|Jul 27, 2012, 01:07 PM|
I am still around, I just didn't have anything much to talk about yet.
I did get in the Cline regulator, so I plan on doing some testing this weekend.
I want to try a Fox carb on the engine, as well as a cline regulator out and a perry pump maybe too. Different combinations as well to see if anything interesting pops up or not.
|Jul 27, 2012, 01:50 PM|
No problem. Using the engine on my plane...the engines seem to work better on the test stands than in the planes.
First I want to run it to make sure it is working OK. I'll try Michael Chow's more current carb adjustment instructions to see if that does anything different too.
Second, I want to try the Cline regulator out to see what it does using the OEM carb.
Third, is to try the OEM carb and Cline regulator with a Perry pump out and see what happens.
Fourth, I want to bolt on the Fox Butterfly carb, no pump and no regulator and see what it does. I have the old style butterfly carb and the new current carb as well as a MK-X carb so if time permits I hope to see what they do too.
Fifth I want to try the Cline regulator and or the Perry pump and see what they do with the Fox carb.
How far I get depends on the batteries in the plane, if the batteries run down, I have to stop and recharge them before I can continue.
|Jul 28, 2012, 01:34 AM|
EarlWB's Con-Rod Failure - Part 3
This is a reply to Greg's questions in Post 651 But, please feel free to read my reply. This is sort of part 3 to my posts about earlWB's con-rod failure, so it is a mix of Greg's questions and more technical detail. This time the focus is on gas pressures, air/fuel and exhaust because these effect how much power a 2 stroke engine makes and Hp/Torque effects the parts. I have also included links or references to material. I had to load them on my website because there is a 3mb file limit on this forum. If you read and understand all the material I present here then you know a lot more than me and I can't help you. My hope here is to enlighten people and hope they come away with more ideas, and not to degrade them. I also want to applogise to everyone, I said I wouldn't write a book but, if you add up my 3 posts I think they would be a short book. Please feel free to download any files from my site for further reading. I also have links at the bottom of this post. Tunining Files Part 3
I know that Greg knows 2 strokes so ..........
This is from an engine and not a compressor .
Practical root cause analysis of connecting rod bushing failures in a new reciprocating compressor and the theory behind the failure mechanism
Greg I have more on your above questions. I’ll try my best at keeping track of my info, answers, and resources. I’ll also provide attachments or links to more reading material. I’ll also try and keep my answers short, wish me luck.
Time to get Technical:
As I mentioned at the start “at no time is there an exact moment that this or that happens or it’s exactly this or that amount”. 2 strokes are unique because compared to atmospheric pressure you have pressure and vacuum above and below the piston. 4 strokes are only supposed to have these conditions in the combustion chamber, above the piston. These pressures are what Greg was referring to in question 3 above ‘load reversal’. Many believe that in 2 strokes there is always a pressure above the piston so the load on the con-rod never reverses, IMHO that is old school thinking. To properly explain a 2 stroke I would have to write a book and a forum is no place for a book, the length of my posts is bad enough. Well I’m going to provide answers below with explanations to Greg’s above questions and dive deeper into the technical aspects of 2 stroke engines. I hope Greg doesn’t mind but, I think those that want to learn would like a good explanation and read. Greg knows about the term primary compression, so he must know something more than just a little bit about 2 strokes.
Reply to 1 and 2. Above: “Are you implying the gas pressure in the crankcase is greater than the inertial force?” “And so much greater that the greatest pressure exceeds the combustion gas pressure on the power stroke?”
Once again yes and no. Here’s the situation. On the combustion side we go from deflagration to detonation (we’re not talking about pre-ignition), with detonation occurring hopefully somewhere between 0 degrees and about 10 degrees. At this point firing pressure is at max, combustion chamber volume is a little more than minimum, and the crank is being accelerated by the piston/con-rod. On the bottom side of the piston in the crankcase we have primary compression which is raising the crankcase pressure and this is resisting the acceleration of the piston and con-rod, and the intake process is ending.
