LOP in Carbureted Engines

[Walter Atkinson]

Over the years, we have had difficulty operating carbed engines LOP for only one reason--they historically have poor F:A ratio balance. Even TCM and Lycoming have said that Cabred engines can't be run LOP. That is true if one assumes that you cannot balance the F:A ratios. However, as Conusious say, "Do not tell man something impossible when he already do it!"

I have personally run many carbed engines LOP with excellent success and repeatability. I know of several hundred others who are having the same success. Here is the issue.

A carburetor atomizes the fuel into small, medium, and large droplets. These droplets weight differing amounts and take different paths in the induction system. This results in varied F:A ratios among the cylinders. If one can get the fuel droplets VAPORIZED early in their progression, the F:A ratios will be quite even between the cylinders and the cylinders will have the same F:A ratio and therefore the same Hp output and will run smooth along the entire mixture spectrum.

One method to improve this vaporization has been to **** the throttle plate slightly to induce some turbulent airflow into the induction. This has been successful in some applications and not so successful in others. As it turns out, each engine and installation has an optimum carb temp which results in maximum vaporization and minimum heat required. Once that is discovered, simply setting that Carb Temp will result in maximum vaporization and optimum F:A ratios. As soon as that is accomplished the engine will run smoothly LOP.

How to find the optimum carb temp:
1) Set the engine monitor to the DIF function.
2) Add carb heat until the DIF is the lowest number you can get with the least amoutn of carb heat.
3) Note the Carb Temp. This same temp can now be set, winter, or summer, high or low to achieve the optimum fuel vaporization.

In a C-182, this is 10dC (50dF). In a Cherokee 180, it is 40dF. In my Twin Beech, it's 35dF. Each is different, but once you find it, you have it.

If it will not run smooth LOP, you have other problems like induction leaks or weak spark plugs or other ignition issues. ANY conforming engine with good F:A ratios and a good ignition system will run smoothly LOP--or it is not conforming.

Walter Atkinson
Advanced Pilot Seminars

[rv8ch]

Walter,

The "bad word filter" got one of your words, and it seems to be an important one.

One method to improve this vaporization has been to **** the throttle plate slightly to induce some turbulent airflow into the induction.

Was that "crack"?

You also wrote:

Set the engine monitor to the DIF function.

I take it this means without an engine monitor, LOP will be tricky. BTW, what is the DIF function?

Thanks!

[gmcjetpilot]

I noticed no RV numbers or examples (Cherokee and C182).

From my experience the Carb is cooking in a RV's tight cowl bolted to a hot sump of oil. Also the "standard" carb heat to the air box, even though the valve is closed, "spill-out" all over the front of the air box and front of the carb (externally). Carb temps of 80F (or more) all the time. May be at low power in the winter? The idea or running carb heat makes the mixture richer. I don't buy the temp thing, at least in a RV. C180/182 which has a cold carb? may be.

Throttle plate? I hear this for Contenentals but not Lycs. I guess the theory is the partial closure of the butterfly mixes it up better, as you say. However I KNOW it will add to "pumping loss". Meaning you will lose MAP and cause the engine to work (suck) against this restriction. The acronym TANSTAAFL comes to mind. Also you will now forward basis the fuel into the plenum towards the fwd Jugs. If the aft cylinders are lean you are SOL, the throttle plate "don't bend that way bro". The throttle plate tends to be like a hose and the BEST "balance" you can do is have it go straight (WOT), notwithstanding your MIX IT UP theory of the partially closed butterfly. I always thought of putting in some guides or vortex generator in the sump (intake plenum) just above the carb to bias the flow as needed, but the idea of parts bonded in the sump is a bad one. It just is not worth it to me. If I wanted FI I would have bought it for the extra $3,000 retro fit ($2100 if bought new from scratch).

I hear the theory and the words. " I know of several hundred others who are having the same success." 100's! I don't even know a 100 pilots, much less any running carb'ed engines LOP; not sure who these folks are, but I am not one of them Why me, why me, it's always me.

