qazimoto

My Feedback: (1)

Yes I agree, ACLN is clearly the best source of innovation today in the c/l modelling world.

The article's author is a well known Australian/New Zealand engine man who works as a Chemical Research Scientist. He's right on top of this stuff. I think you'll find that he's talking of the thermal efficiency of model F2C diesel engines. The current world champion F2C team is from his club so he'd be right up with developments.

The application Lance describes are in C/L B Class Team Racing where improvements are measured in range increases and airspeed. These are described in other articles. The specific contribution he makes with the article is the notion of reducing the squish height of the OS 25 FX engine when detonation occurs rather than raising it as was the prior practice.

Looking at the two push-pull heads from Parra and Nelson diesels I have sitting on my workbench I'm sure that the notion of squish plays no part in their design.

Ray

Recycled Flyer

This site gives 70% http://www.theengineer.co.uk/video/c...004661.article

This site gives 76.9% peak or 75% average http://nelsonengine.com/Thermal.html

gkamysz

My Feedback: (19)

Yup and everything on the internet is true. If you have money to invest don't invest it with those particular companies. Do your homework and you'll find the flaws in those claims.

pe reivers

Yup, they juggle with percentages and heat loss very nicely. For example, friction loss is not carried off by the cooling loss, but added to it. Their theoretical thermal efficiency also is hard to believe the way they put it.
No wonder the engine never saw daylight.

Recycled Flyer

Yeah I have to admit that I was just stirring the pot back there as within the context of this discussion those research based percentages for thermal efficiencies made my head hurt just by reading about it!

Anyway back on topic, there is a great lack of published work concerning model diesel head shape parameters and I get the feeling most designs are settled on more out of ease of manufacture than anything else - just whack a piece of metal in a lathe, choose your favorite angles and away you go kind of thing.

But the squish band/ hemispherical bowl combination seems to be a good balance between ease of making and performance.

Cheers.

gkamysz

My Feedback: (19)

Production today is handled by CNC machines. A turned head is a simple item. Shape is no more than making a sketch. Even if the guys are running manually programmed machines lathework is 2D and easily programmed by hand and a little trig.

pe reivers

Think of what the squish zone has to do, and you will see hardly any benefit for diesel engines.
The squish creates a superfast turbulence in the combustion chamber, that spreads the flame front emerging from the spark plug. It also PREVENTS compression wave front ignition (knock) outside the combustion bowl. So much for spark or glow ignition heads.
In a diesel it is much different. We do not want to ignite the charge all at once, which is not very easy. So if it does ignite all at once, we do not want the combustion pressure to rise too suddenly. This would cause the engine to run very rough. (AKA square running). The chamber shape without combustion bowl consists of a very narrow slit. (Angled or not angled is more a scavenge item). In this narrow slit, excessive temperature rise is inhibited by the presence of the metal at very close distances. This prevents the dreaded diesel knock.
The question now is, how much of the combustion can find place in a small chamber, raising the temperature/time gradient, and how much should take place in the low gradient "squish zone". Mind you, the squish is not needed to spread the flame front here! The chamber probably runs with a slightly higher thermal efficiency, but at a cost.
Knowing all this, think again.

gkamysz

My Feedback: (19)

Compromise. Still, from perfectly flat to squish with domes, a variety exists. The only thing that will tell you what is best for your application is testing. In some cases there may be no difference. Then, you choose the simplest to produce.

pe reivers

The simplest to produce would be the taper piston top, with a small flat top section. Seen those. MVVS has a shallow bowl in the half diameter counterpiston, probably meant for extreme rpm-application, where temperature-time gradient becomes smaller if expressed as temperature-crankangle gradient (which is the proper way to do it). The shape is not difficult to produce. A flat half diameter counterpiston also would create a central combustion chamber.
All those are as almosteasy to produce as a flat or conical section. In an earlier post: current CNC abundance makes it as easy as changing a few lines in the drawing.
Indeed, testing the desired application would determine the choice.

PS
In most literature and scholar books, temperature-crank angle is not mentioned. They mention pressure rise gradient. But since the Boyle Gay-Lussac laws couple temperature to pressure and volume,all can interchange. With the nearness of cool metal walls, the temperature becomes the governing factor in heat transfer, hence my use of temperature instead of pressure, (Mass density not, or hardlychanging because volume can be considered constant at/near top dead center)

Diesel Fan

Whoa! You guys are REALLY into this stuff!

But you conclude that it can't be figured out by pencil and paper and you must try things and see - and I agree but that takes time and a lot of effort.

pe reivers

yes, it does. But isn't that part of the fun?

Hobbsy

My Feedback: (102)

Hey DF, anything that gives us an excuse to run an engine is a good thing.

Recycled Flyer

Hi again,
Just using my own home grown philosophy on model diesel combustion chamber shape, I see that if the combustion can be multi staged or the ignition point (s?) drawn out over as long a period as possible then its a good thing.

