Mixture / Throttle

Please educate me here...

While I was doing my private pilot training in a Diamond DA20 with fuel injected engine and fixed pitch prop, I was practicing engine-idle landings from various spots in the pattern -- where the instructor pulls power at some point, and you have to put the plane down nicely onto the runway. Anyway, at one point he say's "Let's say the opposite happened, your throttle cable has broken (or whatever) and you can't close the trottle at all. How are you going to land?"

Well, I justed pulled the mixture until the RPM was about where it would be if I'd reduced the throttle for landing, and then I landed normally.

So the question is "How is pulling the mixture really different from closing the throttle?" It seems like using throttle, you're restricting the flow of a fixed mixture of fuel and air. Using mixture, you're restricting the amount of fuel you mix with a constant supply of air. So if I leave my throttle wide open (like I did in my training landing), and restrict the fuel flow with mixture, I end up with a given quantity of fuel to burn, but with maybe some excess air (more than needed to burn that much fuel). If I have a properly leaned mixture, and use my throttle to restrict the flow of the fuel/air mixture into the engine, I suppose I could adjust the throttle to give me the same amount of fuel in the engine, but with just the right amount of air to burn that fuel.

Now as I understand things, common wisdom says that excessively leaning is bad, and will damage an engine. But I don't understand how this happens. It almost seems like I could almost get by with no throttle at all, and use my mixture entirely to control my engine. In some cases it would be excessively rich, and I'd be pumping excess fuel vapor out, while in others, it would be excessively lean, and I'd by pumping excess air out. Why is pumping excess air through the engine a bad thing (when I'm excessively lean)? Or is it a bad thing?

Now remember that my training was done in a fuel injected engine, as is all my current flying. I suppose that means that I might have a nicely balanced amount of fuel reaching each of the cylinders. Things may be different in non FI engines, but I've not experienced it.

I also remember reading the CAFE reports on the RV9A and RV8A. All their fuel flow / RPM / MPG reports used wide open throttle, using mixture to control fuel flow, varying it from very rich to very lean (from 10 gph to 6 gph in the first table for the RV9A). And this was with carbureted engines.

Any wisdom?

Dude, Go read the 'Pelican's Perch' articles by John Deakin on AvWeb. Search for the ones on learning (there are about 100 and only 5 or so that are relevant). What you did is fine for an emergency, but could potentially be quite damaging to your engine (burnt holes in pistons) if you were to do it regularly. The engine will only run within a certain range of mixtures (say about 1 fuel to 15 air, but I can't remember exactly), too rich and it will quit, too lean and it will also quit. Why do you think every one else uses the throttle? If you want to use the mixture go buy a diesel engine! Pete

I file this one under "there are no stupid questions, et al...".

One of the things about flying that is different than, say, driving is that a good pilot should IMHO know a heck of a lot more about the technical workings of his machine that they typical driver who, if his car is lucky, might know what a dipstick is and how to check it and not much else. As the instructor was no doubt trying to point out an internal combustion engine's power output is a function of many variables, one of which is the fuel-air mixture. Fuel flow, RPM, Manifold pressure, air density, phase of the moon, etc all enter into the mix.

In a fixed pitch scenario we have no direct operator control over RPM as it is whatever the engien can produce given the load from the prop for a given airspeed, fuel flow, etc. In a constant speed scenario when operating in a range the prope govenor can maintain a particular RPM you do have such control. You can, in both cases modify the amount of power you engine is producing by changing the amount of gas being sent to it to be mixed with the throttle as well as by the misture control. Think of the throttle setting the amount of air the engine is gulping (which,for a given misture control setting sucks a given amount of gas).

The throttle actually then controls the amount of air sucked, the mixture controls the amount of gas mixed with same air. All are rates - cubic feet per minute for air, gallons per hour for gas, etc.

So if your throttle cable gives up the ghost and you want to come home to momma you can - kill the mag switch and see how well your bird REALLY glides or reduce the amount of gas being mixed with what was previously determined as a "correct" amount of air for whtever power setting you had set before the cable decided not to let you mess with the fuel setting anymore.

