The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is called a compression-ignition engine (CI engine). This contrasts with engines using spark plug-ignition of the air-fuel mixture, such as a petrol engine (gasoline engine) or a gas engine (using a gaseous fuel like natural gas or liquefied petroleum gas).

Introduction

Diesel engines work by compressing only air, or air plus residual combustion gases from the exhaust (known as exhaust gas recirculation, "EGR"). Air is inducted into the chamber during the intake stroke, and compressed during the compression stroke. This increases the air temperature inside the cylinder so that atomised diesel fuel injected into the combustion chamber ignites. With the fuel being injected into the air just before combustion, the dispersion of the fuel is uneven; this is called a heterogeneous air-fuel mixture. The torque a diesel engine produces is controlled by manipulating the air-fuel ratio (λ); instead of throttling the intake air, the diesel engine relies on altering the amount of fuel that is injected, and the air-fuel ratio is usually high.

The diesel engine has the highest thermal efficiency (engine efficiency) of any practical internal or external combustion engine due to its very high expansion ratio and inherent lean burn which enables heat dissipation by the excess air. A small efficiency loss is also avoided compared with non-direct-injection gasoline engines since unburned fuel is not present during valve overlap and therefore no fuel goes directly from the intake/injection to the exhaust. Low-speed diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) can reach effective efficiencies of up to 55%.^[1] The combined cycle gas turbine (Brayton and Rankine cycle) is a combustion engine that is more efficient than a diesel engine, but it is, due to its mass and dimensions, unsuited for vehicles, watercraft, or aircraft. The world's largest diesel engines put in service are 14-cylinder, two-stroke marine diesel engines; they produce a peak power of almost 100 MW each.^[2]

Diesel engines may be designed with either two-stroke or four-stroke combustion cycles. They were originally used as a more efficient replacement for stationary steam engines. Since the 1910s, they have been used in submarines and ships. Use in locomotives, buses, trucks, heavy equipment, agricultural equipment and electricity generation plants followed later. In the 1930s, they slowly began to be used in a few automobiles. Since the 1970s energy crisis, demand for higher fuel efficiency has resulted in most major automakers, at some point, offering diesel-powered models, even in very small cars.^[3][4] According to Konrad Reif (2012), the EU average for diesel cars at the time accounted for half of newly registered cars.^[5] However, air pollution emissions are harder to control in diesel engines than in gasoline engines, so the use of diesel auto engines in the U.S. is now largely relegated to larger on-road and off-road vehicles.^[6][7]

Though aviation has traditionally avoided diesel engines, aircraft diesel engines have become increasingly available in the 21st century. Since the late 1990s, for various reasons – including the diesel's normal advantages over gasoline engines, but also for recent issues peculiar to aviation – development and production of diesel engines for aircraft has surged, with over 5,000 such engines delivered worldwide between 2002 and 2018, particularly for light airplanes and unmanned aerial vehicles.^[8][9]

History

Diesel's idea

In 1878, Rudolf Diesel, who was a student at the "Polytechnikum" in Munich, attended the lectures of Carl von Linde. Linde explained that steam engines are capable of converting just 6–10% of the heat energy into work, but that the Carnot cycle allows conversion of much more of the heat energy into work by means of isothermal change in condition. According to Diesel, this ignited the idea of creating a highly efficient engine that could work on the Carnot cycle.^[14] Diesel was also introduced to a fire piston, a traditional fire starter using rapid adiabatic compression principles which Linde had acquired from Southeast Asia.^[15] After several years of working on his ideas, Diesel published them in 1893 in the essay Theory and Construction of a Rational Heat Motor.^[14]

Diesel was heavily criticised for his essay, but only a few found the mistake that he made;^[16] his rational heat motor was supposed to utilise a constant temperature cycle (with isothermal compression) that would require a much higher level of compression than that needed for compression ignition. Diesel's idea was to compress the air so tightly that the temperature of the air would exceed that of combustion. However, such an engine could never perform any usable work.^[17][18][19] In his 1892 US patent (granted in 1895) #542846, Diesel describes the compression required for his cycle:

