R172K EGT-dial

[BIRK]

Does anyone know the range of the EGT dial on the R172K (cessna hawk XP), i.e. what are the lower and upper limits?

The only numbers I've seen are that the space between each mark represents 25°F, but no word about what the first mark stands for (1000°F?) nor the last :confused:

Thanks

[javelin]

The value is not needed, what is required is to know where the peak is and then you have a reference to run lean or rich of peak EGT.

Fly at 5,000' or so, lean until the needle peaks then you have a reference to work with.

Various thoughts about best power but that is another discussion !

[BIRK]

Yeah I've always used the EGT meter to fly the airplane at ~75°F rich of peak.

However a SR22 pilot suggested that I could also try flying it at around 1370°F, since the SR22 has the best economy at that temp. Apparently the SR22 and R172K have very similar engines (basically the same according to him) so I'm curious to see if my usual mixture setting gives the same temp as the SR22(@ 20in MAP)

[javelin]

You are comparing Flintstones to Star Wars !

Our Dynon on our RV7 will give a digital readout of the EGT but it will be used in the same manner - find the peak and operate rich or lean of it.

To quote a specific temperature depends on where the probes are located, whether you have a fixed or constant speed prop, whether you are carburetted or fuel injected..................... Too many variables to be able to do a like for like comparison.

Go onto Avweb and read up some of the articles on engine management and leaning for best power/economy - you will find all sorts of information which will help make your mind up :ok:

[dont overfil]

The handbook i have seen for this type recommended 50 degees rich of peak for normal operations. It can be leaned at any altitude provided the power is set at less than 75%.
Surely Cirrus has lean assist on the Garmin 1000?

[IO540]

However a SR22 pilot suggested that I could also try flying it at around 1370°F, since the SR22 has the best economy at that temp.Somebody definitely needs to do some serious John Deakin reading!!!!

The absolute value of EGT has little relevance. What matters is the operating point relative to peak EGT.

I won't write reams (again!) but in a nutshell:

Peak EGT gives you best economy. Also called stochiometric. Actually 25F LOP gives you the best economy / best MPG (for almost any normal petrol engine) but the efficiency curve is very flat around there so one doesn't have to be too precise.

100F ROP gives you the best power for the amount of air you've got to play with. This is relevant to a) generating sales brochure performance figures :) or b) high altitude flight in non-turbo engines, say FL150+, where you are air-mass-flow limited, are already at full throttle, and just want the best power.

50F ROP (and the above mentioned 75F ROP is not far away from that) is the worst place to set an engine to (on any continuous basis) because this is the biggest detonation risk area. In reality, one gets away with that because the big old Lycos/Contis don't detonate below some 85% power and some 450F+ CHT which is pretty hot.

Incidentally, fuel injection units tend to be set around 150F ROP with both throttle and mixture levers full forward - this is the takeoff setting and delivers the certified 100% power with a very rich mixture which produces a lower CHT through wasting some unburnt fuel. This is done because the lightweight air-cooled engine is structurally not capable of dissipating the heat that would be generated at the 100% power output if one was running stochiometric.


Apparently the SR22 and R172K have very similar enginesWhat is an R172K? The SR22 is about 300HP, Conti.

[dont overfil]

IO540
I hear what you say, but why do manufacturers recommend this. Even the new ish 182 I fly the recommendation is 40 ROP. Admittedly below 75%
DO.

[IO540]

Because they sometimes haven't got a clue what they are talking about.

Not kidding.

Below 75%, 50F ROP won't do any harm and it will result in a cooler engine (for the given power output) than peak EGT would, but you are burning maybe 10-20% extra fuel in this condition.

And, in cruise, there cannot be a CHT management reason for this, unless this type has exceptionally poor cooling. This would be especially true if it is rated at 100% power continuously (as most GA types are).

You would be better off flying LOP. It's cooler and you get better economy. The only downside is that airflow imbalance between cylinders can cause excessive vibration - this is why people get matched injectors sets from GAMI.

Operating at 50F ROP, or any ROP setting really, masks mismatched airflows and stops pilots moaning about vibration.

[Cobalt]

I hear what you say, but why do manufacturers recommend this. Even the new ish 182 I fly the recommendation is 40 ROP. Admittedly below 75%

The manufacturer recommended settings were not as stupid at the time they were invented as they look now.

The main reason is the shape of the power curve as you change mixture. From 150 rich to peak it is relatively flat, once you lean beyond peak it falls steeply. Think about it - rich of peak you have more fuel than you can burn with the air going into the engine, so having, say, 10% less makes little difference. At peak you have just the right amount, so leaning by 10% you lose 10% power. (The fuel flow at 100 rich is approx. 15-20% higher than at peak, BTW)

Unfortunately, not every cylinder gets the same mixture, but the air flow in the induction system and - in individually fuel injected engines - imprecision in the injector nozzles mean that it varies.

Which means that if you lean towards peak, one or two cylinders will reach it first and then go beyond peak. This means that these cylinders will develop less power than the others and the engine runs rough.

Hence "stone age leaning technique No. 1" - without EGT: lean until the engine runs rough a little, enrichen until it is smooth again, done. Good enough.

Now add an EGT - "stone age leaning technique No. 2". Unless the EGT is at the hottest cylinder (which can vary from aircraft to aircraft) if you lean to peak using it, the engine runs rough because one or two cylinders might be already beyond peak. 50 rich is a safety margin to avoid rough runing.

Makes perfect sense, right?

This is what IO means when saying

Operating at 50F ROP, or any ROP setting really, masks mismatched airflows and stops pilots moaning about vibration

These ancient tehniques are now completely out of date because of two things:

(1) Multi-cylinder engine monitors.
(2) Balanced fuel injector systems and better inductions, reducing the mixture imbalance between cylinders

These allow for more sophisticated leaning techniques. 50F rich in a balanced engine is a really stupid setting - maximum engine stress with little benefit over 100 rich and little more power than peak.

The engine manufacturers are behind the curve because these two things were done in the aftermarket - a good dose of "not invented here". Also, none of the engineers around nowadays has actually designed the basic engines flying now. Little wonder many of them have less of a clue than people who modify them to make them work properly.

[IO540]

Agree with Cobalt.

Another thing is that the mfgs 'cannot' change their recommended procedures because the current ones are 'proven' over decades, which is quite important if it ever came to litigation.

Any change in the official operating conditions would open up a new door for litigation.

Currently, many engines don't make TBO but the owners accept that because ..... many engines don't make TBO and 'everybody else' accepts that as normal.

In recent years, some people have looked into this stuff in more detail.

Lyco are still, right now, doing seminars and presentations in which they pretend that this is all a load of rubbish.

[BIRK]

Thanks for the replies, I guess i should not believe everything I hear :}. I was doubtful about leaning the mixture to hold a constant temperature (1370°F or whatever), so thank for clearing things up :ok:

I guess you never stop learning, so only 100ROP from now on :\

[javelin]

I did say it was another discussion...............:ok: