Diesel engine

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License replica by Langen & Wolf of the first functional diesel engine, 1898 (output about 15 kW)

A diesel engine is an internal combustion engine with compression ignition (self-ignition without spark plug), the fuel- air mixture of which is formed within the combustion chamber ( internal mixture formation ) and the torque of which is set via the amount of fuel injected ( qualitative load influence ). It can run on a variety of fuels, including diesel fuel . Diesel engines are available as two-stroke - or four-stroke - piston engines ; they are characterized by a relatively high degree of efficiency and the possibility of designing them with both small and large power .

The inventor of the diesel engine is the German engineer Rudolf Diesel , who first published his ideas on an engine with particularly high efficiency in 1893 in the work Theory and Construction of an Rational Heat Engine . In the years after 1893 he succeeded in building such an engine in a laboratory at the Maschinenfabrik Augsburg (now MAN ), albeit only by deviating from the concept described in his book. Due to his patents registered in many countries and his active public relations work, he became the namesake of the engine and the associated diesel fuel, a middle distillate .



Four-stroke process in the diesel engine shown schematically

Diesel engines are reciprocating piston engines that convert chemical energy into thermal and kinetic energy. They can be designed as two or four-stroke engines. The diesel cycle invented by Rudolf Diesel is a thermodynamic comparison process for the diesel engine. Because it does not adequately represent the actual combustion process, it is better to use the Seiliger process as a comparison process . (More on this in the section Thermodynamics of the Diesel Engine )

Four-stroke diesel engines suck in a cylinder charge of air during the intake stroke ; In the two-stroke engine, the “flushing process” begins shortly before the piston has reached bottom dead center and ends shortly after it has left bottom dead center again - burned exhaust gas is replaced by fresh air. The fresh air is strongly compressed during the compression stroke (ratio for a four-stroke engine about 16: 1 to 24: 1) and thus heated to about 700–900 ° C ( compression heat ). Shortly before top dead center of the piston, the fuel begins to be injected, which is finely distributed and atomized into the hot air in the combustion chamber. The high temperature is sufficient to ignite the mixture - so there is no spark of a spark plug necessary as in a gasoline engine .

Identification of the diesel engine

  • Auto-ignition : The air is heated up by the (almost) adiabatic compression, and the fuel injected into the hot air ignites without an external ignition aid.
  • Internal mixture formation : fuel and air are first mixed in the combustion chamber.
  • Qualitative mixture regulation : The current output is changed primarily by varying the amount of fuel injected.
  • Heterogeneous mixture : air and fuel are not evenly distributed in the combustion chamber.
  • High air ratio : The diesel engine works with excess air:
  • Combustion flame: The oxygen diffuses into the flame during combustion ( diffusion flame ).
  • Ignitable fuel: Diesel engines work best with high-boiling, ignitable fuels.



In principle, diesel engines are multi-fuel engines and can therefore be operated with all fuels that can be delivered by the injection pump at the operating temperature of the engine , that can be atomized well and that are sufficiently ignitable for a low ignition delay . The degree of ignitability is the cetane number , which should be as high as possible. In addition, the calorific value should be high. As a rule, diesel engine fuel consists of high-boiling and long-chain hydrocarbons (C 9 to C 30 ). In practice, (sometimes viscous) liquid fuels distilled from fossil fuels such as gas oils and tar oils with calorific values ​​between approx. 38.8 and 43.5 MJ / kg meet these requirements. In addition to liquid fuels, gaseous fuels are also suitable. After the First World War, mostly inferior, even cheap oils were used as fuel because they were not taxed. Until the 1930s, gasoline , petroleum , lubricating oil , gas oil, and vegetable oils , as well as mixtures of these fuels, were common. With the advancement of diesel engine technology, better, more ignitable fuels with cetane numbers of 45 to 50 CZ became indispensable. In practice, gas oil, coal tar oil and oil from coal smoldering were used.

There was no standardized diesel engine fuel until the 1940s, when diesel fuel was standardized for land vehicles in DIN 51601 for the first time after the Second World War. Since 1993, diesel fuel in the EN 590 standardized and is simply diesel called, most diesel engines (vehicles, tools) are designed for use with this fuel or can be operated with it; large marine diesel engines are still mainly operated with heavier fuel today (see marine diesel oil ). This fuel is standardized in the ISO 8217 standard. The type of fuel for which a particular diesel engine model is designed can usually be found in the operating manual. Some swirl chamber engines, for example, are designed for operation with non-ignitable fuel with a particularly high ignition delay (such as petrol ). Direct injection diesel engines with the MAN-M process are also principally suitable for operation with 86-octane gasoline. If diesel engines are operated with the wrong fuel, coking of the injection nozzles or knocking (nailing) can occur. Impurities in the fuel such as dust, rust, sand and water also have a detrimental effect on the diesel engine, with contamination from sand being particularly unfavorable.

