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Diesel engine

A diesel engine is an internal combustion engine which operates using the Diesel cycle (named after Dr. Rudolph Diesel). The defining feature of the Diesel engine is the use of compression ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression. This is in contrast to a gasoline engine, which utilizes the Otto cycle, in which ignition is initiated by a spark plug following the aspiration and compression of a fuel/air mixture.

Diesel engines were originally used in very large vehicles such as trucks, locomotives and ships, (and also as a stationary engine), as more efficient replacement for the steam engine. The diesel engines of today are refined and improved versions of Rudolf Diesel's original concept. They are often used in submarines, ships, locomotives, and large trucks and in electric generating plants. Starting in the 1930s and initially developing slowly they have been used in a few automobiles. Since the 1970s diesel engines have developed dramatically and have been used in many more cars, starting with larger on-road and off-road vehicles (especially sport utility vehicles in the USA). As they have been refined they have been used in smaller and smaller cars. Today the smallest cars on the European market have a highly efficient diesel engine option and forty percent of all new car sales in Europe are diesel.


Rudolf Diesel was born in Paris in 1858. His actual nationality was German. His parents were Bavarian immigrants. Rudolf Diesel was educated at Munich Polytechnic. After graduation he was employed as a refrigerator engineer. However, his true love lay in engine design. Diesel designed many heat engines, including a solar-powered air engine. In 1893, he published a paper describing an engine with combustion within a cylinder, the internal combustion engine. In 1894, he filed for a patent for his new invention, dubbed the diesel engine. Diesel was almost killed by his engine when it exploded. However, his engine was the first to prove that fuel could be ignited without a spark. He operated his first successful engine in 1897.

In 1898, Diesel was granted patent #608,845 for an "internal combustion engine".

Though best known for his invention of the pressure-ignited heat engine that bears his name, Rudolf Diesel was also a well-respected thermal engineer and a social theorist. Diesel's inventions have three points in common: They relate to heat transference by natural physical processes or laws; they involve markedly creative mechanical design; and they were initially motivated by the inventor's concept of sociological needs. Rudolf Diesel originally conceived the diesel engine to enable independent craftsmen and artisans to compete with large industry.

At Augsburg, on August 10, 1893, Rudolf Diesel's prime model, a single 10-foot iron cylinder with a flywheel at its base, ran on its own power for the first time. Diesel spent two more years making improvements and in 1896 demonstrated another model with the theoretical efficiency of 75%, in contrast to the ten percent efficiency of the steam engine. By 1898, Diesel was a millionaire. His engines were used to power pipelines, electric and water plants, automobiles and trucks, and marine craft, and soon after were used in mines, oil fields, factories, and transoceanic shipping.

Early history timeline

  • 1862: Nicholas Immel develops his coal gas engine, similar to a modern gasoline engine.
  • 1891: Herbert Akroyd Stuart of Bletchley 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 was in the MAN truck museum in Stockport, and is now in the Anson Engine Museum in Poynton. T.H. Barton at Hornsbys builds an experimental version where the vaporiser was replaced with a cylinder head and the pressure increased. Automatic ignition was achieved through compression alone (the first time this had happened), and the engine ran for 6 hours. Diesel would achieve much the same thing 5 years later, claiming the achievement for himself.
  • 1892: Rudolf Diesel develops the principles of his proposed Carnot heat engine type motor which would burn powdered coal dust. He is employed by refrigeration engineer Carl von Linde, then Munich iron manufacturer 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 first oil engine-powered farm tractor.
  • 1893: August 10th—Diesel builds a working version of his ideas.
  • 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. On February 17, Diesel builds his first working prototype, which narrowly avoids a catastrophic explosion in Augsburg. The engine finally ready for market in 1908, thanks to other people's improvements.
  • 1897: Mirrlees, Watson & Yaryan build the first British diesel engine under license from Rudolf Diesel. This is now displayed in the Anson Engine Museum at Poynton, Cheshire, UK.
  • 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, and patents and licenses it. This engine, pictured above, is in a German museum. Burmeister & Wain (B & W) of Copenhagen, Denmark buy rights to build diesel engines.
  • 1899: Diesel licenses his engine to builders Krupp and Sulzer, who become famous builders.
  • 1902: F. Rundlof invents the two-stroke crankcase, scavenged hot bulb engine.
  • 1902: A company named Forest City starts manufacturing diesel generators.
  • 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 starts building two stroke hot-bulb engines.
  • 1912: First diesel ship MS Selandia is built. SS Fram, polar explorer Amundsen’s flagship, is converted to an AB Atlas diesel.
  • 1913: Fairbanks Morse starts building its Y model semi-diesel engine. US Navy submarines use NELSECO units. Rudolf Diesel died mysteriously when he took a ship (SS Dresden) to cross the English Channel.
  • 1914: German U-Boats are powered by MAN diesels.
  • 1920s: Fishing fleets convert to oil engines. Atlas-Imperial of Oakland, Union, and Lister diesels appear.
  • 1922: Mack Boring & Parts Company is established. Adelmo Lombardini manufactures his first engine.
  • 1924: First diesel trucks appear.
  • 1928: Canadian National Railway employs a diesel shunter in their yards.
  • 1930: Edward McGovern Sr., founder of Mack Boring & Parts Company, opens the first diesel-only engine institute in North America.
  • 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 starts building diesels for their tractors.
  • 1933: Citroën introduced the Rosalie, a passenger car with the world’s first commercially available diesel engine developed with Harry Ricardo. Lombardini company is established in Reggio Emilia
  • 1934: General Motors starts a GM diesel research facility. 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.
  • 1934-35: Junkers Motorenwerke in Germany starts production of the Jumo aviation diesel engine "family", the most famous of these being the Jumo 205, of which over 900 examples are produced into the outbreak of World War II.
  • 1936: Mercedes-Benz builds the 260D diesel car. AT&SF inaugurates the diesel train Super Chief.
  • 1936: Airship Hindenburg is powered by diesel engines.

How diesel engines work

The diesel internal combustion engine differs from the gasoline powered Otto cycle by using a higher compression of the fuel to ignite the fuel rather than using a spark plug ("compression ignition" rather than "spark ignition").

