Manifold injection

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Injection nozzle of a single point injection (Bosch Mono-Jetronic)

The intake manifold injection is a method of external mixture formation in internal combustion engines, particularly in gasoline engines . The fuel is injected into the intake manifold of the engine, where it mixes with the intake air. A fuel-air mixture is sucked in from the piston, in which air and fuel are evenly distributed in a ratio that allows complete combustion of the fuel without leaving any oxygen (homogeneous, stoichiometric mixture). The transition from mixture formation to intake is fluid. The injection can be controlled mechanically or electronically; it can be injected continuously or with interruptions ( intermittent ). In the 1970s and 1980s, intake manifold injection replaced the carburettor in the automotive sector , while since the turn of the millennium, automobile manufacturers have increasingly been using direct gasoline injection instead of intake manifold injection.

There are essentially two types, single-point and multi-point injection ; While single-point injection has only one injection nozzle for all cylinders immediately behind the throttle valve, with multi-point injection an injection nozzle is installed for each cylinder in front of the intake valve. In English, the terms Single Point Injection (SPI) and Multi Point Injection (MPI) are used to differentiate between these two types ; in German, the terms central injection or single point injection are also used for single point injection and single injection for multi-point injection. In the literature of the 1960s, the two types of manifold injection are described as "fuel injection into the intake line" and "fuel injection in front of the intake valve of each cylinder".


Working principle

In the case of intake manifold injection, the fuel is injected into the intake tract of the engine at a pressure of 70 to 1470 kPa (depending on the model) in order to atomize it to such an extent that an ignitable fuel-air mixture is formed when the intake valve opens and the Piston sucks. Gasoline engines with manifold injection work with quantitative mixture regulation , which means that to change the torque , i.e. when accelerating, only the amount of the fuel-air mixture sucked in is influenced, but not its composition. To do this, the sucked in air is throttled as much as necessary and fuel is added to match the mass of the air. The composition of the mixture must be regulated so that the fuel burns completely. The ratio of fuel to air, i.e. the air ratio ( lambda ), must be sufficient for the engine to run properly, and the three-way catalytic converter for exhaust gas purification only works optimally with this air ratio. For the injection system, the mass of the air drawn in must be measured precisely in order to always inject the correct mass of fuel, so that is. With purely mechanically controlled systems, it is not possible to set precisely enough, which is why modern systems have electronic control.


Single point injection

Engines with single-point injection are basically constructed like engines with carburetors and do not differ much from them. Instead of the carburetor as the atomizing element, a single injection nozzle is used, which injects into the throttle valve housing, the injection pressure is built up with a pump. This design offers almost no advantages over the carburetor, so that it was hardly used in the 1960s. With the advent of electronic control units in the 1980s, single-point systems with intermittent injection were built because, thanks to their very low injection pressure of around 70 to 100 kPa, they can be fed with an inexpensive electric fuel pump. A system like the Mono-Jetronic is also known as a mixture of carburetor and injection system. Single-point injections were introduced as a cost-effective option at the transition from the carburetor engine and are considered obsolete because they cannot be used to form the mixture precisely enough.

Multipoint injection

Historic six-cylinder in-line engine with multi-point injection (BMW M88): For each cylinder, the fuel is injected individually into the intake pipes (components with the screwed-on red plastic pipes), which is why there are six injection nozzles. An injection line (white pipes) leads from the injection pump to each injection nozzle.

With multi-point injection, the amount of fuel is metered individually to each cylinder, so an injection nozzle is installed in front of each intake valve. A multi-point injection system consists of an electrically driven fuel pump, a tank feed line, a fuel filter, a fuel distributor, the injection nozzles, a sensor for measuring the air mass and, in modern engines with electronic control, the engine control unit. Since the temperatures close to the intake valve are high, the intake process causes air turbulence and a relatively large amount of time is available for the mixture to form, the fuel does not have to be strongly atomized. Since the atomization quality is essentially dependent on the injection pressure, a lower injection pressure compared to direct injection is sufficient. The low injection pressure results in a relatively low relative speed between fuel and air, which results in relatively large, slowly evaporating fuel droplets. However, the point in time of the injection must be selected precisely enough to avoid unnecessarily high hydrocarbon emissions caused by fuel that has cooled down and is no longer burning properly. This is why each injection valve is activated individually in modern, electronically controlled multi-point injections. Modern manifold injection systems are electronically controlled multi-point systems with intermittent injection. Mechanical systems with continuous injection are considered technically obsolete.

Injection control

Mechanical injection control

Injection pump system Kugelfischer PL 004. The space cam is clearly visible here.

