Ignition Principle

What is the ignition system? To understand Electronic ignition systems we must first understand how an engine works and how conventional ignition systems work.


How the Internal Combustion Engine works


An internal combustion engine is effectively a chemical to rotary motion converter. Fuel and air are fed, at a specific ratio, into the engine. The fuel air mixture is then ignited, this causes the fuel and air mixture to burn and heat the gasses. Expansion of the gases occur and exert force on the piston, forcing it downward. The piston is conneected to the crankshaft via the connecting rod, also known as the con rod. The linear motion of the piston is coverted into a rotational motion by the con rod and crankshaft. So that's a basic overview, lets have a look in more detail to see what exactly is going on and also learn how important the ignition system is. Firstly a would like to describe the internal workings of an engine. The main components of a single cylinder engine are a crank, conrod, piston, combustion chamber, cam, inlet valve, exhaust valve and spark plug The image below shows these components



engine components

There are two main types of an internal combustion engine utilising a resiputateing piston, 4 stroke and 2 stroke. I'm going to describe what actually happens in a 4 stroke engine, however the ignition system is just as important in a 2 stroke engine. The engine is described as 4 stroke, what this means is the piston takes 4 strokes, for one complete power cycle, so in other words the piston goes down,up,down and up, for one power cycle.

In a minuite I'll describe what actually happens on these 4 strokes but before I do I need to introduce the valves. These are seen in the diagram above. The valves are responsible for letting in the fuel and air mixture and letting out the burnt gases(exhaust gases) and just as importantly when they are not open they must form a tight seal. The valve responsible for letting in the fuel air mixture is known as the inlet valve the valve that lets out the burnt gases is know n as the exhaust valve. The opening, closing timing of these valves is critical. This is taken care of by the camshaft, the camshaft is connected to the crankshaft, via a timing chain or belt and is geared such that it turns one full revolution to two full revolutions of the crakshaft.


Now i'll take you through what happens on each stroke of the piston.


1) The inlet stroke
At the start of the cycle the piston is at the top of the stroke with the inlet valve beginning to open. As the piston moves down the bore of the cylinder the fuel air mixture is drawn in through the inlet valve. As the piston moves downward the inlet valve opens even more, allowing easier passage of the fuel air mixture. As the piston approaches the bottom of the stroke the inlet valve starts to close.


2)The Compression Stroke
Just as the piston starts its way back up the cylinder the inlet valve closes tight, at this point the exhaust valve is also shut tight, as the piston continues up it starts to compress the mixture inside the cylinder and continues to compress the mixture.


3)The power Stroke
As the piston approaches TOP DEAD CENTER the sparkplug fires and ignites the fuel air mixture. The mixture burns, heating the gases causing them to expand. The piston continues on its path while this burn action takes place. So it continues past TDC, as the heating gases expand they exert pressure on the piston forcing the piston down the bore.


4) The exhaust stroke
The valves remain shut as the piston is in its power stroke until the piston approaches the bottom of the stroke. As the piston nears the bottom of the stroke the exhaust valve begins to open, the inlet valve remains shut. The piston reaches the bottom of the stroke and starts to move back up the bore, with the exhaust open the position starts to push the exhaust gases out past the exhaust valve. The piston continues up the bore an as the piston approaches the top of the stroke the inlet valve starts to open and exhaust valve begins to close.



The whole process then starts all over again with the inlet stroke.

The ignition spark lasts around .002 seconds(2ms).

At 6000 RPM an engine just takes .010 seconds(10ms) to do 1 revolution.

After ignition, combustion pressure can easily reach 400psi