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Flyrod

Well Known Member
I attended the Sebring LSA fly in this past weekend and had the pleasure of meeting Tim Archer the past CEO of Superior engines and was very instrumental in bringing us the Superior XP engine. He has accepted the CEO position at Powerplant Developments and is promoting their Gemini opposed piston diesel engine. Mr. Archer has been an American aircraft engine executive for many years from Teledyne to Superior to Powerplant Developments. His experience is hard to match and his confidence in the Gemini engine speaks volumes.

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This engine is a most interesting design. I once sailed aboard ships that used this design in generators sets made by Fairbanks. Its successful implementation as a diesel engine has been well documented. According to Mr. Archer, developments in metallurgy have allowed PD to lighten the engine to aircraft requirements.

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This will be one to watch. Tim Archer may be involved in another home run. An aircraft engine that eliminates cam shafts, valve trains, mags and spark plugs, cylinder heads, and vapor lock certainly has my attention. They have over 2,500 hours between three operating engines thus far. These three engines are 100hp and only 15lbs heavier than the Rotax 912S, but stay tuned because the 200hp is in the works and all indications are it will be lighter and smaller than our lycomings!
 
Hopefull!

I am encouraged by this development. I was sad to see Superior get bought out by Lycoming. Now to see this...It's good to see something good come of this. :)
 
Superior

I may be incorrect, however, I Understand, Lycoming purchased ONLY Superior's inventory, not the company. A good way to eliminate competition??
Dick
 
Good way to pick up a company without the burden of assuming the liabilities and warranty claims!

N661DJ said:
I may be incorrect, however, I Understand, Lycoming purchased ONLY Superior's inventory, not the company. A good way to eliminate competition??
Dick
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Brantel said:
Good way to pick up a company without the burden of assuming the liabilities and warranty claims!
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Which is how it should be....................since Lycoming didn't make the profit ( to cover expenses) on each unit previously sold.

L.Adamson
 
I'm not sure the added super charger and or turbo charger plus liquid cooling may make the total moving parts count as much or more than a conventional engine. The fuel economy and lack of needing 100LL is definitely a big plus.
 
Vibration harmonics

Petrol engine combustion burn, i.e. (slowly) against diesel explosion (fast). Prop engine combinations would need to be well tested for harmonics. The age old method to dampen harmonics is to add mass or in the case of the Thiebert was a prop shaft damper.

just my tuppence worth

I do hope that they suceed

Rob
RV8 VERY slow build
 
rv8or said:
Petrol engine combustion burn, i.e. (slowly) against diesel explosion (fast). Prop engine combinations would need to be well tested for harmonics. The age old method to dampen harmonics is to add mass or in the case of the Thiebert was a prop shaft damper.

just my tuppence worth

I do hope that they suceed

Rob
RV8 VERY slow build
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I believe the Gemini reduces this in two ways; one is because it is a two-stroke (twice as many power pulses) although that only makes the 2-cylinder act like a 4-cylinder 4 stroke, and because it is a geared engine (again increasing the total number of power pulses per prop revolution).

But you are right - a lot of testing will need to be done. Still, I can get excited about the power-to-weight ratio. Maybe by the time I need an overhaul these toys will be ready for prime time. I would love to shove 275 turbonormalized ponies in in place of the 170 normally aspirated ones I have now!
 
160 - 180 hp?

I hope this continues to move forward successfully. Interestingly they have skipped the 160 - 180 HP range. Seems like it would have made a nice engine for the RV-9. (I know you can do the -9 with 125... but that is a topic for other threads)
DJ
 
A three-cylinder six-piston 2-stroke would fire every 120 degrees of crank rotation. A 4-cyl 4-stroke fires every 180 degrees. Given the same RPM, the Gemini's range of available forcing frequencies would be higher up the scale.

The total number of power pulses per prop revolution has no direct bearing on propeller blade vibration. The natural frequencies of each blade are determined mostly by blade mass, mass distribution, and stiffness.

Gearing is a factor in the natural frequencies of the rotating system, but that is another subject area.

The opposed piston layout might introduce one interesting new factor. Cylinder volume increases at twice the rate compared to a conventional engine. I suspect firing events result in less periodic variation of crankshaft torque; cylinder pressure is falling rapidly just as crank angle mechanical advantage is rising. Amplitude of the forcing frequency may be reduced compared to a conventional layout. First glance thinking, no research. There might be some gas-pressure-vs-crank-position plots in Taylor.
 
DanH said:
The total number of power pulses per prop revolution has no direct bearing on propeller blade vibration. The natural frequencies of each blade are determined mostly by blade mass, mass distribution, and stiffness.
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I don't think "propeller vibration" is the limiting factor with diesels - I believe rather it is the "power pulses" which can tear the hub apart. Having more pulses per revolution of the propeller by using gear reduction means each power-pulse is smaller.

You may be on to something with the increased volumetric increase during the power stroke - this may indeed reduce the max pulse torque. Maybe that's why the Germans were able to use this design in commercial service so many decades ago.
 
