Leaner can mean cooler running, mixture experiments Options

[lapuwali]

In another thread, TimT posted a graph produced by Pratt & Whitney showing the relationship of CGT, EGT, mixture, and power in air-cooled aero engines. Despite the often held assertion here and elsewhere that "our" engines need to run rich to stay cool, the chart showed that CHT fell as the mixture got leaner from "best power" (about 13:1). It also fell as it got richer from best power. There was a question in my mind how well this generalized to the Type IV.

My 1.7 w/ Webers had been, I knew, running rich on the idle circuits, and I was waiting on a set of leaner idle jets. These showed up, and I installed them this morning (50s down from 55s). Crisper running, and I hope much better gas mileage (was below 20mpg). My CHT gauge held the real surprise, however. It's installed on the hottest cylinder (No. 3), and previously warmed up very quickly to 300dF after just 30 seconds or so after a cold start. It would climb to 350dF as soon as any load was put on the engine, and up to 400dF when pulling up a two-mile long 6% grade near my house. With the leaner idle jets and no other changes, it was much slower to warm up, and never exceeded 275dF idling and puttering around the neighborhood at 25mph. Up that long hill, it just barely touched 350dF.

I don't have a wideband mixture meter (yet), so I have no idea if I'm still rich or somewhat lean. Drivability is still good, so it can't be all that lean. I haven't yet tried to do a plug check (used to be sooty black). In any case, it's one data point that seems to support the P&W chart.

[r_towle]

what size carbs are you running???

Rich

[lapuwali]

40IDFs, 28mm venturies. 50 idles, 115 mains, 200 airs.

[Jake Raby]

your jetting is spot on for that combo of venturi.... Its not lean unless you are running in Death valley, on a cold night at wide open throttle.

Lean+ hotter unless it is so rich that its running super inefficient.

Please be advised that CHT can vary from day to day with changing conditions on the gauge, you may have gotten aslight false indication... Observe it for several days and then make a hypothesis.

[TimT]

Jake,

lean is cool too

have a look at this article.... from the AVWeb site...These guys know a wee bit about air cooled egnines...If they have problems they fall out of the sky..They also hate paying for more fuel than they have to..

The gist of the article is go rich ( accelerating or climbing), and lean out the mixture ( at level flight, or cruising) so as to save money...

Also the graph show EGT, and CHT temps falling as the mixture leans.... after peak power and efficiency is achieved... this is easy...

there is less fuel to throw in the fire...no fuel = no fire=less heat

The Pratt&Whitney info is good solid info... they helped write the book....

[lapuwali]

I'm pretty familiar with the characteristics of the CHT gauge, too. They mostly change temps based on the temperture of the "cold end" of the thermocouple, which will read lower if the cold end is hotter. In this case, thanks partly to the very consistent California weather, the conditions from yesterday (when I ran with the "rich" jetting) were identical: 70dF air temp, same route. The cold end hasn't moved from where I installed it. So, either somehow the air temp in the engine bay was 50-75dF hotter than it was yesterday, or the leaner jetting cooled the engine temp.

I'll admit this is still just one data point, and I'm not prepared to draw any real conclusions from it. However, it does fit the data in the article TimT pointed to.

[F4i]

The graph is correct and you will run cooler at lean of peak. However the gain VS potential loss is not really worth it. Not many flat four P&W's out there. Lycoming has even published a publication on the matter called the "new" old leaning technique. SSP700. Will buddy warranty your engine after the bang?
BTW this is not an attack on TimT who posted the link Just more fuel for the fire (IMG:style_emoticons/default/happy11.gif)

[airsix]

lean is cool too

have a look at this article.... from the AVWeb site...These guys know a wee bit about air cooled egnines...

What F4i and lapuwali said...

You can learn things from this data, but you can't make a 1:1 comparison between this data and our engines. There are too many differences. First, they operate at a max of around 2,700rpm. They also have static-timed mageto ignition (dual plug) that has higher spark intensity as rpm increases vs our Ketering ignition that loses intensity as rpm increases. They run higher octane leaded fuel. Basically there are more differences than similarities.