The first diesel engine was designed for the use of mineral oil , but was also suitable for operation with petroleum , motor gasoline and ligroin . Rudolf Diesel tested the use of fuel based on vegetable oils as part of the world exhibition in 1900 . He reported it to a lecture to the Institution of Mechanical Engineers of Great Britain: "... at the Paris World Fair in 1900, a small diesel engine was the gas engine factory Deutz AG of Nikolaus Otto shown, at the request of the French government with peanut oil ran (peanut oil) , and it worked so smoothly that very few people saw it. "


The diesel engine is essentially regulated by the amount of fuel injected. If the amount is increased, more torque is given off, and the combustion air ratio decreases at the same time . In the case of turbo engines, the amount of air can also be increased by increasing the boost pressure.

Fuel injection

Not divided up combustion chamber of a common rail diesel engine
Technical drawing of the cylinder head of a swirl chamber diesel engine with a divided combustion chamber. The combustion chamber can be seen in the middle of the drawing and consists of the spherical swirl chamber , which is marked with three arrows clockwise representing the swirling of the air, and the associated main combustion chamber in the piston at the bottom right, which is flat in the upper part of the piston .

As a matter of principle, diesel engines have fuel injection into the combustion chamber (internal mixture formation), model engines and auxiliary bicycle engines ( Lohmann engine ) with carburettors and compression ignition are not counted among the diesel engines. The fuel is injected shortly before the end of the compression stroke when the air has been compressed sufficiently and has heated up as a result. The course of the injection process depends on the design of the injection nozzle and the pump element as well as the geometric relationship between the injection line and the relief valve. During injection, the liquid fuel enters the combustion chamber as a cloud of finely distributed droplets, with the air already providing ignition conditions. Only a small part of the fuel is vaporous in this phase. The individual fuel droplets have different sizes and are not evenly distributed (heterogeneous mixture). In order for ignition to occur, thermal energy from the compressed air must be transferred to the fuel droplets, so that the individual droplets evaporate on their surface and a layer of vapor forms around the fuel droplets, which can mix with the air. The mixture is only ignitable from a local air ratio of . The period from the start of injection to the start of ignition is known as the ignition delay.

Stationary diesel engine with air injection and an output of 59 kW from 1915. Due to its principle, this motor has a high mass and large dimensions with low output

Various injection processes have been developed for diesel engines, which differ essentially in the design of the combustion chamber and the injection pump. On the one hand there are engines with a compact combustion chamber and direct injection , on the other hand there are engines with a subdivided combustion chamber and indirect injection in a chamber upstream of the main combustion chamber. Because of its lower efficiency, this design is considered outdated. The oldest method, blowing in with compressed air, became obsolete after the First World War. Furthermore, the design of the fuel injection pump is an essential feature of the injection system, and conventional injection pumps can usually be combined with both combustion chamber forms. Modern diesel engines for passenger cars usually have direct injection; the cylinders have a common high-pressure pump and a high-pressure line (common rail) that is constantly under pressure and common to all cylinders; Injection is initiated by opening the injection valves, which are controlled electronically. In engines without electronic engine control, the injection is initiated purely mechanically. The injection quantity is determined by the injection pump, which consequently has to deliver an exactly defined quantity of fuel under high pressure to the injection valve for each cylinder. In the early days of diesel engine construction, the fine distribution of fuel could only be achieved by blowing in compressed air. If diesel engines are operated with gaseous fuel, the engine can either be a dual-fuel diesel engine or a pure gas-diesel engine. Dual-fuel engines draw in a gas-air mixture that is ignited by a small amount of injected conventional liquid fuel that burns (pilot ignition) and then ignites the gaseous fuel-air mixture. This type of engine can also operate in pure liquid fuel mode. All-gas diesel engines have high-pressure fuel injection that does not require pilot ignition. They cannot be operated with liquid fuel.

Types of injection pump

  • Fuel metering pump (with air injection)
  • In-line injection pump
  • Distributor injection pump
  • Single ram pump
  • Pump-nozzle unit
  • High pressure pump (with common rail)

Indirect injection processes

Immediate injection process


The work process of internal combustion engines is complex. In order to describe them mathematically and to make them accessible to a calculation, idealized, theoretically greatly simplified comparison processes are used. The comparison processes are circular processes and, in contrast to the actual engine, assume that an ideal gas in the engine is heated and then cooled again in order to perform mechanical work. According to DIN 1940, it is assumed for a perfect engine that the combustion proceeds according to given model principles, that there is only pure charge without residual gases, no flow and leakage losses occur, the charge exchange is modeled by a defined heat dissipation and the engine is otherwise heat-tight. In an actual engine, unlike in the model, there is no isentropic compression and expansion, but flow losses and slow combustion, which take a certain amount of time. Furthermore, the load exchange and the degree of delivery must also be taken into account.

