Dynamic Analysis of a 4-Stroke Scooter Exhaust System using FEM 2001-01-1867
Proving of an exhaust system design against structural failures has always been a great challenge in a new 4-stroke scooter development. Also the development time of exhaust system is dependent on how quickly we solve the failure problems. The design has to take care of both static & dynamic failures. However, the experience of previous failures clearly indicates the chief cause as resonance taking place in the speed range of the vehicle.
Finite element (FE) model of the first proposal is made using the Solid models available. The natural frequencies are extracted from the FE model consisting of shell, beam & mass elements. The design of exhaust mounting bracket is found to be crucial in manipulating the frequencies. The physical prototype is made and the natural frequencies are found experimentally. These are compared with the FE predicted Eigen values. The FE model is fine tuned to match the predicted natural frequencies with the measured ones.
The validated FE model is used to try various design options for mounting bracket, tube thickness etc. to achieve the desired frequency pattern. Ideally, optimal design is the one where the first natural frequency does not fall in the speed range of the Scooter. However, this is quite impossible to achieve in most of the practical cases. Alternatively, the optimal design is achieved by placing the natural frequencies at non-critical speeds. This manipulation has become much more difficult in 4 stroke Scooter, as the excitation comes from both rotational & firing frequencies. Incidentally, the firing frequency is half of the rotational frequency in case of 4 stroke engines.
After no. of iterations using FE models, optimal frequency pattern is achieved. The prototype is made and tested both in laboratory & road under actual usage conditions as well as extreme conditions. As there are no failures observed in these tests, the pattern of frequencies proposed also got validated. Subsequently, when the exhaust system is redesigned for performance requirements, the bracket is redesigned achieving the same proven frequency pattern using FE analysis. The new prototype is only tested to verify the measured frequencies with the predicted ones. This design is then productionised, without going through extensive test procedures.
Thus, the total development time for the exhaust system has been considerably reduced due to the systematic analysis done using FEM.