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PowerPedia:Diesel engine

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Stationary One-cylinder Single-action Diesel Engine (MAN, Augsburg, 1906, 12 HP) first generation.
Stationary One-cylinder Single-action Diesel Engine (MAN, Augsburg, 1906, 12 HP) first generation.

The diesel engine is a type of internal combustion engine; more specifically, it is a compression ignition heat engine, in which the fuel is ignited by being suddenly exposed to the high temperature and pressure of a compressed gas, rather than by a separate source of ignition, such as a spark plug, as is the case in the gasoline engine. This is known as the diesel cycle, after German engineer Rudolf Diesel, who invented it in 1892 based on the hot bulb engine 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 (such as biodiesel).

The modern diesel engine is a combination of two inventor's creations. In all major aspects, it holds true to Diesel's original design, of the fuel being ignited by compression. However, nearly all present-day diesel engines use the so-called 'cold-fuel' injection system invented by Herbert Akroyd Stuart, inventor of the hot bulb engine (a similar type of engine where compression-ignition is assisted by a metal 'hot bulb' in the combustion chamber which is pre-heated before starting and then is kept hot by the ignition process). 'Cold-Fuel' injection is where the fuel is raised to extreme pressures by mechanical pumps and delivered to the combustion chamber by pressure-activated injectors. Diesel's original engine injected fuel with the assistance of compressed air, which atomised the fuel and forced it into the engine through a nozzle. The compressed air heated the fuel slightly, hence the name 'hot-fuel' injection. Cold-fuel systems are lighter, simpler and allow much faster engine speeds, and so are universally used for automotive diesel engines. Hot-fuel systems provide very efficient combustion under low-speed, high-load conditions, especially when running on poor-quality fuels, and so some large 'cathedral' marine engines use hot-fuel injection.

Contents

Diesel engine operation

Diesel Engine Timeline
  • 1862 - Nicolaus Otto develops his coal gas engine. Similar to a modern gasoline engine.
  • 1891 - Herbert Akroyd Stuart, of Bletchley (also home of the world’s first electronic computer) perfects his oil engine, and leases rights to Hornsby of England to build engines. They build the first cold start, compression ignition engines
  • 1892 - Hornsby engine No. 101 is built and installed in a waterworks. It is now in the MAN truck museum in Northern England.
    The original patent cover page of Rudolf Diesel for the Diesel-engine
    The original patent cover page of Rudolf Diesel for the Diesel-engine
  • 1892 - Rudolf Diesel develops his Carnot heat engine type motor which burnt powdered coal dust. He is employed by refrigeration genius Carl Linde, then Munich iron manfacturer MAN AG and later by the Sulzer engine company of Switzerland. He borrows ideas from them and leaves a legacy with all firms.
  • 1892 - John Froelich builds his oil engine powered farm tractor.
  • 1894 Witte, Reid, and Fairbanks start building oil engines with a variety of ignition systems.
  • 1896 - Hornsby builds diesel tractors and railway engines.
  • 1897 - Winton produces and drives the first US built gas automobile; he later builds diesel plants.
  • 1898 - Busch installs a Rudolf Diesel type engine in his brewery in St. Louis. It is the first in the United States. Rudolf Diesel perfects his compression start engine, patents and licences it. This engine, pictured above, is in a German museum.
  • 1899 - Diesel licenses his engine to builders Burmeister & Wain, Krupp and Sulzer who become famous builders.
  • 1902 - F. Rundlof invents the two stroke crankcase, scavanged hot bulb engine.
  • 1903 Ship Gjoa transits the ice filled, Northwest Passage, aided with a Dan kerosene engine.
  • 1904 - French build the first diesel submarine, the Z,
  • 1908 - Bolinder/Munktell start building two stroke diesel engines.
  • 1912 - First diesel ship MS Selandia is built. SS Fram, polar explorer Amundsen’s flagship, converted to a AB Atlas diesel.
  • 1913 - Fairbanks Morse starts building its Y model semi-Diesel. US Navy submarines use NELSECO units.
  • 1914 - German U-Boats are powered by MAN diesels. War service proves engine's reliability.
  • 1920s - Fishing fleets convert to oil engines. Atlas-Imperial of Oakland, Union, and Lister diesels appear.
  • 1924 - First diesel trucks appear.
  • 1928 - Canadian National Railways employ a diesel shunter in their yards.
  • 1930s - Clessie Cummins starts with Dutch diesel engines and then builds his own into trucks, and a Duesenberg luxury car at the Daytona speedway.
  • 1930s - Caterpillar tractor start building diesels for their tractors.
  • 1934 - General Motors starts a GM diesel research faciility. It builds diesel railroad engines--The Pioneer Zephyr, and goes on to found the General Motors Electro-Motive Division, which becomes important building engines for landing craft and tanks in the Second World War. GM then applies this knowledge to market control with its famous ‘Green Leakers? for buses, and railroad engines.
  • 1936 - Mercedes-Benz builds the 260D diesel car. A.T.S.F inaugerates the diesel train Super Chief
  • 1936 - Airship Hindenburg powered by diesel engines.

When a gas is compressed, its temperature rises; 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 combustion causes the gas in the chamber to heat up rapidly, which increases its pressure, which in turn forces the piston downwards. 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.

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. Other engines utilize small electric heaters called glow plugs inside the cylinder to warm the cylinders prior to starting. A small number 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) 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.

A vital component of older diesel engine systems was the governor, which limited the speed of the engine by controlling the rate of fuel delivery. Unlike a petrol (gasoline) engine, the incoming air is not throttled, so the engine would overspeed if this was not done. Older injection systems were driven by a gear system from the engine (and thus supplied fuel only linearly with engine speed). Modern electronically-controlled engines apply similar control to petrol engines and limit the maximum RPM 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 (the "start of injection") through electric or hydraulic actuators to maintain engine speed.

Controlling the timing of the start of injection of fuel into the cylinder is key to minimising the emissions, and maximising the fuel economy (efficiency), of the engine. The exact timing of starting this fuel injection into the cylinder is controlled electronically in most of today's modern engines. The timing is usually measured in units of crank angle of the piston before Top Dead Center (TDC). 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 deg BTDC. The optimal timing will depend on both the engine design as well as its speed and load.

Advancing (injecting when the piston is further away from TDC) the start of injection results in higher in-cylinder pressure, temperature, and higher efficiency but also results in higher emissions of Oxides of Nitrogen (NOx) due to the higher temperatures. At the other extreme, very retarded start of injection or timing causes incomplete combustion. This results in higher Particulate Matter (PM) and unburned hydrocarbon (HC) emissions and more smoke.

