Carb Heat for better fuel atomization?

After installing an EDM-900 in my p.ponk 0-470-50 equipped Cessna 180 and flying it for over 50 hours I have developed a theory to explain what I can only describe as “unusual instrument indications”. Prior to the install of the EDM-900 I had a six cylinder egt with 25 degree incrimination. This was difficult to read and virtually impossible to determine which cylinder hit peak egt first. I also had no indication as to what the current fuel flow was. I have checked the intake manifold and it has NO leaks.
My first surprise from the EDM-900 was my fuel flow at cruise power (2400 RPM/23 inches m.p.). At 1000 feet and full rich the fuel flow is about 18 gallons an hour. The hottest cylinder is almost always number 1 at about 1450 degrees egt. The problem I had was that cylinder would peak at 1490. This was something I just could not get my head around. Full rich operations were only about 40 degrees rich of peak egt. I was unable to get a 75 degree rich of peak egt (recommended best power) at any power setting. Climbing up to 3000 feet helped to get a little more fuel to the fuel/air mixture. The number one cylinder was able to maintain almost 75 degrees rich of peak but required a full rich mixture setting with a fuel flow of about 18 gph. EGT temperature differential was about 250 degrees. The EDM manual indicates a carbureted engine should have a temp differential of about 200 and a fuel injected engine about 100.
I also noted some “odd” reading when applying carb heat. There is a definite increase in fuel flow when carb heat was applied. The increase was small but notable. The more puzzling indication was the EGT temps. With no other movement of any engine control, with partial carb heat at 40 degrees the EGT of the number one cylinder LOWERED from 1450 to less than 1325. This now allowed leaning the engine to get a 75 degree rich of peak egt setting with a fuel flow of LESS than 16 gph. Another “odd” reading was the cylinder egt temperature differential went from 250 degrees to less than 70.
My only explanation for this is the carb heat is allowing the fuel to better vaporize and burn more efficiently. Basically, with the use of partial carb heat, my engine is running about 150 degrees cooler, using 2 gallons LESS per hour and has the efficiency (egt temperature differential) better than one would expect from fuel injection.
Normal carb temperatures with no heat are about 15 degrees. So, only about a 25 degree rise in carb temps changes the efficiency of this engine from day to night. I would have never known this without an advanced engine monitor. Live and learn.

There is another factor taking place besides a increase air temp, the air flow profile entering the carb. Without carb heat the air flow is flowing into the sharp upturned bend into the carb from one direction. This will cause a non uniform flow entering and going through the carb. When you put on a bit of carb heat, the flow direction at the sharp bend changes to where the air flow is approaching the carb from two opposing directions. With airflow entering into the upturned bend from both the rear and front, you will end up with better flow symmetry.

bubb2,
What you have described is the method which Max Conrad used for leaning his engines for maximum endurance on his many long distance flights. I demonstrated an airplane to Max when he was in the process of planning his flight to the Antarctic.
http://www.southpolestation.com/trivia/igy2/conrad/conrad1.html

Bubb, carb heat causes the air to be less dense, therefore enrichening the fuel/air charge and lowering cylinder temps...just sayin, in case that hadnt come to mind. (This is not an explanation for your increased FF with carb heat applied, I dont get that one)

i am running 40 F carb temp on my 0470A with an EDM 900. Fun to play with.

There's a discussion of this in "Fly the Engine" by Kas Thomas (2nd ed.)...
J

I would think by your description that #1 is running lean due to an induction air leak.
Adding carb heat will lower egts because the air is less dense thus creating a richer mxture. It also can be used to help run LOP on some carbureted engines.
What do the other egts run at and how much ROP do they run at full rich?

I would also suggest that at 70% power you run 125deg ROP on the leanest cylinder. This is the recommendation of most of the engine experts including the makers of Gamijectors. The bottom line is that at 75deg ROP at the higher power setting is too lean for these engines due to several factors when running ROP.