As well as the high level of compression allowing combustion to take place without a separate ignition system, a high compression ratio greatly increases the engine's efficiency. Increasing the compression ratio in a spark-ignition engine where fuel and air are mixed before entry to the cylinder is limited by the need to prevent damaging pre-ignition. Since only air is compressed in a diesel engine, and fuel is not introduced to the cylinder until the start of the combustion process, detonation is not an issue and compression ratios can be much higher.

In the diesel engine, air is compressed adiabatically with a compression ratio typically between 15 and 20. This compression raises the temperature of the air in the combustion chamber to the ignition temperature of the atomized fuel–air mixture, which is formed by injecting fuel once the air is compressed. The injected droplets vaporize from their surfaces and this vapor ignites and burns. The droplets then burn from their surface towards their centers, until all the fuel in the droplets are consumed.

The ideal air-standard cycle is modeled as a reversible adiabatic compression followed by a constant pressure combustion process, then an adiabatic expansion as a power stroke and an isovolumetric exhaust. A new air charge is taken in at the end of the exhaust, as indicated by the processes a-e-a on the diagram.

Since the compression and power strokes of this idealized cycle are adiabatic, the efficiency can be calculated from the constant pressure and constant volume processes. The input and output energies and the efficiency can be calculated from the temperatures and specific heats.

Cold weather


In cold weather, diesel engines can be difficult to start because the mass of the cylinder block and cylinder head absorb the heat of compression, thus preventing ignition. Spark ignition engines have the same problem, but they have the benefit of a spark plug to help cause ignition. The main reason diesel engines take a long time to warm up in cold weather is the lack of a throttle. Spark ignition engines are throttled, so only the right amount of air comes in at a time. This is less efficient, but spark plugs only work near the stoichiometric mixture of fuel and air (the ratio of air to fuel that allows complete and most efficient combustion). Diesel engines accept a cylinder full of air and measure in the right amount of fuel. So each time the intake valve on a diesel opens, a full charge of cold air enters the cylinder. This cools the cylinder back down. The heat gained from each combustion process therefore can only cause a gain in temperature that is much, much smaller than it would be in a spark ignition engine.

Some engines use small electric heaters called glow plugs inside the cylinder to help ignite fuel when starting. Some even use resistive grid heaters in the intake manifold to warm the inlet air until the engine reaches operating temperature. Engine block heaters (electric resistive heaters in the engine block) connected to the utility grid are often used when an engine is turned off for extended periods (more than an hour) in cold weather to reduce startup time and engine wear. In the past, a wider variety of cold-start methods were used. Some engines, such as Detroit Diesel engines and Lister-Petter engines, used a system to introduce small amounts of ether into the inlet manifold to start combustion. Sabb marine engines, Field Marshall tractors (among others) used slow-burning solid-fuel 'cigarettes' which were fitted into the cylinder head as a primitive glow plug. Lucas developed the 'Thermostart', where an electrical heating element was combined with a small fuel valve. Diesel fuel slowly dripped from the valve onto the hot element and ignited. The flame heated the inlet manifold and when the engine was turned over the flame was drawn into the combustion chamber to start combustion. International Harvester developed a WD-40 tractor in the 1930s that had a 7-liter 4-cylinder engine which ran as a diesel, but was started as a gasoline engine. The cylinder head had valves which opened for a portion of the compression stroke to reduce the effective compression ratio, and a magneto produced the spark. An automatic ratchet system automatically disengaged the ignition system and closed the valves once the engine had run for 30 seconds. The operator then switched off the gasoline fuel system and opened the throttle on the diesel injection system.

Such systems fell out of favor when electrical glow plug systems proved to be the simplest to operate and produce. Direct-injection systems advanced to the extent that cold-starting systems were not needed and then electronic fuel injection systems rendered most cold-start systems unnecessary.


Diesel fuel is also prone to "waxing" or "gelling" in cold weather, terms for the solidification of diesel oil into a partially crystalline state. The crystals build up in the fuel (especially in fuel filters), eventually starving the engine of fuel and causing it to stop running. Low-output electric heaters in fuel tanks and around fuel lines are used to solve this problem. Also, most engines have a "spill return" system, by which any excess fuel from the injector pump and injectors is returned to the fuel tank. Once the engine has warmed, returning warm fuel prevents waxing in the tank. Due to improvements in fuel technology, with additives waxing rarely occurs in all but the coldest weather.

Fuel delivery

A vital component of all diesel engines is a mechanical or electronic governor, which limits the speed of the engine by controlling the rate of fuel delivery. Unlike Otto-cycle engines, incoming air is not throttled and a diesel engine without a governor can easily overspeed, resulting in its destruction. Mechanically governed fuel injection systems are driven by the engine's gear train. These systems use a combination of springs and weights to control fuel delivery relative to both load and speed. Modern, electronically controlled diesel engines control fuel delivery and limit the maximum rpm by use of an electronic control module (ECM) or electronic control unit (ECU). The ECM/ECU receives an engine speed signal, as well as other operating parameters such as intake manifold pressure and fuel temperature, from a sensor and controls the amount of fuel and start of injection timing through electric or hydraulic actuators to maximize power and efficiency and minimize emissions.

Controlling the timing of the start of injection of fuel into the cylinder is a key to minimizing emissions, and maximizing fuel economy (efficiency), of the engine. The timing is usually measured in units of crank angle of the piston before top dead center. For example, if the ECM/ECU initiates fuel injection when the piston is 10 degrees before TDC, the start of injection, or timing, is said to be 10° BTDC. Optimal timing will depend on the engine design as well as its speed and load.

Advancing the start of injection (injecting before the piston reaches TDC) results in higher in-cylinder pressure and temperature, and higher efficiency, but also results in elevated engine noise and increased oxides of nitrogen (NOx) emissions due to higher combustion temperatures. On the other hand, delayed start of injection causes incomplete combustion, reduced fuel efficiency and an increase in black exhaust smoke, containing a considerable amount of particulate matter (PM) and unburned hydrocarbons (HC).

