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Definition of lop and rop

nels

By nels
in Vintage Mooneys (pre-J models)

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nels Mentor

nels

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I've been reading alot of references to lop and rop. All rather new to me. Do I understand this correctly: peak is the peak combustion temp you can achieve and is the result of total burn of fuel and oxygen with no leftovers. This condition should be avoided. rop results in lower combustion temp due to unburned fuel or incomplete combustion, lop is complete combustion with unspent oxygen which results in cooling the combustion process due to the excess oxygen/air. 


How is the transition made going from rop to peak to lop? I would assume any time at peak is detrimental to the pistons, rings and cylinders especially under load. To transition to lop from rop is it necessary to reduce the prop pitch/load during the transition? Are fuel injected engines better balanced fuel wise?


OR----- am I in left field and need some schooling?  I am here to learn.

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MooneyMitch Grand Master

MooneyMitch

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I do apologzie for my silly humor.  It's just that we have so many indepth discussions on topics, such as LOP vs. ROP and those can be found if you search the forum for such.   Sometimes the discussions get a bit off course and lenghty and even a little heated.


If you search not only on this site, but search John Deakin on the web, you will find tons of great information regarding your question.


Stand by!

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nels Mentor

nels

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Quote: Mitch

I do apologzie for my silly humor.  It's just that we have so many indepth discussions on topics, such as LOP vs. ROP and those can be found if you search the forum for such. 

If you search not only on this site, but search John Deakin on the web, you will find tons of great information regarding your question.

Stand by!

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Cruiser Grand Master

Cruiser

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you are pretty much on track. Some would word it differently or add some for specific details but you've got most of it. A couple of general guidlines.


1. You should have an engine monitor with fuel flow to monitor your settings.


2. Most, not all, recommend you do NOT attempt LOP above 65% power.


3. At altitudes above 7000' (because at those altitudes you can't get more than 65% power) you can make mixtures changes with little risk of doing anything that would hurt your engine.


4. The first thing you need to know is how balanced the fuel flow is in each of your cylinders. Do a GAMI test to determine the range of fuel flow from leanest cylinder to richest cylinder. More than .5 gal/hr imbalance will cause difficultly in setting LOP


5. Don't touch the prop or power when adjusting mixture. Carburetors are often difficult to achieve LOP because the fuel distribution to each cylinder is not well balanced. (see GAMI test)


6. Keep learning and don't be afraid to ask questions.

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MooneyMitch Grand Master

MooneyMitch

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"2. Most, not all, recommend you do NOT attempt LOP above 65% power."


I do believe Deakin's Red Box allows for LOP above 65% but it does seem push to the extreme limits of the parameters.  My IO-550G does well at 50 LOP or better so I do run it in those areas above 65% power.

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danb35 Mentor

danb35

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To try to answer your questions concisely and impartially...  ROP and LOP refer to rich and lean of peak EGT.  Peak EGT may or may not be a bad place to run, depending mostly on your power output--90% power at peak EGT is probably not good for your engine; at 50% power the mixture doesn't make a difference.  It's pretty widely held that 60%-65% is the point below which mixture just doesn't matter--that is, you can't hurt the engine no matter what you do with the mixture control.  This is where Cruiser's recommendation comes from--even if you do it wrong, at or below 65% power, you aren't going to hurt the engine.


Even a mixture setting that's undesirable for your power setting isn't going to hurt things if you pass through it reasonably quickly.  If you're running at 80% power, you don't want to be running at 50 ROP, for example.  But it won't hurt you to lean through that to get LOP.


An engine monitor with fuel flow is a good idea for any engine.  It's not particularly more necessary or desirable when running LOP than when running ROP, except that the fuel flow makes it easy to lean to a particular point once you have your settings dialed in.  For example, 90+% of the time, I cruise between 7000-9000.  I simply lean to 9 gph, rather than looking for a particular LOP or ROP value.


 

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Hank Grand Master

Hank

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Quote: nels

I've been reading alot of references to lop and rop. All rather new to me. Do I understand this correctly: peak is the peak combustion temp you can achieve and is the result of total burn of fuel and oxygen with no leftovers. This condition should be avoided. rop results in lower combustion temp due to unburned fuel or incomplete combustion, lop is complete combustion with unspent oxygen which results in cooling the combustion process due to the excess oxygen/air. 

This is correct, and very clearly stated. Congratulations!

How is the transition made going from rop to peak to lop? I would assume any time at peak is detrimental to the pistons, rings and cylinders especially under load. To transition to lop from rop is it necessary to reduce the prop pitch/load during the transition? Are fuel injected engines better balanced fuel wise?

OR----- am I in left field and need some schooling?  I am here to learn.

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1964-M20E Grand Master

1964-M20E

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Since peak EGT is maximum power available at any given MP, RPM and altitude if you listen carefully you can hear the engine and prop governor respond to changes in mixture on either side of peak.  This is more apparent at higher altitudes and or lower MP (< 22”).  Some what like you would do with a model airplane engine only it does not have a constant speed prop.

