Optimization of a Fast Light-off Exhaust System for Motorcycle Applications

Emissions standards for two- and three-wheeled powered vehicles are getting more and more stringent, and measurement procedures require to perform driving cycles with engine cold start. Therefore, a fast activation of the exhaust catalytic converter is of primary importance. In this work a numerical and experimental study of the exhaust system layout of a 125cc scooter has been carried out with the main objective of reducing the catalytic converter light-off time, without affecting engine performance and component cost. First, a 1D engine model has been developed to evaluate the impact of the component modification on engine performance. Then, a CFD-3D analysis has been performed to assess and evaluate the velocity and temperature fields of the gases inside of the muffler. After the numerical study, several prototypes have been designed and built for experimental tests. The engine has been installed on the dynamometric bench and instrumented. The exhaust system prototypes have been tested focusing on the engine brake performance and on the exhaust temperatures during warm-up transients. The latter has been monitored in several points inside the muffler, in order to obtain information about the catalytic converter operating conditions. The best prototype configurations have been installed on the vehicle and further road tests. The vehicle experimental results in terms of exhaust gas temperatures at the catalyst inlet and outlet highlight the improvements with the best exhaust prototype compared to the original configuration.