Leaning and Fuel Injection Engines

chris

Well-Known Member
Hey guys,

Got some Qs regarding mixture control and fuel injection. These come after reading the "mixture magic" article on avweb (pelican's perch).

1. Let's say I'm flying at 9,000 ft and I've leaned to peak EGT. If I change my power, will I have to relean it or enrichen it again? I always thought it would be necessary to enrichen your mixture with power increases (more power= more air= more fuel needed). Or, if its a fuel injection engine, will the fuel air control unit take care of this? Does it differ for carb engines?

2. Why is it that fuel injection engines pose more problems with hot starts? What makes them more prone to fuel vaporization? ie. why is it that it is recommended to wait 45 minutes to restart a fuel injected engine after shutdown?

3. The author of the article suggested that if you have problems trying to start a hot fuel injected engine, you should turn the fuel pump on and bring the mixture to ICO for 60-90 seconds- how does this do anything? I understand that if the mixture was rich you would certainly flood the engine, but if the mixture is at ICO, can you still get fresh cool fuel flowing thru the lines (i.e if the mixture control is at ICO, I thought all fuel flow is stopped?) I've never heard of this technique before and I found it odd.

4. Why is it that when starting the plane, if its a cold start, the mixture control is full rich, and if it's a hot start, the control is at ICO? There's probably a simple answer to this one which I am completely overlooking.

Also, if anyone read the article, what do you think of it? Do you agree with his recomendations and opionion? I really liked his article on Manifold Pressure and Props, but this one was a little tougher to follow.

Thanks in advance.
 

pilot602

If specified, this will replace the title that
1. Any power change will require you to "re-lean" (which may include richening the mixture). And, for lower power (up to about 250hp engines) the "lean-till-rough" method is just as good and probably better than using the EGT method.

2.Fuel in hte injection lines vaporizes from the heat ofthe engine block/ambient temperature. When the whole thing cools down the fuel returns to a liquid state. Carbs don't have this problem because they don't utilize fuel lines to each individual cylinder.

3. & 4. I have a basic understanding of these but since I fly a carb'ed aircraft I'm not too famililar with injection SOPs. I.E. Someone else can answer these!
 

EatSleepFly

Well-Known Member
[ QUOTE ]
3. The author of the article suggested that if you have problems trying to start a hot fuel injected engine, you should turn the fuel pump on and bring the mixture to ICO for 60-90 seconds

[/ QUOTE ]

Well, this lowly flight instructor and frequent pilot of a high performance, injected aircraft is suggesting that you follow the procedures for the particular airplane you're flying.

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4. Why is it that when starting the plane, if its a cold start, the mixture control is full rich, and if it's a hot start, the control is at ICO? There's probably a simple answer to this one which I am completely overlooking.


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Those are not set-in-stone starting procedures. It all depends on the airplane.

I don't really care for these articles that generalize everything. Talk to a mechanic and a pilot who are both experienced in whatever particular airplane you will be flying. That is your best bet for getting good advice and answers.
 

Alchemy

Partner, Ally, Friend
[ QUOTE ]
3. The author of the article suggested that if you have problems trying to start a hot fuel injected engine, you should turn the fuel pump on and bring the mixture to ICO for 60-90 seconds

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Never heard of this before, and don't know what good it would do. Maybe it would "circulate" normal temperature fuel from the tanks through the fuel lines to help cool them down without flooding the engine (since the mixture is at ICO). You would probably get Fuel flow in the lines between the fuel pump and the fuel control unit but not in the lines between the fuel control unit and the fuel discharge nozzles. So it could concievably help cooling the lines between between the fuel pump and the FCU, but wouldn't help much with the lines closest to the cylinders, which are the ones that get heated up the most and are the worst culprits of vapor lock.

[ QUOTE ]
4. Why is it that when starting the plane, if its a cold start, the mixture control is full rich, and if it's a hot start, the control is at ICO? There's probably a simple answer to this one which I am completely overlooking.

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I've been taught to ALWAYS start lycoming fuel injected engines with the mixture at ICO, boost pump on, starter engage, and mixture rich when the engine fires. The airplanes POH checklists and our mechanic agree with this method. In fact, I've see people who tried starting it with the mixture rich (cold) and couldn't get it started.

Never flown a continental fuel injected, but you should refer to the aircraft's POH and the Engine Manufacturers reccomendations to determine the proper engine start sequence.
 

E_Dawg

Moderator
Hey Chris... That article was pretty good and well researched; that guy's got some fancy engine monitoring equipment with those GAMIjectors that allow him to really fine tune his lean of peak operation. On most planes if you try to run lean of peak like he describes you'll get the roughness he talks about because one of the cylinders will produce less power than the others.

