Unusual variations on two-stroke head design.

[Owen_N]

Regarding Steamchick post:
This is not a performance mod at the top end.
The main objective is to smooth out bottom end and mid range. This mod is no use if you are turning 5000 rpms plus.
I surmise that the exhaust gas is not fully mixed with the incoming charge, for most transfer port layouts.
This may not apply to all loop charged engines.
I want to squish and fire in sequence from best mix to worst.

It would be nice to have access to a high end simulation system such as AVL Fire to verify this stratification, and adjust the transfer ports to suit.
The centre throat may not survive adjustment of the clearance volume- I have a couple of CCs to find.
There may be a "Jet Firing" effect if the second chamber fails to fire, but if the second chamber contains a good burning mix,
the sparkplug will help eliminate the slow burn and tendency to detonate.
It would be nice if a model aircraft two-stroke ran more like a four-stroke, for firing smoothness.

I think the lifespan of these things is limited- in 10 years new internal combustion engines of all kinds will not be available to most people.
There may even be a lot of limitations on running existing engines, and a severe restriction of suitable liquid fuel.
I don't know how this will play out for the current automotive fleet, with a 20-30+ year life expectancy.

Re: octane ratings: I think many normal motorcycle motors were run at 6.5:1 nominal ratio, as the oil mix tends to promote knock.

- It is a bit "suck it and see" to see if it will run on 91 octane (NZ) at 9:1 comp. I think 91 corresponds to a lower test number in USA.

 

[Bazzer]

"It would be nice if a model aircraft two-stroke ran more like a four-stroke, for firing smoothness."

That of course is a matter of personal opinion !!

"There may even be a lot of limitations on running existing engines, and a severe restriction of suitable liquid fuel."

Alcohol burning model aircraft engines will be running for decades yet, the beauty is that you can potentially make the alcohol fuel and vegetable based oils yourself.

 

[Owen_N]

Maybe this is peculiar to single cylinder model aircraft two-strokes.

Misfiring causes massive torsional vibration with a light airframe.

I have ordered a two-stroke twin after dissatisfaction with the TP60 single.

Hopefully having two cylinders evens out power delivery a bit, even though they fire at the same time,
and it has better balance, though it does have a large rocking couple.

The TP60 is also father fragile. It will make a good test bed for a cylinder head, though.

I want to use it in a contra-rotating hovering platform similar to the Hiller one, and this requires running at part throttle.
A 3-cylinder Saito radial 80cc four stroke would be better, but they are fairly heavy and much more expensive.

This got me interested in two-strokes, and I saw an opportunity to fire in the order of best to worst mixture at part throttle.

Most people don't really need the smoother low-speed running, and motorcycle installations seem to absorb the vibration.

The application is unusual in that weight saving and thrust to weight ratio is more important than usual for
model aero engines.

I have moved to IC from electric to get a sensible flight duration. 2 minutes is getting a bit marginal.

I can go up to 30 minutes with an IC engine, though 10 minutes is long enough.

Electric is OK for Helicopters, but I wanted to do something different.

Regarding liquid fuel, I think car fuel will be available for a while to support the aging fleet.

It is only in certain areas that IC cars and motorcycles will be banned, like city centres.
I see that motorcycles are already banned in some cities.

Use of small IC engines may be penalised more easily, though, depending on the whims of politicians, and the opinions of the
majority.

It is possible that we will be forced to stop running existing small IC engines and buy electric powered devices such as lawnmowers,
model airplanes, motor scooters-light aircraft??

In theory, small petrol engines could be converted to alcohol, depending on "which way the wind blows".

 

[Owen_N]

Twin plug head- Revision 1 31-12-21.
--------------------------------------------
1) Chambers adjusted to 30 degrees conic.
This is very close to spherical, adjusted for plug clearance.

2) angled plug moved inboard.

3) vertical plug raised for piston clearance.

4) fin arrangement revised - one more full width next to plug, loop inflow side fins shortened,

tiny fins removed, exhaust side fin adjusted to clear bolt bosses.

5) centre channel dimensions adjusted to line up with other views.

6) more dimension lines added.

