Fuel Systems

Fuel systems deliver the right amount of fuel at the right time. Learn carburettors vs injection, port fuel injection, gasoline direct injection, diesel common rail, injector sizing and pulse width, injection pressure and atomisation, fuel pumps and evaporative systems.

Modern engines meter fuel with microsecond precision through electronic injectors — at pressures from 3–5 bar (port injection) to 350 bar (petrol direct injection) and over 2500 bar (diesel common rail).

Higher pressure means finer droplets, faster evaporation and cleaner, more efficient combustion.

An analogy

A fuel injector is like a precision spray bottle that fires tens of times a second — the harder you squeeze (pressure), the finer the mist.

From carburettor to injection

Carburettors used venturi suction to draw fuel — simple but imprecise. Electronic fuel injection (EFI) meters fuel via solenoid injectors controlled by the ECU, enabling λ = 1 control and catalysts.

Port vs direct injection

Port fuel injection (PFI) sprays onto the intake valve at 3–5 bar — good mixing, low cost. Gasoline direct injection (GDI) sprays into the cylinder at 200–350 bar — charge cooling allows higher compression and turbocharging, but can create particulates (hence GPFs). Many engines combine both (dual injection).

Diesel common rail

A high-pressure pump feeds a shared rail at up to 2500+ bar; solenoid or piezo injectors fire multiple injections per cycle (pilot, main, post) to control noise, NOx, soot and DPF regeneration.

Sizing, dynamics and evaporative control

Injectors are sized to deliver full-load fuel within ~80–85 % duty cycle at max rpm; dead time compensates opening delay versus battery voltage. Rail pressure control, pressure-wave compensation and injector-to-injector trimming improve accuracy. EVAP systems capture fuel vapours in a charcoal canister and purge them into the intake.

Formulas

• Fuel per injection (4-stroke, 1 inj/cycle): m_inj = ṁ_f / (n_cyl × N/120)
• Pulse width: PW = m_inj / Q_s + t_dead
• Spray velocity: v = C_d √(2Δp/ρ_f)
• Duty cycle: DC = PW / (120 000/N ms) × 100 %

Worked example: Pulse width

Given: Fuel flow 3.61 g/s, 4 cylinders, 3000 rpm; GDI injector 15 mg/ms (ignore dead time)

Find: Fuel per injection and pulse width.

• Injections per cylinder per second = 3000/120 = 25
• m_inj = 3.61/(4 × 25) = 36.1 mg
• PW = 36.1/15 = 2.4 ms

Answer: 36 mg, 2.4 ms

Worked example: Common-rail spray velocity

Given: 2000 bar, diesel ρ = 830 kg/m³, C_d = 0.7

Find: Exit velocity.

• v = 0.7 × √(2 × 2 × 10⁸/830) = 486 m/s

Answer: ≈ 490 m/s — faster than sound in air

Common mistakes

• Wrong: Running injectors near 100 % duty. — Right: Keep ≤ 80–85 % so injectors can close and control remains linear.
• Wrong: Ignoring dead time at low battery voltage. — Right: Compensate opening delay vs voltage or fuelling goes lean.

Applications

• Petrol engines: PFI, GDI and dual-injection systems.
• Diesel engines: Common-rail with piezo injectors and multiple injections.
• Performance tuning: Injector upgrades must match fuel pumps and ECU calibration.

Key takeaways

• PFI (low pressure), GDI (200–350 bar), diesel CR (2000+ bar).
• m_inj = ṁ_f/(n_cyl × N/120); PW = m/Q + dead time.
• Higher pressure → finer sprays, better combustion.
• Keep duty ≤ 80 %; compensate dead time.

Going further

• Piezo injectors: Sub-millisecond, multiple precise injections per cycle for low noise and emissions.
• Rate shaping: Controlling injection rate profile to tailor heat release.
• Flex-fuel: Ethanol sensors adjust fuelling for E0–E85 blends (≈ 30 % more fuel on E85).