There are many various types of internal combustion engine, the most common types being the four-stroke petrol and the four-stroke diesel. Others include two-stroke petrol or two-stroke diesel, the Wankel rotary engine and Gas turbine engines.
Firstly let’s look at the two-stroke petrol engine. This is a very simple engine design which is widely used in today’s mo-peds or small generators, although it used to be used in some car applications. The operating cycle of this engine uses two strokes of the piston, a stroke meaning each full downward or upward movement of the piston.
As I have just explained above ‘two-stroke’ means simply one upward movement then one downward movement of the piston. Lets look a little closer at each stroke and each component involved.
In this diagram we can see that the piston is in its bottom most position, known as bottom dead centre, and that there are exhaust gasses flowing out of the cylinder, shown by the brown arrow. Looking at this diagram I know that the piston has just completed its power stroke and the process happening know is know as scavenging. Scavenging is when after the power stroke exhaust gasses are pushed out of the cylinder by a fresh mix of fuel, air and oil, shown by orange mass on the diagram. The fresh fuel, air and oil mix must pass through a reed valve in the engines crank case, then through a port on the side of the cylinder to enter the combustion chamber. Scavenging is achieved by the fresh mix being compressed in the crankcase so it has enough pressure, when entering the combustion chamber, to expel the remaining exhaust gasses.
In this next diagram you can see that the piston is on its upward stroke, otherwise known as the compression stroke. You may notice, on the diagram, that where the exhaust gasses are expelled from the engine we now have a small amount of fresh fuel, air and oil mix also being expelled. This shows that the combustion chamber is now full of the fresh mixture which is ready to be compressed. As the piston rises and compresses the mixture, both the inlet port and the exhaust port in the cylinder are covered by the piston skirt. This ensures that maximum pressure can build up within the combustion chamber. Before we move on to ignition and the power stroke, you may notice that the reed valve is now open. This is to let more fresh mixture into the crankcase ready to be compressed and let into the combustion chamber after the power stroke.
Combustion happens after the piston rises to compress the fuel, air and oil mixture, at which point the mix is extremely hot and volatile. To start combustion the mixture needs to be ignited. This is achieved by a spark plug, seen at the top of the engine in this diagram. As the piston reaches top dead centre a powerful electric pulse is sent to the spark plug, creating a spark across two electrodes. This ignites the compressed mixture and combustion takes place. The piston is forced down as combustion causes gasses to expand rapidly due to immense heat from the chemical reaction that happens when the mixture combusts.
As the piston is pushed down it uncovers the exhaust port and exhaust gasses from the combustion begin to leave the chamber and the whole process begin again.
NOTE: All pictures used in this blog where found on this web-page from "How Stuff Works" - (Click Here to view)
Please feel free to leave any comments or questions below. Also if you wish to add more to the basics i have just described please do so, this way my knowledge will expand with yours. Thanks!
