Unusual variations on two-stroke head design.

[Owen_N]

"These engines are no good to me unless I can build a contra-rotating prop drive, for a vertical takeoff aircraft."

You do not need a contra rotating prop for a VTOL prop driven model aircraft, just go on the web and see the 1000's of youngsters prop hanging electric RC models .

I would suggest that you stand less chance of VTOL with a contra rotating prop just due to the weight complexities and complications in the system.

If I built a tailsitter, then 2x 250 watt electric motors running 7 inch props would be sufficient.

Using these largish petrol engines, the prop torque is quite high.
Possibly a "Monocopter" layout would work, with large deflector vanes.
I have seen it done in electric.

Altitude control would be problematic, as the petrol engine does not provide the rpm control
that electrics are quite good at.
Additional fast-acting thrust deflectors would be needed.

An engine under 2 kgs has over 10 kgs static thrust, so there is a lot of reserve.

Possibly a reversing system with some acetyl gears would be light enough- maybe another 2 kgs.

I have one designed, but I need a better lathe and possibly a mill to make it.

For satisfactory lift performance, at least 1.5 to one thrust to weight is needed to gain altitude.
1.8:1 would be better.
Contra-rotation is more efficient at thrust to power ratio so that makes up for some weight gain.

The front support bearing runs at 12-13,000 rpm, and possibly needs a circulating oil system to control heat build up.
I would also want to fit an electric start, at about 500g .

The whole assembly extends about 160mm out from the normal prop position, so the gear case needs a lot of bracing back to the engine.

I have an electric start system on hand, but it is not a simple fit to the engine, and it gets in the way of the main case for the contra-rotating gearbox, and makes a larger diameter casing necessary.
There is a large steel gear that sits behind the prop drive flange, on a one-way freewheel.
The gear has large holes in it and is quite narrow, so it is not that heavy, though.

You don't want to handle an engine setup with a vertical axis with 20 inch props while it is running!

 

[Owen_N]

I’ve done giant scale Rc models for years some very high performance some just for boring holes in the sky.

spark plug holes can generally be repaired with heli coils. Mc master Carr has then many auto parts stores have then unless you are into radically different sizes. I have Tig welded some heads years ago but unless it’s not replaceable it’s not a cheap or easy fix. It maybe that it’s just easier to make a new custom head with your mods built into the design.
Most of my larger gas motors ran about 7k rpm adjusting a little by prop size generally the top is running at a stalled speed so it takes a lot of power to get much rpm change on the ground other than pitch or diameter change. My smaller glow engine ran a lot faster but much smaller props. But the same factor applied. It took a lot of nitro to get much rpm change on the ground. In the air the props began to act more like a screw snd you could get some real speed or pulling power.
mid bevinterested in seeing your counter rotating device. I’ve seen it done but not for a long time. There have been a number of constant speed devices planned but I’ve not seen one that really worked as the real ones do.
byron

I couldn't find heli coils in the 10mm size.

The nearest was 10mm to 14mm brass adapters, but it has to be bought overseas , with long lead times.

I also need a suitable tap. I have ordered the parts, but the longest lead time is mid april.

In the mean time, my 11.5mm bush, swaged to 12.7mm (half inch, ) and pinned out to 12.7mm, could be later repaired if needed.

A problem with this is that there is only about 13mm diameter clearance in the chamber, and a tap will leave cuts in the chamber wall.
It may work OK.
I should swage around the pins, to make sure they don't drop into the cylinder.

 

[kf2qd]

Schnuerle porting with a hemispherical head with squish band all the way around. You adjust the size of the hemispherical section and the squish band to fine tune how it runs. Schnuerle porting means you use a flat top piston and simplify the design. The more complex, the more variables, the more variables, the less chance of control.

 

[Owen_N]

Schnuerle porting with a hemispherical head with squish band all the way around. You adjust the size of the hemispherical section and the squish band to fine tune how it runs. Schnuerle porting means you use a flat top piston and simplify the design. The more complex, the more variables, the more variables, the less chance of control.

1) an offset chamber seems to work OK for these lower revving engines.

