Direct injection

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Different bowl shapes for diesel engine pistons

The direct injection is a method for fuel injection for diesel engines and gasoline engines . The fuel is injected directly into the combustion chamber from an injection nozzle .

Direct injection in diesel engines

Characteristics of direct injection in diesel engines are the undivided combustion chamber and the smaller combustion chamber surface compared to the prechamber and swirl chamber engine. This results in lower heat and flow losses and thus lower specific consumption and higher efficiency . However, the injection pressure must be higher in order to distribute the fuel finely enough.


Herbert Akroyd Stuart designed the hot-bulb engine in 1886 , in which the fuel was injected through the cylinder chamber into the hot-bulb. In hot-head engines, the requirements for injection pressure and time are low. Rudolf Diesel tried direct injection, but he did not succeed in building a pump that on the one hand could deliver the necessary high pressure and on the other hand could be regulated precisely enough. So he used a low pressure metering pump and an air compressor ( air injection ) to finely distribute the fuel in the combustion chamber with compressed air. Such motors proved themselves in stationary drives, from 1903 in inland shipping and from 1910 in ocean-going ships.

Prosper L'Orange received a patent ( DRP 230 517 ) on March 14, 1909 for the pre-chamber process with needle injection nozzle and a controllable injection pump . The heavy and fragile air compressor was eliminated and after the First World War lighter diesel engines could be installed in trucks.

In 1920, the Swedish engineer Jonas Hesselman patented a multi-fuel engine, the Hesselman engine , with direct injection and external ignition using spark plugs . The Swedish manufacturers Scania-Vabis, Tidaholms Bruk and Volvo built trucks with Hesselman engines. These engines were started with gasoline and could be switched to petroleum or diesel after warming up . Scania used them until 1936, Volvo until 1947. They were replaced by diesel engines.

In 1931 Harry Ricardo developed the vortex chamber process, which made engines that rotate even faster and lighter.

In diesel engines with an antechamber or swirl chamber, the beginning of the combustion drives the fuel-air mixture through a narrow firing channel into the cylinder, where it continues to burn (lower pressure increase and longer burning time, thus lower maximum speed and smoother engine running).

In the early 1940s, MAN developed the D 0534 G vehicle engine with direct injection into a spherical piston recess inclined towards the injection nozzle. This 4.5 liter engine has an output of 51.5 kW and a specific fuel consumption of 218 g / kWh, an extremely low figure for the 1940s. Initial fears that the spherical piston bowl selected for direct injection would encourage the compression rings to stick or melt out the piston material turned out to be unfounded.

In the mid -spherical engine from MAN from the 1950s, the fuel was injected into an almost spherical recess in the piston, on the wall of which it was partly deposited and only evaporated during combustion and was carried away by the air vortex. Only a small part burned suddenly, igniting the rest in a controlled manner.

In modern diesel engines with direct injection, this principle has been reversed: the fuel is injected in the center of an air vortex in a flat piston recess and burns on the way to the edge. To minimize noise, it is injected at different times. The pre-injection or pilot amount is a very small amount at the beginning in order to initiate a “gentle” combustion. Only then does the main injection follow. By optimizing the shape of the trough (compared to truck engines) the noise development could be reduced even further.

Saurer MH4 with direct injection and aluminum engine block 1945

After direct injection had prevailed in heavy trucks, Ford and Fiat laid the foundation stone in 1984 for the introduction of the so-called high-speed diesel engines with direct injection. The Ford Transit and Fiat Ducato models were first offered with direct injection diesel as standard. You have to take into account that these are commercial vehicles or light trucks. However, it was the introduction of direct diesel injection in this vehicle segment in large-scale production. It plays a role here that the engines are specific to passenger car engines.

A diesel engine with common rail direct injection was successfully tested in 1985 in the GDR on a modified W50 truck in continuous road traffic, but development was discontinued in 1987 due to a lack of capacity to launch production. The engine prototype can be viewed today in the Chemnitz Industrial Museum .

