Power Setting Observations

Having a Percent Power computation and display on my EFIS makes for the ability to see some interesting (if subtle) engine operating behaviors. Recently, I have taken some time to really observe what happens as I am setting cruise power, and thought there might be a little interest. What?s really neat is that you can actually see the relationship between Manifold Pressure, RPM, and Mixture settings ? all reacting just as you think they would, based on the simplest understanding of how the engine should work.

There are two basic things that I wanted to ?prove? to myself about the Percent Power calculation. The first was that the horsepower actually does go up with both RPM and Manifold Pressure (as shown by the Lycoming graphs). The second was that the power peaks when you lean for ?best power?. The first is ?Duh!? obvious?.so I really hoped the Percent Power computation reflected it properly. The second is simply a logical conclusion for an engineer. The engine?s combustion should be the best at the ?proper? (stoichiometric) air fuel mixture. Best combustion should be the point where you get the most power out of the fuel. Best power kind of implies ?most efficient? to me?.so that as you get off of the ?best mixture?, you should lose power. (I'm using the term "best" here in a fairly imprecise way - not implying best power or best economy.)

So?.does it work? You bet! Here?s how I set power, and what I see?

After take-off, I usually reduce RPM to about 2550, but leave the throttle all the way in. I climb this way, with the mixture full rich, until 5,000?, when I lean a bit ? not all the way, but just enough to smooth things out (Interestingly enough, the old rule of thumb that you can lean above 5,000? has you leaning at about 82% power?.). I usually target for cruise between 9-12K, or higher if I can get good winds. When I reach a suitable altitude in this range, I am usually already down to 75% power or less with the throttle all the way in, but if I level off lower due to headwinds aloft, I?ll bring the power back to about 77%, then reduce RPM to about 2350, which seems to be my smoothest point (where we dynamically balanced the prop to the accuracy of the instrument). As I bring the prop back, the % Power drops right down to 75% - I found that 2% drop through trial and error.

The next thing is leaning. Now I am not going to start another whole LOP/ROP/Peak argument ? I generally lean until all of the EGT?s match each other on the moving graph shown by the GRT monitor screen. This usually happens right about peak. I can generally lean a little further and still have a smooth running engine, but sometimes not?..I usually like to run where it?s smooth, simply for comfort. This also corresponds to a little less than the fuel flow shown on the Lyc graphs for the particular % power, so I feel like I?m doing well. Now the important thing for this discussion is that as I lean, with the throttle and prop controls fixed, the % power usually climbs by 1 or 2 percent (along with the MAP) ? confirming to me that you get more power as the mixture approaches stoichimetric perfection. Which is exactly what I wanted to prove to myself.

It?s nice to know that the physics works!

Paul

Paul, an excellent notaion of engine operations!

For those who want more to gnaw on, check out Manifold Pressure Sucks by John Deakin.

He writes for www.avweb.com and is SUPER knowledgeable!

If you haven't logged onto avweb before, you owe it to yourself to do so. You are required to sign up, but it's free and you will thank me later.

Best,

CJ

Paul: How does the monitor calculate %HP? Look-up table? Curious.

Semantics

Ironflight said:

The engine?s combustion should be the best at the ?proper? (stoichiometric) air fuel mixture. Best combustion should be the point where you get the most power out of the fuel. Best power kind of implies ?most efficient? to me?.so that as you get off of the ?best mixture?, you should lose power. Paul

Click to expand...

Paul we may be having semantic problems here. Best power in a Lycoming is that mixture setting that produces maximum HP for any given RPM / MAP condition (ie RPM and MAP are not adjusted). Best economy is that mixture setting that produces maximum Brake Specific Fuel Consumption (BSFC) for any given RPM / MAP condition.

In reality a chemically correct mixture provides neither the best Power NOR the best economy.

I would expect that about 50 to 75 degrees rich of peak EGT should produce highest percentage power and that as you leaned towards peak EGT (with fixed RPM and MAP) the percentage power should drop away. If the GRT does not show that then it could be a shortcoming in the logic upon which the percentage power algorithm is based.

Stoichiometric mixture provides a better BSFC than 50 degree ROP but less power. I suspect that is due to the fact that mixtures that are slightly rich actually have a faster flame front than chemically correct mixtures and therefore produce a higher Internal Combustion Pressure ( and therefore higher CHTs).