2) there is a 2mm dome across 45mm bore. on the piston.

3) there are a lot of variations on Schnuerle loop layouts. Very wide inlet curves on the side transfers seem to be beneficial.
I am limited by the standard model aeroplane case layout, and close-in base studs.

For high revving engines, wrapping the exhaust ports around the sides with bridges, and dropping the front side transfers down, seems to work.

Look at the "2Stroke stuffing" engine, YouTube. It makes about 20 HP at the crank at 17500 rpm, 50cc, reasonably wide power band.

This is as good as any GP engines have ever done., and the port layout seems biased to rear ports and not so much loop.

My mods are much milder than that, and I am aiming at a much lower rpm level.

Later mods are to examine the effects of raised transfers and side reed valves, as well as a controlled compression release bleed for low rpm
engine loading. These may well have no effect, or just damage peak torque at the rpm range of interest.
I have to try it to see. Transfer upper edge profile can be adjusted back with J_B weld, as long as I make sure the piston ring
doesn't hang up in the port.
So far, an angled top and upper widening at the rear seems to work for transfers.
The bad head was causing problems, so I have changed that.

Plug height or angle away from the piston top seems to be fairly critical. - I guess there is a quenching effect that makes starting difficult.

 

[Owen_N]

I was looking at getting a race-capable 2-stroke 125 motorcycle engine, and building on that, but they seem to a bit like hens teeth-hard to find.

Not many disc valve water-cooled 2-stroke 125cc motorcycle engines around.

The idea of building something like Mr "2stroke-stuffing" (M2SS) seems a bit weird.
At least start with a motorcycle transmission?

125 seems a better starting point than a 50cc. - It gives better performance, but is still a small engine.
It is also capable of up to 55 hp, which is better than a lot of mid-sized motorbikes.

"over-the-counter" engines were more like 35 hp, though.

It takes some extreme tuning to get to 55 hp, and lots of replacement parts.

The bearings don't last that long, and you need a top end rebuild every couple of race meetings?

Sounds like drop-in dry sleeves might be a good idea??
- and a ready source of pistons and rings.

I was wondering about SG iron as a barrel material.
This stuff machines like mild steel- wouldn't a steel liner be just as good?

I would be iffy about running aluminium pistons on plain steel, if the built-up edge I get on tool steel is any indication.

Those gold tungsten tool tips seem to be better for non-stick.
Is that a kind of electroplated titanium compound on them?
That would be good on a cylinder liner.
I will google that.

If you want it to rev to 18,000 rpm, the initial drive ratio could be tricky.- A centrifugal starter clutch could be good there.
otherwise you could need a large gear spread.

You also don't want to slip a manual clutch that much.
M2SS's vario drive is pretty much 2-speed as he uses it on the dyno.

With road use it could give better better ratio variations.

Starting with a road transmission also would give problems with the clutch speed.

You want a huge step down ratio to the clutch.
This is difficult to fit on a 125-sized clutch. A 500 GP clutch gear ring is a lot bigger.
- maybe a 2-stage gear reduction?

Also, the engine would be pretty much unusable below 8000 or so rpm, as you would be fighting the pipe,
and possibly blowing a lot of exhaust gas back into the cylinder.

These engines don't seem to start pulling smoothly until they have been loaded up- maybe why the riders like
constantly "zooming" the engine up , to clear out any misplaced exhaust gas.
It is amazing they don't backfire into the crankcase more-not good for reed valve engines!

 

[Bentwings]

If I built a tailsitter, then 2x 250 watt electric motors running 7 inch props would be sufficient.

Using these largish petrol engines, the prop torque is quite high.
Possibly a "Monocopter" layout would work, with large deflector vanes.
I have seen it done in electric.

Altitude control would be problematic, as the petrol engine does not provide the rpm control
that electrics are quite good at.
Additional fast-acting thrust deflectors would be needed.

An engine under 2 kgs has over 10 kgs static thrust, so there is a lot of reserve.

Possibly a reversing system with some acetyl gears would be light enough- maybe another 2 kgs.

