Jim Thomerson

Does diesel combustion chamber shape make any difference?

pe reivers

I wonder.
Have run diesels with narrow conical shapes, and have tried own design squish heads.

AMB

Pe and the rest of the group, think there are several factors, a complete even charge ready for ignition, this of course determined by the intake ports pushing up the charge and getting a complete even fresh fuel air mixture ready for ignition if you do not have an even charge, an issue, the combustion area consiting of the head, cylinder walls and of course the top of the piston which is the movable part (down) It is desirable to have the main force of course on the piston center, the main variable here being the head containing the contra piston
and this also applies to the classics with the contra in the cylinder thus the squish design is critical, Davis has solved this with his designs which convert many modern glows to diesel
and even most classics built as diesel take this into consideration ,We are assuming that the mixture is correct not lean or rich, which will upset combustion if not on the money
Its quick, lets pick an RPM number say 9000 RPM which may be a little low but most engines can spin and of course much more but even at 9000 RPM divide that by 60 and thats
150 firings (PER SECOND) everything has to be on the money for a good runner, Again I am not a combustion engineer, and will leave this to the experts, Davis, Owens, and Burford
for better explaination
It is interesting that after decades of production Davis was inducted into the AMA hall for fame for his contribution to diesel martin

Recycled Flyer

Well I would have to say 'yes' to that.

It seems, in model sizes anyway, that a 6 thou squish band to increase the turbulent intensity in the combustion chamber is a good place to start. The squish band increases the burn rate of the fuel by forcing the fuel charge to the centre and
the width of the band can increase or decrease the charges timing somewhat, too much squish can kill high preformance speed whereas too little does the opposite.

I assume that combustion is spherical in shape so a matching spherical head will direct that charge like a lense down against the piston crown.

I have it on good athority that there is no advantage in using a domed piston crown and since its far easier to make it flat topped, most manufacterers go this way for ease of machining.

If the contra is an insert in the head (like most diesel conversions are) it's important that the 'run' setting does not allow it to protrude below the insert ring that houses it. If the run setting dictates this then its very difficult to achieve a good state of tune s the gas flow will be forced to flow over over extended contra edges.

Thats how I see it but I am sure that others will chime in on this.

There is a link on theglow forums that you participaetd on years ago mate, here it is -
http://www.rcuniverse.com/forum/m_33...tm.htm#3341100

This contains some good information also, cheers.

AMB

ReCycled flyer Some good points, in some full size diesels they do have a dished piston to take advantage of this , wants to expand in this area first, even in munitions the charge is shaped by its container to direct it straight forward to penetrate the thick armor of say a tank,
to punch thru it rather than just a random blast
any disturbance to this area poor design or geometry can be an issue this may explain the issue with the pings in 4 stroke conversions
I do remember a film clip of a full size engine with that had a quartz window in the head or cylinder and the combustion pattern filmed with a high speed camera martin

gkamysz

My Feedback: (19)

What happens in a full size diesel is very, very different than a model diesel. Large amounts of swirl are used to help the injected fuel burn more completely.

The model diesel combustion process is the same as full size HCCI engines.

pe reivers

From theory, or did you try it yourself? I found NO difference in rpm on the same prop. Tuning compression was easier with the squish head and adjustable central section. That's all.

Recycled Flyer

Hi mate,
From theory of course as even engine designers in large automotive plants have trouble proving exactly how much turbulance is going on inside a combustion chamber.

But I back up what I say from what has been told to me by Team Race flyers, but personally - I suspect much but know little about advanced gas flow technonolgy. But as with most things in life I go with my instinct, that and ask those who are a bit closer to knowing than me and then form an opinion.

And I am certainly open to changing my mind on this, so you doubt that a squish band actually increases turbulance? Why?

steve111

When testing the prototypes of his T2.5, Dave Owen tried both conical and flat-topped pistons, with the intention of using whichever one worked best. In theory, the conical one was supposed to give easier handling (don't ask me why - I've got no idea, and I don't want to go there!). In practice, starting and handling were just as easy with either piston, and the flat-topped one gave better performance.