But, there are limits as they say. If you lean the engine too much it won't have enough gas to keep running which is exactly what you do on shutdown when you pull the mixture to idle cutoff. As the mixture gets leaner and leaner at some point the power starts dropping while the temperature of the flame gets hotter and hotter. That is what all the charts and graphs in teh engine manuals show and provide all sorts of fodder for arguments about runnign lean of peak versus rich, etc. If you've ever adjsuted a welding torch you can see what happens as ratio of oxygen (air) to fuel is adjusted in this manner.

At some point the temperature gets too hot and bad things like valves melt, pistons burn, etc. So as a general rule it is not wise to make mixture adjustment your sole power adjustment mechanism - it is too easy to damage your engine, But in an emergency the rule is get on the ground safely so all bets are off.

In cars today all this sort of thing is handled automatically via electronic controls and, unless you are climbing Pike's Peak, we rarely have to worry about the mixture being leaned in a car.

I applaud your curiosity and willingness to ask about something that a lot of people probably don't really understand - me included.

If my explanation helps great - if the true MEs out there have issue with the EE educated answer about a Mechanical engineering subject please pile on and educate me as well.

Richard

penguin said:

Dude, Go read the 'Pelican's Perch' articles by John Deakin on AvWeb. Search for the ones on learning (there are about 100 and only 5 or so that are relevant). What you did is fine for an emergency, but could potentially be quite damaging to your engine (burnt holes in pistons) if you were to do it regularly. The engine will only run within a certain range of mixtures (say about 1 fuel to 15 air, but I can't remember exactly), too rich and it will quit, too lean and it will also quit. Why do you think every one else uses the throttle? If you want to use the mixture go buy a diesel engine! Pete

Click to expand...

I have read Deakin's articles, although it's been a year or two since I did so. I also spent some time discussing them with my instructor, who had not only studied them, but had attended Deakin's workshops, and spoke very highly of them (and him). However, I seem to remember that the whole point of LOP is that you are leaner (cooler) than the peak temperature. You lean past the peak temperature that is so damaging to the engine, and your engine stays healthy. I also seem to recall that Deakin especially likes nicely balanced fuel injectors for LOP operations, so that the peak temperature happens at the same mixture setting for each of the cylinders.

The point is, if LOP is healthy for you engine as he suggests (providing your fuel system delivers a balanced fuel mixture to each cylinder), does something bad happen if you continue leaning past that point? Since there is less fuel to the same quantity of air, it seems like the temperature is not going to get hotter, so you're not going burn up your engine. In fact, I expect it would get cooler, up to some point. With less fuel, your engine will develop less power, just like it does when you close the throttle.

I realize that if you continue leaning eventually you'll completely shut off the fuel supply and kill the engine. I do this at the end of every flight. But what about the range between "just nicely leaned" and fuel cutoff? If this range is unhealthy for the engine, what makes it so, and how is this different from leaving the mixture "just nicely leaned" and reducing power by gradually closing the throttle?

[At this point, I sat and thought for a bit.]

After thinking for a bit, I seem to remember something from my Deakin or my discussions with my instructor. If you lean too far, the fuel droplets (or vapor or whatever) are too spread out, and the burn doesn't progress efficiently from the spark to all the far reaches of the cylinder. Or it doesn't progress as quickly as it normally does when properly leaned. So some of the fuel ends up not starting to burn until the piston starts back up, and it's still burning during the exhaust stroke. Normally it should be all burned well before the exhaust stroke starts. In any case, it's probably not a good thing to send the still burning fuel out the exhaust systems. It seems like it might not be good for the exhaust system.

But on the other hand, when I reduce power, my engine (not prop) RPM slows down, so each stroke of the piston takes longer, and there will be a longer interval for flame front of my slower burning extra lean mixture to reach alll the far reaches of my cylinder during the power cycle, and perhaps maybe be all burned up before the start of the exhaust cycle.

So I guess maybe it comes down to the speed of the flame front versus the speed of the piston. Perhaps it makes sense to lean to the point where the flame front is still sufficiently faster than the piston, so that it will provide hot expanding gas during the power stroke. But if you lean past that point, the hot expanding gas will be too late for power stroke (or too late in the power stroke), and will interfere with the exhaust stroke, and maybe still be burning as it is pushed out the exhaust system.

Does this make sense? If so, is it something that could be detected with a good engine monitor? For example, if I continue leaning past a "leaned just right" LOP state, would I at some point see exhaust temperatures rise, indicating that I was now starting to send still burning fuel out the exhaust system?

