Cirrus SR20 Fuel Injection, Mixture Settings, Altitude Compensation

[Tantalum]

Yes, I read the POH but I'm hoping some of the resident geniuses here, like maybe @Ted DuPuis can give me a more human and qualitative answer to my question

QUESTION: If Cirrus's fuel injection system has an altitude compensating mechanism than why is there a manual mixture control? CFIs have failed to give me a good answer, other than it altitude compensates and one even said that leaving in lean at 12K cruise and then descending to land at sea level is a non-event since it will automatically richen itself via the fuel control. I don't trust that enough to try it, but it made me think.

From the POH
"Engine Fuel Injection The multi-nozzle, continuous-flow fuel injection system supplies fuel for engine operation. An engine driven fuel pump draws fuel from the selected wing tank and passes it to the mixture control valve integral to the pump. The mixture control valve proportions fuel in response to the pilot operated mixture control lever position and automatically provides altitude compensation to supply the proper full rich mixture at any altitude. From the mixture control, fuel is routed to the fuel-metering valve... "

 

[Ted]

The altitude compensating mechanism basically adjusts full rich based off of altitude. As you increase altitude and there's less air, the same full rich air/fuel ratio will require less fuel. Older systems don't have this compensating mechanism and so you will need to lean manually to keep full rich from being "too rich" on a naturally aspirated engine. Once you get to cruise then you'll want to lean and have a different air/fuel ratio. Air flow stays the same, fuel decreases, you get better efficiency.

You don't need a mixture knob if you're going to go full rich all the time, but you probably want to lean your mixture in cruise at least.

 

[Tantalum]

Thanks, so basically, even if I leave it at "full rich" it won't be getting the same amount of fuel as it did on the ground. However, you still want to lean per the book, etc., and leaving it leaned out for your descent is not a wise move

 

[Ted]

Thanks, so basically, even if I leave it at "full rich" it won't be getting the same amount of fuel as it did on the ground.

Correct. On a naturally aspirated engine for full throttle, constant RPM (assume max RPM), full rich, your max fuel flow will go down as you go up and have lower manifold pressure.

However, you still want to lean per the book, etc., and leaving it leaned out for your descent is not a wise move

I always left it leaned for descent and adjust the mixture as required. However I flew aircraft that didn't have the altitude compensation fuel pumps (basically on those the mixture got lean enough to start getting rough and then you judge it rich). As you go down you'll also be adjusting your manifold pressure. But if you're flying a Cirrus under warranty, do what they say since you don't want your warranty voided.

Also as a matter of clarification, calling it Cirrus's fuel injection system is a bit inaccurate - it's just a standard Continental fuel injection system and the same altitude compensating fuel pump (or same style) is used on plenty of other naturally aspirated aircraft, and has been for decades. Basically it's just a fuel pump for a turbo (which references boost to adjust max fuel flow), but since there is no turbo it just makes fuel flow go down with altitude and the associated reduction in atmospheric pressure. Those fuel systems have the following inputs:

1) Engine RPM (which determines how much fuel the pump will flow)
2) Throttle position (which acts as an orifice/restrictor)
3) Mixture position (acts same as throttle from a fuel system perspective
4) Atmospheric pressure or upper deck pressure (pressure before the throttle butterfly)
5) Turbo engines often have a fuel limiter which just acts as a ceiling on fuel flow because 1-4 will result in more fuel than what is desired for full rich. Honestly I wish we could remove the fuel limiters on the engines that have then as they just add complexity and more fuel on those turbo engines is typically more better.

It's worth noting that Lycoming mechanical fuel injection systems are designed completely different from Continental's. Both styles have their pluses and minuses.

Now I'm probably getting too technical.

 

[BigBadLou]

Yikes, that is one departure from standard procedures. (I am not saying it is wrong, merely stating that it is different)
I hope that pilots get good transition training for this airplane and read the POH religiously. And I sure hope they placarded it on the mixture control itself to make it very obvious.

Good explanation, Ted, thanks.

 

[Tantalum]

Now I'm probably getting too technical.

Not at all, I appreciate the insight and mechanics behind it. Wish the POH went into that kind of detail actually. I've been flying it as trained and as the POH and checklists suggest and use the lean assist in cruise, however it always bothers me when there is a system on the airplane that I don't fully understand or comprehend. The Lycomings are what I "grew up" with so know those pretty well, but the mixture in the Cirrus was always kind of an enigma to me.

You mentioned manifold pressure, for what its worth I'm flying an SR20 now, and while it does show the manifold pressure there is no prop control, it is automatically governed and the transition program here flies the planes based on the "percent power" that it shows you on the Avidyne. Full power for cruise, 25 for descent, 50 in the pattern, 20 on final, etc. etc.

 

[Ted]

Yikes, that is one departure from standard procedures. (I am not saying it is wrong, merely stating that it is different)
I hope that pilots get good transition training for this airplane and read the POH religiously. And I sure hope they placarded it on the mixture control itself to make it very obvious.

Are you referring to the method on the Cirrus being a departure from standard procedures or something I said? I think Cirrus, but wanted to confirm.

I'd argue that it's not really that much of a departure from standard procedures, it's just a different standard procedure. Like I said, many aircraft for decades have had the altitude compensating fuel pumps on them. One A36 Bonanza I used to do some instruction in (believe it was an '84 model) had the same pump on it and same procedure. However I do agree that it's not always very well understood by the people who fly the planes now, many years after they were originally built. They end up requiring some additional training or learning from someplace or another.

A lot of people actually end up swapping the altitude compensating fuel pumps for standard, because they like the extra fuel cooling they get in the climb with standard. On the 310 this was important.

Good explanation, Ted, thanks.

