Gemini Diesel Engine Attracts Industry’s Eye

By Scott Spangler on August 18th, 2008

Jetwhine_Gemini Engine_profile At first glance the Gemini 100 looks like a compact water-cooled aircraft engine with four opposing cylinders. Narrow and not very tall or deep, it’s roughly the same size as a Continental O-200 and has the same output, 100 hp. That’s where the similarities end, says Tim Archer, president and CEO of Powerplant Developments, the Gemini’s manufacturer. With three cylinders and six pistons, the Gemini burns diesel/Jet A.

Come again?

Jetwhine_Gemini Engine Like a lot of pilots, I’m not much of a gearhead, but this caught my attention. The Gemini is an opposed piston engine, meaning the combustion chamber is formed between two pistons in the same cylinder, Archer explained. The intake and exhaust ports are orifices in the cylinder wall and the piston’s movement opens and closes them.

Parallel crankshafts transfer the power to the reduction gearing built into the front of the case. Being shorter, these cranks are not subject to the torsional loads imposed on the longer cranks in traditional engines. To ensure reliability, the Gemini cranks are made of EN40 steel, the same steel often used to make cranks for Formula 1 engines, which routinely run at 19,000 rpm, Archer said.

Apparently I’m not the only one attracted to this innovative powerplant. Powerplant Developments has signed a memorandum to supply a pre-certification Gemini 100 and two turbocharged Gemini 125 engines to an Italian aircraft manufacturer, Tecnam. And during EAA AirVenture, seven other airframers expressed interest in the engine, especially the forthcoming 180/200 hp and 300 hp versions. The company has completed design studies for Gemini engines up to 600 hp.

With liquid cooling, weight has always been an issue with aviation diesels. This innovative design seems to have found the solution. The installed weight of the 1,600-cc/98-cubic-inch Gemini 100 is 166 pounds. Installed, the O-200 weighs nearly 200 pounds, and the 112-hp Lycoming O-235 tips the scales at 242 pounds. 

JetWhine_Tecnam_P92 EagletTecnam will evaluate the Gemini 100 in its P92 Eaglet, an LSA powered by a 100-hp Rotax 912 ULS2, which has an installed weight of roughly 170 pounds. The Gemini 125s will power the  P2006T twin.

Fuel is another consideration of weight. The Gemini 100 has a specific fuel consumption of .38. Archer reluctantly converted that to 3 to 3.5 gph, depending on the airframe and prop, other variables in the equation. It’s interesting that a prolonged Internet search for the specific fuel consumption numbers for avgas engines provided no joy.

JetWhine_Gemini Engine_head-on Archer said the Gemini 100 will retail for $18,000 and the 125 will be $23,500. “TBO looks like it will be 2,000 hours,” he said, adding that with operational data on the engine it may grow to 3,000 hours. He foresees no pre-TBO inspections, such as on the reduction gears, and the three cylinders won’t need to be replaced until the third overhaul.

All of it sure sounds good, but the skeptic in me says engineers have been trying to develop a workable aviation diesel since World War II. Designs come and disappear, but few fly. Tecnam will give the Gemini some test wings, and it will be interesting to see if the other manufactures interested in the bigger engines follow through with their initial interest. — Scott Spangler

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30 Responses to “Gemini Diesel Engine Attracts Industry’s Eye”

  1. Alan larson Says:

    You may not have found specific fuel consumption for avgas, but it only took a couple minutes with google to find numbers for the Rotax 912 engines.
    The Rotax 912 (100 hp) engines burn from some references 4.9 gal/hr at 75% power, for a sfc of
    4.9*6/75 = .392 lbs/hp-hr.
    The .38 figure for the Gemini would give 4.25 gal/hr for 75% power (6.7 lbs/gal), nowhere near
    the 3-3.5 gal/hr claimed. The Powerplant Developments web site shows sfc at 75 hp to be near .41 lbs/hr giving a fuel consumption of about 4.6 gal/hr at 75% power.
    As such, the Gemini does nowhere near as well as claimed, and is only slightly ahead of the Rotax 912 S in gallons/hour, and the Rotax actually burns less when measured in pounds/hour.
    While the idea of an efficient Diesel engine for LSA seems wonderful, the numbers for this engine don’t really seem to deliver.

