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Manifold injection
Manifold injection
Manifold injection
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Manifold injection is a mixture formation system for internal combustion engines with external mixture formation. It is commonly used in engines with spark ignition that use petrol as fuel, such as the Otto engine, and the Wankel engine. In a manifold-injected engine, the fuel is injected into the intake manifold, where it begins forming a combustible air-fuel mixture with the air. As soon as the intake valve opens, the piston starts sucking in the still forming mixture. Usually, this mixture is relatively homogeneous, and, at least in production engines for passenger cars, approximately stoichiometric; this means that there is an even distribution of fuel and air across the combustion chamber, and enough, but not more air present than what is required for the fuel's complete combustion. The injection timing and measuring of the fuel amount can be controlled either mechanically (by a fuel distributor), or electronically (by an engine control unit). Since the 1970s and 1980s, manifold injection has been replacing carburettors in passenger cars. However, since the late 1990s, car manufacturers have started using petrol direct injection, which caused a decline in manifold injection installation in newly produced cars.

There are two different types of manifold injection:

  • the multi-point injection (MPI) system, also known as port injection, or dry manifold system
  • and the single-point injection (SPI) system, also known as throttle-body injection (TBI), central fuel injection (CFI), electronic gasoline injection (EGI), and wet manifold system

In this article, the terms multi-point injection (MPI), and single-point injection (SPI) are used. In an MPI system, there is one fuel injector per cylinder, installed very close to the intake valve(s). In an SPI system, there is only a single fuel injector, usually installed right behind the throttle valve. Modern manifold injection systems are usually MPI systems; SPI systems are now considered obsolete.

Description

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Continuously injecting mechanical MPI system Bosch K-Jetronic (ca. 1980s)

The part on the right with red fuel lines coming out of it is the fuel distributor; the part on the left is a vacuum-driven piston used for determining the amount of air currently sucked into the engine

In a manifold injected engine, the fuel is injected with relatively low pressure (70...1470 kPa) into the intake manifold to form a fine fuel vapour. This vapour can then form a combustible mixture with the air, and the mixture is sucked into the cylinder by the piston during the intake stroke. Otto engines use a technique called quantity control for setting the desired engine torque, which means that the amount of mixture sucked into the engine determines the amount of torque produced. For controlling the amount of mixture, a throttle valve is used, which is why quantity control is also called intake air throttling. Intake air throttling changes the amount of air sucked into the engine, which means that if a stoichiometric () air-fuel mixture is desired, the amount of injected fuel has to be changed along with the intake air throttling. To do so, manifold injection systems have at least one way to measure the amount of air that is currently being sucked into the engine. In mechanically controlled systems with a fuel distributor, a vacuum-driven piston directly connected to the control rack is used, whereas electronically controlled manifold injection systems typically use an airflow sensor, and a lambda sensor. Only electronically controlled systems can form the stoichiometric air-fuel mixture precisely enough for a three-way catalyst to work sufficiently, which is why mechanically controlled manifold injection systems such as the Bosch K-Jetronic are now considered obsolete.[1]

Main types

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Single-point injection

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Single-point injection fuel injector of a Bosch Mono-Jetronic (ca. 1990s)

As the name implies, a single-point injected (SPI) engine only has a single fuel injector. It is usually installed right behind the throttle valve in the throttle body. Single-point injection was a relatively low-cost way for automakers to reduce exhaust emissions to comply with tightening regulations while providing better "driveability" (easy starting, smooth running, freedom from hesitation) than could be obtained with a carburetor. Many of the carburetor's supporting components - such as the air cleaner, intake manifold, and fuel line routing - could be used with few or no changes. This postponed the redesign and tooling costs of these components. However, single-point injection does not allow forming very precise mixtures required for modern emission regulations, and is thus deemed an obsolete technology in passenger cars.[1] Single-point injection was used extensively on American-made passenger cars and light trucks during 1980–1995, and in some European cars in the early and mid-1990s.

Single-point injection has been a known technology since the 1960s, but has long been considered inferior to carburettors, because it requires an injection pump, and is thus more complicated.[2] Only with the availability of inexpensive digital engine control units (ECU) in the 1980s did single-point injection become a reasonable option for passenger cars. Usually, intermittently injecting, low injection pressure (70...100 kPa) systems were used that allowed the use of low-cost electric fuel injection pumps.[3] A very common single-point injection system used in many passenger cars is the Bosch Mono-Jetronic, which German motor journalist Olaf von Fersen considers a "combination of fuel injection and carburettor".[4]

The system was called Throttle-body Injection or Digital Fuel Injection by General Motors, Central Fuel Injection by Ford, PGM-CARB by Honda, and EGI by Mazda).

