Full throttle not always best when LOP?

[aaronk25]

Cylinder pressure directly correlates to fuel flow when lop. What's your FF at these altitudes?

Use 10gph for 75% abd 8.2gph for 55%. Would the oil get dirtier quicker if running the same fuel volumn but more mp pressure compared to same fuel but lower, mp pressure, assuming both settings are LOP.

Does the extra air being compressed lead to higher cylinder pressures or is it only the gph that dictate icp?

[jetdriven]

Update on the fuel injection. Backing it off WOT at low altitudes raises EGT but only about ten degrees.I'd call my own hypothesis of an enrichment circuit as bogus.

Second, ten GPH at 1500' is ten GPH, but I have noticed pretty substantial variations in airspeed due to the degrees lean of peak. It really falls down badly leaner than 80 LOP. Depending on manifold pressure, 10 GPH could be 100 LOP, peak, or even rich of peak EGT. If I couldn't get it smooth at WOT (where it is sure to be on the lean side of the curve) I would set it to 27" and use that. However, depending on density altitude it can result in running very close to peak EGT with corresponding higher CHTs as well.

In short, it is a lazy method and at best a guess. I decided to fool with it some yesterday. 1000' density altitude, 67 degrees F, 2500 RPM. I will qualify this further that our EGT values usually peak at ~1520 in cruise. Peak EGT at this low of an altitude can be a but higher.

WOTLOP 10 GPH was a 1420 EGT which is 100 LOP. The airspeed dropped off 5 mph to 165. There was some cycle-to-cycle variability which was barely noticeable. CHT was very low for a M20J, averaging around 280-300. The engine was putting out power but it was down noticeably for the 10 GPH and it just felt too lean. Backing it off to 25" and richening back up to 10 GPH got the 5 mph airspeed back but EGT was pretty near peak and CHTs began to rise. On a hot summer day it would likely go to 380-400.

So now, the a-hah moment. Use MP to set EGT. RPM is still 2500, WOTLOP 10.0 GPH 1420 EGT. Back the throttle off until EGT hits 1470. For this particular day it was around 28-29". Moving the throttle lever back will lower fuel flow due to the design of the RSA fuel injection. So you richen it back to 10.0 GPH and re-adjust the MP for your target EGT of 1470. This does two things, it puts the engine right near the familiar region of best efficiency (~25 LOP), and it is a known safe EGT for cylinder pressure and temperature, and its easily repeatable.

You can go fast, lean, and safely, plus its a repeatable and easy to do procedure. Give it a look.

[carusoam]

Byron, once again for the M20A pilots... .??? (the slower folks in the ranks)

Comparing to the rule of 47... Aka the "key" number rule"

MP + rpm/100 <= 47 <= 65% HP. ( a safe place to operate, LOP). ("<=” is less than or equal to)

Byron's example 28" + 25rpm = 53. ( a number higher than 47, indicating higher than 65% )

How does it become safe to operate or what makes it safe to operate LOP at higher power settings?

What did I miss???

We're going to need to re-define the complex airplane to include higher levels of math to go with the gear and prop control...

Thanks and best regards,

-a-

Note the key number rule comes from the MAPA traing and the number 47 is specific to the model J.

Other planes may vary a bit...

[201er]

So now, the a-hah moment. Use MP to set EGT. RPM is still 2500, WOTLOP 10.0 GPH 1420 EGT. Back the throttle off until EGT hits 1470.

 

 

Well duh! On the LOP side, throttle controls cooling (air), prop controls power (rpm), and mixture controls degrees LOP (cooling and power).

 

Flying full throttle LOP at 10GPH below about 3,000ft is going to put you excessively far LOP and thus losing speed and running unnecessarily cool. The optimal solution is to go higher and stay WOT to improve TAS. Another option is to increase FF above 10GPH and take the extra power.

 

This is why Mike Busch is wrong with his FF/CHT only based approach without EGT. There come a million caveats and exceptions with his method which make it go from simple to needlessly complex. His method is good for when ATC briefly levels you at an altitude but with a further expected climb soon. Sure, pull back by FF and get it close enough for now. But when you level off for prolonged cruise, leaning LOP by CHT ensures you're in a safe place and optimizes the power you are going to get relative to that altitude, temperature combination using EGTs.

