Diesel engine

The diesel engine is a type of internal combustion engine; more specifically, a compression ignition engine, in which the fuel is ignited by the high temperature of a compressed gas, rather than a separate source of energy (such as a spark plug).

It was invented and patented by Rudolf Diesel in 1892. Diesel intended the engine to use a variety of fuels including coal dust. He demonstrated it in the 1900 World's Fair using peanut oil. It was later refined and perfected by Charles F. Kettering

How diesel engines work

When a gas is compressed, its temperature rises (as stated in Charles' 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. 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. (See direct injection vs indirect injection for a discussion of the types of fuel injection.) To fully realize the capabilities of a diesel engine, use of a turbocharger to compress the intake air is necessary; an aftercooler/intercooler to cool the air after compression 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.

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. 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 shutdown for extended periods (more than an hour) in cold weather to reduce startup time and engine wear.

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

Types of diesel engines

There are two classes of diesel engines: two-stroke and four-stroke. Many larger diesels operate on the two-stroke cycle. Smaller engines generally use the four-stroke cycle.

Normally banks of cylinders are used in multiples of 2, 4, 6, or 8, 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 is also common.

Advantages and disadvantages versus spark-ignition engines

Diesel engines are more massive than gasoline/petrol engine 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 mile-for-mile the diesel produces less carbon dioxide (CO₂). 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 unburnt carbon compounds. Other problems associated with the exhaust gases (high particulates, 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 are 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.

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 on 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, petrodiesel

Diesel applications

The vast majority of modern heavy road vehicles, ships, long-distance locomotives, large-scale portable power generators, and most farm and mining vehicles have diesel engines. However, they are not nearly as popular in passenger vehicles as they are heavier, noisier, have performance characteristics which makes them slower to accelerate, and more expensive than petrol vehicles.

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 gasoline and diesel vehicles in the areas mentioned. One anecdote tells of Formula One driver Jenson Button, who was arrested 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.

High-speed (approximately 1200 rpm and greater) engines are used to power lorries (trucks), buses, tractors, cars, yachts, compressors, pumps and small generators. The largest diesel engines are used to power ships along the sealanes. These monstrous engines have power outputs up to 90,000 kW, turn at about 60 to 70 rpm and are 15 m tall. They often run on cheap low grade oil, which needs extra heat treatment in the ship for tanking and before injection. Companies such as Burmeister & Wain and Wartsila NSD (e.g. Sulzer Diesels) design such large slow speed engines. They are unusually narrow and tall due to the addition of a crosshead bearing.

Large electrical generators are driven by medium speed engines, optimised to run at a set speed and provide a rapid response to load changes.

A few airplanes have been built that use diesel engines, such as the Junkers-powered Blohm & Voss Ha 139 of the late 1930's. 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 far superior 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 whom still use carburettors. Combined with Europe's very favourable tax treatment of diesel fuel compared to gasoline, these factors have seen considerable interest in diesel-powered small general aviation planes, and several manufacturers have recently begun selling diesel engines for this purpose. See aircraft engine.

Diesel 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).

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