Multi-point injection

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Straight-six engine BMW M88

This example shows the basic layout of a multi-point injected engine – each cylinder is fitted with its own fuel injector, and each fuel injector has its own fuel line (white parts) going straight into the fuel injection pump (mounted on the right hand side)

In a multi-point injected engine, every cylinder has its own fuel injector, and the fuel injectors are usually installed in close proximity to the intake valve(s). Thus, the injectors inject the fuel through the open intake valve into the cylinder, which should not be confused with direct injection. Certain multi-point injection systems also use tubes with poppet valves fed by a central injector instead of individual injectors. Typically though, a multi-point injected engine has one fuel injector per cylinder, an electric fuel pump, a fuel distributor, an airflow sensor,[5] and, in modern engines, an engine control unit.[6] The temperatures near the intake valve(s) are rather high, the intake stroke causes intake air swirl, and there is much time for the air-fuel mixture to form.[7] Therefore, the fuel does not require much atomisation.[2] The atomisation quality is relative to the injection pressure, which means that a relatively low injection pressure (compared with direct injection) is sufficient for multi-point injected engines. A low injection pressure results in a low relative air-fuel velocity, which causes large, and slowly vapourising fuel droplets.[8] Therefore, the injection timing has to be precise to minimise unburnt fuel (and thus HC emissions). Because of this, continuously injecting systems such as the Bosch K-Jetronic are obsolete.[1] Modern multi-point injection systems use electronically controlled intermittent injection instead.[6]

From 1992 to 1996 General Motors implemented a system called Central Port Injection or Central Port Fuel Injection. The system uses tubes with poppet valves from a central injector to spray fuel at each intake port rather than the central throttle body[citation needed]. Fuel pressure is similar to a single-point injection system. CPFI (used from 1992 to 1995) is a batch-fire system, while CSFI (from 1996) is a sequential system.[9]

Injection controlling mechanism

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In manifold injected engines, there are three main methods of metering the fuel, and controlling the injection timing.

Mechanically controlled

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Mechanic fuel injection pump system "Kugelfischer"

This system uses a three-dimensional cam

In early manifold injected engines with fully mechanical injection systems, a gear-, chain- or belt-driven injection pump with a mechanic "analogue" engine map was used. This allowed injecting fuel intermittently, and relatively precisely. Typically, such injection pumps have a three-dimensional cam that depicts the engine map. Depending on the throttle position, the three-dimensional cam is moved axially on its shaft. A roller-type pick-up mechanism that is directly connected to the injection pump control rack rides on the three-dimensional cam. Depending upon the three-dimensional cam's position, it pushes in or out the camshaft-actuated injection pump plungers, which controls both the amount of injected fuel, and the injection timing. The injection plungers both create the injection pressure, and act as the fuel distributors. Usually, there is an additional adjustment rod that is connected to a barometric cell, and a cooling water thermometer, so that the fuel mass can be corrected according to air pressure, and water temperature.[10] Kugelfischer injection systems also have a mechanical centrifugal crankshaft speed sensor.[11] Multi-point injected systems with mechanical controlling were used until the 1970s.

Not injection-timing controlled

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In systems without injection-timing controlling, the fuel is injected continuously, thus, no injection timing is required. The biggest disadvantage of such systems is that the fuel is also injected when the intake valves are closed, but such systems are much simpler and less expensive than mechanical injection systems with engine maps on three-dimensional cams. Only the amount of injected fuel has to be determined, which can be done very easily with a rather simple fuel distributor that is controlled by an intake manifold vacuum-driven airflow sensor. The fuel distributor does not have to create any injection pressure, because the fuel pump already provides pressure sufficient for injection (up to 500 kPa). Therefore, such systems are called unpowered, and do not need to be driven by a chain or belt, unlike systems with mechanical injection pumps. Also, an engine control unit is not required.[12] Unpowered multi-point injection systems without injection-timing controlling such as the Bosch K-Jetronic were commonly used from the mid-1970s until the early 1990s in passenger cars, although examples had existed earlier, such as the Rochester Ramjet offered on high-performance versions of the Chevrolet small-block engine from 1957 to 1965.

