rocketbob

rocketbob

Well Known Member
Normally I like to run a constant speed RV at 2300/25" in cruise. Lower RPMs help eliminate tip losses and increase efficiency, and also reduce wear on the engine.

I'm wondering if anyone's done any testing at lower RPM's for cruise settings at 1800 RPM or so and 25"-26"-whatever.

If not I'm going to do some experimentation.
 
I seem to remember Lindbergh doing something like that in WW2 to stretch the range of our planes. I think he was teaching the concept to the pilots.

Details have faded, but the concept still resides in my poor tired old noggin.
 
rpm

Rocketbob,

Do the experimenting and post your results. I have thought of this often but have never practiced it because of the old wives tales. I'll run a two to three increment difference every now and then for a short flight, but I've been reluctant to do it for a long three hour flight. I never have had anyone explain to me why running "oversquare" would damage the engine.
 
Lindbergh invented the concept of LOP operations at low RPM's.

What I am interested in finding is at what RPM the peak volumemetric efficiency occurs which should coincide with the peak torque RPM.
 
How far over square is OK?

Looking at the Lycoming manuals, they provide operating curves for part-power operation. The chart I'm referring to is unfortunately not in the linked document. For each manifold pressure, they show the range of RPM that they think the engine can be run at. I don't have it in front of me so I am just going off the top of my head, but for example, the WOT curve goes down to something like 2200 RPM. The 25" MAP curve goes down to something like 2000 RPM, etc. (Again, those are not the real numbers, I'm just making them up as notional examples of what the curves show).

Anyway, point 1 is, Lycoming is fine with running over square.

point 2 is that there is a lower bound of how far over square, as a function of MAP. This is essentially a boundary on what the crankshaft and bearings can take -- in cars it is referred to as "lugging the engine". In extreme cases you can get pretty nasty crankshaft vibration from putting too much load at too low an RPM.

point 3. The benefit of going to very low RPM depends a lot on the particular propeller design. Running below the design RPM the efficiency will drop. Not as precipitiously as running above the design RPM, but it does drop off.

point 4. The full-throttle specific fuel consumption curve (which is in the linked document, first chart) shows that it levels off around 2200 RPM, so there is no compelling reason to go below that anyway.
 
It's possible

rocketbob said:
Normally I like to run a constant speed RV at 2300/25" in cruise. Lower RPMs help eliminate tip losses and increase efficiency, and also reduce wear on the engine.

I'm wondering if anyone's done any testing at lower RPM's for cruise settings at 1800 RPM or so and 25"-26"-whatever.

If not I'm going to do some experimentation.
Click to expand...

Hey Bob:

The data you seek is in the 540 operators' manual. 1800/25" is the lowest setting, with 2000 turns allowing 27". Looks like the lower setting is about 137HP, but I can't see a fuel flow number there. 2000/27" gives about 172HP. The chart notes say the mixture is a max power setting, so it's gonna be thirsty there! Another chart sez 2000/27"/170HP is 12.5GPH @ max economy, and 1800/25"/138HP is about 9.5GPH.

BFSC of .44 on the 2000 setting, and about .41 on the 1800RPM setting.

These look like low altitude numbers, considering the MP numbers.

At altitude, it's gonna be way different as you will need to spin 'er up to get enough HP to get anywhere. Look at 55% & LOP as a good target, and run the lowest RPM/WOT/LOP (if possible) to get the 55% number. At 17500, you won't be running LOP as the power would be too low, with an IAS to match.

Make sure your prop is up to the job at the lower RPMS - Hartzell can help there. As for the prop question: will it work efficiently at low RPMs, you will be the first to know!

Carry on!
Mark
 
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low rpms

About 30 years ago, flying mag had a road test of an observation aircraft built in Europe. Cockpit was a bubble similar to a helicopter. It had a Lycoming engine and used low rpm/high mp for efficient loiter speeds. Seemed like it was 1200/25. Maybe someone can dig up more info.
 
As a kid playing with 2 stroke motorcycles, we used to design our power band around the "mission". Drad racing, road racing or dirt bikes all used a different band peak and width. We would use port sizing, port timing, carb. venturi velocity and then custom build a set of exhaust chambers for our desired power band. We could build an engine that would be a dog to 6,000 rpm and then fall off the planet at 8,500 rpm.
My point is that I would assume that the Lycoming is cam'ed, carbed, timed and exhausted for our standard rpm's. Anything more or less is probably less efficient. Easy to see from the power and torque curves.
It will be interesting to see how these Lycomings do at the lower rpm's.
 
