M20J vs. M20R comparison

[DonMuncy]

Intuitively, it would seem that higher power is harder on an engine, but has anyone ever done any studies on that fact. It would seem that the gurus in Ada OK may have done some work on that. I know they have done a lot of stuff on cylinder temperatures and pressures, but it would be very interesting to know if full power without getting into the high pressure/high temperature area results in measurable or significant effect on engine longevity. 

[Andy95W]

5 hours ago, kmyfm20s said:

Power is harder on an engine. In a NA engine the only way to increase power is to increase RPMin higher altitudes. I really don't think a engine cares if it's at 75% power at a high or low RPM. There is a slight bit of power that is lost to overcome friction at higher RPM and with friction there is more wear.

 

4 hours ago, gsxrpilot said:

Technically it's internal cylinder pressure along with heat, that kills engines. But since we don't have a way to monitor ICP in flight, we do it with a proper combination of power/mixture/rpm.

I agree with both the above with one addition: vibration.  Smoother is better, especially for the airframe and pilot fatigue.  If 2550 is smoother than 2650, I'd go for smoother operation and not worry about a slight increase in ICP.

[jlunseth]

There is a locational issue to the durability question, which makes it hard to pin down.  For example, LOP, to speak in generalizations, results in an EGT that is about 100 dF higher than an equivalent ROP power setting.  So one question that was raised early on, was whether that would result in reduced valve life because the valves are operating in a higher temperature environment.  Or reduced durability of the collector which is going to run hotter.  In my aircraft I run the turbo harder to produce a particular LOP setting, in an effort to get the mix lean enough.  I will generally run at 34" MP instead of the 28-30" I would see at a ROP cruise setting (I vary the power LOP by adjusting fuel flow, not MP).  Further, the TIT will be higher because the EGT's are higher.  GAMI says that the turbo will be fine at cruise operations up to the redline.  I try to keep it at or below 1600, but that is still hotter than it would be if I ran ROP, and the RPM's are going to be higher.  

So, what part of the engine might be affected, and whether that part is able to withstand the heat and/or stress, is the issue.  Valves hold up fine according to GAMI, so long as engine temps are in a normal operating range.  The turbo seems to be capable of running all day at 1600+.  The collector is probably cherry red, but does not appear to deteriorate any faster at the higher temps.  At least that is what I have seen so far in my engine after a few hundred hours, and the GAMI people are interested in, but not concerned about these issues.

The ICP issue is a little different.  Having watched ICP's on a running engine at GAMI, with the wrong mixture setting the ICP gets very spikey, a hammer blow rather than a push, and while the absolute ICP is important, that pre-detonation or in-detonation blow is what hurts the engine most.  The whole emphasis of GAMI's advice on mixture control, whether ROP or LOP, is to control the rate of combustion.  

[ArtVandelay]

With fixed timing you will have higher CHT's at lower RPM and can more susceptible to detonation at high power settings. I should have added to my previous statements as long as your not in the red box.

That's equivalent to being in high gear going up a hill. There is a happy medium which my ears tell me is 2200-2600, higher or lower than that and the engine doesn't sound happy.

[jetdriven]

21 hours ago, jlunseth said:

There is a locational issue to the durability question, which makes it hard to pin down.  For example, LOP, to speak in generalizations, results in an EGT that is about 100 dF higher than an equivalent ROP power setting.  So one question that was raised early on, was whether that would result in reduced valve life because the valves are operating in a higher temperature environment.  Or reduced durability of the collector which is going to run hotter.  In my aircraft I run the turbo harder to produce a particular LOP setting, in an effort to get the mix lean enough.  I will generally run at 34" MP instead of the 28-30" I would see at a ROP cruise setting (I vary the power LOP by adjusting fuel flow, not MP).  Further, the TIT will be higher because the EGT's are higher.  GAMI says that the turbo will be fine at cruise operations up to the redline.  I try to keep it at or below 1600, but that is still hotter than it would be if I ran ROP, and the RPM's are going to be higher.  

50 degrees LOP and 50 degrees ROP are the same temperature of EGT.  Actually the hottest valve face temp is 50 ROP.  "LOP burns valves" is a myth.
 

 

[Cruiser]

Byron, 

I am not so sure. What you say is true, but not actually what is happening in real operations from what I can tell. What I mean is most people when running ROP are running 100 to 150 ROP in their normal operations.(at least I do) This will put the EGT 50 to 100°F less than running the 50 LOP setting.

While I know that GAMI says the actual combustion event produces valve face temps much, much higher than that, the continued operation of EGTs in excess of 1500°F (LOP) might have an accelerated effect on valve guides and seats. 

