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Diesel engine

From Academic Kids

The diesel engine is a type of internal combustion engine; more specifically, it is a compression ignition engine, in which the fuel is ignited by being suddenly exposed to the high temperature and pressure of a compressed gas containing oxygen (usually atmospheric air), rather than a separate source of ignition energy (such as a spark plug), as is the case in the gasoline engine.

This is known as the diesel cycle, after Rudolf Diesel, who invented it in 1892 and received the patent on February 23, 1893. Diesel intended the engine to use a variety of fuels including coal dust. He demonstrated it in the 1900 Exposition Universelle (World's Fair) using peanut oil (see biodiesel). It was later refined and perfected by Charles F. Kettering.
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Caterpillar C15 15.2L 435-625HP (324-466Kw)
Contents

How diesel engines work

When a gas is compressed, its temperature rises (as stated in Charles's Law); a diesel engine uses this property to ignite the fuel. Air is drawn into the cylinder of a diesel engine and compressed by the rising piston at a much higher compression ratio than for a spark-ignition engine, up to 25:1. The air temperature reaches 700...900 C, or 1300...1650 F. At the top of the piston stroke, diesel fuel is injected into the combustion chamber at high pressure, through an atomising nozzle, mixing with the hot, high-pressure air. The resulting mixture ignites and burns very rapidly. This contained explosion causes the gas in the chamber to expand, driving the piston down with considerable force and creating power in a vertical direction. The connecting rod transmits this motion to the crankshaft, which is forced to turn, delivering rotary power at the output end of the crankshaft. Scavenging (pushing the exhausted gas-charge out of the cylinder, and drawing in a fresh draught of air) of the engine is done either by ports or valves. To fully realize the capabilities of a diesel engine, use of a turbocharger to compress the intake air is necessary; use of an aftercooler/intercooler to cool the intake air after compression by the turbocharger further increases efficiency.

A vital component of any diesel engine system is the governor, which limits the speed of the engine by controlling the rate of fuel delivery. Modern electronically-controlled engines achieve this through the electronic control module (ECM) or electronic control unit (ECU) - the engine-mounted "computer". The ECM/ECU receives an engine speed signal from a sensor and then using its algorithms and look-up calibration tables stored in the ECM/ECU, it controls the amount of fuel and its timing (start of injection) through electric or hydraulic actuators to maintain engine speed.

Due to the lower volatility of the fuel, diesel engines do not operate well when the cylinders are cold. Some engines utilize small electric heaters called glow plugs inside the cylinder to warm the cylinders prior to starting. Others use resistive grid heaters in the intake manifold to warm the inlet air until the engine reaches operating temperature. In the past diesels would have been pre-heated with a blow-torch. Once the engine is operating the combustion of fuel in the cylinder keeps the engine warm effectively. Engine block heaters (electric resistive heaters in the engine block) plugged into the utility grid are often used when an engine is shut down for extended periods (more than an hour) in cold weather to reduce startup time and engine wear.

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Detroit Diesel 8V92 2 cycle 12L turbo and super charged V8 550HP(410kw) Diesel Engine.

Indirect injection was a primary method for delivering fuel into combustion chamber, but recent diesel engines employ direct injection. Modern designs use a very highly pressurised common rail fuel supply line, which replaces the older mechanically more complicated, noisy combined pump and selector valve assembly. Electronically-controlled diesel engines also advance injection timing to improve cold startability and reduce white smoke (unburned fuel in the exhaust) under cold start conditions.

In very cold weather, diesel fuel thickens and increases in viscosity and forms wax crystals or a gel. This can make it difficult for the fuel injector to get fuel into the cylinder in an effective manner, making cold weather starts difficult at times, though recent advances in diesel fuel technology have made these difficulties rare. A commonly applied advance is to electrically heat the fuel filter and fuel lines.

Alternatively, due to the very low fuel consumption of the diesel engine at idle, many operators merely leave the engine idling when not in use in cold weather, even for long periods of time.

Types of diesel engines

There are two classes of diesel engines: two-stroke and four-stroke. Most diesels generally use the four-stroke cycle, with some larger diesels operating on the two-stroke cycle.

Normally, banks of cylinders are used in multiples of two, although any number of cylinders can be used as long as the load on the crankshaft is counterbalanced to prevent excessive vibration. The inline-6 is the most prolific in medium- to heavy-duty engines, though the V8 and straight-4 are also common.