MarkW said:
Anything more or less is probably less efficient. Easy to see from the power and torque curves.
It will be interesting to see how these Lycomings do at the lower rpm's.
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That is likely true in a test cell or a given prop. But with newer prop designs such as the BA Hartzell, the efficiency of the prop at a lower RPM could potentially offset the less-than-peak point on the power curves.

What I'm really after is what is the best power setting for max speed to fuel consumption ratio. In other words the most fuel efficient economy power setting.

I have in the past run the engine around 1400 RPM flying with some friends with Supercubs where I was burning 3.6 GPH at their speeds.
 
Low RPM - High MP Cautions

Some reference documents.

Most deal with problems in Continentals but also valid for any pendulum tuned "counterweight" crankshaft. This includes all 6 cylinder Lycoming and Continental engines, and some 4 cylinder 360 Lycomings.

Cape Air/Hyannis Air Services Inc.'s engines slinging pendulum absorbers through the crankcase
--------------
Continental Service Bulletin SS107-5,
-----------------
Or this from Continental:

SUBJECT: MINIMUM CRUISE RPM LIMITS
PURPOSE: To inform operators of the possible long term effects of low engine RPM in cruise conditions. To establish limitation of minimum engine RPM in cruise.
-------------------
High MP low RPM Continental TSIO520 engine (or Lycoming engines)

Lycoming or Continental Counterweight Detuning
 
You might consider becoming familiar with the concept of "Carson's Speed", if you aren't already.

http://www.eaa1000.av.org/technicl/perfspds/perfspds.htm

http://www.flyingmag.com/very-best-speed-fly

CARSON'S SPEED - The term Carson?s speed refers to the paper ?Fuel Efficiency of Small Aircraft? (AIAA-80-1847, 1980) by Professor Bud Carson of the U.S. Naval Academy, which, using prior work by Gabrielli and von Karman, defines this speed as the maximum speed per unit of fuel burned. Carson?s speed can be calculated as 1.316 times the speed for maximum lift to drag ratio, which, in turn, is 1.316 times the speed for minimum power and minimum sink rate. Carson?s speed is also defined as the tangent point on a line that is tangent to the drag polar and passes through the origin.


Hope this helps,


Lee...
 
Hmmm. Sparks a flyin'

I was pondering recently about the "ignition timing effect" of low rpm operation.

If one assumes the flame front speed to be constant, then lowering rpm might have similar effect of advancing the ignition simply because there is more time between spark and TDC for the cylinder pressure to rise.

This thought was not in the vein of "danger Will Robinson" as much as a possible method for Mag drivers like myself to manually obtain some ignition advance that is available to the Electronic ignition owners.

So... I am curious what kind of advance values are purported by the common EI systems in use today?

A change from 2700 down to 2000 could be similar to an advance of roughly 8 degrees from a burn time perspective.
 
Bill is onto it.

The peak pressure pulse timing needs to be around 16-18 Degrees after TDC so by slowing down the rpm you move that closer to TDC and become less efficient.

So the happy medium from the sweet spot of prop and engine is probably around the 2100-2300 RPM mark.

If you want to gain the best MPG, run around 2100-2300 or even 2400, wherever the engine is smoothest, and run LOP. Say 10F LOP at cruise altitudes.

Remember when LOP the MP and RPM don't matter, it is all about fuel flow that determines power, so WOT and a nice happy RPM, then fuel flow does the rest.

If you want really get best range, it probably means slowing right down to 110 knots or something....but where is the fun in that!:D

A couple of days in George Braly's engine room would help the understanding no end ;)

PS: There is no such thing as "over square" by any real definition. The nickname came from some comparison or two independent parameters that became a rule of thumb. I really hate that expression!!
 
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It's true that Pmax needs to be about 12°-15° for maximum brake torque (MBT) so on a fixed ignition system engine, there will be an engine speed where this will be so. Unfortunately, it will depend on air-fuel ratio, altitude (trapped residual exhaust gas) and load.

The burn rate (flame front speed and other stuff) does have an effect on efficiency as a slow relative burn rate 'rounds off' the peak in a p-V indicator diagram. Theory assumes infinite burn rate (which would be incredibly noisy!) but slowing the engine down brings reality closer to theory, since the combustion occurs over a smaller crank angle sector.