  2. Mike Says:

    Hope you realise that here in Europe we use litres, and if we have to use gallons they are of the Imperial variety. Be sure you are not comparing US gallons per hour with Imperial gallons per hour. That might account for the disparity.

  3. Alan Larson Says:

    Thanks Mike for the reminder about the Imperial gallons. My calculations were using US gallons and pounds for both the Rotax and the Gemini. Clearly when he mentioned 3.5 gph, it could have been Imperial gallons.
    Had they specified both in liters it would have been more clear, but mixing liters and horsepower seems a bit strange, too.
    I remain hopeful that an engine such as this can give us an aircraft engine that is significantly more efficient, and include a long TBO (and lower maintenance cost).

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  5. phil Says:

    I think allot of Us loose sight on the innovation that alternative fuels
    brings and can be brought to the aviation industry. A multi fueled engine
    for aircraft. Have not seen that to often! Anybody know the torque values
    of the Gemini? I bet it’s doubled the Rotax! Torque turns propellors does
    it not? And if it’s doing twice the work for the same amount of fuel, that
    sounds more efficient to me….

  6. Antoine Says:

    Interesting analysis Alan. I fully agree with your findings.
    Actually, it does not make much sense to compare fuel burn in Gallons (of any sort) or even liters.
    We should work with pounds or kilograms of fuel weight. Diesel has the same energy content per unit of weight as AVGAS, so any differences would be due to the quality of the implementation.
    In my view the only real benefits of diesel engines are:
    – eliminate the need for AVGAS a costly, environmentally inadequate fuel
    – less volatile and explosion-prone in case of accident.
    It is well known that internal combustion engines have very similar efficiencies. No free lunch…
    The quantum leap in fuel economy IS possible, but we should not expect it from the engine side. How about making the airframe MUCH lighter? Last week I have flown a (true) 4 seater at 120 Knots cruise, burning 17 liters (4.5 USG) of AVGAS per hour. I won’t mention the plane because it is not the purpose of this post, but it had an empty weight of 330 Kg (700 pounds or so). The engine was a Rotax 912S.

  7. Eamon Says:

    The final sentence is somewhat ironic. This engine is based on a WWII era Junkers (German) engine. There is another developer in the UK developing a similar engine and I believe there is some dispute between the two manufacturers. Two points. 1. Diesel is Heavier but Specific fuel consumption is the MOST IMPORTANT performance marker of an aviation engine (Do I really have to elaborate?) 2. The rate of compression and number of power strokes per-cycle is what makes this engine so uh-mazing. You can talk on and on about unproven technology but the bottom line is these projects are always a matter of the capital (K)of the organization marketing it. Too bad we’re in a recession although this engine is so compact it could fit UNDER the rear seat of a compact car giving a pretty rocking rmr layout. New Beetle? pleeease

  8. rodger musso Says:

    The 100 hp Gemini engine is supercharged and this consumes ~30 % of the energy. The 125 hp Gemini model is turbocharged which only consumes 5 % of the energy(same engine and delivers 125 hp). This is a significant improvement in efficiecy. Rodger

  9. Eamon Says:

    really, a supercharger pulls 35 odd horsepower?

  10. rodger musso Says:

    The gemini engine produces ~130 hp. The supercharger consumes about 30 hp leaving a total of 100 hp available for the prop. The turbocharged version requires ~ 5 hp for the turbocharger leaving 125 hp available for the prop. The turbocharged version does have a supercharger on it but is used for starting purposes and then cuts out.

  11. Eamon Says:

    You are stating this as a matter of fact. Are you developing the engine? (I’m actually asking)I realize the engine has not intake stroke but even so, I beleive both devices improve power. The engine would not make 130hp without either, otherwise, why use them? There’s a funny idea among the older generation that turbos do not actually increase power. No device is perfectly efficient, therefore turbos and superchargers cannot improve engine performance for free but the idea that they SUBTRACT power is kind of absurd.

