📋 Case Study

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

🏗️ Project Overview

High-yield corn harvest in Iowa under 95°F ambient conditions

🎯 Challenge

Ammonia slip > 25 ppm triggering fault code SPN 4334, causing derate and reduced throughput

🔧 Design Approach

Urea injector pulse-width optimization + dual-point NOx sensor recalibration + exhaust residence time modeling

📐 Design Diagram

_in NOx_out Dual-point recalibration Exhaust Out Ammonia Slip AS = 28.6 ppm (k × (Urea_Dose − 1.2×NOx_Load)) SPN 4334 AS > 25 ppm Exhaust flow Urea injection NOx sensing

AI-generated project design illustration

📐 Key Calculations

Residence Time at 220°C

τ = V_reactor / Q_exh

Result: 0.42 sec

Below minimum 0.5 sec required for full hydrolysis

Ammonia Slip Prediction

AS = k × (Urea_Dose − 1.2 × NOx_Load)

Result: 28.6 ppm

Validated root cause as overdosing

📊 Results

Ammonia slip reduced to < 5 ppm; no derates observed across 3-week harvest window

💡 Lessons Learned

• •Urea dosing maps must be load- and temperature-compensated—not just speed-based
• •Single-point NOx sensor calibration is insufficient for SCR tuning

✅ Key Takeaways

• 1Urea dosing maps must be load- and temperature-compensated—not just speed-based
• 2Single-point NOx sensor calibration is insufficient for SCR tuning