With the introduction of stringent emission norms for two wheeler applications, and need to reduce green house gases emission, it is a challenging task to improve the performance of the engine, especially at part throttle conditions. The typical drawback of spark ignition engine is poor part throttle performance and the Indian driving cycle (IDC) predominantly covers these part throttle operations. Contrary to popular belief, carbureted two wheelers with engine capacity up to 250cc are able to meet regulation Euro III with add-ons like secondary valve and catalyst. Vehicles with EFI systems are more expensive and they are not able to provide any fuel economy benefit or improvement in drivability. Under these circumstances, carburetor designers are working continuously to improve its performance.
In order to improve the performance, optimization of slow speed to high speed fueling circuit dividing ratio is of immense importance. Greater slow speed fueling circuit contribution at low speeds helps to improve fuel economy and reduce mass emission. However, extending the slow speed fueling circuit contribution can end up in compromising the drivability and cold start recovery in the process.
Based on the above considerations, a concept was designed to introduce a path to guide the flow around the throttle valve. In the carburetor different combination paths were conceived for carrying out the trials to confirm the improvement in performance. Before making the proto samples for measuring the performance, numerical tool was used to understand the flow behavior in order to finalize the concepts. Results from the numerical tools were used to optimize the sizing of the flow path. Samples were tested experimentally both in flow bench and as vehicle. The vehicle results showed significant improvement in slow speed circuit region, without sacrificing the top end power, drivability and cold starting.