Advantages and disadvantages versus spark-ignition engines

Diesel engines are more efficient than gasoline/petrol engines of the same power (by approx. 15%), resulting in lower fuel consumption.

Diesel engines are more massive than gasoline/petrol engines of the same power because of the heavier construction required to withstand the higher combustion pressures needed for ignition. Yet it is this same build quality that has allowed some enthusiasts to acquire significant power increases with turbocharged engines through fairly simple and inexpensive modifications. A gasoline engine of similar size cannot output a comparable power increase without extensive alterations because the stock components would not be able to withstand the higher stresses placed upon them. Since a diesel engine is already built to withstand higher levels of stress, it makes an ideal candidate for performance tuning with little expense.

The addition of a turbocharger or supercharger to the engine greatly assists in increasing fuel economy and power output. The higher compression ratio allows a diesel engine to be more efficient than a comparable spark ignition engine, although the calorific value of the fuel is slightly lower at 45.3 megajoules/kilogram to gasoline at 45.8 megajoules/kilogram. The increased fuel economy of the diesel over the petrol engine means that the diesel produces less carbon dioxide (CO2) per unit distance. The recent development of biofuel alternatives to fossil fuels has unleashed the ability to produce a net-sum of zero emissions of CO2, as it is re-absorbed into plants and then comes full circle, being used to produce the fuel.

Diesel engines can produce black soot from their exhaust. This consists of unburned carbon compounds. Modern diesel engines catch the soot in a particle filter, which when saturated is automatically regenerated by burning the particles. Other problems associated with the exhaust gases (nitrogen oxide, sulfurous fumes) can be mitigated with further investment and equipment.

The lack of an electrical ignition system greatly improves the reliability. The high durability of a diesel engine is also due to its overbuilt nature (see above) as well as the diesel's combustion cycle, which creates less-violent changes in pressure when compared to a spark-ignition engine. Unfortunately, due to the greater compression force required and the increased weight of the stronger components, starting a diesel engine is a harder task. More torque is required to push the engine through compression.

Either an electrical starter or an air start system is used to start the engine turning. On large engines, pre-lubrication and slow turning of an engine, as well as heating, are required to minimize the possibility of damaging the engine during initial start-up and running. Some smaller military diesels are started with an explosive cartridge that provides the extra power required to get the machine turning.

Automobile racing

Although the weight and lower output of a diesel engine tend to keep them away from automotive racing applications, there are many diesels being raced in classes that call for them, mainly in truck racing, as well in types of racing where these drawbacks are less severe, such as land speed record racing. Diesel engined dragsters (http://www.cumminsracing.com/) even exist, despite the diesel's drawbacks being central to performance in this sport. In 1952, Cummins Diesel (http://www.cummins.com/eu/pages/en/whoweare/cumminshistory.cfm) won the pole at the Indianapolis 500 race with a supercharged 3 liter diesel car, relying on torque and fuel efficiency to overcome weight and low peak power, and led most of the race until the badly situated air intake of the car swallowed enough debris from the track to disable the car.

Fuel and fluid characteristics

Diesel fuel is a product of crude oil, although other oils can be burned inside an adapted engine. Good-quality diesel fuel can be synthesised from vegetable oil and alcohol.

Diesel engines can work with thicker, heavier oil, or oil with higher viscosity, as long as it is heated to ease pumping and injection. These fuels are cheaper than clean, refined diesel oil, although they are dirtier.

Diesel fuel is more difficult to ignite than gasoline because of its higher flash point, but once burning, a diesel fire can be extremely fierce.

The use of low-grade fuels can lead to serious maintenance problems.

See also: biofuel, biodiesel

Diesel applications

The vast majority of modern heavy road vehicles (trucks), ships, large-scale portable power generators, most farm and mining vehicles, and many long-distance locomotives have diesel engines. However, in the U.S. they are not as popular in passenger vehicles as they are in Europe as they are perceived as being heavier, noisier, of having performance characteristics which makes them slower to accelerate, and of being more expensive than petrol vehicles.
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Mercedes Benz MBE 4000 350-450HP (261-336Kw)

In Europe, where tax rates in many countries make diesel fuel much cheaper than petrol, diesel vehicles are very popular and newer designs have significantly narrowed differences between petrol and diesel vehicles in the areas mentioned. One anecdote tells of Formula One driver Jenson Button, who was arrested while driving a diesel-powered BMW coupe at 230 km/h (about 140 mph) in France, where he was too young to have a petrol-engined car hired to him. Button dryly observed in subsequent interviews that he had actually done BMW a public relations service, as nobody had believed a diesel could be driven that fast. The BMW diesel lab in Steyr, Austria is led by Ferenc Anisits and is considered to be a leader in development of automotive diesel engines. Similarly, Mercedes Benz had a successful run of diesel-powered passenger cars in the late 1970s and 1980s. After a hiatus in the 1990s with relatively few diesel cars in its lineup, Mercedes Benz has revived diesel cars in it's newer ranges with an emphasis on high performance versus the older models' lack thereof.