(p-V Diagram - theoretical)
PVdiagram2.jpg


(p-V Diagram - real life-ish - note rounded corners which reduce the area of the curve. Clockwise area represents work output. Anticlockwise area represents losses as in the gas-exchange loop at the bottom)
fig59_01.jpg


Two other things are happening that are engine speed dependant. First is engine friction - there is a n² function happening here, where going slower decreases friction. Secondly, heat transfer from the combustion chamber to the environment is a function of time, so in this case, speeding up reduces heat losses.

These three factors (and others) drive a minima somewhere between 0rpm and 2700rpm. The test engineer's job is to fly some measurements to find it! :)

So, in summary:

- Going slower increases effective burn rate (+)
- Going slower increases mechanical efficiency (+)
- Going slower increases thermal losses (-)

Also,

- Going leaner* lowers peak temperature and therefore thermal losses (+)
- Going leaner* increases ratio of specific heats (gamma) and increases thermal efficiency (+)
- Going leaner* slows burn rate therefore retards Pmax angle (-) go slower to compensate

Most importantly! Going slower with higher loads increases the risk of knock (a.k.a. detonation, pinking etc) Take care!

Also, If you get the chance to fiddle with the ignition angle, then LOP becomes meaningless and you need to target an air-fuel ratio as well as optimum ignition angle. Not necessarily more complicated - just a different and probably better optimum to find.

A

* leaner in the sense of LOP - i.e. real lean! Not leaner than full rich, which is starting on the other side of the hump, so it will get hotter for a bit before it gets cooler!
 
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Andy_RR said:
Going leaner* slows burn rate therefore retards Pmax angle (-) go slower to compensate
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Yeah..... Thats the trick isn't it? I have to think about what feedback mechanism the pilot is to use for setting RPM vs mixture (other than counting the parts flying out of the cowl :eek::D:D)

Thanks for sharing all the Thermo charts.
 
This is a nice thread. Every time I think of one of the variables, I refresh and someone comes up with elaboration. So to find the sweet spot, would you fly to find max mpg then change timing a bit and fly to find new max mpg?
 
aerhed said:
This is a nice thread. Every time I think of one of the variables, I refresh and someone comes up with elaboration. So to find the sweet spot, would you fly to find max mpg then change timing a bit and fly to find new max mpg?
Click to expand...

I think this has to be broken down two ways. First being the most efficient cruise speed of the airframe (Carson's number, thanks Lee) and once that is verified, try various power settings at that speed to see what the most efficient fuel consumption numbers can be attained. In other words, fly the airframe at the speed at which L/D is greatest (x1.32), then find the most efficient power setting at that speed.

Last night I realized that Jack Norris' book "Logic of Flight, The Thinking Man's Way To Fly" is nothing more than a rehash of Carson. As much as it pains me to read his prose I'm going to pick it up and read it.

One thing I'm doing in my Rocket is having the ability to control the timing advance, for two purposes: auto fuel and slow RPM PPP (peak pressure point) adjustment.
 
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re-invent the wheel?

Hey Bob:

Most of the big round stuff cruises at 1800/25-28", LOP. You can try the same if you can get the MP high enough, and likely come up with the same efficiency. Think forced induction for this path. The 2600s on the B25 have 7.2:1 compression with timing at 25 deg BTDC, and operate at such settings smoothly. Of course, the power pulses are quite a bit closer, making things easier on the prop. Then again, the blades have pendulum weights in 'em too...why would that be? Maybe because of the low RPM cruise pulse amplitude? Some research might be in order here...

We hit our 'critical altitude' on low blower at about 9500MSL in the 25 (the throttles are at the stops), and to keep the desired HP, we have to turn the RPMs up a bit to maintain the same IAS. Shifting to high blower only slows us down - about 10MPH IAS. We don't get much higher than 12500MSL ever, as someone in the back always complains about the cold. Geez.

Timing would only need advancing as MP drops off (RPM being the same), but a cheaper method to regain the HP is to spin the engine up a bit, which ALMOST negates the requirement for adjustable timing.

Suggestion: consider how much of your time will be spent at altitudes about 12500MSL (about the top of the efficiency curve for your wing) before you spend the energy on a timing module. Read the EAA CAFE report on adjustable timing too, regarding their researched payoff altitude. It might not be cost effective to have another point of failure in your ignition!

