A novel rotary engine design.

Peter Twissell · Jul 27, 2024

Having built the Whittle V8, I have developed a taste for scale (ish) engines for flying models.
I am attracted to early aircraft, so inevitably a rotary will happen sooner or later.
I have looked at designs for 'true' scale rotaries, such as the Bently BR2 and the Gnome. However, I would like to build something at a smaller scale, as I don't have the space or inclination for 1/4 scale model aircraft.
Thus, I embarked on a project to design a somewhat simplified rotary with the intention that it should be practical for use in a flying model.

My design looks like a Gnome 9N monosoupape. It has a displacement of 28.6cc and is 5" (125mm) in diameter.
The original Gnome engine was 4 stroke with induction through complex valves in the pistons. Later versions used piston ported induction through a series of holes in the base of the barrels. Both variants had no effective throttle control, the engine being slowed by selectively cutting ignition to some cylinders. None of this lends itself to use in a flying model, so I set about creating a more conventional system within the GNome layout.

What I have come up with is a uniflow 2 stroke. The single valve in the cylinder head controls exhaust exit. Induction to the cylinder is controlled by piston porting, using transfer ports similar to conventional 2 strokes.
The rotary arrangement does not produce any nett crankcase displacement to draw in mixture and compress it for transfer to the cylinders, so I had to create a mechanism for that function.

The design uses short connecting rods and crosshead guides similar to steam engines. The crosshead connects to the piston by a round shaft which passes through a bush in a plate which isolates the lower part of the barrel from the crankcase space. Thus, when the piston rises and falls, the space between the underside of the piston and the bush plate acts as the induction pump. Admission of fuel / air mixture to this pump chamber is controlled by a port in the hollow crankshaft, as seen in most model aircraft 2 stroke engines.

The carburettor will be fitted to the rear end of the hollow crankshaft, which is of course stationary and will also carry the engine mount to the airframe.

Peter Twissell · Jul 27, 2024

A section through one cylinder shows the induction chamber below the piston and the port from the crankshaft.
The exhaust valve is operated by a cam which is fixed to the forward part of the crankshaft.
The piston needs no skirt, as it is supported by the crosshead. It need only be thick enough to support a piston ring.

The 'big end' arrangement of the rods is currently a close copy of the original Gnome 'slipper' design. This allows the apparently very short rods to have a greater effective length, as all of them articulate around the crankpin axis. With a master and slave rod arrangement, the slave rods articulate around the axes of the pins in the master rod, which also swings to create very large angles between slave rods and cylinder axes.

Peter Twissell · Jul 27, 2024

It is my intention to first build a single cylinder non-rotary prototype with a conventional big end arrangement.
This approach will allow me to develop the 'top end' design before building the whole engine.

petertha · Jul 27, 2024

That's very cool. Can you show a front view section of the rod assembly? I have conventional radial master rod / link rod assembly on the brain & cant visualize how the pistons are making the equivalent TDC BDC motion with cylinders rotating around.

Peter Twissell · Jul 27, 2024

Hi Petertha,

This is what an individual rod looks like at the moment. The whole set of rods will be made as a turned 'disc' to get the curvature of the 'foot' to properly match the crankpin.

When assembled to the crankpin, two rings fit over the 'feet' of the rods - for clarity, I have not shown the front ring in this image, but the rear ring is in place.
The 'foot' of the rod looks relatively delicate at this stage, but that has more to do with the rest of the rod being somewhat oversize. I tend to design components large and clunky, then slim them down later.

Peter Twissell · Jul 27, 2024

a41capt · Jul 28, 2024

Very interesting “work around” to enable a 2 stroke non-crankcase charged type of system. Will you run into any type of charge cutoff backflow with the rotary valving, ie. Hollow crankshaft to porting cutoff? Also, will there be multiple cylinder transfer ports to speed the flow of air/fuel charge to the cylinder on intake?