John Deakin has a series of articles which explain this in detail.
There are other articles but I think his are the best.

Tom3holer said:

I would think by your description that #1 is running lean due to an induction air leak.
Adding carb heat will lower egts because the air is less dense thus creating a richer mxture. It also can be used to help run LOP on some carburated engines.
What do the other egts run at and how much ROP do they run at full rich?

Tom

Click to expand...

Tom,

I have checked the intake for leaks using air pressure and soapy water, No leaks found. Under most conditions the second "hottest" cylinder is number two (occasionally number 2 will peak first). Cylinder 2 egt is always close to the egt of cylinder 1. I would have to download the data out of the edm to see what the other 4 cylinders are running, but I recall 3,4,5, and 6 to be well over 100 degrees cooler, than 1 and 2. This kinda fits my theory of better fuel vaporization (perhaps due to the warmer air or perhaps as pointed out above due to the "disturbed air" entering the carb), as the number one and two cylinders are the first cylinders to be served the fuel/air mixture. If that fuel/air mixture has significant fuel globules (as described in one report I read) perhaps the first two cylinders gets the lion share of those globules while the other down stream cylinders get the more atomized fuel. Perhaps the globule fuel delivered to cylinder one and two is unable to burn thus is just expelled out the exhaust (explains excessive fuel burn with high temps). The part that really got my attention was the temperature variation between cylinders, from about 250 degrees to less than 70.

Dogday said:

There is another factor taking place besides a increase air temp, the air flow profile entering the carb. Without carb heat the air flow is flowing into the sharp upturned bend into the carb from one direction. This will cause a non uniform flow entering and going through the carb. When you put on a bit of carb heat, the flow direction at the sharp bend changes to where the air flow is approaching the carb from two opposing directions. With airflow entering into the upturned bend from both the rear and front, you will end up with better flow symmetry.

Click to expand...

That exact point was mentioned in one of the reports I read. I was thinking of doing a flight test with the carb heat scat tube from the exhaust shroud removed and then testing with the carb heat valve deployed. Unfortunately this engine is an ice maker so flying it with no carb heat may be asking for trouble.

What does the egt run at TO and climb power on #1 and #2.
Not sure what you mean by "The part that really got my attention was the temperature variation between cylinders, from about 250 degrees to less than 70."

I added a comment about cruise power setting when you were typing I think.

Tom

Tom3holer said:

I would think by your description that #1 is running lean due to an induction air leak.
Adding carb heat will lower egts because the air is less dense thus creating a richer mxture. It also can be used to help run LOP on some carbureted engines.
What do the other egts run at and how much ROP do they run at full rich?

I would also suggest that at 70% power you run 125deg ROP on the leanest cylinder. This is the recommendation of most of the engine experts including the makers of Gamijectors. The bottom line is that at 75deg ROP at the higher power setting is too lean for these engines due to several factors when running ROP.

Click to expand...

Not sure on the engine experts recommendations, The excerpt below is from Continentals Service Bulletin M89-18. While I agree cooler is better, I hate wasting gas.
My statement about 250 degrees and 70 degrees is the total differential between my hottest cylinder and my coldest cylinder in cruise. With no carb heat the differential is about 250 degrees. (I just checked the EDM pilot guide, it says the normal range for a carbureted engine is 120 to 150 degrees and 70 to 100 degrees for fuel injection.) With the carb heat set at 40 degrees my cylinder egt differential is less than 70 degrees. With partial carb heat it would appear each cylinder is getting almost the exact same fuel/air mixture. Not so much with the carb heat cold.

