Manifold fuel injection is a widely used fuel delivery system in internal combustion engines where fuel is introduced into the engine's intake manifold or intake ports before it enters the combustion chamber. This method is fundamentally characterized as a system that uses external mixture formation, meaning the fuel and air combine outside the engine's cylinders.
Understanding External Mixture Formation
In an internal combustion engine, mixture formation refers to how fuel and air are combined. With external mixture formation, fuel is sprayed and mixed with the incoming air prior to the mixture being drawn into the cylinders for combustion. This contrasts with internal mixture formation, such as in direct injection systems, where fuel is sprayed directly into the combustion chamber.
Types of Manifold Fuel Injection Systems
There are primarily two types of manifold fuel injection:
1. Port Fuel Injection (PFI)
Port Fuel Injection is the most prevalent form of manifold injection today. In a PFI system:
- Each cylinder has its own dedicated fuel injector.
- The injector is strategically positioned in the engine's intake manifold runner, just upstream of the intake valve for that specific cylinder.
- Fuel is sprayed in a fine mist directly into the intake port, where it mixes with the incoming air.
- This precise placement allows for better control over the fuel-air mixture entering each cylinder.
2. Throttle Body Injection (TBI) / Central Fuel Injection (CFI)
Throttle Body Injection is an older, less common form of manifold injection. In a TBI system:
- One or two fuel injectors are centrally mounted in the throttle body, which is located at the top of the intake manifold.
- These injectors spray fuel into a common area, and the fuel-air mixture then travels through the manifold runners to reach the individual cylinders.
- TBI is less precise than PFI because the fuel has a longer distance to travel and can lead to less uniform fuel distribution among cylinders.
How Manifold Fuel Injection Works
The process of manifold fuel injection involves several key components working in concert:
- Fuel Delivery: An electric fuel pump draws fuel from the tank and delivers it under pressure to the fuel rail.
- Injection: The fuel rail distributes the pressurized fuel to individual fuel injectors.
- Control: The Engine Control Unit (ECU), often referred to as the engine's "brain," precisely controls when and for how long each injector opens. This determines the amount of fuel sprayed.
- Mixing: As fuel is sprayed into the intake manifold or port, it atomizes (breaks into fine droplets) and mixes with the incoming fresh air.
- Combustion: This homogeneous air-fuel mixture is then drawn into the combustion chamber when the intake valve opens, ready for ignition by the spark plug.
Key Advantages and Disadvantages
Manifold fuel injection offers a balance of performance, cost, and reliability.
Advantages
- Cost-Effectiveness: Generally simpler in design and less expensive to manufacture compared to direct injection systems.
- Reliability: It is a well-established and proven technology with robust designs.
- Reduced Carbon Buildup on Intake Valves: Since fuel is sprayed into the intake port, it washes over the intake valves, helping to prevent the accumulation of carbon deposits that can be an issue with direct injection systems.
- Good Mixture Formation: The longer travel path through the intake manifold allows more time for fuel vaporization and thorough mixing with air, particularly beneficial for cold starts and lower engine speeds.
Disadvantages
- Less Precise Fuel Delivery: Fuel film formation on the intake port walls can lead to slight delays in fuel delivery and less precise control compared to direct injection, especially during rapid throttle changes.
- Lower Efficiency/Power: Generally results in slightly lower thermal efficiency and power output compared to direct injection, particularly under high load conditions, due to less precise charge cooling and potential for knocking.
- Emission Control: While effective, achieving ultra-low emissions can sometimes require additional technologies compared to direct injection, which offers more precise control over combustion.
Manifold Injection vs. Direct Injection: A Comparison
To better understand manifold fuel injection, it's helpful to compare it with its primary alternative, Direct Fuel Injection (GDI - Gasoline Direct Injection).
| Feature | Manifold Fuel Injection (PFI/TBI) | Direct Fuel Injection (GDI) |
|---|---|---|
| Mixture Formation | External (fuel mixes with air before cylinder) | Internal (fuel injected directly into cylinder) |
| Injector Location | Intake manifold/port or throttle body | Directly into the combustion chamber |
| Complexity/Cost | Simpler, generally lower cost | More complex, generally higher cost |
| Fuel Delivery Control | Good, but less precise than GDI | Highly precise, allows stratified charge and very fine control |
| Efficiency | Good, but often slightly lower than GDI | Higher, especially at partial loads due to charge cooling and precise control |
| Carbon Deposits | Less prone on intake valves | More prone on intake valves (as fuel doesn't wash over them) |
| Fuel Pressure | Relatively lower (40-60 psi) | Very high (hundreds to thousands of psi) |
Modern Applications and Evolution
While direct injection has become the dominant technology in many new engine designs due to its efficiency and performance benefits, port fuel injection (PFI) remains widely used. It is particularly common in smaller engines, less performance-oriented vehicles, and as a secondary system in dual injection setups. Dual injection engines combine both PFI and GDI to leverage the advantages of both systems across different operating conditions, offering optimized performance, efficiency, and emissions control.
Manifold fuel injection continues to be a robust and reliable technology, forming the backbone of countless automotive engines worldwide.
