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Port fuel injection (PFI) is a type of fuel injection system that has replaced carburettors in cars due to its efficiency and lower maintenance requirements. It is recommended for low-viscosity and highly unstable fuels such as alcohols and petrol. Port injection provides a better, more stable air/fuel mixture at lower engine speeds, resulting in a smoother operation upon startup. It is also simple and inexpensive to produce engines with port injection as it requires fewer moving parts and less development time and materials. Examples of cars that use port injection include Ford's EcoBoost engine and Mazda's SkyActiv engine.
Characteristics and Values of Port Fuel Injection
| Characteristics | Values |
|---|---|
| Definition | A type of fuel injection where fuel is injected prior to the actual cylinder where combustion happens |
| How it works | Fuel is delivered to the engine directly into the intake manifold or cylinder head; fuel is sprayed on the valve, which then uses the heat from the valve to atomize the fuel further |
| Advantages | Simple and inexpensive to produce; less complex with fewer moving parts; requires less development time and materials; fuel vaporization; cleans the valves leading to less carbon build-up over time; can run effectively at high RPMs |
| Disadvantages | Less efficient; delivers more fuel to the cylinder |
| Use cases | Recommended for low-viscosity and highly unstable fuel such as alcohols and petrol; used in some of the automotive industry's most popular vehicles for consumer sale; used in turbocharged and naturally aspirated V-6 and V-8 gasoline engines |
| History | Port fuel injection has been around since the 1980s; it replaced carburettors in cars because of its efficiency and lower maintenance requirements |
| Dual injection | Some modern engines combine port and direct fuel injection to improve performance, fuel efficiency, etc. |
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What You'll Learn
Port fuel injection vs manifold injection
Port fuel injection (PFI) and manifold injection are both out-of-cylinder injections, but they differ in the number of injectors and their injection locations. In a manifold injection system, air and fuel are mixed outside the combustion chamber, and the mixture is then sucked into the engine. This type of injection is common in petrol-fuelled engines such as the Otto and Wankel engines.
Port injection is a form of fuel injection where fuel is injected prior to the actual cylinder where combustion occurs. The injector is incorporated into the intake manifold, very close to the intake valve. The fuel and air mixture will become homogeneous in the intake manifold before the intake valve is opened. Then, the mixture is drawn into the combustion chamber. This process is similar to the working principle of a carburettor.
Port injection is simple and inexpensive to produce because it requires fewer moving parts and less development time and materials. Port injectors are also less complex and cheaper. Another advantage of port injection is fuel vaporization. As the fuel is injected before the valve and cylinder, it gets time to fully vaporize before entering the combustion chamber, allowing for a more reliable burn. The fuel also cleans the valves in port injection engines, leading to less carbon build-up over time. Port injection is also capable of running effectively at high RPMs because fuel can be sprayed at any time during the combustion cycle, eliminating the risk of fuel starvation.
However, port injection is less efficient than direct injection because not all the fuel that gets sprayed enters the combustion chamber. This can lead to wasted fuel and buildup in the intake manifold, which, in turn, results in increased emissions and reduced fuel economy. Port injection is more efficient than direct injection at low speeds, but direct injection is more efficient across the whole RPM range. A final disadvantage of port injection is lower engine compression ratios. Because fuel is in the chamber for longer, it becomes much hotter than fuel in a direct injection engine, making it more likely to spontaneously combust. This causes engine knock and has the potential to ruin port injection engines.
In conclusion, port injection is a simple, inexpensive, and effective method of fuel injection. However, it has some disadvantages, such as reduced fuel economy and the risk of spontaneous combustion, that direct injection does not have.
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Port injection vs direct injection
Port fuel injection (PFI) and direct fuel injection (DFI) are two of the most common types of fuel injection systems, which, despite both getting fuel into the engine, do it in different ways, each with its own pros and cons.
Port injection is a form of fuel injection where fuel is injected prior to the actual cylinder where combustion happens. In port injection engines, fuel enters the cylinder during the intake stroke, but the fuel can be sprayed during any of the four engine strokes. Port injection also only requires a single fuel pump. Port injection is recommended for low-viscosity and highly unstable fuel such as alcohols and petrol. It is also simple and inexpensive to produce engines with port injection, as they require less moving parts and are less complex. Port injectors are also less complex and cheaper. Another advantage to port injection is fuel vaporization—since the fuel is injected before the valve and cylinder, it gets time to fully vaporize before entering the combustion chamber. This allows for a more reliable burn. The fuel also cleans the valves in port injection engines, leading to less carbon build-up over time. Finally, because fuel can be sprayed at any time during the combustion cycle, port injection engines can run effectively at high RPMs without worrying about fuel starvation.
