qazimoto

My Feedback: (1)

It would seem that the Eifflaender's, John Oliver, and Tom Ridley have come to the same conclusion about conical/domed pistons.

I just checked a few late model PAW 15 GTS and they all have flat top pistons (as expected).

I checked two R250's, numbers 17 and 18 (from the first batch). They had moderately domed pistons originally , but after their first rebuilds by SMR came back with flatties.

One of the domed R250 piston and rod went straight into an Oliver Tiger liner with a conical contra and it runs great. I was impressed how the inside piston crown had a curve that matched the dome. I'm sure that it was a curve rather than a conical shape.

I took delivery of some stuff from Tom Ridley yesterday. A new MK2 Cub and a spare p/l (see pic), both pistons were flatties. I replaced the p/l of the other mid 70's cub with a new one from John Oliver about 2005. The original was conical, the replacement was flat.

I'd agree that production costs is perhaps not so much a factor especially after watching a CNC work centre cutting a piston. Once it's programmed a few more steps isn't going to matter. I'd say that the evidence points to it not affecting performance, and that Ockham's Razor (simplest is best) has been applied.

It should be pointed out that the Oliver style ports are highly directional. Some people mightn't realise this. I had a conversation with Andy Kerr quite a while ago about the topic. He suggested that if you placed a bit of suitably sized wire in each port, then all four would meet at a point just under the contra at about it's optimum position give or take the diameter of a mosquito's urine stream. Apparently after much experimentation the stock position was found to be best.

brokenenglish

I don't recognise the Olivers shown, but I see reference to a Mk II Tiger Cub.

Please let me know what this engine called in Oz, so that I can correct the name...

Recycled Flyer

Could that be a Moki?

Recycled Flyer

[QUOTE=qazimoto;11809206]

It should be pointed out that the Oliver style ports are highly directional. Some people mightn't realise this. I had a conversation with Andy Kerr quite a while ago about the topic. He suggested that if you placed a bit of suitably sized wire in each port, then all four would meet at a point just under the contra at about it's optimum position give or take the diameter of a mosquito's urine stream. Apparently after much experimentation the stock position was found to be best.

/QUOTE]

Hi Ray,
it seems that the latest technology in full sized two strokes is about getting the incoming transfer streams to collide together at roughly the centre of the cylinder volume.

The point here is to get the charge to linger longer by temporarily stagnating before fleeing out the open exhaust port.

Probably shows that Oliver and Arden style porting was way ahead of its time!

Cheers.

qazimoto

My Feedback: (1)

Why would you want to correct the name? It looks exactly like what a mk2 Olly Cub with a serial number of 274 should look like.

The engines in my pic are all genuine Oliver Tiger Cub MK2.

Are you referring to the old mk2/mk3 canard?

ffkiwi

I am in general agreement when it comes to directional porting in a 360 degree environment-my earlier point was in the context that the wide number of examples I gave did NOT have directional transfers and hence would have gained some benefit from a conical piston. It is hard to isolate the various effects occurring of course.-and factors such as piston weight, transfer area, bore accuracy and the like. Many of you here (esp Aust and NZ members) will know of Harvey Westland-our best engine designer (a pity he's dropped out of the game in favour of gunsmithing!)-I interviewed him about a decade ago for 'Model Engine World' -and during the course of that interview (we were discussing his pylon engines at that particular stage) he commented that as little as 2 microns-that's 0.002 mm or 0.000078" (yes 78 millionths of an inch) out of round was detectable as a loss of performance. I doubt anybody in the 50s or 60s was machining diesels to that sort of precision...
The other issue is that there are at least 4 independent factors contributing to the effect-crankcase compression volume, transfer passage volume, transfer port area, and transfer port orientation (ie angling)-defective or excessive design in any of these areas will lead to reduced performance-and all 4 interact to achieve an overall process.

One rigorous way of putting the concept to the test would be to set up a gas sampling experiment (as is done with car tuning) and measuring the unburnt fuel content of the exhaust gases. If my reasoning is correct-a flat top piston in an engine with poor transfer angling should allow more charge to be lost out the exhaust than one with a conical piston under the same conditions....

I have no difficulty at all with the observation that a modern replica utilising 360 degree porting with angled ports would show no difference with a conical vs flat top piston. That being said, I doubt we have given anything like the same amount of attention to the combustion chamber shape in diesels that has been done with glow engines....

ChrisM
'ffkiwi'

gcb

My uneducated guess is that the small extra combustion surface created by the cone would not produce enough power increase to justify the extra production costs over a flat-top piston.

