MooneyMaint

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About this blog

Let’s talk about tips, tricks and things that help us know our airplane better ... from the inside out. This is a ‘basics' BLOG where engines, avionics and owner maintenance are discussed and stories are told.

About me: I own Van Allen Airmotive, a new aircraft maintenance and management operation in Allentown PA at Queen City Airport KXLL. I’ve been pilot since 1982, with instrument and twin ratings and I own a 1996 Mooney M20M Bravo.

My early background was in engines (non-aircraft), turbocharging and racing. I later enjoyed years of electrical engineering work, including audio processing and RF transmitter and antenna design. The last 20 years has been in Internet security, cloud hosting and SaaS technologies - for the most part I have been a serial entrepreneur and the founder/CEO of a public Internet company. I am a paramedic and cardiac care instructor, and I fly for Pilots n’Paws as often as I can. 

I am an avid promotor of the aircraft owner being an integral part of the maintenance, and if possible, doing his or her allowable maintenance all the time. I think Mike Busch at Savvy Aviator is helping to change the way we think about maintenance and repairs, and George Braly and the folks at GAMI are pushing the industry out of the stone ages by using science and testing to disprove ol’ wives tales and prove how you can be safer and save money by understanding your engine a bit more.

Finally I’m a big fan of the way Jason Schappert of MZeroA teaches airmanship and promotes continued education for pilots.

So what do you like? What do have to share?

Entries in this blog

DVA

TLS Bravo Power Settings

I have updated the following with some more specific information, mainly due to some questions raised here on the M20M power output and on what MAP is required for takeoff.

 

TL;DR

  1. The TIO-540-AF1A/B is making more power than most people realize; the AF1B is NOT a detuned version of the TIO540, it is turbo-boosted up-tuned version (per Lycoming).
  2. The MAX MAP of 38” is the high-limit on the density controller, normal Full Throttle takeoff MAP should be 35-36” if the density controller is adjusted properly
  3. On take-off you’ll likely be making well over 110% power from this engine
  4. 2725 RPM and 35” MAP on a standard day at SL is where that engine is making 100% rated power of 270HP according to Lycoming

 

I’ve updated the power charts below, (lines start with a | where I do that and I have included some additional supporting Lycoming data)

DVA

-------

 

 

Hi!

 

I’ve seen a lot of threads that have good information on power settings when it applies to the TIO-540-AF1B, but they are buried and disjointed making it hard for someone new to glean an understanding. So I thought I’d try to put something together to help. Fire away.

 

For those who are struggling to understand % of power on turbocharged vs normally aspirated engines, allow me to take a stab at an explanation: (Experts will say I am oversimplifying here, but I think they will agree in principle):

 

A normally aspirated (NA) engine, like the IO 540 will be build to make a certain amount of horsepower (torque, actually) at a certain RPM, Mixture, Air Temp, Altitude and MP. NA engines are rather predictable when it comes to computing the power fall-off from 100% as the airplane gains altitude. With a turbocharged engine it is not as easy, but it can be done.

 

Let’s start with some basics: You can only get 100% power from a NA engine under one general set of conditions: That being at max design RPM, at a certain air temp (usually Std day), at sea level and at the highest practical atmospheric barometer; the latter condition is important to understand.

 

For the sake of this discussion's simplicity - experts please indulge me - your NA engine's highest possible MP, at sea level, will equal the barometer reading at that location. So if you are on the beach, in Florida, sea level 0 feet, and the barometer is 30.00 (nice day) - your NA engine, at full throttle will only produce 30" MP on the ground. If the next day is rainy and nasty and the barometer reads 28.50, then your NA engine will only make 28.5" MP on the ground. 

 

At wide open throttle, MP in a NA engine is very close to the real barometric pressure no matter where the engine is, and barometric pressure decreases by about 1" for every 1000' of altitude. So if your NA engine made 30" of MP on take off at sea level with wide open throttle, and if everything else remains constant, at 1000' you would have 29" of MP,  at 2000' you would have 28" of MP,  at 5000' 25" and at 10,000' 20". And as you climbed and the MP decreased, the engine would be making less and less power by virtue of the fact that the air is less dense as you go higher and higher.

