Torsional Dampening

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Billrsv4

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Thanks for sticking with this discussion. I, for one, really appreciate you letting us 'take advantage' of your expertise. :)

Did he tell you what (if anything) he was using for dampers when he was breaking the gearsets? What the low freq tuning was? Or whether, as Ross mentioned, he might have been running metal props at the time? I'm no genius (but not completely stupid either) and have no training/expertise in this field. But when approaching the data logically, we've got:
1. an inherently low torque engine
2. peak torque that has to be less than 1/2 that of an equivalent HP 4 cyl 4 stroke piston engine, since rpm is over twice as high and the net torque never goes negative like it does with a 4 cyl engine
3. 2 per rev power impulses, like a 4 cyl 4 stroke (but with no torque reversals; see #2)
3. a gearset designed for 600 lbs/ft average torque from a diesel V-8; I assume that it isn't a 2 stroke, so 4 per rev power impulses that will be significantly higher than 600 lbs/ft

So, please help me wrap my mind around the processes that could result in resonance when driving a prop. Everett obviously had problems, but where? Metal prop? Grossly misdesigned dampers? misalignment in assembly? Allowing the system to operate 'unloaded' for extended periods? Too low an idle, causing gear slap?

Charlie
Charlie,
All the failures were running an un-damped system. Once the pendrulous damper was designed the system ran perfectly. There are some torque reversals BTW they are of course worst at idle. The peak torque has little to do with the worst aspect of torsional vibration. Somebody linked a great video on the main forum, I believe it follows Billski's original TV post. (not in auto engines) It shows a super simple oscillating spring and actuator system. There are 2 springs and a weight, plus the actuator which can be tuned, (changing the rate of pulses), to input from 0 to about 60Hz. The movement of the actuator is about 3/8". he starts the system up and the weight moves about 3/8". When he tunes the system to get amplification it goes crazy and the weight starts bottoming the springs in both directions. The amplitude of the actuator never changes, the movement is the same. The force is minimal. The only change is frequency. That is what we are talking about here. It isn't the AMOUNT of torque, it is the frequency of the pulses that causes the problem. If you hit a frequency that isn't dampened or cancelled through WHATEVER means you use you will break parts.

T.O. Bill
 

rv7charlie

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Also if anyone has any more to tell about George Graham's experiences with the Mazda and the tranmission locked in second gear for a reduction drive please share. I thought had to be the cheapest setup one could have for a homebuilt.
I met & talked with George a few times at Tracy's rotary flyins while he was still healthy & flying. The story he told was that he never had a problem with the trans, until one day the engine started on only one rotor. He wasn't specific about why it happened, but 'stuff happens'. Running on one rotor halves the impulse frequency, and allows huge torque reversals that don't happen when both rotors are firing properly. It quickly stripped the transmission teeth before he got it shut down. )More evidence of how destructive resonance can be.)

After that incident, he installed Tracy's drive and as far as I know, operated successfully until he could no longer fly due to health reasons. He even talked about flying the plane on a long cross country once without adding the oil premix (required for apex seal lube/cooling) to the gas (testimony to how tough the engine can be).

He was a get-r-done kind of guy, with a thousand hilarious stories. We miss him.

Charlie

PS: The trans trick has been done on other engines; there was (is?) a 2 seat 'ultralite' type a/c here in Mississippi a number of years ago running a Honda CBR600 & using the trans for reduction, and a 1:1 motorcycle chain up to a prop shaft in industrial pillow block bearings. He even had a shifter so he could change gears for solo vs carrying a passenger. No kidding. Wish I'd gotten his name; I'd like to know if it's still flying.
 

Autodidact

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...help me wrap my mind around the processes that could result in resonance when driving a prop. Everett obviously had problems, but where? Metal prop? Grossly misdesigned dampers? misalignment in assembly? Allowing the system to operate 'unloaded' for extended periods? Too low an idle, causing gear slap?

Charlie
To put it most simply, if the engine/PSRU system is not running, you can twist the crank one direction and the propeller in a direction that twists the PSRU's input shaft the other direction, and then let go of both ends and the system will oscillate (vibrate) back and forth at its natural frequency just like a tuning fork or a piano string. That natural frequency does not change, and if the power pulses of the engine are at the same frequency, then after the system winds up, lets go and winds the other direction and then rebounds to the original wind up, you have roughly 90% of the original torque left that caused the original wind up, and now at that point you have another power pulse and that force adds to the leftover 90% to give 190%, next time it's 280%, then 370%, and on and on until the machine breaks... That's resonance.

