fuel-injection

name	fuel-injection
description	Fuel injection systems — GDI/PFI/DI diesel, injector spray characterization (SMD/spray cone), common rail pressure, injection timing/duration, nozzle flow coefficient, atomization theory, emissions impact, Bosch/Delphi systems, OBD.
metadata	{"priority":7,"promptSignals":{"phrases":["fuel injection","direct injection","GDI injector","diesel injection","common rail","injector spray"],"minScore":3}}

Fuel Injection Systems — Complete Skill

Fuel Injection System Types

Port Fuel Injection (PFI / MPI)

Location: injector upstream of intake valve; spray impinges on back of intake valve Pressure: 3–5 bar (low pressure); simple pump system Advantages: good mixture preparation; charge cooling from evaporation; low particulate Disadvantages: wall wetting → HC emissions; slow transient response; fuel economy limited vs. GDI

Gasoline Direct Injection (GDI)

Location: injector directly in combustion chamber; sprays into cylinder during intake or compression Pressure: 100–350 bar (high pressure GDI); enabled by high-pressure cam-driven pump Advantages: charge cooling → higher compression ratio; stratified charge at part load; better fuel economy 5–10% Disadvantages: particulate formation (PN — particle number) from fuel impingement; injector tip deposits; carbon buildup on intake valves (no fuel wash) EU6/California standards: PN ≤ 6×10¹¹ #/km → requires DPF or OPF in modern GDI systems

Diesel Direct Injection (DI) — Common Rail

Common rail system: fuel pressurized in accumulator rail; each injector draws from rail on command Rail pressure: 1500–2500 bar (modern passenger car); 1000–1800 bar (heavy duty truck) Injector types:

• Solenoid: 100–150 μs response; 5–6 injections per cycle possible
• Piezoelectric: 50–70 μs response; faster; 8+ injections per cycle; higher cost Multiple injection strategy:
• Pilot injection: 1–2 mg before main; reduces NOx and combustion noise (softer pressure rise)
• Main injection: most of fuel; controls power output
• Post injection: oxidize soot in-cylinder; DPF regeneration assist

Injector and Nozzle Design

Nozzle Flow Coefficient

Mass flow rate through injector nozzle: ṁ = C_d × A_nozzle × √(2 × ρ_fuel × ΔP) [kg/s; C_d = discharge coefficient ≈ 0.65–0.85; A_nozzle = nozzle hole area; ΔP = injection pressure - cylinder pressure]

Hole diameter and number: d_hole = 0.1–0.25 mm (diesel); 0.05–0.15 mm (GDI) → precision EDM drilling N_holes = 5–10 (diesel); 5–7 (GDI); hole pattern determines spray cone angle and impingement

Nozzle hole area: A_hole = π/4 × d_hole² [each hole]; A_total = N × A_hole Hydraulic flow at 100 bar: Q = C_d × A_total × √(2 × 100 × 10⁵ / ρ) × 60,000 [cc/min at standardized conditions]

BOSCH standard flow rate: QS (quantitative flow rate) measured at fixed test conditions (100 bar, standard fuel)

Spray Characterization

Sauter Mean Diameter (SMD or D₃₂): diameter of droplet with same volume/surface ratio as whole spray For pressure atomizer: SMD = C₁ × (σ/(ρ_fuel × U²))^0.6 × (μ_air/μ_fuel)^0.2 × d_hole [C₁ = empirical constant; σ = surface tension; U = relative velocity] Typical SMD: 10–30 μm (diesel at 1800 bar); 20–50 μm (GDI at 200 bar)

Penetration length: For non-evaporating spray: S = C₂ × (ΔP/ρ_air)^0.25 × (d_hole)^0.5 × t^0.5 [m; t = time after injection start] Target: spray reaches bowl rim but not cylinder wall (impingement → HC and PN emissions)

Spray cone angle: θ = 2 × arctan(C₃ × (ρ_air/ρ_fuel)^0.5) [full cone angle; C₃ from experiment ≈ 0.27] Typical: 10–20° included angle for diesel; 60–90° for GDI (wider to fill cylinder)