 

[xpylonracer]

I maybe wrong but I understood Owen, the OP, was not seeking more performance in power output but he wanted to change the characteristics of the engine to make it sound like a 4 stroke.

xpylonracer

 

[Owen_N]

The sound will not be the same as a 4-stroke, because you don't get the contrast between intake and exhaust cycles,
and two-valve poppet setups have quite a unique "tone" as well.

It may sound a little closer to a wankel engine at low revs, but more even firing and less torsional vibration are the main aims.
It is really only of interest to RC model airplane people.

It would make a nicer running car engine as well, but those are dying out now.

Direct airblast head injection would work, but is complex and expensive.

High pressure finely atomised direct injection without the air could give smoother low speed running, and allows
use of jet turbine fuel or diesel with spark ignition.

With proper controls, this could also prevent detonation. Possibly rapidly pulsed injection.
The problem with staged injection is that it takes some time, and is more suitable to a constant pressure diesel cycle.

There have been trials blowing metered air into the transfer ports as well, but this sounds mechanically tricky.
Maybe a displacement blower and a separate valving system could do it?? You might as well just use a four-stroke.

 

[lohring]

As was pointed out in the article I posted, the goal of one of the head designs was to enable a 2-4-2 pattern where the engine alternates between firing every and firing every other revolution. Model four strokes are also widely used.

I agree that electric power is rapidly replacing IC engine power starting with the smaller power plants. IC engines will always be built by modelers. Steam power is only used in very special applications where it is unlikely to be replaced; nuclear powered ships and submarines. Land based steam power plants seem to be more expensive than greener alternatives and are fading. Otherwise only model builders preserve the heritage.

Lohring Miller

 

[Owen_N]

Proposed twin-spark head.
A compromise head design with one plug could be a continuation of the first chamber with an extension based on a slanted cylindrical section.
Volume is biased towards the first chamber, but the majority of the squish is directed to a line extending away from the sparkplug.
This still has quite a long flame front, similar to a normal offset chamber, but does not direct all the mixture straight at the plug.
This allows the possibility of a good firing mix close to the plug.

This variation could be worth a tryout after the first version.

 

[Owen_N]

Variation 3., revision 2.

Adjustment to get 6.5cc head clearance, align second plug to be square with piston top.
this gives 5mm sparkplug electrode clearance to the piston.

The cross-chamber now has vertical sides, and 12mm width, and about 10 to 11 mm depth at the plugs.

there are some questions:
1) Is there enough side clearance on the firing zones?

2) Can the electrodes go closer to the piston?
Say, 4mm clearance?

3) Presumably a bit of outwards taper around the sparkplugs is desirable to reduce wall quenching?

Using a rounded form both sides ( cool side and exhaust side) will reduce volume a little.

4) That volume for the taper form has to removed from some other region.
5) The cross chamber could be necked more in the middle.
Will this make scavenging significantly worse?

 

[Steamchick]

From my amateur seat.... I would site the spark centrally between head surfaces and piston surface at TDC.... to permit max sphere of flame to develop before the gases hit cold metal, and where shock waves can reflect. But swirl will dramatically distort the "natural centre" location, so your guess is better than mine! You can only guess at where the swirl will be centred, and how the flames will distort as a result. I understand the expert view is for high swirl and flame turbulence to prevent (deter) detonation and feed fresh cool charge into the combustion zone for optimum combustion. But the chamber is shaped differently at the ignition point (advanced) to that at TDC and thereafter, as the piston approaches and recedes. So modelling what really goes on at max revs is not a simple task....
K2

 

[Owen_N]

Part of the problem is that the chamber is an odd shape.

"hemispherical" chambers do not seem to be half a sphere.

I would have thought that part hemisphere, part straight sides would be better.
This makes the potential flame front more like a ball.
A downside of this is the flame has to turn round sharper corners to spread out into the squish zone at 20 deg atdc.
and travel a bit further.
However, without high end simulation, I just have to try it.

I can pull the plug up a bit and keep the chamber diameter at say 18mm,
or push the plug down a bit and open the chamber out to a full hemisphere at 24mm
Possibly the higher plug is more efficient in this case.

I can reshape the centre of the twin chamber to a more triangular shape, which gives me material to go from 12mm out to 18mm
around the plugs, I think.

It is hard to calculate the actual volume, so I will leave the centre fillets in, keep the 5mm electrode tip clearance, and
measure and adjust to the final shape.