[Shadrach]

Byron, when on the deck I set for about 45 LOP WOT. Runs smooth, runs cool (CHTs <340 in the summer), goes fast (~157KIAS depending on temp and pressure). This is how I broke it in, 1800MSL, WOTRAO, varied RPM 2350-2700 and 40LOP.

[aaronk25]

Byron, when on the deck I set for about 45 LOP WOT. Runs smooth, runs cool (CHTs <340 in the summer), goes fast (~157KIAS depending on temp and pressure). This is how I broke it in, 1800MSL, WOTRAO, varied RPM 2350-2700 and 40LOP.

Back mp pressure down a couple inches or go higher and run 25lop at same FF and you will go faster.

[aaronk25]

Byron, when on the deck I set for about 45 LOP WOT. Runs smooth, runs cool (CHTs <340 in the summer), goes fast (~157KIAS depending on temp and pressure). This is how I broke it in, 1800MSL, WOTRAO, varied RPM 2350-2700 and 40LOP.

Back mp pressure down a couple inches or go higher and run 25lop at same FF and you will go faster.

[Shadrach]

Byron, once again for the M20A pilots... .??? (the slower folks in the ranks)

Comparing to the rule of 47... Aka the "key" number rule"

MP + rpm/100 <= 47 <= 65% HP. ( a safe place to operate, LOP). ("<=” is less than or equal to)

Byron's example 28" + 25rpm = 53. ( a number higher than 47, indicating higher than 65% )

How does it become safe to operate or what makes it safe to operate LOP at higher power settings?

What did I miss???

We're going to need to re-define the complex airplane to include higher levels of math to go with the gear and prop control...

Thanks and best regards,

-a-

Note the key number rule comes from the MAPA traing and the number 47 is specific to the model J.

Other planes may vary a bit...

Key number method no worky when lean of peaky....

When LOP, FF is the only way to derive power. The only problem is that in order for the FF derived power number to be accurate, you must remain in the lowest part of the BSFC curve. Further leaning or additional air from THE POINT OF max BSFC slows the combustion event so much that power falls off rapidly, which is germane to the thread title. I guess the short answer is, when LOP, pick the appropriate FF and then find the MP setting that corresponds to ~50LOP for high power ops, for low power ops do the same thing, but closer to peak will be more efficient as the BSFC curve falls off more rapidly at low power (when LOP).

[Bob - S50]

I guess I don't understand.  I see no reason to believe that FF is a good indication of power.  FF should only correlate with power if the ratio of fuel to air remains constant.  When ROP, put more fuel in and you are just dumping the extra out the pipe with no increase in power.  On the LOP side though, fuel becomes the limiting factor and any reduction in fuel will directly reduce power available.

 

To my mind the only things that affect power are the volume of air being pumped through the engine and the ratio of fuel to air.  High RPM and high MP give you maximum volume.  At full throttle and RPM, only mixture will affect power, with max power coming at about 50 to 100 ROP.  Any thing you do to reduce airflow either by pulling the throttle or the RPM will reduce power.  If you keep the mixture the same, FF should reduce proportionally.  However, according to Mike or John, changing the RPM has little impact on the mixture.  But pulling the throttle does, it should make the mixture richer which will require a mixture adjustment to get back to the same ratio.  If you are running LOP and you pull the throttle, the EGT should go up.  If you are running ROP and you pull the throttle, the EGT should go down.

 

Everything I've read says that 80 LOP may greatly reduce your burn but it will also greatly reduce your power and speed.  Supposedly right around 20-30 LOP +/- is supposed to be the most efficient (most HP/LB of fuel).

 

I don't think Mike says to set power with just CHT and FF, he just says the actual EGT number doesn't matter.  He just wants the CHT kept under control and you can do that either lots ROP or a little LOP.  Why waste the gas?  He just says that because of so many variables, EGT isn't important except in setting the mixture.

 

Am I confused?

 

Bob

[Shadrach]

Back mp pressure down a couple inches or go higher and run 25lop at same FF and you will go faster.

If I'm above 3k I run 25LOP if I'm above 7k I run 15LOP, as I climb I get closer to peak and then going ROP for ops above 12K unless I've got a good tail wind.

