- Key People:
- Rudolf Diesel
diesel engine
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What is a diesel engine?
A diesel engine is an internal combustion engine that compresses air to a high enough temperature to ignite diesel fuel injected into the cylinder, where combustion and expansion actuate a piston. It converts the chemical energy stored in the fuel into mechanical energy to power freight trucks, large tractors, locomotives, and marine vessels. Diesel engines operate on a two-stroke or four-stroke cycle. Diesel engines are also called compression-ignition engines because the start of combustion relies on air heated by compression rather than an electric spark.
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How does a diesel engine work?
A diesel engine is an internal combustion engine that compresses air to a high enough temperature to ignite diesel fuel injected into the cylinder, where combustion and expansion actuate a piston. It converts the chemical energy stored in the fuel into mechanical energy to power vehicles and equipment.
Unlike a gasoline engine, the diesel engine only intakes air into the combustion chamber on its intake stroke and lacks a spark-ignition system. Diesel engines are called compression-ignition engines because combustion relies on air heated by compression rather than an electric spark. Diesel engines are more efficient than gasoline engines because they use higher compression ratios and do not rely on throttling the intake mixture to control power.
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How is a diesel engine different from a gasoline engine?
Diesel and gasoline engines are both internal combustion engines, but they differ in how they ignite fuel. In a gasoline engine, an electric spark ignites a volatile liquid fuel, like gasoline or ethanol. In contrast, a diesel engine compresses air to a high enough temperature that it ignites the injected diesel fuel. Diesel engines don't use a spark-ignition system. Diesel engines are also known as compression-ignition engines because they rely on air heated by compression to initiate combustion, rather than an electric spark.
Compared to gasoline engines of similar horsepower, diesel engines tend to be heavier and more expensive but have a longer lifespan. They also typically operate at a lower cost per horsepower-hour because they burn less fuel.
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What are the main parts of a diesel engine?
The diesel engine is an intermittent-combustion piston-cylinder device that operates on a two-stroke or four-stroke cycle. Unlike gasoline engines, it intakes only air into the combustion chamber on its intake stroke.
Key parts include:
* Cylinder block The main structural member, housing cylinders and supporting the crankshaft.
* Piston and cylinder The piston slides within the cylinder, driven by or against fluid pressure. The cylinder head typically contains the fuel nozzle.
* Fuel injection system Sprays fuel directly into the cylinders, relying on heat from compressed air for ignition, rather than spark plugs. Precise control of fuel injection is critical to performance.
* Valves Intake and exhaust valves are located in the cylinder head in a typical four-stroke-cycle engine.4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Why are diesel engines commonly used in trucks and ships?
Diesel engines are commonly used in freight trucks, large tractors, locomotives, and marine vessels because they convert the chemical energy stored in fuel into mechanical energy to actuate a piston. They are favored in trucks for long-distance hauls since World War II. Diesel engines offer superior operational efficiency, require less fuel energy, and have about four times greater thermal efficiency compared to steam. Furthermore, they provide more rapid train acceleration and can sustain higher speeds with reduced track damage.
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How do diesel engines affect the environment?
Diesel engines have a significant impact on the environment, particularly regarding air pollution. They discharge high levels of particulate matter (soot) and reactive nitrogen compounds (NOx) compared to spark-ignition engines. The use of hydrocarbons in diesel fuel contributes to global warming because the combustion process produces carbon dioxide, a greenhouse gas. While diesel engines are more efficient, their emissions pose environmental challenges.
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diesel engine, any internal-combustion engine in which air is compressed to a sufficiently high temperature to ignite diesel fuel injected into the cylinder, where combustion and expansion actuate a piston. It converts the chemical energy stored in the fuel into mechanical energy, which can be used to power freight trucks, large tractors, locomotives, and marine vessels. A limited number of automobiles also are diesel-powered, as are some electric-power generator sets.
Diesel combustion
The diesel engine is an intermittent-combustion piston-cylinder device. It operates on either a two-stroke or four-stroke cycle (see ); however, unlike the spark-ignition gasoline engine, the diesel engine induces only air into the combustion chamber on its intake stroke. Diesel engines are typically constructed with compression ratios in the range 14:1 to 22:1. Both two-stroke and four-stroke engine designs can be found among engines with bores (cylinder diameters) less than 600 mm (24 inches). Engines with bores of greater than 600 mm are almost exclusively two-stroke cycle systems.
