Turbines, Stirling Engine, Electric Motors

The Stirling engine has an energy efficiency of 67%, which is the maximum theoretical efficiency possible for any fuel-burning engine, and is far more efficient than an ordinary 2-stroke engine. The Stirling engine is also the quietest engine. Both of these features would be useful for a paramotor. But its drawback is that it’s supposed to run at a constant rate, and isn’t designed to quickly change its power output according to varying demand, which is something needed for aviation engines, including a paramotor.

Suppose you had a Stirling engine that could steadily output 11 hp of mechanical power to a propeller. Attached to that propeller shaft would be a dynamo, which siphons off 1 hp to generate electric current. That current would charge up some batteries, so that those batteries could occasionally be used to run an electric motor, which would also be attached to the propeller shaft. While the propeller would have a baseline power output of 10 hp, it would be able to able to run at higher power due to assistance from the battery-powered electric motor. The idea would be to allow your paramotor to cruise on the 10 hp supplied from the Stirling engine, while occasionally using the battery-powered electric motor to boost the power during those times when you need it (ie. “takeoff power” vs “cruising power”)

Another alternative configuration is to have the Stirling engine acting as a generator, generating 11 hp of electric power, but sending 10 hp of that electric power to an electric motor which runs the propeller. Meanwhile 1 hp of electric power would be sent to charge the batteries, which would be available to supply additional power to the electric motor when the situation required it. (again, “takeoff power” vs “cruising power”)

The first configuration would be using the Stirling engine for hybrid propulsion. The second configuration would be using the Stirling engine as a range-extender.

I’d like to have some discussion here (including with pdwhite) on whether either of these configurations might be possible/practical to implement for a paramotor.
What do you all think the challenges and obstacles would be?

I have been able to play with stirling engines quite a bit and have looked into using big stirling engines for many projects that I have dreampt up.

In theory it would be amazing to do what you are proposing, but the reality is that it wouldn’t work well. The biggest problems that plague stirling engines today is that they they weigh a lot and need a VERY big displacement for their power output. So a 10 hp stirling engine would likely weigh well over 30 pounds not to mention the crazy big radiatior that you would need to keep the cold side cool enough to have any decent efficiency. Other problems with the stirling engine include bad throttle response, they need to use helium or hydrogen as the working fluid to be efficient which comes with many additional challenges, and very little development has been made with the stirling engine.

Efficiency wise, gasoline spark ignition engines have a theoretical energy efficiency of between 56 and 61%, a paramotor engine at a cruise is somewhere in the range of 15% efficient and the world’s most efficient spark plug gasoline engines are about 35% efficient (not counting any engine with a pre-chamber or compression ignition). So just because a stirling engine may have a 67% theoretical efficient doesn’t mean we are anywhere close to it. As far as I know the most efficient stirling engine was made by nasa and it was “only” about 40% efficient.

So by the time you have a 10hp engine with a massive radiator, you will be over 60 pound if you are really luck and you would still need about an extra 40 pounds for the frame and electric system plus 15 pounds of fuel. All for a paramotor that would be more maintenance intensive, less reliable, and less powerful than a 35 pound vittorazi moster engine.

Here are a few stirling engines you can look into to understand what sort of power a stirling engine in the weight and displacement range of a 25hp vittorazi moster is producing:

This philips stirling engine only produces 300 watts of power (not even 1/2 horsepower) and I know it weighs well over 40 pounds.
This one was custom built. It is 127cc and produces 700 watts of power (almost one horsepower) and it weighs about 40 pounds

Thanks for the informed response, bob27. So is it possible to come up with another powerplant that will not impose the mass penalty of a Stirling engine?

What about a turbine? A turbine is certainly more lightweight than a piston engine, because it operates in a steady state mode. There are microturbines used in R/C model aircraft, and even in UAV drones too. For example:

Couldn’t this approach be used to improve paramotors?