Early fuel injection systems

The modern diesel engine is a combination of two inventors' creations. In all major aspects, it holds true to Rudolf Diesel's original design, that of igniting fuel by compression at an extremely high pressure within the cylinder. However, nearly all present-day diesel engines use the so-called solid injection system invented by Herbert Akroyd Stuart for his hot bulb engine (a compression-ignition engine that precedes the diesel engine and operates slightly differently). Solid injection raises the fuel to extreme pressures by mechanical pumps and delivers it to the combustion chamber by pressure-activated injectors in a dense jet. Diesel's original engine injected fuel with the assistance of compressed air, which atomized the fuel and forced it into the engine through a nozzle (a similar principle to an aerosol spray). This is called an air-blast injection. The size of the gas compressor needed to power such a system made early diesel engines very heavy and large for their power outputs, and the need to drive a compressor lowered power output even more. Early marine diesels often had smaller auxiliary engines whose sole purpose was to drive the compressors to supply air to the main engine's injector system. Such a system was too bulky and inefficient to be used for road-going automotive vehicles.

Solid injection systems are lighter, simpler, and allow for much higher speed, and so are universally used for automotive diesel engines. Air-blast systems provide very efficient combustion under low-speed, high-load conditions, especially when running on poor-quality fuels, so some large marine engines use this injection method. Air-blast injection also raises the fuel temperature during the injection process, so is sometimes known as hot-fuel injection. In contrast, solid injection is sometimes called cold-fuel injection.

The vast majority of diesel engines in service today use solid injection and the information below relates to that system. In the diesel engine, only air is introduced into the combustion chamber. The air is then compressed to 40 bar (about 600 psi)) compared to 14 bar (about 200 psi) in the gasoline engine. This high compression heats the air to 550 °C (about 1000 °F). At this moment, fuel is injected directly into the compressed air. The fuel is ignited by the heat, causing a rapid expansion of gases that drive the piston downward, supplying power to the crankshaft. In Diesel's manuals, he described the supply of compressed gas into the cylinder to promote the final burn.

Advantages of the diesel engine are numerous. It burns considerably less fuel than a gasoline engine performing the same work. It has no ignition system to attend to. It can deliver much more of its rated power on a continuous basis than can a gasoline engine. The life of a diesel engine is generally longer than a gasoline engine. Although diesel fuel will burn in open air, it will not explode unless compressed.

Mechanical and electronic injection

Older engines make use of a mechanical fuel pump and valve assembly that is driven by the engine crankshaft, usually from the timing belt or chain. These engines use simple injectors that are basically very precise spring-loaded valves that open and close at a specific fuel pressure. The pump assembly consists of a pump that pressurizes the fuel and a disc-shaped valve that 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, which controls the amount of fuel injected.

This contrasts with the more modern method of having a separate fuel pump which supplies fuel constantly at high pressure to each injector. Each injector has a solenoid, is operated by an electronic control unit, which enables more accurate control of injector opening times that depend 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.

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 push starting a vehicle using the wrong gear.

Indirect injection

An indirect injection diesel engine delivers fuel into a chamber off the combustion chamber, called a prechamber or ante-chamber, where combustion begins and then spreads into the main combustion chamber, assisted by turbulence created in the chamber. This system allows for a smoother, quieter running engine, 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 speeds 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 5%–10%. Indirect injection engines were used 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. In road-going vehicles most prefer the greater efficiency and better controlled emission levels of direct injection.

Direct injection

Modern diesel engines make use of one of the following direct injection methods:

Distributor and Inline pump direct injection

The first incarnations of direct injection diesels 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. The problem with these vehicles was the harsh noise that they made and particulate (smoke) emissions. 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.

This type of engine was transformed by electronic control of the injection pump, pioneered by the Volkswagen Group in 1989. The injection pressure was still only around 300 bar (4350 psi), but the injection timing, fuel quantity, EGR and turbo boost were all electronically controlled. This gave more precise control of these parameters which made refinement more acceptable and emissions lower. The technology trickled down to the mass market with cars being both more economical and powerful than indirect injection competitors.

Unit direct injection

Unit direct injection also 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 a Pumpe-Düse-System—literally "pump-nozzle system") and by Mercedes Benz ("PLD") and most major diesel engine manufacturers in large commercial engines (CAT, Cummins, Detroit Diesel, Volvo). With recent advancements, the pump pressure has been raised to 2,050 bar (205 MPa, 30127 psi), allowing injection parameters similar to common rail systems.

Common rail direct injection

In common rail systems, the distributor injection pump is eliminated. Instead, a high-pressure pump pressurises fuel at up to 2,000 bar (200 MPa, 30000 psi), in a "common rail". The common rail is a tube that branches off to computer-controlled injector valves, each of which contains a precision-machined nozzle and a plunger driven by a solenoid or piezoelectric actuators.



Rudolf Diesel intended his engine to replace the steam engine as the primary power source for industry. As such, diesel engines in the late 19th and early 20th centuries used the same basic layout and form as industrial steam engines, with long-bore cylinders, external valve gear, cross-head bearings and an open crankshaft connected to a large flywheel. Smaller engines would be built with vertical cylinders, while most medium- and large-sized industrial engines were built with horizontal cylinders, just as steam engines had been. Engines could be built with more than one cylinder in both cases. The largest early diesels resembled the triple-expansion reciprocating engine steam engine, being tens of feet high with vertical cylinders arranged in-line. These early engines ran at very slow speeds—partly due to the limitations of their air-blast injector equipment and partly so they would be compatible with the majority of industrial equipment designed for steam engines; maximum speeds of between 100 and 300 rpm were common. Engines were usually started by allowing compressed air into the cylinders to turn the engine, although smaller engines could be started by hand.

In the early decades of the 20th century, when large diesel engines were first being used, the engines took a form similar to the compound steam engines common at the time, with the piston being connected to the connecting rod via a crosshead bearing. Following steam engine practice, double-acting four-stroke diesel engines were constructed to increase power output, with combustion taking place on both sides of the piston, with two sets of valve gear and fuel injection. This system also meant that the engine's direction of rotation could be reversed by altering the injector timing, so the engine could be coupled directly to the propeller without the need for a gearbox. While it produced large amounts of power and was very efficient, the double-acting diesel engine's main problem was producing a good seal where the piston rod passed through the bottom of the lower combustion chamber to the crosshead bearing. By the 1930s it was found easier and more reliable to fit turbochargers to the engines, although crosshead bearings are still used to reduce the stress on the crankshaft bearings, and the wear on the cylinders, in large long-stroke main engines.


