Recycled Flyer

I have always considered that the baffle on any piston top will be the hottest place in the entire combustion chamber and when used in a model diesel will cause localised pre- ignition well before the piston rises to its correct place before TDC .

And its not a matter of simply reducing compression to retard the ignition to remedy this problem because to any attempt to wind back the comp as far as the baffle will advance it results in a very poor state of tune.

Does anyone see this differently?

And is it the reason that baffle top piston engines are not more readily adopted for model diesel use?

Thanks.

1QwkSport2.5r

Was there ever a widely produced cross flow (is that the proper term for the baffle piston engines?) diesel? I'm not very "in the know" with diesels, but I do read the forums quite a bit and haven't seen much written about any baffled diesels. I'm curious.

gcb

I believe it was OS (possibly Enya) that had a baffled diesel .15 back in the late fifties. To do this you must pin the contra so that it will not rotate as you change compression.

The baffle is used to cause the fuel mixture to swirl for better combustion...and just inside the baffle on the piston top was the hottest part of the engine and was often referred to as "hell's kitchen". When Supertigre came out with the flat top piston in ~1960 and used the baffles to direct the swirl other manufacturers started switching also.

Of course most diesel manufacturers were already using either a flat-top or conical piston but not "traditional" baffles.

George

ffkiwi

To the best of my knowledge the OS was the only diesel to use a classical 'fence' baffle-and it was a resounding debacle for OS-who undoubtedly lost a lot of money on it....I suspect (quite apart from the problems of keeping the contrapiston alignment correct at diesel running head clearances (which are about half or less compared those of a glow of the same size) the baffle would a others have suggested act as a hotspot and cause preignition leading to poor power and overheating. Some diesels DID use a step baffle-where the edge of the piston is machined away to form a step-the Mills 75 being a good example-and others such as the ED Comp Special had a scallop out of the piston crown which achieved the same sort of effect.
I'm not sure I'd agree with gcb about the role of the baffle though-the PRIMARY role of the baffle is to prevent the incoming fuel charge from the transfer port escaping out the exhaust by deflecting the incoming fuel jet upwards whilst at the same time deflecting the hot exhaust gases downwards and outwards in the opposite direction. Effectively its an wedging action separating two hot and cold gas streams.... Any swirling motion occurs later and is as much affected by head shape geometry as by the baffle.

In any case, the conical piston achieves the same effect as a baffle and is applicable to a range of porting arrangements not just cross flow....

ChrisM
'ffkiwi'

brokenenglish

I always thought that the step on the front of the Mills 75 and Comp Special pistons was simply to make the transfer port open earlier and close later than it would with a uniform flat-top piston, i.e. the step dimension allows the transfer to be adjusted independently of the induction and exhaust timing.
Perhaps I misunderstand the full meaning of the term baffle... I thought it concerned combustion chamber gas flow...

Recycled Flyer

The issue as I see it is that the piston crown is the hottest place in any IC engine so it makes sense to minimise that area - part of the reason that all modern engines have a flat crown.

By introducing topography to the hottest part of the engine you will vary the heat soak, some areas will begin to attract more than their fair share of that heat (just think griddle lines inside of a cast iron fry pan) and thus could cause ignition points independent of compression.

A conical piston crown seems to be a fair compromise but offers no advantage against its lighter flat topped cousin, the Mills system was supposedly done to maximise atomisation from the transfer port by providing a breakwater that smashed the incoming charge into finer particles.

But a true vertical fence would seem to be the worst of all piston evils for a compression ignition engine as it (as previously mentioned) could allow, by virtue of localised heat, the ignition timing to be pulled away from where the compression or degrees of engine rotation directs it to be.

Oh, and a 'baffle' to me has its sole purpose at BDC so is the furthest in degrees away from the combustion point and the furthest away from combustion chamber gas flow. To me the baffle is all about the advantages of directing cylinder filling but suffers from the disadvantages at the point of ignition.

In other words, you don't include a baffle on a piston crown with combustion gas flow in mind, you do it to prevent charge loss during transfer.

Thanks.

ffkiwi

I'd both agree and disagree with recycled flyer-a conical piston DOES offer advantages over a flat topped one-for 360 degree porting....it is however, more expensive to make and requires a matching contra piston-thereby adding two additional machining steps in the production, many manufacturers dispensed with it in the interests of production economics. I can think of several examples-(the Allbon Dart is a good example) where early models with a conical piston crown outperform later models with a flat crown-and no alteration to porting has otherwise occurred. In cross flow scavenged and loop scavenged layouts it offers no advantage (which is why the Super Tigre and Enyas didn't use it-and now that schneurle porting is virtually universal, again flat topped pistons are the norm.
Also pertinent is that diesel ignition is bordering on a detonation, not a defined burn as in a glow engine, and you have another reason for not wanting localised hotspots.
Recycled and I agree on the main purpose of a baffle.....the straight fence one though is dictated purely by production economics-no one would argue that it is an efficient design. The better performng cross flow engines (think McCoy 60, Dooling 61 etc) had quite complex piston crown shapes-and the baffles were curved and contoured quite considerably.....all in the interests of better gas flow.