Furthermore, people are even using 3D printers to make their own turbines and turbo-props:

You could either have a turbine which is optimized for a fixed power output, and use that as a turbo-electric generator. Or else you could have a turbine with variable power output (slightly less efficient), but which could be used for a turbo-prop.

What do you think about that? Is there anybody out there working on a turbine solution for paramotors?

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There are some documented working examples of paramotors with turbines out there, e.g. https://www.youtube.com/watch?v=xJqHkBfGBIE

Usually turbines qualify out for paramotors due to their high fuel consumption and inconvenient sound.

Just to give you some data I can offer first hand:
I flew 200 km / 125 miles with 15 liter / 4 gallons with a Freeride 19 reflex wing and my stock Air Conception Nitro 200 this year (check XContest.org). As far as I know no one ever got anywhere close to that efficiency using a turbine.

Hi, thank you for your great reply. But in that video you’re citing, the turbine is being used as a turbojet, whose high exhaust velocity is a complete mismatch with the travel velocity being sought, and therefore comparatively inefficient. A high-bypass turbofan might have better fuel economy, and a turbo-prop would likely have even better fuel-economy than that – which is the reason why militaries around the world are trying to come up with small turbines as propulsion systems for UAV drones.

As an alternative to the turbine, what about the rotary piston engine? The most common type of rotary piston engine is the Wankel rotary engine. It is known to produce less vibration and noise compared to reciprocating piston engines, while also having a higher power-to-weight ratio. But the Wankel rotary engine tends to have inferior compression ratio, rooted in problems maintaining its seals. However, a company called Liquid Piston has recently come out with a new design for the rotary engine featuring an inverse geometry.

The US military and DARPA have funded development of this engine for possible use in drones and auxiliary power generators.

Here’s more on the X Engine developed by Liquid Piston:

I have been interested in making a turbine engine powered paramotor just to say that I did, but turbine engine use a ton of fuel and micro turbine engines like what we need have very short rebuild intervals.

I was thinking of using this jet cat turboshaft engine as it produces 15kw power (20hp) and it only weighs 10 pounds. A 9 liter fuel tank(about 2.5 gallons) would last only 90 minutes at an idle and 16 minutes at full throttle. So flight times would be really bad.

On the rotary engine topic, a few companies have offered wankel engine paramotors before. Parajet had a 40hp cyclone paramotor powered by a rotron engine, scout made a prototype 60 hp rotary paramotor, and on an old website flat top paramotors had listed for about $10k a rotary paramotor with 50hp. The problem was that they didn’t have that much more thrust than their piston powered counterparts, but they were all heavier, consumed a lot more fuel, and were more maintenance intensive.

I will be interested to see what happens with liquid piston as they claim to have worked out some of the issues, but I first started hearing about them like 5 years ago and last I heard they still didn’t have an engine in production.

You are right @sanman, there might be more efficient thermal engine designs out there.

The practical challenge I see here is the $$$ behind it in our market. Unfortunately we cannot drive innovation in hardware design with a low 2 digit million $ world wide spend on paramotor engines a year (just guessing based on paramotor serial numbers, I have no prove for this value). E.g. EOS is building a new 4 stroke paramotor engine (“Quattro”, some youtube video about it), similar to the Bailey V5. I flew the prototype this year and it is definitely interesting, but still had some room for improvement as well. It takes years to get from a basically well known concept to a reliable production grade product.
Quoting Elon Musk: “production hell”

That said I really hope we could just buy and try suitable motor alternatives to the classical 2 stroke engines. The liquid piston engine looks really interesting, but I doubt we can get it in the 2 - 4.000 $ price range (engine only) of our 2-strokes. Besides being able to buy it small numbers, we also need to get spare parts in small charges for many years. Many manufactures will prefer the opposite: a small number of large customers, e.g. like the few big companies building military gear.