 

You can think of a MP gauge as a 'air density' gauge. To add a little more precision here, it's the oxygen in the "air" that the engine wants so it can support combustion, and as you go higher in the atmosphere, while the percentage of O2 in the "air" remains the same (apx 21%), the density (or partial pressure) of that O2 gas gets less and less.

 

Take-away: MP directly affects the engine's ability to make and maintain power - more MP equals more power.

 

So do other outside conditions affect the NA engine's ability to make power? Yes, but some conditions are not to the same degree as MP.  For instance, the temperature and humidity of the air that is sucked into the NA engine will affect power; colder dryer air will help make a little more power, where warmer moist air hurts power. Commonly, about 1% / 10 degrees +/- is a good ratio. For the sake of this discussion, intake air temp for the NA engine is not that meaningful or significant when compared with MP, mixture, and RPM. However, you will see why intake air temperature becomes more influential with a turbo in a moment.

 

Take away: Cooler intake temperature and lower humidity help an engine make more power. Warmer and/or moist air hurts power.

 

Speaking of RPM: Cars, motorcycles, boats, etc all have much higher RPM where max HP is made; my car makes maximum HP at nearly 7000 RPM.  Why do aircraft engines have such low maximum RPM, commonly below 2900? 

 

It's all about the propeller. For a lot of good reasons a prop just can't spin faster than about 3000 RPM without the tips going transsonic and the efficiency of the blade (really a spinning wing) falling off rapidly. Given that limitation, direct-drive aircraft engines have to be made to produce adequate torque at a relatively low RPM. This is not as easy as you'd imagine. Higher RPM does a lot to contribute to overall torque when you design an engine. Side note: torque is the product of a running engine. Horsepower is simply a calculation derived from measured torque.

 

Take away: If everything else is equal, If you lower RPM you lower torque and thus you lower horsepower.

 

Then there is mixture. Four-stroke internal combustion engines designed around the Otto cycle create combustion (and thus power) identically. Car. boat, airplane, lawn mower, it doesn't matter. There is nothing special about an airplane engine except it is simple and unsophisticated compare to a modern car engine but they all work the same way.

 

While this is a whole nother' topic, mixture basics are simple: If you want to make the most power from an simple internal combustion engine, everything else considered equal, you want to have a mixture that will produce an exhaust gas temperature that is 80-100dF rich of peak at any particular power setting. This is where your Briggs and Stratton lawnmower will make the most power and run its CHT coolest for that power. If you want your lawnmower to cut more grass because the fuel lasts longer, then you want to run your lawnmower at exactly Peak EGT or better yet, slightly Lean of Peak EGT. These mixture settings will reduce power somewhat, but will save fuel and lower the CHT for that setting. You never want to run your lawn mower near full power at less than 80dF ROP - say 30 or 50dF ROP as some manuals suggest - because you are then operating the lawn mower in a combustion regime that will make excessive heat, with the highest internal pressures, at LESS power. Your lawnmower will hate you! So will your airplane. (Follow the POH for mixture settings, unless you know what you're doing. This discussion is theoretical)

 

Take away: Mixture affects power to a great degree - proper mixture setting is very important. Author's note: You should really read as much as you can about the Red Knob and how it affects your engine, and what effects it has on your engine. Proper mixture control, in a properly tuned engine with proper engine instrumentation will properly serve you. I cannot overemphasize this.

 

Lets summarize the NA engine now in flight, this assumes a good solid working engine with no hidden problems.

 

It's a standard day at sea level, 59dF and pressure 29.92 and the atmospheric lapse rate is also standard.  We take off with full throttle (close to 30" MP), mixture rich, prop full forward (if applicable) 2575 max RPM and fuel flow, fuel pressure and all temperatures within limits. We can assume we have 100% power as we lift off and we are climbing at Vy at, say 750FPM. 

 

By 3000' we notice our MP is down to just below 27" and our VS is now 500FPM, Why the loss of VS? We naturally lost power due to the air density going down as we climbed. What can we do about? Nothing really, every engine control is already full forward.