PS, the above numbers are exaggerated to illustrate the point.
 
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plncraze

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Thanks for sharing about George and his experiences. He did a lot with a little (money that is).
 

wsimpso1

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Did he tell you what (if anything) he was using for dampers when he was breaking the gearsets? What the low freq tuning was? Or whether, as Ross mentioned, he might have been running metal props at the time?
I was trying to get him to think about soft systems, but he was beyond that by then. I don't believe we ever talked details with soft systems.

1. an inherently low torque engine
2. peak torque that has to be less than 1/2 that of an equivalent HP 4 cyl 4 stroke piston engine, since rpm is over twice as high and the net torque never goes negative like it does with a 4 cyl engine
3. 2 per rev power impulses, like a 4 cyl 4 stroke (but with no torque reversals; see #2)
I do not know what you mean by inherently low torque. 200 hp at a prop speed of 2500 rpm is always 420 foot pounds, that is what the prop flange does on both.

A two rotor Wankel and a four-cylinder four-stroke piston both have 2/rev power pulses that last about 1/2 a revolution. The discussion of not having torque reversals is nonsense. I discussed it earlier. Both are making big positive torque for half a turn, then negative torque for half a turn, then repeating. They very definitely accelerate, then decelerate twice per turn. The torque traces and how smooth they are is entirely a function of: flywheel inertia; spring rate between engine and load, inertia of the load, and torque sensor response time constant. Yeah, the torque read on a particular dyno might show larger or smaller fluctuations, but that is mostly an artifact of the set up and instruments.

4. a gearset designed for 600 lbs/ft average torque from a diesel V-8; I assume that it isn't a 2 stroke, so 4 per rev power impulses that will be significantly higher than 600 lbs/ft.
Actually, the C6 gearsets were designed for 429 and 460 cubic inch V8's. The racers certainly got more rpm, and somewhat more torque, and the torque converters have torque ratios around 2:1 so input shaft torque could approach 1000 ft pounds. Only later did the diesels come along, and the true capacity of the base gear sets were established, then upgraded to handle 600 ft-lb engines. Even into the 5R110 (short description is a 3 speed C6 with a 2 speed planet set and a couple more clutches, it had six speeds, but two of them were so close together, it was a five speed) handled the big diesels with electronic torque management.

Yes, four firing pulses per rev, but the planetaries never had to see much in the way of torque fluctuations in an automatic tranny. The C6 was an open torque converter trans, no TC clutch, and the hydrodynamic drive is a fantastic vibration isolator. You do not see power pulses in the trans input shaft - just smooth. 4R100 and 5R110 that followed had TC clutches and dampers as well as higher top gears, all in the name of fuel economy. Soft systems isolate the engine from the downstream stuff. At low rpm, you might have torque fluctuation at + and - 10% of mean torque, and it becomes vanishingly small with rising engine rpm. So no, the C6 gearsets were not designed to handle big peak torques of any engine...

So, please help me wrap my mind around the processes that could result in resonance when driving a prop. Everett obviously had problems, but where? Metal prop? Grossly misdesigned dampers? misalignment in assembly? Allowing the system to operate 'unloaded' for extended periods? Too low an idle, causing gear slap?
I talked about much of this before. An even-fire four will make big 2/rev, so will a 2 chamber Wankel. The four makes 4/rev at about 25% of the power pulses, but a 2 chamber Wankel apparently makes 4/rev at close to the power pulses (rotors are heavy). A four will also make 8/rev, but it is usually close to background noise for amplitude, but in the Wankel, this also appears to be significant. In even firing fours, you have to either make the lowest resonance greater than about 6x firing or insert a soft element to take first resonance to 2/3 of idle firing or lower. With the Wankel and its big 4x and significant 8x, you would have to make the soft system do the same thing, about 2/3 of idle firing or go stiff and get first resonance safely above the 8x, say 12x. That is four times the stiffness of a 6x system of the same inertia. Just to make the soft system harder to make work, when you put in a soft spring to drive first resonance low, you are hoping that all of the other resonance modes stay out of range high. If they are, you are cool, but if any of the higher modes ends up in the operating range, you have problems. And just getting the whole system stiff enough to put resonance above max speed x12 is no mean feat. Stiff system, gearing, real gears must have some lash. Of additional concern is that 2 chamber Wankels (and four cylinder piston jobs) will also make 1/rev and 0.5/rev, and these can cause problems if they coincide with a resonance mode...

