Colin Hall Engine...

Well done Colin -- I like the wording generally : and "... air swallowed..." -- no mistaking what that means --

--- but I feel a bit sick at not having done anything much in the way of 2-stroke-engine development: I was just managing the cycle parts and doing the development testing -- we had no dyno facility available so it was `suck-it-&-see´ testing at Brands which I thoroughly enjoyed anyway and conducted with great dedication....

55 horses shoving one along... Phwew! I had 45 bhp at 5,000 rpm from my JAP speedway `Short-4´ engine -- on the Grass -- which felt awsome but of course, it was the TORQUE that was the real kick up the jacksee. That makes me wonder what this 55horsepower at 13000 rpm must be like to ride. My Icarus-1 Bantam was worrying enough, buzzing at 8,400 seemed a lot of revs and then there was the dismal ´disaster, Iacrus-2 (I `sort of´ tuned this engine) , screaming its nuts off at at 3000 MORE revs than Icarus-1 -- not much torque and not always for very long though -- followed by Andy Boyle´s 250 ABS which did have some real push and 5 gears anyway. Pity that ambition (to become an elitist) got in the way.

I wonder at how much the development of reliability of these high-revving engines has cost the backers?

Just musing -- please excuse me for being boring -- yaaaawwwnnn!

Cheers!

A little unfinished business.

The Aprilia engine has a disk valve to achieve asymmetric inlet timing. Another way of introducing asymmetric timing into a piston-ported engine is to offset the cylinder axis relative to that of the crankshaft. Some work has been done with four stroke engines with regard to thermal efficiency I seem to remember – of no concern to a racing engine unless you are worried about fuel consumption.

A quick play with a pair of compasses, protractor and ruler indicates a useful degree of asymmetry with a modest 10mm offset. Also of interest is that if you correct say the exhaust port height to give the same crank angle opening timing you close that port considerably earlier. The same goes for the transfer ports and inlet timing. Not quite a disk valve but a step away from the restrictions of symmetrical timing.

Has anyone tried this?

As Derek observes, a two stroke is mechanically a simple machine, exploiting its full potential is another matter.


Colin

Colin!
I´ve seen several articles in the past with offset piston pins on other than Bantam engines and I am sure there was at least one Bantam that had an offset barrel at sometime or another -- so someone must know -- yet I seem to remember the advantages were small and on one proprietary motorcycle it was done for reduction of piston slap....

Re thermal efficiency. I had a picture on here sometime ago showing that the thermal efficiency gain was big from 8:1 to 12:1 CR with decreasing Ne from 12:1 on bcause the Ne curve bends over rapidly -- and after 16:1 to 22:1 Ne gain becomes very small . My JAP Speedway motor´s compression-ratio was 16,5:1 running on Victor Martin "Track Fuel" -- which was methanol with 2% Puridin. I take it the latter was a trade name for Tetra Ethyl Lead -- which just means it was a doped methanol.

Have you heard of Puriden or Puridin?

On the subject of compression-ratio I was surprised to discover some successful two stoke engines are on a much lower compression ratios than I had thought. Supposedly to get them to rev-on. It was also said that the dynamic CR is way above the measured static CR...

Probably a repeat ...

Cheers!

John

I don’t know of Purden/din, but I guess the TEL was there to keep the exhaust valve seat from burning out.

Maximum cylinder pressure is a key limiting factor when traded against engine life. All else being equal an engine that can trap a higher density charge of ‘good quality’ will produce more power but at a lower CR (to keep max. cylinder pressure under control). You could say that one indicator of successful racing engine development is the reduction over time of CR.

The attraction of static CR is that it is easy to measure and that is all you can say about it.

Colin

Colin ,
The following is a quote from one of the two engineers responsible for design and development of the much admired Aprilia engine .