I don't worry about it, at all. I go for 75-100 ROP and try and fly closer to 55% than 75% power.

Don't take my rejection or surrendering further LOP attempts as not appreciating your excellent write up. We can talk about fuel molecules dancing in the induction and all kind of wonderful things, but the engine is not "stochiometrically" balanced any way from a hardware (mechanical) stand point. In fact it's the ability of fuel injection to provide (precision) unbalanced F:A to each cylinder to make up for these inherent shortcomings, such as exhaust. If balancing a FI was easy they would balance the injectors in the shop and be done. However with Lycs at least you have to "adjust" each based on what they are doing on the engine.

Although the classic Lycoming cross-over is pretty good at producing fairly low exhaust restriction with moderate savaging, it's far from balanced. Just measure the pipe lengths, primary and collector for each jug. Also, each cylinder has differnt cooling, compression and induction path lengths. Thus the mixture (F:A) and power is different between each cylinder. Fuel trim between jugs to balance these hardware based issues is key.

I find from my experience and your description the effort is a bit hit-N-miss with a dash of luck. Some carb engines no matter how you "crack" the throttle or cook the carb will NEVER run smooth LOP. I find your comment that if you can't get LOP operation there is something else wrong. That is a bold statement and think you would have a hard time convincing me that this is true. I think you can have a normal, healthy Lyc that can not run LOP.

My latest creation, is a RV-7 with a O-360-A1A. I'll give LOP a shot again like a true optimist, but I fully expect it to run rough before I can get all the cylinders at least 40F LOP. I'll try the throttle bump and carb heat methods, but I am lazy and it sounds like too much work.

As AC/DC said, "For those about to rock, we salute you". I say, "For those about to run LOP with Carbs, I salute you". One day the fuel atomization angels may shine upon my induction and bless me with smooth LOP operations on my Carburetor Lyc. I LOVE MY CARB IT IS STATE OF THE ART TRACTOR TECHNOLOGY. Yea Verily!

[Walter Atkinson]

Mickey:

Yes, the word was either "crack" or "c.o.c.k" the throttle plate. It is the less desirable approach. The hot-running lycs may not need this carb heat trick until the dead of winter.

Walter

[Walter Atkinson]

gmcjetpilot:

You didn't sign your post and I doubt that's your real name, so I don't know who to politely adderss this to, but here gos! <g>

**I noticed no RV numbers or examples (Cherokee and C182). **

There are many of those carbed engines working right. I've personally flown two different RVs and have been successful with these technics. I do not recall the optimum carb temp, so I did not post it.

**I guess the theory is the butterfly as you say mixes it all up better. However I KNOW it will add to "pumping loss". Meaning you will lose MAP and cause the engine to work (suck) against this restriction. **

If you see ANY MP change, you've moved the throttle further than necessary in my experience.

**Also you will now forward basis the fuel into the plenum towards the fwd Jugs. If the aft cylinders are lean you are SOL, the throttle plate "don't bend that way bro". **

See above!

The cylinder SUCK in what they can, unless you are supercharged, the assumed airflow in the induction is not a factor.

**I hear the theory and the words. " I know of several hundred others who are having the same success." 100's! I don't even know a 100 pilots, much less any running carb'ed engines LOP; not sure who these folks are, but I am not one of them Why me, why me, it's always me. **

Well, I visit eight other forums almost daily and have personally trained over 1000 pilots in the details of engine management. The number of a hundred is low.

**I don't worry about it, at all. I go for 75-100 ROP and try and fly closer to 55% than 75% power. **

Then these issues are not something for you to worry about. Others may have a different approach where this information is of value to them.