I have one designed, but I need a better lathe and possibly a mill to make it.

For satisfactory lift performance, at least 1.5 to one thrust to weight is needed to gain altitude.
1.8:1 would be better.
Contra-rotation is more efficient at thrust to power ratio so that makes up for some weight gain.

The front support bearing runs at 12-13,000 rpm, and possibly needs a circulating oil system to control heat build up.
I would also want to fit an electric start, at about 500g .

The whole assembly extends about 160mm out from the normal prop position, so the gear case needs a lot of bracing back to the engine.

I have an electric start system on hand, but it is not a simple fit to the engine, and it gets in the way of the main case for the contra-rotating gearbox, and makes a larger diameter casing necessary.
There is a large steel gear that sits behind the prop drive flange, on a one-way freewheel.
The gear has large holes in it and is quite narrow, so it is not that heavy, though.

You don't want to handle an engine setup with a vertical axis with 20 inch props while it is running!

if you can get JB to do the job great . Tell us about it , pictures.
You might look at a site called project farm . He tests every thing . You may even get a direct response from him I know he has tested all sorts of glues . His tests are worth watching , he tells what’s happening and shows test results compare results against claims very well done videos

byron

 

[Owen_N]

if you can get JB to do the job great . Tell us about it , pictures.
You might look at a site called project farm . He tests every thing . You may even get a direct response from him I know he has tested all sorts of glues . His tests are worth watching , he tells what’s happening and shows test results compare results against claims very well done videos

byron

(233) The Ultimate Epoxy Competition--Which Epoxy is the Best? - YouTube
J-B weld stands up OK in this test.
I quite like the other one, too -Devcon-, but the smaller quantities are not available in NZ.

They claim J-B is good up to 330 degrees F , or 165 degrees C.

I find air cooled cylinder head temperatures get above this, and J-B becomes soft and powdery.

I presume the particular item you want to know about is the piston inserts.

On 4-stroke engines, I have seem nylon and PTFE buttons used in the thrust faces , and nylon is good up to 150 degrees C continuous,
or 175 degrees C intermittent.
I expect the edge right next to the ring land will soften, but most of the J-B should be good.

I have used it inside the combustion chamber, right out at the edge of the squish area, to fill an accidental cut, and it slowly erodes.
Closer in to the sparkplug, it vanishes quite quickly.


I will post before run and after run photos.
<edit>
J-B original formula -sustained 500 deg F, 3960 psi- 27 MPa. Ultimate.
This is higher strength than most epoxies, and may be under special conditions, such as a tensile machined test piece.
500 F = 260 deg C.
I would suspect that 180 deg C is more realistic, according to surface temperature infra-red readings.

Devcon is only rated to 120 degrees C, as are most of the others.

 

[Owen_N]

The TP-60 mod engine lives again!

the poor head chamber shape was the problem.

It maxes around 5500 -5800 rpm with my test prop, fast run-ups, and no sustained running.

A change in behaviour from the original porting is a jump from idle up to around 4000 rpm with a very small throttle advance.
I suspect draw-through from low pressure in the exhaust.

There is about a 1.5mm lift in the exhaust port, and bigger, re-shaped transfers- more trapezoidal-shaped
to help stop ring catching.
Blowdown offset between exhaust and transfer ports is slightly smaller.

These are similar results to the 70cc twin, once run in, so this can be my benchmark for ongoing mods.

No obvious improvements compared to standard porting.

 

[Steamchick]