Direct injection is a form of fuel injection where fuel is sprayed directly into the cylinder during the compression stroke of the engine at incredibly high speeds and pressures. Direct injection engines are significantly more complex than port injection because of the increased pressure of the cylinder and the split-second timing required. Direct injection engines use two fuel pumps to ensure that the cylinders are getting highly-pressurized fuel at all times. One of the benefits of direct injection is that it is very precise, allowing for great emissions benefits. Direct injection is more efficient across the whole RPM range. Direct injection also diminishes the chance of detonation, or premature ignition of the fuel and air mixture, because the phase-change cooling effect takes place during the compression stroke just before ignition. Lowering the combustion chamber’s surface temperatures enables a higher compression ratio and improved efficiency whether the engine is naturally aspirated or boosted.
While port injection and direct injection each have their own advantages, using both in conjunction is ideal for both maximum economy and maximum performance. Toyota, for example, introduced the D-4S injection system, which combines both port and direct injection, on a V-6 more than a decade ago and now uses it on several of its engines. Ford is also a dominant player in this technology, which it calls dual-fuel, high-pressure direct injection (DI) and lower-pressure port injection (PI).
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Dual injection engines
Port fuel injection (PFI) is a type of fuel injection where fuel is injected prior to the actual cylinder where combustion happens. Carburetors used to be the main fuel mixing and feeding device in motor vehicles, but they have been gradually replaced by advanced electronic injection technology, such as port fuel injection. PFI is the simplest and most straightforward method for external mixture preparation. The low-pressure fuel injector mounted in the intake port supplies the exact quantity of fuel to mix with air just after the intake valve is opened. This is different from the TBI approach, where the mixing of air and fuel takes place behind the throttle plate. The mixture of air and fuel in the PFI system becomes homogeneous in the intake manifold before the intake valve is opened. Then, the mixture is drawn into the combustion chamber, just like the working principle of a carburetor.
Port injection is popular because it is simple and inexpensive to produce. Port injection requires fewer moving parts, making these engines less complex and requiring less development time and materials. Port injectors are also less complex and cheaper. Another advantage of port injection is fuel vaporization. Because the fuel is injected before the valve and cylinder, it gets time to fully vaporize before entering the combustion chamber, allowing for a more reliable burn. The fuel also cleans the valves in port injection engines, leading to less carbon build-up over time.
However, there are some downsides to port injection. One downside is that fuel droplets are sometimes deposited on the intake port walls, upsetting the intended fuel-air ratio. Another downside is that port injection requires more time for homogeneous mixture preparation.
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Homogeneous mode
The homogeneous mode is a feature of gasoline direct injection (GDI) engines, which have become increasingly common in road-going production vehicles. GDI engines enable both stratified-charge and homogeneous modes of operation, which is not possible with port injection. However, GDI engines have been found to suffer from carbon build-up problems on the intake valves due to the lack of fuel hitting the backs of the valves. This is where port injection can be beneficial as it can wash off the valves to reduce carbon deposits.
The maximum degree of homogenisation is obtained by port fuel injection assisted with intake charge preheating. This technique improves volumetric efficiency and fuel distribution over the carburetion system. The mixture enters the cylinder during the suction stroke, and the turbulence generated by the intake flow further improves homogeneity. However, the onset of combustion cannot be controlled by the injection timing in this approach.
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Multi-point injection
The multi-point injection system is a proven, robust, and simple technology. It is recommended for low-viscosity and highly unstable fuels such as alcohols and petrol. It also allows for a clean intake tract as the fuel inherently cleans the intake valves, leading to less carbon buildup over time.
The Bosch Motronic multi-point fuel injection system was the first mass-produced system to use digital electronics. It was introduced in the 1970s and used until the mid-1990s by various car manufacturers. By the early 1990s, electronic manifold injection systems, including multi-point injection, had replaced carburettors in most new petrol-engined cars sold in developed countries.
Today, many modern engines feature dual injection, which combines both a port injector and a direct injector. At lower RPMs, the engine control unit (ECU) will use the port injector to get the most out of the engine. Then, as the engine progresses through the RPM range, the ECU will switch to the direct injector to improve performance and fuel efficiency.
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Frequently asked questions
Port fuel injection (PFI) is a type of fuel injection where fuel is injected prior to the actual cylinder where combustion happens. It is recommended for low-viscosity and highly unstable fuels such as alcohols and petrol.
Some examples of cars that use port fuel injection include Ford's EcoBoost engine family and Mazda's SkyActiv.
Port fuel injection is simple and inexpensive to produce. It also has better fuel vaporization and can run effectively at high RPMs.
Direct fuel injection (DFI) involves shooting fuel directly into a cylinder's combustion chamber at a much higher pressure than port fuel injection. Port fuel injection provides a better and more stable air/fuel mixture at lower engine speeds, resulting in smoother operation upon startup.