And here I have always considered (rightly or wrongly) that the use of a squish band would provide the primary ignition source for combustion due to that particular area having the greater turbulance, the highest spike in compression ratio and then the superheated gases would migrate into the combustion bowl proper to complete the process.

So I don't see the classic squish/bowl chamber as igniting the charge all at once due to a pressure rise lag (or time difference) that exists between the two areas - mind you its not much of a lag but its still there.

Probably why these types of engines are easier to tune and an analogy that I use is that you now have at your disposal an instrument that plays a full chord instead of one note - the more notes in the chord (the longer ignition migration) then the more you will have to play with, so to speak.

Again, I can't supply proof of any of this, its just how I resolve it in my mind.
Cheers.

Recycled Flyer

A good link back to squish bands in regards to model engines.

http://web.me.com/flyingkiw1/Model_A...of_Squish.html

pe reivers

We have had this discussion before. In a model diesel engine, combustion chamber and squish band design work totally different from gasoline or glow engine designs  that have a central ignition initiating point. Large scale diesels have fuel injected in hot compressed air. In these designs squish and chamber shape play very large roles in spreading the flame front, and the squish zone stays cool enough to keep part of the charge from detonating at high compression ratios.

This latter token also can be important in model diesel design. The squish zone stays cooler, so will ignite later than the mixture in the rest of the combustion chamber, thus allowing for less harsh running and wider CR adjustment acceptation. In model diesel engines, the plame fronts originate from the individual fuel froplets, so choosing high swirl chamber designs will not improve combustion speed at all. That is why model diesels run perfectly well with a parallel wall combustion chamber like in the Webra Mach1. The cone shape is for scavenging purposes only.

Recycled Flyer

Hi Pé,
that linked article in its opening drawing shows what happens to gas flows PRIOR to ignition and as such is independant of what type of igntion system or fuel is used.

It attempts to show the benefits of squish as an atomisation tool.

I agree that AFTER combustion has taken place it gets a bit murky in regards to model diesel engines.

And I thought that its at least an interesting read mate.

Thanks.

DeviousDave

I was thinking about how mixture burns in a model diesel on the way home from work tonight..

It would seem to me that combustion would start on the periphery of the combustion chamber and burn it's way inward. My reason for thinking this is that the air fuel mixture doesn't compress at the same rate in the combustion chamber, so the edges of the head which are closest to the bore diameter will ignite first because the air won't have time to equalize pressure with the center of the combustion chamber before ignition starts. I agree that there really isn't a flame path or swirling in a model diesel due to the ether, once combustion starts and pressure starts to spike, the rest of the ether molecules start getting with the program pretty quick.

Makes sense to me at least.

ETA: This of course assumes even air/fuel distribution-asymmetric loading of the cylinder with associated turbulence due to port positions are going to have more say where the party starts, especially at higher RPM.

pe reivers

Over the years, this discussion has been deeply dug out in the forae diesel sections. From this the following emerged:
1) fuel is present in the form of droplets.
2) Some of the ether evaporates at the droplet surface before combustion starts and cools the droplets in the process.
3) in the gas swirl/flow, the droplets are stationary relative to the surrounding oxygen
4) Ether starts burning as soon as the temperature is high enough. (influence of pressure = ?)
5) in a squish zone, the gas is cooled by the metal walls, so the etherwill ignite later.
6) The gas thattransfers from Squish zone to the central chamber is cooler,but has very high speeds.
7) This speed is transformed into heat againinside the central chamberdue toeddy current friction.

There you have it. Droplet burning technology is well researched in fuel burning.

gkamysz

My Feedback: (19)

Recycled, that is the same article you posted earlier in this thread is it not?

Lou Crane

Just decided to look through this long and long-lived thread...

Quick impression is that combustion chamber shape, for our compression ignition, smaller, engines has either little significance, or it is very hard to determine whether there is any significant difference. Two thoughts:

1. A squish zone in the immediate vicinity of TDC may change the rate of charge compression. Compression pressure rise is relatively free to equalize over the full cylindrical shape until the volume is abruptly reduced as the 'land' approaches the head/contra-piston... Not merely turbulence, iow, but change in the available volume to "compress into." This may work well with faster burning combustibles, like alcohols, which also show greater charring inside the smaller, sharply domed, ultimate combustion chamber volume form. Kerosene, I read somewhere, burns much slower than methanol - and that allows us to enjoy a longer useful 'push' through the power stroke. Small squish-chamber, squish-band glow engines seemed to vibrate more, as if the combustion pressure spike had hit the piston like a hammer, not a prolonged push...

2. The intent of a conically domed piston, with matching contra-piston shape, may have had more to do with scavenging than with combustion. The traditional axial array of ports still open in the order: First, exhausts; second transfers. With the conical formed piston, the lag time between port openings may have been intended to initiate flow toward the exhaust ports before the transfer charge enters the cylinder. ...Hopefully, up the middle of the volume, to spread like a mushroom above the burnt gases and help expel them. Whether that makes any difference or not, at least glow engines with the peripheral porting 'seem' (to me, anyway) to make louder exhaust noise than other porting types.