Finally, I assume this will be a standard electric ignition circuit, although I’d guess with a proper power transfer ring system, glow plugs could be used.

I love the design and I’m looking forward to seeing further work!

John W

Peter Twissell · Jul 28, 2024

Hi John,
Not sure what you mean by charge cutoff.
The port in the crankshaft will control the timing of intake to the 'pump chamber', opening the intake passage after BDC when the transfer ports close and closing off the intake passage as the piston arrives at TDC.

I have four transfer ports in each cylinder.

At this point I am planning glow ignition, with a slip ring connection. Glow plugs are reduced to 3/16" thread. Not sure I can make effective spark plugs at that size.

petertha · Jul 28, 2024

I initially assumed the piston ran inside a ported liner & the pockets were relieved in a separate outer cylinder so the piston was always running on the straight liner or port webs like a regular 2S. But it looks like you are counting on the lower pink part for accurate centering during when the piston is completely below the ports unsupported by the cylinder wall, or am I misinterpreting? So that negates a compression ring for sure. Ring-less model glow engines typically have a tapered liner & cylindrical piston, maybe that's part of design?

Peter Twissell · Jul 28, 2024

The piston runs in a cylinder which has 4 transfer ports like a conventional 2 stroke. The piston is still supported between the transfer ports, although that is not clear because of the way the section is cut across the ports.
The piston will have a ring, with a locating dowel to prevent the ends of the ring from getting into the ports.

The pink part is the crosshead and provides lateral support against the load from the rod angle. The crosshead has a relief angle on each side to prevent interference with it's neighbours. Again, the shape of the crosshead is not clear due to the section location.

a41capt · Jul 28, 2024

Peter Twissell said:
Hi John,
Not sure what you mean by charge cutoff.
The port in the crankshaft will control the timing of intake to the 'pump chamber', opening the intake passage after BDC when the transfer ports close and closing off the intake passage as the piston arrives at TDC.

I have four transfer ports in each cylinder.

At this point I am planning glow ignition, with a slip ring connection. Glow plugs are reduced to 3/16" thread. Not sure I can make effective spark plugs at that size.

Excellent, 4 transfer ports should get the charge up into the cylinder adequately, especially since you're adding an active poppet valve. On typical motorcycle piston port engines, unless they're very low revving, more is better! the addition of that active exhaust valve will also negate the use of an expansion chamber to add the needed back pressure wave in a piston port exhaust.

For maximum "squish" are you considering a domed and center dished type of 2-stroke piston?

I agree that the glow setup will be a more effective ignition system. Hopefully it will also be a simpler system than a timed ignition!

My concern with the rotary valve port system is strictly based on the duration of the intake cycle and availability of timing change, nothing more. When I used to build 2 stroke racing engines for my motorcycles, my experimentation in rotary valve timing cost me many, many rotary valves (as a teen age engineer, and with very little technical training/knowledge, trial and error was king!) before I achieved what I was looking for.

I look forward to seeing your project as it progresses, as a 2 stroke fan from way back, i think it's REAL cool!

John W

Peter Twissell · Jul 29, 2024

Hi John,

At this stage, I am going with a flat topped piston and concical combustion chamber.

I'm not looking to optimise performance yet. I anticipate that the engine will need to run at relatively low RPM in order to limit the radial acceleration forces on the components and the gyroscopic effects when installed in a model aircraft. At almost 30cc it has plenty of potential for power, given that a model aircraft to this scale (approx 1/8) would normally fly with a 7.5cc engine.

It may be that it is not practical to fit a large enough prop to produce the required thrust at low RPM, or that the torque reaction on the airframe is excessive. If this is the case, I have a design for a planetary gear drive to speed up the prop.

My priority now is ease of manufacture. You may have noticed that the design has a one piece head and barrel. Compression is at 6:1 for glow ignition, achieved with a simple combustion chamber and no squish.

kf2qd · Jul 29, 2024

A uniflow is going to need a turbo charger. The crankcase is going to be close to atmospheric pressure so you need something to push the air/fuel/oil mix through the engine.