Hi Owen,
I would be interested to see a porting diagram (360degree timing would be OK) to understand these changes better. - But I understand if you don't want to publish "intellectual property" on this "theft ridden" means of communicating! (Interweb: www = "What's Worth-it Walks!").
My only real experience of re-porting 2-strokes was to raise the (theoretical) power of my 250 MZ by 8Hp. Made a huge difference to top end on the motorway, but I didn't notice any change to "town-riding" acceleration torque. The bike was very good at mid-range revs before and after. Before I had the mod (a tuning company's design) Itook a transfer of the poerting, and again afterwards, and the changes were quite small and subtle. But included a very small transfer port through the piston, just beneath the rings, opposite to the exhaust port, directing a 3rd jet of intake gases up into the combustion chamber.
I also owned a "stage 3 race tuned" LC350 Yamaha. Good for over 12500rpm... and over 125mph ... but had 17:1 compression measured at tick-over. It eroded the pistons to seizure if I ran at 55mph for more than a moment (a big flat spot that entailed a gear change to get past). I was told I needed 105 octane fuel. But otherwise trouble free ("it went like stink!"). When I reduced the compression to ~15:1 with addition of a second head gasket, the problem disappeared using pump 98 octane fuel. That engine had more than double the transfer port CSA, exhaust port "squared-out" compared to original. The exhaust port timing was 1~2mm higher (top edge) that original, or so I was told by my local tuning shop, who told me that the engine had been modified (by one of their track rivals) for Production racing... and the "stage 3" tuning should not be on the road! It was as quick as the 500cc 4-cylinder road bike (race replica) that Yamaha made later.
Thanks
K2

 

[Owen_N]

Hi Owen,
I would be interested to see a porting diagram (360degree timing would be OK) to understand these changes better. - But I understand if you don't want to publish "intellectual property" on this "theft ridden" means of communicating! (Interweb: www = "What's Worth-it Walks!").
My only real experience of re-porting 2-strokes was to raise the (theoretical) power of my 250 MZ by 8Hp. Made a huge difference to top end on the motorway, but I didn't notice any change to "town-riding" acceleration torque. The bike was very good at mid-range revs before and after. Before I had the mod (a tuning company's design) Itook a transfer of the poerting, and again afterwards, and the changes were quite small and subtle. But included a very small transfer port through the piston, just beneath the rings, opposite to the exhaust port, directing a 3rd jet of intake gases up into the combustion chamber.
I also owned a "stage 3 race tuned" LC350 Yamaha. Good for over 12500rpm... and over 125mph ... but had 17:1 compression measured at tick-over. It eroded the pistons to seizure if I ran at 55mph for more than a moment (a big flat spot that entailed a gear change to get past). I was told I needed 105 octane fuel. But otherwise trouble free ("it went like stink!"). When I reduced the compression to ~15:1 with addition of a second head gasket, the problem disappeared using pump 98 octane fuel. That engine had more than double the transfer port CSA, exhaust port "squared-out" compared to original. The exhaust port timing was 1~2mm higher (top edge) that original, or so I was told by my local tuning shop, who told me that the engine had been modified (by one of their track rivals) for Production racing... and the "stage 3" tuning should not be on the road! It was as quick as the 500cc 4-cylinder road bike (race replica) that Yamaha made later.
Thanks
K2

I can take a print off the barrel on the next strip-down, and draw round it.
I am not sure how that corresponds to angle. The rod ratio is about 1.8:1,
exhaust is around 180 degrees, transfer about 120 degrees about bottom centre - pretty conventional.
the only notable change are the wider trapezoidal ports.
I have a diagram I can tidy up a bit.

I doubt if there is anything really unusual.

I thought the idea of the inlet-side power-valve was neat! "2stroke-stuffing" has 2 separate tracts on a modified disc valve case.
Accidently discovered by breaking and jamming the intake disc during a dyno run.
No-one else seems to have documented it.

Most reed valve mounts are on the removable barrel, but the top of the case is accessible if the cylinder leans forward a bit.

I have seen, on YouTube, a re-made Kawa Mach 4 upper case with reed valve mounts straight on the case.

It may be possible to get 2 reasonable tracts in there using the same carb.
A deliberately smaller reed valve block may be needed- maybe one half of a big one?

A blocking mechanism that rotates like a front power valve could open up the direct tract.
This would be a "Pipe only" type mod.
<edit>
This modified print looks fairly accurate.