[FONT=Arial,Bold][FONT=Arial,Bold]Guidelines for Fuel Management Using E.G.T.
[/FONT][/FONT][FONT=Arial,BoldItalic][FONT=Arial,BoldItalic]In specific installations where aircraft manufacturers have provided specific instructions,
those instructions should be followed.
[/FONT][/FONT][FONT=Arial,BoldItalic][FONT=Arial,BoldItalic][/FONT][/FONT][FONT=Arial,BoldItalic][FONT=Arial,BoldItalic][/FONT][/FONT]A.
[FONT=Arial,BoldItalic][FONT=Arial,BoldItalic]Take-off and Climb Power Settings. [/FONT][/FONT][FONT=Arial,BoldItalic][FONT=Arial,BoldItalic][/FONT][/FONT][FONT=Arial,BoldItalic][FONT=Arial,BoldItalic][/FONT][/FONT]On normally aspirated engines, use full rich
mixture. Lean only to avoid engine roughness or noticeable power loss, at elevated
fields. Remember that a turbocharged engine is an altitude compensated engine, and
can achieve full sea level power up to a very high critical altitude. Therefore, the mixture
must be full rich at full throttle up to critical altitude.
B.
[FONT=Arial,BoldItalic][FONT=Arial,BoldItalic]Maximum Cruise. (Approximately 75% Power). [/FONT][/FONT][FONT=Arial,BoldItalic][FONT=Arial,BoldItalic][/FONT][/FONT][FONT=Arial,BoldItalic][FONT=Arial,BoldItalic][/FONT][/FONT]For economy of fuel and extended
range, do not lean the mixture below 50 degress F. rich of peak. Note: All turbocharged
engines are limited to 1650 degrees F. or 1750 degrees F. maximum Turbine Inlet
Temperature (T.I.T.) Peak T.I.T. operation is allowed for 60 seconds to determine peak
T.I.T.
C.
[FONT=Arial,BoldItalic][FONT=Arial,BoldItalic]Economy Cruise. (Approximately 65% Power or Lower). [/FONT][/FONT][FONT=Arial,BoldItalic][FONT=Arial,BoldItalic][/FONT][/FONT][FONT=Arial,BoldItalic][FONT=Arial,BoldItalic][/FONT][/FONT]Lean out to peak E.G.T. is
approved at these low power settings as the value of peak E.G.T. is below that which is
allowed at the corresponding propeller load maximum cruise power point at 50 degrees
F. rich of peak E.G.T.

Tom3holer said:

John Deakin has a series of articles which explain this in detail.
There are other articles but I think his are the best.

Click to expand...

Where would one find these articles?

Google "Pelican's Perch"

Increased fuel flow on carb heat: When you bypass the restrictive air filter the airflow should increase leading to more venturi effect leading to pulling more fuel from the emulsion tube or whatever its called. Or perhaps the "more symmetrical airflow" means the local pressure at the tube is reduced relative to the average over the whole venturi cross section. Do you see a change in MP when you pull some heat?

Why not contact the STC holder for your -50 engine conversion and talk with them?
Something very odd is going on and they may have heard of it before.

Tom

Tom3holer said:

Why not contact the STC holder for your -50 engine conversion and talk with them?
Something very odd is going on and they may have heard of it before.

Tom

Click to expand...

I have. The result of that conversation was; the fuel "flow" was as it should be (24 gph at full power), go try and lean it like we did in the old days (you know, like when we had no data) lean until rough, richen until smooth, ad a little more fuel to be sure. That resulted in cylinder 1 and 2 being well LOP while 3,4,5, and 6 were ROP.
As recommended above I looked up Pelican Perch and read all the articles relating to mixture control, leaning, power settings, etc. and discovered I merely discovered something that had been discovered years ago by someone with an EDM-700. Those articles explain, in detail, exactly what I am seeing. This said it all! http://www.avweb.com/news/pelican/182583-1.html News Flash for Carbureted Engines
Highly recommend these articles as they talk about real data and what is going on in your engine.

Skywalker said:

Increased fuel flow on carb heat: When you bypass the restrictive air filter the airflow should increase leading to more venturi effect leading to pulling more fuel from the emulsion tube or whatever its called. Or perhaps the "more symmetrical airflow" means the local pressure at the tube is reduced relative to the average over the whole venturi cross section. Do you see a change in MP when you pull some heat?