Electronic control unit

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Bosch LH-Jetronic

An electronic engine control unit has an engine map stored in its ROM and uses it as well as sensor data to determine how much fuel has to be injected, and when the fuel has to be injected

Engines with manifold injection, and an electronic engine control unit are often referred to as engines with electronic fuel injection (EFI). Typically, EFI engines have an engine map built into discrete electronic components, such as read-only memory. This is both more reliable and more precise than a three-dimensional cam. The engine control circuitry uses the engine map, as well as airflow, throttle valve, crankshaft speed, and intake air temperature sensor data to determine both the amount of injected fuel, and the injection timing. Usually, such systems have a single, pressurised fuel rail, and injection valves that open according to an electric signal sent from the engine control circuitry. The circuitry can either be fully analogue, or digital. Analogue systems such as the Bendix Electrojector were niche systems, and used from the late 1950s until the early 1970s; digital circuitry became available in the late 1970s, and has been used in electronic engine control systems since. One of the first widespread digital engine control units was the Bosch Motronic.[13]

Air mass determination

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In order to mix air and fuel correctly so a proper air-fuel mixture is formed, the injection control system needs to know how much air is sucked into the engine, so it can determine how much fuel has to be injected accordingly. In modern systems, an air-mass meter that is built into the throttle body meters the air mass, and sends a signal to the engine control unit, so it can calculate the correct fuel mass. Alternatively, a manifold vacuum sensor can be used. The manifold vacuum sensor signal, the throttle position, and the crankshaft speed can then be used by the engine control unit to calculate the correct amount of fuel. In modern engines, a combination of all these systems is used.[5] Mechanical injection controlling systems as well as unpowered systems typically only have an intake manifold vacuum sensor (a membrane or a sensor plate) that is mechanically connected to the injection pump rack or fuel distributor.[14]

Injection operation modes

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Manifold injected engines can use either continuous or intermittent injection. In a continuously injecting system, the fuel is injected continuously, thus, there are no operating modes. In intermittently injecting systems however, there are usually four different operating modes.[15]

Simultaneous injection

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In a simultaneously intermittently injecting system, there is one single, fixed injection timing for all cylinders. Therefore, the injection timing is ideal only for some cylinders; there is always at least one cylinder that has its fuel injected against the closed intake valve(s). This causes fuel evaporation times that are different for each cylinder.

Group injection

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Systems with intermittent group injection work similarly to the simultaneously injection systems mentioned earlier, except that they have two or more groups of simultaneously injecting fuel injectors. Typically, a group consists of two fuel injectors. In an engine with two groups of fuel injectors, there is an injection every half crankshaft rotation, so that at least in some areas of the engine map no fuel is injected against a closed intake valve. This is an improvement over a simultaneously injecting system. However, the fuel evaporation times are still different for each cylinder.

Sequential injection

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In a sequentially injecting system, each fuel injector has a fixed, correctly set, injection timing that is in sync with the spark plug firing order, and the intake valve opening. This way, no more fuel is injected against closed intake valves.

Cylinder-specific injection

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Cylinder-specific injection means that there are no limitations to the injection timing. The injection control system can set the injection timing for each cylinder individually, and there is no fixed synchronisation between each cylinder's injector. This allows the injection control unit to inject the fuel not only according to firing order, and intake valve opening intervals, but it also allows it to correct cylinder charge irregularities. This system's disadvantage is that it requires cylinder-specific air-mass determination, which makes it more complicated than a sequentially injecting system.

History

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The first manifold injection system was designed by Johannes Spiel at Hallesche Maschinenfabrik.[16] Deutz started series production of stationary four-stroke engines with manifold injection in 1898. Grade built the first two-stroke engine with manifold injection in 1906; the first manifold injected series production four-stroke aircraft engines were built by Wright and Antoinette the same year (Antoinette 8V).[17] In 1912, Bosch equipped a watercraft engine with a makeshift injection pump built from an oil pump, but this system did not prove to be reliable. In the 1920s, they attempted to use a diesel engine injection pump in a petrol-fuelled Otto engine. However, they were not successful. In 1930 Moto Guzzi built the first manifold injected Otto engine for motorcycles, which eventually was the first land vehicle engine with manifold injection.[18] From the 1930s until the 1950s, manifold injections systems were not used in passenger cars, despite the fact that such systems existed. This was because the carburetor proved to be a simpler and less expensive, yet sufficient mixture formation system that did not need replacing yet.[14]

In ca. 1950, Daimler-Benz started development of a petrol direct injection system for their Mercedes-Benz sports cars. For passenger cars however, a manifold injection system was deemed more feasible.[14] Eventually, the Mercedes-Benz W 128, W 113, W 189, and W 112 passenger cars were equipped with manifold injected Otto engines.[19][20]

From 1951 until 1956, FAG Kugelfischer Georg Schäfer & Co. developed the mechanical Kugelfischer injection system.[18] It was used in many passenger cars, such as the Peugeot 404 (1962), Lancia Flavia iniezione (1965), BMW E10 (1969), Ford Capri RS 2600 (1970), BMW E12 (1973), BMW E20 (1973), and the BMW E26 (1978).[21]

