PRE-IGNITION
AND DETONATION
BEFORE we consider the use of other additives to the fuel we must get a clear
understanding of the problem introduced by two well known conditions that occur,
especially when you are seeking high performance from the engine, and in particular
using fuel of the type we have been discussing.
We have in mind of course Pre-ignition and Detonation and we did state we would
explain these two conditions in detail. Let us therefore look at, perhaps, the
easier one of the two to explain and understand, that of Pre-ignition.
The name itself is self explanatory. The fuel is being ignited before it should
be, causing all sorts of trouble. To understand we must go back to our simple
heat engine and once again consider just what takes place, taking the compression
stroke as our starting point, assuming that up to that moment of time the engine
has been running satisfactorily.
That being so, we have the piston commencing to travel up the cylinder bore,
starting to compress the fuel ready for ignition by the spark at the plug.
IGNITION SETTING
Depending on the ignition setting, the spark should occur at just the right
time to allow the mixture to ignite, the resultant explosion being so timed
that its force is applied to the piston just as it is ready to commence its
downward stroke.
As we have explained with Methanol for example, as compared to Petrol, the ignition
setting point has to be advanced since this fuel is slower in igniting, taking
longer to burn, hence the need to commence the operation just that bit sooner
so as to get the force of the explosion at the right moment looking at it from
the piston point of view.
If the explosion takes place too late, then the piston has already started to
descend, so the force of the explosion is reduced since there is now so much
more room so to speak in the chamber.
On the other hand if it occurs too soon, the force of the explosion meets the
piston on its way up the bore, trying to force it down, so power is lost and
a genera! state of opposing forces exists.
It is just this that makes it necessary to time the ignition setting to agree
with the type of fuel in use so as to get the maximum effect, also to have an
ignition system that will ignite as much of the mixture as possible in the very
short time it has to do so.
DETONATION
Having we hope established a reasonable understanding of Pre-ignition we must
now turn to the other troublesome condition known as Detonation, which again
as its name implies, is an explosive force and as such destructive.
Detonation is caused by the actual compression of the mixture to a level where
it reaches the Auto-ignition point, becoming an uncontrollable explosion, the
point at which this takes place varying from fuel to fuel, hence the use of
additives to vary this point.
The explosion takes place without the aid of any local hot spots, including
the plug itself, and again is out of time with piston movement.
A further cause and a frequent one at that is a small pocket of fuel, after
normal ignition has taken place, getting further compressed by the explosion
in the cylinder head in addition to that of the mechanical compression, then
igniting, after the normal ignition point, so out of time, causing the well
known "pinking" effect and in a severe case mechanical destruction
of the engine.
The amount of destruction is to some extent dependent on the actual shape of
the cylinder head and the space available for the pocket of fuel to collect.
If the pocket that is formed is relatively large, then the force of this very
highly compressed fuel exploding can do mechanical damage, but if on the other
hand it is small it may not do so, but it can, and will, form a local heat spot,
which in turn will cause pre-ignition.
SPARK PLUGS
Right away the first item that leaps to mind is the plug itself, which after
all has just the essential job to do of igniting the mixture.
If this gets overheated and then retaining the heat, becomes hot enough to ignite
the fuel itself, without the aid of the spark across the electrodes, then it
will do so as soon as the fuel is introduced into the cylinder and is directed
by the upward motion of the piston towards the head and the plug, obviously
well before the correct moment of time.
This means that we must be selective in our choice of plug and use the correct
grade, the so-called "hot" type being out, further to that the condition
of the plug must be first class.
It is quite useless and in the long run expensive, to waste time with poor plugs,
so just remove the one you pinched from the lawn mower and treat yourself to
the correct grade of "cold" plug right away.
ENGINE TEMPERATURE
Let us now assume we can forget the plug situation and say all is well. We now
have to consider engine temperature as the next possible cause of preignition,
or some part of the cylinder head becoming so hot in itself that it acts as
the plug, igniting the mixture all out of time with the piston movement.
This means at once forces opposing each other in the engine, producing still
more heat and so the whole thing getting into a vicious circle.
The obvious possible cause would be weak mixture as a start since this can cause
an increase in temperature due to the combustion of the fuel being more complete,
eliminating the cooling effect of any fuel that may be left over in normal conditions,
which in the case of Methanol could well be in liquid form, and sometimes when
considerable overlap timing is used, can be seen ejected from the exhaust ports.
VALVES
Remember we did point out that in the case of Methanol you had the advantage
of a lot of fuel being introduced to the cylinder acting as a coolant, to the
valves in particular.
This being so it follows that if you do have a weak mixture, you are almost
certain to have the valves reaching high temperatures, especially the exhaust
valves which in fact can reach a high enough temperature to ignite the fuel
thus causing pre-ignition.
Now let us say we have the right plug, correct ignition setting for the fuel
in use and adequate mixture being introduced to the cylinders.
We can still suffer pre-ignition, however should there be a rough part of the
head, say a small ridge by the plug hole, which, since it is so small in mass,
can build up and retain enough heat to become a small red hot mass, again taking
over from the plug and doing its work all at the wrong time.
Yet again another cause could be faulty valve operation or incorrect tappet
settings so that the fuel mixture, although in itself rich enough, is unable
to be placed in the cylinder head at the right time, again causing the effort
of the eventual explosion, when it does take place, to be out of time with the
piston movement.
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