Diesel engines are a class of internal combustion engine in which the fuel is burned internally and the combustion products are used as the working fluid. Unlike the spark-ignited (SI) engines found in the majority of today's automobiles in which the premixed fuel-air mixture is ignited by an electric spark, diesel engines are characterized by a spontaneously initiated combustion process where the ignition is brought about by very high temperature compressed air. A small amount of diesel fuel is injected at the end of the compression stroke into the cylinder where the fuel auto-ignites. Because of their higher actual operating efficiencies, as compared with SI engines that require pre-ignition, diesel engines are primarily used in heavy-duty vehicles such as trucks, ships, locomotives, etc.
Diesel engines were first developed by Rudolf Diesel (1858-1913) in the late nineteenth century. The original concept was to build a multifuel engine and to use coal as a primary fuel. However, for some reason, coal-fueled diesel engines so far have gained only occasional interest from the industry (e.g., when fuel-oil prices are high), most of the diesel engines currently being used rely on petroleum fuels. They are four-stroke cycle engines, and operate from several hundred up to around one thousand rpm. In addition to pistons, cylinders, crankshaft, and various valves, diesel engines are also equipped with controlled fuel injection systems, exhaust systems, cooling systems, and so on. Sufficient lubrication is required to prevent excessive wear of various parts in engines. Since pre-ignition is not required, the compression step can be continued to reach a higher pressure or a higher compression ratio than that in SI engines. This results in compressed air with a temperature exceeding the ignition point of the injected fuel for auto-ignition. To achieve high combustion efficiency, the fuel jets must draw in and mix well with air, ignite, and burn, all within less than one millisecond, when they impact on the cold combustion chamber walls. Engine performance is closely related to compression ratio, piston speed, supercharging, turbo-charging, etc., and engine size is normally in terms of power rating (i.e., horsepower; for instance, 20,000 hp applicable for ship propulsion). In principle, the same engine frame can be designed for different output by varying the number of cylinders (10, 12, 16 cylinders, and so on).
In reality, because the fuel-air mixture is burned and the products of combustion are emitted, the process for work production via combustion in diesel engines is complex and not cyclical. However, in order to analyze it, the actual operation is frequently represented approximately by a cyclical process, called "Diesel cycle." From the point of view of thermodynamics, the working fluid is assumed to be air, the compression and expansion stages are assumed to be adiabatic (without the loss or gain of heat) and reversible, and the combustion and exhaust strokes are replaced by constant-pressure heat-absorption and constant-volume heat-rejection stages. As shown in Figure 1, a typical pressure-volume (P-V) diagram for air-standard diesel engine operation, after the intake, air is compressed adiabatically along the path 1-2 and its temperature is increased substantially. At point 2 where the piston begins to reverse its motion, the fuel is injected and added slowly so that combustion is initiated and sustained at constant pressure following the path 2-3. After completion of the combustion, there is the work stroke, i.e., along the path 3-4 where the high-temperature and high-pressure products of combustion are expanded to produce mechanical work. Then the exhaust valve is opened, the spent combustion products and waste heat are exhausted, and the pressure is rapidly reduced as the path 4-1. This, therefore, completes typical four strokes in each cycle of engine operation. The thermal efficiency of the cycle can be obtained from the net work produced divided by the heat absorbed during the entire cyclical process.
Overall, diesel engines can be viewed as a piston-and-cylinder assembly and the work-producing machine. Their operation cycle is similar to that in SI engines which are based on the Otto (after the German inventor for the first internal-combustion engine produced in the mid 1860s) cycle; however, the latter require an external combustion initiator and have combustion occurring under an almost constant-volume condition, which is different from the path 2-3 as shown in Figure 1. In these engines, the chemical (molecular) energy of the fuel (hydrocarbons) is released by a combustion process. Energy is evolved as heat and part of the heat is subsequently converted into useful work or mechanical energy. Because of the loss of heat during the process, research and development efforts have been made constantly in chamber design, new coatings for rings and liner, emission control, alternative fuels, and associated compressor and turbine technologies to improve the conversion efficiency. As can be expected, diesel engines will continue finding a variety of applications in the future, such as power generation as well as land, marine, and aircraft transport.
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