Experience with Subaru at 358 Hours

[Denase]

I have been following this thread. Dan your profile shows you are a commercial car and truck dealer, are you also an engineer?

[LoopDLoop2]

Originally Posted by rv6ejguy

Welcome here and I appreciate your interest. The piston is JE as described on the web page. The oil ring tension, well, frankly never paid attention to this as I have never had an oil consumption problem before- tried to duplicate the same same successful recipe I've used to build engines professionally for quite some time. So, the answer is- I have no idea.

I don't own a blowby meter but I could show the the belly of the aircraft- blowy at the end was "severe". But when built originally, the belly was completely dry after 35 hours and stayed that way for over 200 hours.

Chrome got me home for 35 years building turbocharged engines... and was the choice of the Fuji engineers who designed this engine. Perhaps old school but it still works very well in this application.

This was honed at a pretty flat angle which I have found works fine too.

CK10 also dated like me but again, gave good results with a good operator, engines worked fine.

Subaru engines do not use head bolts into the block deck like most other engines. A torque plate would have minimal effect IMO. Of course "round" and "straight" are relative terms and I agree, the rounder and straighter, the better. With old equipment and measuring stuff, I do the best I can.

What is TBO on a Pro Stock engine? Curious. What BMEP at torque peak and power peak?

TBO on a NHRA 500 CI Pro Stock Engine is about 16 to 18 runs. The make peak torque at around 7900 to 8200 depending on manifold and peak the power around 9800 to 10100 depending on the same. They make in the 3 hp per ci.

I looked at the bearing pictures and if I had to say right off what caused the flaking it would by detionation. This would corolate with the loss of ring seal. If you are going to use the same top ring part # you could measure the free gap. This would be what the end gap is on the bench after you fit it to the bore. Then measure the one that came out of the motor. Just because the piston is not burnt up it does not mean it is not beating it self to death.

[DanH]

Originally Posted by Denase

I have been following this thread. Dan your profile shows you are a commercial car and truck dealer, are you also an engineer?

I have little talent for advanced math, thus did not pursue engineering as a profession. Sometimes I regret it, sometimes not.

This particular subject became interesting after bits of a drive system went streaming past the cockpit in the summer of '98. One thing led to another....education and recreation run amuck.

[SvingenB]

Originally Posted by DanH

Then the subjects all have similar inertia and stiffness values.



We stopped because an accurate model requires the inertia and stiffness values for the Marcotte gearbox. All you had was a drawing sans dimensions, and you were not interested in taking it apart for measurements.

However, even a fictional model with typical numbers is illustrative. Let's lump inertias into three elements and then make changes to the crank/flywheel inertia and the stiffness connecting the crank/flywheel and the gearbox inertias. I'll use some values taken from an early Suzuki model, with pure guesswork for Marcotte inertia and propshaft stiffness.



Units are inertia in kg m^2 and stiffness in N m/rad.

Aluminum flywheel and stiff coupler:

Inertia of disk 1 = 0.39
Stiffness = 2.7e+004
Inertia of disk 2 = 0.03
Stiffness = 1.898e+004
Inertia of disk 3 = 0.07

Natural Frequencies
f1 = 0 Hz
f2 = 67.76 Hz
f3 = 207 Hz

F2 mode shape
1
-1.618
-4.878

Steel flywheel (2.5x alum flywheel), stiff coupler:

Inertia of disk 1 = 0.39
Stiffness = 2.7e+004
Inertia of disk 2 = 0.03
Stiffness = 1.898e+004
Inertia of disk 3 = 0.175

Natural Frequencies
f1 = 0 Hz
f2 = 48.28 Hz
f3 = 202.5 Hz

F2 Mode Shape
1
-0.3293
-2.172

Steel Flywheel (2.5x alum flywheel), soft coupler:

Inertia of disk 1 = 0.39
Stiffness = 2.7e+004
Inertia of disk 2 = 0.03
Stiffness = 2576
Inertia of disk 3 = 0.175

Natural Frequencies
f1 = 0 Hz
f2 = 22.08 Hz
f3 = 163.1 Hz

F2 mode shape
1
0.7221
-2.352

The first model approximates what you have now. We know it's not an accurate model of your current Subaru system because the predicted F2 is 68hz, meaning it would hammer at about 2000 RPM, not the 1250 ballpark you report. Still it is useful to compare with models 2 and 3.

#2 swaps the flywheel. I've merely increased inertia 3 (the crank/flywheel) by a factor of 2.5

#3 is the heavy flywheel plus a soft connecting element between the flywheel and the gearbox.

Compare the examples and you see what you can expect, in very general terms:

Increased flywheel inertia is good. A reduction in connecting stiffness is more effective at lowering frequency. The mode shapes change; models 2 or 3 greatly reduce crank oscillation at the accessory end (remember your beat-up vac pump drive?). The F3 in model 3 is creeping into the top of the operating range; it may resonate at 163 hz, or 4890 RPM for your 4-cyl 4-stroke.

Sure beats cut and try

Interesting. I assume the f3 mode shape is one of + - + ? What happens if you keep the original setup, but decrease the stiffness of the coupling between the flywheel and marcotte?

[rv6ejguy]

Originally Posted by LoopDLoop2

TBO on a NHRA 500 CI Pro Stock Engine is about 16 to 18 runs. The make peak torque at around 7900 to 8200 depending on manifold and peak the power around 9800 to 10100 depending on the same. They make in the 3 hp per ci.