 

A quick check of the POH graph at this point may show that 2575 RPM at 27" MP is 80% power in this example airplane.  By 5000' ASL our MP is under 25" and our VS is 350FPM and the graph may show 70% power - still nothing we can do to make more power. In fact we will continue to lose power, VS and *indicated* airspeed the higher we go, until such a point as the airplane will no longer climb due to lack of sufficient power; this is the engine's 'critical altitude’ and a term you also hear used in a turbocharged engine's POH. The critical altitude (sometimes this is also the Service Ceiling) is simply the point at which the engine can no longer make enough power for the aircraft to climb.

 

Let's add a turbocharger to the above airplane. (I used the Bravo's 2575 RPM in the example above on purpose)

 

Recall above that the NA engine on take off made 30” of MP at 2575 RPM with all controls full forward, and the manufacturer called that 100% rated power. If we took that identical engine and flight conditions, and simply bolted on a turbocharger could we use that turbo to make MORE than 100% power? Yes we could! 

 

By simply using the turbo to boost the MP past 30" - anything past 30" - we will make more power. Another question: If we used the turbo to just keep 30" as we ascended (so not lose MP as a NA engine does) could we essentially keep 100% power as we go to 3000', then 5000' and beyond? Indeed! That’s how a Turbo-Normalized engine works. It doesn't make more MP than the engine has naturally at sea level, it just makes up for the natural lose of MP as you ascend.

 

But that is not what the we have on the M20M - we have a turbocharged engine that indeed makes more than 100% power at times (over a identical sized and configured NA engine)

 

The TIO-540 AF1x engine uses its turbo to over-boost the intake air pressure well above atmospheric pressure, up to as much as 8" or so. Its MP maximum is 38" and normal POH cruise is 30-32".  At any altitude that is way over the natural atmospheric pressure, making this engine a very high performance, hot, power plant. As you will will see, running this engine at high cruise power of 34”/2400 125dF ROP (like what the green visor suggests) is running this engine at approximately 93% of rated power. Lycoming and Mooney have certified this engine to run at percentage of power levels higher than you'd expect and that's what makes the M20M go so fast. But should you routinely run this engine at that high power level?  You can, it’s certified to do it. I don’t, however.

 

You have to look in both the POH and in the Lycoming Engine Operating Manual to put together a % of Power Chart for this engine. I have done some of that work below by averaging out the power charts, but first I think a little turbo talk is in order.

 

|The Turbocharger will make-up for the loss of air density as the plane ascends, and allow the engine to make more power, higher up. This gives us service ceilings of FL250 in the M20M and faster forward speeds ... that’s the up-side. The downside is that a turbocharged high performance engine also makes more heat - a lot of it - which causes faster wear and stress on engine and exhaust parts. Not to mention that the turbo and its parts adds to the cost and complexity of the engine. Running a turbocharged engine incorrectly can also cause premature failure and wear of the engine.

 

|IMHO, to get the most time and efficiency out of the AF1B, for cruise at altitude, you should routinely run this engine at a power point where it is not over-boosting but rather normalizing. That would be in the 28-30” of MAP as a max at cruise. From the chart below you’ll see that is 75-80% of rated power. I run mine at 28-30“ and 20-30dF LOP often, and it goes fast and uses a lot less fuel. (I have GAMI injectors and use Tempest fine wire plugs to help achieve smooth running LOP) If I am running at a higher power level with this engine, which I do occasionally, I will always run 100dF ROP TIT, and never at Peak TIT.

 

I find this to work well, you might have a better plan:

 

35-38” / 2575 / Full rich for takeoff                                                  =   ~100%-114% of rated power (270-308 HP)

  34” / 2400 / Full rich @ 120 for initial climb                                     =   ~93% power

  32" / 2400 / 100dF ROP @ 130 enroute climb                                =   ~87% power

  29" / 2400 /  30dF LOP for cruise (airspeed varies with altitude)    =   ~75% power

 

Here is the interpolated chart, supported by the graphs below derived from the Lycoming power plant manual. Please note that the power curves are not linear for some settings, higher up you’ll make more power.