 

[Steamchick]

I agree that "hemi" heads are not spherical, nor even half of that, but usually more like 40%-ish of the sphere. but considering (with a single combustion chambered head) the squish band forms a dough-nut shaped ring of swirling gases around the central point, the chamber contains the large majority of gases (Less than 10% remain in the squish zone?). So igniting this - before TDC so the ball of flame achieves max pressure soon after TDC - the flame tends to be a jagged and haphazard flame curling around the "surface of the doughnut" (from what I have seen from simiulations and high speed film off the web). Then as the piston travels down the stroke, towards the "cylindrical shape" (not exactly, because of the head shaping) the ball of flame expands out so in theory at least, it reaches all the cooler surfaces simultaneously. (Hence the desirability in modern 4-stroke engines of a bore = stroke, and near-flat head-shape). for the 2-stroke you are studying, the exhaust opening is determined by othe parameters, so you should consider how the flame can expand from a doughnut around each spark-plug, in 10 degree crank intervals from ignition to exhaust opening....
If you can estimate the max rpm of the engine, you can estimate the time for each 10 degree step, and crudely estimate the chamber pressure, - hence speed of flame propagation for each step, and guess how the "doghnuts" of gas will burn at each stage.
This is a very complex problem, - and I don't envy you the work involved to get something like sensible results - but this is a way to manage the steps required...
I hope my amateur fumblings intoi your expertise don't disturb your thoughts, but help to clarify them?
Do tell me to "shut-up!" if you think I am "off my trolley" on this one...
I won't be offended,
K2

 

[Owen_N]

I have seem mention of a package called AVL Fire which does engine simulation, but that is supercomputer stuff, I think.

I think I will just make something. It will either sort of work, or give me more ideas.

There is the prospect of deliberately stratifying the chamber at part throttle, and limiting exhaust gas mixing,
to help the two-stage burn.

It is likely that the flame behaviour will be less than optimum for full throttle running, though.

 

[Steamchick]

Had a browse of the AVL cover page...
https://www.avl.com/fire-mIt looks like interesting software for your study? But do AVL sell this FIRE-M software? - Can it be used on a "simple" PC?
It wasn't clear to me how you get it.
Have fun!
K2

 

[Owen_N]

AVL Fire
If you have to ask, you can't afford it!

This looks like a four valve two stroke simulation! -image-
I would suppose this engine would be intended to run as a 4-stroke, then go to two-stroke for full power.
This could be tricky to manage as a changeover, and emissions management could be an issue.
This could boost output by 35% at least at the same rpm.
With timed valve control it could be a lot more.

A downside is you want an advanced valve system similar to what Koenigsegg is looking at.
(electro-pneumatic)
It is difficult to go from 4-cycle to two-cycle otherwise.
I wonder what the power penalty is using pneumatic power to operate the valves?

You end up blowing all that compressed air out the exhaust.
Maybe 5 or 6 hp for a 4 cylinder engine at high rpm??
This is more suited to a high performance vehicle, where the extra cost does not matter.

Presumably power losses can be recovered at lower rpm by varying the inlet vs exhaust duration (Atkinson cycle).
and otherwise adapting the valve operating cycle.

 

[Owen_N]

The head metal has turned up, so I measured the existing head.
the distance from the plug seat to the piston is 17mm, vs my 16 and 14mm.
The volume is 5ml just about exactly, vs my estimate of 6.5ml.

This gives a nominal CR of 12:1 !

This seems quite high, for a two-stroke on regular gas.

Possibly the propeller load/rpm characteristic allows them to get away with this.

Trying to pull full torque at 3500 rpm would likely cause detonation.

This motor seems to settle at 5500-5800 rpm at present with the 23x10 prop.
it would rev higher with a 23x8 prop.

I will need to revise my twin plug layout quite a bit to get to this point.
I will have a look at this tonight.