Please apply the KISS principle, and think about why no one else is doing what you are thinking of doing. Any shortcuts would have been found by now, dontcha think?

If the low RPM/high MP cruise is actually dangerous to the crank/prop assy, but you won't know it until far into the program, when the crank nose breaks or the prop hub comes apart, or you shed a blade. The damage would be cumulative up to that point, and nearly invisible to an eyeball inspection.

The there are the crank pendulum weights popping out of the crankcase issue...same cause....

Seems to me you'd better explain all this to any potential passengers, so they know the risks. Just sayin'!

So, fly at the Carson speed if you want max range, but do it at a slightly higher RPM, please. Specifics? I'm OK with 20 squared as a bottom number: the margins are wide enough for me to fly over water or mountains with a loved one in the back seat. Oversquare is OK with me too, but limit it to 5", tho my personal limit is 3".

Carry on!
Mark
 
First off I'm doing this in a RV with a non-counterweight O-360, so that eliminates any concerns about counterweights flying off.

Second, the CAFE report wasn't conducted with mogas and since the octane is lower, the burn is much faster hence PPP occurs earler. Retarding the timing is appropriate in that case. The vast majority don't experiment with this stuff because most people are content to just kick the tires and go.
 
HA!

I thought you were planning for the Rocket! DOH! Thanks for giving us ALL the information.

Best you talk to Les Dowd about non-CW vs CW cranks & their effects on the prop & crank. Their is a pulse amplitude limit on every prop, with the wood versions tolerating a much higher amplitude.

In ANY case, use a F/P wood prop so your crank & prop are up to the job. I would recommend against doing this with a metal prop of any type.

Carry on!
Mark
 
Tonight I went for a little jaunt around the area and did some very unscientific testing. Starting at the normal cruise settings I use 2300,25" I pulled the prop back to 2000 and back to 25" due to the expected rise in MP, and all seemed well. CHTs stayed the same, EGTs dropped considerably. When I pulled back to 1800 the EGTs dropped 200+ degrees. The CHTs never budged (analog steam gauges unfortunately.) Pulled the prop all the way out and it got down to 1600 RPM. Thats as far as it will go.

The EGT drop was not unexpected but I was surprised by the amount.

Everything felt fine, a little bit more of a slow shake at 1800. 2000 seemed to be about perfect.
 
F1Boss said:
Hey Bob:

Most of the big round stuff cruises at 1800/25-28", LOP.
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Seems to me the most of the big round stuff like the R-2800 have planitary gears and the tach is reading prop rpm if memory serves.Depending on application that engine was built with one of several gear ratios.
:)
 
The "big round engines" that I have flown all read engine RPM. (R-3350 in a Skyraider, R-2800 in a Corsair.) V-engines read engine RPM as well.
 
If you have a smartphone...

consider using a vibration meter ap for this testing. I haven't done any low rpm testing, but I clearly documented improvement after balancing my prop with an android ap called "vibrometer". I placed my phone on the glare shield propped up against the center support brace (slider). I'm sure there is a similar ap for the apple folks. High amplitude can't be good at any frequency we're talking about..
 
I had some time just jollying around the traps today, at 28+" MP and 2400/2300/2200 it did not make any significant difference, and I must qualify this.

All operations were LOP, so this takes MASS AIRFLOW being the dominant factor out of the equation.

Now I know there is subtle theoretical differences in drag and prop efficiency, but to be honest it was hard to tell.

2300 and 2400 were just fine, in fact I found 2400 was smoother although it may be just prop balance. even though we balance at 2300, but this was 100 hours ago and due for checking.

Find a sweet spot for smooth operation ;)
 
Some of us also have prop limits:

DO NOT OPERATE ABOVE 22 INCHES
MP BELOW 2350 RPM. O-360-A1A:
OPERATION ABOVE 2600 RPM IS
LIMITED TO TAKEOFF, IOF-360-A1A:
MAXIMUM ENGINE SPEED LIMITED TO
2650 RPM

When I'm just tooling around I find 18-20" works well with about 2300, anything less than that is just not as smooth.
 
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Bob, give yourself an edge. Contact Lycoming and ask for guidance on a knock sensor installation. You're an electronics guy so it shouldn't be hard to do. Vibratory and detonation risks are real; a knock sensor would give you a handle on one of them.