I have no evidence for this or any data to support the thought............ just sayin'

[Cruiser]

In support of my statement above, the term "burned valve" is critical. 

Valves burn because of uneven cooling. The uneven cooling is not caused by temperatures in the cylinder or the combustion event. The uneven cooling is caused by failure in the cooling cycle. The valve seat draws heat away from the valve face on every contact. If the valve is not seated properly (many causes) the heat dissipation will not be evenly spread across the entire surface resulting in more cooling in the contact areas and LESS cooling in the others. The area of the valve face that does not have good heat transfer will deteriorate faster than the other parts of the valve. The tell tale pictures of "burnt valves" show this condition very clearly. Higher exhaust temperatures will (in my theory) accelerate that deterioration.

If the valve train is in good shape without wear to the guide and face to seat contact maintains it's integrity, adequate cooling across the entire valve will be sufficient and valves will NOT burn. (even at higher EGTs)

 

........ just sayin'

[ArtVandelay]

I would not worry about absolute EGT temperatures, it's the cylinder temperature that's critical. There is boundary layer that protects the metal and the piston is only subjected to those temperatures 25% of the time, as long as they can shed the heat . I believe the peak temperatures are much higher than 1500° anyway.

[gsxrpilot]

57 minutes ago, Cruiser said:

40 minutes ago, Cruiser said:

Valves burn because of uneven cooling. The uneven cooling is not caused by temperatures in the cylinder or the combustion event. The uneven cooling is caused by failure in the cooling cycle. The valve seat draws heat away from the valve face on every contact. If the valve is not seated properly (many causes) the heat dissipation will not be evenly spread across the entire surface resulting in more cooling in the contact areas and LESS cooling in the others. The area of the valve face that does not have good heat transfer will deteriorate faster than the other parts of the valve. The tell tale pictures of "burnt valves" show this condition very clearly. Higher exhaust temperatures will (in my theory) accelerate that deterioration.

While I know that GAMI says the actual combustion event produces valve face temps much, much higher than that, the continued operation of EGTs in excess of 1500°F (LOP) might have an accelerated effect on valve guides and seats. 

I have no evidence for this or any data to support the thought............ just sayin'

Of course, on the other hand, GAME does have both evidence and lots of data. And they say that the actual number on the EGT scale is meaningless. 

If your valves are not seating correctly and therefore not transferring heat to the cylinder head efficiently, a difference of even a couple of hundred degrees is meaningless.  By the time the exhaust gasses get to your EGT probe, they've already cooled significantly. So the numbers you read on the EGT gauge are already much less then what your valves are dealing with.

My money's on GAMI and their data...... just sayin'  

[jlunseth]

1 hour ago, jetdriven said:

50 degrees LOP and 50 degrees ROP are the same temperature of EGT.  Actually the hottest valve face temp is 50 ROP.  "LOP burns valves" is a myth.
 

 

Yes, but that is not how it works.  50 degrees LOP and 50 degrees ROP are not the same power setting.  Very roughly speaking, and in my engine, 30 LOP is what I use to achieve the same power setting as I do with 125 ROP.  The power fall off is very steep on the LOP side, I am not sure the engine would even run at 125 LOP.  So again, roughly speaking, the difference between equivalent LOP and ROP power settings is about 100 dF.  50 LOP is, again, speaking in generalities, not in the red box, and you would have to work pretty hard to get, say, 75% power out of a 50 LOP setting.  I doubt you could do it at all.  50 ROP is about dead center in the red box and you could probably get a 100% power setting to work at 50 ROP.  That would be hard on the engine, but you could get the engine to do it.

I agree on your conclusion though, it does appear that LOP, although it results in generally higher EGT's, does not burn valves.  It appears to actually be kinder to the valves, they stay clean and do not coke up.

[gsxrpilot]

Of course.  All Byron said was that 50 ROP and 50 LOP are exactly the same temperature... which has to be true based on the definition peak.

You are also obviously correct that the power is not even close to being the same.  The best HP is at about 75 ROP. Once LOP, the HP drops off pretty steeply. But it's also true that efficiency (Fuel/HP) is best at about 65 LOP.

[bonal]

I love this kind of discussion.  Reciprocating engines are really quite amazing.  Just wondering if the turbo charged and normalized engines have different internal materials than their N/A counter parts.

Edited January 13, 2017 by bonal

[carusoam]

The fun part is...

we measure LOP and ROP at an arbitrary point after the exhaust valve.