Peter Twissell · Jul 30, 2024

kf2, I have described the induction system in some detail above.

Basil · Jul 30, 2024

A different take on this.

Peter Twissell · Jul 30, 2024

Yes, I saw that when I was researching for my design. It seems a pity that they went to all the trouble to do that, but didn't make it a uniflow. The designer focuses heavily on not burning 2 stroke oil, but makes no mention of how the piston and bush are lubricated.

Peter Twissell · Jul 31, 2024

A little more on the porting and valve timing:

As with the original Gnome engines, the exhaust valve opens directly to atmosphere. With the engine spinning, the airflow velocity past the exhaust port creates low pressure and effectively extracts the exhaust from the cylinder, allowing for full scavenging.

It will take some experimentation to find the optimum exhaust valve closing point to allow full scvenging without throwing some of the induction charge straight out of the exhaust port.

Fortunately, the rotary design means that the exhaust cam is stationary when the engine is running. With a temporary hollow prop driver, it will be possible to arrange a means to adjust the exhaust cam position while the engine is running. I may even make a development cam in two 'slices' so I can maintain the same exhaust valve opening timing while adjusting the exhaust valve closing timing.

Inlet timing is controlled by the height of the transfer ports. At this point, I have made the transfer ports just high enough to give them an open area at BDC equivalent to their cross section area. While this gives an intake duration shorter than many conventional 2 stroke engines, it may be adequate in this design, as I have the advantages of both forced the scavenging mentioned above and a greater volumetric efficiency than crankcase induction, due to the much reduced 'dead' volume (the additional volume in the induction pump chamber which is not swept by the piston).

On top of all that, there will be some degree of 'supercharging' effect due to the fact that the spinning engine exerts a centrifugal force on the induction charge, all the way from the crankshaft port to the cylinder top.

The more I look into the rotary engine design, the more I have come to appreciate the genius of the original designers, Louis and Laurent Seguin.

I do foresee a couple of potential issues with the use of my engine in a model:

Firstly, the power output, even at the low RPM at which I intend to run it, may be excessive for a model to the same scale as the engine. As mentioned before, this engine is around four times the displacement of a conventional 2 stroke which would fly the same size model.

Secondly, with the exhaust open directly to atmosphere, there is no scope for silencing.

I may be able to address both these issues by artificially restricting the engines output. Reducing the transfer port size will limit power and timing the exhaust valve for late and/or slow opening should reduce noise.

Apologies to John W - I know this must seem like sacrelidge to a 2 stroke tuner!

It may well be an interesting excercise to see how much power I can get out of the design, but ultimately I want to be able to fly it in a scale model.

a41capt · Jul 31, 2024

Peter Twissell said:
A little more on the porting and valve timing:

As with the original Gnome engines, the exhaust valve opens directly to atmosphere. With the engine spinning, the airflow velocity past the exhaust port creates low pressure and effectively extracts the exhaust from the cylinder, allowing for full scavenging.

It will take some experimentation to find the optimum exhaust valve closing point to allow full scvenging without throwing some of the induction charge straight out of the exhaust port.

Fortunately, the rotary design means that the exhaust cam is stationary when the engine is running. With a temporary hollow prop driver, it will be possible to arrange a means to adjust the exhaust cam position while the engine is running. I may even make a development cam in two 'slices' so I can maintain the same exhaust valve opening timing while adjusting the exhaust valve closing timing.

Inlet timing is controlled by the height of the transfer ports. At this point, I have made the transfer ports just high enough to give them an open area at BDC equivalent to their cross section area. While this gives an intake duration shorter than many conventional 2 stroke engines, it may be adequate in this design, as I have the advantages of both forced the scavenging mentioned above and a greater volumetric efficiency than crankcase induction, due to the much reduced 'dead' volume (the additional volume in the induction pump chamber which is not swept by the piston).