Recycled Flyer

Hi Steve, that was my source as well and Ifully believe him.

Ihave always considered the conical topped piston absorbed more heat due to the greater surface area and this was to the detriment of the contra piston where the heat should be, again simply my opinion here.

Thanks.

Jim Thomerson

I think having the charge well mixed is a good idea. My thinking has been that temperature needs to be more or less uniform throughout the combustion chamber, so that the charge fires all at the same time. On the other hand, I wonder if having a hot spot, similar to the glow plug in a glow plug engine, might be a good idea. This might give a more protracted and complete burn, starting at the hot spot and propagating from there.

Recycled Flyer

I believe that carbon deposits can create hot spots in model diesels and this tends to be a bad thing.
Most fliers remove carbon for this reason.

And would not a baffle topped piston create a similar hotspot?

Recycled Flyer

Actually Pe, now I come to think of it, I can offer up some proof of the squish band creates turbulance towards the centre of the combustion chamber.

A lot can be gleaned from the carbon patterns on the piston crown and when a band is used you will find that the area on the piston under it will be rather free of deposits when compared to the area under the combustion bowl.
This tells me that there is little going on in the band area and the fuel/air mixture has been migrated by pressure into the central area. The mixture has in fact been 'squished' by turbulence away from the cylinder walls.

Now I believe that if you over do the ratio of squish band to combustion bowl then you in fact get a reverse effect happening and the burn pattern starts to favour the entire piston crown once more.
Please accept that I have not done any physical research in this area but it does seem logical to me.

Thanks.

gkamysz

My Feedback: (19)

I think we all understand squish creates turbulence. What effect it has in a model diesel engine is in question. Like anything else in an engine, it's a compromise. Turbulence requires crank power to create. More turbulence means greater heat transfer. If turbulence delays autoignition in a spark engine what does it do in a model diesel? When you actually draw a diesel combustion chamber to scale, you'll find the ratio of squish volume to clearance volume is small, especially in a smaller engine. If you shim the head to run the contra flush, it kills any squish that was there.

By proof I think Pe was asking for some observed change in performance.

I've not done any testing in this regard, but it's on my list of tests. For sport I doubt you'll see much difference.

pe reivers

@ recicledguy
"And I am certainly open to changing my mind on this, so you doubt that a squish band actually increases turbulance? Why?
"
This is NOT what I said. I said I found no gains (or loss for that matter) in my head shape experiments. Did you? or did you not test yourself?

Hobbsy

My Feedback: (102)

One thing that has always been a great mystery to me when discussing model airplane engines is the term "handling". When I say that my ole 95 F-150 with 170,000 miles is a lousy handler all of you would know exactly what I'm referring to. Its big and long, weighs 4,000 pounds and turning quickly is not its forte. This term when applied to an engine just doesn't make sense to me.

PS, the ole F-150 is always our choice when going on a trip, quiet and smooth are its forte.

I'll post some pics of some conversion heads and y'all can discuss why some are the way they are.

gkamysz

My Feedback: (19)

Handling in a model airplane engine refers to the ease of which a good tune can be achieved, ie. tractability. I think the term comes from European translations. You might say a Saito FA-80 on 30% nitromethane doesn't handle as well as an OS FS-40 pre Surpass series on 10%. If you are patient and observant any engine can be made to behave. On the other hand, some have trouble even with the most docile engines.

Hobbsy

My Feedback: (102)

Here are some combustion chamber shapes to gaze upon, Fox .15 and .50 Davis heads, SuperTigre 40/51 Davis heads, Irvine .40 stock Diesel head. #4 is a Fox .74 glow head with perfectly flat squish area, Fox .60 glow head tapered all the way and a Davis Fox .60 conversion that is half flat and half tapered. Go figure. If I mixed up the sayin and the pics, let me know.

