...

I usually set my power according the Lyco tables, helped by the magnificent and free AircraftPower iOS app by Jim Petty. I then pull the mixture to 50 LOP (electronic ignition), and leave it there. Leaning more will result in light roughness and marked decrease in RPM/power.
Settings as in the attached screenshot result in a FF of 5.3-5.5. Not sure were the AFR is at this point, but sure would be interesting to know...
 

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Nice and interesting exercise Mickey.
I have a carbed engine and basic EFIS only, but was also toying with an AFR display idea. Seeing no direct practical value for me, it has been put on the back burner...

I usually set my power according the Lyco tables, helped by the magnificent and free AircraftPower iOS app by Jim Petty. I then pull the mixture to 50 LOP (electronic ignition), and leave it there. Leaning more will result in light roughness and marked decrease in RPM/power.
Settings as in the attached screenshot result in a FF of 5.3-5.5. Not sure were the AFR is at this point, but sure would be interesting to know...

Dan, that's a nice visual, and thanks for the App reference. I've just downloaded it. Note: it seems to be available for the iPad only. I've just started flying with a new engine with a CS prop and find this discussion very helpful. Thanks.
 
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Thanks - any idea how to get to a definitive number? Here's the numbers dropping VE to 0.8:

View attachment 24017

Sorry, but don't know how it is calculated, though I speculate that it is done by measuring airflow or using math to back into it. I am surprised by the .875 number that Dan posted, as I expected less. The angle valve engines gets it's extra power through better airflow, so we know the PV engine has a lower VE. No idea how much lower. Doing some math between the two will get you in the ballpark now that we know VE for the AV. A stock 360 PV is 180 and a 360 AV is 200 HP.

Larry
 
Good morning. 470 hours on a Bosch wideband here running 100LL in my 360 without any sensor issues :)


Mike, are you running unleaded autofuel? the O2 sensor mas a pretty short life running on leaded avgas and I wonder how it is holding up if you are running avgas?

I helped another builder install and set up an SDS system on his Onex and it works beautifully, but he runs only autofuel because of the O2 sensor.
Ed
 
Good morning. 470 hours on a Bosch wideband here running 100LL in my 360 without any sensor issues :)

Thanks for the pirep - do you find it useful? What kinds of AFR do you typically run? Where did you install it? I've got the NTK ready to install, just need to get the bung welded on.
 
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