Fuel injection in diesel engines

Mechanical and electronic injection

Older engines make use of a mechanical fuel pump and valve assembly which is driven by the engine crankshaft, usually via the timing belt or chain. These engines use simple injectors which are basically very precise spring-loaded valves which will open and close at a specific fuel pressure. The pump assembly consists of a pump which pressurizes the fuel, and a disc-shaped valve which rotates at half crankshaft speed. The valve has a single aperture to the pressurized fuel on one side, and one aperture for each injector on the other. As the engine turns the valve discs will line up and deliver a burst of pressurized fuel to the injector at the cylinder about to enter its power stroke. The injector valve is forced open by the fuel pressure and the diesel is injected until the valve rotates out of alignment and the fuel pressure to that injector is cut off. Engine speed is controlled by a third disc, which rotates only a few degrees and is controlled by the throttle lever. This disc alters the width of the aperture through which the fuel passes, and therefore how long the injectors are held open before the fuel supply is cut, controlling the amount of fuel injected.

Older diesel engines with mechanical injection pumps could be inadvertently run in reverse, albeit very inefficiently as witnessed by massive amounts of soot being ejected from the air intake. This was often a consequence of Bump starting a vehicle using the wrong gear.

This contrasts with the more modern method of having a separate fuel pump (or set of pumps) which supplies fuel constantly at high pressure to each injector. Each injector then has a solenoid which is operated by an electronic control unit, which enables more accurate control of injector opening times depending on other control conditions such as engine speed and loading, resulting in better engine performance and fuel economy. This design is also mechanically simpler than the combined pump and valve design, making it generally more reliable, and less noisy, than its mechanical counterpart. Both mechanical and electronic injection systems can be used in either direct or indirect injection configurations.

Injection Pumps

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Images of injection pumps
via Google Image

An Injection Pump is the device that pumps fuel into the cylinders of a diesel engine or less typically, a gasoline engine. Traditionally, the pump is driven indirectly from the crankshaft by gears, chains or a toothed belt (often the timing belt) that also drives the crankshaft on overhead-cam engines (OHC). It rotates at half crankshaft speed in a conventional four-stroke engine. Its timing is such that the fuel is injected only very slightly before top dead-centre of that cylinder's compression stroke. It is also common for the pump belt on gasoline engines to be driven directly from the camshaft.

Because of the need for positive injection into a very high-pressure environment, the pump develops great pressure - typically 15,000 PSI or more on newer systems. This is a good reason to take great care when working on diesel systems; escaping fuel at this sort of pressure can easily penetrate skin and clothes, and be injected into body tissues with serious consequences. Earlier diesel pumps used an in-line layout with a series of cam-operated injection cylinders in a line, rather like a miniature inline engine. The pistons have a constant stroke volume, and injection volume (ie, throttling) is controlled by rotating the cylinders against a cut-off port that aligns with a helical slot in the cylinder. When all the cylinders are rotated at once, they simultaneously vary their injection volume to produce more or less power from the engine. Inline pumps still find favour on large multi-cylinder engines such as those on trucks, construction plant, static engines and agricultural vehicles.

For use on cars and light trucks, the rotary pump or distributor pump was developed. It uses a single injection cylinder driven from an axial cam plate, which injects into the individual fuel lines via a rotary distribution valve. Later incarnations such as the Bosch VE pump vary the injection timing with crank speed to allow greater power at high crank speeds, and smoother, more economical running at slower revs. Some VE variants have a pressure-based system that allows the injection volume to increase over normal to allow a turbocharger or supercharger equipped engine to develop more power under boost conditions. All injection pumps incorporate a governor to cut fuel supply if the crank speed endangers the engine - the heavy moving parts of diesel engines do not tolerate overspeeding well, and catastrophic damage can occur if they are over-revved. Mechanical pumps are gradually being phased out in order to comply with international emissions directives, and to increase performance and economy. Alternatives include common rail diesel systems and unit direct injection systems. These allow for higher pressures to be developed, and for much finer control of injection volumes compared to mechanical systems.

Indirect injection

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Images of indirect injection
via Google Image

In an internal combustion engine, the term indirect injection refers to a fuel injection method which does not inject fuel directly into the combustion chamber. Gasoline engines are usually equipped with indirect injection systems, where a fuel injector delivers fuel at some point before the intake valve. An indirect injection diesel engine delivers fuel into a chamber off the combustion chamber, called a prechamber, where combustion begins and then spreads into the main combustion chamber. The prechamber is carefully designed to ensure adequate mixing of the atomized fuel with the compression-heated air. This has the effect of slowing the rate of combustion, which tends to reduce audible noise. It also softens the shock of combustion and produces lower stresses on the engine components. The addition of a prechamber, however, increases heat loss to the cooling system and thereby lowers engine efficiency. Aside from the above advantages, early diesels often employed indirect injection in order to use simple, flat-top pistons, and made the positioning of the early, bulky diesel injectors easier.

An indirect injection diesel engine delivers fuel into a chamber off the combustion chamber, called a prechamber, where combustion begins and then spreads into the main combustion chamber, assisted by turbulence created in the chamber. This system allows smoother, quieter running, and because combustion is assisted by turbulence, injector pressures can be lower, which in the days of mechanical injection systems allowed high-speed running suitable for road vehicles (typically up to speed of around 4,000 rpm). The prechamber had the disadvantage of increasing heat loss to the engine's cooling system and restricting the combustion burn, which reduced the efficiency by between 5-10% in comparison to a direct injection engine, and nearly all require some form of cold-start device such as glow plugs. Indirect injection engines were used widely in small-capacity high-speed diesel engines in automotive, marine and construction uses from the 1950s, until direct-injection technology advanced in the 1980s. Indirect injection engines are cheaper to build and it is easier to produce smooth, quiet running vehicles with a simple mechanical system, so such engines are still often used in applications which carry less stringent emissions controls than road-going vehicles, such as small marine engines, generators, tractors, pumps. With electronic injection systems, indirect injection engines are still used in some road-going vehicles, but most prefer the greater efficiency of direct injection.

Direct injection

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Images of direct injection
via Google Image

The first incarnations of direct injection diesels was the distributor pump direct injection and used a rotary pump much like indirect injection diesels, however the injectors were mounted in the top of the combustion chamber rather than in a separate pre-combustion chamber. Examples are vehicles such as the Ford Transit and the Austin Rover Maestro and Montego with their Perkins Prima engine. The problem with these vehicles was the harsh noise that they made and particulate (smoke) emissions. This is the reason that in the main this type of engine was limited to commercial vehicles— the notable exceptions being the Maestro, Montego and Fiat Croma passenger cars. Fuel consumption was about fifteen to twenty percent lower than indirect injection diesels, which for some buyers was enough to compensate for the extra noise. One of the first small-capacity, mass-produced direct-injection engines that could be called refined was developed by the Rover Group. The '200Tdi' 2.5-litre 4-cylinder turbodiesel (of 111 horsepower) was used by Land Rover in their vehicles from 1989, and the engine used an aluminium cylinder head, Bosch two-stage injection and multi-phase glow plugs to produce a smooth-running and economical engine while still using mechanical fuel injection.

