Diesel << DEE zuhl or DEE suhl >> engine is a type of internal-combustion engine used chiefly for heavy-duty work. Diesel engines drive huge freight trucks, large buses, locomotives, tractors, and heavy roadbuilding equipment. They also power submarines and ships, the generators of electric-power stations in small cities, and emergency electric-power generators. Some automobiles are powered by diesel engines.
How a diesel engine works.
There are two main types of internal-combustion engines—gasoline engines and diesel engines. The gasoline engine, found in most cars, is a spark-ignition engine. It uses an electric spark to ignite a fuel and air mixture in the engine’s cylinders (see Gasoline engine). The diesel engine is a compression-ignition engine. It compresses the air in the cylinders, causing the temperature of the air to rise. Fuel injected into the hot, compressed air ignites explosively.
During the combustion process, the chemical energy stored in the fuel is released and converted to thermal (heat) energy. The temperature in each cylinder rises as high as 4500 °F (2480 °C) and creates pressures of up to 2,300 pounds per square inch (162 kilograms per square centimeter). These pressures push against the top of the piston, resulting in a downward stroke of the piston. Each piston is connected by a rod or other mechanism to a crankshaft, which turns as the pistons move up and down. In this way, a diesel engine supplies mechanical power to drive vehicles and other machines.
To ignite the fuel, the compressed air must reach a certain temperature. The degree to which the temperature of the air rises depends on the compression ratio, the ratio between the volume of the uncompressed air and that of the air after compression. The compression ratio necessary to ignite the fuel is typically between 12 to 1 and 24 to 1. Direct-injection engines, in which fuel is injected into the cylinder above the piston, operate with lower compression ratios. In indirect-injection engines, also called prechamber engines, fuel is injected into, and ignited in, a chamber connected to the cylinder. These engines require higher compression ratios.
For the fuel and air to mix well, the fuel is injected under high pressure as a spray. Fuel injection occurs near the end of the piston’s compression stroke. Combustion usually starts just before the end of this stroke. The power of diesel engines can be increased by supercharging, or forcing air under pressure into the cylinder. See Fuel injection.
Diesel engines have a high thermal efficiency, or ability to convert the stored chemical energy in the fuel into mechanical energy, or work. In a spark-ignition engine, the power output is determined by a throttle that regulates how much air and fuel enter the cylinder. A partly closed throttle reduces the engine’s air intake and efficiency. Diesel engines have no throttle. Their power output is determined by the amount of fuel injected. Higher compression ratios and throttleless operation make diesel engines more efficient than gasoline engines.
Diesel engines are suited for heavy-duty work because they are heavier and more fuel-efficient than gasoline engines of equal power and therefore can better withstand heavy loads. In addition, diesel fuels require less refining and are cheaper than gasolines and other higher-grade fuels.
Kinds of diesel engines.
There are two main types of diesel engines—the four-stroke engine and the two-stroke engine. These types differ according to the number of piston strokes required to complete a cycle of fresh-air intake, air compression, power, and exhaust.
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 is the compression stroke. 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 piston downstroke is the power stroke, and the upstroke is the compression stroke. A two-stroke engine typically has no intake valves. It instead uses a blower, also called a supercharger, to push air into the cylinder. The rising piston closes the intake ports during the compression stroke. The fuel is injected near the end of this stroke. The fuel and air mixture then ignites, and the piston moves down through the power stroke. The exhaust process occurs near the end of this stroke. Because the two-stroke engine has twice as many power strokes as the four-stroke engine, it provides more power for a given weight and size.
History.
The diesel engine is named for Rudolf Diesel, the German engineer who invented it. Diesel patented his design for the engine in 1892 and built his first engine in 1893. The engine exploded and almost killed him, but it proved that fuel could be ignited without a spark. He operated his first successful engine in 1897. Later, Sir Dugald Clerk of the United Kingdom developed the two-stroke diesel.
See also Diesel, Rudolf; Fuel injection; Gasoline engine; Locomotive; Starter.