Before 1987, direct injection diesel engines were only available in commercial vehicles or as large engines. They were not used in cars because their rough running (“ nailing ”) contradicted the comfort requirements and the latter was seen as more important than the lower specific consumption. Diesel engines with direct injection did not gain acceptance in passenger cars until the 1990s.

A direct injection diesel engine was first in automotive high-volume from the spring of 1987 in the of Fiat offered Fiat Croma TD id used. The engine was developed in a Fiat research center in Naples . For this purpose, an engine known from the commercial vehicle sector was equipped with an electronic injection control, which improved the running smoothness to a level that was usable for car conditions. The Fiat Croma was initially only available on the Italian market, as Fiat wanted to rule out possible "teething problems" associated with the development on the foreign markets.

The second car of this type was the (Austin) Rover Montego in 1988 , the engine of which was developed by Rover in collaboration with Perkins Engines . These engines developed a high torque even at low engine speeds and thus enabled both good driving performance and lower consumption.

In 1990, the Audi 100 C3 2.5-liter TDI, the first passenger car diesel from a German manufacturer with direct injection, came onto the market. As a five-cylinder, it ran more smoothly than a four-cylinder. The engine (1T) was also used in the Audi 100 C4 (which appeared in late 1990 and was built until July 1994).

Ford, Fiat, Austin / Perkins and Audi / VW provided the first generation of diesel direct injection engines in passenger cars. This worked with a distributor injection pump, i.e. with an injection pump that supplied all cylinders. Ford Transit and Fiat Ducato were not yet supercharged, Fiat Croma, the later Audi 100 and Audi 80 had turbochargers and intercoolers . The distributor injection pump in the Audi 100 still worked with a pressure of up to 800 bar.

In 1997, the Fiat group launched the Alfa Romeo 156 JTD, the first series-produced passenger car with common rail injection. In this system, pressure generation and the injection process are spatially separated. A high-pressure pump permanently builds up the almost constant pressure and feeds the line common to all cylinders (hence common rail), which stores the fuel. The injection processes are triggered by the engine electronics. To do this, it actuates valves that are assigned to the cylinders. There are also several generations of this technology. The common rail system works with pressures of up to 2000 bar and meanwhile up to five injection processes per combustion process. Occasionally, with engines from BMW and Mercedes-Benz , serious series production problems arose with valve injection nozzles of the common rail systems. Sometimes the shut-off process of the valve did not work; the common rail emptied into a cylinder via the defective valve. However, as of 2011, these problems are a thing of the past.

In 1998, VW launched another TDI generation in the VW Passat B5 . Their pump-nozzle system has neither a central injection pump nor a common pressure build-up with a high-pressure pump for all cylinders. Each cylinder has its own unit in the cylinder head with an injection pump and nozzle. This made it possible to increase the injection pressures again; the pump-nozzle system works with up to 2500 bar pressure. However, a maximum of three injection processes per combustion can be implemented here because the control of the injection is not independent of the pressure build-up due to the system. The pump-nozzle units are controlled via the camshaft , which also controls the valve train. The advantage of the pump nozzles was initially the higher possible pressure and thus lower consumption compared to distributor pumps and the earlier common rail systems. Disadvantages were always the noise development and the high construction costs in the cylinder head and in the pump-nozzle elements, accompanied by cost disadvantages. Finally, the two camshafts controlled four valves per cylinder and the pump. With the common rail system pressures increasing, the pump-nozzle system lost its advantages. In addition, the increased requirements for environmental protection could only be met with even more partial injections - a development with which the pump-nozzle technology could no longer keep up. In the meantime, Audi and VW moved away from the pump-nozzle principle at the end of 2007 and adapted their diesel engines for common rail model by model.

Technical status

Engines of this type have been manufactured with injection pressures of up to 2500 bar since 2010. The common rail technology has established itself because of its simple structure, the lower costs as well as the achievement of the Euro 5 standard .