Rudolf Diesel had the idea of ​​the diesel engine based on the Carnot cycle , which he wanted to realize with a machine. In the Carnot cycle, the heat is supplied at a constant maximum temperature and dissipated at a constant minimum temperature, that is to say isothermally : "Isotherms are changes in the state of the gas in which the temperature remains constant while the pressure and volume of the gas change." Carnot cycle the maximum possible efficiency for a given temperature gradient . The diesel devised based on the Carnot cycle and in the book Theory and construction of an efficient heat engine described diesel cycle is a constant-pressure process , that is, that the heat is isobaric fed into a gas, thus remaining the same, the maximum pressure, while the volume changes . The heat is withdrawn from the process at constant volume, i.e. isochorically, while the pressure changes. Between these two phases there is isentropic compression and expansion, in the order of compression, heat supply, expansion, heat removal. Since the diesel cycle is a cycle, these four phases can be repeated as often as required.

In fact, the working method originally devised by Rudolf Diesel does not work with a real engine, since the necessary gas state changes are not possible and the compression for ideal efficiency would be so great that the engine would have to do more compression work than it could supply itself. Diesel recognized this problem and in May 1893 wrote a manuscript with the title Conclusions about the working method of the engine to be definitely chosen for practice , in which he described a modified working method. The most important changes were reduced compression and more fuel used for combustion. The Seiliger cycle is now used to describe this changed working method, after all diesel engines are working, in a simplified thermodynamic model.

The Seiliger cycle is a mixture of a constant pressure and a constant space process. First of all, air is sucked in and isentropically compressed, then part of the heat is fed to the gas with an almost constant volume (isochoric). When the maximum pressure is reached, the remainder is supplied isobaric as in the diesel cycle, i.e. with a variable volume but constant pressure. In the calculation model, this should map the combustion, which occurs more slowly in a real diesel engine than in a gasoline engine. The gas expands isentropically for the rest of the work cycle. The volume of the combustion gas increases, the pressure in the cylinder and the temperature decrease. In the ideal process, the gas is cooled to its initial state at bottom dead center, and in the real engine the exhaust gas is expelled and replaced with fresh air. The process starts all over again. In a real diesel engine, heat can be supplied to the gas at least approximately isobarically and extracted approximately isochorically. As a result of the isobaric heat supply, the diesel engine has a lower thermal efficiency than the Otto engine. However, since the diesel engine can be operated with a significantly higher compression ratio, thanks to the mixture of fuel and air only after compression, its actual efficiency is not worse than that of a gasoline engine, but better. As a result of the development in gasoline engine technology with new mixture formation processes and controlled auto-ignition, “far-reaching convergence” of the cycle processes of gasoline and diesel engines can be expected in the future.


In his work Theory and Construction of a Rational Heat Engine to Replace the Steam Engine and the Combustion Engines known today, Rudolf Diesel states that the thermal efficiency of an ideal diesel engine is 73%, but that this value is not achieved in reality. Diesel estimated the effective efficiency of a diesel engine to be "6 to 7 times that of today's best steam engines (...) and later correspondingly more" . With an efficiency of 7.2% of a composite steam engine , this corresponds to an efficiency of 43.2% or 50.4% - in fact, today (2014) two-stroke large diesel engines achieve effective efficiencies of up to 55%. In passenger car diesel engines with direct injection and exhaust gas turbocharging, the degree of efficiency is somewhat lower, at the best point it is around 43%.

Exhaust gases

The possible fuels of the diesel engine are primarily composed of the chemical elements carbon and hydrogen , the oxygen required for combustion comes from the intake air. Since the air mainly contains nitrogen , it cannot be ignored. In the combustion chamber of the diesel engine, a chemical reaction takes place between the fuel and the intake air, during which the energy bound in the fuel is converted. The fuel molecules burn with the oxygen contained in the air, producing exhaust gases. If the theoretical model of the ideal diesel engine is used and this is operated with an ideal excess air ratio, then all combustible components of the fuel are brought to the final stage of oxidation by an optimal supply of oxygen - the combustion is complete. The exhaust gas then consists of carbon dioxide , water , nitrogen and optionally the excess oxygen. Incompletely burned components are therefore not found in the diesel engine exhaust of the ideal engine . In practice, however, there is an incomplete combustion condition in which some fuel components are not completely converted. The reason for this can be a lack of air, insufficient mixing of fuel with the air or incomplete combustion due to partial cooling of the combustion chamber.


