Bruce,
We could probably carry this thread on infinitem, but I have more than 40
years of custom, high performance engine design behind me. I specifically
avoided the causes of detonation, but preignition is a cause of detonation,
not a dissimilar event. This phenomena can be best explained as glow
plugging if you will, which is created by everything from hanging gasket
material to a glowing carbon deposit, but the result is the collision of a
flame front from the glowing ember colliding with the one created by the
timed spark. Detonation can also be caused by compression ignition causing
the secondary flame front. Please remember the purpose of the compression
stroke is to raise the fuel mixture close to the self ignition temperature,
but not to cross it, as correct ignition timing is reserved for the spark .
In the case of air cooled engines, this temperature thing is further
agravated by a 150 F degree higher head temperature than a water cooled
head, hence the lower compression ratio. So, any abnormality that creates a
higher than designed chamber temperature can easily move the combustion
enviroment beyond the designed fuel envelope resulting in detonation. As
mentioned earlier, detonation also occurs in fire arms and is almost always
caused by too little propellant in too large of a case volume. If this light
charge happens by chance to be distributed to different areas within the
case and the primer fires, it is possible that sparks from the primer starts
two flame fronts that collide. This condition is referred to as a squib load
and it is catastrophic and has caused mechanical firearm failures resulting
in serious personal injury to the shooter. I am sure you can see the
similarities.

We are just touching on a very high tech subject and it is far too technical
to do true justice to it in a NG thread, but there is quite a bit of this
information readily available on the net. If this subject interests you, I
highly suggest you do some reading on the web. I find combustion engineering
facinating and I really enjoy playing around with it. My current experiments
involve electrorheology for efficient combustion in internal combustion
engines. If you like contact me off-line, as it would be a bit off topic.
Steve

"Bruce In Bangkok" wrote in message
...
On Sun, 5 Jul 2009 11:11:52 +0200, "Steve Lusardi"
wrote:

Bruce,
Lets define detonation first, just so we are both on the same sheet of
music. Detonation is the collision of two simultaneous spreading flame
fronts sweeping across the combustion chamber. When the two flame fronts
collide, the remaining fuel burns at an explosive rate causing both a very
rapid rise in cylinder pressure and temperature. Under normal operation
the
fuel/ air charge ignites from a single point in the squish area of the
combustion chamber and ignition propagates at a controlled rate so that
all
the fuel is burning before the piston starts its downward travel. This is
timed so that the maximum cylinder pressure achieved from the burn occurs
at
the most advantageous crankshaft angle. The start of the ignition cycle is
supposed to be controlled by spark timing on gasoline engines and injector
timing on diesels. The extent of the damage caused by this phenomena is a
product of the remaining fuel yet unburned at the time of the collision.
You
are correct when you stated that detonation can be observed in many other
engine types. In point of fact, it can also occur in firearms. All fuels
are
designed to burn at a controlled rate and temperature, but when those
parameters are exceeded, the burn characteristics of the fuel change.

Disregarding the firearms for a bit :-) Detonation, as I learned it,
is simply the uncontrolled combustion of the charge. It may, or may
not, be the product of two simultaneous flame fronts but is usually
not described that way was as it is the nearly instantaneous
combustion of the charge. An explosion, if you will.

Ignition is not always initiated from a squish area - think flat head
where the squish area is the cylinder area and ignition is effected in
the valve pocket.

Be careful that you do not confuse detonation with pre-ignition. They
are two different activities and may occur either separately or in
combination. Detonation often times occurs after the normal ignition
event.

I disagree with your statement that your experiences are at odds with
mine.
I don't think they are. Your experience in Iran as the first example, I

Irian Jaya - in Indonesia :-)

suggest, was not detonation, it was overheating caused by the lean
condition. This violated the stoichiometric ratio of fuel and air that
gasoline is designed to burn within and the burn temperature rose to
abnormal levels overheating the piston causing thermal expansion beyond
the
available space for this (piston/cylinder clearance), hence the seizure.