There are many different mechanical systems that function differently, three systems are briefly described below as examples.

Historical system Bosch

All cylinders are controlled together, so that (with a six-cylinder engine) an injection pump with only two pump plungers can be used. The engine map is mapped using a space cam. When accelerating, the throttle valve is opened and the room cam is rotated; it is moved axially by a centrifugal regulator. A roller scans the space cam, which moves the control rod of the injection pump with a lever via an intermediate lever. The intermediate lever can be used to make various corrections to the injection quantity using other control variables ( air pressure , cooling water temperature) if this is necessary.

System puffer fish

In the Kugelfischer system, too, the map is mapped using a space cam, but there is a separate injection pump piston for each cylinder and therefore each injection valve. The amount of fuel is determined by the throttle valve position and the crankshaft speed . A camshaft driven by a timing chain or toothed belt in time with the valve camshaft actuates the individual plungers of the injection pumps via cams; the start of the stroke of the injection plungers is adjusted using a control lever controlled by the space cam. The space cam itself is rotatably mounted on a shaft and is moved axially by a linkage from the throttle valve, the dependence of the injection quantity on the speed is ensured by a speed sensor.


In the K-Jetronic developed in the 1970s, fuel is continuously injected into the intake tract, the position of the intake valves is not taken into account during injection. It is not necessary to drive the K-Jetronic by means of a toothed belt from the crankshaft or camshaft of the engine, as the fuel is fed to the system via an electric feed pump at a pressure of around 500 kPa. For this reason, the system is called non-powered . A mechanical baffle plate air flow meter adjusts the amount of fuel to be injected via the fuel flow divider. The injection valves have no metering function on the K-Jetronic. In the mid-1990s, the K-Jetronic no longer met all of the exhaust gas regulations applicable at the time and disappeared when new passenger cars were developed.

Electronic regulation

Opened engine control unit of a Bosch LH Jetronic

With electronic control, there are no mechanical components that map the map; instead, an electronic control unit is used. Here the map is mapped using electronic circuits. The control unit processes signals from the individual sensors (e.g. air mass meter, engine speed sensor, temperature sensor, accelerator pedal position) in an integrated analog circuit or a digital computer and uses them to calculate the correct fuel mass. The control unit then sends an electrical signal to the injection nozzles and the injection pump so that the correct amount of fuel is injected.

Determination of the air mass

So that the fuel-air mixture can be set correctly, the mass of the air drawn in must be known as precisely as possible so that it can be used to calculate the correct amount of fuel. For this purpose, in modern systems, a so-called air mass meter is built into the intake manifold before the throttle valve. This sensor measures the air mass flow and sends an electrical signal to the engine control unit, from which the engine control unit calculates the correct mass of the fuel to be injected. As an alternative to the air mass sensor, there is the so-called intake manifold pressure sensor , which measures the negative pressure in the intake manifold and also sends an electrical signal to the engine control unit. The mass of the intake air can also be calculated from this signal, the throttle valve position and the crankshaft speed. Modern systems often have both. Electronic manifold injection systems of the 1970s (such as the L-Jetronic) were usually equipped with a so-called air flow meter . In even older mechanical systems, the air mass is only determined indirectly via the negative pressure in the intake manifold via a membrane, which acts as a correction variable on the control rod of the injection pump.



The injection nozzles have a relatively low opening pressure of about 100 to 1470 kPa because the atomization fineness does not have to be very high. In general, needle injection nozzles are used that have no sealing guide and therefore no leakage lines. Today's electromagnetic needle injection nozzles consist of a valve housing with a magnetic coil and a connection for controlling it, the valve seat with the injection disc and the valve needle with magnetic armature mentioned at the beginning, which can be moved by the magnetic field built up by the coil and lifted from the valve seat so that fuel comes out of the injection nozzle can escape. In the rest position, the valve needle is pressed onto the valve seat by a spring and the fuel pressure. A small screen filter is also built into the injection nozzle to protect the sensitive parts from dirt. An O-ring is installed on the upper and lower edge; they seal the injection nozzle from the fuel rail and the intake manifold. The injection nozzles must be positioned in such a way that the jet does not hit the wall of the suction line so that there is as little temperature transition as possible between the suction pipe wall and the fuel.

Jet cone

The injection jet has a decisive influence on the correct formation of the fuel-air mixture. There are three commonly used beam shapes, the cone beam, the two-beam and the tilted beam. The cone jet has a cone that is formed from the sum of all fuel jets. The engine control unit can adapt the angle of the cone beam to the requirements of the respective operating state. It is particularly suitable for engines with an inlet valve. The two-jet has two jet cones and is mainly used for engines with two inlet valves, where a jet cone sprays in front of an inlet valve. Here too, the engine control unit can adjust the jet cone angle. The tilted jet is a sharply inclined fuel jet that is used when the installation conditions are unfavorable.