An important advantage of the common rail method is the possibility of pre (or pilot) injection. After a small amount of fuel has been injected, the nozzle closes again to give the fuel time to ignite. Only then is the main amount injected. At the time of ignition, there is only a small amount of fuel in the combustion chamber that burns suddenly. The rough running typical of diesel engines due to the ignition delay can thus be largely avoided and the engines sometimes run almost as smoothly as gasoline engines .

In addition, the main injection can be divided into several injection processes in order to delay the combustion (especially at low and medium engine speeds) and thereby reduce the pressure rise and the peak pressure. This reduces the noise and the mechanical stress on the engine. Post-injection can increase the combustion end temperature and thus the exhaust gas temperature; this is necessary to regenerate a diesel particulate filter , but even without a filter, less soot is formed in the engine.


Direct injection engines with turbocharging are offered by many manufacturers. Everyone has their own abbreviation, mostly protected by trademark law. However, their tenders are usually not protected and not always used consistently:

  • CDI (Common Rail Direct Injection): Daimler, Mercedes-Benz
  • CDTI (Common Rail Diesel Turbo Injection): Opel
  • CRD (Common Rail Direct Injection): Jeep
  • CRDi (Common Rail Direct Injection): Hyundai, Kia
  • d (Diesel) : BMW, Jaguar, Rover, Infiniti, Mercedes
  • D or SD (Diesel or SportDiesel): Mini, Subaru, Volvo
  • dCi (Diesel Common Rail Injection): Dacia, Nissan, Renault
  • DDiS (Direct Diesel injection System): Suzuki (Common-Rail)
  • DI-D (Direct Injection Diesel): Mitsubishi (Common Rail)
  • DiTD (Direct Injection Turbo Diesel) : Mazda
  • dTI (Direct Turbo Injection): Renault
  • DTI (Direct Turbo Injection): Opel
  • D-4D (Direct Injection 4-stroke Diesel): Toyota (Common-Rail)
  • EDIT (Ecotec Direct Injection Turbo): Opel
  • HDi (High Pressure Direct Injection): Citroën, Peugeot, DS (Common Rail)
  • i-CTDi (intelligent Common Rail Turbo Diesel Injection): Honda
  • i-DTEC (intelligent Diesel Technology Electronic Control): Honda
  • JTD or JTDM (Jet Turbo Diesel Multijet) or only Multijet : Alfa-Romeo, Fiat, Lancia, Jeep (Common-Rail)
  • SD4 / TD4 / eD4 , TD6 / SDV6 / TDV6 , SDV8 : LandRover
  • SIDI (Spark Ignition Direct Injection): Opel
  • SKYACTIV-D : Mazda
  • TDCi (Turbodiesel Common Rail Injection): Ford
  • TDDI (Turbo Diesel Direct Injection): Ford
  • TDI (Turbodiesel Direct Injection): Audi, Seat, Škoda, Volkswagen
  • XDi or e-XDi : Ssangyong

Direct injection engines have been the standard in passenger car diesel engine construction since the late 1990s.

The I in the name stands for the English word “injection”. The introduced D stands for “Direct” and indicates that injection takes place directly into the cylinder without an antechamber or vortex chamber . A second D stands for "Diesel". Diesel engines without supercharging are only occasionally used in passenger cars, for example the SDI engines from Volkswagen. Every diesel engine has an injection pump , be it a distributor pump, a pump-nozzle unit or the high-pressure pump of the common rail system.

Pressure rise, noise development, health hazards

Old diesel engines with direct injection have a loud combustion noise, known as pinning, due to the sharp rise in pressure during ignition. Modern diesel engines have pilot injection that prevents nailing so that it practically no longer occurs.

A distinction is made between the injection profile shaping of the older direct injection engines with distributor injection pumps and corresponding injection nozzles with counter cone and real pre, main and post injection with common rail or pump nozzle . This makes the pressure rise more gentle, and the engine runs more quietly and with fewer vibrations. A post-injection carried out if necessary briefly increases the end-of-combustion temperature and thus the exhaust gas temperature, whereby the diesel particle filter is regenerated (at the expense of efficiency ) .