RV10inOz said:
I had some time just jollying around the traps today, at 28+" MP and 2400/2300/2200 it did not make any significant difference, and I must qualify this.
Click to expand...

Full throttle? That's aggressive. I assume you pulled to LOP before reducing RPM?
 
I'm thinking there is little chance of detonation problems using 100LL. I've run auto race engines with 10 to 1 CR and 60 inches of MAP and full ignition advance safely. Auto pump gas is a completely different story however... Be careful with that.
 
rv6ejguy said:
I'm thinking there is little chance of detonation problems using 100LL. I've run auto race engines with 10 to 1 CR and 60 inches of MAP and full ignition advance safely. Auto pump gas is a completely different story however... Be careful with that.
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My guess you were turning it way past the RPM that we operate at.
 
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DanH said:
Bob, give yourself an edge. Contact Lycoming and ask for guidance on a knock sensor installation. You're an electronics guy so it shouldn't be hard to do. Vibratory and detonation risks are real; a knock sensor would give you a handle on one of them.
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Dan, do you have any additional refs on this piece of gear. Sounds like the "det-o-meter" I spoke tounge in cheek about earlier...if there is such an animal, I'd like to learn more. Will call Lyc and Lycon, but interested in any amplifying or additional info or refs you have. Thanks.

DanH said:
Full throttle? That's aggressive. I assume you pulled to LOP before reducing RPM?
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I was also wondering about your technique for getting to LOP at altitudes where you could get to 28" MP. Not a poke...but on the other end of the efficiency spectrum, racers discuss how to safely lean to max power down low, and at high power, without overshooting into detonation. That's a bit of thread drift, but relates a bit, since we're talking about getting the most out of the engine/prop combo.

Bob's initial premise is a very interesting one though...been absorbing all the comments. Pooner, I've also been wondering why the need for such large-delta over squareness. Seems more efficient to go higher, where WOT will be a lower MP, and you can run at lower RPM, LOP, and enjoy low FF and higher TAS. Again, not a poke at all, and perhaps it's a way to find max efficiency down low to avoid headwinds...but just wondering what scenario or need generated the experiment (always trying to learn on this end too!! :)) if it's just to experiment and learn, all good...it's a very interesting discussion!

Cheers,
Bob
 
rvmills said:
Dan, do you have any additional refs on this piece of gear. Sounds like the "det-o-meter" I spoke tounge in cheek about earlier...if there is such an animal, I'd like to learn more. Will call Lyc and Lycon, but interested in any amplifying or additional info or refs you have.
Click to expand...

The circuit below was in my files, but I've not fooled with it. I think it was floating around a Soob or Suzuki list some time ago. I'll bet Rocket Bob can whiz up something better.

Lycoming could tell you the specific knock sensor they use, and where it is located on the engine.

2uh0s2x.jpg
 
Dan, I have some engineer friends that work for Delphi Electronics that over the years I've talked to about knock sensors, and have gotten a fair amount of input on why traditional piezoelectric sensors wouldn't work well on a Lycoming. I've looked at the Lycoming knock sensors on the IE2 system and came to the conclusion that they wouldn't last mounted on a 400 degree cylinder head. Several years ago I prototyped a simple ignition system that I had the intent do ion sensing where a current is applied immediately after the spark, and the voltage is measured in the ionized combustion chamber to determine the peak pressure point and detect detonation (knock). My friends advised me to go in this direction since one of the projects they were working on at the time was an ion sensing ignition for Harley Davidson. Fast forward a couple of years, this system is now standard on fuel injected Harleys and works very well; but from the software side of things is not easy to implement due to the complex algorithms involved to accomplish this task.

Some info here on how the system works: http://delphi.com/manufacturers/auto/powertrain/gas/ignsys/ionized/
 
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Wesael said:
My guess you were turning it way past the RPM that we operate at.
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Yep but the Lycoming chamber would have to be REALLY bad to knock naturally aspirated at altitude at such low BMEP by comparison. I can't believe you can make an 8.5 CR Lyc knock on 100LL unless you had really stupid amounts of ignition timing in it. We run some pretty stiff manifold pressures on the Lycs and Contis at Reno too on 100LL and they are not breaking from detonation even before the ADI kicks in.

Would be interesting to instrument one though...
 
Detenation margin loss

I've spoken with engine shops about how far RPM can be taken below MP, and the "rule of thumb" is no more than 2. So 23"/2100 RPM; 20"/1800 RPM, etc. The reason is a larger spread effectively advances the timing, which ya'll hit on earlier. Don't kid yourself, engines running 100LL have destroyed themselves in a matter of minutes from detenation.