Operating modes

There are two ways of injecting the fuel, continuously , i.e. without interruptions, and intermittently , i.e. with interruptions between individual injections. The operating mode is always the same for continuously injecting manifold injection, while four different operating modes are possible for intermittent injection:

Simultaneous injection

With simultaneous injection, all cylinders are injected simultaneously, regardless of the position of the intake valves. Since this does not result in an optimal injection point for some cylinders, the fuel quantity is divided into two halves and half the fuel quantity is injected once per crankshaft revolution. The time it takes for the fuel to evaporate is different for each cylinder.

Group injection

With group injection, two injection valves of an engine are combined into a group. One group injects the entire amount of fuel with every second crankshaft revolution; the groups are switched in such a way that an injection takes place alternately with each crankshaft revolution. In some areas of the map, it is thus possible to avoid injecting into the open intake port. In this operating mode, too, the evaporation time is different for each cylinder.

Sequential injection

With sequential injection, the fuel is injected for each cylinder individually at the right time in the ignition sequence, but the injection times are synchronized with each other: If the top dead center is considered for each piston individually, the injection takes place in exactly the same time period.

Individual cylinder injection

In the case of cylinder-specific injection, the injection time for each cylinder can be selected as required, independently of the other cylinders. It simplifies the compensation of irregularities in the cylinder charge, which can improve the emission behavior in the cold start phase . However, this operating mode means that the air ratio must be determined individually for each cylinder .

Advantages and disadvantages

Compared to a carburetor, manifold injection offers the advantage that there can be no more carburetor icing . Since the suction line no longer has to be heated and the suction line cross-section can also be selected larger, the cylinder filling increases and thus the performance of the engine. When the engine is overrun with the throttle valve closed, the fuel delivery can be cut off above idling speed ( overrun cut-off ), which lowers fuel consumption. Fuel metering and thus mixture preparation are significantly better with intake manifold injection compared to the carburettor, so that it works together with a three-way catalytic converter.

Compared to a carburetor, manifold injection has a higher construction cost, and maintenance is also more complicated; a historic manifold injection wears out faster than a carburetor. In terms of the pollutants it contains, the raw exhaust gas from gasoline engines is not significantly worse than that from intake manifold injectors, provided that the air ratio is the same for both engine types.

In engines with manifold injection, the mixture formation reacts more sensitively to pressure fluctuations, so that air pressure and temperature as well as cooling water temperature often have to be taken into account when regulating the engine speed, which makes manifold injections more complex than carburettors. Furthermore, the temperature of the injection pump must not be too high, since vapor bubbles can already form from a temperature of 40 ° C.

As with the carburettor, the main disadvantage of manifold injection is the principle-related mixture throttling, which considerably worsens the effective efficiency in the partial load range. Compared to engines with gasoline direct injection, intake manifold injection also has the disadvantage that the heat content of the mixture is greater, which has a negative effect on the filling and thus on performance. In addition, the knocking behavior, the idling behavior and the hydrocarbon emissions of an engine with intake manifold injection are worse compared to direct injection. Compared to direct injection, however, manifold injection offers lower particle and nitrogen oxide emissions, since less combustion than diffusion combustion takes place in a homogeneous mixture . The ignition system does not have to be very complex either, because the homogeneous mixture used in the intake manifold injection has good ignition properties.

Part of the fuel is deposited on the intake manifold wall as a film, particularly with central injection, which slowly evaporates. This film must be included in the exact metering of the fuel. The thickness of this film depends on the pressure in the intake manifold and thus the load status of the engine, which means that the air ratio can temporarily assume an unfavorable value in the event of dynamic load changes . In the warm-up phase of the engine in particular, this can lead to increased hydrocarbon emissions, and it also worsens the response behavior. In the start and the post-start phase, there is the problem that intake manifold injection engines have to be operated with a significantly under-stoichiometric mixture in order to ensure reliable engine operation; 300-400% of the full load amount of fuel is common. This does not completely burn the fuel, which leads to a drastic increase in hydrocarbon and carbon monoxide emissions. If no catalytic converter is installed or if it is not at operating temperature (300 ° C), the pollutants are not converted.

In order to increase the output through charge cooling at full load and to reduce the engine's tendency to knock , engines with intake manifold injection are enriched with intake-synchronous injection, which increases pollutant emissions.