In 1957, Bendix Corporation presented the Bendix Electrojector, one of the first electronically controlled manifold injection systems.[22] Bosch built this system under licence, and marketed it from 1967 as the D-Jetronic.[21] In 1973, Bosch introduced their first self-developed multi-point injection systems, the electronic L-Jetronic, and the mechanical, unpowered K-Jetronic.[23] Their fully digital Motronic system was introduced in 1979. It found widespread use in German luxury saloons. At the same time, most American car manufacturers stuck to electronic single-point injection systems.[24] In the mid-1980s, Bosch upgraded their non-Motronic multi-point injection systems with digital engine control units, creating the KE-Jetronic, and the LH-Jetronic.[23] Volkswagen developed the digital Digijet injection system for their "Wasserboxer" water-cooled engines, which evolved into the Volkswagen Digifant system in 1985.[4]

Cheap single-point injection systems that worked with either two-way or three-way catalyst converters, such as the Mono-Jetronic introduced in 1987,[23] enabled car manufacturers to economically offer an alternative to carburetors even in their economy cars, which helped the extensive spread of manifold injection systems across all passenger car market segments during the 1990s.[25] In 1995, Mitsubishi introduced the first petrol direct injection Otto engine for passenger cars, and the petrol direct injection has been replacing the manifold injection since, but not across all market segments; several newly produced passenger car engines still use multi-point injection.[26]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Manifold injection

View on Grokipedia
from Grokipedia
Manifold injection, also known as port fuel injection (PFI) or multi-point fuel injection (MPFI), is a delivery system used in spark-ignition internal combustion engines, where is injected directly into the intake ports near each cylinder's inlet valve, enabling thorough mixing with incoming air before the mixture enters the during the intake stroke. This approach contrasts with direct injection by occurring outside the , typically at low pressures around 6 bar, and relies on electronic control units to precisely time and meter delivery based on engine load and speed. The system consists of key components including fuel injectors mounted in the intake manifold, a low-pressure , a fuel rail for distribution, and sensors integrated with an (ECU) that adjusts injection timing for optimal air-fuel ratios. Introduced as a replacement for carburetors in the late , manifold injection became widespread in passenger vehicles by the and due to its ability to improve , reduce emissions, and enhance engine performance compared to earlier mechanical systems. Among its advantages, manifold injection offers reliable operation with good mixture homogenization, lower particulate matter emissions, reduced noise, and better tolerance for lower-quality fuels, making it cost-effective for engines achieving up to 60 kW/l specific power through downsizing strategies. It also minimizes deposit formation on injectors and valves since fuel is not exposed to temperatures, unlike direct injection systems, though it provides less precise control over fuel stratification and may limit maximum power output. Despite the rise of (GDI) for stricter emissions standards by the , manifold injection remains prevalent in global engines for its simplicity and lower manufacturing costs.

Fundamentals

Definition and basic principles

Manifold injection is a fuel delivery method used in internal combustion engines, particularly spark-ignition engines, where is injected into the intake manifold, allowing it to mix with incoming air before the mixture enters the cylinders for . This process replaces earlier mechanical mixing systems and ensures a controlled introduction of into the air stream. The intake manifold serves as a critical prerequisite component in this system, functioning to evenly distribute the incoming air (and subsequently the fuel-air ) from the throttle body to the individual intake ports leading to each . By channeling air through a series of branched passages, the manifold promotes uniform flow to optimize engine performance and prevent uneven filling, which precedes the phase where the prepared is ignited. At its core, manifold injection operates on principles of fuel atomization, , and formation. is sprayed through injectors positioned in or near the intake ports of the manifold, breaking the liquid into fine droplets that atomize upon contact with the high-velocity . These droplets then vaporize within the warmer environment of the manifold, aided by heat from the , to form a homogeneous air- that is drawn into the chambers during the . In a typical manifold layout, the structure features a central plenum connected to runner tubes leading to each cylinder's port, with injectors mounted upstream of the valves to facilitate even distribution and thorough mixing driven by intake turbulence. This approach is suited for spark-ignition , where creating a uniform, premixed charge supports efficient ; the ideal stoichiometric air- ratio for is approximately 14.7:1 by mass, ensuring complete fuel oxidation without excess air or unburned hydrocarbons. Compared to carburetors, manifold injection provides more precise fuel metering for improved efficiency.

