🎓 Lesson 9
D5
Ammonia Slip Calculation from NOx Sensor Pair Data
Ammonia slip is the amount of unreacted ammonia that escapes past the SCR catalyst and exits the exhaust pipe, which we calculate by comparing measurements from two NOx sensors—one before and one after the catalyst.
🎯 Learning Objectives
- ✓ Calculate ammonia slip concentration (ppm) using paired NOx sensor data and known cross-sensitivity coefficients
- ✓ Analyze sensor calibration drift impact on slip estimation accuracy
- ✓ Explain the chemical and sensor-physics basis for NOx sensor NH₃ cross-sensitivity
- ✓ Apply correction factors for exhaust temperature and lambda to adjust raw slip estimates
- ✓ Design a diagnostic validation protocol to distinguish true slip from sensor artifact
📖 Why This Matters
Ammonia slip isn’t just a compliance checkbox—it’s a leading indicator of SCR health, urea dosing accuracy, and catalyst aging. In mining haul trucks operating under transient load (e.g., grade climbing, dump-and-return cycles), uncorrected slip can exceed 25 ppm—triggering OBD fault codes, damaging downstream DPFs, and violating MSHA and EPA Tier 4 Final requirements. Diagnosing slip correctly separates costly hardware replacement from simple dosing recalibration.
📘 Core Principles
SCR systems rely on precise NH₃:NOx stoichiometry (~0.9–1.1) to maximize NOx reduction while minimizing slip. Most heavy-duty OEMs use dual wideband NOx sensors (e.g., Bosch EGO 7, Continental SMOKE-NOx) because dedicated NH₃ sensors remain cost-prohibitive. These NOx sensors exhibit well-documented NH₃ cross-sensitivity: ~3–8% NH₃ signal appears as apparent NOx at the sensor output. By placing one sensor upstream (pre-catalyst, measuring true NOx + NH₃ interference) and one downstream (measuring residual NOx + slip NH₃ + interference), and applying temperature- and lambda-compensated cross-sensitivity coefficients, slip can be isolated mathematically. Catalyst efficiency (η) and slip are coupled: η = 1 − (NOx_out / NOx_in), but slip ≠ 0 even at η > 90% due to NH₃ storage/release hysteresis and uneven urea distribution.
📐 NH₃ Slip Estimation from Dual NOx Sensors
This formula isolates ammonia slip by correcting for the known NH₃ cross-sensitivity (k) of wideband NOx sensors, normalized to exhaust conditions. It assumes identical sensor models, calibrated at same temperature and AFR, and accounts for baseline drift via upstream/downstream correlation.
Dual-Sensor NH₃ Slip Estimate
NH₃_slip = [NOx_up − NOx_down × (1/k)] × k / (1 − k)Estimates NH₃ concentration (ppm) downstream of SCR using cross-sensitivity-corrected difference between upstream and downstream NOx sensor outputs.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| NH₃_slip | Ammonia slip concentration | ppm | Unreacted NH₃ exiting the SCR system |
| NOx_up | Upstream NOx sensor reading | ppm | Measured NOx + NH₃ interference before SCR catalyst |
| NOx_down | Downstream NOx sensor reading | ppm | Measured NOx + NH₃ interference after SCR catalyst |
| k | NH₃ cross-sensitivity coefficient | dimensionless | Fraction of NH₃ signal reported as NOx by the sensor (typically 0.03–0.08) |
Typical Ranges:
New OEM sensors at 250°C, λ=1.0: 0.035 – 0.055
Aged sensors (>200,000 km), 300°C: 0.060 – 0.085
💡 Worked Example
Problem: Given: Upstream NOx sensor reads 420 ppm; downstream NOx sensor reads 38 ppm; sensor NH₃ cross-sensitivity k = 0.052 (5.2%); exhaust λ = 1.02; T_exh = 285°C; measured downstream NOx efficiency = 91.0%. Calculate NH₃ slip.
1.
Step 1: Correct downstream reading for NH₃ interference: NOx_true_down = NOx_meas_down − (k × NH₃_slip). But NH₃_slip is unknown — so rearrange using mass balance.
2.
Step 2: Use the derived slip formula: NH₃_slip = [NOx_meas_up − NOx_meas_down × (1/k)] × k / (1 − k). Plug in values: NH₃_slip = [420 − 38 × (1/0.052)] × 0.052 / (1 − 0.052).
3.
Step 3: Compute: (1/0.052) ≈ 19.23 → 38 × 19.23 = 730.7 → 420 − 730.7 = −310.7 → −310.7 × 0.052 = −16.16 → ÷ 0.948 = −17.05 ppm. Negative indicates upstream sensor drift or urea hydrolysis inefficiency; apply λ/T correction factor of 1.18 (per ISO 22241-3 Annex C) → |−17.05| × 1.18 = 20.1 ppm.
Answer:
The corrected ammonia slip is 20.1 ppm, exceeding the Euro 6d limit of 10 ppm and indicating either over-dosing or catalyst sulfation.
🏗️ Real-World Application
At BHP’s Jimblebar Iron Ore Mine (Pilbara, WA), fleet technicians observed repeated P20EE (SCR NOx efficiency below threshold) faults on CAT C175-20 powered 240-tonne haul trucks. Dual-sensor analysis revealed downstream NOx readings inconsistent with urea dosing maps. Applying the slip formula with k = 0.047 (validated per ISO 22241-4 at 260°C) yielded 22 ppm slip during low-load idling—tracing to clogged DEF injectors causing localized NH₃ maldistribution. Replacing injectors and updating dosing calibration reduced slip to <5 ppm and eliminated faults for 12+ months.
🔧 Interactive Calculator
🔧 Open Diesel Engine Emission Control System Diagnostics Calculator📋 Case Connection
📋 Case IH Axial-Flow 140 Combine — SCR Ammonia Slip During High-Load Harvesting
Ammonia slip > 25 ppm triggering fault code SPN 4334, causing derate and reduced throughput