How Diesel Engines work

Diesel Engine is a type of internal combustion engine (one from which work is obtained by compression of the fuel within the cylinders themselves) which operates on the constant pressure or diesel cycle principle. Fuel is admitted directly into the cylinder and combustion takes place as a result of the heat of compression.
In these engines, gas pressure in the cylinder acts on the piston, forcing it down during the power stroke to drive the crankshaft through connecting rods. The extreme positions reached by the piston correspond to the top and bottom dead center positions (TDC & BDC) of the crank and are so designated. The inside diameter of the cylinder is the bore. The distance traveled between dead centers (TDC &BDC) is the stroke, corresponding volume is the swept volume, or displacement, of the cylinder. The cylinder volume above piston when piston is at TDC is called clearance volume. Similarly the cylinder volume above piston when piston is at BDC is called cylinder volume. The ratio of the cylinder volume to the clearance volume is the nominal compression ratio.
The greater combustion pressure is the result of the higher compression ratio used by diesel engines. Compression ratio is a measure of how much the engine compresses the air inside the cylinder. In a diesel engine compression ratio ranges from 14:1to as high as 24:1 are commonly used. Higher compression ratios are possible because only air is compressed, and then the fuel is injected. This is one of the factors that allows the diesel engine to be so efficient.