Ameer Randolph
Fuel injection is the process of introducing fuel into an internal combustion engine, most commonly automotive engines, by the means of a fuel injector. The fuel injector is effectively a spray nozzle that performs the final stage in the delivery of fuel into the engine. Single-point injection is the simplest and earliest injection system that replaced carburetors in the 80s. It uses only one nozzle in the throttle body, which sprays fuel to an air intake manifold shared by all cylinders. 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, no engine stuttering) than could be obtained with a carburettor.
| Characteristics | Values |
|---|---|
| Type | Single-point injection |
| Other Names | Throttle-body injection (TBI), Central port injection |
| Injectors | One or two fuel injector nozzles |
| Injector Location | Throttle body |
| Injection Location | Intake manifold |
| Fuel Spray | All cylinders at the same time |
| Fuel-Air Mixture | Mixed outside the combustion chamber |
| Advantages | Low-cost, better "driveability", more precise volume of fuel, less fuel condensation |
| Disadvantages | Disturbs engine performance at high RPMs, causes rough ride quality, some fuel wastage |
| Common Use | Older petrol-engined cars, Otto engine, Wankel engine |
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What You'll Learn
- Single-point injection systems are the oldest and simplest form of fuel injection
- Fuel injectors are controlled electronically or mechanically
- Fuel injection systems are used in petrol and diesel engines
- Sequential fuel injection triggers each injector nozzle independently
- Fuel injection atomises fuel through a small nozzle under high pressure
Single-point injection systems are the oldest and simplest form of fuel injection
Single-point injection systems, also known as throttle-body injection (TBI), are the oldest and simplest form of fuel injection. They are called throttle-body injection because they use one or two fuel injector nozzles in the throttle body, which is the throat of the engine's air intake manifold.
In a single-point injection system, the fuel is sprayed into all cylinders at the same time. This is in contrast to a carburettor jet, where the fuel is sprayed from an injector and can be monitored by an electronic control unit (ECU). While single-point injection systems are less precise than newer systems, they meter fuel with better control than a carburetor and are less expensive and easier to service.
Single-point injection systems were a relatively low-cost way for automakers to reduce exhaust emissions to comply with tightening regulations while providing better "driveability" (easy starting, smooth running, no engine stuttering) than could be obtained with a carburettor. They were used extensively on American-made passenger cars and light trucks during 1980–1995, and in some European cars in the early and mid-1990s. By the early 1990s, they had largely replaced carburetors in most new petrol-engined cars sold in developed countries.
Single-point injection systems have some disadvantages. For example, since they use a single injector, they can disturb the performance of an engine at high RPMs and cause rough ride quality as the required fuel supply isn't fulfilled. Additionally, a small fraction of fuel can get condensed outside the intake manifold of the cylinders, causing fuel wastage.
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Fuel injectors are controlled electronically or mechanically
Fuel injectors are used in internal combustion engines, most commonly automotive engines, to introduce fuel into the engine. The fuel injector is effectively a spray nozzle that performs the final stage in the delivery of fuel into the engine. The fuel injector system is designed to determine the appropriate amount of fuel to be supplied and control the fuel flow to supply this amount.
The two main types of fuel injectors are mechanical and electronic. Mechanical fuel injectors were used in the 1960s and 1970s by many manufacturers on their higher-performance sports cars and sports saloons. In a mechanical system, the injector is spring-loaded into the closed position and is opened by fuel pressure. Mechanical systems are often called petrol injection (PI for short) and the fuel flow is controlled by a mechanical regulator assembly. Mechanical systems are mechanically complex and have a poor response to backing off the throttle.
Electronic fuel injection (EFI) systems, on the other hand, use electronically-controlled fuel injectors which open and close to control the amount of fuel entering the engine. The injector in an electronic system is held closed by a spring but is opened by an electromagnet built into the injector body. The electronic control unit determines how long the injector stays open. The amount of time the fuel injector stays open is called the pulse width and is controlled by the engine control unit (ECU). The ECU uses a formula and a large number of lookup tables to determine the pulse width for given operating conditions.
The L-Jetronic system, introduced in 1974, is an example of an early electronic fuel injection system that used an airflow meter to calculate the amount of fuel required. The Bosch Motronic multi-point fuel injection system was the first mass-produced system to use digital electronics. The Ford EEC-III single-point fuel injection system, introduced in 1980, was another early digital fuel injection system. These electronic systems became more widespread through the 1980s and gradually replaced carburettors in most new petrol-engined cars sold in developed countries by the early 1990s.
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Fuel injection systems are used in petrol and diesel engines
In petrol-engined cars, indirect fuel injection is commonly used, where a fuel pump sends petrol to the engine bay, which is then injected into the inlet manifold by an injector. Depending on the system, there may be a separate injector for each cylinder, or one or two injectors for multiple cylinders. Single-point injection, also known as throttle-body injection, is a relatively low-cost method where a single injector feeds all the cylinders. This system helps reduce exhaust emissions and improves "driveability" compared to carburettors.
