Luis Pinero, age 15, of Miami, Fla., for his question:
'NOW DOES A DIESEL ENGINE WORK?
A diesel engine is 'a type of internal combustion engine. It is used chiefly for heavy duty work, such as huge freight trucks, large buses, tractors and road building equipment. The diesel engine is a compression=ignition engine.
When air confined in an engine 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.
In the gasoline engine, called a spark ignition engine, electricity and spark plugs are used to ignite the fuel in the engine's cylinders.
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 4500 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.
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, the compression ratio 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. 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 engine can be increased by supercharging, or forcing air under pressure into the cylinders.
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 (1) the four stroke cycle engine and (2) the two stroke cycle engine.
In a four stroke engine, each piston moves down, up, down and up to complete a cycle. The first downstroke draws air into the cylinder. The first upstroke compresses the air. The second downstroke is the power stroke. The second upstroke exhausts the gases produced by combustion.
A four stroke engine requires exhaust and air intake valves.
In a two stroke engine, the exhaust and intake of fresh air occur through openings in the cylinder near the end of the down, or power stroke. The one upstroke is the compression stroke. A two stroke engine does not need valves. These engines have twice as many power strokes per cycle as four stroke engines and are used where high power is needed in a small engine.