Variables that affect leaning/mixture

[Draker]

I'm almost embarrassed to ask this, but my private pilot training was done at sea level in Florida and like many flight schools, they pretty much glossed over the mixture knob.

I'm trying to get more methodical and repeatable in my flight testing. One of my goals is to pick a particular fuel economy setting, and hold it constant through a variety of engine power configurations. For example, let's say I'm cruising at 5,000 feet density altitude, 2300 RPM, 23inMP. I lean the mixture to best power with the mixture knob. (Assumption: Fuel injection)

1. Now I change RPM to 2400 without touching the red knob. Am I still at best power?

2. Instead of changing RPM, I change MP to 22in. Am I still at best power?

3. Instead of the above, I descend to 4,000 feet DA. Am I still at best power?

My intuition says 1. Yes, 2. Yes, 3. No, but I have to admit I don't know enough about the mixture control to know for sure. When do you have to re-lean to your desired fuel economy setting? After any change? Only after altitude changes?

Is this also true for other mixture settings? Let's say I lean to 25* LOP. Can I change RPM or MP settings (or altitude) without having to hunt around for 25* again?

This has been an unexpected benefit of taking Phase 1 seriously: It's shaking out and highlighting gaps in my training, which I can correct.

 

[rv8ch]

You will get better answers from the experts I'm sure, but essentially anything that changes the number of oxygen molecules (air) flowing in will impact the amount of fuel that is needed.

Some FI systems are better than others at making this adjustment automatically. Some systems like the EFII and SDS have computers with lookup tables and other inputs to decide how much fuel to squirt in for a particular mass of air coming in.

I have an AFP FM200 and once I set a mixture (AFR) the fuel servo adjusts when the airflow changes, but probably not perfectly. I find that I can play with it a bit (like 1/10ths of GPH) and improve the mixture slightly.

 

[Taltruda]

It’s been my experience that increasing the RPM causes the engine to lean out.. requiring you to enrichen a little. Reducing RPM seems to richen the mixture, allowing you to lean more..

Increasing the MP seems to need more fuel, as does descending into thicker air. What I like to reference are the EGTs that you achieved at “best power” and whenever you make a change to the engine, take note to the change in EGT, if they get hotter, you need to richen to maintain best power.

 

[lr172]

Every different MAP/RPM combination will require a different amount of fuel to maintain a target ratio, as the volume of air is changing. However, the servo does a pretty good job of making the mixture setting somewhat constant across different levels of air flow. However it is not perfectly linear. A carb doesn't really do much of this, except at full rich. Under similar power levels at the same IAT (intake air temp), Peak EGT will remain relatively constant (large changes will move the peak EGT some). therefore, if your target is 80* ROP, you just need to tweak the red knob a bit to get back to whatever EGT you had before the power change.

The thing to watch for is ign timing. With EI, the timing advance can move at different MAP levels and that movement will also move the peak EGT temp. No issue with mags, as thy don't move the advance.

Larry

 

[Avanza]

Cruise Performance power, fuel flow, MAP digram attached.
Note that a lean mixture will reduce power slightly from best power if rpm and MAP stays the same.

Good luck

 

[N49ex]

A way to avoid the confusion

A few years ago I added direct air/fuel ratio monitoring to my engine. It bypasses all the MP and mixture and peak EGT jockeying to set desired operation. Air/fuel ratio is the single number that lets you adjust where you want to be - best power or lean of peak (when allowed, i.e. <= 65% power). Typically A/F around 12.5 for best power and 15.5 for lean of peak, numbers you establish just once for your engine. If you have EIS with your EFIS you also get a pretty accurate number for real time % power you are at.

And yes, it's possible with low lead avgas, using the right oxygen sensor, e.g. NGK, which are tolerant of low levels of lead. I've had A/F monitoring now for over 400 hours and it still works like new. (probably because I bought a spare oxygen sensor just in case, which is now sitting on the shelf!)

https://www.kitplanes.com/adding-direct-air-fuel-ratio-monitoring/