This type of engine was transformed by electronic control of the injection pump, pioneered by Volkswagen Audi group with the Audi 100 TDI introduced in 1989. The injection pressure was still only around 300 bar, but the injection timing, fuel quantity, exhaust gas recirculation and turbo boost were all electronically controlled. This gave much more precise control of these parameters which made refinement much more acceptable and emissions acceptably low. Fairly quickly the technology trickled down to more mass market vehicles such as the Mark 3 Golf TDI where it proved to be very popular. These cars were both more economical and more powerful than indirect injection competitors of their day.

The common rail direct fuel injection is a modern variant of direct injection system for Diesel engines. It features a high-pressure (1000+ bar) fuel rail feeding individual solenoid valves, as opposed to low-pressure fuel pump feeding pump nozzles or high-pressure fuel line to mechanical valves controlled by cams on the camshaft. Third generation common rail diesels now feature piezo injectors for even greater accuracy, with fuel pressures up to 1700 bar. The common rail system prototype was developed in the late 1960s by Mr. Hiber of Switzerland. After that, Ganser of the Swiss Federal Institute of Technology developed the common rail technology further. In the mid-nineties, Dr. Shohei Itoh and Masahiko Miyaki, of the Denso Corporation, a Japanese automotive parts manufacturer, developed the Common Rail Fuel System for Heavy Duty Vehicles and finally turned into its first practical use on their ECD-U2 Common Rail system, which was mounted on the Hino Raising Ranger truck and sold for general use in 1995.

Solenoid or piezoelectric valves make possible fine electronic control over the injection time and amount, and the higher pressure that the common rail technology makes available provides better fuel atomisation. In order to lower engine noise, a small "pilot" amount of fuel can be injected just before the main load, effectively reducing its explosiveness; some advanced common rail fuel systems perform as many as five injections per stroke. Common rail engines require no heating up time, and produce lower engine noise and lower emissions than older systems. In older diesel engines, a distributor-type injection pump, regulated by the engine, supplies bursts of fuel to injectors which are simply nozzles through which the diesel is sprayed into the engine's combustion chamber. As the fuel is at low pressure and there cannot be precise control of fuel delivery, the spray is relatively coarse and the combustion process is relatively crude and inefficient.

In common rail systems, the distributor injection pump is eliminated. Instead an extremely high pressure pump stores a reservoir of fuel at high pressure—up to 1,800 bar (180 MPa)—in a "common rail", basically a tube which in turn branches off to computer-controlled injector valves, each of which contains a precision-machined nozzle and a plunger driven by a solenoid. Driven by a computer (which also controls the amount of fuel to the pump), the valves, rather than pump timing, control the precise moment when the fuel injection into the cylinder occurs and also allow the pressure at which the fuel is injected into the cylinders to be increased. As a result, the fuel that is injected atomises easily and burns cleanly, reducing exhaust emissions and increasing efficiency. In addition, the engine's electronic control unit can inject a small amount of diesel just before the main injection event ("pilot" injection), thus reducing noise and vibration, as well as optimising injection timing and quantity for variations in fuel quality, cold starting, and so on. Most European automakers have common rail diesels in their model lineups, even for commercial vehicles. Some Japanese manufacturers, such as Toyota, Nissan and recently Honda, have also developed common rail diesel engines. Some Indian companies have also successfully implemented this technology, notably Mahindra & Mahindra for their 'Scorpio' lineup of SUV's. They call it the CRDe technology.

The modern common rail system was extensively prototyped in the 1990's, with collaboration between Magneti Marelli, Centro Ricerche Fiat and Elasis. After research and development by the Fiat Group, the design was aquired by the German company Robert Bosch GmbH for completion of development and making suitable for mass-production, whom later in 1997 extended its use for passenger cars. Common rail engines had been used in marine and locomotive applications for some time. The Cooper-Bessemer GN-8 (circa 1942) is an example of a hydraulically operated common rail diesel engine, also know as a modified common rail. The engines are suitable for all types of road car, ranging from city cars such as the Fiat Nuova Panda to large family cars like the Alfa Romeo 159.

The unit direct injection injects fuel directly into the cylinder of the engine. However, in this system the injector and the pump are combined into one unit positioned over each cylinder. Each cylinder thus has its own pump, feeding its own injector, which prevents pressure fluctuations and allows more consistent injection to be achieved. This type of injection system, also developed by Bosch, is used by Volkswagen AG in cars (where it is called Pumpe Düse - literally "pump nozzle"), Mercedes Benz (PLD) and most major diesel engine manufacturers, in large commercial engines (Cat, Cummins, Detroit Diesel). With recent advancements, the pump pressure has been raised to 2,050 bar (205 MPa), allowing injection parameters similar to common rail systems.

Diesel Engines Types

Cycles of operationTheoretical diagram of the engine, the curves correspond to the period of admission and consumption of fuel, the fuel being injected under pressure greater than the pressure at the point of highest compression. By varying the excess of pressure under which the fuel is injected and in the meantime the length or duration of admission of fuel the combustion curve is changed both in it's form or position. In operation cycle one, the fuel is admitted at the highest compression. In operation cycle two, the beginning of admission is variable.
Cycles of operation
Theoretical diagram of the engine, the curves correspond to the period of admission and consumption of fuel, the fuel being injected under pressure greater than the pressure at the point of highest compression. By varying the excess of pressure under which the fuel is injected and in the meantime the length or duration of admission of fuel the combustion curve is changed both in it's form or position. In operation cycle one, the fuel is admitted at the highest compression. In operation cycle two, the beginning of admission is variable.

The Diesel cycle is the combustion process of a type of internal combustion engine, in which the burning of the fuel is triggered by the heat generated in first compressing air in the piston cavity, into which is then injected the fuel - as opposed to it being ignited by a spark plug, as combustion is in the Otto cycle (four-stroke/petrol) engine.

Diesel engines (Heat engines utilizing the Diesel cycle) are used in automobiles, power generation, diesel-electric locomotives, and submarines. There are two classes of diesel (and gasoline) engines:

  • two-stroke
  • four-stroke.

Most diesels generally use the four-stroke cycle, with some larger diesels operating on the two-stroke cycle, mainly the huge engines in ships. Most modern locomotives use a two-stroke diesel mated to a generator, which produces current to drive electric motors, eliminating the need for a transmission. To achieve operational pressure in the cylinders, two-stroke diesels must utilize forced aspiration from either a turbocharger or supercharger. Diesel two-strokes are ideal for such applications because of their high power density--with twice as many power strokes per crankshaft revolution compared to a four-stroke, they are capable of producing much more power per displacement. 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. As a footnote, prior to 1949, Sulzer started experimenting with two-stroke engines with boost pressures as high as 6 atmospheres, in which all of the output power was taken from an exhaust turbine. The two-stroke pistons directly drove air compressor pistons to make a positive displacement gas generator. Opposed pistons were connected by linkages instead of crankshafts. Several of these units could be connected together to provide power gas to one large output turbine. The overall thermal efficiency was roughly twice that of a simple gas turbine.

