Which system primarily helps to reduce NOx emissions in a vehicle?

The system that primarily helps to reduce NOx emissions in a vehicle is exhaust gas recirculation (EGR). The EGR system works by recirculating a portion of the vehicle's exhaust gases back into the combustion chamber. This process helps to lower the peak combustion temperatures, which is a key factor in forming nitrogen oxides (NOx) during combustion. By reducing these temperatures, the EGR system effectively decreases the amount of NOx produced, thereby helping to meet stringent emission standards.

The other systems listed are important for different functions within the vehicle but do not primarily target the reduction of NOx emissions. For example, positive crankcase ventilation (PCV) is aimed at reducing hydrocarbon emissions by recirculating crankcase gases back into the intake manifold, while the fuel injection system's primary purpose is to deliver the correct amount of fuel to the engine efficiently. The mass airflow system measures the amount of air entering the engine to ensure the proper fuel-air mixture but does not directly affect NOx emission levels.