As with gasoline engines, there are two classes of diesel engines in current use: two-stroke and four-stroke. The four-stroke type is the "classic" version, tracing its lineage back to Rudolf Diesel's prototype. It is also the most commonly used form, being the preferred power source for many motor vehicles, especially buses and trucks. Much larger engines, such as used for railroad locomotion and marine propulsion, are often two-stroke units, offering a more favorable power-to-weight ratio, as well as better fuel economy. The most powerful engines in the world are two-stroke diesels of mammoth proportions.

Two-stroke diesel operation is similar to that of gasoline counterparts, except that fuel is not mixed with air prior to induction, and the crankcase does not take an active role in the cycle. The traditional two-stroke design relies upon a mechanically driven positive displacement blower to charge the cylinders with air prior to compression and ignition. The charging process also assists in expelling (scavenging) combustion gases remaining from the previous power stroke. The archetype of the modern form of the two stroke Diesel is the Detroit Diesel engine, in which the blower pressurizes a chamber in the engine block that is often referred to as the "air box." The (much larger) Electromotive prime mover utilized in EMD Diesel-electric locomotives is built to the same principle.

In a two-stroke diesel engine, as the cylinder's piston approaches bottom dead center a passage between the air box and the cylinder is opened, permitting air flow into the cylinder. During this time, the exhaust valves are opened and some of the air flow forces the remaining combustion gasses from the cylinder—this is the scavenging process. As the piston passes through bottom center and starts upward, the passage is closed and compression commences, culminating in fuel injection and ignition. Refer to two-stroke Diesel engines for more discussion concerning aspiration issues with a two-stroke engine.

Normally, the number 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-six cylinder design is the most prolific in light to medium-duty engines, though small V8 and larger inline-four displacement engines are also common. Small-capacity engines (generally considered to be those below five litres in capacity) are generally four or six cylinder types, with the four cylinder being the most common type found in automotive uses. Five cylinder diesel engines have also been produced, being a compromise between the smooth running of the six cylinder and the space-efficient dimensions of the four cylinder. Diesel engines for smaller plant machinery, boats, tractors, generators and pumps may be four, three or two cylinder types, with the single cylinder diesel engine remaining for light stationary work.

The desire to improve the diesel engine's power-to-weight ratio produced several novel cylinder arrangements to extract more power from a given capacity. The Napier Deltic engine, with three cylinders arranged in a triangular formation, each containing two opposed-action pistons, the whole engine having three crankshafts, is one of the better known. The Commer van company of the United Kingdom used a similar design for road vehicles, designed by Tillings-Stevens, member of the Rootes Group, the TS3. The Commer TS3 engine had 3 horizontal in-line cylinders, each with two opposed action pistons that worked through rocker arms, to connecting rods and had one crankshaft. While both these designs succeeded in producing greater power for a given capacity, they were complex and expensive to produce and operate, and when turbocharger technology improved in the 1960s, this was found to be a much more reliable and simple way of extracting more power.

As a footnote, prior to 1950, 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 engines

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) (and occasionally nitromethane to improve performance) 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 Diethyl ether, which is highly volatile and has an extremely low flash point, combined with kerosene and a lubricant plus a very small proportion (typically 2%) of ignition improver such as Amyl nitrate or preferably 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, thus increasing endurance for the amount of fuel carried. They also exhibit higher torque, enabling the turning of a larger or higher pitched propeller 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 economy

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 Group engines, has a combined Euro rating of 38 miles per US gallon (6.2 L/100 km) for the 102 bhp (76 kW) petrol engine and 54 mpg (4.4 L/100 km) for the 105 bhp (78 kW) diesel engine. However, such a comparison doesn't take into account that diesel fuel is denser and contains about 15% more energy by volume. Although the calorific value of the fuel is slightly lower at 45.3 MJ/kg (megajoules per kilogram) than gasoline at 45.8 MJ/kg, liquid diesel fuel is significantly denser than liquid gasoline. This is important because volume of fuel, in addition to mass, is an important consideration in mobile applications. No vehicle has an unlimited volume available for fuel storage.

Adjusting the numbers to account for the energy density of diesel fuel, one finds the overall energy efficiency of the aforementioned paragraph is still about 20% greater for the diesel version, despite the weight penalty of the diesel engine.

While higher compression ratio is helpful in raising efficiency, diesel engines are much more efficient than gasoline (petrol) engines when at low power and at engine idle. Unlike the petrol engine, diesels lack a butterfly valve (throttle) in the inlet system, which closes at idle. This creates parasitic loss and destruction of availability on the incoming air, reducing the efficiency of petrol/gasoline engines at idle. In many applications, such as marine, agriculture, and railways, diesels are left idling unattended for many hours or sometimes days. These advantages are especially attractive in locomotives (see dieselisation).

Weight can be an issue, since diesel engines are typically heavier than gasoline engines of similar power output. This is essentially because the diesel must operate at lower engine speeds. Diesel fuel is injected just before the power stroke. As a result of this, the fuel cannot burn completely until it has encountered the right amount of oxygen. This results in incomplete combustion because not all of the fuel molecules can collide with enough oxygen molecules to react. In the gasoline engine, air and fuel are mixed for the entire compression stroke, ensuring complete mixing even at higher engine speeds.

Diesel engines usually have longer stroke lengths to achieve the necessary compression ratios. As a result piston speeds are higher and more force must be transmitted through the connecting rods and crankshaft to change the momentum of the piston. This is another reason that a diesel engine must be stronger for the same power output.

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 service requirements. These are issues with newer, lighter, high performance diesel engines which are not "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, due to the latter's susceptibility to knock, and the higher compression ratio allows a diesel engine to be more efficient than a comparable spark ignition engine. Because the burned gases are expanded further in a diesel engine cylinder, the exhaust gas is cooler, meaning turbochargers require less cooling, and can be more reliable, than on spark-ignition engines.