Fox .74 glow head is by AJCoholic and is according to the Fliteline Solutions solution.

pe reivers

The chambers of your vast collection are all squish design. In plug engines with a well designed start of combustion, this aids in turbulence troughout the charge.
Diesels are different! They do not have a single spot where combustion starts, and if they do, there is no knowing where! That is why some parallel chambers work so well, be it conical or flat. The very narrow combustion chamber, and vicinity of cold metal to carry off peak heat spikes will control pressure rise to the extent that the engine can run smoothly. We all know the laboring sound of an overcompressed lean engine in search of best rpm, and then quickly reduce compression and increase fuel flow.
Therefore I doubt the advantage of a squish design, though many manufacturers seem to incorporate it just in case. If it don't harm, why not do it?
My nicest running head had a flat counterpiston of half cylinder diameter. Squish was at 0.4mm. When dismantling the head after a good run. the counterpiston was exactly flush with the squish! So no squish at all!

Recycled Flyer

Pe, I agree with you, thats not what you said at all. I took your inconclusive performance results form posts 2 and 7 as 'doubt' that the text that you highlighted in post 7 (to increase the turbulent intensity in the combustion chamber is a good place to start) was valid. In other words I sensed that you doubted that quoted statement.

Now I have indeed answered the question of "Did you? or did you not test yourself?" twice already, once in post 8 with "From theory of course....' and that answer was chosen from the two chioces you presented me with me in post 7 "From theory, or did you try it yourself? (I chose theory of course.)
And further in post 13 with "Please accept that I have not done any physical research in this area but it does seem logical to me." In other words Ihave not done any testing outside of extensive research.

And thanks for the above information concerning squish bands as it makes good reading.

Cheers mate.

gkamysz

My Feedback: (19)

Just so you know, that highlight is in your original post. For some reason the font is different in that part of your post.

Recycled Flyer

Hi Greg, just went back on checked that and you are right - it probably comes from typing in Word firstly and I must have selected varying fonts.

Thanks.
P.S. Here is a good read on squish - http://docs.google.com/viewer?a=v&q=...aW3A-KBEnULqSQ

pe reivers

I have done quite a few experiments with squish shapes, not only in diesel engines.
True, the carbon pattern changes towards the center of the piston. In the squish zone, the temperature cannot rise so much because of the close vicinity of cooling metal, hence less carbon.
Interesting is the fact, that a central counterpiston inside the head has three stages, progressively changing as you srew down that tommy bar:
Normal circumferential squish at low compression.
No squish if the counterpiston is flush with the head
A toroidal chamber with central squish zone at high compression.
All this depends on the way you design the head clearance.
I found best running with the counterpiston flush. Had I chosen less deck clearance, my results might have changed, though from theory I doubt that. The squish would then probably reduce power because some of the charge gets ignited too late.

Recycled Flyer

Hi again Pe,
I have read engine tests done on diesels at a time when silencers were coming into vogue and think that PAW used to make their piston crowns slightly conical in shape with a matching contra piston to suit but when silencers were considered they changed to flat topped pistons with a squish band/bowl shaped combustion chamber.

So perhaps back pressure comes into play here with the added turbulence of the squish band aiding gas extraction against that pressure all the better.

It amuses me to see some of the older and simpler designs, and even some of the home made stuff - they seem to get away with just about anything in regards to gas flow.
As long as the charge can get from A to B thens its going to work and work fairly well at low speeds, and its only at the higher speeds that all this seems critical.

I agree with about the contra piston being level with the squish band as it seems to give the most power when set that way but I still like to see a bowl shape in the centre as to me this would seem to direct gas flow away from any dead spots around the periphery of the chamber.

Interesting stuff and I am sure that there is much to come from free thinkers like yourself.

gkamysz

My Feedback: (19)

It's not the first time I've read about squish clearance of .005". The article was detailed regarding actual calculated figures, but did not describe any changes in consumption or power. Saving plugs is great if your engine uses them. So, we are still unsure of how squish applies to model diesels. One statement is simply incorrect. The article states that thermal efficiency of engines has magically improved to 75%. The enormous Sulzer ship diesels are pushing 50-51% these days. and something like a VW TDI is 41%. It's not even possible to exceed ~70%. I have the both Blair books quoted in the article, good reference material.

Oh, and I read ACLN regularly, but not in the last couple months, many good articles in the past.