For more detail on sources of resonance, I wrote a bunch on it already http://www.homebuiltairplanes.com/forums/showthread.php?t=14215.

I do not know what he ran afoul of. He was past C6 planetaries when I was talking to him. When he was trying to run a stiff system with C6 planetaries, he likely was not stiff enough and ran resonance close to or inside the operating range. Resonate with serious lash in the system, and you will also have big iimpact. Even overbuild won't live a long time under that.

His torsional pendulum system worked, and so did his very low lash stiff system. He did tell me of seizing a bearing on his prop shaft, which he upgraded. The commercial product lived fine, but the parent company died. Now we get to see it again.

Billski
 
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BoKu

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...the hydrodynamic drive is a fantastic vibration isolator. You do not see power pulses in the trans input shaft - just smooth...
Is there any reason that we don't see such drives in aircraft PSRUs? Is it the weight, or are there other factors?
 

rv7charlie

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I was trying to get him to think about soft systems, but he was beyond that by then. I don't believe we ever talked details with soft systems.



I do not know what you mean by inherently low torque. 200 hp at a prop speed of 2500 rpm is always 420 foot pounds, that is what the prop flange does on both.

A two rotor Wankel and a four-cylinder four-stroke piston both have 2/rev power pulses that last about 1/2 a revolution. The discussion of not having torque reversals is nonsense. I discussed it earlier. Both are making big positive torque for half a turn, then negative torque for half a turn, then repeating. They very definitely accelerate, then decelerate twice per turn. The torque traces and how smooth they are is entirely a function of: flywheel inertia; spring rate between engine and load, inertia of the load, and torque sensor response time constant. Yeah, the torque read on a particular dyno might show larger or smaller fluctuations, but that is mostly an artifact of the set up and instruments.



Actually, the C6 gearsets were designed for 429 and 460 cubic inch V8's. The racers certainly got more rpm, and somewhat more torque, and the torque converters have torque ratios around 2:1 so input shaft torque could approach 1000 ft pounds. Only later did the diesels come along, and the true capacity of the base gear sets were established, then upgraded to handle 600 ft-lb engines. Even into the 5R110 (short description is a 3 speed C6 with a 2 speed planet set and a couple more clutches, it had six speeds, but two of them were so close together, it was a five speed) handled the big diesels with electronic torque management.

Yes, four firing pulses per rev, but the planetaries never had to see much in the way of torque fluctuations in an automatic tranny. The C6 was an open torque converter trans, no TC clutch, and the hydrodynamic drive is a fantastic vibration isolator. You do not see power pulses in the trans input shaft - just smooth. 4R100 and 5R110 that followed had TC clutches and dampers as well as higher top gears, all in the name of fuel economy. Soft systems isolate the engine from the downstream stuff. At low rpm, you might have torque fluctuation at + and - 10% of mean torque, and it becomes vanishingly small with rising engine rpm. So no, the C6 gearsets were not designed to handle big peak torques of any engine...



I talked about much of this before. An even-fire four will make big 2/rev, so will a 2 chamber Wankel. The four makes 4/rev at about 25% of the power pulses, but a 2 chamber Wankel apparently makes 4/rev at close to the power pulses (rotors are heavy). A four will also make 8/rev, but it is usually close to background noise for amplitude, but in the Wankel, this also appears to be significant. In even firing fours, you have to either make the lowest resonance greater than about 6x firing or insert a soft element to take first resonance to 2/3 of idle firing or lower. With the Wankel and its big 4x and significant 8x, you would have to make the soft system do the same thing, about 2/3 of idle firing or go stiff and get first resonance safely above the 8x, say 12x. That is four times the stiffness of a 6x system of the same inertia. Just to make the soft system harder to make work, when you put in a soft spring to drive first resonance low, you are hoping that all of the other resonance modes stay out of range high. If they are, you are cool, but if any of the higher modes ends up in the operating range, you have problems. And just getting the whole system stiff enough to put resonance above max speed x12 is no mean feat. Stiff system, gearing, real gears must have some lash. Of additional concern is that 2 chamber Wankels (and four cylinder piston jobs) will also make 1/rev and 0.5/rev, and these can cause problems if they coincide with a resonance mode...