The increased fuel economy of the diesel engine 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. Although concerns are now being raised as to the negative effect this is having on the world food supply, as the growing of crops specifically for biofuels takes up land that could be used for food crops and uses water that could be used by both humans and animals. The use of waste vegetable oil, sawmill waste from managed forests in Finland funded by Nokia venture capital, and the development of the production of vegetable oil from algae, demonstrate great promise in providing feed stocks for sustainable biodiesel, that are not in competition with food production.

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 is caused by "piston slap", the sudden ignition of the diesel fuel when injected into the combustion chamber slamming the cold-contracted piston into the cylinder wall. The tolerances between the piston and cylinder wall are greater at cold temperatures to allow expansion at higher temperatures. A combination of improved mechanical technology (such as multi-stage injectors which fire a short "pilot charges" of fuel into the cylinder to warm the combustion chamber before delivering the main fuel charge), higher injection pressures that have improved the atomisation of fuel into smaller droplets, and electronic control (which can adjust the timing and length of the injection process to optimise it for all speeds and temperatures), have mostly mitigated these problems in the latest generation of common-rail designs, while greatly improving engine efficiency. Poor power and narrow torque bands have been addressed by the use of superchargers, turbochargers, (especially variable geometry turbochargers), intercoolers, and a large efficiency increase from about 35% for IDI to 45% for the latest engines in the last 15 years.

Even though diesel engines have a theoretical fuel efficiency of 75%, in practice it is less. Large diesel trucks, buses, and newer diesel cars can achieve efficiencies around 45%, however they could reach 55% efficiency in the near future.


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 stoichiometric. However, they can produce black soot (or more specifically diesel particulate matter) from their exhaust, which consists of unburned carbon compounds. This is caused by local low temperatures where the fuel is not fully atomized. These local low temperatures occur at the cylinder walls and at the outside of large droplets of fuel. At these areas where it is relatively cold, the mixture is rich (contrary to the overall mixture which is lean). The rich mixture has less air to burn and some of the fuel turns into a carbon deposit.

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. This is only done in specialized applications (such as tractor pulling competitions) where these disadvantages are of little concern.

Likewise, when starting from cold, the engine's combustion efficiency is reduced because the cold engine block draws heat out of the cylinder in the compression stroke. The result is that fuel is not combusted fully, resulting in blue/white smoke and lower power outputs until the engine has warmed through. This is especially the case with indirect injection engines, which are less thermally efficient. With electronic injection, the timing and length of the injection sequence can be altered to compensate for this. Older engines with mechanical injection can have manual control to alter the timing, or multi-phase electronically-controlled glow plugs, that stay on for a period after start-up to ensure clean combustion—the plugs are automatically switched to a lower power to prevent them burning out.

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.

All diesel engine exhaust emissions can be significantly reduced by the use of biodiesel fuel. Oxides of nitrogen do increase from a vehicle using biodiesel, but they too can be reduced to levels below that of fossil fuel diesel, by changing fuel injection timing.

Power and torque

For commercial uses requiring towing, load carrying and other tractive tasks, diesel engines tend to have better torque characteristics. 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 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 engine, making 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 geometry 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.


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, 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 (400,000 km) or more without a rebuild.

Due to the greater compression force required and the increased weight of the stronger components, starting a diesel engine is harder. 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, called a Coffman starter, which 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 18 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; superchargers improve power at lower speeds; and variable geometry turbochargers improve the engine's performance equally by flattening the torque curve.

Quality and variety of fuels

Petrol/gasoline engines are limited in the variety and quality of the fuels they can burn. Older petrol engines fitted with a carburetor required a volatile fuel that would vaporize easily to create the necessary fuel/air mix for combustion. Because both air and fuel are admitted to the cylinder, if the compression ratio of the engine is too high or the fuel too volatile (with too low an octane rating), the fuel will ignite under compression, as in a diesel engine, before the piston reaches the top of its stroke. This pre-ignition causes a power loss and over time major damage to the piston and cylinder. The need for a fuel that is volatile enough to vaporize but not too volatile (to avoid pre-ignition) means that petrol engines will only run on a narrow range of fuels. There has been some success at dual-fuel engines that use gasoline/ethanol, gasoline/propane, and gasoline/methane.

In diesel engines, a mechanical injector system vaporizes the fuel into a pre-combustion chamber (as opposed to a Venturi jet in a carburetor, or a Fuel injector in a fuel injection system vaporizing fuel into the intake manifold or intake runners as in a petrol engine). This forced vaporisation means that less-volatile fuels can be used. More crucially, because only air is inducted into the cylinder in a diesel engine, the compression ratio can be much higher as there is no risk of pre-ignition provided the injection process is accurately timed. This means that cylinder temperatures are much higher in a diesel engine than a petrol engine, allowing less-combustible fuels to be used.

Diesel fuel is a form of light fuel oil, very similar to kerosene, but diesel engines, especially older or simple designs that lack precision electronic injection systems, can run on a wide variety of other fuels. One of the most common alternatives is vegetable oil from a very wide variety of plants. Some engines can be run on vegetable oil without modification, and most others require fairly basic alterations. Biodiesel is a pure diesel-like fuel refined from vegetable oil and can be used in nearly all diesel engines. The only limits on the fuels used in diesel engines are the ability of the fuel to flow along the fuel lines and the ability of the fuel to lubricate the injector pump and injectors adequately. In general terms, inline mechanical injector pumps tolerate poor-quality or bio-fuels better than distributor-type pumps. Also, indirect injection engines generally run more satisfactorily on bio-fuels than direct injection engines. This is partly because an indirect injection engine has a much greater 'swirl' effect, improving vaporisation and combustion of fuel, and also because (in the case of vegetable oil-type fuels) lipid depositions can condense on the cylinder walls of a direct-injection engine if combustion temperatures are too low (such as starting the engine from cold).

At the request of the French Government the Otto company demonstrated a diesel engine at the 1900 Exposition Universelle (World's Fair) which used peanut oil (see biodiesel). The French government were at the time exploring the possibility of using peanut oil as a locally produced fuel in their African colonies. Diesel himself later tested extensively the use of plant oils in his engine and began to actively promote the use of these fuels.