 

[Steamchick]

High Owen,
I have just been reading some earlier discussions and trying to get "back to basics" of what you are trying to do. I am really clueless on the idea of 2-stroke alternating with 4-stroke firing. I don't have any pearls of advice there, except I don't think you can do that sensibly. I have driven motorcycle 2 strokes that would 4-stroke under certain throttle settings, - even "8 or 12-stroke" on over-run with tiny throttle openings above idle. (Who knows what is happening when the engine is not firing every stroke, anyway!). But those were just engine-carburettor quirks, not planned running modes.
On Octane vs. compression. On many of the motorcycles I have owned, the compression with wide open throttle and kicked-over was anything up to 12:1 for regular production engines running on 95 to 99 octane fuel. readings of 8:1 to 11: 1 were typical on engines made from 1960 through to 1980-ish...
But the actual compression is achieved by sucking in a charge from beneath the piston, with stroke from TDC down to transfer port opening, then this is pumped into the top end at transfer when some of the charge escapes down the exhaust port before that closes, when the remainder is compressed to TDC. This means that "calculated" compression is likely to be wrong, and actual compression is somewhere between the area of piston x stoke from transfer port closure to TDC, and the area of piston x stoke from exhaust port closure to TDC.. Or so I reckon?
In fact, I had great difficulty starting one bike (worn rings) that had compression as low as 6:1 on kick-over. - But once running, it kept running until I let the revs drop too low, although the power was well below proper power levels. (maybe 1/3rd or less?). New rings fitted and it was back to FUN again! (~12:1 on my gauge). I only experienced detonation issues at part throttle (~5000rpm) on one engine - 17:1 race tuned but with 99 octane fuel.... That was OK when I dropped the compression to 13:1 by using a double-thickness cylinder head gasket... which made it OK for road-use, even through max power and revs were reduced.

Probably none of this is of any use, but I look forward to reading your next ideas...
K2

 

[Owen_N]

version R4 with 12:1 CR.
Note: 30 degree plug angle and 5mm spacing of electrode to piston retained.

To get more hemispherical, the plug angle would need to be 45 degrees.

For now, the chamber sides are quite straight.

An advantage of this is that squish is not directed straight at the first plug, so there is less mixing
of exhaust gas at part throttle.
I will try this layout, and see if the partly restrained flame has a negative effect on top end power.

Squish clearance is an even 1mm.
I should get away with no squish radius, as the chamber is an extension of the cylinder wall on the cool side.

High Owen,
I have just been reading some earlier discussions and trying to get "back to basics" of what you are trying to do. I am really clueless on the idea of 2-stroke alternating with 4-stroke firing. I don't have any pearls of advice there, except I don't think you can do that sensibly. I have driven motorcycle 2 strokes that would 4-stroke under certain throttle settings, - even "8 or 12-stroke" on over-run with tiny throttle openings above idle. (Who knows what is happening when the engine is not firing every stroke, anyway!). But those were just engine-carburettor quirks, not planned running modes.
On Octane vs. compression. On many of the motorcycles I have owned, the compression with wide open throttle and kicked-over was anything up to 12:1 for regular production engines running on 95 to 99 octane fuel. readings of 8:1 to 11: 1 were typical on engines made from 1960 through to 1980-ish...
But the actual compression is achieved by sucking in a charge from beneath the piston, with stroke from TDC down to transfer port opening, then this is pumped into the top end at transfer when some of the charge escapes down the exhaust port before that closes, when the remainder is compressed to TDC. This means that "calculated" compression is likely to be wrong, and actual compression is somewhere between the area of piston x stoke from transfer port closure to TDC, and the area of piston x stoke from exhaust port closure to TDC.. Or so I reckon?
In fact, I had great difficulty starting one bike (worn rings) that had compression as low as 6:1 on kick-over. - But once running, it kept running until I let the revs drop too low, although the power was well below proper power levels. (maybe 1/3rd or less?). New rings fitted and it was back to FUN again! (~12:1 on my gauge). I only experienced detonation issues at part throttle (~5000rpm) on one engine - 17:1 race tuned but with 99 octane fuel.... That was OK when I dropped the compression to 13:1 by using a double-thickness cylinder head gasket... which made it OK for road-use, even through max power and revs were reduced.

Probably none of this is of any use, but I look forward to reading your next ideas...
K2

A fairly long timed 4 stroke closes inlet 60 deg abdc
vs 2-stroke closing the exhaust 86 deg abdc, so there is a relative comp ratio going on there
0f 1+ 0.5 vs1+ 0.0675, 1.5 ,1.07, % = 75%, 53.5%
<edit> bad trig.
So a 4 stroke nominally 12:1 actually is 9:1, A 2 stroke 12: 1 is actually 6.42:1, a big difference.

