The hot idle

[Flyrod]

Hey Don! Check out the thread below from the Engine section. It is all about Hot Idle and the associated instability noticed by many IO people in the RV community (myself included). It seems many are drawing the conclusion that it is something we just have to live with. I have the AFP system and the idle on a cool morning, first flight of the day is nearly as smooth as a baby's bottom. Later in the day after that $500.00 hamburger with the mid day sun keeping Florida in the mid to upper 80's, its chaos! The loopy idle is all over the place. If it is a long taxi, like arriving at Sun N Fun, you have to be on your toes with mixture and throttle to keep her running.

What is your expert advice, things to check or insulate such as injector feed lines? In my case, all fuel lines are fire sleeved and inline sensors are covered with reflective fiberglass/aluminum tape. The Distributor and stainless feed lines are not covered. The engine mounted fuel pump has a 1" tube blowing on it but no shield. The electrical pump and associated plumbing is all in the cockpit. There is no water separator.

As others have pointed out in the thread, Cessna's, although they have the same hot start requirements, seem to idle good when hot. Does the tighter cowling really make that big of a difference?

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[Don at Airflow]

Hot operation with Bendix or Airflow fuel injection is a problem since the nozzle system runs at pressures around 0.03 to 0.06 PSI at idle. At full throttle the nozzle pressure typically runs 7-10 PSI at max power. Since the maximum engine driven fuel pump pressure is 22-26 PSI the injector nozzle size plus the other pressure drops in the system (fuel control, flow divider etc.) is determined at the full throttle fuel flow. This is where the .028 nozzle size comes from. At idle there is basically no pressure in the nozzle lines. Since Avgas boils at around 130 degrees F at sea level, you can easily see that there can be idling problems when the ambient temps get hot. This is about the time of year the phone starts ringing with hot idle problems. I guess everyone forgot about last summer. As for the 172?s, they have problems too. Just talk to some of the operators in Florida at Emery Riddle. Sitting on the taxi way in the summer time for 20 minutes idling; same problem.

How can this be addressed? Don?t idle the engine when it?s hot outside. Well I guess some people wouldn?t like that idea. But seriously keep the idle speed up as high as possible. If the engine starts to run rough while idling, run the engine up to clear it out. This will get some cooler fuel in the system. Keep the boost pump on. Make sure the installation has the minimum amount of fuel volume in the engine compartment. Fire sleeve all hoses, no solid metal tubing in the engine compartment (except for the injector nozzle lines). Some times it?s possible to run smaller injector nozzles to increase the nozzle backpressure. This is based on the available inlet fuel pressure and maximum fuel flow the engine uses. The cowling design has a lot of influence on these problems too. Pressure cowlings typically are tighter around the cylinders and don?t let as much air in the cowl at idle standing on the ground so these installation may have hot idling issues.

Hope this explains some of these issues. If you have more specific questions about your particular installation, just give me a call at the shop.

Don.

[Bob Brown]

Thanks for the great reply to that post Don. Assuming all the other stuff is done (sleeved hoses, heat shields, short runs, etc) Do you think it makes any difference to lean aggresively on the ground or is that just my imagination? To rephrase that, full rich...full lean...what do you think?

[Don at Airflow]

My theory is there is a idle mixture adjustment for a reason. Why not adjust the idle mixture correctly so that you don't have to lean the engine on the ground. There may be some instances that this is necessary however. That is when your at a high DA other than you own home base. It may be necessary to lean the idle mixture with the mixture control in the cockpit in these instances. We typically find the 50 RPM rise at ICO is too rich. Most of the engines we set here, have no rise at ICO. Set the MAP for the lowest reading at 750 RPM, and as long as the engine takes the throttle off idle your good to go. With the idle mixture set this way there is typically no rise at ICO and you don't have to lean the mixture while taxing around.

Don

[AlexPeterson]

Why not adjust the idle mixture correctly so that you don't have to lean the engine on the ground.

Don, the way I have to do it is to set the idle mixture for minimum rpm rise when the OAT is cold, as in winter operations. It is my understanding that the idle fuel flow is not compensating for air density (as it does at higher throttle settings). So, along comes warmer weather and the idle goes way rich. I haven't found an idle mixture setting which will allow it to idle well in winter yet not "need" leaning in summer. At least when I had aircraft spark plugs...

That being said, now that I have automotive plugs and EI, there is no reason not to run it rich during summer idling. This keeps more fuel flowing through, which delivers fuel that is some unknown amount cooler.

