Fuel Injection (Gasoline)

Key takeaways

Port injection wets the manifold; direct injection meters into the cylinder at up to 350+ bar.
Injection timing and pressure shape mixture preparation — and particulates on GDI.
Many modern engines run both systems, split by operating point.

Gasoline fuel injection delivers precise fuel quantity via port fuel injection (PFI) or gasoline direct injection (GDI). The ECU calculates injector pulse width based on air mass, target lambda, fuel pressure, and multiple correction factors.

Overview

Modern gasoline engines often use GDI for efficiency and power, sometimes combined with port injection for cold start and high-load enrichment. The ECU manages injection timing, pulse width, and fuel pressure.

Controlled Signals

Injector pulse width
Injection timing (GDI)
Fuel rail pressure (GDI)
Air mass / MAP
Battery voltage compensation

Maps Involved

Base Fuel Maps (VE or mass-based)
Injector Flow Characterization Maps
Fuel Pressure Target Maps (GDI)
Cranking Enrichment Maps
Warm-up Enrichment Maps

Logic Sequence

Air Mass Calculation → Target Lambda
         ↓
Base Fuel Calculation
         ↓
Temperature & Transient Corrections
         ↓
Injector Dead Time Compensation
         ↓
Final Pulse Width → Injector Driver

Calibration Objectives

Accurate fuel delivery across all conditions
Maintain target AFR
Minimize fuel trim deviations

Calibration Strategy

Verify injector characterization if upgrading injectors
Adjust fuel maps to match actual airflow changes
Monitor STFT and LTFT for calibration accuracy

Diagnostics

Injector circuit faults
Fuel trim out of range
Fuel pressure deviation (GDI)

Best Practices

Match injector size to power goals
Always verify AFR after fueling changes
GDI systems require quality fuel to prevent carbon buildup

In Calibration Practice

Direct injection buys charge cooling and precision and pays in particulates and HPFP limits; port systems keep valves washed and hardware simple. On DI, high-pressure pump delivery is the quiet ceiling at high rpm — logged rail pressure sagging under demand bounds the tune exactly like its diesel cousin, and no injector duty math overrides it.

Why do some petrol engines have both port and direct injection?

Each covers the other’s weakness: DI enables precise high-load metering and knock-friendly charge cooling, port injection keeps intake valves washed and particulates low at part load. The calibration blends them per operating point.

Does GDI cause carbon build-up on valves?

On DI-only engines, yes over time: no fuel washes the intake valves, so oil vapour deposits bake on. Driving profile and crankcase-ventilation condition set the pace; dual-injection engines largely avoid the problem by design.

Related: these maps and strategies are the daily work of our ECU remapping service. See also ECU Map Types Glossary in the knowledge base.