The great advantage with OpenPPG electro related parts (motors, esc, bms, battery cells…) is that we can use great parts from manufactures that create their main revenue selling these or similar sized parts to other way larger markets. That helps driving maturity up and cost down. E.g. Mad Motors is for sure making most of their revenue from selling to drone markets. With the existing know how and supply chain it is relatively easy to produce a product that is just right for us and thus affordable.

The Whitehead brothers do an awesome job testing / optimising setups and selecting the right parts. I would have loved to get my SP140 this year (just 7h left over here), but I prefer getting it well tested rather than faster.

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That JetCat turboprop looks very interesting. You’ve said its fuel consumption is bad, but their specs cite the consumption at full power. Is the relationship linear? Consider that when you land at the end of your flight, you’ll be considerably lighter than when you took off.
Since it’s essentially a turboshaft engine, what if you used it at a lower power setting to supply electricity to an electric motor?
This lower power setting (eg. 5-10 hp) could be enough for “cruising power”, and could be supplemented by battery power during those moments where you need an extra boost beyond its operational power band (ie. “takeoff power”)
That lower power setting could also be gradually recharging the battery during flight. The result would be that you don’t need to fly with much battery mass.

By knowing in advance what your flight profile will be, you could choose which battery you want to take with you on your flight. If you knew you were going to be having a very sedate/lazy flight, then you’d take a lower-capacity battery, since you won’t be doing high-power maneuvers very much. On the other hand, if you knew you were going to be doing a lot of aerobatics, then you’d take a higher-capacity battery with you, since you know you’ll be doing more high-power maneuvers more frequently.

The high price on that JetCat turbine makes me wonder if a suitable turbine could be 3D-printed more economically.

There are so many problems with using this turboshaft engine on a paramotor. First just at an idle it burns 1.5 gallons per hour and because efficiency isn’t linear it will likely burn upwards of 4 gallons per hour to make 5 hp. That is about 1/4 to 1/6 the efficiency of an “inefficient” 2 stroke engine. On top of that it burns jet fuel which is harder to obtain and still needs oil mixed in the fuel except this oil is a much more specialised and expensive. Next it needs a complete overhaul every 25 hours which they say needs to be done by a jetcat technician. The engine isn’t close to the reliability of a 2 stroke paramotor engine. Another problem is that the engine is only designed to run for about 10 minutes at a time not 2.5 hours like a normal paramotor. Also the engine is not nearly as user friendly as a typical paramotor engine. Finally it costs about twice as much as a more powerful vittorazi moster engine.

The problem with 3d printing is that it needs to be made out of metal which only metal 3d printers can handle and in the end a 3d printed engine would save little to no money. Most of the parts like the can, combustion chamber, turbine, shaft, fuel system, and everything else on the exhaust side can’t be made on a 3d printer because it would make it a lot heavier or the 3d printer can’t print with a metal that could handle the temperature for long term use and multiple hour long durations. Even the pieces that could be 3d printed would still need to be machine finished to decrease air friction to attempt to not kill what little fuel efficiency you do have which would remove almost any benefit over just making the entire part on the same cnc machine.

For me, I would want to build a turbine powered paramotor to pull out once or twice a year for an airshow or fly-in and maybe make a youtube video or two. It would purely be made for the wow factor not to take out and fly for fun. These engines are so inefficient that you can get better flight times and far better overall performance from a completely electric paramotor. So I personally see no purpose in making a hybrid turbine-electric paramotor.

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Some wingsuit-flyer collaborated with BMW, which came up with an electric dual-turbofan which operates at 20 HP for 5 minutes (enough for a wingsuit flight):

The device weighs 26 lbs (12 kg)

I saw that a bit back. First of all that is electric and uses what we typically call an EDF or electric ducted fan so it is not a turbofan.

As a side note, I really wish they would have used twin 215mm EDF’s. That could have doubled their thrust and they could have done far more entertaining stunts.

Bob27, have you heard of SPCCI technology, branded by Mazda as SkyActiv X?