pure atmospheric air is compressed, according to curve 1 2, to such a degree that, before ignition or combustion takes place, the highest pressure of the diagram and the highest temperature are obtained-that is to say, the temperature at which the subsequent combustion has to take place, not the burning or igniting point. To make this more clear, let it be assumed that the subsequent combustion shall take place at a temperature of 700°. Then in that case the initial pressure must be sixty-four atmospheres, or for 800° centigrade the pressure must be ninety atmospheres, and so on. Into the air thus compressed is then gradually introduced from the exterior finely divided fuel, which ignites on introduction, since the air is at a temperature far above the igniting-point of the fuel. The characteristic features of the cycle according to my present invention are therefore, increase of pressure and temperature up to the maximum, not by combustion, but prior to combustion by mechanical compression of air, and there upon the subsequent performance of work without increase of pressure and temperature by gradual combustion during a prescribed part of the stroke determined by the cut-oil.^[20]

By June 1893, Diesel had realised his original cycle would not work and he adopted the constant pressure cycle.^[21] Diesel describes the cycle in his 1895 patent application. Notice that there is no longer a mention of compression temperatures exceeding the temperature of combustion. Now it is simply stated that the compression must be sufficient to trigger ignition.

1. In an internal-combustion engine, the combination of a cylinder and piston constructed and arranged to compress air to a degree producing a temperature above the igniting-point of the fuel, a supply for compressed air or gas; a fuel-supply; a distributing-valve for fuel, a passage from the air supply to the cylinder in communication with the fuel-distributing valve, an inlet to the cylinder in communication with the air-supply and with the fuel-valve, and a cut-oil, substantially as described.^[22][23][24]

In 1892, Diesel received patents in Germany, Switzerland, the United Kingdom and the United States for "Method of and Apparatus for Converting Heat into Work".^[25] In 1894 and 1895, he filed patents and addenda in various countries for his engine; the first patents were issued in Spain (No. 16,654),^[26] France (No. 243,531) and Belgium (No. 113,139) in December 1894, and in Germany (No. 86,633) in 1895 and the United States (No. 608,845) in 1898.^[27]

Diesel was attacked and criticised over a time period of several years. Critics claimed that Diesel never invented a new motor and that the invention of the diesel engine is fraud. Otto Köhler and Emil Capitaine [de] were two of the most prominent critics of Diesel's time.^[28] Köhler had published an essay in 1887, in which he describes an engine similar to the engine Diesel describes in his 1893 essay. Köhler figured that such an engine could not perform any work.^[19][29] Emil Capitaine had built a petroleum engine with glow-tube ignition in the early 1890s;^[30] he claimed against his own better judgement that his glow-tube ignition engine worked the same way Diesel's engine did. His claims were unfounded and he lost a patent lawsuit against Diesel.^[31] Other engines, such as the Akroyd engine and the Brayton engine, also use an operating cycle that is different from the diesel engine cycle.^[29][32] Friedrich Sass says that the diesel engine is Diesel's "very own work" and that any "Diesel myth" is "falsification of history".^[33]

The first diesel engine

Diesel sought out firms and factories that would build his engine. With the help of Moritz Schröter and Max Gutermuth [de],^[34] he succeeded in convincing both Krupp in Essen and the Maschinenfabrik Augsburg.^[35] Contracts were signed in April 1893,^[36] and in early summer 1893, Diesel's first prototype engine was built in Augsburg. On 10 August 1893, the first ignition took place, the fuel used was petrol. In winter 1893/1894, Diesel redesigned the existing engine, and by 18 January 1894, his mechanics had converted it into the second prototype.^[37] During January that year, an air-blast injection system was added to the engine's cylinder head and tested.^[38] Friedrich Sass argues that, it can be presumed that Diesel copied the concept of air-blast injection from George B. Brayton,^[32] albeit that Diesel substantially improved the system.^[39] On 17 February 1894, the redesigned engine ran for 88 revolutions – one minute;^[10] with this news, Maschinenfabrik Augsburg's stock rose by 30%, indicative of the tremendous anticipated demands for a more efficient engine.^[40] On 26 June 1895, the engine achieved an effective efficiency of 16.6% and had a fuel consumption of 519 g·kW⁻¹·h⁻¹. ^[41] However, despite proving the concept, the engine caused problems,^[42] and Diesel could not achieve any substantial progress.^[43] Therefore, Krupp considered rescinding the contract they had made with Diesel.^[44] Diesel was forced to improve the design of his engine and rushed to construct a third prototype engine. Between 8 November and 20 December 1895, the second prototype had successfully covered over 111 hours on the test bench. In the January 1896 report, this was considered a success.^[45]