 

ACTUAL HP corrected for RPM/MP/ALT/TEMP (Std Day (+/- 1% per 10dF from Std)) TIT ROP -125dF Max Power mixture  

 

114%        308        2575           38”    0-FL220 (This is the max authorized MAP and will rarely if ever be encountered when properly adjusted)

106%        285        2575           36”    0-FL220 (this is the normal “max power” takeoff upper limit where most density controllers will be set)

100%        270        2575           35”    0-FL220 (this is the normal “max power” takeoff lower limit where most density controllers will be set)

 

93%          250           2400        34”    0-FL220

87%          235           2400        32"    0-FL230

80-85%    215-225    2400        30"    0-8K 8K-FL250

75-80%    200-220    2400        28"    0-8K 8K-FL250

65-70%    180-195    2400        26"    0-8K 8K-FL250

60-65%    160-180    2400        24"    0-8K 8K-FL250

55-60%    145-165    2400        22"    0-8K 8K-FL250

 

85%          230          2200        34”     0-FL250

80%          215          2200        32"     0-FL250

75%          203          2200        30"     0-FL250

65%          175          2200        28"     0-FL250

60%          165          2200        26"     0-FL250

55%          150          2200        24"     0-FL250

50%          135          2200        22"     0-FL250

 

(Note, I don't normally fly 2200 RPM. I don’t have a good reason why) I changed my mind, I tried it and I like it, I can get 2200 RPM and 31” MAP LOP and she runs nice and quiet.

 

Finally,  recall from the discussion of the NA engine that we illustrated intake air temperature was not a huge contributor to power gain or loss. On a turbo such as the AF1B if can be. As the turbo spins up from the exhaust gases passing by impeller, the intake side of the turbo is sucking in outside air and compressing it, which raises the density of the air. This compression effect also heats the air - a lot.

 

On my Bravo I have a JPI 830 with all the options and I see Compressor Discharge Temps approaching 220dF on some hot days high up. The intercooler after the turbo will take a full 80-100dF out of that hot air and present an Intake Air Temperature of just over 100dF to the engine.  If you look at the fact that every 10dF of hotter air takes away 1% power, that intercooler is helping by at least 10% - that’s a huge power help. 

 

Here are some graphs to look at. The first is from the Lycoming TIO-540 operating manual. It shows us that that the full rated power of the AF1B is 270 HP at 2475 RPM and 35” MAP. This calculation is corroborated many times over in the manual.

 

Screen Shot 2016-11-11 at 2.29.42 PM.jpg

 

Of note, all of these figures are based on the TIO-540 engine, but notice that many variants of the engine make more of less power. Some of the engines are modified with lower compression ratio or different fuel flows along with higher or lower maximum MAP to create the difference in power. Note the Note: The Mooney POH suggests to us that full rated power comes on at 2475 and 38”, but Lycoming counters that with numerous examples, such as this:

 

Screen Shot 2016-11-11 at 2.23.24 PM.jpg

 

And finally, here is the graph used to adjust the density controller for full throttle operation.  Note the fact that the Compressor Discharge Temperature is critical in making this adjustment. From what I have learned, many A&Ps who are not thoroughly familiar with how a density controller works, will simply “eye-ball” this adjustment which is the likely reason why some M20M (and others) pilots have such large variations on the full throttle MAP.

Screen Shot 2016-11-11 at 2.27.24 PM.jpg

 

If you are consistently making at least 35” to around 36” of MAP with a fuel flow of greater than 27.5 GPH at a little less than 2475 at the start of the ground roll - you are in great shape.

 

I hope this helps, I am not an expert but I listen to and learn from them. I'll try to answer any questions as best I can.

 

Enjoy your Turbo Monster 20 M!

Dave

 

DVA

Hi!

I hope this will help other Bravo owners who struggle to run the TIO-540-AF1B at Peak or LOP TIT.  But first, in full disclosure I am not an A&P and this is not advice. I am simply illustrating my experience with this engine, and it or may not apply to you. Always follow the POH when you are not sure that your deviation from that document is in your best interest.