Carbureted compression ignition model

Simple compression ignition engines are made for model propulsion. This is quite similar to the typical glow-plug engine that runs on a mixture of methanol (methyl alcohol) and lubricant (typically castor oil) with a hot wire filament to provide ignition. Rather than containing a glow plug the head has an adjustable contra piston above the piston, forming the upper surface of the combustion chamber. This contra piston is restrained by an adjusting screw controlled by an external lever (or sometimes by a removable hex key). The fuel used contains ether, which is highly volitile and has an extremely low flash point, combined with kerosene and a lubricant plus a very small proportion (typically 2%) of igntion improver such as Amyl Nitrate or preferrably Isopropyl Nitrate nowadays. The engine is started by reducing the compression and setting the spray bar mixture rich with the adjustable needle valve, gradually increasing the compression while cranking the engine. The compression is increased until the engine starts running. The mixture can then be leaned out and the compression increased. Compared to glow plug engines, model diesel engines exhibit much higher fuel economy, increasing endurance for the amount of fuel carried. They also exhibit higher torque, enabling the turning of a larger or higher pitched propellor at slower speed. Since the combustion occurs well before the exhaust port is uncovered, these engines are also considerably quieter (when unmuffled) than glow-plug engines of similar displacement. Compared to glow plug engines, model diesels are more difficult to throttle over a wide range of powers, making them less suitable for radio control models than either two or four stroke glow-plug engines although this difference is claimed to be less noticeable with the use of modern schneurle-ported engines.

Advantages and disadvantages versus spark-ignition engines

Power and fuel econonmy

Diesel engines are more efficient than gasoline (petrol) engines of the same power, resulting in lower fuel consumption. A common margin is 40% more miles per gallon for an efficient turbodiesel; for example, the current model Skoda Octavia, using Volkswagen engines, has a combined Euro mpg of 38.2 mpg for the 102 bhp petrol engine and 53.3 mpg for the 105 bhp — and heavier — diesel engine. The higher compression ratio is helpful in raising efficiency, but diesel fuel also contains approximately 10-20% more energy per unit volume than gasoline.

Naturally aspirated diesel engines are heavier than gasoline engines of the same power for two reasons; the first is that it takes a larger capacity diesel engine than a gasoline engine to produce the same power. This is essentially because the diesel cannot operate as quickly — the "rev limit" is lower — because getting the correct fuel-air mixture into a diesel engine quickly enough is more difficult than a gasoline engine. The second reason is that a diesel engine must be stronger to withstand the higher combustion pressures needed for ignition, and the shock loading from the detonation of the ignition mixture. As a result, the reciprocating mass (the piston and connecting rod), and the resultant forces to accelerate and to decelerate these masses, are substantially higher the heavier, the bigger and the stronger the part, and the laws of diminishing returns of component strength, mass of component and inertia - all come into play to create a balance of offsets, of optimal mean power output, weight and durability.

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 put out 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. However it should be said that any modification that raises the amount of fuel and air put through a diesel engine will increase its operating temperature which will reduce its life and increase its service interval requirements. These are issues with newer, lighter, "high performance" diesel engines which aren't "overbuilt" to the degree of older engines and are being pushed to provide greater power in smaller engines.

The addition of a turbocharger or supercharger to the engine greatly assists in increasing fuel economy and power output, mitigating the fuel-air intake speed limit mentioned above for a given engine displacement. Boost pressures can be higher on diesels than gasoline engines, and 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 MJ/kg (megajoules per kilogram) to gasoline at 45.8 MJ/kg, diesel fuel is much denser and fuel is sold by volume, so diesel contains more energy per litre or gallon. The increased fuel economy of the diesel over the gasoline engine means that the diesel produces less carbon dioxide (CO2) per unit distance. Recently, advances in production and changes in the political climate have increased the availability and awareness of biodiesel, an alternative to petroleum-derived diesel fuel with a much lower net-sum emission of CO2, due to the absorption of CO2 by plants used to produce the fuel.

The two main factors that held diesel engine back in private vehicles until quite recently were their low power outputs and high noise levels (characterised by knock or clatter, especially at low speeds and when cold). This noise was caused by the sudden ignition of the diesel fuel when injected into the combustion chamber. This noise was a product of the sudden temperature change, hence why it was more pronounced at low engine temperatures. A combination of improved mechanical technology (such as two-stage injectors which fire a short 'pilot charge' of fuel into the cylinder to warm the combustion chamber before delivering the main fuel charge) and electronic control (which can adjust the timing and length of the injection process to optimise it for all speeds and temperatures) have almost totally solved these problems in the latest generation of common-rail designs. Poor power and narrow torque bands have been solved by the use of turbochargers and intercoolers.

Emissions

Diesel engines produce very little carbon monoxide as they burn the fuel in excess air even at full load, at which point the quantity of fuel injected per cycle is still about 50% lean of stochiometric. However, they can produce black soot (or more specifically Diesel particulate matter) from their exhaust, which consists of unburned carbon compounds. This is often caused by worn injectors, which do not atomize the fuel sufficiently, or a faulty engine management system which allows more fuel to be injected than can be burned completely in the available time. The full load limit of a diesel engine in normal service is defined by the "black smoke limit", beyond which point the fuel cannot be completely combusted; as the "black smoke limit" is still considerably lean of stoichiometric it is possible to obtain more power by exceeding it, but the resultant inefficient combustion means that the extra power comes at the price of reduced combustion efficiency, high fuel consumption and dense clouds of smoke, so this is only done in specialised applications (such as tractor pulling) where these disadvantages are of little concern. Particles of the size normally called PM10 (particles of 10 micrometres or smaller) have been implicated in health problems, especially in cities. Some modern diesel engines feature diesel particulate filters, which catch the black soot and when saturated are automatically regenerated by burning the particles. Other problems associated with the exhaust gases (nitrogen oxides, sulfur oxides) can be mitigated with further investment and equipment; some diesel cars now have catalytic converters in the exhaust. [edit]

Power torque

For commercial uses requiring towing, load carrying and other tractive tasks, diesel engines tend to have more desirable torque characterstics. Diesel engines tend to have their torque peak quite low in their speed range (usually between 1600-2000 rpm for a small-capacity unit, lower for a larger engine used in a lorry or truck). This provides smoother control over heavy loads when starting from rest, and crucially allows the diesel engine to be given higher loads at low speeds than a petrol/gasoline engine, which makes them much more economical for these applications. This characteristic is not so desirable in private cars, so most modern diesels used in such vehicles use electronic control, variable geometery turbochargers and shorter piston strokes to achieve a wider spread of torque over the engine's speed range, typically peaking at around 2500-3000 rpm.

Reliability

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 as well as the diesel's combustion cycle, which creates less-violent changes in pressure when compared to a spark-ignition engine, a benefit that is magnified by the lower rotating speeds in diesels. Diesel fuel is a better lubricant than gasoline so is less harmful to the oil film on piston rings and cylinder bores; it is routine for diesel engines to cover 250,000 miles or more without a rebuild. 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 amount of engine damage during initial start-up and running. Some smaller military diesels can be started with an explosive cartridge that provides the extra power required to get the machine turning. In the past, Caterpillar and John Deere used a small gasoline "pony" motor in their tractors to start the primary diesel motor. The pony motor heated the diesel to aid in ignition and utilized a small clutch and transmission to actually spin up the diesel engine.