In February 1896, Diesel considered supercharging the third prototype.^[46] Imanuel Lauster, who was ordered to draw the third prototype "Motor 250/400", had finished the drawings by 30 April 1896. During summer that year the engine was built, it was completed on 6 October 1896.^[47] Tests were conducted until early 1897.^[48] First public tests began on 1 February 1897.^[49] Moritz Schröter's test on 17 February 1897 was the main test of Diesel's engine. The engine was rated 13.1 kW with a specific fuel consumption of 324 g·kW⁻¹·h⁻¹,^[50] resulting in an effective efficiency of 26.2%.^[51][52] By 1898, Diesel had become a millionaire.^[53]

Timeline

1890s

• 1893: Rudolf Diesel's essay titled Theory and Construction of a Rational Heat Motor appears.^[54][55]
• 1893: February 21, Diesel and the Maschinenfabrik Augsburg sign a contract that allows Diesel to build a prototype engine.^[56]
• 1893: February 23, Diesel obtains a patent (RP 67207) titled "Arbeitsverfahren und Ausführungsart für Verbrennungsmaschinen" (Working Methods and Techniques for Internal Combustion Engines).
• 1893: April 10, Diesel and Krupp sign a contract that allows Diesel to build a prototype engine.^[56]
• 1893: April 24, both Krupp and the Maschinenfabrik Augsburg decide to collaborate and build just a single prototype in Augsburg.^[56][36]
• 1893: July, the first prototype is completed.^[57]
• 1893: August 10, Diesel injects fuel (petrol) for the first time, resulting in combustion, destroying the indicator.^[58]
• 1893: November 30, Diesel applies for a patent (RP 82168) for a modified combustion process. He obtains it on 12 July 1895.^[59][60][61]
• 1894: January 18, after the first prototype was modified to become the second prototype, testing with the second prototype begins.^[37]
• 1894: February 17, The second prototype runs for the first time.^[10]
• 1895: March 30, Diesel applies for a patent (RP 86633) for a starting process with compressed air.^[62]
• 1895: June 26, the second prototype passes brake testing for the first time.^[41]
• 1895: Diesel applies for a second patent US Patent # 608845^[63]
• 1895: November 8 – December 20, a series of tests with the second prototype is conducted. In total, 111 operating hours are recorded.^[45]
• 1896: April 30, Imanuel Lauster completes the third and final prototype's drawings.^[47]
• 1896: October 6, the third and final prototype engine is completed.^[11]
• 1897: February 1, Diesel's prototype engine is running and finally ready for efficiency testing and production.^[49]
• 1897: October 9, Adolphus Busch licenses rights to the diesel engine for the US and Canada.^[53][64]
• 1897: 29 October, Rudolf Diesel obtains a patent (DRP 95680) on supercharging the diesel engine.^[46]
• 1898: February 1, the Diesel Motoren-Fabrik Actien-Gesellschaft is registered.^[65]
• 1898: March, the first commercial diesel engine, rated 2×30 PS (2×22 kW), is installed in the Kempten plant of the Vereinigte Zündholzfabriken A.G.^[66][67]
• 1898: September 17, the Allgemeine Gesellschaft für Dieselmotoren A.-G. is founded.^[68]
• 1899: The first two-stroke diesel engine, invented by Hugo Güldner, is built.^[52]

1900s

• 1901: Imanuel Lauster designs the first trunk piston diesel engine (DM 70).^[69]
• 1901: By 1901, MAN had produced 77 diesel engine cylinders for commercial use.^[70]
• 1903: Two first diesel-powered ships are launched, both for river and canal operations: The Vandal naphtha tanker and the Sarmat.^[71]
• 1904: The French launch the first diesel submarine, the Aigrette.^[72]
• 1905: January 14: Diesel applies for a patent on unit injection (L20510I/46a).^[73]
• 1905: The first diesel engine turbochargers and intercoolers are manufactured by Büchi.^[74]
• 1906: The Diesel Motoren-Fabrik Actien-Gesellschaft is dissolved.^[28]
• 1908: Diesel's patents expire.^[75]
• 1908: The first lorry (truck) with a diesel engine appears.^[76]
• 1909: March 14, Prosper L'Orange applies for a patent on precombustion chamber injection.^[77] He later builds the first diesel engine with this system.^[78][79]