Direct injection in gasoline engines

Sectional view of a gasoline engine with direct injection

Gasoline engines with direct injection can be operated like conventional gasoline engines with a homogeneous mixture, but also like diesel engines with an inhomogeneous mixture, whereby the mixture is stratified. The mixture throttling is no longer necessary or only partially necessary, the torque is then only adjusted via the injected fuel quantity. The gasoline can be injected onto the piston recess (wall-guided), guided by an air flow (air-guided) or only determined by the injection pressure (jet-guided). Since the gasoline does not evaporate in the intake manifold but in the cylinder, more air is sucked in and the temperature at the end of compression is slightly lower. This reduces the tendency to knock, and the gasoline does not ignite uncontrollably as easily. The compression can be increased by about 1 to 2 units; Compared to carburetor engines and manifold injection engines, the efficiency is thus improved and the power increased, but the proportion of nitrogen oxides in the exhaust gas increases, especially in stratified charge operation.

At the beginning of the First World War, direct injection was tested for the first time in gasoline engines and was first mass-produced in the 1930s for use in aircraft; in passenger cars there was direct injection in two-stroke and high-performance engines in the 1950s. They couldn't prevail. While intake manifold injection was the predominant mixture formation system from the beginning of the 1980s to the 1990s, gasoline direct injection has been used again for passenger car engines since 1995. The first engines with gasoline direct injection had mechanical in-line injection pumps; modern engines usually have common rail injection. While attempts were still being made in the 1990s to operate gasoline engines with a stratified mixture, a switch to homogeneous operation was made due to exhaust gas detoxification, especially at full load or also in the partial load range.


  • Van Basshuysen (Ed.): Otto engine with direct injection. ATZ / MTZ book, Friedr. Vieweg & Sohn Verlag, Wiesbaden 2007, ISBN 978-3-8348-0202-6
  • Hütten, Helmut: Engines - Technology, Practice, History . Motorbuch Verlag, Stuttgart 1982, ISBN 3879433267

See also

Individual evidence

  1. Patent DE230517 : Internal combustion engine for liquid fuels. Registered on March 14, 1909 , applicant: Benz & Cie, Rheinische Gasmotorenfabrik AG, inventor: Prosper L'Orange.
  2. Automotive Engineer : Jonas Hesselman developed gasoline direct injection to help improve his dual-fuel engine ( Memento from July 5, 2015 in the Internet Archive ), July 29, 2011
  3. H. Kremser: The structure of high-speed internal combustion engines for motor vehicles and railcars . In: Hans List (Ed.): The internal combustion engine . tape 11 . Springer, Vienna 1942, ISBN 978-3-7091-5016-0 , p. 135–136 , doi : 10.1007 / 978-3-7091-5016-0 ( limited preview in Google Book search).
  4. Brochure for the TDI from Volkswagen 1995
  5. Chemnitz Industrial Museum: restored six-cylinder diesel test engine with common rail injection system from an IFA W50 L / S test vehicle. Loan from the August Horch Museum, Zwickau
  6. Fiat Croma TDi from 1987  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: dead link /  
  7. Successor engines see here
  8. Robert Bosch (Ed.): Diesel engine management: Systems and components , 4th edition, Springer, Wiesbaden, 2004, ISBN 9783528238735 , p. 33
  9. Richard van Basshuysen (Ed.): Otto engine with direct injection and direct injection: Otto fuels, natural gas, methane, hydrogen, 4th edition, Springer, Wiesbaden, 2017. ISBN 9783658122157 , p. 178
  10. Richard van Basshuysen (Ed.): Otto engine with direct injection - process · systems · development · potential , 3rd edition, Springer Vieweg, Wiesbaden, 2013, ISBN 9783658014087

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