" I would not bother with offsetting cylinder or piston pins to achieve asymmetrical timing , the effect is so small , and who needs the complication ?
In the calculations I did on the on the rsa 125 , I gave the offset no less than 10% of the of the stroke to enhance the effect . It is obvious that the maximum con rod angle becomes asymmetric , but what is less obvious , is the increase in stroke for a given crank stroke ! It is also remarkable that it takes more than 180 degrees from TDC to BDC , and less than 180 degrees from BDC to TDC , so , just where is TDC ? "

As far as bantams are concerned the most sensible control of the inlet cycle , and , bearing in mind the serious limits imposed by only 3 gears being available , has to be the pressure responsive Reed valve . It is closed when you need it to be and opens early in the crank cycle after BDC , and shuts when crankcase pressure dictates , and with typical inlet open periods of 270 degrees , it seems to me , as a win , win option ! At least that is what I have found on my own reed equipped engine , and the events can be further enhanced by exhaust system manipulation . Despite having asymmetrical inlet cycle the disc timings are still fixed in their phases , as is the piston port ,
the reed is not . In absolute power production potential the Rotary Disc reigns supreme , but with Bantam engines producing much less than half of level of the Aprilia ,
the best option will remain the reed system !
Trevor

Rather pessimistic Trevor -- surely there´s got to be more?


... who is going to find it?

Cheers!

Immer Optimistisch sein!

.... even if the rules are being broken -- just a little --

-- in the interests of scienc, of course, and not personal vanity....

Trevor

According to my rough drawing, there is no change in stroke. With a 10mm offset, the piston crown is short by ~1mm at TDC (geometric) and by the same amount at BDC (geometric). Therefore, you would need to drop the barrel by ~1mm.

TDC is advanced by ~5 deg and BDC by ~12 deg. As both of these local shifts are in the dwell zone of the connecting rod throw I am not convinced if this is of any importance especially at BDC. The ignition timing would need resetting but other than that? Typically, you would expect to see 50% of the fuel mass fraction burn by ~8 deg after TDC and given that laminar flame speed is a strong function of the unburned – end – gas temperature (flame speed increases with burn time) then you would achieve most (?) if not all of the burn at constant volume so no change there.

I thought it of interest not from replicating a disk valve but with regard to transfer and exhaust port timing. You are probably right about the reed valve.

Colin

Here are a few raw numbers to illustrate the crank / con rod geometry associated with offset in the rsa125 engine , with 54.5 stroke !

piston pin offset = 5 .45mm TC lowered by .10 mm BC lowered by .16mm

top centre offset = 2.1* Effective stroke = 54.56mm

bottom centre offset = 3.4* Max rod angle ( 120mm long) 15.8*at 92.1* before TC , and 10.5* at 87.9* after TC

angle from TC to BC = 181.2*

angle from BC to TC = 178.8*

The phrasing may be quaint , but translating from Dutch is a touch tricky !

As Frits was suggesting , and reinforced by the numbers , the effort seems not to be worth the potential reward , quite where a piston with sufficient offset could be found is questionable and , is anyone up for machining the engine , just to experiment ?

Not withstanding all of that , it is a fascinating subject with detractors and supporters in equal measure , and the VW Horex 6cylinder engine is well worth having a look at as a biking example !

As for me , there is always great merit in keeping things simple !


Trevor

Trevor

Thanks for this. Pity there is no data on transfer/exhaust port timing shift.

This is the sort of ‘what if’ problem when engine software packages such as Ricardo WAVE comes into its own. The problem here is that you would need the data from a fully instrumented baseline engine in the test cell to populate the software. If you had that, you would no doubt have far more significant areas in need optimisation. Plus ca change!

Colin

Sir Andrew Boyle of Chelmsford and RE GP5 fame (who, once upon a time when the Ides of March or some other madnes took him, lent me his ABS 250 to play with & break...) sent me some bits on John Hogan that appeared in The Vintage Motorcycle Mag for April 2013...

Quite fascinating were the statements that Red-X was used in the gearbox with the level such that only the largest gear would dip its teeth into the `lubricant´ and that he fitted an ignition cut-out button which he used when he felt the need of boost of power ("boost of power" are my words .. ´not the article´s...).

This obviously indicated that even in those late Forties-early Fifties times John Hogan was reducing friction losses by every means possible and wherever possible.

The cutting out of ignition -- with wide-open-throttle -- comes round to near what you were saying Colin but the article said it was done to cool the crankcase and increase the charge... It also suggests something happening in the Boundary Layer -- or have I missed something?

Relative the above and what Colin said earlier has got me thinking. I am having thoughts (quite dangerous ...) on the boundary layer and a really positive way of boosting power. Only those prepared to fully pay my funeral costs need apply for this secret knowledge.