<<I don't think "propeller vibration" is the limiting factor with diesels - I believe rather it is the "power pulses" which can tear the hub apart. Having more pulses per revolution of the propeller by using gear reduction means each power-pulse is smaller. >>

Gear reduction means decreased RPM and increased hub torque.
 
DanH said:
Gear reduction means decreased RPM and increased hub torque.
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And, thinking a bit about it, I believe the slope of the power spike will be much steeper, thus making the spike "sharper"----I.E., more violent.

Illustration, ---- say the power stroke happens over 90* or crank rotation. With a 2:1 reduction, the same power stroke is applied to the crank over 45* rotation, but in the same time period .

Or, am I just nuts????
 
I dont know with it being a two cycle engie with ports in the cylinder but a four cycle diesel has a power stroke that is a much longer duration than a gas engine and the start of the combustion is slower and there for less of a shock on the down line components. fuel is injected into the cylinder for a longer period of time. that is where a diesel engine gets its torque.
 
DanH said:
<<I don't think "propeller vibration" is the limiting factor with diesels - I believe rather it is the "power pulses" which can tear the hub apart. Having more pulses per revolution of the propeller by using gear reduction means each power-pulse is smaller. >>

Gear reduction means decreased RPM and increased hub torque.
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For any single given pulse - yes. Torque & HP care about how many pulses per revolution of the propeller to determine how powerful each individual pulse needs to be to generate a given hp. Twice as many pulses = 1/2 pulse power needed to achieve the same hp. That's another reason why the DeltaHawk, Wilksch, and this design all chose 2 stroke vs 4 stroke - that, and to increase the hp/weight ratio.

There are some other wierd dynamics I don't understand that cause a high compression Lycoming, for example, that you don't simply calculate the pulse accelleration, but also have to factor in compression decelleration to figure out how destructive the forces are on the prop hub. On a lower compression engine mostly only the "plus" force acts on the prop (the air slows the prop down as quickly as the compression stroke would). This is usually worse on a diesel, but "more small ignitions" are better than a few "big bangs."
 
Mike S said:
And, thinking a bit about it, I believe the slope of the power spike will be much steeper, thus making the spike "sharper"----I.E., more violent.

Illustration, ---- say the power stroke happens over 90* or crank rotation. With a 2:1 reduction, the same power stroke is applied to the crank over 45* rotation, but in the same time period .

Or, am I just nuts????
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I'm already over my head - but just for argument's sake, let's say the power pulse acts over 150 degrees (spiking in the middle). With 3 cylinders each firing each revolution, there would actually be overlap at the start and end of each cylinder's ignition stroke, so the end of one cylinder's stroke would occur at the beginning of another cylinder's. Since the "finishing" cylinder is nearly out of steam and in a condition of reducing mechanical advantage, power output would be low. Since the "firing" cylinder is a TDC, mechanical advantage is also low - however, for that overlapping time the two forces are additive. The "finishing" cylinder concludes just about as the "firing" cylinder is approaching maximum mechanical advantage - which is being opposed by maximum compression of the third cylinder.

This makes my head hurt, but I think the three cylinder version would actually be a very smooth engine...

:confused:
 
The question of vibration

I found through the replies here and previous Gemini threads that there does seem to be substantial concern about vibration. This is understandable considering the large power pulse loads of traditional 4 stroke diesel engines and the relatively large pulses of our own Lycomings. I asked Mr. Archer about this and the following is his reply:

"FYI on the vibration question; The basic operating cycle of an opposed
piston engine requires that one piston slightly leads the other and cannot
be achieved by varying the port positions. The offset, however, is too small
to have any significant effect on the vibration level of the engine. For
virtually the whole of the cycle the reciprocating parts are moving in equal
and opposite directions, thus canceling out the kinetic forces generated by
their movement.

A much more significant factor is torque variation throughout the cycle.
This, in certain other diesel aircraft engines and indeed in some gasoline
engines, has led to unacceptable levels of torsional vibration. Other
elements of the Gemini design however specifically address this issue
(please see attached excerpt from Gemini Power Notes Dec. 2007 on vibration)."

http://www.pbase.com/flyrod/image/108718443/original
 
Well, that gives us some information. 4.8 piston firings per prop revolution / 2 firings per piston crank revolution = 2.4:1 gear ratio between the piston cranks and the prop (I assume he is using as an example the existing 2-cylinder model). So the "peak rotational accellerations" might be higher for a high-compression diesel but for the same horsepower they divide the power into 2.4 times the number of smaller pulses. For the 3 cylinder generating 200hp, that would be 3.6 times the number of firings as a 4-cylinder 4-stroke creating the same hp.

Well, it all sounds good but I'm not shelling out a deposit just yet...

:D
 
Phyrcooler said:
I hope this continues to move forward successfully. Interestingly they have skipped the 160 - 180 HP range. Seems like it would have made a nice engine for the RV-9. (I know you can do the -9 with 125... but that is a topic for other threads)
DJ
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I'm sure you could downrate their 200hp, which appears to weigh in at less than an IO-320.
 
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