Mike Busch has done some good webinars on FWF/engine ops/maintenence issues, including the issue of this thread. https://www.savvymx.com/index.php/resources/webinar

A long term healthy engine is the most important thing you can nurse on your airplane. "Exploring" engine limits brings to mind, "Danger danger Will Robinson".......
Personally, I choose to stand on the shoulders of those before me who have been there done that rather than risk my neck and investment over something they already learned the hard way. My 2 cents.....
 
If we can't have more than a two inch spread how is it that all the fixed Pitch engine combos survive? Every one goes way over squared on take and climb out unless you are flying out of a high altitude airport.
 
Under takeoff conditions you're operating full rich so that acts to provide cooling and retards flame front propagation. That's why lack of "excessive" rich during takeoff can reduce margin to detenation. Advanced timing "error" is doubtless the biggest initiator of detenation. I'm just say'n be "informed" before jumping.
 
I've spoken with engine shops about how far RPM can be taken below MP, and the "rule of thumb" is no more than 2. So 23"/2100 RPM; 20"/1800 RPM, etc. The reason is a larger spread effectively advances the timing, which ya'll hit on earlier. Don't kid yourself, engines running 100LL have destroyed themselves in a matter of minutes from detenation.
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I beg to differ slightly, Engines that have been destroyed in minutes are usually from Pre-Ignition, not detonation. You have to get a pretty serious dose of detonation to do the same trick. :eek:

Under takeoff conditions you're operating full rich so that acts to provide cooling and retards flame front propagation. That's why lack of "excessive" rich during takeoff can reduce margin to detenation. Advanced timing "error" is doubtless the biggest initiator of detenation. I'm just say'n be "informed" before jumping.
Click to expand...

Just to be accurate, Fuel does NOT cool. Fuel only ever adds heat, unless you flood it and the fire stops altogether, and you know that is not burning anything then.

Richer mixtures do slow down the flame front, effectively moving the PPP further from TDC to something later. This means less PPP and less disturbance of the boundary layer in the cylinder, and therefore less heat transfer to the head/cylinder.

Ross,
I'm thinking there is little chance of detonation problems using 100LL. I've run auto race engines with 10 to 1 CR and 60 inches of MAP and full ignition advance safely. Auto pump gas is a completely different story however... Be careful with that.
Click to expand...

You are a smart cookie, so people will read what you say with some level of assurance. I can assure you that you can have detonation with 100LL in a piston aero engine, the difference you have with a auto race car engine is it is water cooled. Go see George Braly run an engine in detonation on his test stand, his pressure sensor (1Mhz) shows it nicely.

So lets not confuse the two scenario's ;)

The myths about only a split of 2" over RPM is also just another OWT rule of Thumb. As already pointed out mayn't radials and forced induction engines run happily that way all the time. The concept of 29.5" and 1700 RPM might move the PPP too close to TDC, but it is probably not so efficient anyway. I do not have a definitive answer on this, but those guys in Ada seem to think the most usable range is probably around 2100-2700 and unless you are at low powers there is not much point in using lower RPM.

Dan
Full throttle? That's aggressive. I assume you pulled to LOP before reducing RPM?
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Aggressive??? No not at all. You also assumed wrong, to find the peak EGT it is done by setting the RPM and MP(which I did not touch) then do your leaning to find the reference peak. And in my case I do a BMP right back to the lean side then sneak up on the first to peak, then go back leaner again.

Here is what John Deakin had to say recently on an engine break in topic elsewhere when I posted this photo, http://i849.photobucket.com/albums/ab58/jaba430/photo2-3.jpg

David Brown wrote:
Sorry it is a bit blurry, but here is a photo of a really good way to break in cylinders and I am sure John, George and Walter would agree.

IO540 260HP, this is 1000' and WOT, 29.6"MP and 2430RPM and about 90% power (when ROP). High QNH day and I get some RAM AIR effect!

So we leaned to about 60-70F LOP and look at the result, 50 LPH, going fast and the CHT's are beautiful. Result was 79% power when LOP. Cop that flat earthers!


http://i849.photobucket.com/albums/ab58 ... oto2-3.jpg
(In case there's any doubt, that means approval from here.)