Comparison to other fuel injection systems

Manifold injection, also known as port fuel injection (PFI), delivers into the intake ports near the intake valves, enabling indirect mixture formation in the intake manifold. In contrast, direct injection (GDI) sprays directly into the at much higher pressures, typically 100-200 bar compared to 3-5 bar for PFI, allowing for stratified charge operation and improved control but requiring more complex high-pressure pumps and injectors. Throttle body injection (TBI), a single-point , injects upstream at the throttle body, resulting in less precise distribution and atomization across cylinders than multi-point manifold injection, which leads to inferior air- ratio control and power output. Compared to carburetion, which relies on mechanical venturi effects for passive fuel-air mixing, manifold injection uses electronic metering for precise delivery, yielding better throttle response, economy, and adaptability to varying conditions without the need for manual adjustments. Performance trade-offs of manifold injection include its simpler, lower-cost design with fewer components than GDI, making it easier to manufacture and maintain, though it is susceptible to wall-wetting where films form on ports and valves, potentially causing incomplete and transient rich mixtures during . In terms of efficiency, manifold injection exhibits higher (BSFC), approximately 10-13% worse than GDI at part-load conditions, due to less optimal charge cooling and stratification, though it avoids the carbon deposits on valves common in GDI systems. Manifold injection was widely applied in engines from the through the . Its use has declined since the in many regulated markets in favor of GDI or dual-injection systems to meet stricter fuel economy and CO2 standards, though it remains in use globally as of 2025, particularly in cost-sensitive applications. Environmentally, it produces lower emissions than GDI owing to evaporative cooling of the charge that reduces temperatures, but it generates higher hydrocarbon (HC) emissions from wall-wetting and incomplete mixing, contributing to elevated unburned fuel in the exhaust.

Types

Single-point injection

Single-point injection, also known as throttle body injection (TBI), features a centralized design where a single fuel injector—or occasionally two for V-configured engines—is mounted directly in the throttle body assembly, positioned similarly to a traditional . This injector sprays into the incoming air stream within the throttle body, creating a fuel-air mixture that flows through the common intake manifold to distribute to all cylinders. The system relies on a fuel rail or accumulator to supply pressurized to the injector, with the throttle body housing additional components like the idle air control valve and for basic regulation. In operation, fuel delivery is primarily synchronized with position and engine load, using an (ECU) to pulse the intermittently based on inputs from sensors such as angle, temperature, and manifold absolute . Pressure regulators maintain a constant fuel supply, typically at 0.7-1 bar (10-15 psi) for most TBI systems. A representative example is the Bosch Mono-Jetronic system, introduced in 1983, which employs a solenoid-operated for intermittent metering and includes a servo-motor for idle control, enabling straightforward adaptation to four-cylinder engines without complex per-cylinder timing. This configuration offers key advantages, including mechanical simplicity with fewer components than multi-point systems, resulting in lower manufacturing and maintenance costs. Its design facilitates easy onto carbureted engines, requiring minimal modifications to the intake manifold or wiring harness, which made it a practical upgrade for emissions compliance in the transition era from mechanical to electronic fuel delivery. However, single-point injection has notable limitations, particularly uneven fuel distribution across cylinders due to the shared manifold path, which worsens at idle or under high-load conditions where wall wetting and can occur. Poor fuel atomization from the centralized spraying also contributes to higher hydrocarbon and emissions compared to more precise systems, limiting its efficiency in demanding applications. Although concepts date back to the in experimental automotive designs, single-point injection saw widespread adoption in economy cars, such as those from and Ford, as a cost-effective means to meet tightening U.S. emissions standards before being largely supplanted by multi-point systems.

Multi-point injection

Multi-point injection, also known as port injection, features a dedicated positioned in the intake port immediately upstream of each cylinder's intake valve, enabling precise delivery tailored to individual cylinders. This operates at pressures typically ranging from 3 to 5 bar, which promotes finer atomization of the spray for improved and mixing with incoming air. Compared to single-point injection, where is distributed centrally from a single throttle-body , this per-cylinder setup serves as a decentralized that enhances mixture uniformity across the . In operation, the injectors pulse fuel directly into the respective intake ports during the intake stroke, minimizing wall-wetting by limiting the distance fuel travels before reaching the and reducing liquid film formation on port surfaces. This sequential or grouped pulsing allows for optimized air-fuel ratios per cylinder, with systems like employing electromagnetic solenoid injectors to control injection duration based on engine conditions. Similar implementations appear in Delphi's multi-port systems, which integrate comparable port-mounted injectors for engines. The primary advantages stem from this individualized delivery, yielding even fuel distribution that supports more complete and better cold-start through targeted enrichment without excess overall fueling. Emissions benefits include reduced unburned hydrocarbons (HC), with multi-point systems achieving lower HC levels compared to single-point setups due to enhanced atomization and reduced wall-wetting. This configuration became standard in passenger vehicles to comply with Euro 2 and Euro 3 emissions standards, which demanded tighter controls on pollutants like HC and CO. Despite these gains, multi-point injection introduces higher complexity from the multiple injectors and associated , elevating and costs relative to simpler systems. Additionally, the finer spray patterns increase susceptibility to injector from impurities or deposits, necessitating regular filter and high-quality .

