Secondary Air System: Notes

As on N73 engine, the N74 is equipped with a secondary air system. Blowing additional air (secondary air) into the exhaust gas duct in the cylinder head during the warm-up phase initiates thermal post-combustion that leads to a reduction in the unburned hydrocarbons (HC) and carbon monoxide (CO) contained in the exhaust gas. The energy generated here heats up the catalytic converter faster in the warm-up phase and increases its conversion rate. The catalytic converter response temperature (light-off temperature) of 300°C is reached only a few seconds after the engine is started.

What is new is that there is one pressure sensor before each secondary air valve. The function of the secondary air system is monitored by registering the pressure conditions.