Direct injection, on the other hand, is more common in diesel engines but is also found in high-performance petrol vehicles. This method bypasses the intake manifold and introduces fuel directly into the cylinder. While direct injection offers improved performance and fuel efficiency, it requires meticulous maintenance for optimal engine operation.
The fuel injection system consists of various components such as injectors, fuel pumps, sensors, and an electronic control unit (ECU). The ECU plays a crucial role in managing the fuel injection process, using input from sensors to determine the appropriate amount of fuel required by the engine. The fuel pump transfers fuel from the tank to the injection system, ensuring a steady supply. The injection pump then pressurises and meters the fuel before it is injected into the engine's cylinders.
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Sequential fuel injection triggers each injector nozzle independently
Sequential fuel injection, also called sequential port fuel injection (SPFI) or timed injection, is a type of multiport injection. Basic multi-point fuel injection (MPFI) systems employ multiple injectors, but they all spray their fuel at the same time or in groups. This can cause the fuel to hang around a port for as long as 150 milliseconds when the engine is idling. Sequential fuel injection, on the other hand, triggers each injector nozzle independently. This means that the fuel is sprayed immediately before or as the intake valve opens, improving efficiency and emissions.
In a sequential fuel injection system, the injectors are timed like spark plugs. This ensures that the fuel is sprayed with precision, just before it is needed. This system provides greater control over the amount of fuel entering the engine, resulting in more efficient combustion and reduced emissions.
Sequential fuel injection is an improvement over basic MPFI systems, which can suffer from fuel "hang time" due to their simultaneous or group injection methods. By triggering each injector independently, sequential fuel injection eliminates this issue and ensures that fuel is only sprayed when and where it is needed.
While sequential fuel injection offers advantages over basic MPFI, it is important to note that it is a less common type of fuel injection system. The most common automotive continuous injection system is the Bosch K-Jetronic system, which was introduced in 1974 and used until the mid-1990s. Sequential fuel injection, as a more advanced system, may be more expensive and complex to implement.
Overall, sequential fuel injection's ability to trigger each injector nozzle independently offers improvements in fuel efficiency and emissions. By spraying fuel immediately before or as the intake valve opens, this system ensures that fuel is used more effectively and reduces the amount of fuel lingering in the port.
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Fuel injection atomises fuel through a small nozzle under high pressure
Fuel injection systems are used to deliver fuel into the engine cylinders of internal combustion engines, most commonly automotive engines. The primary difference between carburetion and fuel injection is that fuel injection atomises fuel through a small nozzle under high pressure, while carburetion relies on suction created by intake air accelerated through a Venturi tube to draw fuel into the airstream.
The fuel injector is effectively a spray nozzle that performs the final stage in the delivery of fuel into the engine. The injector is located in the combustion chamber, inlet manifold, or, less commonly, the throttle body. The nozzle is opened and closed with a solenoid-operated needle valve. The injector simply acts as a spray nozzle to break up the fuel into a fine spray. The amount of fuel sprayed is increased or decreased by a mechanical or electrical control unit. The main types of injection systems include pump-line-nozzle, unit injector, and common rail.
In a common-rail system, fuel from the fuel tank is supplied to a common header (called the accumulator) and then sent through tubing to the injectors, which inject it into the combustion chambers. The accumulator has a high-pressure relief valve to maintain pressure and return excess fuel to the fuel tank. The fuel is sprayed with the help of a nozzle that is opened and closed with a solenoid-operated needle valve.
The injectors are of two types, depending on the injection system. The first system uses continuous injection, where the fuel is squirted into the inlet port all the time the engine is running. The second popular system is timed injection (pulsed injection), where the fuel is delivered in bursts to coincide with the induction stroke of the cylinder.
In petrol-engined cars, a fuel pump sends the petrol to the engine bay, and it is then injected into the inlet manifold by an injector. There is either a separate injector for each cylinder or one or two injectors into the inlet manifold. Single-point injection uses one injector in a throttle body mounted similarly to a carburettor on an intake manifold.
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Frequently asked questions
Single-point injection, also known as throttle-body injection, is the oldest and simplest form of the fuel injection system. It uses one or two fuel injector nozzles in the throttle body to spray fuel into all cylinders at the same time.
In a multi-point injection system, each cylinder has its own dedicated fuel injector. This is more complex and expensive than a single-point system, which uses a single injector for all cylinders.
In a single-point injection system, the fuel is sprayed into the throttle body and mixed with air before entering the intake manifold and combustion chamber. The amount of fuel sprayed is controlled by a mechanical or electrical unit.
Single-point injection is a relatively low-cost way to reduce exhaust emissions and improve "driveability" (easy starting, smooth running, no engine stuttering) compared to carburettor systems.
Single-point injection systems can cause a rough ride at high RPMs as the required fuel supply is not fulfilled. They also have a slight fuel wastage issue, with a small fraction of fuel condensing outside the intake manifold of the cylinders.
