We have shown how a cylinder with a compressed fuel/air mixture can be ignited with up to a 20,000 volt spark from a 6 or 12 volt electrical system and we have emphasized that it must be ignited at the proper instant.
But when do we ignite it? What is the proper instant? Exactly when the piston is at TDC (Top Dead Center) ? Just before TDC or just after TDC ?
In order to decide when to fire the mixture of fuel and air compressed in the cylinder, let’s think a little bit about the explosion that takes place. Understanding this, we should be able to determine when to ignite the mixture.
First of all, what is an explosion? Well, according to Webster, it is a sudden bursting, a violent expansion, or the sudden production or release of pressure.
What happens when the fuel/air mixture compressed in the cylinder is ignited? It burns very rapidly. A chemical reaction takes place which produces H2O (water) CO2 (carbon dioxide) and CO (carbon monoxide). These are the products of combustion.
It is the heat resulting from the chemical reaction during the burning of the fuel and air which produces these products that is important to us. (The heat of combustion produces extremely high pressure.)
The breakdown of fuel and air and formation of these new products is in the form of an explosion the mixture burns so rapidly, it is often called an explosion. There is a great deal of heat produced and a tremendous pressure is built up. It is important to realize that it is burning which actually takes place.
The burning takes place from the spark plug gap where the spark occurs out into the rest of the mixture. It is not an instantaneous thing.
It takes a certain amount of time for the entire mixture to burn and produce the total pressure obtainable from the mixture. The rate at which the wall of flame travels through the mixture is called the rate of flame propagation.
The point we are trying to make here is that we’ve got to give the mixtures time to burn completely if we are to obtain the entire effect of the pressure produced by the explosion.
Now we can answer the questions we asked earlier. In order to get the full force of the hot expanding gas exerted upon the moving piston, enough time must be allowed for it to burn.
It will have to be ignited at a point early enough before TDC so that the greatest pressure is produced Just past TDC.
In order to obtain the greatest effort of the expanding hot gasses, ignition must take place further before TDC as engine speed increases. The mechanisms to provide this are called spark advance mechanisms.
These mechanisms vary the spark timing for different engine operating conditions. Because the timing of the spark varies according to the load and speed of the engine, an automatic spark advance mechanism, sensitive to the load and speed of the engine, is incorporated in the distributor.
A centrifugal governor advances the position of the distributor cam according to the speed of the engine and a vacuum diaphragm controls the position of the breaker plate assembly according to the load on the engine.
Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.