Control Systems

Mechanical control methods

Mechanical control methods in manifold injection systems rely on purely analog hardware to meter and deliver into the manifold without electronic computation, predominant in pre-ECU automotive applications during the mid-20th century. These systems typically employ throttle-linked mechanical injectors, where delivery is modulated by the position of an air flow sensor plate or diaphragm directly connected to the , ensuring flow scales with air volume. A key example is the Bosch K-Jetronic continuous injection system, which uses a mechanical distributor to allocate proportionally to airflow, with injectors maintaining constant open flow into the manifold ports ahead of the . Fuel metering in these systems often incorporates vacuum or pressure-based mechanisms, such as a control that maintains differential pressure (typically around 3-5 bar system pressure with lower control pressure) to balance fuel delivery against air flow. Operationally, the —often electric in continuous-flow systems like K-Jetronic—supplies pressurized (around 5 bar for ) to the metering components, with excess fuel recirculated to maintain system pressure. Regulation occurs through the air flow sensor plate in the metering head, where the position deflects based on air volume to adjust fuel distribution proportionally, providing basic compensation for load through air mass but limited adaptability to transient conditions like or altitude variations, resulting in higher fuel consumption compared to electronic systems. These methods offer advantages in reliability, particularly in harsh environments like applications, as they eliminate electronic components vulnerable to failure from , , or electrical issues. However, limitations include poor adaptability to varying conditions, leading to suboptimal air-fuel ratios across operating conditions. Mechanical control methods were used in early manifold injection applications, such as in 1970s and 1980s vehicles before widespread electronic adoption.

Electronic control systems

Electronic control systems for manifold injection rely on an (ECU) that processes inputs from key sensors to precisely manage delivery. The ECU integrates signals from mass air flow (MAF) or manifold absolute pressure (MAP) sensors to determine air intake, oxygen sensors in the exhaust to monitor air-fuel ratio, and crankshaft position sensors to track engine speed and timing. These inputs enable the ECU to calculate and output pulse-width modulated signals to the fuel injectors, controlling the duration each injector remains open to deliver the appropriate volume. In operation, the ECU employs closed-loop feedback using data to maintain a stoichiometric air- ratio of = 1 under normal conditions, adjusting delivery in real-time to optimize efficiency and reduce emissions. algorithms update trims based on long-term sensor feedback, compensating for variations in quality, , or component wear to ensure consistent performance. Common control algorithms include speed-density, which uses and speed to estimate air mass, and alpha-N, which relies on throttle position and speed for load , with speed-density preferred for its accuracy in naturally aspirated engines. These systems provide precise fuel metering with accuracies typically within ±2%, enabling better and power output compared to mechanical methods. Integration with (OBD-II) supports emissions compliance by monitoring and reporting deviations in real-time, facilitating regulatory standards for reduced pollutants. Despite their advantages, electronic control systems incur higher costs due to the complexity of and processing hardware, and they remain vulnerable to electrical faults, such as wiring issues or failures, which can disrupt fuel delivery. Their widespread adoption began in the 1980s, driven by electronic fuel injection (EFI) mandates in to meet stringent emissions requirements. The core of control is the calculation, which determines the time τ the is energized per cycle. The basic is: τ=mfqi×K\tau = \frac{m_f}{q_i} \times K where mfm_f is the required per cycle, qiq_i is the flow rate (typically in per unit time), and KK incorporates correction factors for battery voltage, dead time, and environmental conditions. To derive this, first compute mfm_f from mam_a (measured via MAF or estimated via speed-density) and target air- ratio (AFR): mf=ma/AFRm_f = m_a / \text{AFR}. The flow rate qiq_i is calibrated under standard conditions, adjusted for drops. Correction factors include voltage compensation (since flow decreases at lower voltages) and lag (dead time, often 0.5-2 ms). For closed-loop operation, short-term trims from oxygen sensors multiply KK to fine-tune toward = 1, while long-term adaptive trims update base maps. This derivation ensures metering precision across operating conditions.