High-Speed
High-speed (approximately 1200 rpm and greater) engines are used to power lorries (trucks), buses, tractors, cars, yachts, compressors, pumps and small generators.
Medium-Speed
Large electrical generators are driven by medium speed engines, (approximately 300 to 1200 rpm) optimised to run at a set speed and provide a rapid response to load changes.
Low-Speed
The largest diesel engines are used to power ships. These monstrous engines have power outputs over 80,000 kW, turn at about 60 to 100 rpm, and are up to 15 m tall. They often run on cheap low-grade oils, which require extra heat treatment in the ship for tanking and before injection due to their low volatility. Companies such as Burmeister & Wain and Wrtsil (e.g., Sulzer Diesels) design such large low speed engines. They are unusually narrow and tall due to the addition of a crosshead bearing. Today (2005), the Wrtsil-Sulzer RTA96C turbocharged two-stroke diesel engine is the most powerful and most efficient prime-mover in the world, with cylinder bores of 960 mm (37.8 in.) and stroke of 2500 mm (98.4 in.), producing up to 80,080 kW in the 14-cylinder configuration.

The zeppelins Graf Zeppelin II and Hindenburg were propelled by reversible diesel engines. The direction of operation was changed by shifting gears on the camshaft. From full power forward, the engines could be brought to a stop, changed over, and brought to full power in reverse in less than 60 seconds. This was done before reversible pitch propellers for aircraft had been perfected.

A few airplanes have been built that use diesel engines, such as the Junkers-powered Blohm & Voss Ha 139 of the late 1930s. This is quite rare because of the high importance of power-weight ratios in aeronautical applications, and the development of kerosene-powered jet engines and the closely-related turboprop engines. However, this may change in the near future. The newer automotive diesels have power-weight ratios comparable to the ancient spark-ignition designs common in general aviation aircraft, and have better fuel efficiency. Their use of electronic ignition, fuel injection, and sophisticated engine management systems also makes them far easier to operate than mass-produced spark-ignition aircraft engines, most of which still use carburettors. Combined with Europe's very favourable tax treatment of diesel fuel compared to petrol, these factors have led to considerable interest in diesel-powered small general aviation planes, and several manufacturers have recently begun selling diesel engines for this purpose. The Diamond Twin Star is currently one of the very few general aviation aircraft manufactured with diesel engines. It is can be twice as efficient as a comparable twin aircraft due to the diesel engines made by Thielert. See aircraft engine.
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General Motors Electromotive Division V16 264.7 L 6300 hp (4700 kW) Rail Locomotive engine

Dieseling in spark-ignition engines

A gasoline (spark ignition) engine can sometimes act as a compression ignition engine under abnormal circumstances, a phenomenon typically described as "pinging" or "pinking" (during normal running) or "dieseling" (when the engine continues to run after the electrical ignition system is shut off).


See also

  • Napier Deltic - A high-speed, lightweight diesel engine used in fast naval craft and some railway locomotives.
  • Junkers Jumo 205 - The most successful of the first series of production diesel aircraft engines.
  • Elsbett - An improved multi-fuel diesel engine design

Bibliography

  • Rudolf Diesel, Die Entstehung des Dieselmotors. Erstmaliges Faksimile der Erstausgabe von 1913 mit einer technik-historischen Einfhrung., Steiger Verlag, Moers, 1984. ISBN: 3921564700
  • Max J. Rauck, 50 Jahre Dieselmotor: zur Sonderschau im Deutschen Museum, Leibniz-Verlag, Mnchen, 1949. ISBN: B0000BMMSD


External links

de:Dieselmotor es:Motor disel fr:Moteur_à_explosion#Moteur_diesel_4_temps nl:Dieselmotor no:Dieselmotor id:Mesin diesel ja:ディーゼルエンジン pl:Silnik wysokoprężny sv:Dieselmotor

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