A diesel engine will produce excess amounts of nitrogen oxides (NOx) when

• A. It is idling
• B. Under heavy load
• C. At high RPM
• D. During cold start

Answer: (A)

NOx forms mainly when combustion temperatures are high, because the heat accelerates the chemical reaction between nitrogen in the air and oxygen in the combustion gases. When the engine is under heavy load, it demands more fuel and produces higher peak combustion temperatures and longer high-temperature durations, so more NOx can form. At idle, fuel use is low and the combustion temperature is typically cooler, which reduces NOx production. Cold starts can temporarily raise NOx due to rapid temperature changes and less-than-ideal combustion conditions as the engine warms up, but the dominant factor is peak temperature during combustion, which is greatest under heavy load.