Analysis and Modeling of Heat Transfer in the SI Engine Exhaust System During Warm-Up

In order to meet the severe emission restrictions imposed by SULEV and EURO V standards the catalytic converter must reach light-off temperature during the first 20 seconds after engine cold start. Thermal losses in the exhaust manifold are driven by the heat transfer of the pulsating and turbulent exhaust flow and affect significantly the warm-up time of the catalyst. In the present paper an investigation concerning the gas-side heat transfer in the exhaust system of a spark ignited (SI) combustion engine with retarded ignition timing and secondary air injection into the exhaust port is reported. Based on this analysis, the warm-up simulation of a one-dimensional flow simulation tool is improved for an evaluation of different exhaust system configurations.

Advanced heat transfer measurements with high resolution heat-flux-sensors and gas temperature measurements with thin-wire-thermocouples were performed on a cooled exhaust pipe in order to obtain a better understanding of the gas-side heat transfer during the warm-up phase. By means of a thermography camera, the influence of pipe bends and the disturbing effects of the thermocouple installation on heat transfer were examined. The pulsating turbulent flow in the exhaust manifold was further visualized with the Particle-Image-Velocimetry (PIV) measurement technique. Investigations on the exothermic reactions in the exhaust manifold were carried out with a transparent exhaust manifold pipe and a light-intensified CCD camera.

The proposed heat transfer model for the turbulent pulsating exhaust flow is able to predict the gas-side thermal flux along the exhaust pipe during engine cycle with an increased accuracy. A new validation method for simulated mass-averaged gas temperatures adopting resolution-restricted gas temperature measurements with thermocouples is presented and a post-oxidation model for the exhaust flow is applied. The research work shows that engine warm-up with retarded ignition timing and secondary air injection in the exhaust port can be simulated with the proposed heat transfer model taking into account the exothermic effects of post-oxidation in the exhaust manifold. The improved model is able to support the development process in an early stage, reducing significantly the number of investigated exhaust system configurations.