The fuel injection system is a crucial component in modern petrol engines. It controls how much fuel is delivered and when, directly affecting performance, emissions, and fuel efficiency.
There are different types of injection systems, each with its own way of delivering fuel — from Single Point to Multi Point to Gasoline Direct Injection — reflecting ongoing innovations in engine technology.
In this article:
Single Point Fuel Injection (SPI) :
Also called: Throttle Body Injection (TBI)
How It Works:
- A single injector sprays fuel into the throttle body (similar location to a carburetor).
- The air-fuel mixture flows through the intake manifold to all cylinders.
Features:
- Simpler and cheaper than MPFI and GDI.
- Better fuel control than a carburetor but still relatively basic.
Advantages:
- Simpler design, easier to maintain.
- Cheaper to manufacture.
- Improved emissions over carburetor systems.
Disadvantages:
- Less precise fuel distribution (some cylinders may run richer or leaner).
- Lower fuel economy and power compared to MPFI or GDI.
Multi Point Fuel Injection (MPFI) :
Also called: Port Fuel Injection (PFI)
How It Works:
- Each cylinder has its own injector positioned near the intake valve.
- Fuel is injected into the intake port just before the valve opens.
Features:
- More precise fuel metering for each cylinder.
- Controlled by an Electronic Control Unit (ECU).
Advantages:
- Better fuel-air mixture consistency across cylinders.
- Improved fuel efficiency and power output.
- Lower emissions compared to SPI.
Disadvantages:
- More complex and costlier than SPI.
- Maintenance of more components (e.g. multiple injectors).
Gasoline Direct Injection (GDI)
How It Works:
- Fuel is injected directly into the combustion chamber, like in diesel engines.
- High-pressure fuel system controlled by ECU optimizes injection timing and quantity.
Features:
- Very precise control of fuel delivery.
- Supports stratified charge combustion (lean burn at light load).
Advantages:
✅ Best fuel efficiency and performance.
✅ Lower CO₂ emissions.
✅ Supports turbocharging and higher compression ratios for more power.
Disadvantages:
❌ Most complex and expensive system.
❌ Potential for carbon buildup on intake valves (since no fuel washes them).
❌ Requires high-pressure pumps and injectors, increasing maintenance cost.
Comparison Table:
| Feature | Single Point (SPI) | Multi Point (MPFI) | Gasoline Direct (GDI) |
|---|---|---|---|
| No. of Injectors | 1 for entire engine | 1 per cylinder | 1 per cylinder (high pressure) |
| Injection Location | Throttle body | Intake port (near valve) | Direct into combustion chamber |
| Fuel control precision | Low | Medium | High |
| Fuel efficiency | Low-moderate | Moderate-high | High |
| Emissions | Better than carburetor | Low | Very low |
| Cost | Low | Moderate | High |
| Complexity | Simple | Moderate | Complex |
| Maintenance cost | Low | Moderate | High |
Summary:
SPI (Single point) → Basic, better than carburetor, but outdated for modern standards.
MPFI (Multi-point) → Reliable, efficient, common in most petrol cars today.
GDI (Gasoline Direct Injection) → Advanced, delivers the best performance and economy, but costlier and more complex.
Conclusion:
While Single Point Injection is simple and cost-effective, Multi Point Injection offers greater control and improved engine performance. Gasoline Direct Injection stands out for its ability to maximize power and reduce emissions, albeit with higher complexity and cost.
The choice of injection system depends on engine design, performance goals, and manufacturing preferences.
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