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 “auto-ignition” temperature of the fuel spontaneously reacts with the oxygen in the air and burns. Air temperatures are typically in excess of 526 °C (979 °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 engine's 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.

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 (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 heterogeneous—that 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.

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 NO_x), 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 source—about 1.7 to nearly 2.4 megapascals—to 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 engine's 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.

There are three basic size groups of diesel engines based on power—small, medium, and large. The small engines have power-output values of less than 188 kilowatts, or 252 horsepower. This is the most commonly produced diesel engine type. These engines are used in automobiles (automobile), light trucks (truck), and some agricultural and construction applications and as small stationary electrical-power generators (such as those on pleasure craft) and as mechanical drives. They are typically direct-injection, in-line, four- or six-cylinder engines. Many are turbocharged with aftercoolers.

Medium engines have power capacities ranging from 188 to 750 kilowatts, or 252 to 1,006 horsepower. The majority of these engines are used in heavy-duty trucks. They are usually direct-injection, in-line, six-cylinder turbocharged and aftercooled engines. Some V-8 and V-12 engines also belong to this size group.

Large diesel engines have power ratings in excess of 750 kilowatts. These unique engines are used for marine, locomotive, and mechanical drive applications and for electrical-power generation. In most cases they are direct-injection, turbocharged and aftercooled systems. They may operate at as low as 500 revolutions per minute when reliability and durability are critical.

Use of the two-stroke cycle can eliminate the need for one or both valves in the engine design. Scavenging and intake air is usually provided through ports in the cylinder liner. Exhaust can be either through valves located in the cylinder head or through ports in the cylinder liner. Engine construction is simplified when using a port design instead of one requiring exhaust valves.

Petroleum products normally used as fuel for diesel engines are distillates (distillation) composed of heavy hydrocarbons (hydrocarbon), with at least 12 to 16 carbon atoms per molecule. These heavier distillates are taken from crude oil after the more volatile portions used in gasoline are removed. The boiling points of these heavier distillates range from 177 to 343 °C (351 to 649 °F). Thus, their evaporation temperature is much higher than that of gasoline, which has fewer carbon atoms per molecule. In the United States, specifications for diesel fuels are published by the American Society of Testing and Materials (ASTM). ASTM D975 “Standard Specification for Diesel Fuel Oils” covers specifications for five grades of diesel fuel oils:

Water and sediment in fuels can be harmful to engine operation; clean fuel is essential to efficient injection systems. Fuels with a high carbon residue can be handled best by engines of low-speed rotation. The same applies to those with high ash and sulfur content. The cetane number, which defines the ignition quality of a fuel, is determined using ASTM D613 “Standard Test Method for Cetane Number of Diesel Fuel Oil.”

Rudolf Diesel (Diesel, Rudolf), a German engineer, conceived the idea for the engine that now bears his name after he had sought a device to increase the efficiency of the Otto engine (the first four-stroke-cycle engine, built by the 19th-century German engineer Nikolaus Otto (Otto, Nikolaus August)). Diesel realized that the electric ignition process of the gasoline engine could be eliminated if, during the compression stroke of a piston-cylinder device, compression could heat air to a temperature higher than the auto-ignition temperature of a given fuel. Diesel proposed such a cycle in his patents of 1892 and 1893.

Originally, either powdered coal or liquid petroleum was proposed as fuel. Diesel saw powdered coal, a by-product of the Saar coal mines, as a readily available fuel. Compressed air was to be used to introduce coal dust into the engine cylinder; however, controlling the rate of coal injection was difficult, and, after the experimental engine was destroyed by an explosion, Diesel turned to liquid petroleum. He continued to introduce the fuel into the engine with compressed air.

The first commercial engine built on Diesel's patents was installed in St. Louis, Mo., by Adolphus Busch (Busch, Adolphus), a brewer who had seen one on display at an exposition in Munich and had purchased a license from Diesel for the manufacture and sale of the engine in the United States and Canada. The engine operated successfully for years and was the forerunner of the Busch-Sulzer engine that powered many submarines (submarine) of the U.S. Navy in World War I. Another diesel engine used for the same purpose was the Nelseco, built by the New London Ship and Engine Company in Groton, Conn.

The diesel engine became the primary power plant for submarines during World War I. It was not only economical in the use of fuel but also proved reliable under wartime conditions. Diesel fuel, less volatile than gasoline, was more safely stored and handled.

At the end of the war many men who had operated diesels were looking for peacetime jobs. Manufacturers began to adapt diesels for the peacetime economy. One modification was the development of the so-called semidiesel that operated on a two-stroke cycle at a lower compression pressure and made use of a hot bulb or tube to ignite the fuel charge. These changes resulted in an engine less expensive to build and maintain.

One objectionable feature of the full diesel was the necessity of a high-pressure, injection air compressor. Not only was energy required to drive the air compressor, but a refrigerating effect that delayed ignition occurred when the compressed air, typically at 6.9 megapascals (1,000 pounds per square inch), suddenly expanded into the cylinder, which was at a pressure of about 3.4 to 4 megapascals (493 to 580 pounds per square inch). Diesel had needed high-pressure air with which to introduce powdered coal into the cylinder; when liquid petroleum replaced powdered coal as fuel, a pump could be made to take the place of the high-pressure air compressor.