If the combustion in the diesel engine is incomplete due to a lack of air or low temperatures, carbon components of the fuel are not converted and what remains as diesel soot , the combustion of the engine becomes smoking . Such a combustion, however, has an unfavorable effect on the operating properties of the diesel engine due to the heavy contamination of the combustion chamber, which is why a diesel engine must not be operated with a lack of air. Even an ideal diesel engine, more generally any engine with an inhomogeneous mixture formation , cannot burn the combustion chamber filling free of soot. The injected fuel is in the form of the finest droplets that ignite from the outside to the inside. The expansion of the combustion gases that occurs in the process prevents the adequate flow of further combustion air. Even if there is a large excess of air at the start of combustion, viewed in its entirety, it cannot be fully used. This always creates some soot. The particle mass tends to decrease as a result of finer atomization and a large excess of air. On the other hand, the inhomogeneous mixture formation is the necessary prerequisite for igniting a combustion chamber filling with a large excess of air, since volume elements can always be found in which an ignitable mixture is present. In engines with a homogeneous mixture formation, this state must be set by stratified charging .

Nitric oxide formation

In the ideal diesel engine, the exhaust gas consists of CO 2 , H 2 O, N 2 and O 2 , as described above . However, this condition would only be found at low combustion temperatures. In a real diesel engine, high combustion temperatures arise that change the chemical equilibrium; the nitrogen contained in the intake air dissociates and nitrogen oxides are formed .

Exhaust gas composition

Raw emissions of a passenger car diesel engine from various sources and at various operating points. The left column shows an operating point with a low load (approx. 25% and a combustion air ratio of 4). In the right column an ​​operating point close to full load, with a combustion air ratio of 1.1).

Exhaust gas composition
Exhaust gas components Weight% Volume-%
Nitrogen (N 2 ) 75.2% 72.1%
Oxygen (O 2 ) 15% 0.7%
Carbon dioxide (CO 2 ) 7.1% 12.3%
Water (H 2 O) 2.6% 13.8%
Carbon monoxide (CO) 0.043% 0.09%
Nitrogen oxides (NO x ) 0.034% 0.13%
Hydrocarbons (HC) 0.005% 0.09%
Aldehydes 0.001% (not specified)
Soot particles ( sulfates + solids) 0.008% 0.0008%

The distribution changes strongly depending on the load condition and slightly also with the humidity. The air humidity is usually calculated back from the proportions of the fuel, as it is rarely measured.

Torque curve and power output

Diesel engines have a physically determined speed limit due to the ignition delay ; Theoretically, swirl chamber motors can rotate up to approx. 5000 min −1 , direct injection engines up to approx. 5500 min −1 . In terms of design, however, not all motors are designed for operation at the theoretical upper speed limit.

In order to achieve the same performance compared to a gasoline engine, a diesel engine must have a larger displacement or a supercharging (= higher mean internal pressure), since the torque of a diesel engine must be higher due to the smaller speed range:

.. power [W]; .. torque [Nm]; .. speed [s −1 ]; .. angular velocity [rad s −1 ] ( )

Calculation example

A gasoline engine delivers a torque of 160 Nm at a speed of 6000 min −1 (100 s −1 ) , which corresponds to an output of approx. 100,000 W. An ordinary diesel engine cannot reach this speed, which is why its torque must be greater in order to achieve the same performance. In order to achieve an output of 100,000 W at a speed of 3000 min −1 (50 s −1 ) , the torque must be 320 Nm.

Advantages and disadvantages of the diesel engine

Advantages of the diesel engine

The diesel engine has a good degree of efficiency due to the high compression (degree of expansion) . The lower throttling results in lower gas exchange losses in the diesel engine and therefore a lower specific fuel consumption, especially in the partial load range . That makes the diesel engine particularly economical. In addition, the fuels used are easier to manufacture and less dangerous, as they evaporate more slowly (the flash point of diesel fuel is at least that of gasoline ). Diesel engines are also well suited for turbocharging in the low speed range, because the fuel cannot ignite in an uncontrolled manner due to the internal mixture formation during the compression stroke and the torque output is adjusted by changing the composition of the fuel-air mixture ( quality change ), but not its quantity.

Disadvantages of the diesel engine

Typical combustion noise of a historical industrial engine with direct injection, type MWM AKD 112 Z

The combustion noise of the diesel engine is louder and the specific power is lower than that of a gasoline engine. In order to be able to withstand the high pressures, utility diesel engines must be built to be comparatively robust; this leads to a greater mass of the motor. Furthermore, nitrogen oxides are produced during combustion , which may require a complicated exhaust gas aftertreatment system because the three-way catalytic converter does not work in diesel engines. This makes the diesel engine significantly more expensive to purchase and less economical to operate compared to a diesel engine without an exhaust gas cleaning system.

Starting and stopping a diesel engine

Incandescent filament display on the dashboard of a diesel vehicle. The engine can be started when the lamp goes out.

In order to start a diesel engine, the injection pump must be set in such a way that sufficient fuel pressure can be generated, then the crankshaft must be set in a sufficiently fast rotary motion so that compression starts auto-ignition. The crankshaft can be turned by hand using a crank or a cable pull, a starter motor or compressed air , for example . With simple motors, electrical components are only used for monitoring.