On top of all that, there will be some degree of 'supercharging' effect due to the fact that the spinning engine exerts a centrifugal force on the induction charge, all the way from the crankshaft port to the cylinder top.

The more I look into the rotary engine design, the more I have come to appreciate the genius of the original designers, Louis and Laurent Seguin.

I do foresee a couple of potential issues with the use of my engine in a model:

Firstly, the power output, even at the low RPM at which I intend to run it, may be excessive for a model to the same scale as the engine. As mentioned before, this engine is around four times the displacement of a conventional 2 stroke which would fly the same size model.

Secondly, with the exhaust open directly to atmosphere, there is no scope for silencing.

I may be able to address both these issues by artificially restricting the engines output. Reducing the transfer port size will limit power and timing the exhaust valve for late and/or slow opening should reduce noise.

Apologies to John W - I know this must seem like sacrelidge to a 2 stroke tuner!

It may well be an interesting excercise to see how much power I can get out of the design, but ultimately I want to be able to fly it in a scale model.

No apologies necessary, as stated earlier, I love the experimentation aspect of engine design, and especially on something as unique as a gnome concept!

I can’t wait to see what you’ll come up with and I’m looking forward to hearing you fire it up.

John W

Basil · Aug 1, 2024

Peter Twissell said:
A little more on the porting and valve timing:

As with the original Gnome engines, the exhaust valve opens directly to atmosphere. With the engine spinning, the airflow velocity past the exhaust port creates low pressure and effectively extracts the exhaust from the cylinder, allowing for full scavenging.

It will take some experimentation to find the optimum exhaust valve closing point to allow full scvenging without throwing some of the induction charge straight out of the exhaust port.

Fortunately, the rotary design means that the exhaust cam is stationary when the engine is running. With a temporary hollow prop driver, it will be possible to arrange a means to adjust the exhaust cam position while the engine is running. I may even make a development cam in two 'slices' so I can maintain the same exhaust valve opening timing while adjusting the exhaust valve closing timing.

Inlet timing is controlled by the height of the transfer ports. At this point, I have made the transfer ports just high enough to give them an open area at BDC equivalent to their cross section area. While this gives an intake duration shorter than many conventional 2 stroke engines, it may be adequate in this design, as I have the advantages of both forced the scavenging mentioned above and a greater volumetric efficiency than crankcase induction, due to the much reduced 'dead' volume (the additional volume in the induction pump chamber which is not swept by the piston).

On top of all that, there will be some degree of 'supercharging' effect due to the fact that the spinning engine exerts a centrifugal force on the induction charge, all the way from the crankshaft port to the cylinder top.

The more I look into the rotary engine design, the more I have come to appreciate the genius of the original designers, Louis and Laurent Seguin.

I do foresee a couple of potential issues with the use of my engine in a model:

Firstly, the power output, even at the low RPM at which I intend to run it, may be excessive for a model to the same scale as the engine. As mentioned before, this engine is around four times the displacement of a conventional 2 stroke which would fly the same size model.

Secondly, with the exhaust open directly to atmosphere, there is no scope for silencing.

I may be able to address both these issues by artificially restricting the engines output. Reducing the transfer port size will limit power and timing the exhaust valve for late and/or slow opening should reduce noise.

Apologies to John W - I know this must seem like sacrelidge to a 2 stroke tuner!

It may well be an interesting excercise to see how much power I can get out of the design, but ultimately I want to be able to fly it in a scale model.

Looking forward to this build. Good luck.

The Ignoble Troll · Aug 2, 2024

I must be missing something, with no forced induction to the crankcase, what keeps the air mix from bouncing from retreating pistons to extending pistons?

Is their valving to lock the charge air under the retreating piston so that it is forced into the cylinder?

If not, wouldn't it just flow between cylinders, staying in the crank, producing no proper induction?

Sorry, I'm not particularly clever but radials fascinate me.

A novel rotary engine design.

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