For more detail on sources of resonance, I wrote a bunch on it already http://www.homebuiltairplanes.com/forums/showthread.php?t=14215.

I do not know what he ran afoul of. He was past C6 planetaries when I was talking to him. When he was trying to run a stiff system with C6 planetaries, he likely was not stiff enough and ran resonance close to or inside the operating range. Resonate with lash in the system, and you will also have impact. Even overbuild won't live a long time under that.

His torsional pendulum system worked, and so did his very low lash stiff system. He did tell me of seizing a bearing on his prop shaft, which he upgraded. The commercial product lived fine, but the parent company died. Now we get to see it again.

Billski
Are we talking about prop torque, or input torque to the redrive? I thought the planetaries were rated by input torque. Radical difference in torque for 200hp@6000+rpm fat the input to the redrive vs 200hp@2700 rpm direct, right? I do understand that torque at the prop is the same. In any case, I was trying to draw a comparison of loading between the Renesis (~200 lbs-ft of torque) vs the input torque of the big V-8s the gearset was designed for.

Would you indulge my 'no torque reversals' nonsense? Here's a link to ROTARY ENGINES by Kenichi Yamamoto (not the martial artist; the chief of Mazda's rotary engine dept). Torque discussion starts on pg 83.
http://www.rx7.net.nz/REbyKenichiYamamoto-1971.pdf

Thank you for your patience,

Charlie
 

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I'm not Billski (by any means), but Figure 7.6 on page 85 very clearly shows torque dipping into the negative range, while at 360, 720, and 1080 deg the overlapping torques are also going negative.
 

rv7charlie

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I'm not Billski (by any means), but Figure 7.6 on page 85 very clearly shows torque dipping into the negative range, while at 360, 720, and 1080 deg the overlapping torques are also going negative.
Keep reading; we're discussing a 2 rotor (or more) engine.

edit: The rotary isn't a piece of cake to understand; the graph you're referring to shows the 3 faces of one rotor.

Charlie
 

Autodidact

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Keep reading; we're discussing a 2 rotor (or more) engine.

edit: The rotary isn't a piece of cake to understand; the graph you're referring to shows the 3 faces of one rotor.

Charlie
OK. I do not mean to upset anyone by what I'm going to say, and I do not claim to be an expert in IC engine design but it is an interest of mine and I have grappled with trying to understand it, which means that I have examined a fair amount of material related to it. I'll try to be brief and concise as I am capable (ie, not very). I have looked at torque curve charts in several different sources on design; the chart in Yamamoto that I assume you are looking at to bolster your assertion that two rotor rotaries have no torque reversal is Figure 7.7 on page 86. Figure 7.7 also has a torque curve for an inline six which is an even fire six and Yamamoto likens the torque curve of the two rotor engine to that of the even fire six and indeed neither the two rotor or the inline six have any negative torque in that chart. But I have also seen a torque curve for an even fire six on this web page - http://epi-eng.com/piston_engine_technology/torsional_excitation_from_piston_engines.htm - that does dip into the negative implying (if it is correct) that there is some variable that can cause this fundamental difference in the torque curves of otherwise similar engines. I have had conversations with friends about this and some are of the opinion that the information on epi-eng dot com is not necessarily reliable and I cannot prove that it is reliable, although I think that it is. The reason that some might not trust epi is that many engine design texts show a different curve for the same number of cylinders than epi does. One of the books I have is Charles Fayette-Taylor and in it there is a chart comparing cylinder gas pressures for different compression ratios and the variation in those pressures at differing compression ratios was not linear and while I do not fully understand the necessary calculations to predict max/mean torque ratios, it was my understanding from reading this book as well as another earlier engine design book, that the charts are examples only and that that mean and max torque values as well as the max/mean torque ratios had to be calculated for any specific engine design and couldn't be known from charts such as the one on the epi site and in Yamamoto's book. In other words, I believe that max/mean torque ratios and also whether there is any negative torque or not is dependent upon the compression ratio and possibly other factors like bore/stroke and/or others. That doesn't mean that I didn't make a mistake in my last post, I did, and you caught me there - the chart does imply that two rotor engines have no negative torque. But it is my contention that, depending on design parameters (compression ratio), it may be possible for some two rotor engines and also some even fire sixes to have torque curves that dip into the negative. I have been as guilty as anyone in assuming that these torque curve graphs commonly seen in the textbooks and elsewhere were set in stone, but I now believe that they are only examples and that for a particular engine they must be calculated in order to find the information needed to more easily do something like design a vibration isolator for a PSRU/engine combo. If that's true, then it is yet another complication making it difficult for experimenters who are not professional powertrain engineers to design these things.

