2 Stroke Diesel Engine
As you can see from the image below with the air inlet port uncovered the exhaust gasses are forced out as well as the cylinder filling with air (blue). The piston will also begin moving upwards now as the momentum of the flywheel will force it to do so –
When the piston covers the inlet port while on its way back up the cylinder it covers the air inlet port. At the same time the engine has been timed to close the exhaust valve by means of a camshaft. This now results in the cylinder becoming completely airtight, as a result the piston will begin to compress the air that is now trapped as it continues moving up in the cylinder –
As i have already mentioned a diesel engine does not have any spark plug, instead it relies on the immense heat created by compressing the air to ignite the fuel. When you halve the volume of a gas you double its temperature. A diesel engine compresses the original air that came in through the air inlet port to 1/18th of its original volume. This results in a massive increase in temperature – several hundred degrees, and enough to spontaneously ignite the fuel.