How many people are interested in a GOOD safe psru for the rotary?

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wsimpso1

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Building a PSRU is not all that difficult
The main difficulty is isolating the power impulses from the engine.
I have been talking about it for a long time here. It has to be engineered. Here is the thread I started and explained a lot of this stuff:


Now I make my point .... is an automatic transmission available for the Mazda rotary ?? ... if so use the torque converter between the engine and gearset which will produce a very smooth power transfer.
Also been talked about. I have 23 years doing automatic transmissions for Ford and then Chrysler/FCA, so I KNOW this stuff. Listen carefully.

An open torque converter will throw away roughly 20% of the engine power that goes in. That power gets thrown away as heat. We have to turn over the oil in the converter several times a minute and circulate it through a suitably large cooler. We need not only the torque converter, but the sump, oil seals, oil pump, cooler circuit, and pressure controls. They are part of the automatic tranny, so it is doable.

How to avoid the big losses? Well, we could do what the car companies have been doing since the 1980's (and Packard first did in the 1940's) - Put in a clutch to bypass the hydrodynamic drive. These clutches add some more requirements. We have to also keep the valves and controls to turn the clutch on and off, pressure control to run the clutch at a proper pressure for the rest of the torque converter. These also come with auto trans built since the mid 1980's.

The things is, once we have the clutch, we still have to get the spring rate and travel and torque capacity right so that we isolate the vibration from running the engine. And all the other parts of the torque converter and its control and cooling become irrelavant - all we really need is the spring set between engine and gearset... So, we are back to where I was in the article cited above. You have to engineer the gear set, shafts, bearings, and pick isolation (the spring set) appropriately for the torque and inertia on both ends of the system.

And therein lies the rub...

Billski
 
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wsimpso1

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Perhaps it would also be helpful for us non-engineers to put example frequency numbers to the principles.

For instance, with a 'soft' system, if we idle a 4 cyl 4 stroke engine at 1500 rpm, would lowest fundamental be 3000/minute / 60 = 50 Hz? Does that mean resonance should be somewhere below ~20 Hz (>1 octave below)?
Correct - here's the detail. A single cylinder four stroke engine makes one firing pulse every other revolution of the crank shaft. An even fire four cylinder makes two power pulses per rev. ff= n*c/2/60. At 1500rpm * 4/2 /60 = 50 Hz. That is also the firing frequency of a Wankel two rotor at the same rpm.

To be safely below that you generally need to be at least half an octave below. A full octave above is double the frequency, and full octave below is half the frequency. So you will need to be about a quarter of 50 hz lower, or about 37.5 Hz or lower. Ideal is about as many octaves below idle as it is octaves above cranking rpm. Cranking rpm is 150 for an engine of this size, so 5 Hz. Maybe you want you soft system frequency at more like 15-20 Hz.

For a 'stiff' system on the same engine, if it runs at 5000+ to 8000+ with the Yamaha sled engines, would the fundamental be somewhere above:
16000/minute / 60 = 266 Hz? Do we also need to consider 3rd order, 4th order, etc frequencies? If only the fundamental is of concern does the math show that resonance should be 266 * 2 * 2 = somewhere above 1100 Hz(>2 octaves above)?
I do not know the cylinder count of the engine you have in mind... 267 Hz is firing frequency at 8000 rpm of a four cylinder four stroke engine. The reason we usually talk in terms of having the first fundamental frequency 2-1/2 octaves higher is that four stroke engines have the pistons going up and down in the bores, giving the second largest vibe at twice firing frequency. This engine will have big vibratory input at firing frequency of 267 Hz and another that is 1/4 or so as big as firing at 533 Hz. All internal engine resonance in durable engines must be safely above both of these, or 667 Hz.

Making a stiff system downstream? It too must have its first frequency above 667 Hz.

Difference in spring rates for this soft vs this stiff system? f is proportional to square root of k/m. IF we can keep inertia the same with the stiff system (usually inertia will go up some too). So we want frequency to go up 667/20 = 33.3. So this system will have to be 1100 times stiffer torsionally to go from safely soft to safely stiff if it does not gain any weight in the process. Conversely, to go from a safely stiff system to a safely soft system means we would need to be 1/1100 as stiff... Hmmm. Fortunately, we know how to design springs made from everything from titanium alloys to rubber.

One other thing to talk about - The Wankel rotary engine is a bit unusual. I know that it has one firing pulse per rotation of the eccentric per rotor, so a two rotor has the same firing frequency vs rpm relationship as a four cylinder four stroke. But I do not know about its other significant orders. I have looked and not found it documented anywhere, and never had occasion to put a Rotec on one in my whole career... So maybe the stiff 7000 rpm Wankel made by Mazda needed 350 Hz (1.5 octaves) or maybe it needed 583 Hz. But I do not know...

Now, which method is harder/heavier/more expensive to achieve? My money is on a soft spring between the engine flywheel and the rest of the gears, shafts, bearings, etc.

If one or more cylinders' output fails while in flight, does either method offer any better protection, or would it even matter?
Well, that depends on how we lose the cylinder. If either ignition or fuel is interrupted, we still get a compression stroke before firing. Our 4 cylinder engine still has a 2nd order or rotation vibe on all four cylinders. But one of them is smaller than the other three, so now we also get a negative pulse at 1/2 order:
  • One cylinder drops out at 8000 rpm has content at 67Hz. The soft system with resonance at 25 Hz will not resonate. And our stiff system at 667 Hz is cool too;
  • If instead, we drop a valve on that cylinder, we lose the compression stroke as well as the power stroke, but we still have the two per rev and 1/2 per rev safely away;
  • Now let's have a steady misfire at 3200 rpm. Bang, the 1/2 order input is right at 25 Hz, and we have a potentially destructive resonance;
  • The soft system can still have resonance. The stiff system hardly knows a cylinder has dropped out.
Now let's look at the Mazda built Wankel. If we lose spark or fuel to one of two rotors, we too still have two per rev plus a one per rev. If we have a soft system, have a one octave margin below idle, and we go to one rotor, we may be only at idle, but we are also bang on for resonance. There is a story out there about a guy with a 13B that had a steady misfire on one and tore things up RIGHT NOW!

Maybe the stiff system seems like the way to go, but let's remember that several things happen to get to a stiff system:
  • Making all of the soft elements stiffer adds weight to them;
  • Making a stiff system means that the entire system sees all of the firing pulses, and that adds weight to everything;
  • Making it a stiff system means that lash anywhere in the system makes impact through the system, so we need as close to a zero-lash system as we can get.
Have fun guys. I sure ain't building one...

Billski
 
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Cardmarc

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The late Paul Lamar had rotary engine vibration isolators all figured out. Using them on the flywheel to couple to the planetary. He published plans. Several iterations. I’m sure someone has those plans and can post them? Don’t re-invent the wheel.
And Tracy Crook is still flying his as well!
 

rv7charlie

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I'm probably the person who mentioned loss of a rotor killing a gearbox. The builder had used the stock Mazda 4-spd transmission case with 2nd gear engaged (I assume the other gears that could be removed were removed). It flew that way successfully for several years, until one day, for some reason, it started on only one rotor. It destroyed the gears at not much above idle rpm. He then switched to Tracy's drive, and the plane was still flying when the builder succumbed to cancer years later.

Thanks, Bill. My hypotheticals were for the same 'real world hypothetical' 4 cyl 4 stroke, like the larger Yamaha sled engines. They are 4 cyl inline, though I assume that pulse count/intervals would be the same for all even fire 4c/4s engines; like the Rotax 9xx engines.

I've never seen any info on the higher order vibes for a rotary, either, but I'll do some digging if you're willing to look at it. I do know Tracy Crook (designer of the RWS planetary drives. As I mentioned earlier, the most recent iteration of the RWS drive directly couples the planetary input shaft to an ~8 lb aluminum flywheel, with no soft elements. It's restricted to lighter weight props; he's running a 76" dia wood prop on a 2.85 ratio drive.
Quote from Tracy on the Flyrotary list, back in 2014:
On 1/11/2014 7:45 PM, Tracy wrote:

[The flywheel is] 8 1/2 pounds without the plate. I don't know if the added weight is responsible but the drive feels even smoother than it did with the flexplate and damper. Remember, I didn't use the damper assembly with this flywheel. The drive spline is bolted directly to the flywheel on the RD-2.

Tracy
I should probably say that the rotary's eccentric shaft (crank) is incredibly stiff compared to a piston engine crank. On the drive side, the planetary input shaft is maybe 8" long including the gear & splines, a bit over an inch in dia. The output shaft is derived from a truck axle, IIRC, and is probably about the same length; somewhat larger dia. So the only 'soft' element is the lash in the planetary assy. IIRC, Tracy says his will idle down around 1500 rpm without the drive gears 'rattling' (which I assume would be the onset of resonance). It's been flying that way for about a decade in his RV4.

BTW, to my knowledge, Paul Lamar never once flew anything related to reduction drives that he 'figured out'. I'm not sure he ever even built any of that stuff.
 

FinnFlyer

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Charlie, remind me what the secret is for Tracy's PSRU working and lasting.
We do know that he rarely runs at full power (except during take-off).
Pulses from engine are more gradual and longer in duration that a piston engine's?
A "softer" transfer from rotor to e-shaft?
The rubber "donuts" are bigger than theoretically need be?
The V6 planetary gear set is designed to handle way more force than it will ever see in our applications?
Wood prop is more "soft" when it comes to absorbing pulses?
Just lucky?

Trying to understand all this without getting into the math of it.

But I'm repeatedly impressed by Tracy's common sense and engineering skills. I'm all into sensors, instrumentation and data logging. Case in point: when discussing engine and prop balancing he said: "Did you try to put a finger or hand on the engine to feel the vibration?" and "Try to rest the control stick just against the end of the slack in the system when flying". It's amazing what we can actually feel, hear and smell. I had put a 5g weight on the flywheel and was running the engine as he stopped by. He put a finger on the engine and told me that's more vibration than he'd like to see. I removed the weight and ran it again and he said that's more like what's he'd expect. Perhaps very unscientific but a little feeling, listening and smelling goes a long way.

Finn
 

mm4440

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I'm probably the person who mentioned loss of a rotor killing a gearbox. The builder had used the stock Mazda 4-spd transmission case with 2nd gear engaged (I assume the other gears that could be removed were removed). It flew that way successfully for several years, until one day, for some reason, it started on only one rotor. It destroyed the gears at not much above idle rpm. He then switched to Tracy's drive, and the plane was still flying when the builder succumbed to cancer years later.

Thanks, Bill. My hypotheticals were for the same 'real world hypothetical' 4 cyl 4 stroke, like the larger Yamaha sled engines. They are 4 cyl inline, though I assume that pulse count/intervals would be the same for all even fire 4c/4s engines; like the Rotax 9xx engines.

I've never seen any info on the higher order vibes for a rotary, either, but I'll do some digging if you're willing to look at it. I do know Tracy Crook (designer of the RWS planetary drives. As I mentioned earlier, the most recent iteration of the RWS drive directly couples the planetary input shaft to an ~8 lb aluminum flywheel, with no soft elements. It's restricted to lighter weight props; he's running a 76" dia wood prop on a 2.85 ratio drive.
Quote from Tracy on the Flyrotary list, back in 2014:
On 1/11/2014 7:45 PM, Tracy wrote:


I should probably say that the rotary's eccentric shaft (crank) is incredibly stiff compared to a piston engine crank. On the drive side, the planetary input shaft is maybe 8" long including the gear & splines, a bit over an inch in dia. The output shaft is derived from a truck axle, IIRC, and is probably about the same length; somewhat larger dia. So the only 'soft' element is the lash in the planetary assy. IIRC, Tracy says his will idle down around 1500 rpm without the drive gears 'rattling' (which I assume would be the onset of resonance). It's been flying that way for about a decade in his RV4.

BTW, to my knowledge, Paul Lamar never once flew anything related to reduction drives that he 'figured out'. I'm not sure he ever even built any of that stuff.
(3) TIME TO CLIMB world record, short version! 1'40"! - YouTube Paul helped develop this powertrain. Bell 47 gears and silicone rubber element damper.
 

wsimpso1

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IIRC, Tracy says his will idle down around 1500 rpm without the drive gears 'rattling' (which I assume would be the onset of resonance). It's been flying that way for about a decade in his RV4.
While resonance can result in the gear train going through the lash in the splines and gear teeth, it is not necessarily why you get get rattle through the gear train. Rattle in the gears happens when cyclic swing of the engine exceeds the elastic deformation through the powerteam, and gear teeth lift off. As rpm comes down, swing increases - it is inverse squared with firing frequency. On the accel during the next firing stroke, the lash closes and that is impact. Bad if energy is high enough, fine if energy is small.

Billski
 

wsimpso1

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The V6 planetary gear set is designed to handle way more force than it will ever see in our applications?
Not V6, C6!

The C6 is a three speed automatic transmission designed by Ford for use with big block engines and with trucks. Ford's part numbering scheme is first digit decade, second digit was planned model year, third and fourth digits gave the vehicle program for introduction. Then there was a hyphen and transmissions all used a leading 7. All of this transmission's original part numbers were C6xx-7yyyy-zz. The C6 was produced into the mid 1990's. It was superceded by a four speed made by adding an overdrive planetary and clutches to the C6 to make the E4OD, which was followed on by the 4R100 and 5R110 up through the 2000's. There were many C6xx part numbers soldiering on right up to the end of 5R110. Lots of upgrades both from the factory and the aftermarket, mostly for racing the C6.

Billski
 

rv7charlie

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(3) TIME TO CLIMB world record, short version! 1'40"! - YouTube Paul helped develop this powertrain. Bell 47 gears and silicone rubber element damper.
If he really had anything to do with that gearbox, I suspect it was to copy what had been run in a push-pull twin rotary powered Reno Racer from decades earlier, which used the Bell 47 gears. The plane was displayed at SNF many years ago; he even had pics of it on his website before he died. I can't find any messages in my archive detailing him changing the damper/coupler assy from the original in the TTC RV, which was far along before he got involved. Do you have any records of him making those claims?
edit: Found PL's comment about the damper in the time-to-climb record setting RV in my archive; he credits John Lynch with design & construction:

Here is another relatively low cost way to make a shaft coupling.
Our gear boxes need shaft couplings on the prop shaft and prop
flange and between input shaft and sun gear. I call it the "Pin"
shaft coupling.

This is the one we are using on the 650 HP Racing Beat time to climb
engine between the rubber coupling and the Bell 47 GB sun gear.
As you can tell it works with dissimilar metals. In our case
billet aluminium rubber coupling and the steel sun gear input
shaft. Takes an NC mill to make the parts accurately or a very
good rotary table in a drill press. The pins 3/16 hardened steel.

This gear box combination is good for 1000 HP if we need a turbo 3 rotor
to dominate the Reno sport class racing if we can come up with the needed
money

John Lynch came up with this idea and made the parts.

Paul Lamar


I can attach images if anyone cares, but there's nothing unusual about it; just 6 elastomeric wedges arranged between a trio of thick 'blades' on both the engine output and planetary input shafts.
 
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dwalker

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Here is the drawing for the Ross Redrive I currently have, and it is remarkedly similar to the Tracy redrive I will have soon. In the drawing you can see the "damper plate", which looks to me to be a sprung center clutch disc. I am positive I read that either Ross or Tracy or both at some point used an elastomer of some type in addition to the damper plate. I am fairly certain I have seen a picture of an aluminum flywheel/damper in a Tracy redrive.

In any case, I think there is probably still room for improvement, and I am very interested to see what the future might hold.
 

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Billrsv4

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The really big deal is architectural. Get the the first torsional mode at least half an octave below firing frequency at your minimum engine speed and it will isolate well with small vibration everywhere else. A prop speed of 1000 rpm is around 1800 engine rpm. A four-banger at 1800 rpm idle works out to a vibrational swing of about 0.5 degrees each way around mean deflection. Not much... Take it up to big torque and 4500 rpm in flight, and the swing is more like 0.2 degrees. Lots of rubber elements last a long time time at those levels. So do springs, etc.

Viscous dampers? Bad idea. They only really work well at large deflections. You get large deflections at or near resonance. If you are operating near resonance, you already have BIG vibration problems, and then converts power to heat instead of propulsion, adding to your cooling drag.

In vibe control you have two main options:
  • Get stiffnesses high enough to put operation safely above max firing frequency - 2-1/4 octaves above is generally considered enough - and build to stand the power pulses through the system. This is called a "stiff system", and was used by Ev Hatch in PowerSport;
  • Get a soft element between the engine and rest of the powerteam to put operation safely below min firing frequency - 1-1/4 octaves below is generally considered enough - make the system lash free until after the isolator, and peak torques are only moderately higher than mean torque. This is called a "soft system" and is WIDELY used in powerteams of all sorts from chain saws to power generation turbines to container ships.
Everything else is much harder to make work, heavier, and adds to your cooling load. Ugh.

Billski

Billski, As you are probably aware the Original Powersport design uses no rubber anywhere. This design has run with everything between a wood prop to a constant speed without problems. I would suggest a ground adjustable prop rather than a weighty CS. Steve Winseril (sic) did his masters thesis on these tests. but went on to the EPS diesel. The rotary works great with a stiff system. Not enough concern is paid by most to torsional vibration issues. Take note of the fact that the rotax 912is is having some issues. Not the carb version, but the injected version. I believe it starts too quickly causing pulses at very low RPM in the first order RPM. Everyone that thinks these isssues are trivial has their head in the sand. I'm including a jpeg of my update design so folks will know what we are talking about.
Bill Jepson
 

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Billrsv4

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I'm probably the person who mentioned loss of a rotor killing a gearbox. The builder had used the stock Mazda 4-spd transmission case with 2nd gear engaged (I assume the other gears that could be removed were removed). It flew that way successfully for several years, until one day, for some reason, it started on only one rotor. It destroyed the gears at not much above idle rpm. He then switched to Tracy's drive, and the plane was still flying when the builder succumbed to cancer years later.

Thanks, Bill. My hypotheticals were for the same 'real world hypothetical' 4 cyl 4 stroke, like the larger Yamaha sled engines. They are 4 cyl inline, though I assume that pulse count/intervals would be the same for all even fire 4c/4s engines; like the Rotax 9xx engines.

I've never seen any info on the higher order vibes for a rotary, either, but I'll do some digging if you're willing to look at it. I do know Tracy Crook (designer of the RWS planetary drives. As I mentioned earlier, the most recent iteration of the RWS drive directly couples the planetary input shaft to an ~8 lb aluminum flywheel, with no soft elements. It's restricted to lighter weight props; he's running a 76" dia wood prop on a 2.85 ratio drive.
Quote from Tracy on the Flyrotary list, back in 2014:
On 1/11/2014 7:45 PM, Tracy wrote:


I should probably say that the rotary's eccentric shaft (crank) is incredibly stiff compared to a piston engine crank. On the drive side, the planetary input shaft is maybe 8" long including the gear & splines, a bit over an inch in dia. The output shaft is derived from a truck axle, IIRC, and is probably about the same length; somewhat larger dia. So the only 'soft' element is the lash in the planetary assy. IIRC, Tracy says his will idle down around 1500 rpm without the drive gears 'rattling' (which I assume would be the onset of resonance). It's been flying that way for about a decade in his RV4.

BTW, to my knowledge, Paul Lamar never once flew anything related to reduction drives that he 'figured out'. I'm not sure he ever even built any of that stuff.
Guys the "solution" Paul Lamar had to the gearbox problem was to run a Bell helecopter gearbox, (read super expensive and rare to boot) and cast a bunch of silicone rubber blocks and paddles. This was the shotgun solution. the cast blocks and paddles were exactly like the rear final drive on most motorcycles, which I had suggested previously! I was told I was crazy and banned from the newsletter for a time. I believe now as I did then that the rubber blocks are a stopgap solution that will wear quickly and bring a lot of heat into the system. The PowerSport system was solidly bolted together and works great. I didn't invent it, I was redesigning it for new tooling. The rotary 2 rotor has no resonances in the normal range, the 3 rotor a small resonance easily tuned out with a small damper. I never worked it out for a 4 rotor.
Bill Jepson
 