Best...
John Deakin
Advanced Pilot Seminars
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And as WALT has pointed out, in some combinations there are prop limits that must be observed.

www.advancedpilot.com is a really good place to go folks. And no I am not on any sort of commission. :)
 
RV10inOz said:
PS: There is no such thing as "over square" by any real definition. The nickname came from some comparison or two independent parameters that became a rule of thumb. I really hate that expression!!
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Someday I'm going to reprogram my EFIS to display MAP as kiloPascals and RPMs as radians/sec.

Revs/min is pretty bad, but inches of *mercury*? How are we still stuck with that? :)
 
Someday I'm going to reprogram my EFIS to display MAP as kiloPascals and RPMs as radians/sec.
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POST OF THE YEAR AWARD

How true. Flat earthers will never believe you, they will find another formula.

Just like the stupid combined CHT/EGT formula ...... and what is worse a major manufacturer published it years ago. When challenged to justify it with some science.....10 years of silence followed :rolleyes:
 
RV10inOz said:
Aggressive??? No not at all. You also assumed wrong, to find the peak EGT it is done by setting the RPM and MP(which I did not touch) then do your leaning to find the reference peak.
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If we set vibratory issues aside, I have no concern for 28" and 2200 RPM when well LOP. Assuming reasonable CHT it won't detonate there.

I am curious about how you get to your lean setting. If you set WOT, then pull RPM to 2200, then lean, you very likely pass through a pro-detonation region at little rich of peak. Here's a dyno example, 28.5 MAP, 2400 RPM, with a mixture pull from full rich to quite lean. It detonates 10%, maybe 12% at ~25 ROP, a little lean of best power.

dexxf7.jpg


Two caveats; (1) this example is an angle valve and you're running a parallel 540, and (2) CHT and intake temp are both quite high per the FAR test standard. Still, I would think there's some illustration here.

So....I assume your mixture pull is quite rapid (Deakin's "big pull") to get on the lean side with minimal time spent in the pro-detonation mixture range. In doing so you accept a little momentary detonation, which doesn't hurt anything. Then you fish for peak (for reference) working from the lean side. About right?
 
NA Detonation

RV10inOz said:
You are a smart cookie, so people will read what you say with some level of assurance. I can assure you that you can have detonation with 100LL in a piston aero engine, the difference you have with a auto race car engine is it is water cooled. Go see George Braly run an engine in detonation on his test stand, his pressure sensor (1Mhz) shows it nicely.

So lets not confuse the two scenario's ;)
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The engine that APS uses to demonstrate detonation on the GAMI test stand is (or used to be) a high power turbocharged lycoming (I forget the model though) operating on 91 octane fuel. APS teaches that detonation is virtually impossible on a typical normally aspirated aircraft engine operating on 100LL at reasonable temperatures (CHT < 400F) and with no internal problems. In fact, I quote from the APS class material:

"For conforming Normally Aspirated Engines, operating on conforming 100LL fuel, there are essentially zero incidents of true detonation."


With that said, I believe the OP referred to operating "over square" on mogas, so the risk of detonation might be slightly higher in this case.

Skylor
RV-8
APS Graduate
 
skylor said:
The engine that APS uses to demonstrate detonation on the GAMI test stand is (or used to be) a high power turbocharged lycoming (I forget the model though) operating on 91 octane fuel. APS teaches that detonation is virtually impossible on a typical normally aspirated aircraft engine operating on 100LL at reasonable temperatures (CHT < 400F) and with no internal problems. In fact, I quote from the APS class material:

"For conforming Normally Aspirated Engines, operating on conforming 100LL fuel, there are essentially zero incidents of true detonation."


With that said, I believe the OP referred to operating "over square" on mogas, so the risk of detonation might be slightly higher in this case.

Skylor
RV-8
APS Graduate
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Yes, it simply does not make sense that we can run double the cylinder pressure on Reno race engines as a run of the mill atmo Lycoming at altitude without detonation problems. I'll stand by my statement and agree with the statement above. 91 octane is a whole different story.

RV10 in OZ, I'll dispute your statement about fuel not cooling either. While most engines on the dyno make best power between 12.5 to 12.8 AFR, going richer ALWAYS drops EGTs and CHTs as well as power. Detonation margins are also increased by going richer however getting much richer than 11.5 will often cause rich misfire so we really can't operate there. Where there is not enough oxygen for complete combustion of the fuel to occur, you have additional cooling same with an excess of air- more cooling LOP. MAP limits for radials using auto lean and auto rich settings are usually fairly different to maintain detonation margins- same reason.
 