Even more unusual was an International Harvester design in which the diesel motor had its own carburetor and ignition system, and started on gasoline. Once warmed up, the operator moved two levers to switch the motor to diesel operation, and work could begin. These engines had very complex cylinder heads (with their own gasoline combustion chambers) and in general were vulnerable to expensive damage if special care was not taken (especially in letting the engine cool before turning it off). As mentioned above, diesel engines tend to have more torque at lower engine speeds than gasoline engines. However, diesel engines tend to have a narrower power band than gasoline engines. Naturally-aspirated diesels tend to lack power and torque at the top of their speed range. This narrow band is a reason why a vehicle such as a truck may have a gearbox with as many as 16 or more gears, to allow the engine's power to be used effectively at all speeds. Turbochargers tend to improve power at high engine speeds, intercoolers do the same at lower speeds, and variable geometry turbochargers improve the engine's performance equally (or make the torque curve flatter).

Spark-ignition Engine Dieseling

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). This is usually caused by hot carbon deposits within the combustion chamber that act as would a "glow plug" within a diesel or model aircraft engine. Excessive heat can also be caused by improper ignition timing and/or fuel/air ratio which in turn overheats the exposed portions of the spark plug within the combustion chamber. Finally, high-compression engines that require high octane fuel may knock when a lower-octane fuel is used.

Fuel and fluid characteristics

Diesel or diesel fuel is a specific fractional distillate of fuel oil (mostly petroleum) that is used as fuel in a diesel engine invented by German engineer Rudolf Diesel. Petroleum derived diesel is composed of about 75% saturated hydrocarbons (primarily paraffins including n, iso, and cycloparaffins), and 25% aromatic hydrocarbons (including naphthalenes and alkylbenzenes). The average chemical formula for common diesel fuel is C12H26, ranging from approx. C10H22 to C15H32. The term "diesel" typically refers to fuel that has been processed from petroleum, but increasingly, alternatives such as biodiesel or biomass to liquid (BTL) or gas to liquid (GTL) diesel that are not derived from petroleum are being developed and adopted. Diesel engines can operate on a variety of different fuels, depending on configuration, though the eponymous diesel fuel derived from crude oil is most common. Good-quality diesel fuel can be synthesised from vegetable oil and alcohol. Biodiesel is growing in popularity since it can frequently be used in unmodified engines, though production remains limited. Petroleum-derived diesel is often called "petrodiesel" if there is need to distinguish the source of the fuel. The engines can work with the full spectrum of crude oil distilates, from compressed natural gas, alcohols, gasolene, to the "fuel oils" from diesel oil to residual fuels. The type of fuel used is a combination of service requirements, and fuel costs.

Diesel is produced from petroleum, and is sometimes called petrodiesel (or, less seriously, dinodiesel) when there is a need to distinguish it from diesel obtained from other sources such as vegidiesel (biodiesel) derived from pure (SVO) or recycled waste (WVO) vegetable oil. As a hydrocarbon mixture, it is obtained in the fractional distillation of crude oil between 250 °C and 350 °C at atmospheric pressure. The density of diesel is about 850 grams per liter whereas gasoline has a density of about 720 g/l, about 15% less. When burnt, diesel typically releases about 40.9 megajoules (MJ) per liter, whereas gasoline releases 34.8 MJ/L, also about 15% less. Diesel is generally simpler to refine than gasoline and often costs less (although price fluctuations sometimes mean that the inverse is true; for example, the cost of diesel traditionally rises during colder months as demand for heating oil, which is refined much the same way, rises).

Diesel powered cars generally have greater fuel economy than gasoline powered cars, which is due to the greater energy content of diesel fuel and also the intrinsic efficiency of the diesel engine. Proponents of diesel powered automobiles often cite this advantage as a way to reduce Greenhouse gas emissions. However, diesel's 15% higher volumetric energy density results in 15% higher greenhouse gas emissions per liter compared to gasoline[1], which offsets the increased fuel economy. In other words, a petrodiesel powered engine must have greater than 15% more fuel efficiency than a gasoline engine in order to reduce greenhouse gas emissions.

Also, diesel fuel often contains higher quantities of sulfur. European emission standards and preferential taxation have forced oil refineries to dramatically reduce the level of sulfur in diesel fuels. In contrast, the United States has long had "dirtier" diesel, although more stringent emission standards have been adopted with the transition to ultra-low sulfur diesel (ULSD) starting in 2006 and becoming mandatory on June 1, 2010. U.S. diesel fuel typically also has a lower cetane number (a measure of ignition quality) than European diesel, resulting in worse cold weather performance and some increase in emissions. High levels of sulfur in diesel are harmful for the environment. It prevents the use of catalytic diesel particulate filters to control diesel particulate emissions, as well as more advanced technologies, such as nitrogen oxide (NOx) adsorbers (still under development), to reduce emissions. However, lowering sulfur also reduces the lubricity of the fuel, meaning that additives must be put into the fuel to help lubricate engines. Biodiesel is an effective lubricant.


"Residual fuels" are the "dregs" of the distilation process and are a thicker, heavier oil, or oil with higher viscosity, which are so thick that they are not readily pumpable unless heated. Residual fuel oils are cheaper than clean, refined diesel oil, although they are dirtier. Their main considerations are for use in ships and very large generation sets, due to the cost of the large volume of fuel consumed, frequently amounting to many tonnes per hour. The poorly refined biofuels straight vegetable oil (SVO) and waste vegetable oil (WVO) can fall into this category. Moving beyond that, use of low-grade fuels can lead to serious maintenance problems. Most diesel engines that power ships like supertankers are built so that the engine can safely use low grade fuels. Normal diesel fuel is more difficult to ignite than gasoline because of its higher flash point, but once burning, a diesel fire can be fierce.

Diesel applications

Diesel Engine applications
High-speed
High-speed (approximately 1200 rpm and greater) engines are used to power trucks (lorries), 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 80MW, turn at about 60 to 100 rpm, and are up to 15 m tall. They often run on cheap low-grade fuel, which require extra heat treatment in the ship for tanking and before injection due to their low volatility. Companies such as Burmeister & Wain and Wärtsilä (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 (2006), the 12 cylinder MAN B&W Diesel K98MC turbocharged two-stroke diesel engine build by MAN B&W Diesel licencee Hyundai Heavy Industries in Korea is the most powerful diesel engine put into service, with a cylinder bore of 980 mm delivering 74.8 MW (101,640 bhp). When put into service in 2007, the 14 cylinder Wärtsilä-Sulzer RTA96-C will become the most powerful diesel engine with 80 MW (110,000 hp).

