Blockage of the air-to-air after cooler would most likely increase which exhaust pollutant?

• A. Oxides of nitrogen
• B. Carbon dioxide
• C. Hydrocarbons
• D. Carbon monoxide

Answer: (A)

Blockage of the air-to-air aftercooler raises the temperature of the charge air entering the engine. Hotter intake air increases the in-cylinder peak combustion temperature. When combustion temperatures are higher, nitrogen and oxygen in the hot flame react more readily to form nitrogen oxides (NOx) through the high-temperature (thermal) NOx mechanism. So, despite possibly lower air density due to the heat, the rise in peak temperatures drives more NOx formation than the other pollutants. Carbon monoxide and hydrocarbons tend to be higher with incomplete combustion or poorer mixing, which is not the primary effect of hotter charge air, and carbon dioxide depends on the total amount of fuel burned, which can be reduced if the air is less dense. The key idea is that higher combustion temperatures from hotter intake air promote NOx production.