  12. Eamon Says:

    i guess I’m asking you to cite your sources

  13. captbilly Says:

    Actually turbochargers do not use engine power but harness waste heat in the exhaust gases. Superchargers take a suprising large amount of power, I wouldn’t find 30 HP for the supercharger on a 100HP diesel surprising at all. In the last generation of big piston engine aircrat engines the turbochargers actually harnessed enough power to not only run the supercharger but also to supply power to the propellor.

    The reason why many turbocharged gasoline engines actually use more fuel per HP than non-turboed engines is because of the requirement to decrease compression ratio to avoid detonation. In a diesel there is no potential for detonation so compression ratio and boost pressure do not have adverse effects on fuel efficiency (in fact they both increase efficiency). By the way diesels are almost always more efficient than gasoline engine because of the extremely high compression ratios that are possible/required. Diesels can also run with less cooling because hot spots in the combusion chamber don’t cause detonation. Part of the reason that gasoline engine have to run as cold as they do is to reduce detonation potential.

    In the good old days when 130 octane and sometimes even higher octane fuel was available at airports (for the military and commercial airliners) the issues with detonation were less limiting, but with 100 octane and aircooling many piston aircraft engines run as low as 7:1 compression. Compare this with a modern liquid cooled automotive engine running 11:1 (but requiring retarded ignition at high power settings for detonation suppression) and 20-25:1 on a diesel. For those of you who took thermodynamics in college you know that the theoretical efficiency of an internal combustion engine is directly related to compression ratio, so it is the diesel’s compression ratio rather than the direct injection/comprssion ignition that increases the efficiency.

    I would say that the main reason that diesel aircraft engines have not been developed is simply that essentially no new aircraft engines have been developed since WW2. We are flying with more or less the same engine we had in 1945. All the money went into developing turbine engines and piston engines were simply slightly modified versions of older engines. Theilert and a few others are finally beginning to produce significant numbers of diesel engines (their financial issues not withstanding), and hopefully we will see a significant improvement in aircraft efficiency and reliability as a result.

  14. rodger musso Says:

    Just look at a hotrod or dragster and you can see a very big,wide belt with ribs driving the supercharger,. Those 6 inch wide belts are designed to transfer many dozens of horsepower to the compressor. The total horsepower may double in a supercharged engine so a net gain of hp is achieved.

  15. Eamon Says:

    OK, I think we both understand but if a “net gain of horsepower” is acheived through forced induction, your second comment here is false because it implies that the engine is more powerful WITHOUT forced induction. I wasn’t trying to correct you so much as see if there was a unique aspect of the engine you were privy to (like a negative torque phase) that required a supercharger that did not necessarily improve power.

  16. rodger musso Says:

    The reference to a potential 130 hp engine is the theoretical power available using: pounds of fuel/horse power/hour equations.Most people are interested in the available horsepower to the prop or driveshaft as tested on a dynomometer. There are many ‘tricks’ to increase available horsepower but some are expensive or heavy or reduce engine life,or reduce reliability,etc.Turbochargers are about 6 times better(more efficient) than superchargers.If you buy a car, turbos are quite common options and I doubt if you can find a supercharger as an option on common cars.

  17. Gemini Engine Moving Toward Production - Jetwhine: Aviation Buzz and Bold Opinion Says:

    […] that more challenging. But the three-cylinder, six piston diesel/Jet A  Gemini 100 (see Gemini Diesel Engine Attracts Industry’s Eye) is gaining on that goal, says Tim Archer, president of Powerplant Development […]

  18. BillB Says:

    I am designing a 3 seater amphibian and am looking at a range of engines. But one of the criteria is bio fuel compatibility (peak oil and all of that). This engine is not tested for that yet but the scope is there for biodiesel compatibility. The Gemini looks like a very good engine partner choice beside the Rotax. If the service periods pan out then there is sufficient advantage there as well. I am quite pleased that this engine is available.