I can tell you first hand that yes, you can dramatically drop EGT by leaning out the mixture. However by the time you've leaned it out enough to significantly drop the EGT the power is dropping off like Niagra Falls. The power is less than when the EGT is at the same level in an over-rich condition. In otherwords you can get the same EGT by being too lean or too rich, but you'll have more power when it's at the same temp and rich. Peak power is very near peak EGT temp (on an aircraft engine). I know this from sitting on the ramp and tuning the mixture for max power using the tach and EGT guage as a guide. (Airplanes have a built-in dyno we call a 'prop' (IMG:style_emoticons/default/wink.gif) ). We typically lean out for max rpm and max EGT, then richen it back just a hair. Remeber, that procedure works great on a static timed aircraft engine turning 2,750rpm with dual-plug mageto ignition running on high-octane leaded fuel. The same technique might be 'less effective' on a typical typeIV.

-Ben M.

[F4i]

(IMG:style_emoticons/default/agree.gif) Thats how I fly em and thats how I reccomend it done as an aircraft mechnic. It can be done the other way and will work. Just risky and less power. For like 2 gal. an hour.

[lapuwali]

I suppose the real thing to take away from this is not so much that leaner is cooler but that richer does not always equal cooler. A rich engine is not always a cool one, which is something I've heard over and over again.

[Bleyseng]

I think what has been tossed around is not to tune a type 4 to 14.7 to 1 which is what most modern cars are set to. The factory ran them at 13.7 to 1 for many reasons.


Geoff (IMG:style_emoticons/default/biggrin.gif)

[Jake Raby]

Put an aircooled engine at 17.7:1 and watch it fry.. Most of my best power and cooling stettings are at 13:1

Aircraft have a totally different group of settings. For one that are at higher elevation and have different ambient air temperatures and pressures AND the engine is working just inside a cowling with fresh air blowing through keeping charge temperatures down.

They also work with less load that a vehicle typically sees. Yes a huge prop creates load but at the same time you are only keeping it loaded during changes in airspeed, climbing and doing maneuvers... at cruise and constant airspeed the load is lower . This is exactly why they recommend in the article to enrich during these times and lean out during cruise.. Most of my aircraft experiences have been with Turboshafts, but I have been trained with piston engines as well.

Keep in mind that "avgas" is 100 octane low lead and is filled with its own coolants to enhance the burn with the cylinder pressures that typical aircraft engines have- this changes everything and is one reason it cannot accurately be compared to a ground vehicle on 91 octane pump gas which is in many cases "oxygenated"

Keep the engine at the point where it wants to run coolest and make the most power and you'll be just fine- sorry but you'll have to have a dyno to find that without dialing in for a week... I dial my engines in with use of a wide band O2 meter and a fuel flowrator to measure BSFC and plot it.

[lapuwali]

Jake, are you willing to divulge one of those CHT/AFR/HP/BSFC graphs for the Type IV? I'd certainly understand if you were reluctant to, but that would be a useful tool for those of us who don't have the hardware and are still trying to put together the money for one of your engines. If you can't spare the time, but are willing to spare the raw data, I'd be happy to work it up in pretty graph form and repost it for you.

[TimT]

Well I knew the air frame mechanics would come out and say this and that..

Yes they have magneto ignitions with static timing... and they dont operate at over XXX rpm....I hope your car has full ignition advance at 55mph...

However there are some things the .49cc engine you had as a kid (your model airplane engine), the engine in your 914, and the lycoming engine, or any water cooled engined all have in common...

Laws of thermodynamics.....

AVweb has a host of incredibly informative articles a portion is quoted here

     
ie #7:
Aggressive leaning results in burned valves and detonation.
Fear of the red knob is one of the most pernicious areas of misinformation among general aviation pilots. Most pilots operate way too rich most of the time, and do so because of the mistaken belief that leaning will harm their engine. The result is usually trouble: fouled spark plugs, accelerated exhaust valve guide wear, and stuck exhaust valves.

Lycoming has long authorized leaning to peak EGT at any cruise setting up to 75% power. TCM authorizes leaning to peak EGT up to 65%, and its latest recommendations even endorse lean-of-peak operation for many big-bore engines, provided the engines will run smoothly when operated that lean.

Contrary to popular belief, aggressive leaning doesn't cause burned valves. Most burned valves are the result of excessive valve guide wear or valve stem contamination.

Aggressive leaning doesn't cause detonation, either. Most of our engines are incapable of detonation at cruise power settings, provided that we don't exceed CHT red-line or try to burn contaminated fuel. Furthermore, recent tests on Lycoming engines by ASTM revealed this fascinating result: detonation is most likely to occur at a mixture setting 11% richer than stoichiometric (i.e., substantially richer than peak EGT).