Injection Timing and Duration

Injection timing: Diesel: main injection at 5–15° BTDC; pilot at 20–40° BTDC GDI stratified: injection at 40–60° BTDC; creates rich zone near spark plug GDI homogeneous: injection during intake (as PFI); better mixture preparation

Injection duration: t_inj = m_fuel / ṁ [ms; ṁ from nozzle flow equation] Diesel typical: 1–3 ms main injection at full load; < 0.3 ms pilot injection

Electronic control (ECU solenoid): Injection pulse width: τ = C × m_target / (C_d × A_nozzle × √(2 × ΔP × ρ)) [μs per injection] Injector dead time (opening delay): 0.1–0.3 ms; must be compensated in ECU map

Common Rail System Hydraulics

Rail pressure control: High-pressure pump: cam-driven; 3× compression per revolution; flow varies with cam lift + metering valve Pressure regulator: reduces rail pressure by returning fuel to tank; PID control by ECU Rail capacity: 40–80 cm³ (designed to limit rail pressure fluctuation < 5 bar between injections)

Pressure ripple: ΔP_ripple = ṁ_injected_per_pulse / C_rail [C_rail = rail hydraulic capacitance = V_rail / (c_fuel²/ρ_fuel)] Acoustic wave attenuation: rail geometry and damping elements to minimize pressure waves at sequential injections

Fuel temperature: High-pressure compression heat: ΔT ≈ ΔP × ρ × β_T / c_p [β_T = isentropic compression factor] Fuel return cooling: recirculated fuel cools injectors; temperature controller prevents vapor formation

Atomization Theory

Rayleigh breakup (low velocity jets): Jet breaks into droplets when λ > π × d_jet [λ = instability wavelength; droplet size ≈ 1.89 × d_jet]

Atomization regime (high velocity — diesel): Weber number: We = ρ_air × U² × d / σ [U = relative velocity; σ = surface tension] Ohnesorge number: Z = μ_fuel / √(ρ_fuel × σ × d) High We (> 200), low Z: atomization regime → SMD << d_hole

Secondary breakup mechanisms: Bag breakup: We = 10–100 Stripping: We > 100 (shear from relative velocity) Catastrophic: We >> 100; all mechanisms

Emissions Impact

NOx vs. soot trade-off (diesel): High temperature + excess O₂ → NOx ↑; rich zones → soot ↑ Injection timing retard: reduces T_max → NOx ↓; but soot ↑ (less time to oxidize) EGR: dilutes charge → T_max ↓ → NOx ↓; soot trade-off (complex)

Particulate (GDI): Spray impingement → liquid fuel film → diffusion flame → soot Avoid: optimize injection timing; increase pressure; reduce SMD; avoid wall wetting OPF (Otto Particulate Filter): DOC + particulate filter; PN reduction 90%

OBD-II (On-Board Diagnostics — SAE J1979): Monitor injector performance; misfire detection (ISO 15031); fuel trim (long/short term); evap system P0200–P0299: fuel injector codes (P0201: cylinder 1 injector circuit)

Standards

Standard	Scope
Bosch Automotive Handbook	Common rail, GDI, injector design reference
SAE J1979 / ISO 15031	OBD-II diagnostic communication
EU Regulation 715/2007 (Euro 6)	Emission limits (NOx, PM, PN)
SAE J1835	Fuel injector electrical test — static flow
ISO 4113	Calibration fluid for diesel fuel system testing

Output

Provide: injection system type (PFI/GDI/diesel CR), injection pressure [bar], number of holes and hole diameter [mm], nozzle flow rate [cc/min at 100 bar], mass flow rate ṁ [mg/stroke] at operating conditions, spray SMD [μm], spray penetration [mm] at x ms after SOI, spray cone angle [°], injection timing (SOI [°BTDC]) and duration [ms], multiple injection strategy (pilot/main/post timing and masses), rail pressure fluctuation [bar], ECU injector pulse width [μs], emissions impact (NOx/PN trade-off), OBD fault code if applicable, and applicable standard (Euro 6, SAE J1979).