Target bull's-eye pasted on. Awaiting comments...

Recycled Flyer

Hi Greg,
Basically you are right but the above mentioned diagram seems to be missing from that extract, so when I sourced the original I wondered about what happens before top dead centre as oppossed to what happens after.

There seems to be far more information concerning real diesel and gasoline engines than glow or compresssion ignition engines but all that only matters post ignition.
What we now see here is what happens pre igntion and simply deals with gas flow in model engines
regardless of what gas it is and how it is to be later dealt with.

And I agree that gas flow from a squish band would be cooler than that of the centre of the chamber, so would it ignite later than the gas in the central chamber?

I am not so sure now since both residual heat and compression will give rise to combustion, and the squish band seems to trade off residual heat for a greater compression spike.

As to a kerosene/ ether fuel burning slower, I thought it was the opposite since combustion is completed well before the exhaust port opens whereas on a glow engine you can get flames exiting.
This is why diesels are quieter in operation and need less of a muffler or am Imissing something here?

Anyway, always learning!

AMB

Hi guys, theory only (mine) .. large diesels were always in a state of pre-igniton which puts on hellava load on the engine, it was overcome by "overbuilding the engines rods shafts to take the stress hence their mass and size of course influenced by the timing, some has dished pistons which maybe the force of combustion found an easier path for its direction down rather than the sides of the cylinder average injection pressure maybe 1500PSI out the nozzle , then comes along piezo electric injectors and still a high rail pressure to injector maybe 1500 psi these build to a very high pressure before popping maybe 15000 PSI or higher so instead a coarse spray you have a "fog". with this set up maybe the timing can be reduced a bit still for a good burn The pre-ignition rattle is reduced. no much louder than a gas engine yes you still have that diesel sound but a lot quieter
Our model diesels using ether as the igniter may be a carrier for the kero to mimic a similar effect.(samaller particle size) It has been point out the combustion is over no flames out the exhaust port'
compared the glow or gas model engine (which does still burning). Of course the engine must be set to avoid pre-igniton by overcompression again a factor of timing

well just an idea martin

of course squish band shape and area are still a big player. Its really gets interesting when you look at the variety of heads Davis has available for many size engine conversions
it appears the squish area may be designed for each engine not a generic shape, one fits all

Lou Crane

Hi, Recycled,

Don't know if you caught my 'alternative explanation' post before your recent about (in effect) flame propagation speeds between kero and methanol...

The flames occasionally spewed from glow exhausts, imho, may relate more to the excessively rich mixture settings we use:

- to assure there is ample methanol to burn - or if not, to cool...

- may burn the remainder of unburnt methanol released to the atmosphere at port opens, while kerosene (paraffin - does anyone still call it that?) has burned more of its weight present, more slowly, and has less flaming energy when ExOpens. I "feel" that diesels' quieter exhaust may be more due to the greater extraction of combustion force prior to ExOpens. And, of course, at hundreds of 360° rotations per second, the somewhat less than a quarter-rotation while the burnables burn takes very little clock-time... The difference is relative, not absolute. My stopwatch-thumb might not catch the time difference...

- For squish&bowl glow combustion chambers, the turbulence from closing the 'land to head' volume may help a MORE rapid burn during the very brief clock-time the combustion chamber is in that optimal condition. Which very rapidly changes, as the piston descends, to a much larger volume. Doesn't that usually mean a drop in pressure? Within a few shaft degrees ATDC, the volume, into which combustion heating is expanding, changes to much larger...

I also note that recent sorta-squish band heads are more common in glow engines than those with a flat band parallel to the piston top and a small central 'cup' as the bowl. By sorta-squish, I am referring to the tapered form inside a very narrow ring, rising to the combustion bowl - if one is present.

Somewhere along in this thread, mention was made of, in effect, 'shaped demo charges.' Armor-piercing charges were often conical, large opening toward the target. An expanding explosion event 'rises' along the path to the open end and extends the (brief) time the explosive forces are aimed, as the reflector in a flashlight (USA for "torch"?) aims light, along the desired axis in the desired direction. That may have been part of the intent of a conically domed piston and matching contra-piston form, but I still feel that the initial 'guidance' to exhaust flow may also have been intended.

AndyW

My Feedback: (1)

Well, here's one experiment to try. A very shallow dish shape at the top of the piston and the same at the bottom of the contra-piston. A domed piston seems counter-intuitive to me. A dished piston and CP would "scoop" the combustion pressure towards the middle and perhaps create a more efficient initiation of ignition. Or has this already been tried?

Recycled Flyer

Andy,to me shaping a piston top is bad because -
It adds wieght to a reciprocating part,
Adds surface area for heat and I don't won't my pistons too hot,
Adds machining complexity,
And if the crown is dished out it could interupt the upwards transfer flow into the head.

But who knows, it may work!