posted September 04, 2001 10:43 AM
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.
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.
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.
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.
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.
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.