The 94 deg atdc is quite late for a two-stroke, probably not intended to use a tuned pipe.

A modern 4-valve motorbike multi uses quite high compression, but some of the older English bikes were much lower.
8.5:1 seems a common number, but probably shorter timing??
8.5:1 was common on older V8 truck engines, too.- 1960s and 1970s...

The Triumph 500 twin and 750 triples had quite high compression.

The engine alternating 2 and 4-stroke timing has never gained popularity to the point of production, as it needs a turbocharger to run, and you can get the extra power just by winding up the boost, but less efficiently.

Possibly turbo plus variable compression is a better target?

Anyhow, I am not trying to do this, - my version is an "enhanced stratification two-stage burn" in a normal two stroke.
This is intended to give smoother running at part loading and lower revs, without taking much off the top end.

The poor running is quite noticeable with a model aero two-stroke with throttle control.

I could take further measures to calm chamber turbulence if needed, by:
1) blocking the boost ports.
2) re-angling the side transfer ports, with a short "sweeper section" to blow flat across the top of the piston.
this probably only has to be 1/4 or less of the whole port width.
This part has a flatter roof angle.
The idea is to ensure exhaust gas does not get sucked up the back of the cylinder and mixed into the good mixture.

 

[Steamchick]

Thanks Owen. Interesting.
Post WW2, Although tetra-ethyl lead was available (most aviation engines of WW2 needed it!), it wasn't commercially cheap enough for the limited gasoline in Europe for decades....
I remember in 1960s compressions were around 6:1, or 7:1 in car engines, for "Regular" fuel ~88 ~91octane. And most 4 stroke motorcycles, we're the same, until late 1960s when the benefits of performance of 8.5:1 compression - with 96~99 octane Premium fuel became more commonly used. By early 1970s, some performance cars & motorcycles were available up to 10:1 compression and needed super 100+ octane fuel.
When the star rating was introduced, I thinK 4 star was 99 octane, 5 star 105 octane, and 2 star 92 octane. After lead was removed... 1992? - compression ratios were dropped from 10:1 to 8.5 or 9: 1, before fuel injection became more widespread, and knock sensors could retard ignition automamically.... and 4 star became 95 octane fuel!
But this is my memory, so don't quote me, do your research on what you can get today!
Keep up the good work!
K2

 

[Owen_N]

Update on new head: 21-01-22

1)The engine has been built and test run on one plug.
There is a smoothness benefit between 3000 and 4500 rpm with this engine.

It really needs two sparkplugs.
The first plug location is not that good over 5000 rpm.

I am waiting for my twin-fire CDI unit, so I can run both plugs.

2) I would like to add an offset skullcap insert, for some reshaping.
The sparkplug entry paths seem to be OK, but the chamber could be shallower and rounder.
Can I just glue this in place, or does it need screws? I can probably add screw seating on the outside.
<> Part of the sparkplug thread stays in the outer head. I will have a go at drilling and retapping it like this.
I think there is enough head material to recess like this, otherwise I may need to add a taper to the insert and the outer head.
The material in place is fairly dome-shaped.

3) I have a new idea on adaptable transfer ports.

The low speed ports need less exhaust blowdown timing, but can be aimed further towards the rear of the cylinder and higher up.

I was thinking of using reed valve blocks cut in half and remounted, but a downside is increased crankcase/transfer volume.

I can cut the whole sides of the cylinders out down to the base flange and use a plain 6063 aluminium insert.
This should take the basic ring loading, no combustion pressure.
the main transfers can have reduced timing.
I will need to order some bigger blocks of aluminium, I think.
30x30x30 blocks may be big enough.

There are two problems that I see:
3a) the rear transfer location does not get much air cooling. (relative to the propeller).
3b) the reed valve flow will drop off at higher rpm, leading to a lack of reed and reed block cooling.
Is this a real problem? are there any cures?

I have been following a two-stroke builder on You Tube, titled "Two Stroke Stuffing", and he uses this idea
on his "blown", disc valve exhaust , engine.
He has better mixture cooling of the reed valves, however.

I am going to trial deflector vanes to get more airflow behind the cylinder.