Most large marine diesels (often called cathedral engines due to their size) run on heavy fuel oil (sometimes called "bunker oil"), which is a thick, viscous and almost un-flammable fuel which is very safe to store and cheap to buy in bulk as it is a waste product from the petroleum refining industry. The fuel must be heated to thin it out (often by the exhaust header) and is often passed through multiple injection stages to vaporize it.

Fuel and fluid characteristics

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. The engines can work with the full spectrum of crude oil distillates, from compressed natural gas, alcohols, gasoline, to the fuel oils from diesel oil to residual fuels. The type of fuel used is a combination of service requirements, and fuel costs. 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. Recently, Biodiesel from coconut, which can produce a very promising coco methyl esther (CME), has characteristics which enhance lubricity and combustion giving a regular diesel engine without any modification more power, less particulate matter or black smoke, and smoother engine performance. The Philippines pioneers in the research on Coconut based CME with the help of German and American scientists. Petroleum-derived diesel is often called petrodiesel if there is need to distinguish the source of the fuel.

Pure plant oils are increasingly being used as a fuel for cars, trucks and remote combined heat and power generation especially in Germany where hundreds of decentralised small- and medium-sized oil presses cold press oilseed, mainly rapeseed, for fuel. There is a Deutsches Institut für Normung fuel standard for rapeseed oil fuel.

Residual fuels are the "dregs" of the distillation 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, but can be viable fuels on non common rail or TDI PD diesels with the simple conversion of fuel heating to 80 to 100 degrees Celsius to reduce viscosity, and adequate filtration to OEM standards. Most converted vehicles start and shut down on standard diesel fuel. 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

Use of the diesel engine is highly dependent on local conditions and the specific application. Applications which require the diesel's reliability and high torque output (such as tractors, trucks, heavy equipment, most buses etc.) are found practically worldwide (obviously these applications also benefit from the diesel's improved fuel economy). Local conditions such as fuel prices play a big part in the acceptance of the diesel engine—for example, in Europe most tractors were diesel-powered by the end of the 1950s, whilst in the U.S. diesel did not dominate the market until the 1970s. Similarly, around 40% of all the cars sold in Europe (where fuel prices are high) are diesel-powered, and are promoted as the low CO2 emission option, while practically no North American private cars have diesel engines, (apart from a small number of VWs and Audis), because of much lower fuel costs and a poor public image.

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-boats known as E-boats (Schnellboot) of the Second World War were also 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 had 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. Other car manufacturers followed, Borgward in 1952, Fiat in 1953 and Peugeot in 1958.

In the United States, diesel is not as popular in passenger cars as in Europe. Such cars have been traditionally perceived as heavier, noisier, having performance characteristics which make them slower to accelerate, sootier, smellier, and of being more expensive than equivalent gasoline vehicles. From the late seventies to the mid-eighties, General Motors' Oldsmobile, Cadillac, and Chevrolet divisions produced a low-powered and unreliable V8 diesel engine which generally serves as the prime example for this reputation. Dodge with its ever-famous Cummins inline-six diesels optioned in pickup trucks (since about the late 1980s) really revitalized the appeal for diesel power in light vehicles among American consumers, but a superior and widely-accepted American regular-production diesel passenger car never materialized. Ford Motor Company tried diesel engines in some passenger cars in the 1980s, but to not much avail. In addition, before the introduction of 15 parts per million ultra-low sulfur diesel, which started at 15 October 2006 in the U.S. (1 June 2006 in Canada), diesel fuel used in North America still had higher sulfur content than the fuel used in Europe, effectively limiting diesel use to industrial vehicles, which had further contributed to the negative image. Ultra-low sulfur diesel is not mandatory until 2010 in the US. This image 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. After the introduction of ultra-low sulfur diesel, Mercedes-Benz has marketed passenger vehicles under the BlueTec banner. In addition, other manufacturers such as Ford, General Motors, Honda, Subaru, Audi, Volkswagen, BMW, and Nissan plan to sell Diesel vehicles in the US in 2008-2010, designed to meet the tougher emissions requirements in 2010. Recently, in early 2008, Honda has stated that they plan to offer their 50 state compliant 2.2 liter i-DTEC diesel engine in the new 2009 Acura TSX for the US market.

In Canada, Smart Fortwo was first introduced in 2004 with a diesel engine, up until 2008.

In Japan, newly registered Diesel vehicles were less than 1% in 2005. Honda and Mercedes-Benz have made plans to offer Diesel vehicles in the future, with Mercedes-Benz having already started selling the Mercedes-Benz E320 CDI in autumn 2006.

European governments tend to favor diesel engines in taxation policy because of diesel's superior fuel efficiency.

In Europe, where tax rates in many countries make diesel fuel much cheaper than gasoline, diesel vehicles are very popular (over half the new cars sold are powered by diesel engines) and newer designs have significantly narrowed differences between petrol and diesel vehicles in the areas mentioned. Often, among comparably designated models, the turbodiesels 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 road car 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 (eMB), 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.

A few civilian motorcycles have been built using adapted stationary diesel engines, but the weight and cost disadvantages generally outweigh the efficiency gains in this application. NATO though, has a single vehicle fuel policy and because of its fuel efficiency and safety advantages on the battlefield, NATO has selected diesel as that fuel. NATO and the United States Marine Corps have been developing a diesel military motorcycle based on a Kawasaki off road motorcycle, with a purpose designed naturally aspirated direct injection diesel at Cranfield University in England, to be produced in the USA, because motorcycles are the last remaining petrol/gasoline powered vehicle in their inventory.

Engine speeds

Within the diesel engine industry, engines are often categorized by their speeds into three unofficial groups:

High-speed engines

High-speed (approximately 1200 rpm and greater) engines are used to power trucks (lorries), buses, tractors, cars, yachts, compressors, pumps and small electrical generators.