In principle there is no need to start a diesel engine. If the engine is warm, it starts immediately even at low temperatures. If the engine is not at operating temperature, however, it may have to be preheated . The air temperature from which an engine must be preheated depends on its design. This is approximately at prechamber , in the swirl chamber engines and with direct injection . In the case of small diesel engines (displacement less than 1000 cm³ per cylinder), glow plugs are used which are built into the secondary combustion chamber; with direct injection, they protrude into the main combustion chamber. In large commercial vehicle engines, a flame starter system is installed instead of glow plugs . In addition to their function as starting aid, the glow plugs in modern engines are also activated by the control unit when the engine is not starting, which increases the combustion chamber temperature, for example to support the regeneration of the particulate filter system.

Some engines also have a change in valve timing as a jump start. The simplest design is the “decompression lever”, which, when activated, causes the cylinder exhaust valves to remain open until the crankshaft and its flywheel have reached the starting speed. After closing the decompression lever, the exhaust valves close again, the momentum should lead to the onset of the initial ignition. In the pre-chamber diesel engine XII Jv 170/240 from Ganz & Co., the timing of the intake camshaft is changed during the starting process, so that the intake valves only open very late. This creates a negative pressure in the combustion chamber, which ensures that the incoming intake air experiences a temperature increase due to the sudden increase in pressure; in this way, the ignition temperature in the engine can be achieved without glow plugs.

Since no ignition and in some designs generally no electrical system is required to keep the engine running, switching off the electrical system cannot stop the engine in such engines. In older vehicles with diesel engines, even removing the key does not stop the machine. To stop the engine, the exhaust brake is activated until the engine dies or the fuel supply to the injection nozzles is interrupted by means of a valve flap. In modern vehicle engines, this is electronically regulated so that the behavior of the ignition key in a modern diesel car does not differ from that of a car with a gasoline engine.

Special features of engines for driving motor vehicles

Throttle valves

With the principle of the diesel process, throttle valves are in principle not required and, because of the throttle losses (enlargement of the gas exchange loop), do not make sense for the efficiency. However, there are throttle valves in modern diesel engines: In engines with two intake ports, one intake port is designed as a filling port and the other as a swirl port. A throttle valve called a "swirl valve" is installed in the inlet channel, which is designed as a filling channel and is closed in the partial load range. This improves the mixing of air and fuel, which is used to reduce exhaust emissions. The throttle valve is also increasingly used to improve the intake air flow noise behavior ( English sound design ).

There are examples in history of diesel engines that were equipped with a throttle valve for another reason. So z. B. the OM 138 from Daimler-Benz from 1936. Up until the 1980s, Daimler-Benz built throttle valves in diesel engines because the Bosch injection pump used earlier was pneumatic, ie. H. was controlled by a slight negative pressure in the intake tract . This type of control is, however, quite susceptible to the formation of black smoke in some operating states: The engine is over-greased with too much diesel fuel, which does not burn completely and produces soot.

Injection techniques

In the case of diesel engines for passenger cars, despite their poorer efficiency, indirect injection of the fuel was initially used, as it is favorable in terms of exhaust gas and noise emissions. Only at the end of the 1980s was there an increasing switch to direct injection. Modern direct-injection diesel engines for passenger cars generally have common rail injection .

Exhaust aftertreatment

Diesel engines emit soot particles, with modern vehicle engines emitting significantly less soot particle mass than older vehicle engines. The ejected soot particle mass correlates with the soot particle quantity; the size of the particles has not decreased in recent years . In 1993, the size of the soot particles was predominantly between 0.01 and 0.1 µm and 0.3 µm; in 2014 this range was unchanged. Some of the particles are in the respirable area. The core of the soot particles can have a carcinogenic effect. In the Federal Republic of Germany around 72,000 t of diesel soot were emitted annually at the end of the 1990s, of which 64,000 t came from traffic and 42,000 t came from commercial vehicles; "This causes around 1000 deaths annually" (for the year 2000). Results from studies carried out in the USA in the 1980s show that the risk of becoming fatally ill from the exhaust gas from diesel engines is very low; city ​​dwellers are about as likely to be struck by lightning and die as a result. According to the study, road workers, on the other hand, have a significantly higher risk of being fatally ill from the exhaust gases. In order to reduce the total particle emissions, soot particle filters are built into passenger cars as standard; they achieve separation capacities of over 90%. The soot particles are oxidized in the particle filter.

Unregulated oxidation catalysts have been installed in diesel cars since 1990. This can reduce the emission of some pollutants: hydrocarbons by up to 85%, carbon monoxide by up to 90%, nitrogen oxides by up to 10% and soot particles by up to 35%. Since the torque output in the diesel engine is adjusted by changing the air ratio ( ) and the engine is usually operated with excess air ( ), a conventional controlled three-way catalytic converter that requires an air ratio of around cannot be used . Work around 2010 dealt with the use of perovskite in vehicle catalysts for diesel engines. The doping of perovskite-containing catalysts with palladium increases the resistance to "poisoning" by sulfur.

