emission-control

name	emission-control
description	Emission control — NOx (SCR, EGR, LNT), particulate matter (DPF, GPF), CO/HC (TWC, DOC), evaporative emissions (EVAP), Euro 6/EPA Tier standards, OBD, on-board diagnostics, emissions certification testing.
metadata	{"priority":7,"promptSignals":{"phrases":["emission control","NOx reduction","SCR system","DPF filter","three-way catalyst","Euro 6 emissions"],"minScore":3}}

Emission Control Systems — Complete Skill

Regulated Pollutants

Primary engine exhaust pollutants:

Pollutant	Source	Regulated by
NOx (NO + NO₂)	High T combustion	Euro 6, EPA Tier
PM (particulate matter)	Diesel soot	Euro 6, EPA Tier
HC (hydrocarbons)	Incomplete combustion	All standards
CO (carbon monoxide)	Rich combustion	All standards
N₂O	Catalytic convertor (cold)	EPA
NH₃	SCR slip	Monitored

Three-Way Catalyst (TWC) — Gasoline/Petrol

Function: simultaneous oxidation of HC and CO, and reduction of NOx Reactions: 2CO + O₂ → 2CO₂ [CO oxidation] HC + O₂ → CO₂ + H₂O [HC oxidation] 2NO + 2CO → N₂ + 2CO₂ [NOx reduction; requires CO as reductant]

Window operation: must operate at stoichiometry (λ = 1.00 ± 0.01) λ (lambda) = actual AFR / stoichiometric AFR; λ = 1 → stoichiometric; λ < 1 rich; λ > 1 lean Oxygen sensor (UEGO/HEGO): closed-loop λ control; wideband (UEGO): accurate λ measurement; narrow-band (HEGO): binary switch at λ = 1

TWC efficiency vs. temperature: Lightoff temperature: T_lightoff ≈ 200–250°C (50% HC conversion); full efficiency: > 350°C Cold start (first 100–200 s): 70–90% of total trip HC emissions (catalyst cold) Electrically heated catalyst (EHC): preheat to 300°C at start → reduce cold-start emissions

TWC composition: Washcoat: alumina + ceria + zirconia (OSC — oxygen storage capacity); Pt, Pd, Rh (precious metal) OSC: ceria buffers λ fluctuations; absorbs O₂ rich → releases O₂ rich (helps NOx reduction) Rh: primary NOx reduction; Pt, Pd: HC and CO oxidation

Diesel Oxidation Catalyst (DOC)

Function: oxidize HC, CO, and NO → NO₂ (for DPF regeneration) NO + ½O₂ → NO₂ (important for passive DPF regeneration) Efficiency: 90%+ HC conversion; 90%+ CO conversion at T > 200°C; NO to NO₂ conversion: 40–60% Composition: Pt/Pd on alumina washcoat; no Rh (no NOx reduction needed; O₂ present in diesel exhaust)

Diesel Particulate Filter (DPF)

Function: collect PM from diesel exhaust; periodic regeneration required Filter media: silicon carbide (SiC) wall-flow monolith; cordierite (lower cost, lower T limit) Collection efficiency: > 99% (PM mass); > 85% (PM number)

Regeneration: Passive regeneration: NO₂ from DOC oxidizes PM at T > 250°C; continuous if exhaust hot enough Active regeneration: inject fuel after engine → increase exhaust T > 550°C → burn soot; triggered at 45–50 g/L soot loading Soot loading limit: 15–25 g/L before forced regeneration (pressure drop triggers)

DPF pressure drop: ΔP = k_w × Q_gas × μ × L_filter / (w_w × k_w_w) [simplified; increases with soot loading] Fresh DPF: ΔP ≈ 10–30 mbar; loaded: 50–150 mbar (alarm)

DPF sizing: Volume ratio: DPF volume / engine displacement = 1.5–2.5× (longer if heavy-duty, high PM)

Selective Catalytic Reduction (SCR) — NOx Reduction

Reaction with urea (AdBlue/DEF): Urea → 2NH₃ + CO₂ (urea hydrolysis at T > 160°C) 4NH₃ + 4NO + O₂ → 4N₂ + 6H₂O [standard SCR reaction; fastest] 4NH₃ + 2NO + 2NO₂ → 4N₂ + 6H₂O [fast SCR; most efficient; NO:NO₂ = 1:1 optimal] 8NH₃ + 6NO₂ → 7N₂ + 12H₂O [slow SCR]

