A diesel engine is an internal combustion engine which operates using the diesel cycle (named after Dr. Rudolph Diesel). The defining feature of the diesel engine is the use of compression ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression. This is in contrast to a petrol (gasoline) engine, which uses the Otto cycle, in which a fuel/air mixture is ignited by a spark plug.
Diesel engines are manufactured in two stroke and four stroke versions. They were originally used as a more efficient replacement for stationary steam engines. Since the 1910s they have been used in submarines and ships. Use in locomotives, large trucks and electric generating plants followed later. In the 1930s, they slowly began to be used in a few automobiles. Since the 1970s, the use of diesel engines in larger on-road and off-road vehicles in the USA increased. As of 2007, about 50 percent of all new car sales in Europe are diesel.
How diesel engines work
The diesel internal combustion engine differs from the gasoline powered Otto cycle by using a higher compression of the air to ignite the fuel rather than using a spark plug ("compression ignition" rather than "spark ignition").
As well as the high level of compression allowing combustion to take place without a separate ignition system, a high compression ratio greatly increases the engine's efficiency. Increasing the compression ratio in a spark-ignition engine where fuel and air are mixed before entry to the cylinder is limited by the need to prevent damaging pre-ignition. Since only air is compressed in a diesel engine, and fuel is not introduced into the cylinder until shortly before top dead center (TDC), premature detonation is not an issue and compression ratios are much higher.
In the diesel engine, air is compressed with a compression ratio typically between 15 and 22 raising the air temperature to 1,000 F (538 C). At this temperature, the air in the combustion chamber will ignite the atomized fuel-air mixture. The injected fuel spray vaporizes, ignites and burns.
In cold weather high speed diesel engines, which are mostly prechambered, can be difficult to start because the mass of the cylinder block and cylinder head absorb the heat of compression, preventing ignition because of the higher surface to volume ratio. Prechambered engines therefore make use of small electric heaters inside the prechambers called glow plugs. These engines also generally have a higher compression ratio of 1-19 to 1-21. Low speed and compressed air started larger and intermediate speed diesels do not have glowplugs and compression ratios are around 1-16. Some engines use resistive grid heaters in the intake manifold to warm the inlet air until the engine reaches operating temperature. Engine block heaters (electric resistive heaters in the engine block) connected to the utility grid are often used when an engine is turned off for extended periods (more than an hour) in cold weather to reduce startup time and engine wear. In the past, a wider variety of cold-start methods were used. Some engines, such as Detroit Diesel engines and Lister-Petter engines, used a system to introduce small amounts of ether into the inlet manifold to start combustion. Saab marine engines, Field Marshall tractors (among others) used slow-burning solid-fuel 'cigarettes' which were fitted into the cylinder head as a primitive glow plug. Lucas developed the 'Thermostart', where an electrical heating element was combined with a small fuel valve. Diesel fuel slowly dripped from the valve onto the hot element and ignited. The flame heated the inlet manifold and when the engine was turned over the flame was drawn into the combustion chamber to start combustion. International Harvester developed a WD-40 tractor in the 1930s that had a 7-liter 4-cylinder engine which ran as a diesel, but was started as a gasoline engine. The cylinder head had valves which opened for a portion of the compression stroke to reduce the effective compression ratio, and a magneto produced the spark. An automatic ratchet system automatically disengaged the ignition system and closed the valves once the engine had run for 30 seconds. The operator then switched off the gasoline fuel system and opened the throttle on the diesel injection system. Such systems fell out of favor when electrical glow plug systems proved to be the simplest to operate and produce. Recently direct-injection systems advanced to the extent that prechambers systems were not needed using a common rail with electronic fuel injection.
Diesel fuel is also prone to "waxing" or "gelling" in cold weather, terms for the solidification of diesel oil into a partially crystalline state. The crystals build up in the fuel line (especially in fuel filters), eventually starving the engine of fuel and causing it to stop running. Low-output electric heaters in fuel tanks and around fuel lines are used to solve this problem. Also, most engines have a "spill return" system, by which any excess fuel from the injector pump and injectors is returned to the fuel tank. Once the engine has warmed, returning warm fuel prevents waxing in the tank. Due to improvements in fuel technology, with additives waxing rarely occurs in all but the coldest weather. A mix of diesel and kerosene should be used to start a cold vehicle in the coldest weather.
A vital component of all diesel engines is a mechanical or electronic governor which limits the speed of the engine by controlling the rate of fuel delivery. Unlike Otto-cycle engines, incoming air is not throttled and a diesel engine without a governor can easily overspeed, resulting in its destruction. Mechanically governed fuel injection systems are driven by the engine's gear train. These systems use a combination of springs and weights to control fuel delivery relative to both load and speed. Modern, electronically controlled diesel engines control fuel delivery and limit the maximum rpm by use of an electronic control module (ECM) or electronic control unit (ECU). The ECM/ECU receives an engine speed signal, as well as other operating parameters such as intake manifold pressure and fuel temperature, from a sensor and controls the amount of fuel and start of injection timing through electric or hydraulic actuators to maximize power and efficiency and minimize emissions. Controlling the timing of the start of injection of fuel into the cylinder is a key to minimizing emissions, and maximizing fuel economy (efficiency), of the engine. The timing is usually measured in units of crank angle of the piston before top dead center. For example, if the ECM/ECU initiates fuel injection when the piston is 10 degrees before TDC, the start of injection, or timing, is said to be 10 BTDC. Optimal timing will depend on the engine design as well as its speed and load.
Advancing the start of injection (injecting before the piston reaches TDC) results in higher in-cylinder pressure and temperature, and higher efficiency, but also results in elevated engine noise and increased oxides of nitrogen (NOx) emissions due to higher combustion temperatures. On the other hand, delayed start of injection causes incomplete combustion, reduced fuel efficiency and an increase in exhaust smoke, containing a considerable amount of particulate matter (PM) and unburned hydrocarbons (HC).