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Dan, you are correct, I do a BMP and sneak up from the lean side. You can spend a few seconds in a mild detonation scenario, and it cleans the deposits out vey nicely. I did a high power LOP run the other day, and when boroscoping after 650 hours it looked like a new engine. Very clean.

Even if you lean slowly over say 30 seconds, the amount of time you spend in any detonation margin zone is very small, and you would need to have high CHT's as well, which of course I don't. And nor should anyone else.

Skylor
Yes that is true, it is a chieftain engine, however when Ross was talking about engines before he was quoting 40"MP, just like a chieftain.

I can induce detonation in the IO540 at takeoff power. I have done it and John Deakin and I took a day or two to diagnose what was going on. So you must bare in mind, that this is all a bit variable. APS will say this too. Some engines operate closer to the mark than others. The 300HP IO540 is obviously closer than my 260HP. Remember the 180HP IO360 is just 2/3's of my engine ;)

Ross, you make factual statements about what you observe when you go richer or leaner, but you are missing the real reason why the temperatures change. The things you see on EGT and CHT and temperature at the valve were studied back in 1943 and the results of that NACA report were identical to when Lycoming did the same thing in th 60's. The truth is that extra fuel does not change the temperature by some cooling effect like poring water on a hot saucepan, that is immeasurable. It does it as you know by changing the flame front and the PPP and the theta of that peak pressure. Same on the lean side. The notion of extra air cooling is an OWT, the amount of air sucked into each event is pretty much the same cycle to cycle, so any cooling effect is the same, and a combustion event in the 3000+ range is unaffected by the small variation in real terms.

Time the OWT's were killed off.

Skylor will probably remember this stuff too.

Here is a little snippet I have copied courtesy of Walter Atkinson, while it is in response to similar questions there are a few relevant points.
3) EGT has no measurable effect on exhaust valve temperature. If one thinks it might, one must explain why as the EGT goes up from 25dF ROP to Peak EGT, that the exhaust valve temperature is going down. That little fact screws up that idea. In addition there are times when the exhaust gasses are actually cooling the exhaust valve. I'll not go into those details in this post.
4) The reason the CHTs are lower at higher FFs does not have anything measurable to do with the lower EGTs at properly adjusted max FFs.
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Ross, if you wish to stand by your statement, can you show some data to back it up please. I am happy to be wrong, and learn something from it. I will be seeing George and co in a week or so, and rather than take my word for it, I will ask him to explain the differences and report back.

Cheers!
 
I have asked this question before but so far have never been able to get a direct answer or find someone running my setup LOP. I have a IO360 with dual lightspeeds and the ECI fuel injection system. I have 10 to 1 compression pistons. Do you have any data on LOP operations with high compression pistons? The engine seems to run fine LOP. I have been running very low power settings at 3500 hundred feet and LOP. I see about 6.6 GPH with about 19 inches of MAP. This makes for a very economical just cruising around operation. I see about 145 knots true in the above situation.
Normal cruise at 8500 feet is 10.8 GPH when 75 ROP at 182 knots true with full throttle. I can go about 50 LOP and ff drops to 7.8 GPH and I am still getting 165 knots true. I would love to make that my normal cruise setting for cross countries but the high compression pistons keep worrying me and other then short term experimentation I have not done that on a long flight.
What are your thoughts?

George
 
My thoughts are...... an APS course would help you understand all this stuff a lot better. Then you can make your own informed decisions.

There is no single post that can tell you all you need to know.

There is no single magic bullet.

Every time someone asks a single point question...Where should I run #####

The answer is ....It depends. #####

Your higher compression pistons are say a little bit like a Turbo engine, its just jamming a bit more into the cylinder at a higher pressure. Yes I know its not the same but bare with me.

The rules of combustion don't care, so the same rules apply.

At 8500' and at a 50LOP you are very conservative, in fact squeeze a bit more fuel in for more speed, maybe 10-20LOP.
 
RV10inOz said:
EGT has no measurable effect on exhaust valve temperature. If one thinks it might, one must explain why as the EGT goes up from 25dF ROP to Peak EGT, that the exhaust valve temperature is going down. That little fact screws up that idea.
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Would you care to show us the figure Walter refers to above? If we could see it perhaps we might explain it.
 
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