I'm pretty sure that there is somebody lurking around who can set us straight on this.

Bret
 
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wsimpso1

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Is there any reason that we don't see such drives in aircraft PSRUs? Is it the weight, or are there other factors?
Weight, bulk, other hardware, cooler, poor efficiency. Modern automatic transmissions still have torque converters, they are a great device for getting a vehicle moving. Modern torque converters include a clutch to by-pass the low speed turbo-machinery and a damper system to drive first resonant order down around idle for engine isolation. For a huge majority of EPA or Euro fuel economy cycles, the clutch is on and the damper is working.

Details: A decent torque converter for a 200 hp, 7000 rpm engine will be on the order of 10" in diameter, 25 pounds, will require about 6" longitudinal packaging, will need an oil pump moving over a gallon a minute to take away waste heat, will require an oil cooler substantially larger than the engine's oil cooler, and will divert on the order of 15% of the engine power to the oil.

Can you get way with smaller? Yeah, but efficiency will likely drop. Bigger will be more efficient, but weigh more. You get to choose, but it is big complicated and inefficient...

Billski
 

wsimpso1

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Are we talking about prop torque, or input torque to the redrive? I thought the planetaries were rated by input torque. Radical difference in torque for 200hp@6000+rpm fat the input to the redrive vs 200hp@2700 rpm direct, right? I do understand that torque at the prop is the same. In any case, I was trying to draw a comparison of loading between the Renesis (~200 lbs-ft of torque) vs the input torque of the big V-8s the gearset was designed for.

Would you indulge my 'no torque reversals' nonsense? Here's a link to ROTARY ENGINES by Kenichi Yamamoto (not the martial artist; the chief of Mazda's rotary engine dept). Torque discussion starts on pg 83.
http://www.rx7.net.nz/REbyKenichiYamamoto-1971.pdf

Thank you for your patience,

Charlie
We are talking about a 200 hp Mazda turning 7000 rpm, and trying to make it drive a prop at 2.8 tiimes the torque and 1/2.8 times the rpm, while tolerating all of the torsional vibration the engine can make. Charlie wants to talk about doing this with a piece of an obsolete gearbox, originally designed in the early 1960's that had a specific role in that tranny, and that saw only highly smoothed output of the engine. Now it might be OK in its new home.

When you hear a story, you need to ask yourself "what is he selling?" Yamamoto was trying to sell his brainchild to the world.

IC engines, even when their rpm is changing rapidly are running at quasi-steady state speed compared to everything else that they are doing. The engine speed at the end of an engine turn is basically the same as it was at the beginning of that engine turn. So, with net speed change of zero and acceleration in some part of the cycle, it had to decelerate in other parts. They do this.

For actual torque, you gotta go to the torque computed from pressure. If you look at page 85 and sum the pressure curves you can get fooled. You have to compute the torque for all three spaces captured by the rotor in each chamber. Remember also that the pressure in the compression stroke, the intake stroke, and the exhaust stroke are ALL negative torque and negative energy. Only the power stroke is positive torque. Yeah, you have exhaust and compression and intake actions working against the powerstroke. You have that in the four-banger too. The character is a little different in the Wankel, but not hugely so. So, with the engine speeding up and slowing down two times per rev, what moderates pressure based torques? Well, put a big flywheel on it and a damper spring set, and hang a big dynamometer inertia on it, filter your torque signal a bit to take off the noise in the signal (which makes it respond to changes more slowly), and any engine can look really smooth on the dyno output.