I am a lean of peak fan, always have been. It comes back from my days of working on non-aircraft internal combustion engines and proving that an engine run LOP operates cooler, cleaner and lasts longer than a similar engine run ROP.  I have taken the Advanced Pilots Seminar course on advanced engine management http://www.advancedpilot.com and had numerous discussions with Lycoming engineers, the folks at GAMI and engine builders, and I have used this knowledge to come to a few conclusions about this engine that I would like to share. I am not poking the sleeping “ROP vs LOP” dog. :-) and I realize that Lycoming - in some instances but not all - does not recommend operating LOP.  I also believe that if they could, they would revise that language to say:

If you have a good engine monitor, tuned injectors, and a knowledge of how your engine operates, you should run LOP whenever your heart desires - except on take off.

My opinion is that Mooney, when they introduced the TLS, continued their fine mission to make the fastest commercial SE piston airplane. To do this, they needed a lot of power -and- in a weight package that would not cause the flight envelope of the long body to get too forward on the CG, the TIO-540 was the answer. Bravo owners know that the airplane is already pushed forward CG and many have Charlie weight for aft ballast installed (which lowers the already skinny useful load). The TIO-540 is a complex high performance engine and should not be grouped with most other ground boosted engines for performance discussions,  The reasons for this, IMHO, are due in part to two things: 1) a complicated (but effective) turbocharging controller system, and 2) the requirement that the engine runs at very high percentage of power levels to make book speeds. I did a post a few weeks ago on the Bravo’s power percentage here.

Because this engine is normally operated at greater than 80% power during cruise by most people, this engine is very working hard and making a lot of heat for a lot of the time. It is also doing so with rather loose factory tolerances on the precision of fuel flow to the cylinders which makes it extremely difficult to run this engine in an wide and efficient range of power settings.

The POH states only two settings: 1) ROP TIT by at least 125dF for “best power"; and 2) Peak TIT as long it’s below 1750dF (1650dF at high altitudes) for “best economy" - the latter is sometimes impossible to achieve with this engine at higher power levels (30” MAP and above) because of poor fuel distribution which causes engine roughness. When near peak TIT (or EGT) the roughness is normally due to some cylinders running leaner than others. The leaner cylinders produce less power than do the richer cylinders which give you the impression that there is something wrong because you feel that power imbalance as roughness. (Note: that slight roughness is not a bad thing, your engine won’t fly apart, it really doesn’t care, only you do.) Spark plugs play a key role in this too - more on that in a bit.

Here’s the rub... Because most of the TIO-540-AF1Bs have unequal cylinder fuel distribution, when Bravo owners try to run the engine per the Sun Visor chart at Best Economy (Peak TIT) they may find an disconserting “roughness” and feel a slight loss of power.  That combo causes some consternation, and when that happens, some operators I’ve spoken with will simply dial the Bravo’s red knob in just a little richer and go slightly rich of peak TIT just enough to cure the roughness. Thinking that they are now 'just fine’ they fly the engine at that setting - when in fact they are not “just fine." They are now operating the engine “slightly ROP TIT” at a mixture setting that causes the most cylinder head heat, the highest internal combustion pressures and at a place where the engine can easily begin to exhibit detonation. (See graph below, which was taken from the Lycoming Flyer publication) The Mooney POH does not say it is OK to run the engine “slightly” ROP TIT because both the factory and Lycoming know that is a very bad mixture setting.  All of the experts I’ve spoken to agree that no internal combustion engine should be operated “slightly” (10-60dF) rich of peak. If you can’t make Peak TIT for whatever reason, better to just go greater than 80-100dF and accept the extra fuel costs and keep things in the engine cooler and happier.

I have not found anyone who disagrees that sustained high heat weakens, fatigues and shortens the life of the metals used in engines, and that’s why we see all kinds of advice about keeping cylinder head temps below 400dF. The Bravo’s POH advises that you use a combination of gauges when setting power - TIT and CHT as the most prominent. The POH also says that the CHT redline for this engine is 500dF - which everyone (experts or not) agrees is simply ridicules.

If you have an older Bravo, and especially one where a field AF1A to AF1B conversation was done, you may want to check to see on which cylinder the panel’s CHT temp probe is located. The AF1A probe was located on cylinder #5, and Mooney Service Instruction M20-101C states that it should be on cylinder #3 for the AF1B. Check yours, especially if you rely on the single panel CHT gauge, I’ve spoken to three Bravo owners where the CHT probe was still on #5 (mine was too). There is a big difference in the cooling between #3 and #5 - #3 being as much as 50dF hotter.

