Design and optimization of a throttle body assembly by CFD analysis

Throttle body assembly plays a vital role in metering the airflow. It mainly consists of a butterfly valve to vary the flow area to control air flow rate through it. There is hardy any established procedure to design a throttle body assembly based on the engine specifications. In order to bridge the gap, this study, design and optimization of a throttle body assembly for a single-cylinder engine used in two-wheeler application has been analyzed along with the investigation of critical flow through various sub systems using computational fluid dynamics (CFD). To start with, the throttle bore and bypass passage diameters are calculated from the basic flow equations. Using CFD, best possible throttle shaft profile is arrived at, which will enhance airflow to the engine. The airflow rate for different throttle openings is predicted taking into account the distribution of main and bypass flow. It is observed that the airflow through main and the bypass passage are almost same around 12% throttle opening and the airflow through main passage takes over beyond 25% opening. The novelty of this study is that airflow through the bypass is also predicted for different screw positions. From the analysis of results, it is found that with around two turns of bypass screw opening, the required amount of air flow rate could be achieved through the bypass passage to run the accessories of the engine at idling and also to meet the required performance and emissions levels as per the design target. In addition, there is a good agreement of CFD predictions with experimental results with an error of about 6%. Finally, it is concluded that the procedure adopted in this study to design the throttle body as per engine specifications will be very useful for the engine designers and in this aspect, CFD plays an important role.