Using exhaust gas recirculation is nitrogen oxide emissions of the diesel engine, although positively influenced, but it has here a compromise between acceptable nitrogen oxide and particulate values are entered in the exhaust gas as drop at high exhaust gas recirculation rates, although engine power and nitrogen oxide emissions, but diesel particulate emissions increases in intolerable degree. Nevertheless, the average nitrogen dioxide emissions from passenger car diesel engines under real conditions on German roads are well above the permitted limit values. While the limit values ​​for the Euro 4, Euro 5 and 6 emission standards are 250, 180 and 80 mg NO x per km, respectively, diesel passenger vehicles in Germany emit an average of 674 (Euro 4), 906 (Euro 5) and on average when they are actually driving 507 (Euro 6) mg NO x per km. Overall, almost a third of the diesel vehicles used for heavy haulage and more than half of the diesel vehicles used for light transport in the most important markets exceed the applicable limit values, which leads to an additional 38,000 premature deaths each year. The nitrogen oxide emissions of a diesel vehicle without exhaust aftertreatment systems are lower than the nitrogen oxide emissions of a vehicle with a gasoline engine without a regulated three-way catalytic converter. If, on the other hand, one compares a diesel vehicle with an unregulated oxidation catalytic converter with a gasoline vehicle with a regulated three-way catalytic converter, the nitrogen oxide emissions in vehicles with a gasoline engine are lower.

Wankel diesel engine

In the 1960s and 1970s there were attempts to construct a compact and lightweight rotary piston engine using the diesel process as a motor vehicle drive. The attempts failed because of the high compression ratio that could not be implemented, so that the built prototypes could only run with externally supplied, pre-compressed air, but not on their own.

Areas of application

Modern diesel engines are used in many areas of application due to their high economic efficiency. A disadvantage for their use is their unfavorable mass / power ratio - they are rarely used where high power with low weight is essential, such as in airplanes or motorcycles . Diesel engines can be designed for both large and small power ranges; The power spectrum ranges from the four-digit watt range to the double-digit megawatt range: the world's most powerful diesel engine, the fourteen - cylinder ship engine Wärtsilä RT-flex96C , has a displacement of 1.8 m³ per cylinder and develops a nominal output of more than 80 MW - the smallest at the time The world's commercial diesel engine, a stationary engine from RH Sheppard , has a displacement of 460 cm³ and develops an output of approx. 2800 W. Modern diesel engines for passenger cars achieve a liter output of around 50–58 kW.

historical development

Rudolf Diesel (1883)
Patent for Rudolf Diesel dated February 23, 1893
Second prototype of the diesel engine from 1894. With this engine, the first idling was achieved on February 17, 1894.
First functioning diesel engine from 1896.
Bore × stroke: 250 mm × 400 mm (displacement: 19,635 cm³,
power: 13.1 kW (at speed: 154 min −1) ),
torque: 812 N · m (at speed: 154 min −1 ),
specific fuel consumption : 324 g / kWh

Diesel's theory

In 1878, Rudolf Diesel, then a student at the Munich Polytechnic , attended the thermodynamics lectures given by Professor Carl von Linde . Linde explained to his students that a steam engine only converts 6–10% of the heat given off by the fuel into effective work, but in the Carnot process all heat is converted into work. Diesel states that this should be his key experience in developing a machine that could realize the Carnot cycle. Initially, Diesel worked on an ammonia steam engine in his laboratory in Paris, but this did not lead to practical implementation. Instead, he realized that normal air could be used in place of ammonia if the fuel burns in that air. Diesel applied for a patent for such a machine and published his thoughts on the engine in the work Theory and Construction of a Rational Heat Engine .

On February 23, 1893, he received the patent RP 67207 "Working method and design for internal combustion engines" and the collaboration with the Augsburg machine factory and the establishment of a laboratory for testing various working principles with the aim of achieving a high level of efficiency began. At that point, Diesel had not yet realized that his theory was flawed and that the engine described in his book would not work because it would require more compression work than he could provide himself. Rudolf Diesel only became aware of this in the spring of 1893. Between May and September 1893 he designed a modified working method that managed with far less compression and a lower air ratio ; this working process, now known as the diesel process, is functional and the basis of all diesel engines. Diesel's notes show that he had already worked out the most important part of this changed working procedure before the start of the tests in Augsburg. It is therefore considered proven that Diesel himself invented the diesel engine and the associated working process, although it differs from the theory and construction of a rational heat engine in his work . Diesel did not publicly admit his mistake, as he had a patent on the non-functional working method described in his book, but not on the actual working method of the diesel engine. Diesel only applied for a patent for this actual working process in November 1893 (RP 82168).

