Thermal mass and external insulation of exhaust manifolds and down pipes have always been known as main factors that could significantly influence the inlet gas temperature of catalytic converters. However, heat loss from exhaust gas to the environment is first influenced by the internal gas flow. This indicates the importance of gas-to-wall heat transfer.
In the present work, a basic heat-transfer analysis was performed to assess and rank the relative impact of thermal mass, external heat insulation and internal gas flow on the exhaust manifold outlet temperature.
For a Euro 4 application, where the light-off time is about 15-30 s, the results show that internal flow plays the most important role in heat loss from the exhaust gases. Thermal mass and external heat insulation start to be important only once the material temperature is high, beyond the first 30 sec. Therefore improving internal flow of the exhaust manifold will be crucial to minimizing light-off time.
The conclusions derived from our investigation are significant with regard to the trade off between fabricated and cast iron exhaust manifolds and will allow a better selection of the technology type depending on the application.