Click to expand...

Yes. As expected the use of carb heat reduces the MP about an inch in cruise. I did give thought to the "restrictive air filter: theory" but don't see the likelihood of that. The air filter is 6 inches by 12 inches and is exposed to direct ram air. The carb heat air is delivered by a 2 and a half inch scat tube ducted through a muffler shroud, the air entrance for the carb heat is perpendicular to direct ram air. Maybe I can try a flight with no air filter and see what happens when I apply carb heat.

Tom3holer said:

Why not contact the STC holder for your -50 engine conversion and talk with them?
Something very odd is going on and they may have heard of it before.

Tom

Click to expand...

I did. Although the conversation was about the excessive fuel flow in cruise and my inability to get enough egt rise out of cylinder one. I was told the carb was supplying enough fuel (24 gph at take off) and to try leaning like we did in the old days (lean until rough, richen until smooth, add a little more to be sure) and see what the fuel flow is. I did just that. It resulted in cylinder 1 and 2 to be well LOP while 3,4,5, and 6 were well ROP. The engine seemed to run fine with a fuel flow of about 15 GPH (what would be considered normal). The EDM data though indicates this engine is NOT running fine.
As recommended above I read the Pelican Perch articles about power settings leaning, etc. These articles validate everything I am witnessing. My favoriteelican's Perch #65
Where Should I Run My Engine? (Part 3 -- Cruise) side article "News Flash for Carbureted Engines" Anyone who runs a carburetor should read that article. The author also states in one of his articles (although I can't re-find it) that the Continental 0-470 has the worst fuel delivery system than any other certificated engine (or something to that effect).
BULLSEYE!

This is a great article for anyone running with a carburetor. Describes exactly what I am seeing.

Pelican's Perch #43:
Detonation Myths
I think the use of the 0-470 fuel delivery system in what is really an 0-520 may make that delivery system work even worse than described in the article.

Why don't you install a fuel injection system? It will even out the temperatures and lower the fuel burn. That engine should cruise between 14- 15 gph.

Not all O-520 inductions behave the same. Whether talking about temperature spreads or induction ice the pireps about O-520s are not consistent. With all we know about engines I find that very curious, how nobody has it figured out.

Here's a snapshot of my EDM. My temps are very consistent at 24/2400 and 15.5 gph, which is my go-to setup. I fine tune from there. I prefer to burn a little extra fuel to keep CHTs below 380* and as long as I run 15+ gph that's what I get. EGTs have never been much of a concern. I never use carb heat for temp management. I added a carb temp probe with the EDM and experimented with it a bit. It didn't prove important.

Bubb2,

Glad to see you have done your homework. Those articles and several others are a must read for anyone operation the larger engines.
The fact that you are running ROP on some jugs and LOP on others in not right. You mentioned it ran fine and that just not compute with that wide in peak on the cylinders there should be noticeable vibration. I wonder if the egt probes on 1&2 have a problem? If there is a way to swap either one with another that would tell if the probes were at issue. I looked at a 180 with a O-470-50 conversion and it exhibited none of those systems.
We have a 182 based on the field with a new factory reman engine. The operator complained about one or two jugs running high EGT and Cyl temps. The IA looked it over a couple of times and did not find anything. Upon the ops insistence he removed the induction system and guess what he found. Not one but two intake gaskets that were misplaced and actually drilled through with new holes by the bolts. Piss poor quality control. My own 2009 FREM IO-520 showed small staining on the intake pipe on #2 and indeed it ran somewhat higher temps than the others. Same mechanic knew just where to look and yep misplaces intake gasket again. Different engine, 6 years different assembly time, so this shoddy assembly has been going on for some time. You may have Jug issues such as a crack/s gone unseen.