TThurston said:

Does this make sense? If so, is it something that could be detected with a good engine monitor? For example, if I continue leaning past a "leaned just right" LOP state, would I at some point see exhaust temperatures rise, indicating that I was now starting to send still burning fuel out the exhaust system?

Click to expand...

Yes - but the distance between this point and the point where the mixture is so lean that it won't ignite is pretty short. EI will keep igniting it beyond the point that magneto ignition would, and may give you enough advance (lighting it earlier in the compression stroke) to still generate a decent amount of power in the downstroke. The location of max EGT (not to be confused with Peak EGT, which occurs at a stoichiometric mixture) will move from the cylinder to the manifold to the exhaust header, so you will see EGT's rise and CHT's fall just before you get to the point of lean cutoff.

True, this is probably not a place you want to put your exhaust system into on a regular basis - but we are talking about a failure mode that you might see once in 500,000 operating hours on average. If that failure mode occurs, you could also simply run your mixture between cutoff and full rich, delivering "bursts" of power to adjust your energy as needed to make a deadstick landing to a runway. That's not good on the engine from the aspect of shock heating and cooling, but again we're talking about a very rare event.

With any failure mode in an aircraft that has the potential to kill you, the remaining life service of the aircraft (and all it's components) only has to be long enough to get you safely on the ground. After that I don't care what's wrong with it - it belongs to the insurance company now.

As Greg said, the distance between running comfortably LOP and the engine quitting due to an overly lean mixture is not very far and doesn't allow very good control of engine power. I suggest you go back and re-read Deakin in detail, this is a complex subject to understand what is going on.

Let's assume you open the throttle on the take-off run and then decide to control engine power with mixture. If you only climb to (say) 2000' then the engine can produce nearly rated power. The mixture is set rich of ideal to aid in engine cooling at lower altitudes & high power settings - by running rich the temperatures don't climb to what they might at stochiometric mixture settings. Also, especially with a fp prop, the detonation margin is lower at higher power settings. So after leveling at 2000' you start bringing the mixture back, power output increases as the mixtures leans up, and you enter Deakin's 'red zone' at about 50F ROP, you continue leaning until LOP and set the power you require. This sounds great, but what if you have a carb? On most RVs its difficult to get a carb'd engine to run LOP (due to poor fuel distribution), it is possible, sometimes. So now you have potentially a rough running engine to deal with. Throw in some traffic and ATC instructions and your workload is getting high - why not just push the mixture in and use the throttle?

Using the mixture might sound like a great idea, but in practice it is rarely possible, especially at lower altitudes and especially with a carb. You will run the risk of leaving your engine in a potentially dangerous place if you are distracted (look at Deakin's discussion of 'parking the engine'). And you will find it difficult to precisely achieve the power settings you are looking for with a smooth running engine. Apart from that, go practice, find out how easy or difficult it is, tell us what you find out.
Pete

penguin said:

Using the mixture might sound like a great idea, but in practice it is rarely possible, especially at lower altitudes and especially with a carb. You will run the risk of leaving your engine in a potentially dangerous place if you are distracted (look at Deakin's discussion of 'parking the engine'). And you will find it difficult to precisely achieve the power settings you are looking for with a smooth running engine. Apart from that, go practice, find out how easy or difficult it is, tell us what you find out.

Click to expand...

I hope I didn't come across as suggesting that one ought to try and control power exclusively with mixture. I'm just trying to make sure I really understand the issues about why it may not be good practice. I think I understand the issues better now than I did when I first asked the question.

I should also note that my home airport is at 4500 MSL, so my normal flying is at 7000 MSL or above, usually with substantial leaning. In fact, with my limited instrumentation, my normal practice is to start leaning as soon as I leave the pattern. I pull the mixture until I see a slight RPM drop (usally a LONG pull on the mixture), and then enrich just a tad. I normally repeat the procedure again on reachng cruise altitude, and maybe mid-climb I'm climbing a long way. If the OAT is high, I might actually perform the procedure before takeoff, pushing in a little more (more rich) until I leave the pattern.

When I reach cruise, I normally not only lean, but also pull the throttle just a bit so that I'm 100-200 RPM less than full throttle. I guess mostly what I've been wondering is, what I used mixture to get that 100-200 RPM drop at cruise instead of using the throttle? Would I hurt my engine? Would I get better fuel economy? What I was trying to achieve a real maximum range fuel economy?