Down on the deck, which is to say below 2k I do what's needed to keep CHTs where I like them. With the ram air open on a cool fall day I'm pulling better than 28" at 1800 ft. I'm think 45LOP is right where I want to be for that power setting, but I'll try your recommendation and see what happens.

[Bob - S50]

Let me slightly modify what I said earlier.  It doesn't matter if you are LOP or ROP, if you pull the throttle back very much the EGT will drop because you are pumping less energy through the engine.  In theory, if you are ROP it should drop more than if you are LOP.

 

Bob

[Shadrach]

Let me slightly modify what I said earlier.  It doesn't matter if you are LOP or ROP, if you pull the throttle back very much the EGT will drop because you are pumping less energy through the engine.  In theory, if you are ROP it should drop more than if you are LOP.

 

Bob

Everything you've stated in both of your posts is correct. Deriving Power from FF when LOP is not perfect, but it's pretty close. The reason is because all of said FF is contributing to making power when on the lean side of peak vs. ROP where a portion of the FF is used to slow the combustion event and is not consumed during the power stroke

 

When LOP, the additional air from raising MP at a given fuel flow has minimal impact on power produced until one leans outside the flat part of the BSFC curve (see attached). Power then drops of pretty rapidly due the combustion event occurring so slowly that the power pulse delivered to the piston occurs well after the optimal time period. See the attachment, note that BSFC remains pretty flat between about 20 and 60 LOP. That tells us that for a given FF, any MP setting that yields an EGT within that flat range is going to give about the same HP.

Those of you experiencing a significant drop off in power within the flat part of the BSFC curve likely have other cylinders that are outside of the curve on the lean side...

[scottfromiowa]

Byron, when on the deck I set for about 45 LOP WOT. Runs smooth, runs cool (CHTs <340 in the summer), goes fast (~157KIAS depending on temp and pressure). This is how I broke it in, 1800MSL, WOTRAO, varied RPM 2350-2700 and 40LOP.

What is the HP% doing this? No worries about detonation? What is/was your fuel flow? I always thought there was a concern if over 75%?

[Shadrach]

What is the HP% doing this? No worries about detonation? What is/was your fuel flow? I always thought there was a concern if over 75%?

No worries. An IO360 would be a tough engine to induce detonation in if you tried and I certainly was not trying.  I do not know what my FF was as I have not had my JPI upgraded to accept the input from my recently installed transducer yet. What I do know is that my hottest CHT (#3) was around 335df. the next hottest was #2 about 318df, 1 and 4 were hovering just below 300df. These are pretty comparable to full rich CHTs if not a tad higher. There is no way, no how the engine was even close to detonation with those CHT numbers. I believe that the engine was producing well over 80% maybe closer to 90% but the only gauge I have to go by is IAS which was ~156KIAS or just inside the yellow arc (starts at 152KIAS) That's 10kts indicated over what the POH says I can do at 2500ft on 79.6%. On a high pressure fall day, I can pull well over 25" with the Ram Air open at 1800MSL and 2500RPM (maybe my MP gauge is off) but I have no idea what max, full rich power is because the POH does not have anything over 25" for that alt (see below).

 

 

Interestingly, at 2500 RPM the POH only gives power numbers up to 25inches of MP for sea level, 2500ft and 5000ft; all FFs give are for full rich.  This is a learning experience for me, but can anyone explain the following POH numbers to me MP and RPM are constant but FF and % power go up with altitude?  

 

POH numbers:

Sea Level  - 2500RPM and 25" MP equal 77.0% power on 14.2 GPH (full rich), Book speed 139kts @ 2740lbs, 141kts@2300lbs

2500ft        - 2500RPM and 25" MP equal 79.6% power on 14.7 GPH (full rich), Book speed 144kts @ 2740lbs, 146kts@2300lbs

5000ft        - 2500RPM and 25" MP equal 82.1% power on 15.2 GPH (full rich), Book speed 149kts @ 2740lbs, 151kts@2300lbs

 

Perhaps this is due to the servo leaning in a non linear way as it compensates for increased altitude and the 18df drop in temp on a standard day, but I'm inclined to think that the 18df differential does not add 10HP, but maybe I'm wrong... Thoughts?