Also, the EGT gauge is either on one of the cylinders (giving only a partial picture) or on the exhaust manifold for all the cylinders (giving an average picture). So you can't really know what the EGT is for any particular cylinder without the instrumentation that guy has.

Anyway, on the Bendix RSA system (Lycomming engines) it'll measure the pressure in the intake manifold vs the pressure over the venturi (farther down the manifold). That pressure indicates throttle setting and regulates the amount of fuel to the cylinders via the fuel control unit.

The Contennial system does the same thing but it just measures throttle placement directly (and doesn't need the venturi).

So you can change throttle settings and the FCU will adjust the amount of fuel going into the cylinders to keep the same fuel / air ratio. The carbeurator does the same thing, just in a different way.

But then, if you are increasing power you might want more fuel for cooling and would have to richen the mix.

On hot starts I personally have had success by opening the throttle a quarter inch, mix rich, pump on for 10 sec or so. Then you know where you are (fuel in the lines), vs. 'playing with it' and not being sure whether there's fuel there or not. So then it's just mix back throttle full pump off and crank it until the mixture's 'right' and it will fire. Once it does start, throttle back and sloooowly push the mix back in so it doesn't flood again. But then this is on a relatively low power Lycomming IO-360.

And when you take a look at the 'spider lines' from the FCU to the cylinders, they're about 1/8" thick or so... any vaporization will cause bubbles that block the flow of fuel to the cylinders.
 

ananoman

New Member
[ QUOTE ]
1. Let's say I'm flying at 9,000 ft and I've leaned to peak EGT. If I change my power, will I have to relean it or enrichen it again? I always thought it would be necessary to enrichen your mixture with power increases (more power= more air= more fuel needed). Or, if its a fuel injection engine, will the fuel air control unit take care of this? Does it differ for carb engines?

[/ QUOTE ]

It depends...On some engines when you change the throttle setting, the mixture will change too. These engines enrichen the mixture more than necessary for extra cooling at full throttle. If you have one of these and you were running full throttle at altitude, you may have to enrichen the mixture to get the engine to run well at a lesser throttle setting. Just pulling back the throttle from full open to a lower power setting has the effect of 'leaning' the engine. Other engine types will correctly meter fuel automatically when the power is changed. The only way to know for sure what your engine does is by experience.

If you make a power change and it still runs smooth, an adjustment is not really necessary. If you have time, it may be a good idea to relean just to make sure you get best economy. If after flying the aircraft several times, you note that the mixture is close to where it was before releaning you can feel safe that releaning is not required.

On a normally aspirated engine you can not really hurt the engine by leaning at 65% power and below no matter what you do. At 9,000' you should be running at about 73% power when full throttle. It would take a gross error to cause damage at this power. You would notice the engines displeasure by rough operation. No matter what the EGT says, if you have a CHT gauge, this should stay within limits at all times. You should be looking at CHTs of around 400 degrees in cruise during normal operations.

[ QUOTE ]
2. Why is it that fuel injection engines pose more problems with hot starts? What makes them more prone to fuel vaporization? ie. why is it that it is recommended to wait 45 minutes to restart a fuel injected engine after shutdown?

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As others have said, the fuel lines from the flow divider to the individual injectors run right over the hot cylinders and are prone to vapor lock. It would also be possible to have problems with hot fuel vaporizing in the flow divider as well. I have not personally had problems starting hot engines, but most of my experience is with the Lycoming IO-360 and IO-540. If you do what you are supposed to, it will start.

[ QUOTE ]
3. The author of the article suggested that if you have problems trying to start a hot fuel injected engine, you should turn the fuel pump on and bring the mixture to ICO for 60-90 seconds- how does this do anything? I understand that if the mixture was rich you would certainly flood the engine, but if the mixture is at ICO, can you still get fresh cool fuel flowing thru the lines (i.e if the mixture control is at ICO, I thought all fuel flow is stopped?) I've never heard of this technique before and I found it odd.

[/ QUOTE ]

On some fuel injected engines, more fuel is sent to the flow divider than the engine can use. The extra fuel is returned to the fuel tank. By leaving the mixture to cut off and turning on the fuel pump, you are circulating cooler fuel to the divider and purging the main fuel line and the flow divider of vaporized fuel 'bubbles', making starting easier. (It is very important to understand your fuel system. On some of these aircraft you have to use the correct fuel tanks for takeoff and initial cruise. For example, if your aircraft returns this 'extra' fuel to the main tanks and you take off on the aux. tanks, your airplane will be pumping excess fuel into the main tanks. If the mains are full, it will be going overboard out the fuel vents. If you are taking a long flight, you may be suprised to run out of fuel short of your destintation.)

