Diesel engines are categorized as a type of internal combustion engines (ICEs). ICE is simply one where the fuel is burned inside the main part of the engine (the cylinders) where power is generated. Diesel engines possess a wide field of applications as energy converters because of their higher efficiency. The ability of a diesel engine to start rapidly, often in less than 10 s, also makes them expressly attractive. The diesel engine has been widely used in commercial vehicles, industrial applications, and today’s passenger cars and light-duty trucks. Diesel engines also have relatively high efficiency in power generation and can also be utilized in Combined Heat and Power (CHP) systems.
Gas Turbines basically consist of three main sections:
There are different classifications for diesel engines based on various characteristics and definitions. According to the number of crankshaft revolutions per working cycle, diesel engines are categorized as four-stroke engines (two revolutions per cycle) and two-stroke engines (one revolution per cycle). Two-stroke engines are lighter and smaller than four-stroke engines. They are generally more efficient because they generate power during each rotation (unlike four-stroke engines, which produce power during every two rotations). This also implies they need more cooling and lubrication and suffer higher depreciation and damage.
According to emissions standards of engine applications, diesel engines are classified as on-road, off-road, and stationary. The on-road applications include trucks, buses, and automobiles. The off-road applications involve marine, industrial, construction equipment, agricultural, and locomotive. Based on emissions certification methods for on-road applications, diesel engines are classified as heavy-duty and light-duty. Diesel engines are also classified as low-speed (rated speed of crankshaft rotation slower than approximately 300 rpm), medium-speed (300–1000 rpm), and high-speed (faster than 1000 rpm).
According to different fuel injection methods, diesel engines are classified as indirect injection and direct injection. Diesel engines are classified as naturally aspirated, mechanically supercharged, and aerodynamically turbocharged engines by air charging method. By cooling medium, diesel engines are divided into water-cooled and air-cooled. According to in-cylinder NOx emissions control and NOx after-treatment technology, diesel engines are categorized into EGR (Exhaust Gas Recirculation) engine, non-EGR engine, SCR (Selective Catalytic Reduction) engine, and non-SCR engine. Moreover, by different fuels utilized or fuel compatibility, diesel engines are classified as light-liquid-fueled, heavy-liquid-fueled, or multi-fueled engines.
Essential components of internal combustion engines include: 1) Fuel systems, 2) Lubrication systems, 3) Air intake systems, 4) Exhaust systems, 5) Cooling systems, and 6) Electrical systems.
In the engine, fuel reaches the cylinder bore through a path comprised of these parts: Fuel tank, Water separator, Feed pump, Filter, Injection pump, Injector nozzle, and Cylinder. The Air intake system consists of these sub-parts: Air cleaner, Turbocharger, Intake manifold, Inlet port, Inlet valve, and Cylinder bore. Exhaust gases flow through the following way in an engine: Cylinder bore, Exhaust valve, Exhaust port, Exhaust manifold, Turbocharger, and Muffler. Finally, an engine's electrical system comprises the following parts: 1) Starter-Motor, 2) Alternator, and 3) Battery.
Diesel engines are intended to operate on either the two- or four-stroke cycle:
a) Four-stroke cycle: A diesel engine regularly operates by repeating a cycle of four stages or strokes, during which the piston moves up and down twice within a cycle. Air is withdrawn into the cylinder through the open-air inlet valve as the piston moves down in the intake process. Then, in the compression step, the inlet valve blocks, the piston moves up, squeezes the air mixture, and heating it up. Fuel is inserted into the hot gas through the central fuel injection valve and unconsciously ignites. Unlike a gas engine, no sparking plug is needed to onset the ignition. As the air-fuel mixture inflames and burns, it pushes the piston down, driving the crankshaft that conveys power to the wheels. Eventually, the outlet valve opens to let out the exhaust gases, pushed out by the returning piston.
b) Two-stroke cycle: In a two-stroke diesel, the complete process occurs as the piston moves up and down merely once. As a result, there are three stages in a two-stroke cycle: exhaust and intake, compression, and power. The combustion process in a diesel engine is independent—that is, the fuel and air are not premixed before combustion initiation. Consequently, rapid vaporization and mixing of fuel in the air are very important to the burning of the injected fuel. This puts much emphasis on injector nozzle design, especially in direct-injection engines. In the typical four-stroke-cycle engine, the intake and exhaust valves and the fuel injection nozzle are placed in the cylinder head. Often, dual valve arrangements—two intake and two exhaust valves—are employed.