The first engine with intake manifold injection was designed in 1884 by Johannes Spiel at Halleschen Maschinenfabrik. From 1898 Deutz built stationary engines with manifold injection, two-stroke engines from Grade and four-stroke aircraft engines from Wright and Antoinette with manifold injection were mass-produced in 1906. In 1912 Bosch equipped a boat engine with intake manifold injection, for which purpose a lubricating oil pump was converted into an injection pump, which however did not prove to be stable. In the 1920s, Bosch tried to adapt injection pumps for diesel engines to gasoline engines, but it did not succeed. The first functional intake manifold injection for land vehicle engines was built in 1930 by Moto Guzzi for a motorcycle engine. In spite of a number of developments that were not ready for series production from the 1930s to the 1950s, the incentives to build passenger car engines with intake manifold injection were rather small, as the effort was out of proportion to the benefits.

The Daimler-Benz AG began about 1950 with Bosch, one for sports and racing cars direct fuel injection to develop. Use was made (u. A. The construction of such aircraft engines to the collected as early as the mid-1930s experience 601 DB / 605 and DB 603 draw). For everyday passenger cars, however, Daimler-Benz relied on mechanical manifold injection, among others the 220 SE , 230 SL , 300 d and 300 SE were equipped with it. The Schweinfurt-based FAG Kugelfischer Georg Schäfer & Co. began in 1951 with the development of the Kugelfischer injection introduced in 1956 . It was built in larger numbers from 1959 and in the Peugeot 404 (1962), Lancia Flavia iniezione (1965), BMW 2000 tii (1969), Ford Capri RS 2600 (1970), BMW 2002 tii / turbo (1971/1973) and the BMW M1 (1978) used. The Münch-4 TTS-E 1200 (1973) also had this system. Lucas Industries introduced the Mk 2 intake manifold injection system in 1957, which went into series production in 1968 and Bendix developed an electronically controlled injection system called Electrojector around 1957 , which Bosch further developed into D-Jetronic and which was brought onto the market in 1967; At the end of the 1950s, a “stormy development in the construction of injection systems” began.

The D-Jetronic is a regulated multipoint system with intermittent injection using analog technology. Here the negative pressure in the intake manifold was used as a manipulated variable. From 1973 there were the L- and K-Jetronic. While the K-Jetronic works mechanically-hydraulically with continuous injection, the L-Jetronic has similar properties to the D-Jetronic, with the difference that the amount of air sucked in and no longer the intake manifold vacuum is used as the controlled variable. Due to the good developments in intake manifold injection, hardly any further development approaches for gasoline direct injection were advanced in the 1960s and 1970s, so that for a long time intake manifold injection was the predominant injection system in passenger car gasoline engines.

In cooperation with BMW, Bosch introduced the microprocessor-controlled Motronic in 1979 , which for the first time depicts ignition and injection in a three-dimensional map. American manufacturers such as Ford and General Motors initially relied on electronically controlled central injection in the early 1980s. The LH-Jetronic , introduced in 1981, corresponds to the L-Jetronic, but works with a digital engine control unit and air mass measurement. The K-Jetronic has been replaced by the KE-Jetronic , which works like a normal K-Jetronic, but has electronically controlled additional functions. Together with the liquid-cooled boxer engine in the T3, the VW Group introduced a microprocessor-controlled intake manifold injection with the brand name Digijet , which was merged with the map ignition in a next development step . The resulting system went into series production in 1985 as Digifant .

The Mono-Jetronic was introduced in 1987 as an inexpensive single-point system, which injects intermittently and calculates the amount of air using only the throttle valve angle and engine speed. Only cost-effective systems that could also be used economically in medium-sized and small cars ensured that manifold injection became widespread. Since modern exhaust gas limit values ​​in gasoline engines can only be achieved with a three-way catalytic converter and the necessary precision of the fuel-air mixture can only be guaranteed with an injection system, manifold injection finally replaced the carburettor in the automotive sector. In September 1995, Mitsubishi first introduced a common rail system for gasoline direct injection. Since then, manifold injection has been increasingly displaced by gasoline direct injection, whereby the process of market penetration is slower than the displacement of indirect injection through direct injection in diesel engines.

Brand names and manufacturers

Many vehicle manufacturers use their own systems and brand names for intake manifold injection, such as Multec from General Motors . Many supplier companies offer their own, manufacturer-independent injection systems, which, depending on the manufacturer and vehicle, are usually adopted into series production with minor modifications. Such systems were and are produced by Bosch under the brand name Jetronic as well as by Mikuni , Keihin, Dell'Orto , Delphi, Magneti Marelli , Kugelfischer , Spica ( Società Pompe Iniezione Cassani & Affini , now part of Delphi Automotive ) and Lucas Industries .

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