[ QUOTE ]
4. Why is it that when starting the plane, if its a cold start, the mixture control is full rich, and if it's a hot start, the control is at ICO? There's probably a simple answer to this one which I am completely overlooking.

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I never use this procedure, but the theory is sound. When you start a cold engine, the fuel does not want to vaporize. It wants to stay in liquid form, making starting rather difficult. If you engine is 'hot', the fuel will vaporize quickly when you crank the engine and putting the mixture to rich (effectively priming the engine) may not be necessary.

This is not really different than starting a carburated engine in warm weather. If it is cold, you prime. When it is hot you can just crank the engine and start without priming.

[ QUOTE ]
Also, if anyone read the article, what do you think of it? Do you agree with his recomendations and opionion? I really liked his article on Manifold Pressure and Props, but this one was a little tougher to follow.

[/ QUOTE ]

He knows what he is talking about. Unfortunately there is not much written about proper engine operation. We are always told to 'read the manual'. Some times this is ok, but often things in the manual are confusing, poorly explained, or just plain incorrect. I would suggest that you read his other articles on engine management. Most do not apply to the average Skyhawk pilot, but I think it is important as a pilot know what is going on and why.
 

ananoman

New Member
[ QUOTE ]
1. Let's say I'm flying at 9,000 ft and I've leaned to peak EGT. If I change my power, will I have to relean it or enrichen it again? I always thought it would be necessary to enrichen your mixture with power increases (more power= more air= more fuel needed). Or, if its a fuel injection engine, will the fuel air control unit take care of this? Does it differ for carb engines?

[/ QUOTE ]

I would just like to revisit this question. The reason we lean at altitude has nothing to do with the power setting. (Although it is true that we usually do not want to lean above 75% power.) We lean because the carburetor meters fuel and air by VOLUME.

In other words, if at sea level 'X' cubic feet of air are sucked through the carburetor, then a volume 'Y' of fuel will be mixed with the air. The problem is that as pressure altitude increases, the density of the air decreases. So there are alot less oxygen molecules in the same volume 'X' of air sucked through the carburetor than at sea level. But, the carburetor still mixes the same volume 'Y' of fuel with the less dense air. Now we are running too rich, making less power and wasting fuel. Leaning lessens the amount of fuel metered for a given volume of air, restoring the correct balance of fuel and air entering the engine.

In a normally aspirated engine, this reduction of air density causes the loss of about 3% engine power for every 1,000' of pressure altitude above sea level. This is why the best cruise performance in a normally aspirated airplane is usually around 7,000'-8,000'. At these altitudes you will be at full throttle and approx. 75% power. This is the highest altitude you can maintain max cruise power, while still benefiting from the lower drag due to the thinner air.
 

chris

Well-Known Member
Thanks for the responses.

One thing though that is confusing me...

If the Fuel Control Unit (FCU) in fuel injected engines helps maintain the same fuel/air ratio for throttle changes, why then is it necessary to relean or enrichen the mixture with power changes? Shouldn't the FCU automatically take care of it (i.e. if you are flying a 172 and lean it to Peak EGT at 75% power, and then reduce power to 65%, will you not still be at Peak EGT?... doesn't the FCU keep the ratio constant?)

Thanks again.
 

pkloop

New Member
From Alchemy "I've been taught to ALWAYS start lycoming fuel injected engines with the mixture at ICO, boost pump on, starter engage, and mixture rich when the engine fires. The airplanes POH checklists and our mechanic agree with this method. In fact, I've see people who tried starting it with the mixture rich (cold) and couldn't get it started. "


Ditto for me.
 

E_Dawg

Moderator
[ QUOTE ]
If the Fuel Control Unit (FCU) in fuel injected engines helps maintain the same fuel/air ratio for throttle changes, why then is it necessary to relean or enrichen the mixture with power changes?

[/ QUOTE ]

It's not. Just think that you can go from full throttle to idle back to full without killing the engine and without touching the red knob.


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Shouldn't the FCU automatically take care of it

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Yeah, that's the purpose of the FCU and one of the purposes of the carburetor.
But remember that, by design, the FCU / carb only takes the volume of the air into account... but the fuel / air ratio is actually the weight of the fuel divided by the weight of the air.
So as we climb we need to lean even with the FCU / carb because the same volume of air will weigh less at altitude.


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i.e. if you are flying a 172 and lean it to Peak EGT at 75% power, and then reduce power to 65%, will you not still be at Peak EGT?...

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Not sure... you will be near it but you may need to fine tune it a bit.
 
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