Primary compression occurs in the crank case and is caused by the piston coming down and compressing the intake charge. The ratio can be somewhere in the 1:1 ratio and I’ve read as much as 8:1 but. 1.5:1 is more common. The formula is:
Primary compression ratio = Case Volume @ TDC / Case Volume at BDC
Because air/gases are elastic and when moving they have momentum there is the possibility to ‘supercharge’ the intake charge depending on the port and valve timing, rpm, and intake tuning. Ever see an engine that is spitting fuel out the carb at idle? Well that shows you that there is a lot of intake timing, with higher rpm and momentum of the air the spitting will go away. My O.S. MAX-H 60FRG has a rotary valve timing of ‘opens at 45 degrees ABDC and closes at 50 degrees ATDC, so even though the piston is coming down and is starting to compress the intake charge, the fuel/oil mix is still trying to get in the crankcase at 50 degrees ATDC. The more the primary compression the better the intake charge transfer, crank case to combustion chamber or so we would think. It has been proven that at some point you get diminishing returns when increasing the primary compression. Note: see, page 95-97 of the “Two-stroke Tuner's Handbook - Gordon Jennings.pdf”, “The Effect of Crankcase Volume and the Inlet System - crankcase_volume6.pdf” by Fujio Nagao, “A Study of the Delivery Ratio Characteristics - delivery_ratio6.pdf” by Kazunari Komotori and Eiichi Watanabe
Ok back to my reply. During all this time the crankpin has been trying to force the con-rod to the outside, more to the right according to my drawings, and the con-rods momentum is resisting that force. We still have the rod driving the crank down, the crank is trying to turn the prop, and the prop doesn’t want to turn. The resistance to all these forces is robbing the engine of power and is putting more pressure on the oil film. As the crankpin, piston, and rod continue downward they reach maximum velocity @ 90 degrees. But because we have already had detonation and the combustion chamber volume is still increasing, the combustion chamber pressure is dropping. As we go past 90 degrees the crankpin is now trying to pull the con-rod in (more to the left), and the piston/rod is starting to decelerate (slow down). Also at this time the crankcase pressure is increasing, making it harder for the piston to push down on the con-rod. Start of the red zone. As we hit 112 degrees ATDC on a O.S. MAX-H 60GP the exhaust ports are being uncovered, combustion chamber pressure goes down big time. Reply to 2: This is the end of the power portion of the “power stroke” even though the piston and rod are still going down. Also the primary compression is apporaching maximum pressure in the crank case and on the bottom of the piston. We still have 68 degrees of crankshaft rotation until we get to 180 degrees. So somewhere between now (112 degrees) and the opening of the transfer port (124.5 degrees) the crank case pressure is going to be higher than the combustion chamber. Why?
One of the biggest jumps in 2 stoke technology has to be accredited to Walter Kaaden wiki Walter_Kaaden, he is considered the inventor of the tuned pipe. Walter Kaaden was an engineer who figured out he could use resonance waves in the exhaust to help boost performance in 1953 on his 125cc racing motorcycle. His work is based on the Kadenacy Effect see: http://en.wikipedia.org/wiki/Kadenacy_effect, , page 99 “The Basic Design of Two Stroke Engines.pdf”, and for very in-depth detail “Kadenacy Effect.pdf”. Michel Kadenacy discovered the basics for tuned exhaust pipes, actually exhaust expansion chambers for scavenging. Kadenacy discovered that a pressure wave is created as soon as the exhaust port is starting to open, and that behind that wave there is a drop in pressure so strong that not only could it create a partial drop but, even a pressure drop that is below atmospheric pressure. See “extractor effects” page 96 “Two-stroke Tuner's Handbook - Gordon Jennings.pdf”. The Kadenacy Effect is based on Bernoulli's principle, in particular the Bernoulli equation for compressible fluids (Gases). The Bernoulli's principle “http://en.wikipedia.org/wiki/Bernoulli's_principle” is great for arguing about how lift is created by an airfoil. Most of the glow and gas 2 strokes made today use porting and mufflers that make use of the above effects. For example my old O.S. MAX-H 60FRG is rated at 1.2hp with cross-flow porting, the O.S. MAX 60FSR with Schneurle porting makes 1.8hp, and my new Aviastar .46 is rated at 1.66hp, all have stock exhaust. Pretty good Hp for a .46 and what a difference from my old .60FRG. That little 46 must make use of the above effects to get that much Hp
Add in reply to 3 here:
So am I “implying the gas pressure in the crankcase is greater than the inertial force”, yes I am but, I’m thinking about the piston and rod. Also it is only for a very brief amount of time, It’s very brief because on the old O.S. MAX-H .60FGP the transfer port is being uncovered at 124 degrees ATDC. There is only 12 degrees between the exhaust port opening and the transfer port opening. Because the negative pressure wave created a low pressure area in the cylinder, the Kadenacy Effect. At this point crankcase pressure is greater than the combustion chamber pressure and this is where the load reversal happens, for 12 degrees. Instead of the piston trying to push the connecting rod down we have negative pressure on top of the piston and positive pressure on the bottom of the piston, so the piston either stalls or is trying to go back up with the con-rod up. Don’t forget that the con-rod is slowing down as it comes to the bottom of its stroke and we also still have the prop slowing down the crank. So IMHO and in some of the documentation the crankcase pressure is greater than the piston/con-rod verticle inertia but, is less than the crank inertia. So I think the con-rod stalls for just a moment because of the crankpin to bushing clearance and then the crank inertia takes over and continues to pull the rod. But this has been debated quite a bit because; what is load reversal? Does it occur anytime the load is removed or reversed for any length of time, or does it have to be something like an intake stroke on a 4 stroke. This little whip in load is why I think most 2 stroke rods I’ve seen break near the bottom of the ‘I’ beam.