The statement " I was told the carb was supplying enough fuel (24 gph at take off)" is a VERY misleading statement. That in no way indicates all cylinders are getting proper distribution.

What is the egt on the offending cylinders at TO power? How far is that from peak? If ts less that 150-200deg you have a problem you need to correct.
There are many 0-470-50 powered 180's around that run fine with that system.
I would first verify your readings with probe swapping then take a very close lnspection of the induction system again.

Tom

I recall the Pponk STC specifying the single EGT probe location. Isn't it #4? How many 470/520 guys see #1 as the hot cyl? I wouldn't expect to find many. But I have to way of knowing. Speak up, guys!

If my hottest cyl was #1 I'd expect to find a leak at the Y pipe. That's been my experience. Even knowing where to look it was very difficult to find my leak. My induction rubbers get inspected and the clamps tightened every time my cowl is off. Use a cutoff wheel to cut a slot across the end of a nut driver to lock onto the clamp. Or do the same to a small socket and use a universal joint and extension to reach in and tighten hard to reach clamps. But that only works when your rubbers are properly seated. At the Y pipe that's easier said than done.

skywagon8a said:

Why don't you install a fuel injection system? It will even out the temperatures and lower the fuel burn. That engine should cruise between 14- 15 gph.

Click to expand...

I am. I have all the components to install an RSA fuel injection system. Waiting for a field approval. Paperwork was submitted to the FAA last month. I hope to install the fuel injection during brake up.

Tom3holer said:

Bubb2,

Glad to see you have done your homework. Those articles and several others are a must read for anyone operation the larger engines.
The fact that you are running ROP on some jugs and LOP on others in not right. You mentioned it ran fine and that just not compute with that wide in peak on the cylinders there should be noticeable vibration. I wonder if the egt probes on 1&2 have a problem? If there is a way to swap either one with another that would tell if the probes were at issue. I looked at a 180 with a O-470-50 conversion and it exhibited none of those systems.
We have a 182 based on the field with a new factory reman engine. The operator complained about one or two jugs running high EGT and Cyl temps. The IA looked it over a couple of times and did not find anything. Upon the ops insistence he removed the induction system and guess what he found. Not one but two intake gaskets that were misplaced and actually drilled through with new holes by the bolts. Piss poor quality control. My own 2009 FREM IO-520 showed small staining on the intake pipe on #2 and indeed it ran somewhat higher temps than the others. Same mechanic knew just where to look and yep misplaces intake gasket again. Different engine, 6 years different assembly time, so this shoddy assembly has been going on for some time. You may have Jug issues such as a crack/s gone unseen.

The statement " I was told the carb was supplying enough fuel (24 gph at take off)" is a VERY misleading statement. That in no way indicates all cylinders are getting proper distribution.

What is the egt on the offending cylinders at TO power? How far is that from peak? If ts less that 150-200deg you have a problem you need to correct.
There are many 0-470-50 powered 180's around that run fine with that system.
I would first verify your readings with probe swapping then take a very close lnspection of the induction system again.

Tom

Click to expand...

Already did the probe swapping thing. Switched cylinder 1 with cylinder 3. It just validated the probes were reading correctly. Engine had been overhauled and ED-900 installed at the same time. After one summer season (about 50 hours) and not understanding what was going on in that engine, compression check indicated 79-80 lbs in all cylinders and no induction of any induction leaks. Carb was overhauled by AvStar. I'm going to pull the data out of the EDM today as I did several T.O's yesterday, with and without carb heat. I do not think I'm getting 150-200 ROP on takeoff with the carb heat cold. Another interesting observation; the increase in fuel flow with the application of carb heat. At cruise power, full rich, no carb heat, fuel flow increased almost 1 gph when I apply carb heat. Can anyone explain that? Skywalkers suggestion about the restrictive air filter may have some validity here as I had installed the Cessna "winter air filter plate" prior to this test (been seeing temps well below zero as of late) to avoid "overboosting" (for lack of a better term) the engine on takeoff. I have adjusted the size of the air filter restrictor to give me a takeoff MP at 20 degrees as about equal to what I see at about 60 degrees. I'll pull the EDM data and compare the FF increase I see without the plate. I do not remember it being that much.