The problem with the Mercury straight sixes was simple enough after we
admitted to ourselves what was happening. Three of the engines were
mounted on a "crew change" boat and because the run to the air field
was nearly four hours someone had built a 100 gallon tank into the
rear of the boat. The little crankcase driven fuel pumps weren't quite
strong enough for full throttle operation and the two top cylinders
were running lean.

As the Indonesian Boat Boys had two positions for the throttles - idle
and full throttle - when I got there we were burning up at least an
engine a trip.

After a few runs (and overhauls) I realized that the problem was
always the two upper cylinders and put pressure gages at the carbs.
then of course it was simple. We installed an electric fuel pump with
pressure regulators at the carbs and never had another problem from
that end of the engine.

This is also the case in racing two stroke motorcycles. In the old days
lube
oil was added to the fuel at fueling time, which made the fuel air ratio
important for lubrication as well as burn parameters. On all later model
high performance two stroke engines, a separate oil pump was employed that
metered oil into the air fuel stream from a separate oil tank eliminating
this fuel/oil dependency. It may be necessary to state that lean is mean.
All internal combustion engines are really hot air engines and it is the
displacement difference of the ingested air at atmospheric temperatures
verses the same air heated by fuel combustion that drives the piston
downward, not the exhaust gases of the fuel burned. So, it stands to
reason
that the more heat generated during the ignition cycle, the greater the
cylinder pressure achieved. Maximum heat is generated in the leaner side
of
stoichiometric, so it can be a fine line between max power and disaster.
Now, I am not saying detonation did not occur, I was not there, but if it
did occur, it was a secondary effect, not primary and in point of fact,
the
addition of excessive oil in the fuel aggravated the condition.

These were the first of the Yamaha racing motorcycles and ran premix.
The problem was augmented by the tuned exhaust that allowed us to get
more power from a 250 cc engine than other people got from nearly
twice that capacity of four stroke. The initial response from Yamaha
was that we were running them too lean and we had to tell them the jet
sizes we were using to argue that we were actually running a bit rich.
Yamaha came to the conclusion that the problem was the hot engine and
leaning the oil quantity when the throttle went closed and one of the
schemes was actually not to use the engine for braking but to depend
on the brakes alone and pull the clutch in, which sounded weird but
worked. It may even be normal practice now.

In the case of Detroit Diesel engines (DDA), yes, they very happily will
run
on their own lube oil, but that is not detonation it is simply compression
ignition, as the self ignition temperature of lube oil and diesel fuel is
similar. This fact supports my original statement that the addition of
lube
oil causes detonation in gasoline engines, because gasoline self ignites
at
a higher temperature than lube oil. Please recall the big warning sign in
the cabs of the military trucks with multi fuel engines stating not to use
high octane gasoline. This is because the compression temperature rise in
not sufficient to ignite that fuel.

I'm not familiar with military multi-fuel engines, other then the
navy's jets that ran on gasoline or jet fuel.

But, the major difference between high and low test gasoline is the
additives to control detonation, which makes the high octane fuels
actually burn slower. As I said, I'm not familiar with the multi-fuel
engines but certainly 115/145 avgas would run in a cushman motor
scooter without detonation. Started fine too....

Please note that I have avoided the causes of detonation because there are
many. This discussion is only about the presence of lube oil in gasoline
and
there is a limit of tolerance before the compression ignition of the oil
takes place prior to the occurrence of the spark. This causes pinging
which
is normally heard by the operator, but if this warning is ignored and wide
open throttle (WOT) is used, you can expect melted parts.
Steve

I'm not sure about this. A normal gasoline engine has relatively low
compression and I can't see compression ignition in this case. My
experience with diesels seemed to indicate that diesel fuel used to
prime the engine worked much better then engine oil.

I really would like to get a better handle on this as nothing in my
experience and nothing I have ever read indicates that detonation is
caused by anything but lean mixture or a heavily overloaded the engine
- both causing the same condition, too hot a combustion chamber.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)