All the load reversal I could find on 2 strokes was to do with diesel engines that had either a blower, blowers, or a combination of a blower with a turbocharger/s. In this case there would always be pressure in the combustion chamber and no load reversal would occur. But we are talking about small naturally aspirated 2 stroke engines here, so one has to be careful what one is reading. I know. I did it myself a few times. I’ve had this argument with mechanics before and they always forget or don’t know about the Kadenacy Effect and so they think all 2 stokes work the same. You have to ask yourself when was the last time you saw a diesel with a set of tuned pipes hang off of it, or a naturally aspirated 2 stroke diesel.
At just past 180 degrees the con-rod is no longer trying to go down but, the crank wants it to go left. Also primary compression has ended but, we still have a bit of crankcase pressure which will help the crank move the con-rod up from about 190 to 200 degrees. From about 200 to 210 degrees is when the crankpin will quickly take up the clearance ahead of it and hit the bushing hard. At about 210 degrees to 220 degrees is when the crankpin will be putting max pressure on the oil film and bushing, max red zone. On some high revving motorcycle engines it was found that sometimes the needle bearings were skidding instead of spinning at this point. At about 220 degrees is when the crankpin will be sliding past the second oil hole pulling in oil, see note below. It is about this time that if a tuned pipe or Mac’s muffler is being used the positive pressure wave is returning to the exhaust port and putting some of that air/fuel charge back into the cylinder. Actually most if not all glow/gas engines use a muffler that provides for some Kadenacy Effect, especially those big bulky O.S. mufflers.
Bushings use a form of lubrication known as Hydrodynamic lubrication:
Hydrodynamic Journal Bearing - BEST DESCRIPTION
If you can understand hydrodynamic lubrication then you’ll pretty much understand why EarlWB’s rob broke, just add in lean burn pre-ignition to complete the picture.
Let’s finish this up. So we got max pressure on the oil film, we are pulling in a new air/fuel charge, the combustion chamber is pretty much filled (still a little more to go), the crankpin is trying to accelerate the con-rod up and push it outward, and none of the gas pressure are helping. We have the bushing maxed out with pressure and with the least amount of oil. See figure 13.7, page 333 of “Journal Bearing Design.pdf” for bearing load. On my old O.S. 60FRG the rotary valve is closed at 230 degrees and the exhaust port is closed at 248 degrees, now secondary compression starts. As I said before, this is all fine and dandy, just don’t run the engine lean.
And that's it. Earl was running a Mac's muffler and there is nothing wrong with that. But it does help to scavenge the cylinder, so when the engine went lean there was less time for Earl to react. I still think the problem with Earl's con-rod was what I previously posted. Crankcase to rod clearance is too much and the con-rod oiling hole/s are in the wrong place for this engine.
Refernces and downloadable files:
The first 2 books below, I consider as bibles when it comes to 2 stroke engines. I started reading them when I was 12 years old. I had doubled the speed on my go kart up to 70 mph and had my dirt bike doing wheelies when the power band hit. I hope this was a good read.