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. Besides their use in merchant ships and boats, there is also a naval advantage in the relative safety of diesel fuel, additional to improved range over a gasoline engine. The German "pocket battleships" were the largest diesel warships, but the German torpedo-boat called "Schnellboot" of the Second World War was also a diesel craft. Conventional submarines have used them since before the First World War. It was an advantage of American diesel-electric submarines that they operated a two-stroke cycle as opposed to the four-stroke cycle that other navies used. Mercedes-Benz, cooperating with Robert Bosch GmbH, has a successful run of diesel-powered passenger cars since 1936, sold in many parts of the World, with other manufacturers joining in the 1970s and 1980s. The second car manufacturer was Peugeot, prior to 1960.

In the U.S., probably due to some hastily offered cars in the 1980s, Diesel is not as popular in passenger vehicles as in Europe. Such cars have been traditionally perceived as heavier, noisier, having performance characteristics which make them slower to accelerate, and of being more expensive than equivalent gasoline vehicles. General Motors Oldsmobile division produced a variation of its gasoline-powered V8 engine which is the main reason for this reputation. This image certainly does not reflect recent designs, especially where the very high low-rev torque of modern Diesels is concerned -- which have characteristics similar to the big V8 gasoline engines popular in the US. Light and heavy trucks, in the U.S., have been diesel-optioned for years. European governments tend to favor diesel engines in taxation policy because of diesel's superior fuel efficiency. In addition, diesel fuel used in North America still has higher sulphur content than the fuel used in Europe, effectively limiting diesel use to industrial vehicles, before the introduction of 15 parts per million Ultra low Sulfur Diesel, which will start at 15 October 2006 in the U.S. (1 June 2006 in Canada). Ultra Low Sulfur Diesel is not mandatory until 2010 in the US.

In Europe, where tax rates in many countries make diesel fuel much cheaper than gasoline, diesel vehicles are very popular and newer designs have significantly narrowed differences between petrol and diesel vehicles in the areas mentioned. Often, among comparably designated models, the Turbo-Diesels outperform their naturally aspirated petrol-powered sister cars. One anecdote tells of Formula One driver Jenson Button, who was arrested while driving a diesel-powered BMW 330cd Coupé at 230 km/h (about 140 mph) in France, where he was too young to have a gasoline-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. Yet, BMW had already won the 24 Hours Nürburgring overall in 1998 with a 3-series diesel. The BMW diesel lab in Steyr, Austria is led by Ferenc Anisits and develops innovative diesel engines. Mercedes-Benz, offering diesel-powered passenger cars since 1936, has put the emphasis on high performance diesel cars in its newer ranges, as does Volkswagen with its brands. Citroën sells more cars with diesel engines than gasoline engines, as the French brands (also Peugeot) pioneered smoke-less HDI designs with filters. Even the Italian marque Alfa Romeo, known for design and successful history in racing, focuses on Diesels that are also raced.

The first diesel aircraft engine was the Junkers Jumo 205, which used an opposed-piston two stroke design. A Mercedes-Benz diesel engine was also used in Zeppelins, including the infamous Hindenberg. Entering service, it was moderately successful in its use in the Blohm & Voss Ha 139 and even more so in airship use. The Zeppelins Graf Zeppelin II and Hindenburg were propelled by reversible diesel engines. This engine proved unsuitable in military applications and subsequent German aircraft engine development concentrated on gasoline and jet engines. Other manufacturers also experimented with diesel engines in this period, such as the French Bloch MB203 bomber prototype. The Clerget diesels used were radial designs. 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. Diesel engines were first tried in aircraft in the 1930s. A number of manufacturers built engines, the best known probably being the Junkers Jumo 205, which was moderately successful, but proved unsuitable for combat use in WWII. Postwar, another interesting proposal was the complex Napier Nomad. Interest in genera over diesel engines in the postwar period was sporadic; the British Air Ministry supported the development of the 3,000 hp Napier Nomad. It was exceptionally efficient but judged too bulky and complex and cancelled in 1955. With fuel available cheaply and most research interest in turboprops and jets for high-speed airliners, interest in diesel-powered aircraft virtually disappeared. The near-death of the general aviation market saw a massive decline in interest in the development of any new aircraft types.

In general, though, the lower power-to-weight ratio of diesels, particularly compared to kerosene-powered turboprop engines, has precluded their use in this application. The very high cost of avgas in Europe, and the advances in automotive diesel technology, have seen renewed interest in the concept. New, certified diesel-powered light planes are already available, and a number of other companies are also developing new engine and aircraft designs for the purpose. Many of these run on the readily-available jet fuel, or can run on both jet fuel or conventional automotive diesel.

Several factors have emerged to change this equation; a number of new manufacturers of general aviation aircraft, with new designs becoming available. Secondly, in Europe in particular, avgas has become very expensive and less easy to obtain. Finally, automotive diesel technologies have improved greatly in recent years, offering higher power-to-weight ratios more suitable for aircraft applications. The first manufacturer to produce a certified design for the general aviation market is 'Thielert GmbH', located at the small town of Lichtenstein, eastern Germany (not to be confused with the principality of Liechtenstein, between Switzerland and Austria). They produce four-stroke, liquid-cooled, geared, turbo-diesel aircraft engines based on Mercedes automotive designs which will run on both Diesel and Jet Aviation fuel (JetA1). Their first engine, A 1.7 litre, 135 hp four-cylinder (based on the 1.7 turbo diesel Mercedes A-class power unit) was first certified in 2002. It is certified for retrofit to Cessna 172s and Piper Cherokees which were originally equipped with the 160-hp Lycoming O-320 Avgas (petrol) engine. Although the weight of the 135 hp Thielert Centurion 1.7 at around 136 kg, is similar to that of the 160 hp Lycoming O-320, its displacement is less than a third of that of the Lycoming. It however achieves maximum power at 2300 prop rpm (3900 crank rpm) as opposed to 2700 for the petrol Lycoming.

The Austrian aircraft firm Diamond Aircraft Industries offers its single-engine Diamond DA40-TDI Star with a Thielert Centurion 1.7' engine and also the Twin Star with two. The Star offers low fuel consumption with a very fuel efficient figure of 15.1 l/h. Several hundred Thielert-powered airplanes are now flying, and the company has certified a 4.0-litre, V8, 310 HP version in 2005. 'Apex aircraft', formerly Robin also offers an aircraft (Ecoflyer) with the Thielert engine Interest in diesel aircraft in the USA has been more limited with the lower taxes on fuel there. A number of other manufacturers are currently developing experimental diesel engines, many using aircraft-specific designs rather than adapted automotive engines. Many are using two-stroke designs, with some opposed-piston layouts directly inspired by the original Junkers design. Examples include:

  • 'Dair', a British company who are developing a twin-piston, two-stroke layout inspired by the original Junkers design. Their engine has flown in test aircraft and airship installations.
  • DeltaHawk Engines, an American company currently developing V-4, 160 and 200 horsepower designs. Also using a two-stroke, ported design, they have also flown a prototype engine in an aircraft and are claiming delivery of non-certified engines in 2006 and hope to achieve certification in 2007.