Medium-speed engines

Large electrical generators are often driven by medium speed engines, (approximately 300 to 1200 rpm) which are optimised to run at a set synchronous speed depending on the generation frequency (50 or 60 Hertz) and provide a rapid response to load changes. Medium speed engines are also used for ship propulsion, and mechanical drive applications such as large compressors or pumps. The largest medium speed engines produced today (2007) have outputs up to approximately 22,400 kW (30,000)bhp). and are supplied by companies like MAN B&W, Wartsila, and Rolls-Royce (acquired Ulstein Bergen Diesel in 1999). Medium speed engines produced today are primarily four-stroke machines, however there are some two-stroke units still in production.

Typical cylinder bore size for medium speed engines ranges from 20 cm to 50 cm, and engine configurations typically are offered ranging from in-line 4 cylinder units to Vee 20 cylinder units.

It should be noted that most liquid fueled medium speed engines operate on either diesel fuel or heavy fuel oil, in the same manner noted below for low speed engines.

It should also be noted that most major manufacturers of medium speed engines make natural gas fueled versions of their diesel cycle engines, which in fact operate on the Otto cycle, and require spark ignition, typically provided with a spark plug.

Low-speed engines

Also known as "slow-speed", the largest diesel engines are primarily used to power ships, although there are a few land-based power generation units as well. These extremely large two-stroke engines have power outputs up to approximately 85 MW, operate in the range from approximately 60 to 200 rpm and are up to 15 m tall, and can weigh over 2000 tons. They typically run on cheap low-grade "heavy fuel", also known as "Bunker C" fuel, which requires heating in the ship for tanking and before injection due to the fuel's high viscosity. The heat for fuel heating is often provided by waste heat recovery boilers located in the exhaust ducting of the engine, which produce the steam required for fuel heating.

Companies such as MAN B&W Diesel, (formerly Burmeister & Wain) and Wärtsilä (which acquired Sulzer Diesel) design such large low speed engines. They are unusually narrow and tall due to the addition of a crosshead bearing. Today (2007), the 14 cylinder Wärtsilä-Sulzer 14RTFLEX96-C turbocharged two-stroke diesel engine built by Wärtsilä licensee Doosan in Korea is the most powerful diesel engine put into service, with a cylinder bore of 960 mm delivering 84.42 MW (114,800 bhp). It was put into service in September 2006, aboard the world's largest container ship Emma Maersk which belongs to the A.P. Moller-Maersk Group.

Typical bore size for low speed engines ranges from approximately 35 cm to 98 cm. So far (2008), all currently produced low speed engines are in-line configurations; no Vee versions are produced.

Unusual applications


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.

Diesel engines were first tried in aircraft in the 1930s. A number of manufacturers built engines, the best known probably being the Packard air-cooled radial, and 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. 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 either jet fuel or conventional automotive diesel. To gain the high power-to-weight ratio needed for an aero engine, these new "aero-diesels" are usually two-strokes and some, like the British " Dair" engine, use opposed-action pistons to gain further power.

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 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 of weight and low peak rpm, specifications central to performance in this sport. However, in 2006, the new Audi R10 TDI LMP1 entered by Joest Racing became the first diesel-engined car to win the 24 Hours of Le Mans.


As early as 1931, Clessie Cummins installed his diesel in the Cummins "Diesel Special" race car, hitting at Daytona and at the Indianapolis 500 race, where Dave Evans became the first driver to complete the Indianapolis 500 without making a single pit stop, completing the full distance on the lead lap and finishing 13th, relying on torque and fuel efficiency to overcome weight and low peak power.

In 1933, a 1925 Bentley with a Gardner 4LW engine was the first diesel-engine 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, Fred Agabashian in a Cummins diesel won the pole at the Indianapolis 500 race with a turbocharged 6.6 liter diesel car, setting a record for pole position lap speed, 222.108 km/h or 138.010 mph. Don Cummins and his chief engineer Neve Reiners recognized that the low center of gravity of the flat engine configuration (designed to lie beneath the floor of a bus) plus the power advantage gained by the novel use of Elliott turbocharging would be a winning combination.

At the start, a slow pace lap (reportedly less than 80 mph) apparently induced what is now referred to as "turbo lag" and badly hampered the throttle response of the Cummins Diesel. Although Agabashian found himself in eighth place before reaching the first turn, he moved up to fifth in a few laps and was running competitively (albeit well back in the field after a tire change) until the badly situated air intake of the car swallowed enough debris from the track to disable the turbocharger at lap 71; he finished 27th.


When turbocharged diesel technology made progress in the 1990s and rule makers supported the concept, BMW and Volkswagen raced diesel touring cars, with BMW winning the 1998 24 Hours Nürburgring with a 320d against other factory-entered diesel competition of VW and about 200 normally powered cars, mainly by being able to drive very long stints. Alfa Romeo even organized a racing series with their Alfa Romeo 147 1.9 JTD models.

In 2006, a BMW 120d repeated a similar result, scoring 5th in a field of 220 cars, many of them much more powerful, a significantly stronger competition than in 1998. The VW Dakar Rally race Touareg 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 TDI in 2006, which is powered by a 650 hp (485 kW) and 1100 N·m (810 lbf·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 transmissions that could stand the torque long enough.

After winning the 12 Hours of Sebring in 2006 with their diesel-powered R10 TDI, Audi obtained the overall win at the 2006 24 Hours of Le Mans, too. This is the first time a sports car could 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 alternative fuels such as diesel.

Audi again triumphed at Sebring in 2007. It had both a speed and fuel economy advantage over the entire field including the Porsche RS Spyders, gasoline powered purpose-built race cars. Audi's diesels won again the 2007 24 Hours of Le Mans, against competition coming from the Peugeot 908 diesel powered racer.

In 2006, the JCB Dieselmax broke the diesel land speed record posting an average speed of over 328 mph. The vehicle used "two diesel engines that have a combined total of 1,500 horsepower (1120 kilowatts). Each is a 4-cylinder, 4.4-liter engine used commercially in a backhoe loader."

In the 2008 BTCC (British Touring car Championship), Jason Plato and Darren Turner are racing factory sponsored SEAT Leon TDI with some success against a variety of gasoline powered competitors.


With a traditionally poor power-to-weight ratio, diesel engines are generally unsuited to use in a motorcycle, which requires high power, low weight and rapid acceleration. 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 cc and produces 21 kW (28 bhp) with a top speed of 85 mph (136 km/h). Hayes Diversified Technologies mooted, but has subsequently delayed, the delivery of a civilian version for approximately USD$19,000.