For brokenenglish-the Mills and Comp Special pistons may well have used the cutouts to alter transfer timing-but with that would inevitably come imposition of directional effects on the charge as the port opens-the incoming high pressure charge sees only a narrow contricted exit which is oriented towards the upper cylinder-which is essentially what is required in any case-we don't want the cool high pressure incoming mixture mixing with the hot downgoing expanding exhaust gases if we can avoid it. By the time the transfer port fully opens, the exhaust is already fully open and gas pressure has dropped significantly. The quest of the 2-stroke designer has always been to maximise cylinder filling, maximise scavenging and minimise fresh charge loss during scavenging-all of which are mutually exclusive to a degree-though I'd argue that modern designers have done a pretty good job to date....

ChrisM
'ffkiwi'

Recycled Flyer

Hi Chris,
what modern high performance 360 degree port model diesel offers a conical top piston?
Oliver, Parra, the Taipan 2.5's (and variants) or PAW don't because they know its pointless.

I would say that if the engine leans on the crutch of needing a conical piston to improve its running then there something really wrong elsewhere - probably with the transfer system.

Adding real estate to the piston crown also adds considerable weight.

I have an old Sokol Russian diesel that used to have a conical piston but now has a flat topped one and matching squish type head.
The piston is now 1/3 lighter and the engine runs FAR better than originally intended.

I am curious mate, what advantages are you alluding to?

steve111

Funnily enough, the topic of conical pistons came up over at Dave Owen's only yesterday, while looking at a couple of dismantled ED Racers. A while ago, Dave did a bit of experimentation with a conical piston in one of his T2.5 prototypes, and it made no discernable difference to either performance or handling. In fact, the only effect was to needlessly complicate the manufacturing process, so he went no further with it. Steve Rothwell has apparently come to the same conclusion.

fiery

My Feedback: (2)

Thanks for that Steve. Chris advised that the Allbon Dart has a conical crown piston. When the Dart became DC badged the piston was changed to a flat crown. The Allbon's are reputedly faster.

It would appear to depend on application. Perhaps the Allbon built Darts just had better fits.

Recycled Flyer

Hi Steve,
forgot to include the R 250 and 320 but similarly Dave mentioned the same to me about piston tops years ago.

It just seems that if you have to have a contoured crown to direct gas flow then the port system can't be doing its job too intelligently.

It 's just that a flat crown is lighter and, as you say, easier to make - but I can see that the cone is visible and 'obvious' in that it physically seems to have a task to do, same with a baffle. Both 'look' purposeful but are they?

123Cat

Hi
I noticed that the MB 40 has a cone type top with a 0.9 mm height and a 61.7 mm radius , why is this ?

fiery

My Feedback: (2)

O.S. believed they had a world beater. 'Twas not to be.

Recycled Flyer

My guess is that it has more to do with the crown not collapsing under the regime of Pylon racing.

123Cat

I see,

so its thicker in the centre so it can take more abuse ,

Recycled Flyer

Possibly, or simply just having a slight even thickness dome will be more resistant to pressure.

My old Yamaha RD 400 had slightly domed pistons -- never really pondered why other than aluminum has poor strength in tension when hot.

gerryndennis

G'day Steve, those would be my Racers, a bit of a dogs breakfast to quote David.

He is going to fit a flat topped piston and contra to one of them for me. Will be fun seeing if any of the old blokes at the club pick it up.

I'm not expecting any difference in the running.

I assumed that the cone in a peripheral port engine worked in a similar manner to the baffle in a cross flow engine ie it helps direct the incoming transfer charge up into the cylinder which helps chase the exhaust gas out. It seems that a flat top piston works just as well (in most cases?) I guess the transfer flow meets in the middle and goes up any way.