I can't imagine I would ever (short of an emergency) ever use the mixture for power reductions for normal descent, approach, and landing. But I really do want to make sure I understand the issues.

But it's still interesting to me that the cruise performance charts in the CAFE reports of the carburated RV9A all use wide open throttle, and control the fuel flow with mixture. The first chart starts at 10.0 gph, shows a peak EGT at 7.5 gph, but continues leaning to 6.1 gph, and shows that as the point with best mpg and best CAFE score. To me, that seems to be a wide range of fuel flows to be controlled exclusively with mixture. I assume the CAFE people knew what they were doing when they ran the tests.

TThurston said:

I should also note that my home airport is at 4500 MSL, so my normal flying is at 7000 MSL or above, usually with substantial leaning. In fact, with my limited instrumentation, my normal practice is to start leaning as soon as I leave the pattern. I pull the mixture until I see a slight RPM drop (usally a LONG pull on the mixture), and then enrich just a tad. I normally repeat the procedure again on reachng cruise altitude, and maybe mid-climb I'm climbing a long way. If the OAT is high, I might actually perform the procedure before takeoff, pushing in a little more (more rich) until I leave the pattern.

Click to expand...

Since we're next door at 4600' msl, that red mixture knob is going to be about 3/8 -1/2" out, right after engine start; and about 5/8" out on the takeoff run. This is around the standard minimum and perhaps more with density altitude.

edit: stretch that 5/8" to 3/4"

L.Adamson --- RV6A/ 0360

TThurston said:

But it's still interesting to me that the cruise performance charts in the CAFE reports of the carburated RV9A all use wide open throttle, and control the fuel flow with mixture. The first chart starts at 10.0 gph, shows a peak EGT at 7.5 gph, but continues leaning to 6.1 gph, and shows that as the point with best mpg and best CAFE score. To me, that seems to be a wide range of fuel flows to be controlled exclusively with mixture. I assume the CAFE people knew what they were doing when they ran the tests.

Click to expand...

To do it right (and safely) you need to run balanced injectors and full EMS instrumentation. Carb'ed engines are notorious for poor fuel distribution, and you can end up cooking your richest cylinder or two while trying to run the engine far LOP, putting one or two cylinders at peak EGT. Assuming you've got balanced injectors and good instrumentation on each cylinder, then yes, it's quite feasible to run deep LOP in cruise.

Get yourself on downwind, set up for landing and kill the engine and glide in. In a failure scenario like that there are just too many other things that you have to think about without having to baby the mixture, watch egts and chts etc. You need to look out for traffic, warn other traffic of your intentions, brief passengers blah bla blah.

I understand that your question was more technical in nature, but I just wanted to make the point that in a real life emergency your brain will shrink and it will be harder to focus on technical details. None of us would remember a thing we read here about mixture, LOP, detonation etc etc. Unload yourself of as much thinking as possible and fly the plane and only do enough radio work to get people out of your way and if you are alone in the circuit screw the radio altogether.

My 2 cents submitted in the hope that it is helpful.

TThurston said:

I hope I didn't come across as suggesting that one ought to try and control power exclusively with mixture. I'm just trying to make sure I really understand the issues about why it may not be good practice. I think I understand the issues better now than I did when I first asked the question.

I should also note that my home airport is at 4500 MSL, so my normal flying is at 7000 MSL or above, usually with substantial leaning. In fact, with my limited instrumentation, my normal practice is to start leaning as soon as I leave the pattern. I pull the mixture until I see a slight RPM drop (usally a LONG pull on the mixture), and then enrich just a tad. I normally repeat the procedure again on reachng cruise altitude, and maybe mid-climb I'm climbing a long way. If the OAT is high, I might actually perform the procedure before takeoff, pushing in a little more (more rich) until I leave the pattern.

When I reach cruise, I normally not only lean, but also pull the throttle just a bit so that I'm 100-200 RPM less than full throttle. I guess mostly what I've been wondering is, what I used mixture to get that 100-200 RPM drop at cruise instead of using the throttle? Would I hurt my engine? Would I get better fuel economy? What I was trying to achieve a real maximum range fuel economy?