Of course if you can continue further LOP the BSFC improves even further (but the percentage power continues to decline). But presumably with a carbie you cannot get out there and retain a smooth running engine.

I'm not sure that peak EGT is a good place to cruise. Peak EGT means hottest exhaust gases at the EGT sensor in the exhaust pipe....which also means hottest exhaust gases flowing over the exhaust valves. Exhaust valves don't like high temps and they are arguably the most susceptible component in a Lycoming engine.

Good Points Bob

Bob,

All of your points are good ones, and I knew I was going to get in a little trouble using the term "best" in terms of engine operations. The post was just running a little long, and I was trying to economize on words!

What I was really trying to say was that there is a "peak" in engine power output as you lean from full rich through idle cutoff (for a fixed throttle and prop setting, power improves for awhile as you lean from full rich, and then it drops off again until the engine quits at full lean). I was actually trying to stay away from the whole debate on where to run....been in those threads before...!

I merely wanted to confirm that the Percent Power computation accurately reflected the physics of what should be happening in a "trend" sense - and it does. Although I don't know the actual algorithm used in the GRT monitor, it appears to be either a curve fit or interpolation from a data table which you enter on a setup page.

Where do I actually run my engine? Wherever folks want me to in order to avoid an argument..... (Most of the time, I lean until I get minimum fuel flow and the engine sounds happiest. This pretty much corresponds to the point we all ran for years before having instrumentation - lean until it is rough, then smooth it out. I haven't toasted an exhaust valve in all my years of flying....)

Paul

Ironflight said:

...lean until it is rough, then smooth it out. I haven't toasted an exhaust valve in all my years of flying...

Click to expand...

Incidently, this is exactly the procedure Lycoming recommends for carburated engines. I have four cylinder egts and sometimes, for my own entertainment, I look at what happens when I lean. As expected all the cylinders are different, some lean of peak, some rich and some at peak. Not much other choice except to ante up for fuel injection.

I have had no engine problems at all in my RV but did have some cylinder cracking and valve swallowing problems in my Piper Pacer. I suspect high CHT problems in that airplane but it didn't have enough instrumentation to know what was going on.

In the RV I do try to keep the CHTs reasonable and like being able to monitor them. Ususally I lean at all altitudes but on a hot summer day I grit my teeth and "cool with fuel" to keep CHTs around 400F in the climb.

Captain Avgas said:

I'm not sure that peak EGT is a good place to cruise.

Click to expand...

Citing contemporary wisdom, I'll argue that peak EGT is less detrimental than 50 ROP. See Deakin's articles galore.

Captain Avgas said:

Best economy is that mixture setting that produces maximum Brake Specific Fuel Consumption (BSFC) for any given RPM / MAP condition.

Click to expand...

Actually, I think that should be minimum BSFC (greatest efficiency).

Captain Avgas said:

I suspect that is due to the fact that mixtures that are slightly rich actually have a faster flame front than chemically correct mixtures and therefore produce a higher Internal Combustion Pressure ( and therefore higher CHTs).

Click to expand...

Precisely - the hydrogen in the fuel molecule is stripped off and burned extremely easily and early on in the actual combustion event, in an initial flame front, leaving carbon and a small quantity of singly hydrogenated-carbon intermediates to scavenge the rest of the oxygen in the chamber. At a stoichiometric mix, this takes a while and results in a less energetic "second flame front". This second flame front is much slower and is the reason we see EGT's rise on leaner mixtures - the carbon is still burning long after the main flame front has expired, and in many cases is still going after the exhaust valves have opened. When engineers speak of flame fronts in combustion chambers, they are almost universally talking about the hydrogen-burning event consuming the hydrogen off the hydrocarbon fuel chain. This flame front is responsible for about 80-85% of the total power extracted. The second carbon-burning flame front is slower and weaker and is usually ignored except for heating concerns. The reason peak EGT and peak CHT do not track identically is because of this second flame front, where the carbon is still burning as the exhaust stroke takes place. Combustion outside the combustion chamber does not contribute to CHT.
Best power mixtures rely only on the hydrogen burning event, and the carbon is basically discarded because it cannot produce power fast enough. In a diesel engine this is what produces the black exhaust (carbon and carbon monoxide) under heavy load. Gasoline has a lower ratio of carbon to hydrogen than diesel, so the blackened exhaust is much less noticeable. Modern auto gasoline engines finish the combustion of the carbon and carbon monoxide in the catalytic converter.

