Michelle Bjornson, age 13, of Willingboro, N.J., for her question:
HOW DOES A DIESEL ENGINE WORK?
There are two main types of infernal combustion engines. One type, found in most automobiles, is called a spark ignition engine. It uses electricity and spark plugs to ignite 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 the heat of the air makes it ignite.
Diesel engines burn fuel oils, which require less refining and are cheaper than higher grade fuels such as gasoline.
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 4,500 degrees Fahrenheit and creates pressures of 1,500 pounds per square inch.
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 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.
Near the end of a piston's compression stroke, the fuel is injected into a cylinder.
In order to have the fuel and air mix well, the duel 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, or forcing air under pressure into the cylinders.
Diesel engines have a high thermal efficiency, or ability to convert the stored chemical energy in the fuel into mechanical energy, or work. They burn cheap fuel and can perform heavy work under highly overloaded conditions. This is one of the reasons why diesel engines are favored for heavy duty work.
There are two main types of diesel engines. They differ according to the number of piston strokes required to complete a cycle of air compression, exhaust and intake of fresh air. A stroke is the distance a piston travels in one direction.
The two engines are: the four stroke cycle engine and the two stoke cycle engine.