The first diesel engine

Diesel explicitly states that he did not invent the principle of compression ignition, but only wanted to find a process with the highest possible heat utilization; such a process presupposes self-ignition. The first test machine, which was built by M. A. N. according to Diesel's specifications, was completed in July 1893 and designed for operation with liquid fuels. It was a four-stroke with crosshead connecting rod and OHV valve control, the bore was 150 mm, the piston stroke 400 mm. On February 17, 1894, this engine ran for the first time under its own power at an idling speed of 88 min −1 for a period of just under a minute after it was rebuilt in January.

However, Diesel had to make a compromise. Diesel favored direct injection of the fuel and for this purpose had provided the principle of accumulation, in which the injection nozzle is fed from an accumulation vessel in which an overpressure is kept constant by means of an air pump. However, despite several improvements, this system did not work well enough due to the unsuitable pumps and the lack of precision of the injection valves, so that Diesel had to replace the air pump with a large compressor instead, which made it possible to omit the accumulation vessel and the fuel was now injected directly . The concept of the compressor came from George Bailey Brayton . However, Diesel preferred to build an engine without a large compressor. Since it did not seem possible to implement this, he ultimately described compressorless direct injection as “impracticable”.

From 1894, Diesel received several patents in various countries for significant improvements to the compression-ignition engine. In particular, he made the engine ready for practical use in years of trials together with Heinrich von Buz , the then director of the Augsburg machine factory, and tried to get development funds for this by propagating the promising principle and attracting donors. Fuels such as crude oil, coal dust and gasoline were also tested during development. It was not until 1897 that Diesel presented an engine that worked with mineral oil and withstood days of endurance testing to its financiers and the global public at the II. Motor and Work Machine Exhibition in Munich. According to recent literature, it had a specific fuel consumption of 258 g / PSh (350.8 g / kWh), which results in an efficiency of almost 24%. Other plants also indicate a fuel consumption of 324 g / kWh. The efficiency exceeded that of all previously known heat engines.

Diesel engine as a land vehicle engine

BMW M21, the first passenger car diesel engine with an electronic engine control unit

Due to its design, the diesel engine could initially only be used as a stationary engine . The first commercially used diesel engine, a two - cylinder four-stroke engine with an effective output of 60 PS e (around 44 kW e ) at 180 min −1 , went into operation in 1898 in the Union match factory in Kempten (Allgäu) . The diesel engine was first used in ships from 1902, and in trucks from 1923 . At the end of the 1940s, the diesel engine had become widely used as a drive for commercial vehicles , rail vehicles and ships.

The basis for the development of the vehicle diesel engine was the prechamber , for which Prosper L'Orange applied for a patent in 1909 . By injecting the fuel into the prechamber, a lower injection pressure was sufficient, which made it possible to dispense with the complex and large air injection system that had previously been necessary . The reduced size and weight of the diesel engine made it possible to install it in land vehicles .

In 1924 MAN presented the first diesel engine with direct injection for commercial vehicles , the output was around 30 kW. In the years that followed, the performance of the engines continued to increase; by the mid-1930s there were engines with more than 100 kW of power for commercial vehicles. In February 1936, the first two German series passenger cars with diesel engines were presented at the Berlin Motor Show - the Mercedes-Benz 260 D and the Hanomag Rekord .

Chamber machines were widespread in the commercial vehicle sector until the 1960s , before the direct injection engine assumed a dominant position here due to its greater economic efficiency. Up until the 1990s, passenger car diesel engines were constructed using the chamber method, as the combustion noise is lower. For a long time, however, passenger car diesel engines were unable to gain acceptance because they were considered to be underperforming. This only changed with the changeover to electronic high-pressure direct injection ( common rail or pump nozzle ) in combination with exhaust gas turbocharging (" turbodiesel "). The passenger car diesel engine was increasingly accepted by consumers, so that in Europe (as of 2017) around every second newly registered car has a diesel engine.

The first electronic control unit for passenger car diesel engines with distributor injection pumps , called EDC , was developed by Bosch and used for the first time in 1986 in the BMW M21 . The common rail principle is today (2014) the most widely used system for vehicle diesel engines. It was developed in 1976 by the ETH Zurich . A first common rail system was successfully tested in the winter of 1985/1986 on a modified diesel engine of the type 6VD 12.5 / 12 GRF-E in continuous road traffic with an IFA W50 truck . The engine prototype can be viewed today in the Chemnitz Industrial Museum .

Passenger car diesel engines worldwide

Percentage of new passenger cars sold in 2014
according to the functional principle:

B: Brazil, Ch: China, E: Europe, I: India,

J: Japan, USA: United States

The spread of the diesel engine for passenger cars worldwide depends on various factors, so that in some markets there are hardly any passenger cars with diesel engines. The main advantage of the diesel engine is that it is more economical due to its higher level of efficiency, but this is only significant when fuel costs are high.