Operation and Fuel Delivery

Air mass and fuel metering

In manifold injection systems, the air mass entering the engine is measured to ensure precise fuel delivery for optimal . Two primary methods are employed: direct measurement using mass air flow (MAF) sensors or indirect calculation via manifold absolute pressure (MAP) sensors combined with engine speed. MAF sensors, commonly based on the hot-wire anemometer principle, directly quantify the mass of air flowing into the intake manifold in grams per second (g/s). A thin wire is electrically heated to a constant , and as air passes over it, the cooling effect requires additional current to maintain the ; this current is proportional to the air mass flow rate, providing a direct and accurate reading independent of air density variations. In contrast, MAP sensors measure the absolute within the intake manifold, typically outputting a voltage signal corresponding to levels from to atmospheric. The speed-density method then computes air mass by integrating manifold , intake air temperature, , and rotational speed (RPM) to estimate and air . This approach is less direct but allows for a simpler intake design without airflow restrictions. Once is determined, metering occurs through stoichiometric calculations performed by the (ECU). The required mass is given by mf=maAFRm_f = \frac{m_a}{\text{AFR}}, where mfm_f is mass, mam_a is , and AFR is the target air- ratio, typically 14.7:1 by mass for to achieve complete with minimal emissions. Adjustments for environmental factors are essential to maintain accuracy. air temperature (IAT) sensors provide data to correct air density, as warmer air is less dense and requires less , while barometric pressure (inferred from MAP or a dedicated sensor) compensates for altitude effects, where lower reduces and thus delivery. These ensure the system adapts to conditions like high-altitude operation or temperature swings without lean or rich mixtures. The (TPS), a mounted on the shaft, integrates with air metering by signaling the ECU about throttle angle, from 0% at idle to 100% at wide-open . This supplements MAF or data for detecting rapid load changes, enabling quick fuel adjustments during . For instance, in many systems, the TPS outputs 0.5-4.5 volts, linearly increasing with throttle opening to indicate driver demand. MAF sensors often produce a nonlinear voltage output curve relative to air flow; at zero flow (engine off), the signal is approximately 0.98-1.02 V, rising to 2-3 V at idle and up to 4-5 V at high loads, calibrated specifically for the sensor's transfer function. In the 1990s, multi-point manifold injection systems increasingly transitioned from MAP-based speed-density to MAF metering, particularly in vehicles like the 1994 Chevrolet Corvette, to achieve better transient response during sudden throttle changes and improved fuel economy. Fundamentally, the air mass flow rate into the manifold is governed by the equation m˙a=ρ⋅V⋅A\dot{m}_a = \rho \cdot V \cdot A where m˙a\dot{m}_a is the mass flow rate (g/s), ρ\rho is air density (derived from temperature and pressure), VV is the air velocity through the throttle and manifold passages, and AA is the effective cross-sectional area of the flow path. In manifold injection, this accounts for restrictions like the throttle plate and intake geometry, with MAF sensors measuring the product directly and speed-density systems estimating it from manifold dynamics.

Injection timing and modes

In manifold injection systems, fuel delivery operates through distinct modes that determine how and when injectors activate relative to engine cycles. Simultaneous injection fires all injectors at the same time during each engine revolution, regardless of individual cylinder positions, simplifying control but potentially leading to uneven fuel distribution across cylinders. Grouped or batch injection activates injectors in sets, such as by cylinder banks (e.g., pairs or groups of two to four in multi-cylinder engines), allowing partial synchronization with intake events while reducing hardware complexity compared to full individual control. Sequential injection, in contrast, pulses each injector independently and timed precisely to the intake stroke of its assigned cylinder, optimizing fuel placement near the intake valve for better atomization and mixing. Cylinder-specific injection extends this by tailoring pulse characteristics to individual cylinder demands, often incorporating real-time adjustments for variations in load or combustion events, though it requires advanced electronic coordination. Injection timing in these systems is governed by the (ECU), which advances or retards the start of injection based on engine load and speed, using signals from and position sensors to align pulses with valve events. Dwell time, the duration of injector opening, is adjusted dynamically to deliver the precise volume, typically ranging from milliseconds at to longer pulses under high load, ensuring complete evaporation in the port. The injection timing angle, denoted as θ\theta, represents the position (in degrees before or after top dead center) at which injection begins and is fundamentally a function of engine RPM and load: θ=f(RPM,load)\theta = f(\text{RPM}, \text{load}), often mapped relative to valve timing to maximize charge homogeneity while avoiding wall wetting. Operationally, sequential and cylinder-specific modes offer advantages in combustion stability and fuel economy, with studies indicating up to 16% gains in brake thermal efficiency over conventional modes due to improved air-fuel mixing and reduced fuel puddling in the manifold. These benefits stem from precise synchronization, which enhances and lowers unburned hydrocarbons, though challenges arise in engines with uneven firing orders (e.g., certain V-engine configurations), where phase mismatches can cause fluctuations if cam-crank signal accuracy is insufficient. Sequential modes became standard in electronic manifold injection systems in the 1990s, driven by stricter emissions regulations, as they enable better control of hydrocarbons and without relying on continuous flow variants common in older mechanical setups. In mechanical systems, timing lacks such precision, often using fixed or throttle-linked mechanisms that do not adapt to per-cylinder events.