That all said, in summary the TIO-540-AF1B is a hot running, high power, high performance engine, different from many others. In the M20M it is asked to operate at the top of its performance range in order to make POH (book) performance numbers, and us Mooney drivers like to go fast! Adjusting the mixture on this engine can be  tricky due to engine’s generally unequal cylinder fuel distribution and, in many cases, the wrong type of spark plug being used.

I wanted my Bravo to act like most other well-tuned and instrumented airplanes I’ve flown. While always keeping the cylinder head temps below 400dF, I want to be able to safely set the engine for maximum power when I want to go fast, and I want the ability to safely save fuel when fast doesn’t matter as much. I don’t want complicated instructions to do this, and I want to feel as if the engine is happy and smooth no matter what.  Before I started this trek, I could not run my Bravo at Peak TIT at MAP higher then 29” without noticeable roughness and/or what I felt was unacceptable power loss. And there was no way this engine would run LOP.  I would flow about 22 GPH of fuel in cruise at 100dF ROP TIT (on hot days I needed to to be richer to keep the CHTs below 400dF).

Here’s what I did.

  • I first ensured that magneto timing was correct. This is very important with high performance engines; you can often get away with inexact timing on lower power engines, but never on engines like the TIO-540. Mine were pretty close, but not exact - they are now. I had new Champion massive plugs installed about a hundred hours earlier, on inspection they looked okay and they passed the tester test. We gapped them at .016.
  • I installed a new set of GAMI TurboInjectors. When I spoke with the factory rep John-Paul he cautioned me that this engine was a hard tune and that I should expect to have to work at, and that there might not be the success that others have with GAMIs on other engines. I love honesty - these guys at GAMI are true pros. 
  • The first set of injectors made a marked and clear difference. I was able to run at Peak TIT smoothly for the first time, but I was unable to run LOP without roughness. I did a GAMI lean spread test and found that my spread was about 1.4GPH, while better, it was not ideal. I contacted the factory and John-Paul immediately sent out two replacement injectors for a better match. After that a test flight or two it showed that I actually picked up about two knots at peak TIT and fuel flow was down a little. I could now get a little bit LOP with a GAMI spread of .9GPH.  Also noticed CHT were generally cooler by about 20dF. This was due to the fact that the better fuel distribution was allowing all cylinders to run more equally, so at Peak TIT all cylinders were closer to their peak EGT. Fuel was saved for the same reason - unnecessary rich cylinders were now leaner for any given mixture setting.
  • Because this engine seems finicky at different MAP/RPM settings, I decide to tune to a specific sweet-spot for the GAMI spread - I picked 29”/2400 for this as it is, according to the Lycoming power graphs, about 75% power on a standard day, at mid altitudes. This might have been the most important step I took in achieving success with this tune, on this engine to allow for good LOP performance.
  • I sent the new GAMI lean test to John-Paul - not satisfied he sent me a single replacement for the one cylinder that was off a bit. (no charge for all of this and no questions asked). We installed that one injector and then did a test flight. The biggest change was that I could get more LOP without roughness, at 2400/29” I could get to about 20dF LOP. I would lose about 9 knots, but I was able to save almost 6 GPH of fuel. While I still couldn't get much past Peak TIT at higher power setting I was happy with the trade off; now I could achieve both fast and efficient settings. My GAMI lean spread was now a very comfortable .3GPH as you can see from the graph below. I thought that was all I needed to do but it wasn’t ...
  • I have a Savvy Aviator account, I upload my JPI engine analyses data there, and I happily buy their yearly analysis service. I uploaded a flight and was looking at the graph and saw something on one of my lean spread tests that I could not understand. During a lean test, you should see all EGTs rise as you get leaner and leaner, then they should all peak (at slightly different times, that’s the fuel flow “spread”) and then they should drop off. On my test, there was a second peak? I submitted the flight for review at Savvy and Paul Kortopates wrote back and explained it, and as soon as I read his explanation I understood: He said "That second "peak" is actually what happens when the mixture goes lean enough to fire only one plug. You are seeing the same rise we would see if you switched off one of the magneto's so that there was only one plug firing- which is what we're seeing here. On one plug alone, combustion is slowed and therefore when the exhaust valve opens we are seeing more of the combustion event and the associated higher EGT because of it”  That’s when we discussed the last step I needed to take to get this whole project right - new plugs - but specifically fine wire plugs. It seems as if the fine wire plugs work better than the massives in two instances 1) older wet and oily engines (not the case here) and; 2) in lean mixtures. They’re expensive, about $80 a shot, but they also are suppose to last hundreds of hours longer.
  • After researching both Champion and Tempest, I opted for the Tempest Fine Wires and installed 12 of them. Paul was right on! From the moment I turned the key I could tell that something was different. The engine started better and ran smoother on the ground and in the air, and I am now able to run LOP at 32” MAP and below if I chose. My CHTs are generally 30dF cooler than when I started this project, and I am saving fuel at every power setting. Where I use to run 22GPH at 2400/32 ROP, I now run 20GPH with the same airspeed, and if I want to throttle back to 2400/29, I loose about 10 knots and run about 15GPH at about 20dF LOP. 