1910s

• 1910: MAN starts making two-stroke diesel engines.^[80]
• 1910: November 26, James McKechnie applies for a patent on unit injection.^[81] Unlike Diesel, he successfully built working unit injectors.^[73][82]
• 1911: November 27, the Allgemeine Gesellschaft für Dieselmotoren A.-G. is dissolved.^[65]
• 1911: The Germania shipyard in Kiel builds 850 PS (625 kW) diesel engines for German submarines. These engines are installed in 1914.^[83]
• 1912: MAN builds the first double-acting piston two-stroke diesel engine.^[84]
• 1912: The first locomotive with a diesel engine is used on the Swiss Winterthur–Romanshorn railway.^[85]
• 1912: MS Selandia is the first ocean-going ship with diesel engines.^[86]
• 1913: NELSECO diesels are installed on commercial ships and US Navy submarines.^[87]
• 1913: September 29, Rudolf Diesel dies mysteriously while crossing the English Channel on SS Dresden.^[88]
• 1914: MAN builds 900 PS (662 kW) two-stroke engines for Dutch submarines.^[89]
• 1919: Prosper L'Orange obtains a patent on a precombustion chamber insert incorporating a needle injection nozzle.^[90][91][79] First diesel engine from Cummins.^[92][93]

1920s

• 1923: At the Königsberg DLG exhibition, the first agricultural tractor with a diesel engine, the prototype Benz-Sendling S6, is presented.^{[94][better source needed]}
• 1923: December 15, the first lorry with a direct-injected diesel engine is tested by MAN. The same year, Benz builds a lorry with a pre-combustion chamber injected diesel engine.^[95]
• 1923: The first two-stroke diesel engine with counterflow scavenging appears.^[96]
• 1924: Fairbanks-Morse introduces the two-stroke Y-VA (later renamed to Model 32).^[97]
• 1925: Sendling starts mass-producing a diesel-powered agricultural tractor.^[98]
• 1927: Bosch introduces the first inline injection pump for motor vehicle diesel engines.^[99]
• 1929: The first passenger car with a diesel engine appears. Its engine is an Otto engine modified to use the diesel principle and Bosch's injection pump. Several other diesel car prototypes follow.^[100]

1930s

• 1933: Junkers Motorenwerke in Germany start production of the most successful mass-produced aviation diesel engine of all time, the Jumo 205. By the outbreak of World War II, over 900 examples are produced. Its rated take-off power is 645 kW.^[101]
• 1933: General Motors uses its new roots-blown, unit-injected two-stroke Winton 201A diesel engine to power its automotive assembly exhibit at the Chicago World's Fair (A Century of Progress).^[102] The engine is offered in several versions ranging from 600–900 hp (447–671 kW).^[103]
• 1934: The Budd Company builds the first diesel–electric passenger train in the US, the Pioneer Zephyr 9900, using a Winton engine.^[102]
• 1935: The Citroën Rosalie is fitted with an early swirl chamber injected diesel engine for testing purposes.^[104] Daimler-Benz starts manufacturing the Mercedes-Benz OM 138, the first mass-produced diesel engine for passenger cars, and one of the few marketable passenger car diesel engines of its time. It is rated 45 PS (33 kW).^[105]
• 1936: March 4, the airship LZ 129 Hindenburg, the biggest aircraft ever made, takes off for the first time. It is powered by four V16 Daimler-Benz LOF 6 diesel engines, rated 1,200 PS (883 kW) each.^[106]
• 1936: Manufacture of the first mass-produced passenger car with a diesel engine (Mercedes-Benz 260 D) begins.^[100]
• 1937: Konstantin Fyodorovich Chelpan develops the V-2 diesel engine, later used in the Soviet T-34 tanks, widely regarded as the best tank chassis of World War II.^[107]
• 1938: General Motors forms the GM Diesel Division, later to become Detroit Diesel, and introduces the Series 71 inline high-speed medium-horsepower two-stroke engine, suitable for road vehicles and marine use.^[108]

1940s

• 1946: Clessie Cummins obtains a patent on a fuel feeding and injection apparatus for oil-burning engines that incorporates separate components for generating injection pressure and injection timing.^[109]
• 1946: Klöckner-Humboldt-Deutz (KHD) introduces an air-cooled mass-production diesel engine to the market.^[110]