The ship's EGT gauge on an O is a combination of three exhaust streams on one side of the engine.  This EGT is hotter than any one single EGT from a single cylinder.  Three power pulses in the same period of time than a single cylinder EGT sees...

The 201's ship's EGT gauge is from a single cylinder.

The TIT is a combination of six cylinders going into the turbocharger... and sees a higher temp than a single cylinder EGT. Six power pulses in the same time that a single cylinder EGT sees.

As long as the fuel is burning, chances are the EGT continues to get hotter...  in an open pipe, the gasses start to expand allowing for significant cooling.  In a TC, a good amount of pressure is still maintained until after the turbine...  expansion cooling is waiting for the expansion.... some reasoning behind why a loose clamp on the turbo is such a blow torch... 

Summary...

1) exhaust valves are hot, but EGTs are still rising after the valve, until the flame/burning is done.

2) LOP most of the fuel is burned inside the cylinder. A very small flame exits the cylinder.

3) ROP there is inches of flame exiting the cylinder.

4) exhaust gasses really cool a whole bunch when allowed to expand.  This is why turbos are in the red hot gasses, not after they are nicely cooled.

5) LOP has the fuel mostly burned prior to the exhaust valve.  Meaning that the max temp is happening earlier in an LOP cycle.  But LOP has the ability to bring in excess air to keep the burn rate slow and cool.

6) ROP uses excess fuel as the internal cooling agent. The burning continues down the exhaust pipe.

7) watch a plane's engine with No exhaust pipe. The flames will be spouting out more when run ROP.... I cool demo if dark out.

Wasn't that fun?  

Stuff I learned around here, not a mechanic...

Best regards,

-a-

 

 

 

Edited January 13, 2017 by carusoam

[ArtVandelay]

Doesn't the fuel burned on exit also depend on the spark timing? More lean the mixture the longer it takes for the mixture to burn, I say this because when my timing changed 25 to 20° I can no longer lean as much...

[M016576]

6 hours ago, bonal said:

I love this kind of discussion.  Reciprocating engines are really quite amazing.  Just wondering if the turbo charged and normalized engines have different internal materials than their N/A counter parts.

They don't, really... and that's part of the "problem" (or complication... or root of OWT's... or whatever you want to call it!)

[carusoam]

TJ,

By starting the burn earlier, more fuel gets finished before it exits. More power, and higher CHTs are the result of moving the timing further before TDC...  Ross gives the best explanation of this phenomena.

 

Bonal,

the TC'd engine gets a very different compression ratio, the Acclaim TN'd version gets a slightly different compression ratio. The materials are a flavor of the day.  You can read about cylinder construction on the Continental web site. 

Best r guards,

-a-

[Shadrach]

On 1/8/2017 at 3:24 PM, Tx_Aggie said:

Recently I had the pleasure of an unrealized comparison flight between the 2000 M20R I currently rent and a friend's 201. Two weekends ago we took a flight in the 201 to San Angelo (SJT) for a $100 hamburger run. At 7000' and 150 TAS, we were seeing about a 10 knot tail wind for a gs of 160 knots. Not bad. On the return, the winds picked up in the opposite direction, with 150 TAS, we were at a painful 120 knots over the ground. This past weekend I took a friend to Stephenville TX (SEP) for arguably the best BBQ around at Hard 8. In the Ovation we went up to 11,000' 180 TAS, and making 200-205 knots over the ground. On the return with the same type of headwinds, I was at 8000', TAS of about 185 and 165 knots over the ground. The fuel burns in the Ovation averaged about 14 gph while the average in the 201 were about 10 gph. I get the efficiency of the 201 at low fuel burn is really good, and good numbers may be in store with a bit of a tail wind. But you always have that wind working against you on the way back. I guess now that I've experienced  the power of the IO-550, it is soooo hard to convince myself a slower, less powerful plane is worth the money. It doesn't seem to me the 4 gph difference in fuel burn Is worth it. Now I see why the Missle and Rocket guys are so amped up about their performance gains. Thoughts? Comments?

Thanks,

Matt 

There is no replacement for displacement.  That being said, you went on a single round trip flight, in one J model, with another pilot flying who may or may not know how to get the best out of his airplane. My observations would be the following:

 1) If I owned a J model that trued at just 150kts at 7000ft, I would be investigating what the problem was. 150kts is the bottom of the barrel for a J model unless he was LOP in which case he should not have been burning 10GPH at 7000ft. That's where I flight plan my box stock geriatric F model and I often see more.