I looked at the bearing pictures and if I had to say right off what caused the flaking it would by detionation. This would corolate with the loss of ring seal. If you are going to use the same top ring part # you could measure the free gap. This would be what the end gap is on the bench after you fit it to the bore. Then measure the one that came out of the motor. Just because the piston is not burnt up it does not mean it is not beating it self to death.

Interesting. Don't take this the wrong way but how does a TBO of 125 seconds relate in any way to prime mover engines especially ring "life" and how do you quantify oil consumption with a total engine life of 2 minutes?

Hmmm, detonation damage on the bearings? I think not. I've been building turbocharged engines for a long time and we can/ have encountered detonation very frequently. NEVER seen a rod bearing damaged by detonation- EVER, pushing 3.5hp/cubic inch and the engines are going 25-50 hours before overhaul and we are doing this down around 7500 rpm so the BMEP is very high, about 50% higher than the Pro Stock engine at power peak. First thing to fail is either top ring lands or head gaskets and that pretty well signs the engine off right there before anything else can be damaged. Clearly this is cavitation damage. On prime movers, if you encounter heavy detonation, because of the the continuous high power, the engine will certainly die. Many engines will last though 7 seconds of heavy detonation, very few will take 1 minute or 5-10 minutes of it. I hold takeoff MAP through 500 feet and make a slight MAP reduction for the climb where the engine is often held here for 10 to 15 minutes. There were zero signs of detonation on this engine and I'm very familiar with the results of this.

The rings were going south for over 150 hours but the oil consumption never got above 1 qt in 6 hours ( about the same as lots of Lycomings I flew). This is not nearly enough oil to significantly lower the octane rating of the fuel to cause detonation.

If you read the web page I already stated that ring gap was the same as when the engine was assembled 9 years ago- in other words no wear on the rings or bore.

[Mike S]

Originally Posted by DanH

....education and recreation run amuck.

Much better mental image than the potato

[DanH]

Originally Posted by SvingenB

Interesting. I assume the f3 mode shape is one of + - + ? What happens if you keep the original setup, but decrease the stiffness of the coupling between the flywheel and marcotte?

Side-by-side comparison, with gearing included. The gear correction means multiplying the faster-rotating inertia and stiffness values by ratio squared. In this case I've assumed 2/1 gearing. So, as compared to previous models, the coupler and crank/flywheel values have been increased by a factor of 4.



Remember, this a fictional system, an example for purposes of illustration. It is similar to but not exactly what Ross is flying. We don't have the real stiffness and inertia values, and the above is a "lump" model (for example, the entire flywheel, crank and accessory sections of the engine are represented by a single inertia). A really good model would use carefully measured values and break the system into several more inertia/stiffness pairs. The only point in posting these values is to demonstrate how applying known principles/procedures is better than randomly changing hard parts.

[David-aviator]

Originally Posted by DanH

Side-by-side comparison, with gearing included. The gear correction means multiplying the faster-rotating inertia and stiffness values by ratio squared. In this case I've assumed 2/1 gearing. So, as compared to previous models, the coupler and crank/flywheel values have been increased by a factor of 4.

Remember, this a fictional system, an example for purposes of illustration. It is similar to but not exactly what Ross is flying. We don't have the real stiffness and inertia values, and the above is a "lump" model (for example, the entire flywheel, crank and accessory sections of the engine are represented by a single inertia). A really good model would use carefully measured values and break the system into several more inertia/stiffness pairs. The only point in posting these values is to demonstrate how applying known principles/procedures is better than randomly changing hard parts.

When MT came to Florida to do the vibration survey of the MT-7 and the Egg H6 engine, they attached strain measuring devices to the engine and prop.

I believe your computer generated data with various components would have to checked in a similar manner to prove its worth.

For sure an airplane can be designed, built and safely flow based on computer data, Boeing does it successfully but flight tests are necessary to verify the end result. As far as I know, the FAA has not certified an airplane without actual flight testing.

Same can be said for your efforts in determining vibration data or whatever you are attempting to determine. It needs to be flight checked.

Unfortunately in this business, flight checking alone is not the answer. Engine-psru-prop combinations are being assembled with little science, briefly flight checked, and off to market we go. The end result can be a less than satisfying experience for many pilots.

[DanH]

Originally Posted by David-aviator

When MT came to Florida to do the vibration survey of the MT-7 and the Egg H6 engine, they attached strain measuring devices to the engine and prop. I believe your computer generated data with various components would have to (be) checked in a similar manner to prove its worth.

Absolutely. Math models are design and diagnosis tools. Live measurement of the flight article validates the model...the vital reality check.

[rv6ejguy]

I'd mention that the drive ratio on this aircraft is 2.2 to 1 and I just want to thank Dan again for his time to help educate us in this field.

I will be doing inertia tests on the prop and any other parts before I re-install then on the aircraft and try to do some reasonably accurate calcs on the internal gearbox parts as far as stiffness and inertias.

The last chart laying everything out side by side shows the relative effects of changes in certain elements. It will be cool if I can gather enough accurate data to be able to have the math model agree closely with practice. We'll be able to have some more confidence in finding a better solution to all this.

It will take a while to do this because of my work and plane work schedule. At the moment, I am working on electrical components, airframe inspection and the new radiator/ cooling system layout. Once I get the flywheel back, I can do some tests and get the engine back in. This is a fairly big project in itself- something over 300 hours I am guessing.