November 1996 Technical Notes

Supplement to " The Engine Analysis Program"
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Last month we started an initial discussion of Part One - Combustion Chamber (Cylinder Heads): The Squish Band.

This month a discussion of some different Combustion Chamber designs and the reasoning behind them. This is a major research program for me this off-season and I will write about my findings in the Spring.


Offset Combustion Chamber

The offset chamber is designed to place a VERY WIDE squish band adjacent to the Hot Exhaust side of the piston, to force a wider thin layer of gasses to remove heat from the piston crown at the exhaust side. We have made a preliminary test of this chamber with no definitive results. I learned about the potential for this type chamber in the SAE Book talked about in last months Technical Notes. I intend to study this head design in detail in the next 4 months.

"The Oblong Chamber"

Oblong Combustion Chamber

This chamber is the work of John Ackerman and Bobby Coleman. This past month Bobby, who works at Navistar International, told John Ackerman he had seen something at work which was brought in by a large customer which might have application to our models. After a discussion, John and Bobby came up with an oblong chamber combustion chamber. After I saw it I thought of how we might apply it to our use. I immediately saw the possibility of more than one glow plug (maybe even 3 for .90 engines) with installation of these plugs made easy by their 90 degree relationship to the squish band. No more angle of the glow plug, a decidedly difficult task. You would have the benefit of the wide squish band talked about on the offset chamber as well as multiple plug configurations. We have only run the head on Ackerman's .90 OS Max engine on the water. We don't have ANY dynamometer runs or data to tell us what we saw. It appeared to me that this head produced very good power and we will study this configuration in detail this Winter. How good was the performance? The wider squish band produced enough power that it lifted the top of the case off of the bottom of the case. This is the first time we have produced this much power. Looks REALLY promising!

"Double and Triple (Bubble) Combustion Chamber"

Double & Tripple Chambers

We have been using double and triple bubble combustion chamber designs for the past 3 or 4 years. This configuration makes it simple to adjust the volume of the chamber without changing the width or proportion of the squish band. This is the head design which I would recommend to the serious 2 cycle tuner who doesn't have a dynamometer or very sophisticated machining capability. You can make VERY FINE ADJUSTMENT to the chamber volume easily. You start out with a much smaller ball mill than you would use for a Hemispherical head chamber and draw it out away from the center. You can shallow the next draw and thus you form the multiple bubble chamber.

You can get into as much detail as you can stand by reading the SAE Book by Dr. Blair & his Research Team- Click Here! (Cost about $180 for the Book and Software or $80 for the software alone) I will very soon have the software that is talked about in the book, which will allow computer simulation of some of these very interesting theories. Also, if you want the data upon which to base building a dynamometer, here is a technical paper prepared by Dr. Kee & Dr. Blair. He outlines how to build an inertial dynamometer. If this is important to you Click Here! This is a VERY detailed publication, and is not for everyone. It will be very confusing if you are not highly engineering oriented. Will let you know how this all works out. Some early plans to produce an inertial dyno so everyone has access to this important tool. Keep your eye on this site for information. Won't be available until at least middle of 1997!

One of my Close Boating Friends - Bob Bonahoorn - This months Feature Article Author of Boat Modeler Magazine - wrote me an E-Mail to add to the discussion of what is at play in the combustion chamber. His comment is as follows:

'I just read your tech tip on squish bands. I have a different understanding on why a thin squish band reduces auto-ignition, (Pre-ignition is a different thing. in spark engines it is caused by hot spots on the spark plug, etc. We can get pre-ignition if we use too hot a plug with high nitro). Auto ignition is what is referred to as knock ". It is caused by the rapidly increasing pressure from the expanding burning gasses acting on the unburned fuel/air that is farthest away from the glow plug. That fuel mixture Is called "end gas". it is heated by the steadily Increasing pressure from the wavefront until it detonates. Normal nitro flamefronts move at about 200-300 meters per second. A detonation moves at more that 2000 meters per second and it can put pits and holes in things like the blast from a high explosive. (Nitro methane is actually a high explosive under certain extreme conditions!) If our squish band is thin, then the end gas will be closer to cooler metal, (head and piston) and it will be very difficult to heat enough to detonate. My fuel experiments were an attempt to find a blend that inhibited auto-ignition, The theory was that the end gases were less likely to auto-ignite, but no one knows what the theory is that causes the improvements. I have been putting a 3 degree cut on all of my buttons. I have been doing it all the way across. After reading your article I am going to try a straight cut. It makes sense that it would help you use higher compression ratios without auto-ignition. By the way, the reason 21 engines like nitro so much is that the chamber is too small to have much auto-ignition because the end gases are closer to the plug and don't have time to heat up to detonation temperatures. Multiple plug heads should help with auto-ignition in big engines. I've never used them. Do you know if they help?"

An important note about the software by Dr. Blair. This program will analyze squish velocity, and other important criteria dealing with the combustion chamber.

ENGINE Analysis Software for the Serious RC Competitor



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