They’re claiming 30% increase in fuel efficiency and a reduction in noise. I assume it can be done for 2-stroke engines as well, in principle.

I have heard a bit about it. The biggest thing is that those engines can change a crazy amount of parameters including compression ratio, fuel mixture, valve timing, spark ignition timing, oxygen percentage in the intake, intake air temperature, intake boost pressure, and so much more. To control the intake temperature and oxygen percentage, some exhaust is cooled and some is not so they can recirculate it in controlled ratios. They also have crazy amounts of sensors including pressure sensors in the cylinder, detonation sensors, intake sensors for oxygen content, barometric pressure, flow rate, and temperature, exhaust sensors for temperature and oxygen content, and more. So theoretically you might be able to do something like this with a paramotor engine, but the weight penalties and the complexity would be very significant. At the same time I believe you would also need to go with something like a free valve 4 stroke to get that much control over everything in a ~250cc size engine.

Here is one idea I have had for a paramotor engine.

It’s an old full scale drawing I made to get some ideas down. A running version would be very different.

My design is based off a combination of a Junkers Jumo 205, Achates 2.7 liter engine, and a few different paramotor engines that I have had experience with.

The first and most unique part is that it has an opposed piston design. Opposed piston engines typically have an efficiency increase over comparable 4 strokes by 10-30% simply because of the opposed piston design. They also have more horsepower than a 4 stroke and typically more horsepower than a 2 stroke at a given rpm. These engines typically also have very wide power band and extremely high torque figures. For example the Achates engine produces 650nm or 480 foot pounds of torque and that is from a 2.7 liter gasoline engine! It also produces 360 horsepower at only 3600rpm. One interesting thing about this engine is that I can theoretically make comparable power to a vittorazi with a similar size engine, but with far lower rpm’s which would hopefully improve reliability, combustion efficiency, and the overall lifespan of the engine.

This engine should be very efficient for a paramotor engine. For perspective, the standard vittorazi with stock carb settings is at about 15% brake thermal efficiency (BTE) at a cruise. The vittorazi silent is at about 18% BTE, and the EOS 4 stroke is right around 21% BTE. I believe that a carbureted version of this engine should see something in the range of 25% BTE for about 4.5 hours of flight with a 2.5 gallon fuel tank. Alternate versions with electronic fuel injection and a few other tricks just might see higher than 30% BTE for peak flight times in the 5.5 hour range with a 2.5 gallon fuel tank.

This engine will have many difficult challenges that come with it. Possible the biggest will be the gearbox that will be required to connect the two crank shafts together. This will mainly be challenging as weight is one of the biggest issues that paramotor engines are designed to overcome and most gearboxes on opposed piston engines are very heavy. The next big challenge will be the oil system. At this point I would like to use a dry sump oil system as I don’t believe a wet sup system would appreciate the height difference that could occur between the two crank cases during maneuvers and on the ground. Another issue is scavenging the engine. These opposed piston engines work on a 2 stroke cycle so they can’t replace the fuel/air like a 4 stroke and they can’t use the crank case like a typical 2 stroke because this engine used uniflow scavenging and that means you could only use one crank case which wouldn’t work. So you need to use a supercharger and a way of regulating back pressure which in my designs has always been a valve in the exhaust to allow for high torque down low without restricting top end power.

I would go into more detail on some of what I want to do, but I would legitimately like to try and develop this engine and maybe put it in production if it somehow manages it work well let alone at all.

Theoretical advantages of this engine:

  • very low fuel consumption
  • low noise
  • nearly no vibration
  • longer lasting piston rings
  • no oil mixing
  • high torque
  • wide power band
  • lower emissions
  • can be modified to have no power loss at high elevations
  • spark plugs last longer
  • lower operating temperatures
  • overall longer lasting engine

These are some rough specs I would like to see from a carbureted model, but they could be miles off:

  • Displacement --165cc
  • Power – 30hp @ 6000rpm
  • Weight – 30 lbs
  • fuel consumption – as low as 1.8 liters/hour

I know as well as anyone else that things never seem to work as well in the real world as they work in theory, but I still want to try developing this engine that I have been fantasizing over for a long time and just maybe it would work well.