Originally Posted by Don at Airflow

Some times it?s possible to run smaller injector nozzles to increase the nozzle backpressure. This is based on the available inlet fuel pressure and maximum fuel flow the engine uses.

I've thought about doing that, but my injectors are pretty well balanced right now so I'm hesitant to monkey around with it. If I were to consider this, how does one determine how much smaller to go?

Also, this brings up a question I've been wanting to ask for a while... I've read numerous posts about people using the smaller restrictors that you sell to balance their injectors for LOP operations. However, the engine books that I've read and Lycoming Service Instruction 1414A all mention that restrictors should be kept with their respective injector bodies (like when they're removed for cleaning).

So... How does one comply with that advice while simultaneously trying new restrictors with smaller holes? Is the advice about keeping restrictors and bodies together outdated?

Thanks.

[Flyrod]

Geoff,

I am not sure but I think they are talking about not mixing and matching used inserts. I think you can put a new one in any injector

Don,

Thanks for the great reply and help. It is great having an OEM that stays in touch with the sometimes stumbling flock.

[Don at Airflow]

Originally Posted by Geoff

I've thought about doing that, but my injectors are pretty well balanced right now so I'm hesitant to monkey around with it. If I were to consider this, how does one determine how much smaller to go?

Also, this brings up a question I've been wanting to ask for a while... I've read numerous posts about people using the smaller restrictors that you sell to balance their injectors for LOP operations. However, the engine books that I've read and Lycoming Service Instruction 1414A all mention that restrictors should be kept with their respective injector bodies (like when they're removed for cleaning).

So... How does one comply with that advice while simultaneously trying new restrictors with smaller holes? Is the advice about keeping restrictors and bodies together outdated?

Thanks.

The fact is that if you have stock nozzles in your engine (all .028 restrictors) they are not matched to the bodies. Nor are they matched to the engine cylinder (we are talking here about Bendix/ Precision or Airflow nozzles only). Used inserts or not they should all be the same. I ought to know, I was the engineer at Bendix when the two-piece nozzle was designed, and worked on the project that developed the machining processes for making the restrictor. If you have tuned the nozzles to get all the EGT?s to peak at the same time, then you have to keep the restrictor matched to the cylinder that it came out of. Records are essential here or you can easily destroy your engine. I?m not going to second-guess why Precision say?s the nozzle restrictors are a matched assembly to the body they come out of. You think it could be a marketing ploy so you have to buy a complete assembly if you loose a restrictor???

You are correct that the idle mixture is a fixed flow after it is set and will not change with DA (but neither will the mixture on a carb), so you have to change it from summer to winter to get ideal mixture. To me it?s not a big deal to give the mixture linkage a half to one flat adjustment when doing an oil change to get the mixture dialed in for the change in seasons. I?m sure you all do a run up with the cowl off to check for leaks after you done changing the oil. Right?

It may be possible to run smaller restrictors, but the size must be determined by the maximum fuel flow the engine uses, and the engine driven pump pressure at the max fuel flow. We can calculate this. And if you have already done the nozzle tuning on the standard nozzles, you?ll have to start all over (a marketing ploy for us). But if you want to come to our shop on a weekend I can probably fix you up with some smaller restrictors.


Don

Originally Posted by Don at Airflow

It may be possible to run smaller restrictors, but the size must be determined by the maximum fuel flow the engine uses, and the engine driven pump pressure at the max fuel flow. We can calculate this. And if you have already done the nozzle tuning on the standard nozzles, you?ll have to start all over (a marketing ploy for us). But if you want to come to our shop on a weekend I can probably fix you up with some smaller restrictors.

I didn't do any tuning. I was just very lucky that my stock .028 restrictors result in a 0.4 gph or less spread at peak EGT. That's good enough for me, so I decided not to mess with it.

I'd love to come by your shop, but since I'm on the opposite coast that probably won't happen any time soon. Thanks much for the offer, though! I may make the trip some day to attend one of your fuel injection classes, though. I hear they're excellent.

Incidentally, my engine driven pump pressure has always been on the low side of the normal range at max fuel flow. Is there a way (or is it necessary) to bump it up a few PSI, or is that all internal to the pump?

Thanks again..

[Walt]

Has anyone installed the .025 nozzles with an O360? Don suggested I try this for the hot idle issue and perhaps enjoy better atomization at the same time, but pump pressures and fuel flow "may" be an issue. Anyone have any experience yet with this setup?