SCR efficiency: 70–99%+ NOx conversion depending on temperature and α ratio Alpha ratio: α = NH₃ / NOx (molar); α < 1.0 → under-dosing (low conversion); α > 1.0 → NH₃ slip Optimal temperature: 250–450°C; below 200°C → poor conversion; above 500°C → NH₃ oxidation to NO

SCR catalyst: V₂O₅/WO₃/TiO₂ (vanadium); Cu-zeolite (Cu-SSZ-13); Fe-zeolite (lower T) Cu-zeolite: better low-T performance; preferred for light duty Euro 6

DEF dosing: m_DEF = m_NOx × M_urea / (2 × M_NO) × η_decomp [grams; η_decomp = 0.85–0.95] Urea consumption: 3–7% of fuel consumption (typical)

Exhaust Gas Recirculation (EGR)

External EGR: route exhaust gas into intake → dilutes O₂ → lower peak combustion T → lower NOx EGR rate: EGR% = V_EGR / (V_fresh + V_EGR) = 5–30% (gasoline: 10–25%; diesel: 20–50%) NOx reduction: 30–50% per 20% EGR (trade-off: higher PM, fuel economy penalty)

Cooled EGR: cool EGR before intake → denser charge → more effective dilution + cooler combustion EGR cooler effectiveness: 70–90% heat removal

Internal EGR: retained exhaust gas from late exhaust valve closing; lower dilution; simpler

Lean NOx Trap (LNT) — Lean Burn Gasoline

Operation: stores NOx as nitrates during lean operation → purge with rich spike → regenerate Lean storage: NO + O₂ + BaO → Ba(NO₃)₂ [storage on barium sites] Rich purge: Ba(NO₃)₂ + reductants → N₂ + CO₂ + BaO [reduction with HC/CO/H₂]

Purge cycle: every 60–120 s (lean) + 2–5 s (rich purge at λ = 0.85–0.95) Sulfur poisoning: SO₂ competes with NO₂ for BaO sites → desulfation at 650°C required

Euro and EPA Standards

Euro 6d (2020+, light duty, WLTP test):

Pollutant	Gasoline	Diesel
NOx	60 mg/km	80 mg/km
PM	4.5 mg/km	4.5 mg/km
HC+NOx	100 mg/km	—
CO	1000 mg/km	500 mg/km
PN	6×10¹¹ /km	6×10¹¹ /km

EPA Tier 3 Bin 30 (US, 2025+): NOx + NMOG combined: 30 mg/mile; PM: 3 mg/mile; CO: 1700 mg/mile

Real Driving Emissions (RDE): portable emission measurement (PEMS on road); conformity factor CF < 1.5× WLTP limit

OBD (On-Board Diagnostics)

OBD-II (US) / EOBD (EU): monitors emission-relevant systems; MIL (malfunction indicator lamp) for faults Key monitors: O₂ sensor (upstream/downstream), catalyst efficiency, EGR, EVAP, DPF, SCR Catalyst monitor: compares λ oscillations upstream vs. downstream O₂ sensor → OSC degradation indicator SAE J1979: OBD data link protocol; DTC (diagnostic trouble code) P0420 = catalyst below threshold

Standards

Standard	Scope
Euro 6d	EU light duty emission limits (WLTP)
EPA CFR 40 Part 86	US heavy duty emission limits
SAE J1979	OBD-II PID definitions
ISO 15765	CAN OBD communication
CARB Low Emission Vehicle III	California LEV III emission limits

Output

Provide: engine type (gasoline/diesel), aftertreatment system components (TWC/DOC/DPF/SCR), current engine-out NOx [mg/km or g/kWh], required tailpipe NOx [mg/km], SCR conversion efficiency η [%], DEF dosing rate [g/km], DPF soot loading [g/L] and regeneration trigger [g/L], catalyst lightoff temperature [°C], EGR rate [%] and NOx reduction [%], TWC/SCR operating temperature range [°C], cold-start emissions fraction [%], Euro 6/EPA Tier compliance status, OBD monitor status (pass/fail for catalyst, EGR, EVAP), and applicable standard (Euro 6d, EPA Tier 3, SAE J1979).