How fuel gets into the combustion chamber matters enormously for power, efficiency, and emissions. The two dominant approaches in gasoline engines — port fuel injection (PFI) and gasoline direct injection (GDI) — each solve the problem differently, and modern engines increasingly use both.

Port Fuel Injection (PFI) places the injector in the intake port, upstream of the intake valve. Fuel sprays onto the back of the valve and mixes with incoming air before entering the cylinder. This gives the mixture plenty of time to homogenize, producing clean, consistent combustion. The injector operates at relatively low pressure — typically 3 to 5 bar (43–72 psi). PFI is simple, reliable, and keeps intake valves clean because the fuel wash prevents carbon buildup.

Gasoline Direct Injection (GDI) places the injector tip directly in the combustion chamber. Fuel is sprayed at high pressure — 150 to 350 bar (2,175–5,075 psi) — directly into the cylinder. This brings several advantages:

• Charge cooling: Fuel vaporizing inside the cylinder absorbs heat, dropping charge temperatures by roughly 20–30°C. This allows higher compression ratios (typically 12:1–13:1 vs. PFI's 10:1–11:1) without knock.
• Precise fuel metering: The ECU can inject fuel at exact moments during the intake or compression stroke, enabling stratified-charge lean burn at part load.
• Better volumetric efficiency: No fuel displacing air in the intake port means more air enters the cylinder.

The tradeoff: GDI engines are prone to intake valve carbon deposits because no fuel washes over the valves. This is why walnut blasting has become a common maintenance item on GDI engines at 60,000–100,000 miles. GDI also produces higher particulate emissions, which led to gasoline particulate filters (GPF) on many post-2018 vehicles.

Real-world example: Toyota's D-4S system (used in the 86/BRZ's FA20 and the Lexus LC500's 2UR-GSE V8) combines both — port injectors for low-load cleanliness and smooth idle, direct injectors for high-load power and efficiency. Ford's 3.5L EcoBoost V6 went from GDI-only to adding port injection in its third generation specifically to combat carbon buildup.

Rule of thumb for the efficiency gain: Switching from PFI to GDI on the same engine typically yields a 3–5% improvement in fuel economy and a 5–10% increase in peak power, primarily from the higher compression ratio that charge cooling enables. You can estimate the thermal efficiency gain from compression ratio using the Otto cycle formula: efficiency ≈ 1 − (1/r^(γ−1)), where r is compression ratio and γ ≈ 1.3 for real exhaust gases. Going from 10.5:1 to 12.5:1 raises theoretical efficiency from about 46% to 49% — a meaningful real-world gain.