Hi, thanks for that Bob27. Yes, I’ve heard of opposed piston engines and some of their benefits. So this would be gas-powered, since you mention sparkplugs. I’d read somewhere that an opposed piston design can burn diesel quite efficiently too, with better emissions.

Does anyone think supercapacitors could be useful for electric motors? Their energy density is less than batteries, but they can dump out lots of power when it’s needed, and allow endless recharge cycles. They might make for a useful buffer reservoir in a hybrid setup.

I’m with you, sanman, regarding supercapacitors that might enable fast response to load changes such as at takeoff and cruise-climb, in a hybrid system that produces a lower output of continuous power. By themselves, however, I’m not convinced they could replace advanced secondary (rechargeable) batteries. But at one trade show within the last year or so one manufacturer was touting a longer-life replacement or extender for batteries, using liquid-hydrogen fuel cells. I haven’t followed up on this research, because it requires an entire hydrogen cycle system to handle the production, storage and transfer of the fuel, as well as a suitably-sized portable package of cells to power electrical motor(s) for a miniature aircraft such as a paramotor. Electrics, of course, are much more efficient in operation than any combustion engine, as are fuel cells – though there are energy losses in the hydrogen production that reduce the efficiency of the overall system.

larger EDFs were also considered. If you fly without propulsion, you have an extreme braking effect that would make wingsiut flying dangerous. the video system is the best compromise between airworthiness and technical implementation. with kerosene turbines there were big problems with experiments of other wingsuit pilots when the thrust was too much on the body. therefore, many only use fixed wings to get the thrust axis more into the center. If the thrust axis is too far outside, a moment occurs that has a negative effect. that is, the pilot pulls out of the ideal flight line. It’s not that dramatic when it comes to climbing. However, as soon as there is a sideways or negative flight on the back, it is difficult for the pilot to counteract.

bob27, here’s an example I found of a homemade opposed piston engine:

Also, had you heard about this free piston engine?

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The biggest thing the opposed piston design gives is the increased expansion ratio. It’s what the Atkinson cycle engine in the Prius “simulates” with the weird valve timing. The latest Prius engine has a peak thermal efficiency of 40%.

As far as a gearbox, for this level of power, designing around a timing belt would probably be the better option. Also at this size it would probably be best to not use pressure lubrication but go with bearing cranks. Take a look at the design of a Honda Supercub engine.

It doesn’t necessarily need to be designed as a 2 stroke. You could have valving at middle and operate as a 4 stroke, but obviously you’d be giving up some power density.

“What about a turbine?”

Garbage at small scale…utter disappointment. They suffer from efficiency losses at extreme rates as they downsize and the crossover point between a reciprocating engine and the turbine is long gone by the PPG level. The big turbofans on airlines run at less than 10,000 RPM, by the time you’ve downsized to a PT-6 you’re up to roughly 30,000 RPM. Below the size of the PT-6 and you’ll start losing efficiency fast…the smallest PT-6 core develops around 600 HP. By the time you get to the size of the PBS TJ100 used on the Subsonex, it has excellent power-to weight, but just demolishes fuel. The hobby sized turbines are hitting around 70,000 - 120,000 RPM.

I always thought that if someone came up with a drop-in, power turbine shaft, replacement for the roughly 200-400HP range of GA aircraft they would demolish the Continental/Lycoming market but at that size the TBO and fuel economy benefits are pretty well gone.

Reciprocating engines seem to start losing efficiency in earnest below around 500cc/cylinder in a roughly square design and drop like a rock <100cc/cylinder. It’s often off-set by the engine running in a more efficient part of its range in the application it’s in.

If you want some other fun engine designs, take a look at the Swashplate and Revolver Cam engines.