1950s

• 1950s: KHD becomes the air-cooled diesel engine global market leader.^[111]
• 1951: J. Siegfried Meurer obtains a patent on the M-System, a design that incorporates a central sphere combustion chamber in the piston (DBP 865683).^[112]
• 1953: First mass-produced swirl chamber injected passenger car diesel engine (Borgward/Fiat).^[81]
• 1954: Daimler-Benz introduces the Mercedes-Benz OM 312 A, a 4.6 litre straight-6 series-production industrial diesel engine with a turbocharger, rated 115 PS (85 kW). It proves to be unreliable.^[113]
• 1954: Volvo produces a small batch series of 200 units of a turbocharged version of the TD 96 engine. This 9.6 litre engine is rated 136 kW (185 PS).^[114]
• 1955: Turbocharging for MAN two-stroke marine diesel engines becomes standard.^[96]
• 1959: The Peugeot 403 becomes the first mass-produced passenger sedan/saloon manufactured outside West Germany to be offered with a diesel engine option.^[115]

1960s

• 1964: Summer, Daimler-Benz switches from precombustion chamber injection to helix-controlled direct injection.^[117][112]
• 1962–65: A diesel compression braking system, eventually to be manufactured by the Jacobs Manufacturing Company and nicknamed the "Jake Brake", is invented and patented by Clessie Cummins.^[118]

1970s

• 1972: KHD introduces the AD-System, Allstoff-Direkteinspritzung, (anyfuel direct-injection), for its diesel engines. AD-diesels can operate on virtually any kind of liquid fuel, but they are fitted with an auxiliary spark plug that fires if the ignition quality of the fuel is too low.^[119]
• 1976: Development of the common rail injection begins at the ETH Zürich.^[120]
• 1976: The Volkswagen Golf becomes the first compact passenger sedan/saloon to be offered with a diesel engine option.^[121][122]
• 1978: Daimler-Benz produces the first passenger car diesel engine with a turbocharger (Mercedes-Benz OM617 engine).^[123]
• 1979: First prototype of a low-speed two-stroke crosshead engine with common rail injection.^[124]

1980s

• 1981/82: Uniflow scavenging for two-stroke marine diesel engines becomes standard.^[125]
• 1982: August, Toyota introduces a microprocessor-controlled engine control unit (ECU) for Diesel engines to the Japanese market.^[126]
• 1985: December, road testing of a common rail injection system for lorries using a modified 6VD 12,5/12 GRF-E engine in an IFA W50 takes place.^[127]
• 1987: Daimler-Benz introduces the electronically controlled injection pump for lorry diesel engines.^[81]
• 1988: The Fiat Croma becomes the first mass-produced passenger car in the world to have a direct injected diesel engine.^[81]
• 1989: The Audi 100 is the first passenger car in the world with a turbocharged, intercooled, direct-injected, and electronically controlled diesel engine.^[81] It has a BMEP of 1.35 MPa and a BSFC of 198 g/(kW·h).^[128]

1990s

• 1992: 1 July, the Euro 1 emission standard comes into effect.^[129]
• 1993: First passenger car diesel engine with four valves per cylinder, the Mercedes-Benz OM 604.^[123]
• 1994: Unit injector system by Bosch for lorry diesel engines.^[130]
• 1996: First diesel engine with direct injection and four valves per cylinder, used in the Opel Vectra.^[131][81]
• 1996: First radial piston distributor injection pump by Bosch.^[130]
• 1997: First mass-produced common rail diesel engine for a passenger car, the Fiat 1.9 JTD.^[81][123]
• 1998: BMW wins the 24 Hours Nürburgring race with a modified BMW E36. The car, called 320d, is powered by a 2-litre, straight-four diesel engine with direct injection and a helix-controlled distributor injection pump (Bosch VP 44), producing 180 kW (240 hp). The fuel consumption is 23 L/100 km, only half the fuel consumption of a similar Otto-powered car.^[132]
• 1998: Volkswagen introduces the VW EA188 Pumpe-Düse engine (1.9 TDI), with Bosch-developed electronically controlled unit injectors.^[123]
• 1999: Daimler-Chrysler presents the first common rail three-cylinder diesel engine used in a passenger car (the Smart City Coupé).^[81]