Two-Stroke TUNER'S HANDBOOK By Gordon Jennings
Two-stroke Tuner's Handbook - Gordon Jennings.pdf
The Basic Design of Two Stroke Engines – Gordon P. Blair – excellent explanations but more than the basics.
The Basic Design of Two Stroke Engines.pdf
Also a good book but, too much 4 stroke stuff.
Tuning for Speed (P E Irving 1965 - Tuning Racing Motorcycle Engines) Both 2 and 4 stroke, Book Page 212 negative pressure, page 233 oiling.
Tuning for Speed - P E Irving.pdf
Rod Loading (More than you ever wanted to know) Hot Rods Inc., General FAQs:
From their website, remember they are talking about the whole rod and not just the bearing.
“The highest stress on a connecting rod occurs at TDC (“just after”). However, on a two stroke engine the highest stress does not occur under full load but when the throttle is closed at high rpm. This removes the gas pressure acting on top of the piston which counteracts the high tensile stress on the rod. When the gas pressure is absent the rod undergoes a severe tensile stress which forces both the small and big end of the rod bores to oval. This always occurs on four stroke engines during overlap and at least explains one of the reasons four stroke rods are beefier. Obviously, the rod bores distort at any RPM or loading, however, the most severe distortion occurs at high rpm and light loading. The ovaling of the rod is why some tolerance is required. If there is not adequate tolerance the needle rollers will be pinched as the bore ovals, ultimately leading to bearing failure. As will be discussed later Hot Rods has developed a rod design that minimizes the rod bore distortion.
As a side note, the general rule for calculating the tolerance of the rod big end is to take the crank pin diameter and divide by 1000 and multiply by 1.5.
For example: For a 24 mm crank pin 24/1000 X 1.5 = 0.036 mm or 0.0014 inches.”
SubsTech they are an engineering technology firm and have excellent info on bushing/bearings and other stuff.
Hydrodynamic Journal Bearing BEST DESCRIPTION
Hydrodynamic lubrication theory Great formulas.
Gas flow through a 2 stroke.
Bernoulli's principle Also good for debating lift from an airfoil.
Kadenacy Effect - Exhaust Gas- Wiki Tuned pipes and exhaust scavenging.
Walter Kaaden Inventor tuned pipes
SURGE PHENOMENA IN ENGINE SCAVENGING – Helmuth W. Engelman
Expansion chamber - Wiki
Bridgestone Motorcycles Documents Some excellent material further down the page.
Have a nice weekend guys, I have to get back to work.
|Jul 28, 2012, 01:58 AM|
Joined Apr 2008
I was reading your references, before the tech article, and I have been meaning to mention the "Uptune
Effect" that GP Blair refers to, only on page 246 is REAL!!!
And it is an incredible boost of power when it occurs. I thought this was something you could use with the
Drag Sleds. Back when the IJSBA was kicking butt in the early 90's, the Uptune Effect was the difference
between finishing in the top 4 vs the top 10 in the Pro Classes.
But, chapter 2 is is the best chapter in the book.
Now I will read your article.
(Greg) “Are you implying the gas pressure in the crankcase is greater than the inertial force?” “And so much greater that the greatest pressure exceeds the combustion gas pressure on the power stroke?”
2 Stroke engine == 6 cycle engine
My opinion (respectfully): Inertia and out of balance forces are always greater than the gas pressure on the power stroke.
I don't believe an engine rotates 360' at a constant speed, and I am skeptical that the piston, rings, pin, bearing, and 1/2 the
rod can act like an effective parachute (especially on the NGH engine, and it's exhaust port design).
But I must admit, I have no idea how much friction is being applied to the assembly, or the amount of drag? the propeller is
producing at any given time. (other forces working against normal rotation, not including gas pressure)
I believe your example might only occur on a well tuned engine with a properly phased pipe, using a 30% to
35% long head-pipe with only a 2' to 3' taper. And more likely on an engine simulator.
But even that might only occur within a narrow rpm range at peak HP.
The Hot Rods section is just speaking about rod stretch and the need for a minimal squish clearance. I think the bearing
clearance reference applies to roller bearings.
Model airplane engines should have a clearance of .0015" to .002". (D Gallbreath, NFFS Symposium #42)
(Greg) "Car and heli engines are used differently than aircraft engines, and so the lubrication needs are different."