 

[Steamchick]

I always liked the idea of a disc valve as it makes the intake timing assymetrical, which is what the reed valve is doing I suppose. Always thought an additional disc valve between the crankcase and transfer port could improve transfer timing... but never had enough time to spend trying to work it all out! Glad to be in touch with someone who is trying some variations!
K2

 

[Owen_N]

Looking at a 125 gp or carting engine, with cast pistons.
these are normally limited to 25 m/s, but some production engines run to 27 m/s average piston speed.
this is 14,000 to 15,120 rpm with 54.5mm stroke.

The Vortex 125 engine is claimed to be 43 HP at 13,900 rpm, and maximum 14,000 rpm.
27 m/s probably shouldn't be used for sustained running.

A 50cc 2-stroke with 36 mm stroke could run to 21,194 rpm as a maximum power point.
2SS uses 17,500 rpm, so he has some leeway.
If his output of 20 hp was applied to a 125, that is 2.5x the volume, a 125 should make 40 hp exactly at 14,000
rpm at the same level of volume efficiency.
I have heard of disc valve engines running at 55 hp for 125cc- I will check that out.
<edit>
I think Aprilia had 55 hp at 13,000 rpm.
quote ex Kevin Cameron, cycle world.

I would suppose everyone else matched that fairly quickly.

This I think would include draw-through by pipe suction, and "supercharging" from the pipe pressure pulses.

Pipes were generally 36mm inlet, 5.1 inches maximum diameter , or 130mm.
This also shows what a difference disc valves makes over reed valves- 30% more power at 94% rpm.
The favoured exhaust is T type with centre bridge.
This is not easy to use with a power valve, but smaller side exhaust ports are not that efficient.
The Honda CR 250 has a complex power valve system that blocks off the two side ports.
The main port valve has a rocking action.

I would like to see if the 2stroke-stuffing guy can match this kind of "average piston pressure" and
capacity to power ratio.
It is early days yet.
I will see if I can find a few port-print type maps.
I have a couple of port cross-section drawings, showing the way a "full" transfer port swings out and back in, top edge angles, and
inside curvature.
I am not sure whether the boost port should be 60 degrees of the horizontal.

A bit of up-angle is good to get the transfer streams to merge centrally, and not chaotically.
Visualising this experimentally is difficult.

That fancy GT Power and AVL-Fire simulation would have to emulate this effect.
I think they dyno-ed the original engine to benchmark the simulations.
energies-11-02739.pdf ex google.

 

[Owen_N]

Hi Owen,
I would be interested to see a porting diagram (360degree timing would be OK) to understand these changes better. - But I understand if you don't want to publish "intellectual property" on this "theft ridden" means of communicating! (Interweb: www = "What's Worth-it Walks!").
My only real experience of re-porting 2-strokes was to raise the (theoretical) power of my 250 MZ by 8Hp. Made a huge difference to top end on the motorway, but I didn't notice any change to "town-riding" acceleration torque. The bike was very good at mid-range revs before and after. Before I had the mod (a tuning company's design) I took a transfer of the porting, and again afterwards, and the changes were quite small and subtle. But included a very small transfer port through the piston, just beneath the rings, opposite to the exhaust port, directing a 3rd jet of intake gases up into the combustion chamber.
I also owned a "stage 3 race tuned" LC350 Yamaha. Good for over 12500rpm... and over 125mph ... but had 17:1 compression measured at tick-over. It eroded the pistons to seizure if I ran at 55mph for more than a moment (a big flat spot that entailed a gear change to get past). I was told I needed 105 octane fuel. But otherwise trouble free ("it went like stink!"). When I reduced the compression to ~15:1 with addition of a second head gasket, the problem disappeared using pump 98 octane fuel. That engine had more than double the transfer port CSA, exhaust port "squared-out" compared to original. The exhaust port timing was 1~2mm higher (top edge) that original, or so I was told by my local tuning shop, who told me that the engine had been modified (by one of their track rivals) for Production racing... and the "stage 3" tuning should not be on the road! It was as quick as the 500cc 4-cylinder road bike (race replica) that Yamaha made later.
Thanks
K2

OOO!
I am jealous! Those LC 350 Yamahas were very desirable!
They are probably on the same price list as restored Kawasaki Mach 4s nowdays!
I saw one of those that went for $50,000 USD.