In all, I have about $2500 invested here, but in fuel savings alone that will pay back in short order and then keep paying back. The big benefit is that I have more power options now with the aircraft and my engine will be much cleaner with less carbon deposits on the heads, the values, the plugs and the exhaust system. 

My flight profiles are not religious LOP, and yours don’t have to be either to get a benefit from the cleaning and cooling aspects of running your engine with a proper mixture, which, for me includes LOP at times. Typically I will run lower power and LOP in tail winds of any speed, because why not? If I loose 10 knots true in LOP but I make up some or all with a tail wind, I’m saving 5-6GPH of fuel AND cleaning the engine as I go.

Thanks for reading! I attached some pics - happy to try to answer any questions.

Dave

 

Second Peak.jpg

GAMI Spread.jpg

Power Curve.jpg

JPI.JPG

 
DVA

Vapor lock comes in varying degrees, so a single technique to purge the fuel lines of “gas air” won’t necessarily work. I break it down by short heat soak (about 10 min or less) and long soak (about 30-60min) and yes, there is that charlie foxtrot area in the middle ~15-30 minutes where anything can happen.

This discussion is for a fuel injected engine.  

During a short heat soak the likely culprit will be the upper fuel lines to the distribution device and the injector lines to the cylinders. Note, these fuel lines often sit atop the hot engine, and since heat rises, the relatively small volume of liquid fuel in these lines atomizes quickly and becomes a vapor (gas air). To fire a mixture off in the combustion chamber, there must be an atomized fuel mist suspended in the surrounding air. If the fuel is too atomized as in a vapor, the fuel density won’t be sufficient for firing, hence a hard start, actually due to a overly lean mixture.

To clear a short soak, you need to pressurize the upper fuel lines with just enough fuel to push out the vapor, and not too much that you flood the intake ports.  This is where most pilots get in trouble with hot starts; a hot engine needs far less fuel to start than a cold engine. The theory is to begin the starting sequence for a short heat soak with NO fuel flow and then ADD fuel slowly until it fires off.  Most often, we do it backwards and that makes things exponentially worse.

During a long heat soak, the entire fuel system comes in to play from the fuel tank feed lines to the fuel pumps, to the pump chamber, to the feeder lines, on up. It generally takes longer for these components to heat up and begin to vaporize after sitting than do the upper fuel lines. The clearing technique here is different than a short soak, as you want to (have to) push the vapor out of the pump circuit and that takes time; sometimes a lot of time.

To clear a long soak, you need to purge the entire fuel circuit of vapor, not just the lines going to the injectors. In a long soak, there is a lack of liquid fuel in the pump circuit (because it got hot and vaporized away) ... and a fuel pump, while good at pumping a liquid, is very inefficient at pumping vapor. So you crank and crank and crank and nothing happens because nothing is happening - no fuel is flowing - because the fuel pump is essentially pumping air. In cases of a long soak, using just techniques that will effectively clear a short heat soak will do little to clear the pump circuit, causing a hard start.

Knowing a little about why it’s hard to start a hot engine often makes it easier to find a solution. 

If this works for anyone send money; I have airplane payments to make and my wife and mistresses want jewelry. 