** the engine is not "stochiometrically" balanced any way from a hardware (mechanical) stand point. In fact it's the ability of fuel injection to provide (precision) unbalanced F:A to each cylinder to make up for these inherent shortcomings, such as exhaust. If balancing a FI was easy they would balance the injectors in the shop and be done. However with Lycs at least you have to "adjust" each based on what they are doing on the engine.**

Nope. You're confusing volumetric efficiency with F:A balance. They are not the same and are not addressed the same way. It is quite possible to balnce the F:A ratios in a poorlyvolumetirically balnced engine. It's done all of the time--regularly. Again, one should not tell another that something is impossible when he's already doing it! <g>

**I find from my experience and your description the effort is a bit hit-N-miss with a dash of luck.**

That's true if one does not have an engine monitor. Once the optimum temp is found, it's pretty consistent.

**Some carb engines no matter how you "crack" the throttle or cook the carb will NEVER run smooth LOP. I find your comment that if you can't get LOP operation there is something else wrong. That is a bold statement and think you would have a hard time convincing me that this is true. I think you can have a normal, healthy Lyc that can not run LOP. **

Could be, but so far, that's not been the case. We find LOTS of engines have insidious induction leaks that cause problems and even more have ignition issues which don't show up ROP that are a problem LOP. We've done an awful lot of research on Lycomings which seem to work fine. I can't address your particular engine.

** I fully expect it to run rough before I can get all the cylinders at least 40F LOP. **

Ah, HA! There's the problem. Why so far LOP? That's waaaay further than you need to be for anything below 75% power. At lower power settings that far LOP you will be feeling the normal cycle to cycle variability in the combustion event as roughness. Heck ,that'd be normal! At, say, 65%, I would be 15-20dF LOP at the MOST. Probably 10-15dF LOP will be good. 40dF LOP at lower power settings is leaner than BSFC(min) and there is no advantage to being that lean. There is rarely any time in cruise where these carbed engines need to be more that 20dF LOP. Being further is counter-productive.

Good luck.

Walter

[Chickenlips]

I wonder if "cowflop" gets through the filter?

When your engine is at less than 75% power pull the mixture until the engine misses a little and shove it in 1/4"... then relax!

cl

[Ironflight]

Originally Posted by Chickenlips

I wonder if "cowflop" gets through the filter?

When your engine is at less than 75% power pull the mixture until the engine misses a little and shove it in 1/4"... then relax!

cl

Ya know....as much as I am a high-tech rocket engineer, I find that more often than not, if I do the EGT and Fuel Flow dance with all my fancy instrumentation, I end up just about at the same spot as if I did it this way.....

Lean to roughness and then smooth it out. My fuel flow then ends up within .2 gph of the "Minimum Fuel FLow" lin on Lycoming's graph, and I'm happy with that!

Still like to play with all the fancy toys when I am bored though...

[R.P.Ping]

So Walter?. I have an O-320, 160hp and when I am running at 10,500 or 11,500ft I ever so slightly play with the throttle to even up EGTs as close as possible. I can bring EGTs on cylinder 1 & 2 down and 3 & 4 up to within 50f of each other. Then I lean until the first cylinder peaks and that is where I run the engine.
So? if I play with the carb heat a bit it may be possible to run LOP? I was at 12,500 one day while flying off my 25hrs and I guess the conditions were just right because I was able to run LOP with no noticeable roughness and was burning about 1 GPH less. I haven?t been able to obtain that again.
Interesting post. No argument here and will let you know my results when I get a chance.


And CL & Paul you are both right on...I was lleaning the other day like I use to (by ear) and I ended up very close to the "exact intrumentation" lean.

Just having fun!!!

[Walter Atkinson]

Roger:

At those altitudes being about 10-15dF LOP is an optimal mixture for the balance between speed and economy. As a general rule, a DIF of 50dF in a carbed engine will be adequate to work well. Each engine will be a bit different.

Walter

[R.P.Ping]

As you can see I can get EGTs pretty even when adjusting the position of the throttle plate just ever so slightly. The 1322 temp is 1 degree rich of peak the other cylinders are all running considerably more rich of peak.