Stewartb, what are you seeing for oil temps? This engine runs what only can be described as very cool (except for those egt's on 1 and 2 with no carb heat). Even before I realized the beneficial effects of carb heat and was running just 40 degrees ROP on cylinder 1, I don't think the CHT on that cylinder ever went over 325 (although I admit I was putting lots of fuel through the engine). Cylinders 5 and 6 (the ones getting direct ram air) never go over 300. I attribute this cool running condition to several reasons, extremely tight baffling, Selkirk cowl caps (much larger air inlet to the cowl than factory), and the cowl flaps on early 180's when closed, allow more air flow than latter 180's do. I flew the airplane yesterday. It was between 15-20 degrees OAT, 2400 RPM, 23 MP, carb temp 40, FF about 16 GPH, leanest cylinder no. 1 was set at 75 degrees ROP, cowl flaps closed, CHT of number one never went over 310. With duct tape over about half of my oil cooler oil temps never went over 165. EGT temperature variation between cylinders was indicating as low as 45 degrees.

stewartb said:

I recall the Pponk STC specifying the single EGT probe location. Isn't it #4? How many 470/520 guys see #1 as the hot cyl? I wouldn't expect to find many. But I have to way of knowing. Speak up, guys!

If my hottest cyl was #1 I'd expect to find a leak at the Y pipe. That's been my experience. Even knowing where to look it was very difficult to find my leak. My induction rubbers get inspected and the clamps tightened every time my cowl is off. Use a cutoff wheel to cut a slot across the end of a nut driver to lock onto the clamp. Or do the same to a small socket and use a universal joint and extension to reach in and tighten hard to reach clamps. But that only works when your rubbers are properly seated. At the Y pipe that's easier said than done.

Click to expand...

I see your point, but how would one explain the almost perfect running condition with carb heat. The leak did not go away just because I moved the carb heat control.

Skywalker said:

Increased fuel flow on carb heat: When you bypass the restrictive air filter the airflow should increase leading to more venturi effect leading to pulling more fuel from the emulsion tube or whatever its called. Or perhaps the "more symmetrical airflow" means the local pressure at the tube is reduced relative to the average over the whole venturi cross section. Do you see a change in MP when you pull some heat?

Click to expand...

Do you have a Brackett induction filter? Skywalker saying "restrictive air filter" brought back some memories. I know of two C150s, a C172, and a Bonanza that did not like a Brackett filter. These airplanes each picked up between a hundred rpm and 225 rpm static and on climbout after switching back to a paper air filter. None had EGTs or CHTs or fuel flow meters to compare performance, only the rpm increase and nobody thought far enough ahead to pull carb heat and check for a performance change. No determination why these carburetted engines behaved this way and they varied from a fresh field OH to high time. This was back when Brackett elements were dripping with oil out of the package. Perhaps a test flight with the filter element removed is in order, if it's a Brackett? jrh

Re: oil temps. I have one strip of duct tape across my oil cooler all summer and two strips in winter. Oil temps are around 165-170.

The restrictive induction comment is interesting. How else could the increased fuel flow with carb heat be explained?

Based on what you are seeing I doubt going to the expense of adding FI will solve your problem.
These -50 mods are fairly common and I have never heard of a complaint to the magnitude you are reporting.
Carb heat is simply enriching the mixture to all cylinders causing the lean ones to run a bit more normal but masking the real problem. The others are now running excessively rich which is not good.
I would call your STC holder again and ask them if running 50deg or whatever you see ROP on #1 and #2 at TO is acceptable. If you don't get a reasonable answer then call one of the other STC holders and ask them if they have seen this before.

T