Car and Heli engines use gear reductions, and have greater loads at higher rpm. Airplanes are direct drive, with greater loads at
lower rpms. But the crankcase of Car engines must be similar to a "swamp" as rpm goes significantly beyond peak HP.
|Jul 28, 2012, 09:35 AM|
In the case of the NGH , I dont think it is a matter of hole placement or hydrodynamic failure . The academics are nice but many engines have survived worse hole locations for years . I had commented about materials earler and how some of the manufacturers tend to use whatever is available and that situation changes from day to day . I have not taken Earl's engine apart but a very reliable, un-named source has told me that he took his new NGH apart and discovered the crank pin is dead soft . This is cause for concern as it could indicate a poor material choice and/or skipping the hardening step . This is why I had inquired about dimensions for these parts , so we could determine wear or poor fitting at the factory in the case of a new engine .
With the help of this thread the whole picture may yet come to view.
|Jul 28, 2012, 02:40 PM|
The bearing load diagram is the standard diagram used for explaining hydrodynamic pressure in journal bearings. Things to note. The force is on the shaft pushing down. This is the case for the main bearing in in the engine, but the opposite for the big end of the rod. It is extremely important to be sure of the force vectors to say where the pressure is in a bearing. Relevant Google search big end polar load diagram
1. I think you are not looking at all of the forces involved correctly. This leads to a misunderstanding of what's happening at the bottom of the stroke. Debate is fine, but there are people who figure this stuff for a living and publish papers which really leave little for debate. If we agree that big end bearing load is greatest at full power (WOT) we need to investigate the forces acting at WOT and high RPM i.e., no significant reversal of load on the rod. At part throttle with a prop the reversal loads are present, but of such magnitude to cause failures? The downward inertia force of the piston alone for the dimensions of the NGH9 from 120°-240° is 20+kg and maximum upward force due to crankcase pressure ~7kg at ~14k. Even though inertia forces on the piston are 50% greater at TDC than BDC combustion pressure peaks at 150kg force pushing the piston down.
2. Combustion pressure in an engine this size is ~35 bar and crankcase pressure maybe 1.5 bar. The period between exhaust open and BDC does have less pressure above the piston than below, but like you said the piston is slowing, so unloading only happens at part throttle and high RPM. The piston is always pushing the rod down toward the crankpin. The journal diagram is inverted from typical. Somewhere before 90° the piston inertia is already pushing down against the crankpin and this doesn't stop until somewhere after 270°.
3. Related to above. car and heli engines spend a good portion of time operating at high RPM where the inertial loads are much greater (reversing the piston motion). Doubling RPM quadruples inertial load. Car engines often run very low oil ratios 10-12%. A dead giveaway is the amount of material surrounding the rod end to keep it together. Their RPM is high so the shear on the oil is much greater. It's easier to maintain the oil wedge when surface speed is high. Those engines just need to get oil into the bearing. So in an engine which actually has load reversal on the rod, getting oil to the top of the bearing is important. I'll agree that there is even significant load reversal at full throttle and high speed ~30k, but we are discussing a sport engine at 12k. Model engines are hardly optimized. A typical FS-26 runs 12k maximum in aircraft configuration, in the car versions guys ran them to 26k without rod failures. The original aircraft rod is much larger than necessary and the same rod was used in the car version. The same rod fits my MVVS .15 two stroke and the cross section of the original rod is virtually the same, it was also rated to 26k, but lacks bronze bearings.
The study you cited, though for a compressor, still proves my point. Where was the bearing wear? At the top of the rod big end, not at your indicated maximum pressure point at the bottom. Note the the drawings are rotated 90° clockwise. IMO whoever designed the first bearing blew it. There was not enough surface area to carry the load. Adding a hole at the top in a model engine is a similar situation, unless the load is small enough that the resulting surface area is still enough to carry the load. In the study it wasn't lack of oil as it was pressure fed, but that the bearing surface area was insufficient to carry the load due to all the supply grooves.
