Diesel Engines – Introduction
Internal combustion engine principles first proposed by Sadi Carnot in the early 19th century. Dr.Rudolf Christian Karl Diesel applied Sadi Carnote’s principle into a patented cycle, that has become known as “Diesel Cycle”. His patented cycle operated when the heat generated during the compression of the air fuel charge caused ignition of the mixture, which then expanded at a constant pressure during the power stroke of the engine.
Dr. Diesel’s first engine ran on coal dust and used a compression pressure of 1500 psi( about 103 bar) to increase its theoretical efficiency. Also there was no provisions for cooling. Consequently, between the extreme pressure and lack of cooling, the engine exploded and almost killed its inventor. After recovering from his injuries, Dr. Diesel tried again using oil as fuel, adding a cooling water jacket around the cylinder, and lowering the compression ratio to about 550 psi(about 35 bar). This combination eventually proved successful. Production rights to the engine were sold to Adolphus Bush, who built the first diesel engine for commercial use. Further lot of inventions affected construction and increased efficiency of the engine. Still now researches are going on for better results and alternatives.
How Does it 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.
REFERENCES
Benson & Whitehouse, “Internal Combustion Engines”, Pergamon.