Historical Development

Early innovations and adoption

The origins of manifold injection trace back to early 20th-century experiments aimed at improving fuel delivery in internal combustion engines. Robert Bosch GmbH began investigating gasoline injection systems as early as 1912, drawing on their expertise in high-pressure diesel pumps developed in the 1920s, though initial adaptations for Otto-cycle engines proved challenging and unsuccessful due to issues with metering and vaporization. These efforts laid foundational knowledge for precise fuel atomization, primarily tested in stationary and aviation applications before automotive use. By the mid-20th century, the first practical gasoline injection system emerged in 1951, when Bosch introduced a single-point setup in the Goliath GP 700, a two-stroke passenger car engine showcased at the Frankfurt International Motor Show; this mechanical system marked an early step toward replacing carburetors with more controlled fuel delivery. Key milestones in the and accelerated the shift to electronic manifold injection. In 1967, Bosch introduced the D-Jetronic system, the world's first production electronic multi-point fuel injection for gasoline engines, which debuted in 1968 on the and 250CE; it used manifold pressure sensors to meter fuel dynamically, improving efficiency and power over mechanical predecessors. In the United States, adoption gained momentum in the late amid emissions pressures, with systems like Ford's EEC-I (Electronic Engine Control), introduced in 1978 and used on 1979 California-market vehicles such as the to optimize carbureted engines and meet stricter standards. In , introduced electronic multi-point injection in 1975 on the , followed by Toyota's EFI systems in the late , driven by similar efficiency and emissions demands. Stricter environmental regulations drove widespread adoption, particularly the U.S. Clean Air Act of 1970, which mandated significant reductions in and emissions, prompting automakers to transition from carburetors to for better combustion control. In and , similar controls and fuel economy demands accelerated the change, with systems like Bosch's K-Jetronic mechanical continuous injection appearing in the 1970s on vehicles from and . By 1980, approximately 30% of new cars globally featured electronic , reflecting rapid market penetration in premium and regulated segments.

Evolution in modern engines

In the 1990s and early , multi-point manifold injection systems evolved significantly through integration with (VVT), which optimized intake and exhaust valve operations to enhance air-fuel mixing and reduce pumping losses, thereby improving overall engine efficiency in spark-ignition engines. These advancements coincided with the adoption of drive-by-wire throttles, electronic systems that replaced mechanical linkages to provide precise airflow control, better transient response, and lower emissions in port fuel injection setups. As precursors to full (GDI), multi-point systems in the facilitated smoother transitions in engine design, with manufacturers like developing blended port and direct injection approaches to balance power, economy, and emissions during the shift to more advanced combustion strategies. Despite the growing dominance of GDI for its superior fuel atomization and efficiency, manifold injection remains prevalent in modern applications, including motorcycles and small-displacement engines where compact design and cost constraints favor port-based delivery over high-pressure direct systems. In cost-sensitive markets, such as emerging economies for two-wheelers, it continues to offer reliable performance without the complexity of direct injection. Hybrid configurations combining port and direct injection have also emerged for stratified charge operation, where port injection provides homogeneous mixtures at higher loads while direct injection enables lean, stratified for improved fuel economy in downsized turbocharged engines. To meet Euro 5 and Euro 6 emission standards introduced in 2009 and 2014, respectively, manifold injection underwent refinements in injector design to achieve finer atomization, targeting droplet sizes around 10-30 μm for better evaporation and reduced particulate matter formation. This allowed compliance with stricter and particle number limits, though adoption of systems began declining post-2010 as GDI offered 10-15% better and lower CO2 emissions in passenger vehicles. In the 2020s, manifold injection has seen updates for electrification, notably in plug-in hybrids like the , which uses dual multi- and direct injection in its 2.0-liter Atkinson-cycle engine to support efficient hybrid operation and . Post-2015 adaptations have focused on compatibility, enabling systems to process higher content for reduced lifecycle emissions in flex-fuel configurations. A key challenge in these evolutions involves ethanol blends like , which demand adaptations such as upgraded injectors and higher rail pressures—often exceeding 5 bar in port systems—to counteract ethanol's lower and ensure adequate and power output without or cold-start issues.

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