The diesel engine gains its energy by burning fuel injected or sprayed into the compressed, hot air charge within the cylinder. The air must be heated to a temperature greater than the temperature at which the injected fuel can ignite. Fuel sprayed into air that has a temperature higher than the āauto-ignitionā temperature of the fuel spontaneously reacts with the oxygen in the air and burns. Air temperatures are typically in excess of 526 Ā°C (979 Ā°F); however, at engine start-up, supplemental heating of the cylinders is sometimes employed, since the temperature of the air within the cylinders is determined by both the engineās compression ratio and its current operating temperature. Diesel engines are sometimes called compression-ignition engines because initiation of combustion relies on air heated by compression rather than on an electric spark.
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In a diesel engine, fuel is introduced as the piston approaches the top dead centre of its stroke. The fuel is introduced under high pressure either into a precombustion chamber or directly into the piston-cylinder combustion chamber. With the exception of small, high-speed systems, diesel engines use direct injection.
Diesel engine fuel-injection systems are typically designed to provide injection pressures in the range of 7 to 70 megapascals (1,000 to 10,000 pounds per square inch). There are, however, a few higher-pressure systems.
Precise control of fuel injection is critical to the performance of a diesel engine. Since the entire combustion process is controlled by fuel injection, injection must begin at the correct piston position (i.e., crank angle). At first the fuel is burned in a nearly constant-volume process while the piston is near top dead centre. As the piston moves away from this position, fuel injection is continued, and the combustion process then appears as a nearly constant-pressure process.
The combustion process in a diesel engine is heterogeneousāthat is, the fuel and air are not premixed prior to initiation of combustion. Consequently, rapid vaporization and mixing of fuel in air is very important to thorough burning of the injected fuel. This places much emphasis on injector nozzle design, especially in direct-injection engines.
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Engine work is obtained during the power stroke. The power stroke includes both the constant-pressure process during combustion and the expansion of the hot products of combustion after fuel injection ceases.
Diesel engines are often turbocharged and aftercooled. Addition of a turbocharger and aftercooler can enhance the performance of a diesel engine in terms of both power and efficiency.
The most outstanding feature of the diesel engine is its efficiency. By compressing air rather than using an air-fuel mixture, the diesel engine is not limited by the preignition problems that plague high-compression spark-ignition engines. Thus, higher compression ratios can be achieved with diesel engines than with the spark-ignition variety; commensurately, higher theoretical cycle efficiencies, when compared with the latter, can often be realized. It should be noted that for a given compression ratio the theoretical efficiency of the spark-ignition engine is greater than that of the compression-ignition engine; however, in practice it is possible to operate compression-ignition engines at compression ratios high enough to produce efficiencies greater than those attainable with spark-ignition systems. Furthermore, diesel engines do not rely on throttling the intake mixture to control power. As such, the idling and reduced-power efficiency of the diesel is far superior to that of the spark-ignition engine.
The principal drawback of diesel engines is their emission of air pollutants. These engines typically discharge high levels of particulate matter (soot), reactive nitrogen compounds (commonly designated NOx), and odour compared with spark-ignition engines. Consequently, in the small-engine category, consumer acceptance is low.
A diesel engine is started by driving it from some external power source until conditions have been established under which the engine can run by its own power. The simplest starting method is to admit air from a high-pressure sourceāabout 1.7 to nearly 2.4 megapascalsāto each of the cylinders in turn on their normal firing stroke. The compressed air becomes heated sufficiently to ignite the fuel. Other starting methods involve auxiliary equipment and include admitting blasts of compressed air to an air-activated motor geared to rotate a large engineās flywheel; supplying electric current to an electric starting motor, similarly geared to the engine flywheel; and applying a small gasoline engine geared to the engine flywheel. The selection of the most suitable starting method depends on the physical size of the engine to be started, the nature of the connected load, and whether or not the load can be disconnected during starting.