Paul and you other RV GURU's

Great thread. Learned alot.......but......Physics.......you really know that CHEMISTRY rules!!!!!!!!!!!! X moles of fuel X moles of oxygen......you get the idea.

Boy am I a sensitive old, used up, chemistry teacher or what.......

Paul and others thanks for your info and real world knowledge I just can't wait to put it to use..............................

Frank @ SGU RV7A..............still not done $$$$$$$$$$$$$

Don't know how GRT works...

but the engine monitor that I use can't measure percent power directly, but uses a mathematical formula to compute it based off of MP, RPM and I think fuel flow. If any of the inputs are inaccurate or the formula is not correct, you will not get a correct answer.
This is good enough to provide guidance for adjusting engine controls, but can't be used as an absolute measurement of power.
I think that a better indicator would be IAS as to if power is increasing, decreasing or peaking.

Still interesting information.

Kent

IAS is influneced by lift and sink

kentb said:

I think that a better indicator would be IAS as to if power is increasing, decreasing or peaking.

Click to expand...

But, if you maintain altitude, your IAS will increase in lift (rising air) and decrease in sink (descending air). Consequently, IAS would not ba a good indicator of power.

The initial premise of this thread is circular

kentb said:

but the engine monitor that I use can't measure percent power directly, but uses a mathematical formula to compute it based off of MP, RPM and I think fuel flow. If any of the inputs are inaccurate or the formula is not correct, you will not get a correct answer.
This is good enough to provide guidance for adjusting engine controls, but can't be used as an absolute measurement of power.
I think that a better indicator would be IAS as to if power is increasing, decreasing or peaking.

Still interesting information.

Kent

Click to expand...

I am one of the many with the GRT. What the GRT says about Pct. Pwr. is what you tell it to say about some combo of MAP and RPM. If you change the values in the table, the results for a given combo will change. Using GRT readings to confirm that Lycoming or Superior's charts are correct is circular because that's where the table entries are derived from. That said, I find it a useful readout and use it more than MAP or RPM in setting power - along with fuel flow.

airguy said:

Actually, I think that should be minimum BSFC (greatest efficiency).

Precisely - the hydrogen in the fuel molecule is stripped off and burned extremely easily and early on in the actual combustion event, in an initial flame front, leaving carbon and a small quantity of singly hydrogenated-carbon intermediates to scavenge the rest of the oxygen in the chamber. .

Click to expand...

Of course what I meant to say was maximum efficiency BSFC (which coincides with the lowest numerical value). Boy... this is a topic where you have to be so careful with words.

I really appreciated your enlightening description of the combustion event. I always wanted to know why a slightly richer mixture had a faster flame front than a chemically correct mixture...now I know. Good stuff.

As for Danny boy's comment. Well I didn't say 50 degree rich of peak was better than peak. It's probably a choice between the devil and the deep blue sea. At one you get max CHT and at the other you get max EGT....probably the engine doesn't like either at higher power settings.

Probably LOP is the place to be......clean, cool and just sippin' fuel.

Didn't mean to create a Black Hole.....

hevansrv7a said:

I am one of the many with the GRT. What the GRT says about Pct. Pwr. is what you tell it to say about some combo of MAP and RPM. If you change the values in the table, the results for a given combo will change. Using GRT readings to confirm that Lycoming or Superior's charts are correct is circular because that's where the table entries are derived from. That said, I find it a useful readout and use it more than MAP or RPM in setting power - along with fuel flow.

Click to expand...

You're right about what you're saying - the GRT numbers ARE created as a "real-time" implementation of the table. I wasn't trying to prove that the computer matches the table. I guess that what I found really interesting was that if you left RPM and the Throttle Plate fixed, and you leaned from full rich, the Percent Power (caused by a change in MAP, because the other variables are fixed) increases... It really isn't a confirmation of the Computation or the table - it is a confirmation that the engine produces more power as you get closer to "perfect" mixture.

Of course, we all know this because if you start with a full rich mixture at sea level, and leave it there until 8,000', and then lean - you get a measurable increase in rate of climb.....

Just for the Record, go back to the source

I am dizzy from detail, yes me, too much detail. Its shows you guys have an excellent understanding and what homebuilding is all about, learning and teaching. I'm still a simple lean to first to peak (and/or first roughness) and enrich to 100 rop or egt rise, if at or below 75% power (map/rpm).