The magnitude of the load is important. Even though there seems to be a load reversal in a model engine what is the magnitude of this force at the crankpin? Does it really exceed combustion forces at the top of the stroke to be so critical to the design of the rod and bearing? The compressor paper didn't even mention reversal as a problem, the wear occurred at TDC at maximum pressure. The same study with more detail. http://www.recip.org/fileadmin/user_...ct2-6-2010.pdf Pay particular attention to the magnitude of the oil pressure and the direction of the loads. Keep in mind the compressor has only an intake stroke and compression stroke like that of a four stroke.
Kadency effect is an old term used to describe what's happening. Blair covers this quite well.
I also disagree with the rod end to wall clearance supplying the oil to the big end oil holes. It's a great idea if it works. Which engines actually have a very small clearance here? Some high performance engines have very normal or odd setups and aren't getting oil into the rod by scraping it off the crankcase. Most engines have a very significant clearance compared to the expected oil film thickness on the crankcase wall. Rod failures are uncommon today. I'd guess the NGH follows the lineage of borrowing proven OS designs, so why should it be so critical of an oil hole?
Bill, don't get me wrong, the oil ratio is important, but if the manufacturer has risked their existence on 20:1, I would expect they tested it fully and were confident. Do I think it's light on oil? Yes, but Earl's rods failed with much more oil than recommended while other engines seem to do fine. The fact that angular velocity varies during the cycle is well known, due to both the rod angle and crank speed.
I know a bit about engines. I'm not formally trained. I've found these books vital to understanding engines. This is what I keep on my desk. They used to be by the bed until I got married and my wife ousted them.
Be careful about hosting those SAE publications.
|Jul 28, 2012, 04:47 PM|
I'm hoping that with all the material supplied and us putting our minds together maybe we can solve Earl's rod problem. It would not only benefit him but, those you have and maybe thinking about buying the NGH 9GT. Gary has pointed out in some of his posts there might be a problem with the crankpin and inconsistent clearances.
|Jul 28, 2012, 05:07 PM|
P.S. Gary I want to see you put a blower on that Walbro.
|Jul 28, 2012, 05:47 PM|
The dead soft crank pin discovery was not my own but from a very reliable source indeed . I had always suspected materials here more than any particular operator or even mechanical design insufficiency .
|Jul 28, 2012, 08:28 PM|
|Jul 28, 2012, 09:43 PM|
The rest is well taken. I'll see how things go when I enroll at HKU.
|Jul 28, 2012, 11:56 PM|
Joined Apr 2008
I was taught that the 6 cycles of a 2 stroke engine occurred in some sort of linear pattern. That when
fuel-air mixture was pulled/pushed into the crankcase, then it was pushed into the cylinder, compression
began when the exhaust port closed, ignition, power and so on. After the transfer process I thought the
crankcase was nearly empty until the next amount of fuel-air mixture entered the crankcase.
Because of GP Blair, I learned that I could build a pipe that was phased to deliver a surge of negative
pressure close to BDC, and that would pull more fuel-air mixture from the carburetor, through the reeds
or before the rotary valve closed, into the empty crankcase, through the transfer ports while the piston
is moving upwards, to supply my cylinder with more fuel-air mixture, and of course, produce more HP.
This was still a very linear perspective.
At the same time, I learned what Blair was good at, and that there were other subjects that he wasn't
so good at.
It wasn't until I started working on Zenoah's that I realized just how HUGE a 2-stroke crankcase is, and
that it was impossible to empty the contents of the crankcase every revolution. That an unturned engine
like the NGH is lucky to pass half the crankcase contents to the cylinder, and that air-fuel mixture is lucky
if it is actually compressed and ignited.
Now I think about the fuel air mixture in the crankcase, is similar to the song "99 bottles of beer on the wall."
Except that 99 is replaced with infinite.
"infinite bottles of beer on the wall, infinite bottles of beer.
Take one down and pass it around, infinite bottles of beer on the wall."
The crankcase is actually 2 bottles of beer, 1 beer is passed to the cylinder, and one beer is left to cool, and
lubricate the crankcase and cylinder. Then the carb shares another beer with the crankcase, and the process
I refereed to the "swamp" in an rc car engine after peak hp. If you could imagine that there are 3 to 4 bottles
of beer in the crankcase, and only one beer moves to the cylinder, with the remaining beers cooling/lubricating
the bottom end.
It took me a long time to get past what I was taught, and now it's just pressure differences and beer. It's nice to know
that there is always a bottle of beer to cool/lubricate the bottom end.
That makes sense.
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