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 and tractor pulling, as well in types of racing where these drawbacks are less severe, such as land speed record racing or endurance racing. Even Diesel engined dragsters exist, despite the diesel's drawbacks being central to performance in this sport. 1931 - Clessie Cummins installs his Diesel in a race car. It runs at 162 km/h in Daytona, and 138 km/h in Indianapolis where it places 12th. In 1933, A 1925 Bentley with a Gardner 4LW engine was the first diesel-engined car to take part in the Monte Carlo Rally when it was driven by Lord Howard de Clifford. It was the leading British car and finished fifth overall.

In 1952, Cummins Diesel 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. With turbocharged Diesel-cars getting stronger in the 1990s, they were also entered in touring car racing, and BMW even won the 24 Hours Nürburgring in 1998 with a 320d, against other factory-entered Diesel-competition of Volkswagen and about 200 regular powered cars. Alfa Romeo even organized a racing series with their Alfa Romeo 147 1.9 JTD models. The VW Dakar Rally entrants for 2005 and 2006 are powered by their own line of TDI engines in order to challenge for the first overall diesel win there. Meanwhile, the five time 24 Hours of Le Mans winner Audi R8 race car was replaced by the Audi R10 in 2006, which is powered by a 650 hp (485 kW) and 1100 Nm (810 lb·ft) V12 TDI Common Rail diesel engine, mated to a 5-Speed gearbox, instead of the 6 used in the R8, to handle the extra torque produced. The gearbox is considered the main problem, as earlier attempts by others failed due to the lack of suitable transmissons that could stand the torque long enough. After winning the 12 Hours of Sebring in 2006 with their Diesel-powered R10, Audi obtained the overall win at the 2006 24 Hours of Le Mans, too. This is the first time a sports car can compete for overall victories with Diesel-fuel against cars powered with regular fuel or methanol and bio-ethanol. However, the significance of this is slightly lessened by the fact that the ACO/ALMS race rules encourage the use of alternate fuels like Diesel. [edit]

With a traditionally poor power-to-weight ratio, diesel engines are generally unsuited to use in a motorcycle, which requires high power, light weight and a fast-revving engine. However, in the 1980s NATO forces in Europe standardised all their vehicles to diesel power. Some had fleets of motorcycles, and so trials were conducted with diesel engines for these. Air-cooled single-cylinder engines built by Lombardini of Italy were used and had some success, achieving similar performance to petrol bikes and fuel usage of nearly 200 miles per gallon. This led to some countries re-fitting their bikes with diesel power. Development by Cranfield University and California-based Hayes Diversified Technologies led to the production of a diesel powered off road motorbike based on the running gear of a Kawasaki KLR650 petrol-engine trail bike for military use. The engine of the diesel motorcycle is a liquid cooled, single cylinder four- stroke which displaces 584 cm³ and produces 21 kw (28 bhp) with a top speed of 85mph (136kph). Hayes Diversified Technologies mooted, but has subsequently delayed, the delivery of a civilian version for approx USD$19,000. Expensive compared to comparable models. In 2005 the United States Marine Corps adopted the M1030M1, a dirtbike based on the Kawasaki KLR650 and modified with an engine designed to run on diesel or JP8 jet fuel. Since other U.S. tactical vehicles like the Humvee utility vehicle and M1 Abrams tank use JP8, adopting a scout motorcycle which runs on the same fuels made sense from a logistical standpoint. In India, motorcycles built by Royal Enfield can be bought with 650cc single-cylinder diesel engines based on the similar petrol engines used, due to the fact that diesel is much cheaper than petrol and of more reliable quality. These engines are noisy and unrefined, but very popular due to their reliability and economy.

Diesel locomotives

Diesel locomotives became the dominant type of locomotive in rail transport in the mid 20th century in much of the world. Powered by diesel engines, they use a variety of transmissions to convey power to the wheels. Diesel locomotives, in contrast to electric locomotives, do not require catenary or electrified rail installations to run. Therefore, they offer more flexibility in various types of service and are generally predominant in countries which, for historical or economical reasons, have few electrified lines. Since the 1950s, however, they have been superseded by electric locomotives in terms of power, maximum speed, tractive effort, and acceleration. Rudolf Diesel had suggested that his engine could be employed in railroad service, and in 1909 helped to construct an experimental locomotive.

In 1918 diesel electric switching locomotives were put in service in the United States. Sixteen years later, mainline engines began to be produced, at first for passenger service. Custom units were produced at at first for the Chicago, Burlington and Quincy Railroad's Pioneer Zephyr and as a single unit for the Baltimore and Ohio Railroad. Mass production of passenger and freight units soon followed. By 1960, diesel-electrics had displaced steam locomotives on every Class I railroad in the United States of America.

In the 1970s British Rail developed a high-speed diesel-electric train called the High Speed Train or HST. This train consists of two Class 43 locomotives (also known as power cars), one at each end, and a number of "Mark 3" carriages (usually 8). A complete HST set was originally designated as a Class 253 or 254 diesel multiple unit (DMU), but due to the frequent exchanges between sets the power cars were reclassified as locomotives and given class number 43. The unpowered carriages were simultaneously reclassified as individual coaches; the number of a DMU set should identify all its associated carriages as well. The HST holds the world speed record for diesel traction, having reached a speed of 148 mph, although the operating speed in service is 125 mph (200 km/h), hence the name "Inter-City 125". Unlike steam engines, diesel engines require a transmission to power the wheels.

The engine must continue to idle when the locomotive is stopped. The simplest form of transmission is by means of a gearbox, in the same way as on road vehicles. Diesel trains or locomotives that use this are called diesel-mechanical and began to appear after World War I. It has been found impractical to inexpensively build a gearbox which can cope with a power output of more than 400 horsepower (300 kW) without failure, despite a number of attempts to do so. Therefore this type of transmission is only suitable for low-powered shunting locomotives, or lightweight multiple units or railcars. For more powerful locomotives, other types of transmission have to be used.

The most common form of transmission is electric; a locomotive using electric transmission is known as a diesel-electric locomotive. With this system, the diesel engine drives a generator or alternator; the electrical power produced then drives the wheels using electric motors. This is effectively an electric locomotive with its own generating station.

For the first decades the motors were direct current. More recently alternating current has come to be preferred. In either case, a common option is the use of dynamic braking, in which the motors are switched to perform as generators, thus converting the motion of the locomotive into electrical energy, which is then dissipated through heating elements usually mounted on the top of the locomotive. Dynamic braking reduces brake usage in mountainous areas, though it is ineffective at low speeds.

There are special locomotives that can either operate as an electric locomotive or a diesel locomotive (called Electro-diesel locomotive). Dual-mode diesel-electric/third-rail locomotives are operated by the Long Island Rail Road and Metro-North Railroad between non-electrified territory and New York City because of a local law banning diesel-powered locomotives in Manhattan tunnels. For the same reason, Amtrak operates a fleet of dual-mode locomotives in the New York area. British Rail operated dual diesel-electric/electric locomotives designed to run primarily as electric locomotives. This allowed railway yards to remain un-electrified as the third-rail power system is extremely hazardous in a yard area.