Just to keep it real, here is a poor copy of the generic power/egt/cht/ff curve from Lycoming and what Lyc says (click).




This is what the graph says to me very simply and something to keep in mind always:


Just for grins and reference within the thread, this is Lycomings official word.

From Key Reprints: Leaning Textron Lycoming Engines

A. GENERAL RULES

1. Without exception, observe the red-line temperature limits during takeoff, climb and high performance cruise power operation.
2. Whenever mixture is adjusted, rich or lean, it should be done slowly.
3. ALWAYS RETURN MIXTURE SLOWLY TO FULL RICH BEFORE INCREASING POWER SETTING.
4. At all times, caution must be taken not to shock cool the cylinders. The maximum recommended temperature change should not exceed 50oF per minute.

B. LEANING THE NORMALLY ASPIRATED ENGINES

1. Use full rich mixture during takeoff or climb. Careful observation of engine temperature instruments should be practiced to ensure limits specified in Textron Lycoming operator?s manual are never exceeded. Refer to the aircraft POH (pilot?s operating handbook) or AFM (aircraft flight manual) for more specific instructions.

2. For 5000 feet density altitude and above, or high ambient temperatures, roughness or reduction of power may occur at full rich mixture. The mixture may be adjusted to obtain smooth engine operation. For fixed pitch propeller, lean to maximum RPM at full throttle prior to takeoff where airports are 5000 feet density altitude or higher. Limit operation at full throttle on the ground to a minimum. For direct-drive, normally aspirated engines with a prop governor, but without fuel flow or EGT, set throttle at full power and lean mixture at maximum RPM with smooth operation of the engine as a deciding factor.

3. For cruise powers where best power mixture is allowed, slowly lean the mixture from full rich to maximum power. Best power mixture operation provides the most miles per hour for a given power setting. For engines equipped with fixed pitch propellers, gradually lean the mixture until either the tachometer or the airspeed indicator reading peaks. For engines equipped with controllable pitch propellers, lean until a slight increase of airspeed is noted.

4. For a given power setting, best economy mixture provides the most miles per gallon. Slowly lean the mixture until engine operation becomes rough or until engine power rapidly diminishes as noted by an undesirable decrease in airspeed. When either condition occurs, enrich the mixture sufficiently to obtain an evenly firing engine or to regain most of the lost airspeed or engine RPM. Some engine power and airspeed must be sacrificed to gain a best economy mixture setting.

NOTE

When leaned, engine roughness is caused by misfiring due to a lean fuel-air mixture which will not support combustion. Roughness is eliminated by enriching slightly until the engine is smooth.

5. The exhaust gas temperature (EGT) offers little improvement in leaning the float-type carburetor over the procedures outlined above because of imperfect mixture distribution. However, if the EGT probe is installed, lean the mixture to 100oF on the rich side of peak EGT for best power operation. For best economy cruise, operate at peak EGT. If roughness is encountered, enrich the mixture slightly for smooth engine operation.

6. When installing an EGT probe, the probe must be installed in the leanest cylinder. Contact the airframe or kit manufacturer for the correct location. In experimental or custom applications, multiple probe instrumentation is required and several power settings should be checked in order to determine the leanest cylinder for the specific application.

7. During normal operation, maintain the following recommended temperature limits:

(a) Cylinder head temperature - limit listed in the Textron Lycoming Operator?s Manual.

(b) Oil temperature - limit listed in the Textron Lycoming Operator?s Manual.

8. For maximum service life, maintain the following recommended limits for continuous cruise operation:

(a) Engine power setting - 65% of rated or less.

(b) Cylinder head temperatures - 400oF. or below.

(c) Oil temperature - 165oF. - 220oF
------------------------------------------------------------------

All the info you Guys speak to is great but don't forget the basics. Often the ideal power setting does not occur in a Vacuum (no pun intended), like temp, tail winds, head winds and aircraft weight.

There is a fear of 50 rop. I say you just need an awareness. Lycs recommendation is peak or 100 rop or more, no between, with all the overriding caveats (smoothness and limitations). However 50 rop is an esoteric point on the curve that some point to or overemphasis. If you are flying real high (low power) and lean for smooth operations, it may happen to be 50 rop; that is fine, provide you are in the general limits.