Lean as aggressively as the book allows. For Lycomings, that means peak EGT at all cruise power settings to 75%. For Continentals, lean to peak EGT up to 65%, 50°F rich of peak at 75%. For turbocharged engines, also limit TIT to 1600°F.

Lean during all ground operations except for engine start. It is particularly important to lean for taxi and runup. Since EGT is usually off-scale at idle power, the best method is to lean for peak RPM at idle.  

once the engine has climbed the hill... brought the boat onto a plane, or got the plane into the sky,, the engine can "relax" and doesnt need to produce max hp per unit of fuel...Savings of a few GPH cant be ignored...

All this run lean stuff doesnt mean shit with carbs...with EFI I can optimize the mixture at any rpm, manifold pressure etc...

BTW I would only use lean run stuff on a street car..

[Jake Raby]

Tell ya what..
Tomorrow I'll do a few test for ya and show you the differences between a 12:1,13:1 and 15:1 engine Then fully optimized

... Not only in power but also cylinder head and exhaust gas temps..

you'll see the differences as clear as a graph.

I was never an airframes guy, worked in powerplants and played with fuel controls and power management systems mostly... Longe before I touched an engine dyno I played in "test cell" and help run up Turboshafts..
I have done my share of 540" Lycomings and a few Continentals as well. Those engines are as backaward from a TIV than anything that I have ever seen..

Some of the biggest mistakes I have seen with engine combos is guys trying to treat this engine like another engine- its not! The funky head design throws everything otherwise conventional with other engines out the window.

I'm not saying that a TIV is unlike anything else in the world, but it is a totally different animal, its still a 4 stroke overhead valve, and thats about it..

BTW aircraft engines make peak VE about 2600 RPM, their piston speeds are weird because they are designed to be totally over revved at 3600 RPM.

[DNHunt]

First let's make sure everyone is on the same page. Reading these posts 3 reference points were referred to 1) Stoich 2) best power and 3) lowest EGT, CHT.
That gets kind of confusing. I suspect we should be considering best power or best temp as a starting point. That gets Geoff, Jake and even PW on board with the same reference point.

Stoich as a tuning point is a random number important in only in thermodynamics and no more important than any other number you could pick. Nice to shoot for but maybe not attainable.

The other numbers are something that can be worked toward.

I have a few observations gained from tuning 1 engine so take it for what it's worth. My engine needed to be tuned to run around 13.5 and richen to 12.5 on accel.

When trying to tune to leaner running conditions and very light load the duty cycles of the injectors are so short tuning becomes impossible. A/F ratios would bounce between 13 and 17 and the car would buck slightly. Say good bye to lean cruise. Coming off such a lean cruise condition the car would stumble.

Accel enrichment was easy to over cook. Very little went a long way so I ended up with a small increase over a very short interval and the car can run pretty well without any at all.

The dyno helped to smooth the variations out in the A/F ratio. I'm not sure I would have ever been able to do that with the dyno.

I suspect that there is not much room for fuel savings on the lean side max power. No doubt it will run and maybe it will stay cool but with the factory injectors it will not run well. If you use smaller injectors or meter fuel with an variable fuel pressure regulator you might be able to tune in a lean cruise but, I couldn't.

Dave

[Jake Raby]

What good is lean running and saving money on fuel if you are cracking your heads and frying the top end of the engine??

Fuel is a coolant to our engines. Build one for a Zamboni and run it on LP gas and you'll soon see how fuel effects the engine..

[Rockaria]

I think we need a book written called

"Zen and the art of 914 tuning"

A book that describes Head Temps, Oil temps, Mixture, Ambient Temperature and how they all work togther. Oh and throw in Elevation to boot.

Maybe just one really good guide that we can all write and get these facts on paper (or Screen). This topic has made me ponder for the past few years and I have yet to get a definitive answer.

Anyone?

[Jake Raby]

I have been writing a book similar to this since 1999. In about 3-4 more years I should have something to publish.

[Rockaria]

In about 3-4 more years I should have something to publish.

In that time I think my engine will be a molten pile of metal on the side of the road... (IMG:style_emoticons/default/blink.gif)

Just kidding... Does anyone have any High Altitude, Desert information? "Sand and Heat," Not the Chocolate kind (IMG:style_emoticons/default/wink.gif) I can keep my Head Temps between 330 and 365 all of the time and even in hard driving, but my oil temps get to 240 on a 100 degree day...