AndyW

My Feedback: (1)

In our glow, and other engines, how important is the bowl shape in the combustion chamber? What would we lose if this was ALL squish band? Model diesels are done this way out of necessity. Would diesels gain anything with the combustion chamber shaped more like glow engines? As in a dome shaped contra piston matched with a dome shaped piston.

Jim Thomerson

There are a number of diesels which have cone-shaped contrpistons and piston tops. I think the ED Racer, and the various AM diesels are that way (will go check later). The RJL 40 head has a shape very like the usual glow head, with a very shallow "hemisphere". The replacement contrapiston Gene Hempel made for me after the squishband on my RJL contrapiston came loose had a conical indentation in the center. It was there in the piece of stock we used and we just left it. No harm that I could see. Because the charge in a diesel fires more or less everywhere at once, the shape of the combustion chamber is not thought to be important. In a glow engine, combustion starts at the glow plug and goes from there, so considerable changes in performance can be made with changes in head shape.

Bodge

Jim,

Absolutely. With diesels you don't need to generate combustion chamber turbulence via a squish band since the burn does not start at the plug, but evenly throughout the combustion chamber due to the high compression. The ideal shape for a point source ignition (plug) would be spherical - since that is impractical in an engine a squish band helps make up for the deficiencies of uneven flame travel distance.

I think the piston crown shape has little benefit on a diesel - the few diesel manufacturers in existence now tend to use flat crowns as the effort in making hemispherical heads is not justified.

rmh

Supertigre had the "bowl of the month" along with other running changes -
some race setups also were double bubbles---others were trumpet TeeDee and Medallion Cox used these
Which is best? who knows -- they each were done to satisfy a specific goal --which usually changed as did the shape
And if you think these aere all rather arbitrary, try to get agreement among racers ,which Chev head chamber is "best".

AndyW

My Feedback: (1)

Thanks to all for the input. How about a bowl shaped piston top along with a bowl shaped head? Has this been tried? Any advantage?

William Robison

My Feedback: (3)

[b]Andy:

The very first MVVS I eve saw was the property of Dave Cotton, it was a 2.5cc engine, he was running it in c/l "A" speed. 1960 Nats, Grand Prairie Texas. He pulled the head for the rest of us to see, it had a genuine spherical combustion chamber. Or it did when the piston was at TDC anyway. It was fast, but the Shelton and Harris team beat him that year, and as I recall if Bill Wisniewski didn't beat him he was really close.

S/H and Wisniewski had engines they had built themselves, Dave was the fastest of the factory engines. I was down in the ranks with my Super Tigre from John Maloney. The next year I had one of Wisniewsli's engines. But that's another tale for another time.

Bill.

downunder

I just had a look at my old Taipan 2.5BB diesel and the contra piston has got a very definite squish band and a bowl shaped combustion chamber. It's also got a flat topped piston but uses two Schneurle ports and two exhausts.

With glows though and their single ignition point there's an optimum distance from the coil to the top of the piston at TDC. It's a ratio of the distance to the bore size and IIRC it's something like .15:1 or thereabouts. It's to do with getting the flamefront to spread out evenly and as quickly as possible so a fairly high combustion chamber is needed. A hemi is a good shape (although the Cox trumpet seems to be even better, at least with Cox's) and an added advantage is that to get a good shape and high enough compression it leaves you with a nice sized squish band.

The worst combustion chamber I've seen on a Schneurle engine was in my son's old SC 1.08 because it had a quite narrow squish band which quickly stepped up to a totally flat combustion chamber roof. We made a new hemi head with a much wider squish band and this gave an extra 500 revs. It almost certainly wasn't ideal but as a first attempt I was quite pleased with it.

pe reivers

Downunder,
The general rules for squish dome combination are:
narrow squis.flat dome >> high rpm operation
wide squish.high dome >> low rpm operation
That means the SC engine is laid out for higher rpm (quite relative, and not rpm defined), and probably would be prone to detonation at lower rpm. This maybe explaines the gain you got using the new head

It does not always work out that way, because nitro content, squish height and the gas cooling/swirling it affectuatesalso plays a large role, combined with compression ratio. Too many factors to play with.