Under what vehicle condition will excessive NO emissions be tested to duplicate a concern?

Excessive nitrogen oxide (NOx) emissions are typically linked to specific driving conditions that promote high combustion temperatures. Light acceleration is an important condition for testing NO emissions because it often represents a scenario where the engine operates at a relatively high load but not at full throttle. During light acceleration, the fuel-air mixture can become leaner, resulting in higher combustion temperatures and increased formation of NOx as a byproduct of combustion.

In contrast, at idle, the engine is not under significant load, which can result in lower combustion temperatures and reduced NO emissions. Heavy acceleration tends to lead to richer fuel mixtures, which also decreases NOx production. During deceleration, the engine may enter fuel-cut-off mode or operate at a stoichiometric or even rich mixture, further reducing NOx emissions. Thus, light acceleration is a critical condition to duplicate when testing for excessive NOx emissions, as it best simulates the engine conditions under which these emissions may increase significantly.