All the details can lose some of the readers and pilots. It really is very simple. We don't have 350 HP engines, where 1% or 2% fuel savings is a lot. By all means lean and be smart with power settings, but at some point you need to be practical. Lycomings recommendations are the practical.

With Eng Monitors we can fine tune, but frankly you don't need FF, CHT or EGT to get around safely with reasonable economy. It is fun to take about it but a mental picture of that graph above is key and all you need to know, along with Lycs limits and cautions. Although Lyc does not say 75% power is a safe place to start to lean the 5,000 ft density alt should be a clue. Some say you can lean in climb or above 75% power. I would not, but its your engine, do as you like.

Paul:

Your experiment was very interesting, I'm still trying to understand if you learned something that could not be derived from looking at the charts? I think I've been thinking too much about house building lately rather than airplanes...

Anyway, isn't what you really want some form of direct torque meausrement (e.g. strain guages) so that you could see the actual percent power? Unless you are using the same carbs, timing, intake, mags, exhaust it seems like the test has limitations since all the data is pulled out of lycomings tables...

If I understand what you were looking for (peak power between rich and lean) a simple test can show this if your engine is setup correctly. Idle at 1000 rpm full rich and slowly lean, if you have a digital tach you will find a 20-40 RPM rise on the way down and then a dropoff. (my tach has 20rpm resolution) Since RPM is about equal to power this shows the trend you were looking for. If you had an oxygen sensor I *think* you could prove it.

All this discussion about LOP, ROP, etc. usually doesn't address one factor that has always confused the devil out of me. Lean WHAT to LOP, ROP, etc. I have a carb and, watching the GRT monitor closely over a couple of hundred hours shows that there is a significant spread between cylinders - and the distribution of temperatures changes at different altitudes and power settings. So do you lean with reference to the cylinder that has the highest EGT? If so, the others are dragging along behind somewhere with the coolest often 70-80 degrees cooler.

If, for example, you wanted to run at peak for best economy then you'd have one cylinder at peak, one at 20 ROP, one at 50 ROP, one at 80 ROP. Or would you lean until the hottest one is significantly LOP - again with the others distributed back along the curve? I've played with this while cruising at aprox. 65% power and the engine doesn't start getting rough until the leanest cylinder gets 60 + degrees LOP. Is just rich of here where you want to be for best economy? At power settings 65% or below will this hurt anything?

I've also tried the Lycoming "seat of the pants" leaning procedure (lean till rough and then richen "a little" till smooth) and I usually end up pretty much as described above (hottest cylinder LOP and the others dragging along behind).

Sometimes too much information just keeps you confused and worried - and keeps you from looking out the window!

jamiller said:

Lean WHAT to LOP, ROP, etc.

Click to expand...

It's pretty simple...

If you intend to run ROP, lean until a cylinder peaks. Run ROP relative to that cylinder's peak EGT.

If you intend to run LOP, lean until the last cylinder peaks. Run LOP relative to that cylinder's peak EGT.

If you've balanced your injectors, all cylinders will reach peak EGT simultaneously, so LOP/ROP is a trivial matter.

Hottest/coldest EGT has nothing to do with anything imho.

jamiller said:

All this discussion about LOP, ROP, etc. usually doesn't address one factor that has always confused the devil out of me. Lean WHAT to LOP, ROP, etc. I have a carb

Click to expand...

I think Dan has given the definitive answer but I'll add a couple of points. Firstly you have a carbie so LOP operations are probably not viable.

It doesn't matter that the EGTs are all over the place...that may be for any number of reasons such as the EGT sensors being further up or down the exhaust pipe (even a VERY small dimensional difference in sensor placement will result in a temperature difference as the exhaust gases are cooling very rapidly).

What is important for LOP ops is that the EGTs (whatever they are) should peak at virtually the same TIME (regardless of their value). As a rule of thumb it is desirable that as you lean the mixture the fuel flow (as say indicated by a fuel flow guage) is within roughly 0.1 gallon between the first cylinder to peak and the last cylinder to peak. For example if you are leaning out and the first cylinder peaks at 7.1 gals/hr flow, you want the last cylinder to have peaked by 7.0 gals/hr. That 0.1 gal/hr is called the EGT spread.

