A construction site generator set equipped with a Tier 4 final engine is observed emitting a strong ammonia-like odour from the exhaust. The DPF and SCR systems are fully operational, and no engine warning lights are illuminated. What is the most probable cause of this odour?

Question: A construction site generator set equipped with a Tier 4 final engine is observed emitting a strong ammonia-like odour from the exhaust. The DPF and SCR systems are fully operational, and no engine warning lights are illuminated. What is the most probable cause of this odour?

Answer options:

• Excessive fuel regeneration of the DPF.
• Insufficient DEF injection rate by the SCR system. ✅ Over-injection of Diesel Exhaust Fluid (DEF) by the SCR system.
• Failure of the upstream NOx sensor, causing incorrect DEF dosing.

Correct answer: Over-injection of Diesel Exhaust Fluid (DEF) by the SCR system.

Explanation: A strong ammonia odour often indicates 'ammonia slip' in an SCR system, which occurs when too much DEF is injected or the catalyst is not efficient enough to convert all the ammonia and NOx, allowing excess ammonia to pass through the exhaust. This could also be a result of the upstream and downstream NOx sensors being out of calibration, resulting in excessive DEF being injected to meet NOx targets.