There is no way around it though, if it is making speed oscillations, it pumps the vibe system. Any lash combined with resonance will open the lash and give impact. Even when f/fn>1, if the vibe is big enough to unload the gear tooth pairs (how small a part of a degree do you think the gear teeth bend and live a long time?) you will also have impact...And while the gears, particularly if built for much higher torque, can stand some of this abuse, they will have limits. Then impact does excite all resonant modes... Dynamics sure does get complicated.

Gotta go, more later.


Billski
 
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wsimpso1

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I'm not Billski (by any means), but Figure 7.6 on page 85 very clearly shows torque dipping into the negative range, while at 360, 720, and 1080 deg the overlapping torques are also going negative.
Somebody else thinks so too. Thanks Autodidact... I think that while I say that, something else is missing from that plot. Each space around the rotor (there are three) are running at negative torque for most of the rotation. he is only showing two, but over much of the rotor movement, two are making negative torque for much of the time.

I intend to read this treatise closely. I do want to understand the vibration spectrum of these engines.

Billski
 
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Autodidact

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Looking at what few animations I can find that depict two rotor engines, it's difficult to try and see where in the combustion cycle peak torque happens. I am hamstrung by my ignorance of the behavior of the igniting fuel charge. It does appear that the compression action in the Wankel is counteracted by the combustion (talking specifically about the two rotor here...), but as you pointed out, the same appears true in the four cylinder and it usually goes negative. I have noticed that the published torque curve charts that have much lower max/mean torque ratios are also very low compression engines, like in around the 5 to 1 range.
 
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Richard Schubert

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There are 3 faces per rotor, the diagram on page 85 figure 7.6 is clearly labeled as a single rotor with 3 faces. If you superimpose an identical graph delayed by 180 degrees to represent the second rotor (also with 3 faces) you will see the negative torque peak is under a high positive torque area of the second rotor.
 

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Looking at the rotary's eccentric (crank?) shaft, one possibility that looks plausible is to extend a quill shaft through almost its entire length, which would make for a compact and lightweight soft system reducing the transmissability of the forcing vibrations to a very small percentage. I see no reason why the oil supply to the rotor bearings could not flow around the quill; it would need a seal or a tight annular aperture (a bearing on a quill journal?) at the end...
 

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Sorting out the rotary's processes is a bit hair-pulling for me, too, and I've been paying attention since my father bought an RX-4 in 1974.

The key, I think, is combining 7-6, the text, and 7-7. I wish he'd have included a graph overlaying the individual rotor curves, before showing the mean torque in 7-7. Here's a link to a later edition with overlaid individual rotor curves, plus the summed torque:
http://foxed.ca/rx7manual/manuals/REbyKenichiYamamoto-1981.pdf
Fig 1-32 on pg 8

Note that he mentions that it takes 1080 degrees (3 rotations of the crank) to complete the 4 'strokes' of one cycle, for a single face of one rotor. Divide by 4 & you get 270 degrees, more or less (probably closer to 250 for actual power), for each 'stroke', instead of a <180 degree power stroke in a piston engine. But at the 180 degree point, the 2nd rotor fires.

To be honest, I'm still struggling with 7-6, myself. It almost looks like chamber 1's torque line morphs into chamber 3's. Perhaps it's because when chamber 1 is in its 'compression stroke', chamber 3 is firing. Maybe it's supposed to be a composite curve for a single rotor(?).

I'm looking forward to Billski's report once he dives a bit deeper into these docs.

Thanks, Billski,

Charlie
 

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rv7charlie, thanks for making these posts; I am learning things about the rotary I never knew and it's hard not to become a rotary enthusiast - there are so many good things about the engine.
 

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It's not perfect, but it is pretty cool, isn't it? The systems around it (control, cooling, redrive, etc) are problematic, especially for those of us attempting conversions without engineering expertise. But the core engine is fairly potent, power to weight, and incredibly rugged. It is very, very hard to kill, unless you boost it a lot. The Renesis version improves on the BSFC issues, and at least partially tames the noise & high EGT issues. Its weight & power lets it slot nicely into the 180-225 hp niche; a space that's been hard to fill with other alternative engines.