[wishboneash]

I am waiting for the day when one of the engine monitor display graphically shows the red box and where the engine is currently operating. It seems like all the modern engine monitors have all the information to display this, from FF, MP, CHT, EGT, altitude, temperature. Why don't they have this representation?

 

WIthout much calculation or assumptions (I don't have MP, tach info in my engine monitor), if I keep my CHTs under 380 or so am I guaranteed to be out of the detonation range and out of the red box? I also keep EGTs under 1500 or so. I usually rapidly lean towards LOP and enrichen to 50deg or so LOP and FF around 9.5gph. There is no issue above 4500 ft, but below that altitude I am not sure exactly where I am. Hence would CHT give a clue? Thanks, this is an interesting discussion.

[aaronk25]

I am waiting for the day when one of the engine monitor display graphically shows the red box and where the engine is currently operating. It seems like all the modern engine monitors have all the information to display this, from FF, MP, CHT, EGT, altitude, temperature. Why don't they have this representation?

WIthout much calculation or assumptions (I don't have MP, tach info in my engine monitor), if I keep my CHTs under 380 or so am I guaranteed to be out of the detonation range and out of the red box? I also keep EGTs under 1500 or so. I usually rapidly lean towards LOP and enrichen to 50deg or so LOP and FF around 9.5gph. There is no issue above 4500 ft, but below that altitude I am not sure exactly where I am. Hence would CHT give a clue? Thanks, this is an interesting discussion.

I think you would be ok. As I just posted on another thread I will cruse full throttle at 4k ft at 2700rpms at peak egt if temps are 7c or colder, if it starts to get warmer I will lean to about 10-20 LOP. The respective fuel flows are about 11.3 and 10.6gph.

If your running 100ll and your heads are under 380, I would venture to say its impossible to get into detonation that is significant enough to cause and damage, unless you have substantial carbon deposits on the cylinder heads ect (from burning a lot of oil or running excessively rich) as they could act as a glow plug and start detonation.

That's assumes your using correct fuel, in a IO360 200HP. If you were flying a experimental aircraft with the same engine running straight Mogas you would have to limit fuel flow to about 9.2gph of 65% power when LOP. ROP isn't a issue given you are 100rop. I heard from a friend.

100ll is pretty darn good stuff (less the nasty lead sludge it creates in the engine) and pretty hard to make detonate, but high cht temps make thing happen exponentially faster.

I'd say if Cht are less than 360-380 there is no configuration you can out your engine in that would make it detonate on 100ll.

[Shadrach]

I am waiting for the day when one of the engine monitor display graphically shows the red box and where the engine is currently operating. It seems like all the modern engine monitors have all the information to display this, from FF, MP, CHT, EGT, altitude, temperature. Why don't they have this representation?

WIthout much calculation or assumptions (I don't have MP, tach info in my engine monitor), if I keep my CHTs under 380 or so am I guaranteed to be out of the detonation range and out of the red box? I also keep EGTs under 1500 or so. I usually rapidly lean towards LOP and enrichen to 50deg or so LOP and FF around 9.5gph. There is no issue above 4500 ft, but below that altitude I am not sure exactly where I am. Hence would CHT give a clue? Thanks, this is an interesting discussion.

Very unlikely to have detonation at 380df, but that does not mean you're being kind to your engine... If the OAT is in the low 20s, you have to be generating some serious heat to hit 380df CHT in level flight. Were would the same power setting put you on a 90 degree day? If the same power setting at an OAT of 90 would put you @ 425df CHT, then it's a pretty aggressive power setting. The cylinders will run cooler, because you're blasting them with 20 degree air, but that does not mean it's a good place to run your engine...

[aaronk25]

Very unlikely to have detonation at 380df, but that does not mean you're being kind to your engine... If the OAT is in the low 20s, you have to be generating some serious heat to hit 380df CHT in level flight. Were would the same power setting put you on a 90 degree day? If the same power setting at an OAT of 90 would put you @ 425df CHT, then it's a pretty aggressive power setting. The cylinders will run cooler, because you're blasting them with 20 degree air, but that does not mean it's a good place to run your engine...[/quote

In my opinion the limiting factor on these engines is heat. A 360 cubic inch liquid cooled engine can put out 200hp continuous. At 4000 ft were probably getting 180HP.