Short Soak: 

  1. Throttle cracked to the point where it would need to be to have about 1000-1200RPM if the engine were running
  2. Boost pump OFF - (Do not use the boost pump or primer at all)
  3. Mixture Full Rich for about 5 seconds then Idle Cut Off
  4. Begin cranking the engine, wait a 2 seconds then slowly (over 5-10 seconds) move the mixture toward rich. Don’t exceed recommended cranking time.
  5. As soon as the engine starts, keep the mixture at about that point, adjust it and throttle for smooth operation. You should ALWAYS run the engine as lean as possible on the ground.

Rationale: The throttle is cracked open so that when the engine fires, there is proper air flow for the fuel that is being slowly added by the mixture control. The Mixture is open fully for a few seconds first to allow any built up vapor pressure to purge out, then its closed; this gives liquid fuel a clear path down the lines. The mixture is kept closed until the cranking so that you have complete control of how much fuel to add to get the engine lit off - which likely will be different every time - this technique also significantly reduces the changes that you will flood the engine. There is NO boost pump used because the mechanical pump should be able to provide fuel at a rate that keeps excess fuel low, unless you have a hot soak condition... You will know if you have a hot soak event because the above technique will not work after two tries. Summary: There is liquid fuel available at the fuel pump but there is vapor in the injector lines. The vapor does not ignite easily and it blocks liquid fuel from flowing past the vapor area (vapor-lock) causing a hard to start condition. You have to relieve the pressure of the vapor and then slowly add liquid fuel to the lines and the injectors so that the mixture of fuel and air is correct for ignition in a hot cylinder.

Long Soak:

  1. Throttle Closed
  2. Mixture Full Rich for about 5 seconds then Idle Cut Off (Be very sure it is fully at idle cut off)
  3. Boost pump on (or on low if two speed) for 30-60 seconds. (yes, a full half a minute to a minute)
  4. Boost pump OFF
  5. Throttle cracked to the point where it would need to be to have about 1000-1200RPM if the engine were running
  6. Begin cranking the engine, wait a 2 seconds then slowly (over 5-10 seconds) move the mixture toward rich. Don’t exceed recommended cranking time.
  7. As soon as the engine starts, keep the mixture at about that point, adjust it and throttle for smooth operation. You should ALWAYS run the engine as lean as possible on the ground.

Rationale: The Mixture is open fully for a few seconds first to allow any built up vapor pressure to purge out, then its closed; this gives liquid fuel a clear path down the lines. You want to be very sure that the Mixture is fully at idle cutoff because we do not want any fuel to get past the metering circuit. Running the boost pump with the mixture closed will pressurize the fuel circuit and circulate some liquid fuel which will help cool things down and reduce additional vaporization. Excess vapor will be expelled through a vent port and the mechanical pump and the lines leading to the metering circuit will become fresh with cooler fuel. This takes time, and you have no worry of flooding the engine because the mixture is at idle cutoff. You finish by following the same procedure as a short soak. Summary: The mechanical fuel pump has to have the engine cranking to do its job. It would take too long and be too hard on the starter to use this pump to purge vapor. (Hence why I’ve seen pilots cranking the engine for absurdly and dangerous amounts of time). The electric pump is parallel* with the mechanical pump and has the ability to run quickly and more efficiently to do the job of purging the vapor, but its not an easy job - it takes a lot of time therefore you need to run the pump for at least 30-60 seconds. Once the vapor is purged, liquid fuel can flow past the metering circuit, and now you have the situation of a short soak to deal with, see above.

So why not just use a Hot Soak procedure every time?

You could, but after you understand why the damn thing won’t start and you think about it, you can use the technique that works the best.  If I land the plane, shut down hot, and then go for a restart in less than 10 minutes or so (all thing considered equal), I know that my problem is not at the pump, its at the top lines, so why waste time.

Disclaimer: Follow your POH unless you fully understand the pro’s and con’s of using other methods and other’s advice.

DVA

*As Don Kaye correctly pointed out, the electric fuel pump (on the M20M) is physically in “series" with the mechanical pump as shown on the schematic. The point is that neither of the two are dependent on one another and both can participate alone or together, in parallel, to provide fuel flow and pressure. 

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