George

Recycled Flyer

I can see that shaping the combustion chamber away from the pistons line of travel (degrees above or below it being flat) should result in the charge forces following a 90 degree from angle of that shape.

In other words I see the ideal line of force from combustion being straight towards BDC and this is how a flat topped piston, and flat head would resolve the forces of combustion - you want all of the force acting against the angle and shape of the chamber.
But with an angled surfaces the forces will resolve at angles perpendicular to those surfaces just like in shaped charge explosives.

So a cone head piston will resolve the pressures against it with a partial inwards pinch crushing against the piston crown itself and waste some of the energy otherwise useful for power generation - or the more angle you have towards the pistons line of travel the more (in effect) sidewall you will have.

Hence why flat surfaces at 90 degrees to the piston travel are the best for rapid detonation engines - they simply transfer more energy into the correct direction.

And I can well imagine that a lot of attention has been given has been given to model diesel combustion chamber shape but its been belied by its simplicity.

gerryndennis

[QUOTE=Recycled Flyer;11811578

Hence why flat surfaces at 90 degrees to the piston travel are the best for rapid detonation engines - they simply transfer more energy into the correct direction.

[/QUOTE]

Nope, same amount of energy.

You are correct when you say the pressure is at 90° to the piston crown, so in a coned crown piston some of the energy will be wasted squeezing the piston rather than driving it down, however that same piston must have more surface area. Therefore there is more force (same psi X more square inches). The wasted force (squeezing) is exactly balanced by the extra force available due to the extra surface area.

Or to think of it another way. Imagine the cone made up of lots of tiny steps horizontal and vertical. The area of all the horizontal bits ( assume upright cylinder) has to equal the area of a normal flat piston. Same area and same pressure, same energy transfer. Keep making the steps smaller and smaller, the horizontal area remains the same but now you have a smooth cone.

Unless I've misunderstood your post?

Dave H

Recycled Flyer

Hi Dave,
I think that you get what I am trying to say but how does one resolve 'pounds per square inch multiplied by more square inches' and say that it equals the same amount of force?

Surely the more area you have for a fixed amount pounds then the ratio, pounds:inches, drops?

For example the highest psi for any given amount of force will be found contained by the smallest area, practically inside a sphere and conversely the lowest psi will be found in an infinite area, say in space.

So the goal in trying to achieve the highest psi ratio with out resorting to using more energy or pressure is to reduce the inches that it acts upon to a minimum.

Happy to be proven wrong though, thanks.

qazimoto

My Feedback: (1)

Gerryndennis is right Chris.

The "projected" surface area of the conical piston top to a vertical force will be the same as the actual surface area of a flat top piston of the same diameter.

It's a Geometry thing :-)

Recycled Flyer

Ok, what I failed to emphasis last post was the agreement that pressure only acts at 90º to its container, so there is no vertical, no horizontal component, there is no 'top' force - it simply pushes equally everywhere at once. There is in fact is no 'projected 'area as pressure has no point of view or direction as required by projection.

So if you have a piston crown area that is sloped at 45º the pressure acts inwardly at 45º and since its a symmetrical object there will be an equal component on the other side.
Now to resolve this simply draw the lines of force from any two diametrically opposed points on the crown, intersect them and resolve the forces. You will have a vertical component available for piston travel and a self cancelling wasteful horizontal crush component. The more obtuse the angle, the more the horizontal crush component, the less the vertical travel component and therefore more wasted energy.

The same with the combustion chamber, the pressure at the sidewalls does nothing for pistons travel as the resolved forces there are self cancelling, acting in matched pairs at 180º to each other.

The more closely you perpendicularly align the pressure vessels surfaces surfaces to the desired piston travel the more force can be resolved out of it.

The hurdle here seems to accepting that pressure can ONLY be resolved at 90º to the surface being pressurized.

AMB

Ok guys great topic lots of info, now just to add to the thoughts go on google and look up on dished pistons used in full size diesels no point in repeating look it up and the hot rod guys increase their CID for more power in gas engines


martin
now that I have thought about this I do not know if we would get a clean sweep of the burnt charge out the exhaust port a a clean fresh one one the intake on a 2 strk

qazimoto

My Feedback: (1)

Chris,

I'm a bit confused by your post above but I suspect that you're arguing both for and against 500 years of established vector mathematics.

The piston in an IC engine is there to convert the vertical component of the energy generated in the combustion chamber into a downward movement. The overwhelming quantity of this energy will act in that direction 'cause there's no where else for it to go. Yes the rest of it is wasted.