If there is more cooling available why not go faster? 8.7 compression pistons is pretty low, if it will stay cool, and I am in a hurry why not take advantage of the 20f temps. Heat kills not % of HP being used.

[KSMooniac]

In my opinion the limiting factor on these engines is heat. A 360 cubic inch liquid cooled engine can put out 200hp continuous. At 4000 ft were probably getting 180HP.

If there is more cooling available why not go faster? 8.7 compression pistons is pretty low, if it will stay cool, and I am in a hurry why not take advantage of the 20f temps. Heat kills not % of HP being used.

 

I haven't been on this thread in a while and just posted on the latest LOP thread about why you're only understanding half of the equation.  You're thinking the cold air in MN is helping you be kind to the engine, but your 85% power setting at 11.3 GPH is most assuredly NOT kind to the engine, even with cold OAT.  You need to learn about the relationship of the internal pressures to mixture setting.

[wishboneash]

Well, it is getting interesting. Is it heat or the pressure? I guess both are important. In my set up I have no gauge that tells me about being in the red-box and it seems CHT is not a direct indication of how I am treating the engine although it is better than nothing. So it comes down to looking at the tables in the operating manual and calculating my % HP output at a given OAT, altitude and adjusting the power by setting the mixture & MP setting to stay out of the red-box. However it seems like if I keep my CHTs fairly cool and reduce it according to the OAT correspondingly I am going to be safe in terms of ICP.

[Shadrach]

The relationship is more cause and effect. Heat radiated through the cylinders is the result of internal cylinder pressure.  CHT is just an indicator. 

[wishboneash]

-deleted-

gross errors in my conversions!

[wishboneash]

Do you have fuel flow instrumentation? That is all you really need in addition to the OEM stuff to very efficiently (and simply!) cruise LOP at 65% power and below. Upon leveling off, close cowl flaps, set the cruise rpm of your choosing, lean to the LOP fuel flow of your choosing, and you are done. Anything under 8.7 GPH will keep you under 65% as long as all four cylinders are LOP, which they will be at WOT below 8500 feet.

My preferred LOP fuel flow is 8.5 GPH, which will give you 140 to 145 KTAS depending on altitude. You can go a long way in a 201 at that power setting, and your engine will love you for it.

A JPI FS-450 costs about a grand laid in. Well worth it even if only to greatly simplify engine management.

Jim

 

I have the JPI-700. The issue I have is at lower altitudes where I could easily exceed 65% power without knowing it unless I start opening the Lycoming manual. Relying on CHT alone is not going to tell me everything it seems. If I want to operate on the edge of the red-box to get max speed, I would need better instrumentation. So I go conservative like you do as above to be safe.

[aaronk25]

I fly most of the time at around 9.2GPH.....but last night facing 40Kt headwinds I wanted everything I could get without running rich.  I still think the limiting factor of % of HP on these engines is keeping them cool.  I mean a marine engine at 360C.I. is rated at 350HP and easily can cruise at 300HP continuously.  So why, given that the oil and chts will stay cool is damaging to the engine?

 

On the experimental side, the Cool Jugs (liquid cooled cylinders for o-360) claim you can cruise at 100% because they stay cool.  Well 20F air is almost as good as liquid cooled.

 

I've read the Mike Busch stuff and he is more concerned about staying away from 50ROP and High CHTs than he is about internal Cylinder pressures. 

[201er]

I am waiting for the day when one of the engine monitor display graphically shows the red box and where the engine is currently operating. It seems like all the modern engine monitors have all the information to display this, from FF, MP, CHT, EGT, altitude, temperature. Why don't they have this representation?

Because nobody really knows where the red box really is. It's kind of an arbitrary concept with insufficient long term evidence on a lot of real world engines to prove that. However, it's pretty obvious that you want to avoid the worst parts of it and running your engine cooler is safer. I do see your point though. Would be cool if the EGT graph could change colors to like red, yellow, and green depending on what zone you're operating in (even if you could just set your own numbers).