 

[Steamchick]

$50,000 USD !! A crazy price! Mine sold for £1300 in 1998? It was a "cheap and cheerful" commuter back then. Clever "one-touch" indicator buttons, very light, easy riding position, and OOOOHHHH!!! All that power with light and easy gearbox and clutch. etc. But it was going to either kill me, lose my licence for excessive speed, kill someone I hit, or something I didn't want to risk.... so I sold it and bought a Guzzi V50II, to tame the adrenalin and save my nech/sanity/licence! (I still have it). Crude by any comparison with the LC350... (But I have grown to like it).
K2

 

[Bentwings]

Hi Owen,
I would be interested to see a porting diagram (360degree timing would be OK) to understand these changes better. - But I understand if you don't want to publish "intellectual property" on this "theft ridden" means of communicating! (Interweb: www = "What's Worth-it Walks!").
My only real experience of re-porting 2-strokes was to raise the (theoretical) power of my 250 MZ by 8Hp. Made a huge difference to top end on the motorway, but I didn't notice any change to "town-riding" acceleration torque. The bike was very good at mid-range revs before and after. Before I had the mod (a tuning company's design) Itook a transfer of the poerting, and again afterwards, and the changes were quite small and subtle. But included a very small transfer port through the piston, just beneath the rings, opposite to the exhaust port, directing a 3rd jet of intake gases up into the combustion chamber.
I also owned a "stage 3 race tuned" LC350 Yamaha. Good for over 12500rpm... and over 125mph ... but had 17:1 compression measured at tick-over. It eroded the pistons to seizure if I ran at 55mph for more than a moment (a big flat spot that entailed a gear change to get past). I was told I needed 105 octane fuel. But otherwise trouble free ("it went like stink!"). When I reduced the compression to ~15:1 with addition of a second head gasket, the problem disappeared using pump 98 octane fuel. That engine had more than double the transfer port CSA, exhaust port "squared-out" compared to original. The exhaust port timing was 1~2mm higher (top edge) that original, or so I was told by my local tuning shop, who told me that the engine had been modified (by one of their track rivals) for Production racing... and the "stage 3" tuning should not be on the road! It was as quick as the 500cc 4-cylinder road bike (race replica) that Yamaha made later.
Thanks
K2

There is a guy that tests everything on you tube.

Project farm you tube I YHINK is his site he did a test of job weld and several other adhesives JB. Did pretty well .
I think you could get away with it on glow motors as they run cooler gas motors get pretty warm EGT are often 800 + deg F cyl heads around 350 deg F so I YHINK you are pushing the limit there but try it . I certainly would like to see it work . I can’t help much as to port design . I’ve done tuned pipes but even there I’d have to do some searching for data.
Byron

 

[Owen_N]

There is a guy that tests everything on you tube.

Project farm you tube I YHINK is his site he did a test of job weld and several other adhesives JB. Did pretty well .
I think you could get away with it on glow motors as they run cooler gas motors get pretty warm EGT are often 800 + deg F cyl heads around 350 deg F so I YHINK you are pushing the limit there but try it . I certainly would like to see it work . I can’t help much as to port design . I’ve done tuned pipes but even there I’d have to do some searching for data.
Byron

This particular engine may suit a semi-tuned pipe.
The conical expanding part is ok out to around 60-70 mm in diameter, but a 6000 rpm pipe would be too long.
If I got a good torque build from 5000 rpm, I could spin up to possibly 7000 rpm, depending on the torque curve.

Prop torque climbs very quickly. I think static thrust is proportional to the cube of the power, and
has a square relationship to rpm and velocity. T = dens x effy x V sq x A (thrust)
- similar format to drag, only no 0.5 factor.

A pipe on this engine may give 30% more torque, = 30 % more power at the same rpm,

and possibly up to 10% more thrust..

To get from 6000 to 6500 is am 8.3 % increase, 1.083^2 = 1.17 or 17% more thrust.
at say 10% more thrust, velocity increases by 1.05, or 6300 rpm.
This is not exactly correct, but gives some idea.
if I increase speed by 1.05, then power is x 36.5 % more.