In 2005 the United States Marine Corps adopted the M1030M1, an off-road motorcycle based on the Kawasaki KLR650, and modified it with an engine designed to run on diesel or JP8 jet fuel. Since other U.S. tactical vehicles like the HMMWV utility vehicle and M1 Abrams tank use JP8, adopting a scout motorcycle which runs on the same fuels would ease logistics.

In India, motorcycles built by Royal Enfield could be bought with 325 cc single-cylinder diesel engines. Due to the fact that diesel was much cheaper than petrol at the time, and of more reliable quality. These engines were noisy and unrefined, and not very popular because of low performance and weight penalties and also the unique kick-starting techniques. The diesel engine was designed to be used for other commercial applications like Gen-sets, water pump, etc.,

Current and future developments

Already, many common rail and unit injection systems employ new injectors using stacked piezoelectric wafers in lieu of a solenoid, giving 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.

Accelerometer pilot control (APC) uses 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 is 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.

Diesel car history

The first production diesel cars were the Mercedes-Benz 260D and the Hanomag Rekord, both introduced in 1936. The Citroën Rosalie was also produced between 1935 and 1937 with an extremely rare diesel engine option (the 1766 cc 11UD engine) only in the Familiale (estate or station wagon) version.

Following the 1970s oil crisis, Volkswagen introduced the first compact diesel the VW Golf with a 1.5L naturally aspirated IDI engine. this was a redesign (dieselised) version of a petrol engine. Mercedes-Benz tested turbodiesels in cars tested (e. g. by the Mercedes-Benz C111 experimental and record-setting vehicles). The first production turbo diesel car was, in 1978, the 3.0 5-cylinder 115 hp (86 kW) Mercedes 300 SD, available only in North America. In Europe, the Peugeot 604 with a 2.3 litre turbo diesel was introduced in 1979, and then the Mercedes 300 TD turbo.

The biggest single step forward for mass-market diesel cars came in 1982 when Peugeot introduced the XUD engine in the Peugeot 305, Peugeot 205 and Talbot Horizon. This was the class leading automotive diesel engine until the mid 1990s. The first mass market turbo diesel was the XUD powered, 1988 Citroen BX and then the 1989 Peugeot 405, they gave power and refinement approaching petrol engine standards, with the best chassis in their class. These were the cars that started the diesel boom in Europe that has now hit 40% of the market in new car sales.

Many Audi enthusiasts claim that the Audi 100 TDI was the first turbo charged direct injection diesel sold in 1989, but actually it isn't true, as the Fiat Croma TD-i.d. was sold with turbo direct injection in 1986 and two years later Austin Rover Montego. What was pioneering about the Audi 100, however, was the use of electronic control of the engine, as the Fiat and Austin had purely mechanically controlled injection. The electronic control of direct injection really made a difference in terms of emissions, refinement and power. All earlier generation car direct injection diesel engines benefit greatly form the use of biodiesel fuel, which reduces emissions and greatly improves refinement without engine modifications, provided they use compatible 'Viton' type rubber in their fuel systems.

The diesel car markets are the same ones who pioneered various developments (Mercedes-Benz, BMW, Peugeot/Citroën, Fiat, Alfa Romeo, Volkswagen Group), with the exception of Austin Rover, although Austin Rover's ancestor, the Rover Company had been building small-capacity diesel engines since 1956, when it introduced a 2051 cc 4-cylinder diesel engine for its Land Rover 4 × 4. In fact, the 1988 Austin-Rover MDi unit (also known as the 'Perkins Prima') was developed by Perkins Engines of Peterborough, who have designed and built high-speed diesels since the 1930s. It is still in production as a marine engine.

In 1997 first common rail diesel passenger car was introduced, the Alfa Romeo 156.

In 1998, for the very first time in the history of racing, in the legendary 24 Hours Nürburgring race, a diesel-powered car was the overall winner: the BMW works team 320d, a BMW E36 fitted with modern high-pressure diesel injection technology from Robert Bosch GmbH. The low fuel consumption and long range, allowing 4 hours of racing at once, made it a winner, as comparable petrol-powered cars spent more time refueling.

In Spring 2005, Mercedes-Benz unveiled the first application of a mass-produced aluminum block diesel engine for passenger vehicles and commercial use. While aluminum is traditionally considered of inferior strength and temperature resistance to withstand diesel applications, Mercedes engineers made extensive use of CAD/CAM design to arrive at an aluminum block that would meet with Mercedes' rigorous testing and reliability standards. First use was in 2006 model-year vehicles in the E-Class sedan and ML-class and GL-class SUVs. Similar in weight to the five-cylinder it replaced, and considerably lighter than the in-line six cylinder it also replaced, this 3.0L V-6 produces 165 kW (224 hp) at 3,800 rpm and max torque of 510 Nm (376 ft·lbf) at 1,600-2,800 rpm and makes use of a four-valve head. Additionally, fitment of Mercedes-Benz BlueTec system, a concert of emissions control strategies, renders this new diesel 50-state legal in the U.S. beginning in 2008 (stringent NOx limits have made U.S. passenger-car diesels unpopular or impossible in parts of the U.S. in recent years). In 2006, the new Audi R10 TDI LMP1 entered by Joest Racing became the first diesel-engined car to win the 24 Hours of Le Mans. The winning car also bettered the post-1990 course configuration lap record by 1, at 380 laps. However, this fell short of the all-time distance record set in 1971 by over .

The Subaru car company of Japan is preparing to sell its station wagon version of their Legacy mid-size car (called the Subaru Outback in North America) with the world's first 2.0 liter, boxer engine format opposed-four cylinder diesel engine of 110 kW (147 hp) power, and 350 Nm (258 ft-lb) of torque, in the United Kingdom, with sales in continental Europe planned for 2009, and in the United States by 2010.

Today the cars that produce the lowest CO2/km in emissions, lower than hybrids, and much lower than very inefficient electric cars (when generation and grid transmission losses are taken into account), are the most advanced compact European diesel cars.

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