Mercedes Mullins
The intake manifold is responsible for distributing air (and sometimes fuel) to the cylinders of an engine. It is typically made of metal or plastic and is mounted on top of or to the side of the engine block. The fuel injectors are usually mounted onto the intake manifold, where they spray fuel into the combustion chambers of the engine. The fuel injectors are connected to the fuel rail, which delivers pressurized fuel from the fuel pump to the injectors. The fuel injectors can be removed from the intake manifold, although this can be a complex process.
| Characteristics | Values |
|---|---|
| Intake Manifold's Role | Distributes air (and sometimes fuel) to the cylinders of the engine |
| Fuel Injectors' Role | Spray fuel into the combustion chambers of the engine |
| Fuel Injectors' Mounting Point | Fuel injectors are mounted on the engine via the fuel rail |
| Fuel Rail's Role | Delivers pressurized fuel to the fuel injectors |
| Fuel Injection Types | Continuous, Intermittent, Multi-Point, Single-Point, Sequential, Cylinder-Specific |
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What You'll Learn
Multi-point fuel injection systems
Multi-point fuel injection (MPFI) systems, also known as port injection, have a separate fuel injector nozzle for each cylinder, located outside the intake port. The fuel injectors are usually installed in close proximity to the intake valve, allowing them to inject fuel through the open valve into the cylinder. This is in contrast to direct injection, where fuel is injected directly into the main combustion chamber of each cylinder.
In an MPFI system, the fuel injectors are mounted either close to the intake valves or directly into the intake manifold. The intake manifold is a passage that distributes the air-fuel mixture to each cylinder. The injectors spray precise amounts of fuel into the intake manifold or directly into the combustion chamber, based on real-time engine data. This data is provided by sensors such as the crankshaft position sensor (CKP) and camshaft position sensor (CMP), which allow for precise fuel injection timing.
The MPFI system includes a fuel pressure regulator, which is connected to the fuel rail and governs the fuel flow. The fuel rail acts as a distribution channel, supplying pressurised fuel from the fuel pump to all the injectors and ensuring equal fuel pressure. The engine control unit (ECU) controls the opening and closing of the injectors, triggering them to spray the precise amount of fuel required for each cylinder at the appropriate moment during the engine's intake stroke.
There are different types of MPFI systems, including sequential, batched, and simultaneous injection. In a sequential MPFI system, the injectors deliver fuel in a sequential manner, timed to coincide with each cylinder's intake stroke. This results in better fuel atomisation and more accurate fuel delivery. The batched MPFI system, also known as semi-sequential MPFI, groups the injectors into smaller batches or pairs, delivering fuel simultaneously to specific cylinders following a predefined pattern. The simultaneous system injects fuel into all cylinders at the same time.
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Single-point injection systems
In a single-point injection system, there is only one fuel injector that supplies fuel to all the cylinders in the combustion chamber of the engine. This is in contrast to multi-point injection systems, which have a separate fuel injector for each cylinder. The single injector in an SPI system is usually installed right behind the throttle valve, in the throttle body, which is the throat of the engine's air intake manifold.
The fuel injector in an SPI system sprays fuel into all the cylinders simultaneously. The fuel is injected with relatively low pressure into the intake manifold, where it forms a fine fuel vapour or mist. This vapour then mixes with air to form a combustible mixture, which is sucked into the cylinders by the pistons during the intake stroke. The amount of fuel injected is controlled by a flap valve located in the engine's air intake, which responds to airflow changes caused by the opening of the throttle.
One advantage of single-point injection systems is that they are relatively low-cost and provide better "driveability" compared to carburettor systems. They also offer improved control over fuel metering compared to carburettors, resulting in reduced exhaust emissions. However, SPI systems have some disadvantages, such as reduced performance at high RPMs and rough ride quality due to the inability to meet the required fuel supply. Additionally, there is a small amount of fuel wastage as a fraction of the fuel condenses outside the intake manifold of the cylinders.
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Continuous vs intermittent injection
The intake manifold is responsible for distributing air, and sometimes fuel, to the cylinders of the engine. It is typically made of metal or plastic and is mounted on top of or to the side of the engine block. The fuel rail, on the other hand, is part of the fuel delivery system in an engine equipped with fuel injection. It delivers pressurised fuel to the fuel injectors, which are usually housed within the fuel rail and mounted on the intake manifold. The injectors then spray the fuel into the combustion chambers of the engine.