Alternatively, diesel-hydraulic locomotives use hydraulic transmission to convey the power from the diesel engine to the wheels. On this type of locomotive, the power is transmitted to the wheels by means of a device called a torque converter. A torque converter consists of three main parts, two of which rotate, and one which is fixed. All three main parts are sealed in a housing filled with oil. The inner rotating part of a torque converter is called a centrifugal pump (or impeller), the outer part is called a turbine wheel (or driven wheel), and between them is a fixed guide wheel. All of these parts have specially shaped blades to control the flow of oil.

The centrifugal pump is connected directly to the diesel engine, and the turbine wheel is connected to an axle, which drives the wheels. As the diesel engine rotates the centrifugal pump, oil is forced outwards at high pressure. The oil is forced through the blades of the fixed guide wheel and then through the blades of the turbine wheel, which causes it to rotate and thus turn the axle and the wheels. The oil is then pumped around the circuit again and again. The disposition of the guide vanes allows the torque converter to act as a "gearbox" with continuously variable ratio. If the output shaft is loaded so as to reduce its rotational speed, the torque applied to the shaft increases, so the power transmitted by the torque converter remains more or less constant.

However, the range of variability is not sufficient to match engine speed to load speed over the entire speed range of a locomotive, so some additional method is required to give sufficient range. One method is to follow the torque converter with a mechanical gearbox which switches ratios automatically, similar to an automatic transmission on a car. Another method is to provide several torque converters each with a range of variability covering part of the total required; all the torque converters are mechanically connected all the time, and the appropriate one for the speed range required is selected by filling it with oil and draining the others. The filling and draining is carried out with the transmission under load, and results in very smooth range changes with no break in the transmitted power.

Diesel-hydraulic multiple units, a less arduous duty, often use a simplification of this system, with a torque converter for the lower speed ranges and a fluid coupling for the high speed range. A fluid coupling is similar to a torque converter but the ratio of input to output speed is fixed; loading the output shaft results not in torque multiplication and constant power throughput but in reduction of the input speed with consequent lower power throughput. (In car terms, the fluid coupling provides top gear and the torque converter provides all the lower gears.) The result is that the power available at the rail is reduced when operating in the lower speed part of the fluid coupling range, but the less arduous duty of a passenger multiple unit compared to a locomotive makes this an acceptable tradeoff for reduced mechanical complexity.

Diesel-hydraulic locomotives are slightly more efficient than diesel-electrics, but were found in many countries to be mechanically more complicated and more likely to break down. In Germany, however, diesel-hydraulic systems achieved extremely high reliability in operation. Persistent argument continues over the relative reliability of hydraulic systems, with continuing questions over whether data was manipulated politically to favour local suppliers over German ones. In the US and Canada, they are now greatly outnumbered by diesel-electric locomotives, while they remain dominant in some European countries. The most famous diesel-hydraulic locomotive is the German V200 which were built from 1953 in a total number of 136. The only diesel-electric locomotives of the Deutsche Bundesbahn were BR 288 (V 188), of which 12 were built in 1939 by the DRG.

The high reliability of the German locomotives was paralleled by higher reliability of non-German locomotives built with German-made parts compared to that of the same designs built using parts made locally to German patterns under licence. Much of the unreliability experienced outside Germany was due to poor quality control in the local manufacture of engines and transmissions. Another contributing factor was poor maintenance due to staff accustomed to steam locomotives now working on unfamiliar and much more complex designs in unsuitable conditions, and failing to follow the unit-replacement maintenance methods which were part of the German success. It is notable that diesel-hydraulic multiple units, with the advantages of modern manufacturing techniques and improved maintenance procedures, are now extremely successful in widespread use, achieving excellent reliability. In the 1960s, more than 15 diesel-hydraulic locomotives were purchased by the Denver & Rio Grande and Southern Pacific Railroads on a trial basis from the Kraus-Maffei company. Only the outer shell of one of these (converted into a camera car by SP in the 1970s) exists today, the others having all been scrapped.

Diesel-steam locomotives can use diesel or steam power, as needed. When mainline diesels were mass produced in the United States, they were initially sold as multiple unit sets. The engines and traction motors of the day were not capable of the power output needed to pull an entire train with a single unit. These units were controlled through the same type of multiple unit system already in use for electric locomotives. The "American Association of Railroads" standard for multiple-unit control remains the basis for US operation. The Kraus-Maffei diesel-hydraulic units were also equipped with this system. See also Multiple working for UK locomotives.

Modern developments

Already, many common rail and unit injection systems employ new injectors using stacked piezoelectric crystals in lieu of a solenoid, which gives finer control of the injection event. Variable geometry turbochargers have flexible vanes, which move and let more air into the engine depending on load. This technology increases both performance and fuel economy. Boost lag is reduced as turbo impeller inertia is compensated for. A technique called accelerometer pilot control (APC) uses a sensor called an accelerometer to provide feedback on the engine's level of noise and vibration and thus instruct the ECU to inject the minimum amount of fuel that will produce quiet combustion and still provide the required power (especially while idling.) The next generation of common rail diesels are expected to use variable injection geometry, which allows the amount of fuel injected to be varied over a wider range, and variable valve timing similar to that on gasoline engines. Particularly in the United States, coming tougher emissions regulations present a considerable challenge to diesel engine manufacturers. Other methods to achieve even more efficient combustion, such as HCCI (homogeneous charge compression ignition), are being studied.

Fuel passes through the injector jets at speeds of nearly 1500 miles per hour (2400 km/h) – as fast as the top speed of a jet plane. Fuel is injected into the combustion chamber in less than 1.5 ms – about as long as a camera flash. The smallest quantity of fuel injected is one cubic millimetre – about the same volume as the head of a pin. The largest injection quantity at the moment for automobile diesel engines is around 70 cubic millimetres. If the camshaft of a six-cylinder engine is turning at 4500 rpm, the injection system has to control and deliver 225 injection cycles per second. On a demonstration drive, a Volkswagen 1-litre diesel-powered car used only 0.89 litres of fuel in covering 100 kilometres (264MPG {US}, 317MPG {Imperial/English}) – making it probably the most fuel-efficient car in the world. Bosch’s high-pressure fuel injection system was one of the main factors behind the prototype’s extremely low fuel consumption. Production record-breakers in fuel economy include the Volkswagen Lupo 3L TDI and the Audi A2 3 L 1.2 TDi with standard consumption figures of 3 litres of fuel per 100 kilometres (78MPG {US}, 94MPG {Imperial}). Their High-pressure Diesel injection systems are also supplied by Bosch. In 2001, nearly 36% of newly registered cars in Western Europe had diesel engines. By way of comparison: in 1996, diesel-powered cars made up only 15% of the new car registrations in Germany. Austria leads the league table of registrations of diesel-powered cars with 66%, followed by Belgium with 63% and Luxembourg with 58%. Germany, with 34.6% in 2001, was in the middle of the league table. Sweden is lagging behind, in 2004 only 8% of the new cars had diesel engine.

Patents


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