That's why I'm interested in the planetary; I'm pretty sure it's lighter than a stiff system, and it should be significantly cheaper, with mostly off the shelf parts.

Charlie
 

wsimpso1

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Looking at what few animations I can find that depict two rotor engines, it's difficult to try and see where in the combustion cycle peak torque happens. I am hamstrung by my ignorance of the behavior of the igniting fuel charge. It does appear that the compression action in the Wankel is counteracted by the combustion (talking specifically about the two rotor here...), but as you pointed out, the same appears true in the four cylinder and it usually goes negative. I have noticed that the published torque curve charts that have much lower max/mean torque ratios are also very low compression engines, like in around the 5 to 1 range.
Gas engines in both piston and rotor types will extract the most power from a measure of fuel by having peak pressure occur at min combustion chamber volume. If the peak is earlier, it makes some negative power before reaching TDC or the equivalent, as well as potentially damaging knock, etc. If the peak occurs late, you have less work done wit the gas expansion, and more energy leaves in the exhaust gas. One thing done in road going diesels and rotaries is late ignition to keep peak temps down to help reduce NOx emissions, not an issue we worry over in airplanes. Thermodynamic theory tells us the efficiency of a machine goes up when we raise the difference between source temperature and sink temperature, so doing things to reduce source temperature (late ignition reduces peak pressures and peak temperatures) reduces efficiency.

This whole thing about "not going negative" on torque is nonsense. Let's get into why:

You still get accels and decels that ARE periodic vibration, no matter how many firing pulses per rev you get;

Accels at frequencies define the amplitude of the vibration. We are not talking force or torque here, just the movement. The amplitude of this vibration is proportional to acceleration and inversely proportional to frequency squared (amplitude prop to alpha/omega^2);

In a system f/fn>>1, the soft spring system will smooth the engine's output speed change to a modest ripple, and downstream elements will generally stay loaded. Cars/trucks, helicopter anti-rotation tail rotor systems, electric powerplant steam turbine/dynamo systems, jet and turboprop engines, and lots of other systems are soft systems with either very high input vibration frequencies (jet engines) or unavoidably soft systems (helo tail rotor shafts). True, almost all of these have stiff sub-systems in them, but if you put in isolation elements between vibe source and stiff subsystems, there is little vibe input to deal with in the stiff subsystem;

In a system where f/fn<1 the entire system will see some amplification of the engine speed change, depending upon how close f is to fn, As f/fn approaches 1, resonance is happening and amplification gets BIG;

In a stiff system, the system must be very stiff to achieve the needed Fn above highest f from the vibe source, and so has little deflection between unloaded and loaded states. So they are vulnerable to being unloaded during normal vibration. At high speed (f/fn > 0.5) we get some amplification, but the stroke gets smaller. At low speed (f/fn near zero), transmitted vibration is close to input vibration, input vibration stroke can be large even when the acceleration is moderate. Lucky for us, in a system driving a propellor the mean torque is tiny at low speed (prop torque is proportional to speed squared), so we may have only modest amplitudes down there;

In a geared stiff system, the lash is vulnerable to being unloaded. Low lash designs can make the impact energy from unloading tolerable, but still require large gear teeth and beefy surrounding structures. Since this is all consistent with stiff system design, it can work. And since props can only carry low torque where strokes are larger, and can only carry high torque where strokes are reduced, stiff systems, even geared ones, have a chance...

Now in all of this discussion, I talked about vibration movement which makes the local loads that we have to worry about. The only place I talked about negative torque,was in the origin of the engine eccentric or crank vibrating. Once we have the vibration, it is all how far it moves on each cycle.

Smart folks have taken a PSRU that has been successful in a 200 hp piston four, and failed it when applying it to a 200 hp 2 rotor Mazda. If you were to believe Yamamoto, you would say that the Wankel should go easier on the PSRU, but it is not. You think something is going on with the Wankel that is not going on in piston four? I think so. I will read the treatise yet, but there are airplane parts to build and a motorcycle to ride and bedrooms to paint. In the end, I will pick and engine and PSRU on demonstrated success, and let the theory take care of itself when I fly it.

Billski
 
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