Now, manifold-injected engines can use either continuous or intermittent injection systems. In a continuously injecting system, the fuel is injected continuously, and there are no operating modes. Modern multi-point injection systems use electronically controlled intermittent injection. In intermittently injecting systems, there are usually four different operating modes. 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.
Systems with intermittent group injection work similarly to the simultaneously injecting systems, 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. 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 intervals. This allows the injection control unit to inject the fuel according to the firing order and correct cylinder charge irregularities.
Early manifold-injected engines with fully mechanical injection systems used gear-, chain- or belt-driven injection pumps with a mechanical "analogue" engine map to inject fuel intermittently and relatively precisely.
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Injector mounting points
The intake manifold is responsible for distributing air, and sometimes fuel, to the cylinders of the engine. It is usually made of metal or plastic and is mounted on top of or to the side of the engine block. The intake manifold receives air from the air intake system and directs it into the combustion chambers of the engine.
The intake manifold often serves as a mounting point for various components, including fuel injectors, fuel pressure regulators, intake air temperature sensors, and idle air control valves. The fuel injectors are connected to the fuel rail via injector ports or fittings, which allow fuel to flow from the rail into the injectors. The fuel rail is typically housed within the fuel injectors and provides a secure mounting point for them.
In a multi-point injected engine, each cylinder has its own fuel injector, and the injectors are usually installed close to the intake valve(s). The fuel is injected through the open intake valve into the cylinder. This 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.
The amount of fuel injected is controlled by a flap valve located in the engine's air intake. The flap valve rises and falls in response to airflow, altering the position of a shuttle valve within the metering control unit to allow more fuel to be injected into the cylinders. Each injector contains a spring-loaded valve that is kept closed by its spring pressure and only opens when the fuel is injected.
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Fuel distribution
The intake manifold is responsible for distributing air, and sometimes fuel, to the cylinders of the engine. It is typically made of metal or plastic and is mounted on top of or to the side of the engine block. The intake manifold receives air from the air intake system and directs it into the combustion chambers of the engine. In some designs, it also mixes fuel with the incoming air before it enters the cylinders.
The fuel rail is an essential component of the fuel delivery system in modern internal combustion engines, particularly those equipped with fuel injection. It plays a critical role in ensuring that fuel is delivered reliably and at the correct pressure to the fuel injectors for combustion. The fuel rail is a metal pipe that delivers pressurised fuel to the fuel injectors, which are mounted on the engine. The injectors then spray the fuel into the combustion chambers of the engine. The fuel rail receives fuel from the fuel pump and maintains the required pressure to ensure proper fuel delivery to the injectors. The fuel injectors are connected to the fuel rail via injector ports or fittings, which allow fuel to flow from the rail into the injectors.
The fuel injectors are mounted on the intake manifold, which serves as a mounting point for these injectors. The intake manifold is an ideal location for the fuel injectors as it allows for the mixing of fuel and air before the mixture enters the cylinders. This ensures a more even distribution of the fuel-air mixture across the combustion chamber.
The number of fuel injectors can vary depending on the system. In a multi-point injected engine, each cylinder has its own fuel injector, usually installed very close to the intake valve. This allows the injectors to inject fuel through the open intake valve into the cylinder. In contrast, a single-point injection system has only a single fuel injector, typically installed right behind the throttle valve. This type of system is less common and considered obsolete in modern engines.
The amount of fuel injected is controlled by a flap valve located in the engine's air intake. As the airflow increases, the flap rises, allowing more fuel to be released into the cylinders. The fuel is then delivered to each of the injectors, which spray the fuel into the inlet port in the cylinder head.
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Frequently asked questions
The intake manifold is responsible for distributing air (and sometimes fuel) to the cylinders of the engine. It is typically made of metal or plastic and is mounted on top of or to the side of the engine block.
A fuel injector is a component in an engine's fuel delivery system. It sprays fuel into the combustion chambers of the engine.
Fuel injectors are mounted onto the intake manifold. The injectors are connected to the fuel rail, which delivers pressurized fuel to them. The fuel rail is then connected to the fuel pump, which supplies the fuel.
The amount of fuel injected is controlled by a flap valve located in the engine's air intake. As the throttle opens, the flap rises, allowing more fuel to be squirted into the cylinders. The fuel is then sprayed through the injectors into the combustion chambers.