Dave H

ffkiwi

OK-a quick check around the stable: Allbon Dart, Allbon Spitfire (1 and 2), Allbon Javelin, Alag X-3, TWO (!) PAW19Ds, ED Baby, ED Fury, Super Fury and Racer, Webra Winner and Record (all models), CS 147 and 247, Mk-17, all the AMs, Webra Mach-1, Frog 249, all the Taifun diesels, early Frog 150-Amco 3.5, all my Elfins-all with conical pistons and contras............and I'm sure I could find more examples................WHAT do these all have in common? .....well what they have in common is poor transfer ports-with no directional profiling-either the classic 'ED/DC/AM sawn exhaust and sawn transfer one above t'other or slightly staggered-or a milled internal flat topped transfer-in other words-(Webra Mach-1, Elfins , CSregardless of the exact porting method used, there is no attempt to direct the incoming fuel charge by the transfer port shape-this function in these examples is achieved by the conical piston shape.

I'm still not convinced by the 'modern 360 degree' arguments-and whilst I have enormous respect for David-did he try a flat topped piston and contra and a conical ditto in the SAME cylinder, in the SAME engine. If not-then extra variables have been added to the experiment and I question its validity (or rather the validity of the conclusions drawn). It is quite possible that there is no detectable difference-in handling or performance-but I would suggest it would be credulous to think there is no difference in efficiency and combustion process between a flat top and a matching conical piston and contra...the only valid experiment in this situation is using the same engine and changing nothing else....

Finally-the only concrete proof of my position would be to do the reverse option-replace the conical pistons and contras in my examples with flat top ones........

I have however no logical explanation for PAW abandoning the conical setup, other than production economics, as AFAIK the PAW cylinder porting style has remained unchanged since their inception. Note I said porting 'style'-that does not imply port areas and timings have not varied over the various models. Of course I have no argument with a manufacturer changing piston style when porting changes are also made.

ChrisM
'ffkiwi'

Recycled Flyer

Thanks for the reply Chris, I thought that PAW dropped the conical piston around the time the squish band heads were introduced and weren't they a prelude to muffled operation?

Something else with the cone heads besides the extra weight, theoretically they should retain the heat all the better so perhaps they suit low speed and throttled applications better?

ffkiwi

`I'm not sure mate-PAW chronology is a bit of a mystery when it comes to pinning down-these two are vertical intake-but that could place them anywhere from ~1962 through till the mid 80s when the angled intake came in. Apart from the directional effects-which as I indicated earlier are probably only relevant with less developed porting (I should have been more specific-sawn ports and internal flutes machined parallel to the cylinder axis with a flat ended mill (classic Webra, early Frog, early Allbon and DC, Elfinsand anyone else who pinched Webra Mach-1 porting (a lot of guilty b...s out there!)-these are generous transfers-but they stop in a dead end-the gas has to do a 90 degree bend to enter the cylinder-in these instances I think the conical piston is doing something useful. [if you think about it-WITHOUT a conical piston in this scenario the incoming fuel charge shoots straight across the piston and out the exhaust port....with certain negative impact on fuel consumption-at the very least-might not be so much on power if other aspects of design are limiting.

On the ignition side of the house-when you think about it, a flattop piston is compressing a disc of fuel mixture, a conical one is compressing a dished cone shaped volume-I'm entering into pure speculation here-the former is likely to ignite randomly-with the disc area, the latter at the point of the cone and progress outwards-that particular aspect might promote less violent detonation of the charge and smoother running. A high speed camera look at the combustion process in slow motion might be very interesting .....that being said I accept the other arguments about conical pistons being generally heavier (yet presumably they could have thinner crowns than an equivalent flattop one as the conical shape could be said to possess similar strength to an arch structure-and hence less likely to suffer a collapsed crown failurethan a flattop.....

Anyway i doubt anyone is going to get anal enough to check out the range of possibilities and explore some of those issues...

ChrisM
'ffkiwi'

Recycled Flyer

I am sure that the Team Race boys have all over this and back again - its just hard to get any one other than Grant Potter (when he is fired up enough) to comment about any of this kind of stuff.

Another thing to consider with domed/coned pistons in two strokes is that the timing gets kinda fuzzy the more you angle crown and the flatter it is the more likely precise timing figures can be held.

So it seems that a lot can point to having a flat piston crown and I wish that Pe Rievers was still with us to add to all of this - he said that his best home made diesel had a flat piston and head.


Cheers.

steve111

Same cylinder, same engine, Chris.

Recycled Flyer

I would imagine that Steve Rothwell would have left no stone unturned with his R250 either.

controlliner

I had a Super Tigre G.23 glow engine with such high compression, I could run it on 1/2 A diesel fuel. It ran real well.

raglafart

My Feedback: (10)

Even you should realise Chris from all the discussion here and on Barton that the R250 is an Oliver clone and has to be identified as such to qualify for Vintage A t/r comps. It had to have a flat top piston to be legal!
That's not to say any stones were left unturned by Steve as proved in the demand for his engines and the results he has achieved


Ooops..........the MK 3's have a conical head