2) Getting the most out of an airplane requires planning. In a true XC situation, one would climb high to sip fuel with the best tail wind and return lower out of the strong winds. You seem to be implying that 11,000ft is "Ovation Country".  A J model is extremely usable in the 10,000ft to 15,000ft range. Climb rate starts to drop off above 12,500ft  but a good J should average more than 500FPM in a climb to 12,000ft.

3) The actual TAS difference between a well performing J and R at max cruise is about 25kts making the R about 15% faster in a race. The difference in fuel burn is about 5gph. This means that the R is burning about 50% more gas at max cruise.  If you think that cross country block times will be 15% faster, I would bet that is highly optimistic even on longer trips, on short trips you get bragging right for landing a few minutes earlier and burning a lot more gas. Unfortunately, I would bet that block fuel burn will be 50% more for the R all other things being equal.

4) Useful load. I don't know the exact numbers for either specific plane, but both of the Rs I am familiar with weigh in at near 2400lbs with a useful of about 965lbs. I contrast that with my lowly 200hp F's dry weight of 1681lbs with a useful of 1059lbs.

Let's compare a 500nm trip for a typical couple in both planes with 45min reserves.

Using numbers for my F model                                                                                                                                                     pilot and spouse - 305lbs                                                                                                                                                                   bags - 100lbs                                                                                                                                                                                     Fuel - 240lbs                                                                                                                                                                                     Aircraft dry weight 1681lbs                                                                                                                                                              Total payload and fuel 645lbs                                                                                                                                                       Total weight 2326lbs (note that loaded for the trip my plane is lighter than an empty R model).                                                     Power to weight ratio is .086hp for every lb.

For a typical R Model                                                                                                                                                                         pilot and spouse - 305lbs                                                                                                                                                                   bags - 100lbs                                                                                                                                                                                     Fuel -  317lbs                                                                                                                                                                                     Aircraft dry weight 2395lbs.                                                                                                                                                           Total payload and fuel 722lbs                                                                                                                                                           Total weight  3117lbs                                                                                                                                                                       

Power to weight of .09hp per lb.

Not really a huge power to weight ratio benefit for a normal XC mission. If the plane is being flown solo, then the P/W spread is greater, but you should understand that the 180 and 200hp machines do quite well when flown solo as well. Plenty of folks seeing initial climb rates of well over 1200fpm when flying solo in the cooler months of the year; great climb rates are not solely the purview of the big Conti's. I can often hold greater than 1000fpm all the way to 8000 in the cool months.

5) I'm not so sure that any long body has superior short field performance either in or out when compared to a 200hp mid or short bodied bird. Check your POH and compare. I believe that at gross weight as well as lightly loaded, the R is going to use more runway when compared to the lighter short and mid-bodies. My POH says a 200hp F will have a take off roll at MGW on a standard day of 880ft. Configured for the above mentioned 500nm XC trip the POH says 595ft at SL on a standard day.

I am not trying to poo poo the big Conti-engined birds. I think they are great machines, especially if you fly alone a lot and don't care much about efficiency. I don't see them as the fire breathing muscle machines that some make them out to be. I think if one studies the performance specs, one finds an honest plane that goes somewhat faster on a lot more gas. With regard to the J model, I am trying to think of a certified, production 4 place aircraft that goes faster on a gallon of gas and nothing comes to mind...unless I include turbos. A good K model 252 will do better. In fact a 252 will hang with a standard Ovation above 10K and walk away from it above 15K...all while burning less gas.

You might consider spending a bit more real world time in the J (maybe rent one if you can).  I think you'll find that while the performance differences are significant, they're not quite as dramatic as you are portraying them to be.

 

 

 

Edited October 8, 2021 by Shadrach

[Shadrach]

14 hours ago, teejayevans said:

Doesn't the fuel burned on exit also depend on the spark timing? More lean the mixture the longer it takes for the mixture to burn, I say this because when my timing changed 25 to 20° I can no longer lean as much...

You're correct! As is Anthony's statement. Additionally when the mixture is leaned past stoichiometric (peak) the flame front propagates slower. Now there are two factors (ignition timing and reduction in flame front speed) that are moving combustion event pressure not only away from TDC but likely cause a situation where a significant part of the combustion event is occurring after BDC where the exhaust stroke begins. Not efficient. Why did you have your ignition timing retarded to 20°?

[gsxrpilot]

Ross's post should be a sticky. It's 100% correct and should be required reading for anyone shopping for an airplane.

[ArtVandelay]

Why did you have your ignition timing retarded to 20°?