Situation in the USA

New registrations of diesel cars in the USA
between 2011 and 2014 by manufacturer

In the USA, petrol is much cheaper than in Europe, so the advantage of economy does not come into play. In addition, the diesel engine has a bad reputation in the USA due to the Oldsmobile diesel engine from the 1970s and the 2015 emissions scandal . The market share of diesel vehicles in the USA was therefore just under 2.7% in 2017. German automobile manufacturers are the market leaders; most American automobile manufacturers do not offer any diesel vehicles. Volkswagen, with its brands Audi and VW, has also stopped selling diesel cars since the emissions scandal. The supply of diesel vehicles is increasing, however, so that an increase in the diesel car market share was forecast for 2018.

Situation in Germany

Until the 1990s, the prevailing opinion in Germany was that a diesel car was only profitable for frequent drivers because of its higher purchase price. Because of the considerable under-consumption, especially on short trips in the city, and also because of the price difference of the lower taxed diesel fuel (the tax advantage is approx. 22 cents / liter), it was enough for many vehicles - despite the significantly higher vehicle tax (per 100 cm³ displacement: 9.50 € / a for newer diesel vehicles instead of € 2.00 / a for gasoline vehicles) as well as the often higher insurance premium - as of April 2018, less than 10,000 kilometers per year so that the diesel pays for itself .

Emissions scandal and driving bans

In September 2015, the Volkswagen group publicly admitted that the exhaust gas aftertreatment system of its diesel vehicles illegally uses special test bench settings when a test bench run is detected, and that this is the only way that their cars achieve the prescribed low emission levels during the test bench run. This VW emissions scandal brought the diesel engine into criticism as an efficient drive technology. As a result, it became known that many diesel vehicle types from other manufacturers often emit multiples of the permissible pollutants in everyday operation. From 2016, possible driving bans for diesel vehicles in German cities were discussed. As a result, the popularity of the diesel engine in Germany declined, and according to estimates by the business magazine Manager Magazin from 2016 to mid-2017 , the emissions scandal cost Volkswagen around 20-25 billion euros.

At the meeting of the "National Forum Diesel" of the German Federal Ministry of Transport and the Federal Environment Ministry as well as other specialist ministries and representatives of the automotive industry as well as decision-makers from the federal states, a nationwide solution to reduce nitrogen oxide emissions was found on August 2, 2017 after the exhaust gas scandals and the judgment of the Stuttgart Administrative Court on air pollution discussed for diesel cars. The participation of environmental and consumer protection associations in the “National Diesel Forum” was not planned. It was agreed that by the end of 2018 the nitrogen oxide emissions of around 5.3 million diesel cars complying with the Euro 5 and 6 emissions standards should be reduced by around 25-30% through manufacturer conversion measures. However, as of February 2019, this goal has not yet been fully achieved. In addition, the automobile manufacturers should make the switch to environmentally friendly vehicles more attractive through bonuses and, together with the federal government, set up a fund “Sustainable Mobility for the City”. Foreign automobile manufacturers have also been urged to reduce their vehicles' emissions.

On May 23, 2018, for the first time since the emissions scandal, a public authority imposed driving bans on vehicles with older diesel engines with the Hamburg Authority for the Environment and Energy . According to the Hamburg Clean Air Plan , from May 31, 2018, driving bans will apply in parts of Max-Brauer-Allee and Stresemannstraße for vehicles that do not meet at least the Euro 6 emissions standard . The Federal Administrative Court had previously considered such driving bans to be permissible in order to reduce the air pollution with nitrogen oxides . The BUND Hamburg criticized the decision because the traffic and the harmful nitrogen oxides would only be distributed to other streets where no measurements are carried out. Only area-wide driving bans are effective.

Share of diesel cars

Number of cars in Germany by type of fuel, 2004 to 2017

In 1991, 13% of all newly registered cars in Germany had a diesel engine; In 2004 it was 44%. Until 2008, the percentage of diesel cars registered each year remained roughly constant. In 2009, because of the environmental bonus, an above-average number of new small and very small cars were registered in Germany that only rarely had a diesel engine. From 2011 to 2016, the share of newly registered diesel cars was always over 45 percent. In 2017, only 38.8 percent of newly registered cars were diesel cars; One reason for the decline was the diesel emissions scandal and the discussions about driving bans. In 2017, around a third of all cars registered in Germany had a diesel engine.

Share of diesel cars in new registrations in Germany from 1991 to 2017
year 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000
proportion of 13.0% 15.0% 14.9% 16.9% 14.6% 15.0% 14.9% 17.6% 22.4% 30.4%
year 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
proportion of 34.6% 38.0% 39.9% 44.0% 42.7% 44.3% 47.7% 44.1% 30.7% 41.9%
year 2011 2012 2013 2014 2015 2016 2017
proportion of 47.1% 48.2% 47.5% 47.8% 48.0% 45.9% 38.8%

Web links

Wiktionary: Diesel engine  - explanations of meanings, word origins, synonyms, translations
Commons : Diesel Engines  - Collection of pictures, videos and audio files

Individual evidence


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This article was added to the list of articles worth reading on August 7, 2018 in this version .