You mentioned manifold pressure, for what its worth I'm flying an SR20 now, and while it does show the manifold pressure there is no prop control, it is automatically governed and the transition program here flies the planes based on the "percent power" that it shows you on the Avidyne. Full power for cruise, 25 for descent, 50 in the pattern, 20 on final, etc. etc.

Right, Cirrus does things a bit differently in that regard. Ultimately it comes down to about the same thing, just that instead of leaving the prop constant the Cirrus throttle/prop changes both for you at the same time.

 

[Tantalum]

^thanks! and I mis-spoke, it is full power for climb, and then your standard 65-75 percent power for cruise. Depending on the altitude though that generally means the throttle itself is at or near the max setting anyway. Recently with it wide open it was only showing around 67 percent power, but we were high up

 

[Ted]

Yep, that's normal, and one of the reasons naturally aspirated engines last better than turbos. Most of the time naturally aspirated you're not going to be able to do more than 65-70% power.

 

[citizen5000]

@Tantalum You may find this set of slides useful. They discuss the role of the altitude compensating fuel pump along with the mixture issue. http://takeflightsandiego.com/assets/documents/SR22T Engine Management.pdf

 

[Tantalum]

@citizen5000 thanks! Dave's a good guy, I downloaded just about all the PDFs from his site and have them up on Foreflight but didn't get too into the SR22 files yet, I assumed with it being a turbo and all it wouldn't be too applicable to the SR20 and may just risk confusing me, ha! But I will check them out, knowledge is power after all

 

[citizen5000]

@citizen5000 thanks! Dave's a good guy, I downloaded just about all the PDFs from his site and have them up on Foreflight but didn't get too into the SR22 files yet, I assumed with it being a turbo and all it wouldn't be too applicable to the SR20 and may just risk confusing me, ha! But I will check them out, knowledge is power after all

Yea, the slides are more about the turbo but I assume the fuel management is close or the same. All the planes combine the constant speed prop control in the throttle/governor. All the planes have boost/prime pumps, right? And doesn't the SR20 POH discuss ROP/LOP climbs, cruise and descents? Anyway, I couldn't find slides just on the SR20.

One thing I noticed today on a 1k fpm descent from 15k to 5k I left the mixture control at the last cruise LOP position. The indicator stayed just at the bottom of the green block the whole time and the engine seemed happy. OAT was 5c and did not see inordinately cold CHTs either.

 

[BigBadLou]

Are you referring to the method on the Cirrus being a departure from standard procedures or something I said? I think Cirrus, but wanted to confirm.

Cirrus. Sorry for having been unclear.

I'd argue that it's not really that much of a departure from standard procedures, it's just a different standard procedure. Like I said, many aircraft for decades have had the altitude compensating fuel pumps on them. One A36 Bonanza I used to do some instruction in (believe it was an '84 model) had the same pump on it and same procedure. However I do agree that it's not always very well understood by the people who fly the planes now, many years after they were originally built. They end up requiring some additional training or learning from someplace or another.

I was going by the view of most non-professional pilots, you know, us "little guys" who grew up on singles with no altitude compensation equipment.
As I mentioned, one better read the POH carefully when transitioning into an advanced aircraft with such neat features. And one better know what that means as far as engine operations go.

Using the fuel pressure controller from a turbo-charged engine on an N/A engine is pretty nifty. One has to verify, though, that the controller works on both sides of std pressure.
I learned something new today (about the boost controllers). Thanks.

 

[Tantalum]

So another peculiarity, the POH says under the cruise section in 4-20 that mixture should be set to full rich to allow the aneroid to provide auto leaning during all flight conditions... and basically tells you to only do additional leaning if under 75% power and engine temps are within limits, etc. I've always used lean assist, but thought this was peculiar in the POH

 

[Ted]

That's basically just a complex way of saying that you don't need to lean for takeoff at high altitudes because it has the altitude-compensating fuel pump. Saying that leaning is for at or below 75% power is pretty normal for most piston engines.

 

[Tantalum]

Cool thanks Ted! Owe you a beer or two for all the informative posts here!

 

[takair]

So basically, this altitude compensation works with the throttle full forward and the mixture full forward? For those who fly this way, has anybody monitored fuel flow and EGT during a climb to see how well it works? when you come off full throttle, does it tend to go rich, or is it proportional? Thanks...interesting discussion on the differences with the Cirrus models.

 

[Tantalum]

So basically, this altitude compensation works with the throttle full forward and the mixture full forward? For those who fly this way, has anybody monitored fuel flow and EGT during a climb to see how well it works? when you come off full throttle, does it tend to go rich, or is it proportional? Thanks...interesting discussion on the differences with the Cirrus models.

My understanding from Ted's explanation further up and the POH is that the "full rich" setting gets proportionally leaner to always give you "full rich" at the full rich setting that corresponds to that altitude. Leaning from there will still lean the engine out. The POH does have a note about running rich of peak, that you may not see as much of a change in temps, since it is already at "full rich" so to to speak

During climb the only think I have noticed is that the CHTs tend to run on the higher side ~400 and need some babysitting with airspeeds and climb gradients to keep them in a good spot. On the 172 I fly on a hot day you can just leave the mixture full rich to combat this, although I usually will try to start leaning around 3,000 in climb for best RPM in the 172 as well. The mixture makes a big difference on temps

 

[Ted]

The high CHTs in the climb is typically an issue that exists with these systems, assuming that they are calibrated to Continental specs. Continental's max fuel flow numbers are way too lean to keep CHTs in the appropriate range, so it's common to turn the fuel pressure up higher than max spec. In the Twin Cessna world that is extremely common.

The altitude reference will apply at any power/mixture setting, not just full rich.