I can't imagine I would ever (short of an emergency) ever use the mixture for power reductions for normal descent, approach, and landing. But I really do want to make sure I understand the issues.

But it's still interesting to me that the cruise performance charts in the CAFE reports of the carburated RV9A all use wide open throttle, and control the fuel flow with mixture. The first chart starts at 10.0 gph, shows a peak EGT at 7.5 gph, but continues leaning to 6.1 gph, and shows that as the point with best mpg and best CAFE score. To me, that seems to be a wide range of fuel flows to be controlled exclusively with mixture. I assume the CAFE people knew what they were doing when they ran the tests.

Click to expand...

The technique you're describing goes back (at least in aviation lore) to Lindbergh teaching fighter pilots how to extend their range in the Pacific during WW2. If you carefully peruse the Lyc operator's manuals, they will show that at some altitude (typically around 8k'), you need 2700 rpm to make 75% power; something many (most?) pilots don't seem to understand. You constantly hear things like, "Yeah; I only burn 7 gph at 75% power up at 8500 feet, turning 2400 rpm." Yeah; no they don't.

My understanding is that once you're high enough that it requires 2700 rpm to make 75% power, the most efficient way to operate the engine is to control with mixture. It reduces 'pumping losses' in the engine. Probably could be applied to lower altitudes, as well, as long as actual egt's are monitored & kept under control.

Charlie

Not meaning to hijack the thread but the question is related. When departing an airport with a DA of say, 4,000' would you lean to best power on runup and depart or or go full rich?

I was shown the lean to best power by an experienced pilot but I know the Lycoming manual says full rich for any power changes. Injected by the way.

Another option with a runaway engine would be to switch off the magnetos/ignition, and switch it back on briefly if you need a burst of power. That would be my first instinct in a stuck throttle scenario.

That might work in some engines. One of mine has a failure mode if the engine backfires, an intake balance tube blows off and the engine won't run until it's put back on.

So before accepting that as a valid response, ensure that it really is. Which it might be on a Lycoming. Best to check, though.

Dave
C180 flying,
RV-3B skinning the fuselage

penguin said:

Dude, Go read the 'Pelican's Perch' articles by John Deakin on AvWeb. Search for the ones on learning (there are about 100 and only 5 or so that are relevant). What you did is fine for an emergency, but could potentially be quite damaging to your engine (burnt holes in pistons) if you were to do it regularly. The engine will only run within a certain range of mixtures (say about 1 fuel to 15 air, but I can't remember exactly), too rich and it will quit, too lean and it will also quit. Why do you think every one else uses the throttle? If you want to use the mixture go buy a diesel engine! Pete

Click to expand...

Please explaine how too lean of a mixture can burn a hole in the piston.

Larry

David Paule said:

That might work in some engines. One of mine has a failure mode if the engine backfires, an intake balance tube blows off and the engine won't run until it's put back on.

So before accepting that as a valid response, ensure that it really is. Which it might be on a Lycoming. Best to check, though.

Dave
C180 flying,
RV-3B skinning the fuselage

Click to expand...

Please explain how the mixture setting alone (within the range provided by your servo) at any meaningful rpm level can induce a backfire

lr172 said:

Please explaine how too lean of a mixture can burn a hole in the piston.

Click to expand...

Don?t expect any reply. I?m guessing you didn?t notice that the post that you are responding to is from 2008

Erich

erich weaver said:

Don?t expect any reply. I?m guessing you didn?t notice that the post that you are responding to is from 2008

Erich

Click to expand...

And that this basic engine operation knowledge falls under the title of "RTFM". The knowledge base has already been called out, read it and the question answers itself.

FWIW, I had the problem the OP's instructor described in real life during my phase one testing.
I killed one mag in the pattern, flew it down to (very) short final, killed the other mag, and landed.
Turned out a nut on the cable bracket at the carb had loosened itself.

lr172 said:

Please explain how the mixture setting alone (within the range provided by your servo) at any meaningful rpm level can induce a backfire

Click to expand...

My posting wasn't about mixture adjustment but about shutting off the ignition. It followed (but didn't reference) one that mentioned the concept of shutting down the ignition and turning it back on as a method of reducing power. A similar concept was used on rotary engines in WW I.

On my carbureted C180 shutting off (or even checking) the mags in flight are specifically prohibited in the manual, for the reasons I described.

Dave