Is there any way I can get a correct tuned response into a pipe that is only 320-400 mm long?
The idea is that this length would fit into a typical model aircraft, which is maybe 800mm long in the body.
If a short tailless design is considered, the body length would be less.
I am considering wingspan from 60-80 inches, or 1.5 m to 2m.

 

[Steamchick]

I suggest you consider how a contra-bassoon or trombone works, compared to a clarinet or post horn. In musical instruments, columns of air work fine if doubled back on themselves. Or even wrapped in coils - like a French horn or Susaphone! Suppose you take reflecting compression-cone and tail pipe and silencer part of the traditional 2-cone expansion box, then the large end feeds into an outer spiral chamber, that returns along the outside but the spiral is sized for length and cross-section to mimic the length and cross-section of the longer expansion cone of the traditional expansion box. The engine exhaust would then feed into the longer outer expansion spiral chamber and expanded gases would then be compressed and reflected when they pass into the central part. The whole silencer should be less than half the length of a traditional straight expansion box, and fit within a fuselage.
You'll need to cut some cardboard parts and work out the construction, but first decide the "plot" of CSA versus length, and do some sums to work out the spiral and outside can shape to make the chamber around the inner cone simulate the expansion cone.
Ask if you don't understand, and I'll do some more work to explain.
K2

 

[Owen_N]

I suggest you consider how a contra-bassoon or trombone works, compared to a clarinet or post horn. In musical instruments, columns of air work fine if doubled back on themselves. Or even wrapped in coils - like a French horn or Susaphone! Suppose you take reflecting compression-cone and tail pipe and silencer part of the traditional 2-cone expansion box, then the large end feeds into an outer spiral chamber, that returns along the outside but the spiral is sized for length and cross-section to mimic the length and cross-section of the longer expansion cone of the traditional expansion box. The engine exhaust would then feed into the longer outer expansion spiral chamber and expanded gases would then be compressed and reflected when they pass into the central part. The whole silencer should be less than half the length of a traditional straight expansion box, and fit within a fuselage.
You'll need to cut some cardboard parts and work out the construction, but first decide the "plot" of CSA versus length, and do some sums to work out the spiral and outside can shape to make the chamber around the inner cone simulate the expansion cone.
Ask if you don't understand, and I'll do some more work to explain.
K2

You might have to draw me a picture.
I am thinking Archimedean screw, but to mimic the cross-section it would be very fat!
You could change the pitch of the screw as well.
Weren't there mufflers that has an internal screw like this?

Wouldn't making the gas pass though this kind of passage cause more resistance?

I was thinking that the tail cone could be inverted for half its length. What effect would this have?
I think all that does is reduce the crossways travel time, but the lengthways spreading effect would be similar but halved.
The reverse pulse building at the exhaust may be more effective over a narrower rev range.

Some alternate "stinger" inlets are set at the half cone position.
what does the end cone actually do?

1) it decelerates some of the expanded gas from the front cone.
2) it progressively reflects and spreads out the sonic front.
3) the spread out sonic front then reforms back in the front cone, and acts as a compressor at the engine outlet.
This can block further outflow, and even reverse some flow.

There is possibly a sweeping effect back down the front cone.

Do the two cones have to have the same angle?
How long is a 6000 rpm exhaust? I see that the main cones add up to about 1.5 feet on a 17500 rpm cone.
(see 2stroke-stuffing).
is a slower setup proportionately longer? 17.5/6 = 2.9, or 4.4 feet. , 53 inches.
Plus the short header, the stinger, and the muffler.
That is enormous! most motorbikes only have room for about 25 inches of exhaust!
I have see pipes bent at the stinger with the muffler going back the other way.

