What Are the 4 Types of Fuel Injection Systems?

The four primary fuel injection systems used in gasoline engines are: single-point injection (throttle-body injection, TBI), multi-point/port injection (MPFI), sequential multi-port injection (SFI), and gasoline direct injection (GDI). These categories describe where fuel is introduced and how precisely it’s timed, shaping performance, emissions, efficiency, and cost. While modern vehicles increasingly favor GDI or mixed systems that combine port and direct injection, understanding these four foundational types explains how fuel delivery evolved and why today’s engines behave the way they do.

The Four Types at a Glance

The list below outlines each injection type, focusing on injector location, control strategy, and typical use cases across different vehicle generations.

• Single-point injection (Throttle-Body Injection, TBI): One or two injectors spray fuel into the throttle body, replacing a carburetor. Simple and low-cost, common in late 1980s–early 1990s.
• Multi-point/Port Fuel Injection (MPFI, “batch fire”): One injector per cylinder sprays into the intake port, often fired in groups rather than timed to each intake stroke. Ubiquitous through the 1990s–2000s.
• Sequential Multi-Port Injection (SFI): One injector per cylinder sprays into the intake port, with each injector pulsed in sync with its cylinder’s intake event. Improves driveability and emissions vs. batch fire.
• Gasoline Direct Injection (GDI): Injectors spray fuel directly into the combustion chamber at high pressure, improving efficiency, power, and knock resistance—dominant in modern gasoline engines.

These four systems trace a clear progression from simpler, centralized fuel delivery toward precise, cylinder-specific timing and higher pressures that enhance efficiency and emissions control.

How Each System Works

Single-Point Injection (TBI)

TBI places one or two injectors upstream in the throttle body, atomizing fuel before it enters the intake manifold. It modernized carbureted layouts with electronic control, improving cold starts and emissions without major hardware complexity.

Multi-Point/Port Fuel Injection (MPFI, Batch Fire)

MPFI locates an injector at each intake port, providing more even distribution than TBI. In batch-fire configurations, multiple injectors fire together regardless of each cylinder’s intake timing—simpler than SFI but less precise.

Sequential Multi-Port Injection (SFI)

SFI times each injector to the corresponding cylinder’s intake stroke, enhancing mixture control, idle quality, and transient response. It became a mainstream standard as electronic control units improved.

Gasoline Direct Injection (GDI)

GDI injects fuel directly into the combustion chamber at high pressure (often 150–350 bar). By cooling the charge and enabling precise stratification under some conditions, it raises efficiency and power and allows higher compression ratios. Modern engines often pair GDI with turbocharging for strong performance and fuel economy, though they may require particulate control and careful intake valve deposit management.

Key Differences That Matter

These systems differ in three core dimensions that determine real-world behavior and cost.

• Injector location: Throttle body (TBI), intake port (MPFI/SFI), or in-cylinder (GDI).
• Control strategy: Grouped/batch (MPFI) versus cylinder-synchronous (SFI); GDI adds timing flexibility across intake/compression.
• Fuel pressure: Low (TBI/MPFI/SFI) versus very high (GDI) with separate high-pressure pumps and rail design.

Together, these factors drive outcomes like torque response, fuel economy, emissions, noise, and system complexity.

Pros and Cons by Type

The following comparison summarizes trade-offs engineers and buyers typically see among the four systems.

• TBI — Pros: Simple, inexpensive, easier service; Cons: Weaker atomization and distribution, modest efficiency and power.
• MPFI (Batch) — Pros: Better distribution than TBI, proven and reliable; Cons: Less precise fuel timing than SFI, middling transient control.
• SFI — Pros: Cylinder-specific timing improves idle, emissions, and response; Cons: More complex than batch MPFI, still limited by port-wall wetting.
• GDI — Pros: Highest efficiency/power potential, better knock resistance, supports higher compression/turbo; Cons: Cost/complexity, particulate emissions, potential intake valve deposits (often mitigated with dual-injection or maintenance).

In practice, SFI balances precision and cost for port systems, while GDI leads on efficiency and performance but introduces new engineering and maintenance considerations.

Where You’ll See Them Today

Contemporary gasoline engines predominantly use GDI, sometimes paired with port injection (a “dual-injection” setup) to reduce particulate emissions and keep intake valves clean. Many mainstream models from the 2010s onward—especially turbocharged ones—adopt GDI or dual systems, while older fleets still feature MPFI and SFI. TBI remains largely historical in passenger cars.

Related Diesel Injection Types (Context)

Diesel engines follow a different lineage. For context, here are four major diesel injection architectures found across eras and applications.

• Common-rail direct injection (CRDI): A high-pressure rail feeds electronically controlled injectors—today’s dominant diesel system.
• Unit injector (UI/PD): Each cylinder has an integrated pump-injector, giving very high localized pressure.
• Pump-line-nozzle (inline pump): A mechanical high-pressure pump feeds individual lines to injectors—older heavy-duty standard.
• Distributor/rotary pump: A single pump meters and distributes fuel sequentially—common on earlier light-duty diesels.

While these diesel systems are distinct from gasoline types, they illustrate parallel evolution toward higher pressure and finer electronic control.

Summary

The four types of gasoline fuel injection are TBI (single-point), MPFI (batch), SFI (sequential multi-port), and GDI (direct). Each step reflects greater precision and pressure, moving from centralized delivery toward cylinder-specific timing and in-cylinder injection. Today’s market largely favors GDI—often combined with port injection—balancing performance, economy, and emissions in modern powertrains.

What is the most common fuel injection system used today?

The most commonly used fuel system today is the Direct Fuel Injection (DFI) system. This advanced technology improves engine performance by injecting fuel directly into the combustion chamber of the engine, rather than mixing it with air prior to entering the chamber.

What is the difference between GDI and EFI?

GDI is a fuel injection system where the fuel is directly injected into the combustion chamber of the engine. EFI, on the other hand, is a system where the fuel is injected into the intake manifold before entering the combustion chamber.

What are the four basic types of fuel injection systems?

In this day and age, while there are various types of fuel injection systems available, the broad categorization brings them down to four main types: Single point fuel injection, Multi-point fuel injection, Sequential fuel injection, and Direct fuel injection.

Which is better, MPI or GDI?

Compared to conventional MPI engines of a comparable size, the GDI engine provides approximately 10% greater output and torque at all speeds. In high-output mode, the GDI engine provides outstanding acceleration.