There were a number of ways in which a pump could be used. In England the Vickers Company used what was called the common-rail method, in which a battery of pumps maintained the fuel under pressure in a pipe running the length of the engine with leads to each cylinder. From this rail (or pipe) fuel-supply line, a series of injection valves admitted the fuel charge to each cylinder at the right point in its cycle. Another method employed cam-operated jerk, or plunger-type, pumps to deliver fuel under momentarily high pressure to the injection valve of each cylinder at the right time.

The elimination of the injection air compressor was a step in the right direction, but there was yet another problem to be solved: the engine exhaust contained an excessive amount of smoke, even at outputs well within the horsepower rating of the engine and even though there was enough air in the cylinder to burn the fuel charge without leaving a discoloured exhaust that normally indicated overload. Engineers finally realized that the problem was that the momentarily high-pressure injection air exploding into the engine cylinder had diffused the fuel charge more efficiently than the substitute mechanical fuel nozzles were able to do, with the result that without the air compressor the fuel had to search out the oxygen atoms to complete the combustion process, and, since oxygen makes up only 20 percent of the air, each atom of fuel had only one chance in five of encountering an atom of oxygen. The result was improper burning of the fuel.

The usual design of a fuel-injection nozzle introduced the fuel into the cylinder in the form of a cone spray, with the vapour radiating from the nozzle, rather than in a stream or jet. Very little could be done to diffuse the fuel more thoroughly. Improved mixing had to be accomplished by imparting additional motion to the air, most commonly by induction-produced air swirls or a radial movement of the air, called squish, or both, from the outer edge of the piston toward the centre. Various methods have been employed to create this swirl and squish. Best results are apparently obtained when the air swirl bears a definite relation to the fuel-injection rate. Efficient utilization of the air within the cylinder demands a rotational velocity that causes the entrapped air to move continuously from one spray to the next during the injection period, without extreme subsidence between cycles.

In 1914 a young American engineer, William T. Price, began to experiment with an engine that would operate with a lower compression ratio than that of the diesel and at the same time would not require either hot bulbs or tubes. As soon as his experiments began to show promise, he applied for patents.

In Price's engine the selected compression pressure of nearly 1.4 megapascals (203 pounds per square inch) did not provide a high enough temperature to ignite the fuel charge when starting. Ignition was accomplished by a fine wire coil in the combustion chamber. Nichrome wire was used for this because it could easily be heated to incandescence when an electric current was passed through it. The experimental engine had a single horizontal cylinder with a bore of 43 cm (17 inches) and a stroke (maximum piston movement) of 48 cm (19 inches) and operated at 257 revolutions per minute. Because the nichrome wire required frequent replacement, the compression pressure was raised to 2.4 megapascals (348 pounds per square inch), which did provide a temperature high enough for ignition when starting. Some of the fuel charge was injected before the end of the compression stroke in an effort to increase the cycle timing and to keep the nichrome wire glowing hot.

In the meantime many engines of the two-stroke-cycle, semidiesel type were being installed. Some were used to produce electricity for small municipalities, while others were installed in water-pumping plants. Many provided power for tugs, fishing boats, trawlers, and workboats.

In the early 1920s the General Electric Company (General Electric Co.) suggested to the Ingersoll-Rand Company, for whom Price was working, that they cooperate in the building of a diesel-electric locomotive. At that time many of the locomotives in service were powered by gasoline engines. A diesel-electric locomotive with Price's engine was completed in 1924 and placed in service for switching purposes in New York City. The success of this locomotive resulted in orders from railroads, factories, and open-pit mines. The engine used in most of these installations was a six-cylinder, 25-cm (10-inch) bore, 30-cm (12-inch) stroke system, rated 300 brake horsepower at 600 revolutions and weighing 6,800 kg (15,000 pounds).

Many diesel engines were purchased for marine propulsion. The diesels, however, normally rotated faster than was desirable for the propellers of large ships because the high speeds of the huge propellers tended to create hollowed-out areas within the water around the propeller ( cavitation), with resultant loss of thrust. The problem did not exist, however, with smaller propellers, and diesel engines proved especially suitable for yachts, in which speed is desired. The problem was solved by utilizing a diesel-electric installation in which the engines were connected to direct-current generators that furnished the electricity to drive an electric motor connected to the ship's propeller. There were also many installations in which the diesel was connected either directly or through gears to the propeller. When diesel engines with larger horsepower and slower rotation speeds became available, they were installed in cargo and passenger ships.

The diesel engine became the predominant power plant for military equipment on the ground and at sea during World War II. Since then it has been adopted for use in heavy construction machinery, high-powered farm tractors, and most large trucks and buses. Diesel engines also have been installed in hospitals, telephone exchanges, airports, and various other facilities to provide emergency power during electrical power outages. In addition, they have been used in automobiles, albeit on a limited scale. Although diesels provide better fuel economy than gasoline engines, they do not run as smoothly as the latter and emit higher levels of pollutants.