So what's in contention?

Recycled Flyer

The shaping of the combustion chamber.

The contention here is "Does a flat topped piston and flat head result in more useable force than its sloped and angled cousin?"

You seem to agree that the vertical component is useful and the rest wasted, so I see the that more angle means more waste as it resolves itself into being more self cancelling.

Thanks for your help here Ray

steve111

Chris, maybe you're looking at it the wrong way. If you can accept that the combustion chamber pressure is the same for a given point in the ignition sequence, and for a given chamber volume, regardless of the piston shape (I suggest that to you with a due sense of dread and trepidation...) - then you have the same force per square inch. By increasing the area that pressure is acting on, you aren't actually 'diluting' the force available. There is actually more total force, as the same pressure is acting on a larger area. You aren't really getting something for nothing, though, as the horizontal components of that force all cancel out, and you're left with the same vertical component in either case.

Clear as mud?

Recycled Flyer

I can see that his one is going to involve my kids crayons and a bottle of red wine as my head hurts already!

But I kinda get it now, thanks.

As I said, happy to be proven wrong.

steve111

No worries Chris. I sense that perhaps part of the issue here is a concern about conservation of energy. The work done is force x distance - the horizontal force applied against the vertical component of our cone is not acting through any distance, so no work is being done by it and no energy is being expended. The available energy is all converted to work being done by the vertical component of the force against the piston, just as it is with a flat-topped piston.

I suppose there's a small amount of energy lost in heating the slightly greater surface area of the conical piston, but let's not go there!

fiery

My Feedback: (2)

Here we have two cross flow scavenged diesels from Modela.

A 2.0 DFS, and a 1.5 DFS.

The first has a 'flat top' piston. The second has a shallow 'conical top' piston.

If anyone can explain why, please feel free to advise.

wjvail

My Feedback: (3)

Err... Commenting on the topic of this post - baffle piston diesels - it seems to me that a question to asked and answered would be how to key the sub-piston to the cylinder. If you built a diesel with a baffled piston it would have to slot into the head to have anywhere near enough compression. Since most all model diesel engines have a sub-piston and tommy-bar compression adjustment, you'd have to find a way to make sure the sub-piston didn't rotate. Keying the sleeve is the obvious answer but cutting a slot into a sleeve could lead to a stress point and cause the sleeve to go out of round with temperature change.

A solution might be similar to the Drone diesels from many years ago. They didn't have adjustable compression and compression was altered but using head shims. Since I don't own a Drone, I can verify what they had.

Further, the overwhelming drawback to baffled pistons is that they do key into the head and effectively divide the combustion chamber in half. In a glow motor this is a nightmare and in a diesel it could lead to half the fuel air charge not burning.

Anything is possible but I see all kinds of problems with this and there would have to be some considerable advantage to go down this path.

The short answer is I don't know if any model diesel engines have been built with a baffled piston, but if there were, there were not many.

Cheers,

Bill

[email protected]

Bill, earlier in this thread the O.S. .15 diesel was listed a having a piston baffle and adjustable compresion. I had one late 1950's into the 1960's. Never had a problem with this engine. Was a great combo with the original size Flite Streak. Jack

qazimoto

My Feedback: (1)

I gather that the OS 15 Diesel was a good engine. Surprisingly I know quite a few people, maybe four or five who actually own one. I have seen one used and it went well.

However it wasn't a great engine, and it was more expensive, heavier, less economical, and marginally less powerful than the Enya 15D. While the Oliver Tiger mk3 was more expensive and harder to get it was just as powerful, lighter and much more economical. At a time when hundreds (and perhaps thousands) of people flew c/l team race, these things were important. Model engine design moved on quickly and AFAIK there never was another baffled diesel.

Adrian Duncan has written an excellent article on the topic and it's available on the MEN site.

Rather than continuing to flog a dead horse about conical pistons can I just point out that the angle of the cone is only about 10-15 degrees at the edge on the engines I have. These are mostly Taipans, Olivers and Rothwells. That's only a very small cone!

Ray.

wjvail

My Feedback: (3)

Thanks Jack. I saw where there was reference to the OS earlier. Before this thread I was unaware any had ever been made. Out of curiosity, do you remember how OS kept the contra-piston from rotating?

Bill

qazimoto

My Feedback: (1)

It's all explained here in Adrian Duncan's fine article: http://www.modelenginenews.org/ad/max15d.html

Mr Cox

I don't think that's any (major) problem.
Once the transfer port is blocked of it is essentially one combustion chamber, although with a rather odd shape, that's all...