WORKING OF A DIESEL ENGINE
There are two main types of internal-combustion engines. One type, found in most motor cars, is called a spark-ignition engine. It uses electricity and spark plugs to ignite the fuel in the engine's cylinders. The other type, the diesel engine, is a compression-ignition engine. When air confined in a cylinder is suddenly compressed, the temperature of the air rises. In a diesel engine, each piston compresses air in a cylinder. Fuel is injected and forms an explosive mixture, which ignites spontaneously under pressure.
Diesel engines burn fuel oils, which require less refining and are cheaper than higher-grade fuels such as petrol. During the combustion process, the stored chemical energy in the fuel is converted to thermal, or heat, energy. The temperature in each cylinder rises as high as 2,480 °C and creates pressures of about 100 kilograms per square centimetre. The pressure pushes against the tops of the pistons, forcing them to the other end of their cylinders. The pistons are connected by a rod or other suitable connecting mechanism to a crankshaft which they turn. In this way, a diesel engine supplies rotary power to drive vehicles and other kinds of machines.
In order for the compressed air inside the cylinders to ignite the fuel, it must have a certain temperature. The degree to which the temperature of the air rises depends on the amount of work done by the piston in compressing it. This work is measured in terms of the ratio between the volume of uncompressed air and the volume of the air after it is compressed. The compression ratio necessary to ignite the fuel depends on the size of the engine's cylinders. In large cylinders, the compression ratio is about 13 to 1. For small cylinders, it may be as high as 20 to 1. The average is 14.5 to 1.
Near the end of the piston's compression stroke, the fuel is injected into a cylinder. In order to have the fuel and air mix well, the fuel is injected under high pressure as a spray. Combustion usually starts just before the piston ends its compression stroke. The power of diesel engines can be increased by supercharging. This is the technique of forcing air under pressure into the cylinders.