The trouble with a carbie is that the fuel/air distribution to the individual cylinders is so varied that the EGT spread is too wide. So you run into the problem you have described....even before your last cylinder has reached peak the first one to reach peak is so lean it is cutting out.

You really need fuel injection (and well balanced injectors) to successfully run LOP in the most efficient range (out at about 70 to 100 degrees LOP). And to get REALLY good fuel economy out there you probably also need electronic ignition. At very lean mixtures the flame front is so slow that you need some method to advance the ignition timing so that peak cylinder pressure is maintained around 16 degrees after Top Dead Centre which is close to optimum.

Shock cooling

gmcjetpilot said:

4. At all times, caution must be taken not to shock cool the cylinders. The maximum recommended temperature change should not exceed 50oF per minute.

Click to expand...

George, thanks for the valuable input. Intuitively one would think that there must be some truth in the above. I certainly like to practice engine management that does not result in rapid engine cooling. However.....

Many parachutist drop planes (climb full power to altitude, and then plummet back down at very low power...over and over again) regularly go to full TBO. Then there's all those ab initio training planes doing non stop circuits that also regularly go out to full TBO. This would seem to indicate that in reality it is very difficult to shock cool an engine which is still running (even at low power) because so much heat is still being internally generated.

In reality the worst shock cooling of an engine actually occurs when you turn it off at the end of the flight....typically 100 degrees per minute or higher (more than twice the Lycoming recommended cooling rate). Just food for thought.

chuck said:

Paul:

Your experiment was very interesting, I'm still trying to understand if you learned something that could not be derived from looking at the charts? I think I've been thinking too much about house building lately rather than airplanes...

Click to expand...

Another good point Chuck, and I guess that maybe I am just being to subtle. What I proved to myself was that the Percent Power calculation of the GRT system DOES accurately reflect the Lycoming tables. (I actually do believe the Lycoming tables, as they have had a few years to test and perfect them using direct measurements of torque, etc - I am not trying to recreate them.) Now that I have confidence in this real-time displayed number, I can leave the books and charts behind, because once I pick an RPM that is smooth and sounds nice, I can fly the airplane using Percent Power directly, rather than juggling RPM, MAP, and altitude in-flight to figure out where I am running. It greatly simplifies the task of figuring out what numbers to use, because there is basically only one you care about.

That's all - no mystery - not trying to test the engine really - just proving that the system
works.

Paul

shock cooling?

I

n reality the worst shock cooling of an engine actually occurs when you turn it off at the end of the flight....typically 100 degrees per minute or higher (more than twice the Lycoming recommended cooling rate). Just food for thought.

Click to expand...

if i understand the problem of shock cooling correctly (and that is an if, i grant you), then this is not a problem. as i understand it, the problem is the differences in thermal coefficients of the steel barrel and the aluminum piston. if the steel contracts more rapidly due to cooling than the aluminum does, the the gap between the piston and the cylinder wall decreases, which increases wear. naturally, if the engine is stopped, there is no friction, no wear.

i'm sure that someone who understands the issue more clearly will chime in (at least i hope so -- i'd like to think that i'm always open to learning more.)

How to make a VAF thread long...

Someone needs to mention the magic words LOP, ROP or leaning.
I guess this post will double the length of this thread.

In my previous post about using engine monitor for determining power, I think that I was asking a question more then trying to tell the answer.

So this is what I think that I understand:

The engine monitor uses sensor input (MP, RPM, ???) as input to a formula to model the engines power chart.
The power chart was developed by running the engine and directly measuring the power.

Now here is the question part:

If there is error in the sensor(s), won't the power computed be incorrect?
I.E. what if there were a leak in the MP sensor?

If anyone can improve my understanding here it would be much appreciated.

Kent

The shocking truth

johnp said:

I
if i understand the problem of shock cooling correctly (and that is an if, i grant you), then this is not a problem. as i understand it, the problem is the differences in thermal coefficients of the steel barrel and the aluminum piston. if the steel contracts more rapidly due to cooling than the aluminum does, the the gap between the piston and the cylinder wall decreases, which increases wear. naturally, if the engine is stopped, there is no friction, no wear.

i'm sure that someone who understands the issue more clearly will chime in (at least i hope so -- i'd like to think that i'm always open to learning more.)

Click to expand...

