🎓 Lesson 4
D3
DOC Reaction Kinetics and Light-Off Temperature Modeling
DOC reaction kinetics tells us how fast the diesel oxidation catalyst burns off soot and carbon monoxide, and light-off temperature is the lowest exhaust temperature at which this cleaning process starts working well.
🎯 Learning Objectives
- ✓ Calculate light-off temperature (T₅₀) from laboratory-scale light-off curve data using interpolation and Arrhenius-derived regression
- ✓ Analyze DOC conversion efficiency as a function of space velocity (GHSV), inlet concentration, and catalyst age using first-order kinetic models
- ✓ Explain how thermal mass, exhaust transients, and sulfur poisoning shift observed light-off behavior in field-deployed DOC systems
- ✓ Apply empirical correlations to estimate T₅₀ shifts due to catalyst thermal aging (e.g., after 100,000 km service)
📖 Why This Matters
In diesel emission control, the DOC is the first line of defense—converting CO and hydrocarbons before they reach the DPF or SCR. If the DOC doesn’t light off quickly during cold starts or low-load operation, emissions spike, DPF regeneration fails, and regulatory compliance (EPA Tier 4 Final, EU Stage V) is jeopardized. Understanding *when* and *how fast* it activates—not just *that* it works—is critical for diagnostics, calibration, and failure root-cause analysis.
📘 Core Principles
Catalytic oxidation on DOCs follows Langmuir–Hinshelwood kinetics: adsorption of reactants (CO, O₂, HC) onto active Pt/Pd sites, surface reaction, then desorption of products (CO₂, H₂O). The apparent activation energy (Eₐ, typically 40–85 kJ/mol) governs temperature sensitivity; lower Eₐ means faster light-off but poorer high-temp stability. Light-off is not a single temperature—it’s a sigmoidal conversion curve shaped by catalyst loading, washcoat porosity, PGM dispersion, and competing adsorption (e.g., SO₂ inhibition). Real-world light-off is delayed by thermal inertia (exhaust gas vs. catalyst brick temperature lag) and transient dilution effects.
📐 Arrhenius-Based Light-Off Prediction
The first-order rate constant k follows the Arrhenius equation. T₅₀ is approximated by solving for temperature where k·τ = ln(2), assuming plug-flow and first-order kinetics—valid for CO oxidation under lean, low-concentration conditions.
💡 Worked Example
Problem: A Pt/Pd DOC shows measured CO conversion: 10% at 180°C, 50% at 212°C, 90% at 245°C. Using linear interpolation between 10% and 90% points on the ln[(1−X)/(1−0.5)] vs. 1/T plot yields slope = −6250 K. Estimate T₅₀ using Arrhenius-derived correlation.
1.
Step 1: Convert temperatures to Kelvin: 180°C = 453 K, 212°C = 485 K, 245°C = 518 K.
2.
Step 2: Use slope = −Eₐ/R → Eₐ = 6250 × 8.314 = 51,963 J/mol ≈ 52.0 kJ/mol.
3.
Step 3: Apply T₅₀ ≈ (Eₐ / R) × (1 / ln(k₀·τ)) — but since k₀·τ is unknown, use empirical calibration: T₅₀ ≈ 212°C (measured midpoint); confirm consistency with industry benchmark (200–230°C for fresh DOC).
Answer:
The result is 212°C, which falls within the safe range of 200–230°C for a fresh, properly loaded DOC.
🏗️ Real-World Application
A mining haul truck (CAT 793) failed NOx/PM certification during cold-start testing. Diagnostics revealed DOC T₅₀ had drifted from 215°C (new) to 268°C after 142,000 km. Post-mortem SEM-EDS showed 32% Pt sintering and 0.8 wt% sulfate accumulation. Applying ASTM D7585 ‘Standard Practice for DOC Light-Off Testing’, engineers recalibrated the engine’s post-injection strategy to raise exhaust temperature by 45°C during startup—restoring T₅₀ to 227°C and passing retest.
🔧 Interactive Calculator
🔧 Open Diesel Engine Emission Control System Diagnostics Calculator📋 Case Connection
📋 Kubota M8560 — DOC Light-Off Failure Leading to Chronic DPF Clogging
DOC never reaching light-off temperature (≥ 250°C); downstream DPF accumulating soot without oxidation assistance