That aside, I have this feeling that the thickness of the boundary layer and going from laminar to turbulent flow within the Bantam transfer ports has not been fully dealt with for all states & conditions? Smooth and as big as possible? but what of the sudden changes of of direction before the port closes ?
Has the reduction of friction-drag on the intake air charge been taken to its full limit? -- some of the transfers I have seen are enormous.... (Relative Icarus-1 and Icarus.2 -- of course!)...


One picture of the Hogan Bantam showed it with mergaphone exhaust (much like the Walsh bantam) and the article stated that exhaust pipes -- at the time -- for racing had to end by the rear wheel spindle... Also mentioned was a 7/8" carb...???

How things have changed!!

Amazing that the Hogan & Walsh ( in Australia) Bantams went so quick -- or was it that the others were slow?

Cheers!

John

I had quite forgotten Red-X. For younger readers there was a time when engines were de-coked on a regular basis as engine designers had not worked out how to keep lube oil out of the upper cylinder. Red-X was used as a ‘treatment’ to increase upper cylinder lubrication – a mad world.

A far as gearboxes go most of the oil in the box ends up coating the walls or suspended as a micro-fog, so the oil level falls. The remaining oil will be highly aerated offering a reduction in drag. Good modern synthetic oil filled to the lower recommended limit or less if, the micro-fog can be maintained seems a reasonable compromise. Pass that I should think any gains in friction reduction are not measurable.

Using a kill switch on the overrun with wide-open throttle would certainly keep air flowing through the engine. A fuel cut was not mentioned so I fancy you may find you will have a nice little pool fire in the expansion chamber after resumption of normal operation. You would also compromise the spark plug. Denso quote plug fouling if the centre electrode temperature drops below ~450 C, extremely likely even after a modest period of non-fired running. You would cool the engine but equally you then have a time delay to achieve thermal stability after start of normal running. Much better to retard the ignition aggressively to produce no net power and export high levels of heat flux into the expansion chamber to maintain scavenging amplitudes.

As suggested before, fluid flow in the transfer is turbulent with the exception of the very early flow period when the port is just starting to open and then for a very short period. A great property of turbulent flow is its ability to reattach to a surface via the generation of re-circulating vortices. Many years ago, I designed a range of model transfer ports that Les Eggs made up for me. I tested these on a flow rig to assess flow direction, stability and pressure loss. One port that Les did not make for me had a squared off roof with no outside radius. This allowed the flow to generate a re-circulating vortex in the corner and so determine its own flow shape. Interestingly it showed good direction control and stability and from memory no increase in pressure loss. If I had the means to measure its profile it might have suggested an optimal shape?

The real problem is the highly pulsed nature of the flow - steady state testing is suspect to say the least. I would suggest that at the beginning of transfer port opening there is a re-circulating vortex above the inner wall re-entrant curve, extending to the piston crown edge. This would become less elongated as the piston descended and flatten out before BDC.

I don’t think I would worry about the boundary layer until you have achieved a mature port shape. I would certainly not bother polishing a poorly designed port – I would not bother polishing anyway as the fluid film covers a lot of imperfections. However if that’s what makes you happy then keep polishing!

Colin

Sound advice I´d say... I forget who said it -- polishing ports is a waste of time, shaping is not...

I worked as a Lab Technician in the Strength of Materials & Hydraulics laboratories at Walthamstow Tech and built several rigs to measure those weird things like Flow Resistance In Pipes, Standing-Wave, Water-hammer and a Bernoulli-Rig -- and later attended Bristol Uni as an aeronautics student and stll have difficulty with what the hell is going on in the 2-stroke transfer-ports. I don´t really need to know now but I wouldn´t mind finding out before the lights are dimmed ....

Its all happening so quick .... Is there info available on pressure in the transfers during a complete rev...?

John Hogan increased the compression ratio and centralised the plug in the head but I saw nothing about how much -- FOR RACING -- the CR increase was, whereas the Walsh Bantam was at 16:1 CR -- on methanol, of course. Hogan sold cylinder heads for road-going Bantams with 9:1 CR .... -- A picture I have of the Walsh Bantam also shows a megaphomne exhaust -- similar Hogan.... Increase of CR increases the `sucking ability´ which I would have thought very necessary with a 7/8" carb....



Cheers!