In theory rapidly reducing power could cause excessively worn ring grooves accompanied by broken rings (for the reasons you mention above). In practice it seems to be not so much of a problem as evidenced by the meat bomber engines often going to TBO. This reflects the fact that the clearance dimensions between moving parts in a Lycoming engine tend to be VERY generous (read SLOPPY!!). In reality the bigger danger to the rings caused by rapidly reducing power may be caused by ring flutter (letting the prop drive the engine) than actual "shock" cooling.

Another significant risk for Lyc drivers is cracks in the cylinder heads (most commonly found running between the spark plug and the exhaust port) caused by differential cooling leading to high internal stresses in the metallurgy....but this this type of stress is unrelated to whether the engine is running or not and in fact the worst stresses may be induced when the engine is turned off (and the temperature drop is at its highest).

I recommend good engine management practices but at the same time I suspect that the risk of damage caused by so called "shock cooling" of a Lycoming engine in flight is probably greatly exaggerated.

That's my opinion at this time.....but I'm always open to alternative views.

Not my jump plane

Captain Avgas said:

parachutist drop planes (climb full power to altitude, and then plummet back down at very low power...over and over again) regularly go to full TBO.

Click to expand...

I flew jump planes (very old tired C182's), my first real flying job, and we changed cylinders as much as oil. To be fair they would replace the old cracked cylinder with another old or older repaired cylinder (welded).

Actually we did partial power on with banked (2-3 g) descending spirals. However I agree with you about the "exaggeration", cylinder cracks are caused by many things, like design and just thermal fatigue. Old very high time cylinders are likely to crack at some point, regardless of how careful you are (especially with a welded jug). On the other hand one rapid throttle closure and decent will not cause a crack alone.

However (I think) there is some cause and effect or correlation to rate of cooling and cracks, but as you say paranoia is warranted. My EIS has a 50f rate of cooling alarm and I don't bail our or call mayday when it comes on. However I do like to plan descents to keep partial power up, partly for cylinders, partly for carb ice avoidance and part comfort.

power settings observations

Paul,
I have another question for you. I've never flown with as much data as I'm getting from the GRT and I was wondering what EGT temps you are seeing during your climbout? Mine are touching 1500 under high power settings (take off and climb) and go to the high 1300s when cruising. Seems high to me, but everything otherwise is pretty normal. Are you seeing similar numbers?
Rich Zeidman
RV7
O360a1a (field o/h)

High EGT's

RV7RICH said:

Paul,
I have another question for you. I've never flown with as much data as I'm getting from the GRT and I was wondering what EGT temps you are seeing during your climbout? Mine are touching 1500 under high power settings (take off and climb) and go to the high 1300s when cruising. Seems high to me, but everything otherwise is pretty normal. Are you seeing similar numbers?
Rich Zeidman
RV7
O360a1a (field o/h)

Click to expand...

Not answering for Paul, but adding data points - my GRT EGT's at best power - while going from rich to LOP - can go over 1500. I talked to Superior about it. There is really no standard for EGT because of all the variations. They did not see 1500 as a problem. CHT is the one for which there are limits. That said, others advise that you should be full rich in full power climbs and I'd guess 1500 is too high in that mode. Are you able to run rich enough in TO and climb?

Absolute EGT's Vary.....

I really don't pay much attention to absolute EGT's, because it depends so much on where, exactly you place the probes - closer to the exhaust port, they'll be higher, farther away, lower. I have faith that the exhaust valves aren't going to melt unless I operate way outside of recommended parameters, so I use the EGT's to tell me where peak is, and if all four cylinders are doing about the same thing.

That said, I see peaks right at 1400 or so - depends on altitude and ambient temps. I can get all four cylinders within ten degrees of each other, so feel they're as balance as the instrumentation is accurate. I see much lower EGT's in the climb because I am running rich, as was mentioned above. Also as mentioned above, it's CHT's that I actively manage for absolute values, and I usually will take action in a climb if they are headed above 400. Cruise for my engine is usually around 340-350 on CHT's - again, depending on ambient temps and final altitude.

I figure all the instrumentation gives us something to do on a cross-country while the A/P is flying the airplane....

Paul

My EGT probes are mounted 2" from the exhaust flange, per recommendations in the GRT 4000 installation instructions. I generally see peak EGT's of about 1460 -1480. Have seen almost 1500 a couple of times. I worried these temperatures were too hot as well, but after reading about the subject a bit have come to believe all is OK.