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Shadrach Grand Master

Shadrach

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I am going to try and explain the whats and whys LOP ops as simply as I can. I originally posted this about 3 years ago on the red board and edited a bit to try and make it better.  


Simply stated, all of us flying certified aircraft have engines with a fixed timing ignition system. This is to say that the plugs always fire at the same degree of piston rotation relative to top dead center (TDC). In a modern auto, motorcycle or whatever engine the timing will advance and retard automatically to best suit what the operator is asking the engine to do.  This provides a spark at such a time as to minimize detonation and start the combustion event and provide the most pressure at the ideal time (~17ATDC). We don't have such sophisticated mechanisms on our AC engines so we make do with what we have - the mixture. For high power ops, we need to slow the combustion event (flame front) so that the expansion of the ignited fuel air mixture is distributed as evenly as possible through out the power stroke of each piston. Imagine you have the cylinder heads off an engine and want to move the piston from TDC to BDC, would you rather hit it with a hammer when it's close to TDC, or push it forcefully and evenly down through the cylinder to BDC? This is what is being talked about when you see discussions of Theta and Theta PP. 


Theta = angle of rotation of the crankshaft WRT to top dead center. 


ThetaPP = the point of rotation at which the peak internal cylinder pressure occurs.


In the absence of variable timing ignition, we (knowingly or not) control the combustion event with the mixture. The further ROP we get the slower the combustion event becomes. This is because of a surplus of atomized fuel in the combustion chamber as the burn starts. There is not enough oxygen in the combustion chamber for all of the fuel, therefore the flame front is slowed in it's search for combustible pairings of fuel and oxygen. We can do the same thing on the lean side of peak - on this side of peak we are using a surplus of air to slow the flame front.  This slower flame front produces a more even expansion of combustion gasses through out the power stroke.


The highest peak pressure occurs ~40df ROP. Pressure at ~40dfROP are high because at this mixture setting the flame front propigates rapidly; this is like the hammer mentioned above. This is the mixture setting that burns the most fuel atoms and burns them the fastest.  At this mixture setting the the combustion event happens very fast and leads to high pressure and the highest heat generation. Because of the speed of the combustion event, it also happens near TDC (thetapp -again with the hammer reference). This ROP setting does not lead to the highest power output for a given manifold pressure, enricening slightly to ~80ROP slows the combustion event slightly and produces a more favorable ThetaPP and therefore more power. At max MP on a turboed engine or SL MP on a N/A engine 40df ROP will produce the highest CHTs and is downright abusive on an engine. However at 75% power, it may be fine and it may go to TBO - but it is not ideal and your CHTs will show it. 


Imagine you're spinning a large gaming wheel at the county fair. If you really want to get a good spin, would you grab it at the 12o'clock position (TDC) and pull straight down? Of course not, you'd grab and pull it down at the highest place possible after the 12 o'clock position (ATDC) to maximize leverage/mechanical advantage and use steady force to accelrate the wheel.  We know that the ideal place to maximize our efforts is between 15 and 17 degrees ATDC depending on the application. 40 ROP is like whacking the gaming wheel with a hammer at the 12:30 position richer or leaner moves the effort on the wheel clockwise and transitions from a shrp blow to a more even pull. If you go too lean or rich the effort moves too far clockwise and the returns star to deminish rapidly, more so when going lean (more on that below).


Whether we know it or not, this all we're doing with mixture management. ThetaPP can be controlled using surplus air or surplus fuel. In NA engines we have limited amout of air (limited to ambient) to use to control combustion and can make more power using fuel because we can run as much FF as the engine needs to make power. TC/TN engines have an advantage for LOP ops. As they can turn up the boost.


I run LOP when ever practical because air is free and fuel is expensive.  Also, the affects are not linear on both sides. In terms of degrees from peak (which is called stoichiometric or complete combustion), it takes less air than fuel to do the same job. 


 


Sorry for the novella... Hope it helps!


 

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KSMooniac Grand Master

KSMooniac

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You do have a lot to learn, but that isn't a bad thing since you seem willing to do some homework!  Learning about this topic (correctly) on a message forum isn't the best way to go about it.  The absolute best and most efficient way to get thoroughly educated on engine management is to take the Advanced Pilot Semimar (APS) at one of their live weekend sessions in Ada, OK.  Second to that would be to take the on-line version, although you lose out on seeing the engine test stand in action and the very beneficial question-answer sessions.  Check out www.advancedpilot.com.  It is the best money I've spent as an aircraft owner, period.  I learned more in 2.5 days there than I did in many semester-long engineering courses.


John Deakin was mentioned above, and he is one of the principals in APS.  He used to write for Avweb.com, and I believe his columns are still archived there and they are a great place to start.  Read everything he has written about engine management and you'll be much smarter when you're done.

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