Not my choice, I had mags 500 hr service done. I quickly noticed my EGTs were higher and where before I could lean 60-70° LOP, now it complains if I try to lean half that. My engine plate specifies 20°, so I assume they retarded it to 20, is there a service instruction out there that allows it to be 25°? I gotta think the extra 5° matters only at high RPMs, at lower RPMs, the piston is moving slower so there is more time for the combustion event. I complained about tough starts, it seems much better now, does advancing the timing help on starts?

[Shadrach]

12 minutes ago, teejayevans said:

Not my choice, I had mags 500 hr service done. I quickly noticed my EGTs were higher and where before I could lean 60-70° LOP, now it complains if I try to lean half that. My engine plate specifies 20°, so I assume they retarded it to 20, is there a service instruction out there that allows it to be 25°? I gotta think the extra 5° matters only at high RPMs, at lower RPMs, the piston is moving slower so there is more time for the combustion event. I complained about tough starts, it seems much better now, does advancing the timing help on starts?

 You are correct that at lower RPMs piston speed is reduced which means peak cylinder pressures occur closer to the ignition point. However, my experience is that those who have retarded their ignition timing see a decrease in performance across the spectrum; a slight loss in cruise but pronounced loss on take off and in climb where a delta of just ten horspower has a significant effect on performance. There are benefits to retarding the timing. Starts should be easier in theory, but both impulse coupled  SOS ignition systems reduce timing to ~TDC for start up, so retarding timing 5° on start up should not have a dramatic effect on starting. CHTs will be noticeably cooler at high power settings because the 20° engine is less thermally efficient (energy is going out the exhaust rather than contributing to power).  A 20° engine simply will not make as much power at max rpm WOT as a 25° engine.  Lycoming has a service instruction for changing the timing on the data plate. I'm pretty sure that either the engine manual  or a factory SI provides data for changing the timing between 20° and 25°. It is however, necessary that the data plate be modified to match the ignition setting.

We've kind of hijacked the thread here. We should take the discussion elsewhere if it needs to be fleshed out further. I'm sure there have been many threads covering the topic since mooneyspace was brought online.

[aaronk25]

So if you run a J at max cruise 160-165kts, 2700rpm around 10ish gph your not far behind the Ovation, I think the biggest difference is the nicer and longer cabin of the "O".

I think I'd go with a K, if I didn't need the cabin length due to the fact if i need to top weather it's got authority for climb at altitude say picking up ice in the tops at 11k and 15k would be in the clear, but then your sucking O2 so that might be a draw back.

Cost wise it would be cheaper to run the J flat out all the time than maintain the bigger O engine. By the way 2700rpm max cruise on a J for 1,000 hours and all cylinders met "new service limits". So as long as you don't get the engine hot, have good baffles and manually adjust the cowl flap close limits to hang open 1/2 in cruise I'm a firm believer power settings have no effect on cylinders making TBO. Infact on my Sam Jewel said they were in beautiful shape. Just my 2c.


Sent from my iPhone using Tapatalk

[gsxrpilot]

7 hours ago, aaronk25 said:

I think I'd go with a K...

Sorry, I'm using N252AD right now 

I don't use the O2 on every flight and actually probably on only 20% of my flights. But I sure like the ability to go to 24K or even 28K if I need to, if even for a short time.

[Shadrach]

On January 13, 2017 at 1:57 PM, jlunseth said:

Yes, but that is not how it works.  50 degrees LOP and 50 degrees ROP are not the same power setting.  Very roughly speaking, and in my engine, 30 LOP is what I use to achieve the same power setting as I do with 125 ROP.  The power fall off is very steep on the LOP side, I am not sure the engine would even run at 125 LOP.  So again, roughly speaking, the difference between equivalent LOP and ROP power settings is about 100 dF.  50 LOP is, again, speaking in generalities, not in the red box, and you would have to work pretty hard to get, say, 75% power out of a 50 LOP setting.  I doubt you could do it at all.  50 ROP is about dead center in the red box and you could probably get a 100% power setting to work at 50 ROP.  That would be hard on the engine, but you could get the engine to do it.

I agree on your conclusion though, it does appear that LOP, although it results in generally higher EGT's, does not burn valves.  It appears to actually be kinder to the valves, they stay clean and do not coke up.

I don't know what's hard about getting 75% power at 50 LOP.  It's not hard at all for me. The flight regime in which it's possible to have enough air flowing though a normally aspirated engine to do it is limited.  I do it down low the winter time often at 1500-2000 agl with DAs at that altitude ranging from -1000ft to 200ft. My angle valve IO360 will run smooth to around 90LOP, but there is no utility running much past 60LOP.  

[Tx_Aggie]

Good discussion all, Thanks for the input.