  19. Andy Keech Says:

    This engine is a contender for world record setting in Class E3 (Autoguros…see my website).
    What are engine revolutions at cruise power?

    Cnd I call you by phone?

  20. Peter Says:

    The opposed piston 2 stroke was very succesful in German aircraft, up to several thousand horse power and also US submarins and small ships in the the 1930 to 50s. Based on the inlet and exhaust port overlap I doubt you can technicaly super or turbo charge this engine compared to a 4 stroke. What the blower is used for is to replace the inlet stroke on a 4 stroke. When this happens using an engine driven blower, air is lost though the exhaust port wasting power. If a turboer powered blower is used, then this wasted blow through and exhaust energy is recovered, also gives some back pressure causing high cylinder pressures.
    Three cylinders also means power stroke overlap, this means the prop does not drive the engine as it does for 30% of the time on a 4cyl 4 stoke. So reducd harmonics, think 6 cyl 4 stroke. The Wilksch aero diesel, presently a 3 cyl unit single crank shaft design uses normal poppet valves in the head for exhaust and ports for the inlet. This mean they can close the exhaust valves with the inlet port still open and really charge the cylinder as on a 4 stroke. There really no need to ever build a 4 stroke Diesel, better suited to 2 stroke. One nice design is the Zoche air cooled 300hp 8 cyl twin row 2 stroke with a turbo and super charger, weight approx 360lbs.

  21. Eamon Says:

    I believe their is a sliding bit that covers the exhaust port towards the bottom of the stroke and allow whatever forced induc tion system to maintain pressure. I think they’re pretty precise with their scavenging. There must be some overlap but all the older engines of this type solved 90% of the problem you describe.

  22. dave Says:

    Re. remarks on turbo/superchargers;

    To my understanding the Gemini currently uses a centrifugal compressor. Conceptually, this can be taken to be the compression component (half) of a turbo-charger. It produces boosted intake air. It is therefore not a ‘blower’ as this device does not materially increase intake air pressure. A centrifugal compressor requires energy to spin it; in the current incarnation this is provided directly from the crank and the power required must be subtracted from the engine output ( as said before ). The turbo version substitutes the crank input with free exhaust energy ( also said ) and the engine output will therefore rise accordingly. However, there is low exhaust energy when the engine rpm is low and therefore insufficient air flow is produced by the turbo to scavenge the cylinders of exhaust gases ( also said ). An additional engine driven blower is typically employed at low revs and is normally de-clutched in some fashion after about 1200 rpm when the exhaust gases have enough energy to spin the turbo.

  23. Eamon Says:

    A centrifugal compressor does not materially increase intake pressure? You guys have some interesting ideas about physics. This is a fascinating conversation except that noone cites examples or sources. Superchargers produce MORE power than their unsupercharged counterparts. That’s why the Mini S has a supercharger and is faster than the regular mini. Granted a supercharger robs some power but ultimately produces more power by increasing the Pressure of the airflow into the cylinder and therefore the effective displacement of the engine. It also increases effective compression which further increases the efficiency of the engine. I do not believe this is a clutch-disengagement design either. It appears that the supercharger is always running while the turbo produces over-boost. Disesles can generall withstand far more boost than their gasoline counterparts

