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Technical Route – Diesel Vehicles DOC+DPF+SCR

The Complete Diesel Aftertreatment System

Modern diesel vehicles face simultaneous challenges: reducing particulate matter (PM/PN) and nitrogen oxides (NOx) from lean-burn exhaust where traditional three-way catalysts cannot function. The DOC+DPF+SCR technical route integrates three complementary technologies into a complete aftertreatment system.

  • Diesel Oxidation Catalyst (DOC): Oxidizes CO/HC, generates heat, and converts NO → NO₂.
  • Diesel Particulate Filter (DPF): Traps soot and ash.
  • Selective Catalytic Reduction (SCR): Converts NOx → N₂ + H₂O using DEF-derived ammonia.

This three-step architecture achieves >95% CO/HC reduction, >99% PM removal, and 90–99% NOx conversion, meeting Euro 6/7, EPA 2010/2027, and China 6/7 standards.

The DOC+DPF+SCR Pathway

Complete system architecture:
Engine → DOC → DPF → SCR → ASC (optional) → Tailpipe

Step 1: DOC – Diesel Oxidation Catalyst

Function:

  • First active device after the engine.
  • Converts CO → CO₂ and HC → CO₂ + H₂O.
  • Oxidizes NO → NO₂ (critical for downstream passive DPF regeneration and fast SCR reactions).
  • Generates exothermic heat during active DPF regeneration (post-injected fuel burns on DOC, raising exhaust to 600–700°C).

Construction:

  • Cordierite honeycomb (200–400 cpsi, 5–8 mil wall)
  • Pt/Pd washcoat (0.5–3 g/L Pt, 0–1 g/L Pd)
  • Volume: 1–3L (passenger), 5–15L (heavy-duty)

Performance:

  • CO/HC conversion >90% at 200–400°C
  • Light-off (T50): 150–200°C

NO → NO₂ conversion: optimized to 40–60% for downstream synergy

Step 2: DPF – Diesel Particulate Filter

Function:

  • Traps soot and ash particles from exhaust.
  • Wall-flow substrate forces exhaust through porous ceramic walls, capturing >99% of PM (including sub-23nm nanoparticles).
  • Accumulated soot removed via passive (NO₂-based, continuous) or active (fuel post-injection, DOC exotherm) regeneration.

Construction:

  • Wall-flow cordierite or silicon carbide (SiC) substrate (200–300 cpsi, 10–12 mil wall, 40–55% porosity)
  • SiC preferred for heavy-duty (higher thermal conductivity, withstands aggressive regeneration)

Performance:

  • Filtration efficiency >99% (PN <6×10¹¹ #/kWh)
  • Passive regeneration: 250–400°C (requires NO₂ from DOC)
  • Active regeneration: 550–650°C when soot load exceeds 4–6 g/L

Ash cleaning interval: 150,000–300,000 km (passenger), 200,000–500,000 km (heavy-duty)

Step 3: SCR – Selective Catalytic Reduction

Function:

  • Primary NOx removal
  • DEF (32.5% urea) injected upstream → decomposes to NH₃ → reacts with NOx → N₂ + H₂O
  • Standard SCR: 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O (250–450°C)
  • Fast SCR: 2NO + 2NO₂ + 4NH₃ → 4N₂ + 6H₂O (10× faster, requires 50:50 NO:NO₂ from DOC)

Construction:

  • Cordierite honeycomb (400–600 cpsi, 4–6 mil wall)
  • Cu-zeolite (175–500°C) or Fe-zeolite (250–550°C) washcoat
  • Volume: 3–8L (passenger), 15–40L (heavy-duty)
  • ASC (Pt on alumina) often coated on outlet section for NH₃ slip control <10 ppm

Performance:

  • NOx conversion 90–99% in active temperature window
  • Cu-zeolite: low-temperature applications (urban driving, cold climates)

Fe-zeolite: high-temperature durability (long-haul trucking)

System Integration and Synergy

Component

Primary Function

Secondary Function

Downstream Beneficiary

DOC

CO/HC oxidation

NO → NO₂ conversion; exotherm generation

SCR (NO₂ for fast SCR); DPF (passive regeneration)

DPF

PM/PN filtration

Soot storage

SCR (clean gas, no fouling)

SCR

NOx reduction

NH₃ storage

Tailpipe (clean exhaust)

ASC

NH₃ oxidation

N₂O suppression

Tailpipe (odorless, safe)

Key Design Parameters

System Sizing:

DOC: engine displacement × 1.0–1.5

DPF: engine displacement × 1.2–1.8

SCR: engine displacement × 1.5–2.5

Space velocity: 30,000–60,000 hr⁻¹ (passenger), 15,000–30,000 hr⁻¹ (heavy-duty)

Temperature Management:

SCR light-off requires >175°C (Cu-zeolite)

Low-load operation (idle, urban) needs thermal management: close-coupled mounting, exhaust insulation, active heating (eSCR for Euro 7)

DPF regeneration requires >550°C – achieved via late post-injection (fuel penalty 3–5%) or burner system

DEF Dosing Control:

Closed-loop using upstream and downstream NOx sensors

NH₃ storage model prevents slip while maximizing conversion

Dosing rate: 3–6% of fuel consumption

Advantages of the DOC+DPF+SCR Route

We engineer complete DOC+DPF+SCR systems for any diesel application. Whether you require a compact system for a passenger van (Euro 6d), a high-durability system for a long-haul truck (EPA 2027, 700,000+ km), or a sulfur-tolerant system for off-highway equipment (Stage V/Tier 4), our team tailors catalyst formulations, substrate geometries, PGM loadings, and control strategies to your specific engine, duty cycle, and regulatory target.

Partner with us to define your path to compliance – diesel emission control, fully integrated.

The Future of DOC+DPF+SCR

Electric regeneration for PHEV

Active heating systems for low-load operation (plug-in hybrids, urban delivery). 48V electric heaters maintain catalyst temperature without fuel penalty

SCR on filter (SCRoF / SDPF)

SCR catalyst coated directly on DPF substrate. Saves space (one less brick), reduces backpressure, enables passive regeneration + SCR in same can. Preferred for Euro 7 and tight packaging

Dual SCR (Euro 7 heavy-duty)

Two SCR bricks in series (small, fast-light-off close-coupled + large, high-storage underfloor). Reduces cold-start NOx by 70–90% compared to single SCR

Hydrogen combustion (H₂-ICE)

Zero CO₂ diesel replacement retains DOC+DPF+SCR architecture for NOx and PM control. SCR essential for H₂-ICE NOx compliance

The DOC+DPF+SCR technical route is not three separate components – it is an integrated system where each element enables the next. A well-engineered system achieves >99% PM removal, 90–99% NOx reduction, and <10 ppm NH₃ slip across thousands of operating hours. Whether you are certifying a Euro 7 heavy-duty truck or a Tier 4 Final excavator, DOC+DPF+SCR defines the state of the art – and we are ready to engineer your solution.