2000s

• 2000: Peugeot introduces the diesel particulate filter for passenger cars.^[81][123]
• 2002: Piezoelectric injector technology by Siemens.^[133]
• 2003: Piezoelectric injector technology by Bosch,^[134] and Delphi.^[135]
• 2004: BMW introduces dual-stage turbocharging with the BMW M57 engine.^[123]
• 2006: The world's most powerful diesel engine, the Wärtsilä-Sulzer RTA96-C, is produced. It is rated 80,080 kW.^[136]
• 2006: Audi R10 TDI, equipped with a 5.5-litre V12-TDI engine, rated 476 kW (638 hp), wins the 2006 24 Hours of Le Mans.^[81]
• 2006: Daimler-Chrysler launches the first series-production passenger car engine with selective catalytic reduction exhaust gas treatment, the Mercedes-Benz OM 642. It is fully complying with the Tier2Bin8 emission standard.^[123]
• 2008: Volkswagen introduces the LNT catalyst for passenger car diesel engines with the VW 2.0 TDI engine.^[123]
• 2008: Volkswagen starts series production of the biggest passenger car diesel engine, the Audi 6-litre V12 TDI.^[123]
• 2008: Subaru introduces the first horizontally opposed diesel engine to be fitted to a passenger car. It is a 2-litre common rail engine, rated 110 kW.^[137]

2010s

• 2010: Mitsubishi developed and started mass production of its 4N13 1.8 L DOHC I4, the world's first passenger car diesel engine that features a variable valve timing system.^[138]
• 2012: BMW introduces dual-stage turbocharging with three turbochargers for the BMW N57 engine.^[123]
• 2015: Common rail systems working with pressures of 2,500 bar launched.^[81]
• 2015: In the Volkswagen emissions scandal, the US EPA issued a notice of violation of the Clean Air Act to Volkswagen Group after it was found that Volkswagen had intentionally programmed turbocharged direct injection (TDI) diesel engines to activate certain emissions controls only during laboratory emissions testing.^{[139][140][141][142]}

Operating principle

Overview

The characteristics of a diesel engine are^[143]

• Use of compression ignition, instead of an ignition apparatus such as a spark plug.
• Internal mixture formation. In diesel engines, the mixture of air and fuel is only formed inside the combustion chamber.
• Quality torque control. The amount of torque a diesel engine produces is not controlled by throttling the intake air (unlike a traditional spark-ignition petrol engine, where the airflow is reduced in order to regulate the torque output), instead, the volume of air entering the engine is maximised at all times, and the torque output is regulated solely by controlling the amount of injected fuel.
• High air-fuel ratio. Diesel engines run at global air-fuel ratios significantly leaner than the stoichiometric ratio.
• Diffusion flame: At combustion, oxygen first has to diffuse into the flame, rather than having oxygen and fuel already mixed before combustion, which would result in a premixed flame.
• Heterogeneous air-fuel mixture: In diesel engines, there is no even dispersion of fuel and air inside the cylinder. That is because the combustion process begins at the end of the injection phase, before a homogeneous mixture of air and fuel can be formed.
• Preference for the fuel to have a high ignition performance (Cetane number), rather than a high knocking resistance (octane rating) that is preferred for petrol engines.

Thermodynamic cycle

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The diesel internal combustion engine differs from the gasoline powered Otto cycle by using highly compressed hot air to ignite the fuel rather than using a spark plug (compression ignition rather than spark ignition).

In the diesel engine, only air is initially introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15:1 and 23:1. This high compression causes the temperature of the air to rise. At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber. This may be into a (typically toroidal) void in the top of the piston or a pre-chamber depending upon the design of the engine. The fuel injector ensures that the fuel is broken down into small droplets, and that the fuel is distributed evenly. The heat of the compressed air vaporises fuel from the surface of the droplets. The vapour is then ignited by the heat from the compressed air in the combustion chamber, the droplets continue to vaporise from their surfaces and burn, getting smaller, until all the fuel in the droplets has been burnt. Combustion occurs at a substantially constant pressure during the initial part of the power stroke. The start of vaporisation causes a delay before ignition and the characteristic diesel knocking sound as the vapour reaches ignition temperature and causes an abrupt increase in pressure above the piston (not shown on the P-V indicator diagram). When combustion is complete the combustion gases expand as the piston descends further; the high pressure in the cylinder drives the piston downward, supplying power to the crankshaft.