wsimpso1

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Billski, As you are probably aware the Original Powersport design uses no rubber anywhere. This design has run with everything between a wood prop to a constant speed without problems. I would suggest a ground adjustable prop rather than a weighty CS. Steve Winseril (sic) did his masters thesis on these tests. but went on to the EPS diesel. The rotary works great with a stiff system. Not enough concern is paid by most to torsional vibration issues. Take note of the fact that the rotax 912is is having some issues. Not the carb version, but the injected version. I believe it starts too quickly causing pulses at very low RPM in the first order RPM. Everyone that thinks these isssues are trivial has their head in the sand. I'm including a jpeg of my update design so folks will know what we are talking about.
Bill Jepson
I recognize the stiff system is the way Ev Hatch went with it. I have mentioned Ev Hatch's stiff system solution, 8 pitch gears lapped with a bit of a crown to them to give very little lash and good tooth contact, etc. Only Ev Hatch and Alan Tolle flying them convinced me the stiff system could work at reasonable weight. At that, Ev had used small block Ford bottom end bearings (yeah, journal bearings) on the prop shaft, and seized a set in the Sun 60 race. Ev had gone up to big block Ford bottom end bearings after that. I suspect tapered roller bearings are the hot ticket.

I ran some numbers for Ev Hatch way back on expected load on the bearings and the prop flange due to gyroscopic moments, and it all looked doable, even the bolted on flange.

Billski
 

tspear

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Tspear, Not sure what you are asking about here. Please elaborate...
Turbo chargers receive the exhaust in a series of pulses. It is not a constant pressure of exhaust.
So the incoming turbine spinning in the exhaust (many in high thousands of RPMs), would seem to have significant pulses. Which are then passed to the compression side. I assume the pulses end there since air is a poor transmitter of the physical pulse.
So, is the turbo, both incoming turbine and compressing fan usually linked together in a stiff system?

Tim
 

wsimpso1

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Turbo chargers receive the exhaust in a series of pulses. It is not a constant pressure of exhaust.
So the incoming turbine spinning in the exhaust (many in high thousands of RPMs), would seem to have significant pulses. Which are then passed to the compression side. I assume the pulses end there since air is a poor transmitter of the physical pulse.
So, is the turbo, both incoming turbine and compressing fan usually linked together in a stiff system?

Tim
Gotcha. Sort of thread drift on thread drift, but OK.

Turbomachinery are flow machines - the volume flow through the gadget and the momentum change across the gadget is what it responds to. The behaviour has little to do with pressure across it. The header sort of lumps the flow from several cylinders together, and in-the-best-case, has all four flows evenly spaced into one continuous wash. Then the diffuser wraps around the turbine, sort of spreading the flow into the turbine, so the flow variation from a pressure wave is spread around the turbine, further diffusing the flow into the turbine, averaging it out.

Next, the momentum of the turbine-compressor set spinning on the order of 100,000 rpm is big. It would take a lot of change over a fairly long period (compared to firing interval) to slow down or speed up the set.

Between the change in momentum in any one exhaust pulse being modest compared to the momentum of the rotating parts and the tendency of the header and then the turbine' diffuser to spread out any pulses, this piece of turbomachinery serves as an excellent filter for high frequency noise. It just sort of averages that stuff out. All turbomachinery tends to do this and I expect that a turbocharger will behave consistently with other turbomachinery.

I never got into the turbo vibrationally. I do not have any info on their vibe modes and frequencies, but I can imagine that it is a pretty darned stiff assembly with pretty darned high natural frequencies in torsion. The firing rate appears to be pretty darned low by comparison. Now the blades on the wheels, I bet they are sized at least in part for having natural frequencies that are much higher than the variation in flow through them.

Then there is the issue of turbochargers are pretty darned reliable when properly sized and then fed clean feed gas, clean induction air, clean cool oil, and the oil has a good path back to the sump. I am willing to just go with that...

Billski

Billski

Just one engineer's opinion.
 

EzyBuildWing

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Molt Taylor invented a sort of torque-converter with lead-shot and a wavey-plate inside it to to remove pulses/harmonics from his long drive-shaft in his Imp.....if I remember correctly.
 
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