Post Oxidation Study During Secondary Exhaust Air Injection for Fast Catalyst Light-Off

To comply with ever more stringent emission limits, engineers are studying and optimising gasoline engine start-up and warm-up phases. Secondary air injection (SAI) represents one option to reduce emissions by post-oxidizing products of a rich combustion like HC, CO and H₂. With this approach, the faster catalytic converter light-off allowed by the increase in exhaust temperature leads to a significant HC emissions reduction. All the mechanisms involved in post oxidation downstream of the exhaust valve are not well-known. In order to achieve substantial improvements, various SAI strategies were studied with a conventional PFI gasoline engine. Tests have been carried out both on steady-state running conditions and on transient warm-up phases at engine test bench. Various specific experimental devices and methodologies were developed. For example, the use of fast HC and temperature measurements is coupled with exhaust gas flow rate modeled with system simulation. This approach permits to study precisely the initiation and evolution of post oxidation at engine’s cycle scale. 3D CFD calculations have also been performed for understanding fresh air and burned gases mixing during the different exhaust phases as well as post-oxidation initiation and propagation. Additionally, sources and locations of heat release within the exhaust line have been quantified to make the difference between the power actually used to heat-up the catalytic converter and the heat losses. Owing to the better understanding of the complex phenomena taking place, fine tuning of engine and secondary air settings lead to significant improvement in catalyst heating up and HC tailpipe emissions reduction.