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 pre-ignition, which would cause engine damage. Since only air is compressed in a diesel engine, and fuel is not introduced into the cylinder until shortly before top dead centre (TDC), premature detonation is not a problem and compression ratios are much higher.

The pressure–volume diagram (pV) diagram is a simplified and idealised representation of the events involved in a diesel engine cycle, arranged to illustrate the similarity with a Carnot cycle. Starting at 1, the piston is at bottom dead centre and both valves are closed at the start of the compression stroke; the cylinder contains air at atmospheric pressure. Between 1 and 2 the air is compressed adiabatically – that is without heat transfer to or from the environment – by the rising piston. (This is only approximately true since there will be some heat exchange with the cylinder walls.) During this compression, the volume is reduced, the pressure and temperature both rise. At or slightly before 2 (TDC) fuel is injected and burns in the compressed hot air. Chemical energy is released and this constitutes an injection of thermal energy (heat) into the compressed gas. Combustion and heating occur between 2 and 3. In this interval the pressure remains constant since the piston descends, and the volume increases; the temperature rises as a consequence of the energy of combustion. At 3 fuel injection and combustion are complete, and the cylinder contains gas at a higher temperature than at 2. Between 3 and 4 this hot gas expands, again approximately adiabatically. Work is done on the system to which the engine is connected. During this expansion phase the volume of the gas rises, and its temperature and pressure both fall. At 4 the exhaust valve opens, and the pressure falls abruptly to atmospheric (approximately). This is unresisted expansion and no useful work is done by it. Ideally the adiabatic expansion should continue, extending the line 3–4 to the right until the pressure falls to that of the surrounding air, but the loss of efficiency caused by this unresisted expansion is justified by the practical difficulties involved in recovering it (the engine would have to be much larger). After the opening of the exhaust valve, the exhaust stroke follows, but this (and the following induction stroke) are not shown on the diagram. If shown, they would be represented by a low-pressure loop at the bottom of the diagram. At 1 it is assumed that the exhaust and induction strokes have been completed, and the cylinder is again filled with air. The piston-cylinder system absorbs energy between 1 and 2 – this is the work needed to compress the air in the cylinder, and is provided by mechanical kinetic energy stored in the flywheel of the engine. Work output is done by the piston-cylinder combination between 2 and 4. The difference between these two increments of work is the indicated work output per cycle, and is represented by the area enclosed by the pV loop. The adiabatic expansion is in a higher pressure range than that of the compression because the gas in the cylinder is hotter during expansion than during compression. It is for this reason that the loop has a finite area, and the net output of work during a cycle is positive.^[144]

Efficiency

The fuel efficiency of diesel engines is better than most other types of combustion engines,^[145][146] due to their high compression ratio, high air–fuel equivalence ratio (λ),^[147] and the lack of intake air restrictions (i.e. throttle valves). Theoretically, the highest possible efficiency for a diesel engine is 75%.^[148] However, in practice the efficiency is much lower, with efficiencies of up to 43% for passenger car engines,^[149] up to 45% for large truck and bus engines, and up to 55% for large two-stroke marine engines.^[1][150] The average efficiency over a motor vehicle driving cycle is lower than the diesel engine's peak efficiency (for example, a 37% average efficiency for an engine with a peak efficiency of 44%).^[151] That is because the fuel efficiency of a diesel engine drops at lower loads, however, it does not drop quite as fast as the Otto (spark ignition) engine's.^[152]

Emissions

Diesel engines are combustion engines and, therefore, emit combustion products in their exhaust gas. Due to incomplete combustion,^[153] diesel engine exhaust gases include carbon monoxide, hydrocarbons, particulate matter, and nitrogen oxides pollutants. About 90 per cent of the pollutants can be removed from the exhaust gas using exhaust gas treatment technology.^[154][155] Road vehicle diesel engines have no sulfur dioxide emissions, because motor vehicle diesel fuel has been sulfur-free since 2003.^[156] Helmut Tschöke argues that particulate matter emitted from motor vehicles has negative impacts on human health.^[157]

The particulate matter in diesel exhaust emissions is sometimes classified as a carcinogen or "probable carcinogen" and is known to increase the risk of heart and respiratory diseases.^[158]

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