 

[Steamchick]

Hi Owen,
Yes, I have been thinking about something like an Archimedian screw - with variable pitch as the passage progresses along the pipe.
First I need to understand - what length and diameter of header pipe are you planning? Do you have a design of expansion box already worked out?
If not, this guide seems straightforward to design one.
https://www.google.com/search?sxsrf=APq-WBs99ZFZpTf5aO8_20cBuomRxG4QvA:1647859534465&source=univ&tbm=isch&q=design+of+model+aircraft+tuned+exhaust+pipe&client=tablet-android-samsung&fir=3T4uBYf2jAL5zM%2CVKzv8LPCrn-n4M%2C_%3B7-QXKn5d0hRVhM%2CvdFz9FknV5LBkM%2C_%3BdHymh39mLdmBRM%2CVKzv8LPCrn-n4M%2C_%3BHexMFsLilZAHiM%2CC8ecOuVs0cqiWM%2C_%3BZXt_MDn2QLBiDM%2CFhISyNWfxPUUsM%2C_%3BAay5ldBGI4lK9M%2CId6_vQh5tDojIM%2C_%3BmFX5vIkmYaCIbM%2CC8ecOuVs0cqiWM%2C_%3BOqqOK4MKaRRfnM%2CAJHN6HjXjTU5mM%2C_&usg=AI4_-kQ0UuUO8oMkG_6tHn_NjiWxE4v5zQ&sa=X&ved=2ahUKEwjdxMORg9f2AhVGecAKHXA4AWwQ7Al6BAhOEEY&biw=962&bih=601&dpr=1.33#imgrc=IW6z9R50RGvJhM

As gases do not really appreciate cones and tubes, just flat planes that they reflect off, the "spiral" tube should be pretty much OK as would a rectangular tube. In simple terms, the mean diameter of a spiral tube would be akin to the length of the middle of the spiral chamber - that is really just a rectangular tube wrapped into a spiral. But as the cross-section needs to change, the change of pitch of the screw alters the length of one side of the rectangle - thus increasing or reducing the CSA, and simulating the expanding and reducing cones of a twin-cone expansion box.
So I understand you are planning on a 60cc, engine, 6300rpm as the tuned speed. (I may get this completely wrong, but please bear with me?).
So:
we can calculate closely the pipe length. The formula for determining the length is:

Lt = (Eo x Vs) / N English OR (83.3(Eo x Vs)) / N Metric

Where:

Lt = tuned pipe length, in inches OR millimeters

Eo = exhaust open period, in degrees

Vs = wave speed (1700 ft/sec OR 518.16 Meters/sec at sea level)

N = crankshaft speed, in RPM
I.E.
83.3 x (120 x 518.16)/6300 = 822.1mm. But supposing that is around a spiral of mean diameter 50mm: Then we will need 822mm to the Widest point of the divergent and convergent "cones", including the pipe from the engine port to expansion box.
So assuming 100mm from exhaust port to feeding in tangentially to the expansion box, the divergent cone will need to be 722.1mm long (4.6 revolutions of the spiral)... the convergent cone say 362mm long (2.3revolutions of the spiral.
Now supposing the divergent cone has "an expansion angle of 10degrees. - Tan 10 = 0.176: You exhaust port is about 495sq.mm. CSA. so the header (assumed 100mm long) shall be about 25mm diameter. Therefore the widest part of a divergent cone from 25mm dia at 362mm long shall become ~139mm diameter (Bigger than my guess of 100mm diameter, so totally illogical!), or a CSA of 15175sq.mm. On a spiral, mean width 50 mm this would become a rectangle of 303mm long.... (one revolution at mean diameter of the spiral chamber). The converging cone would be half of that. SO: Just the diverging spiral on dia 100mm tube would be longer than the straight cone!
This is becoming ridiculous, so don't bother trying to design that one! It would need to be at least 150mm diameter to start to shorten the overall 1.08m long regular twin cone design...
I think anything larger than a regular twin-cone- design will create more wind drag, as it will increase the CSA of the aircraft.
Back to the head-scratching stage? e.g. - Make a longer fuselage, 6inches diameter, capable of holding a regular twin-cone expansion chamber 40inches long?

K2

 

[lohring]

I wrote a series of articles that cover the development of the high power two stroke. The emphasis is the application of these principles to model size 26 & 35 cc engines that we have a lot of experience with.

Lohring Miller