Experimental and Simulation Study of Zero Flow Impact on Hybrid Vehicle Emissions

Combustion engines in hybrid vehicles start and shut off several times during a typical passenger car trip. Each engine restart may pose a risk of excessive tailpipe emissions in real-drive conditions if the after-treatment system fails to maintain an adequate temperature level during engine off mode. In view of the tightening worldwide tailpipe emissions standards and real-world conformity requirements, it is important to detect and resolve such risks via reliable and cost-effective engineering tools that can perform accurate analysis of the thermal and chemical behavior of exhaust systems. In this work, we present a catalyst model that predicts the 3D thermal and chemical behavior under normal and zero flow conditions. Particular emphasis is given to the phenomena of free convection and thermal radiation dominating the heat transfer at zero flow. Next, we examine the impact of zero-flow duration on the exhaust system temperature and subsequent emissions risk and we validate the obtained results with respective measurements from experimental tests. Overall, the model can accurately predict the temperature distribution inside the catalyst and tail pipe emissions, under a broad range of operating conditions. The model can subsequently be used to study several scenarios of vehicle hybridization schemes, as well as techniques to minimize the risk of zero flow operation by proper system design and control.