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November 1996 Technical Notes
Supplement to " The Engine Analysis Program"
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 CHAMBER 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"
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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"
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.
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