  24. Colin twinjet Says:

    Well guys indeed a very interesting engine project had a good deal of experience with automotive / comercial engines there are anumber of directly opposing problems encountered when chasing a panacea for aero engines weight being not the only factor.
    Car / automotive diesel vs petrol. has until recent advances in high speed diesel by german and japanese manfs been petrol for performance diesel for economy.
    Performance is not only directly linked to compression ratio or more specificly combustion chamber pressure this accounts for turbo charging super charging two stage turbo turbo into super charger
    detroit two stroke diesels have emplyed these technologies for years in truck and bus engines
    vee 16 detroit two vee eights twin turbos into holmes blowers running at 2000 rpm is music like a merlin.
    and theres the rub 2000 rpm thats about right for crank speed 2 /2500 if you hang the prop directly on the crank, prop tip speed stays below the speed of sound and avoids all the aerodynamic problems at those speeds air screws have well documented efficiencies at a proven rpm range excessive rpms not only inefficient but introduce excessive centrefugal / xcentrepetal forces weight in engineering needed to contain.
    Unfortunatly bhp output of any engine increases directly with the increase in rpm that goes for petrol or diesel F1 engines and japanese bike engines chasing 20,000 + rpm
    YES I SAID 20,000 RPM
    Thats why rotax theilet and other small fourstrokes U S. experimental builders employing ve 8 s all use 2.2 2.4 2.7 etc reduction boxes but more weight.
    Gemini i like and i had for many years sketching aluminium blocks with steel liner cylinders,
    blocks machined from aluninium billet like jabiru engines are easily possible with todays machining centres its down to expertise in computers ( programmers.) my ideas drew a great deal from my experience of the TS 3 and TS 4 engines fitted to commer trucks in the 1960s killed of when ROUTES group became part of the CHRYSLER CORPORATION.
    These little supercharged 3 & 4 cylinder oposed piston engines developed phenomenal power for their size but had an unusual pivot rocker linkage conrods to main crank.
    thats why i like the idea of two crankes geared to main gear output shaft this also provides the cxapability of selecting some reduction gearing and by experimentation the best engine output torque curve can be matched to prop speed this type of engine needs to rev to make power / torque on the downside higher revs uses more fuel whatever the throttle setting. it then becomes a matter of matching power to phase of flight ie initial take off particularly heavy full fuel maximum t.o weight and possibly flapped approach emergency situation max to weight something even the most advanced jet liners can not do without fuel jettison
    best regards to Tim Archer and his team i lookforward to siting behind a gemini in my PA 28 Cherokee let me know if you need any flight testing pilots. Col B

  25. Manousos Pattakos Says:

    Take the opposed piston engine of Gemini or Junkers or… and just turn the pistons upside-down to get the OPRE engine (Opposed piston Pulling Rod Engine).
    The pistons do not touch the liners, nor the ports or the port bridges.
    The pistons wait for the combustion to complete.
    The pistons make the scavenging.
    The second order inertia forse is opposite to combustion forse at midstroke.
    The gearing is shorter.
    The lenght and weight lesser.

  26. AirVenture Update: Gemini Diesel Engine - Jetwhine: Aviation Buzz and Bold Opinion Says:

    […] EAA AirVenture Oshkosh 2008 I wrote about Powerplant Development’s Gemini diesel engine (Gemini Diesel Engine Attracts Industry’s Eye), with an update in January 2009 (Gemini Engine Moving Toward Production). Because readers are […]

  27. AirVenture Update: Gemini Diesel Engine | RENT-A-PLANE Says:

    […] EAA AirVenture Oshkosh 2008 I wrote about Powerplant Development’s Gemini diesel engine (Gemini Diesel Engine Attracts Industry’s Eye), with an update in January 2009 (Gemini Engine Moving Toward Production). Because readers are […]

  28. Bill Izard Says:

    hello Are your engines in production yet Regard Bill Izard

  29. Richard Says:

    OK, the Rotax is pretty efficient at converting AV Gas to power, but that’s only at about 100 HP. The 914’s BSFC is a lot worse.

    The LycoSaur’s BSFC are close to twice the Rotax’s, and I have yet to see a Rotax develop more than 115 HP.

  30. Peter Says:

    All though I like this engine, I think this design by Ecomotors is simpler and more forward thinking. Pull rod opposed piston design. I believe a European Helicopter maker is considering it for a light weight chopper.

    http://thekneeslider.com/archives/2009/12/21/ecomotors-opoc-two-stroke-engines-opposed-piston-opposed-cylinder/

    Uses an electric motor/generator boosted turbocharger to remove the need for a supercharger at low speed and remove turbo lag on spool up, also acts as a generator when turbo up to speed, clever. Back in the late 1990s I think Continental did the same on a 2 stroke they designed and ran, pity they canned it. See they have just bought in the French SMA 4 stroke design, bad idea. I think 2 stroke diesels are best.

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