Influence of asymmetry ratio on exhaust pulse characteristics of asymmetric twin-scroll turbocharged heavy-duty diesel engines

The Asymmetry Ratio (ASY), as a defining feature of asymmetric twin-scroll turbines, is instrumental in governing the energy distribution of engine exhaust pulses, which in turn affects overall engine performance and emissions. This paper pioneers an investigation into how ASY affects the exhaust pulse of an asymmetric twin-scroll turbocharged heavy-duty engine. Firstly, the test bench and research methodology required were introduced. Subsequently, the engine’s high-frequency exhaust pressure, alongside other steady-state operational data, was acquired. Finally, an exhaust pulse decoupling model was employed to derive the high-speed temperature, turbine flow, and isentropic energy for the asymmetric twin-scroll turbine, which uncovered the governing principles and effects of ASY on the critical variables of the exhaust pulse. The findings demonstrated that with an increasing ASY, the pulse intensity at the larger scroll inlet was amplified, while the pressure pulse magnitude at the smaller scroll inlet rose sharply. Conversely, the exhaust pulse strength at the smaller scroll inlet diminished, accompanied by a slight increase at the larger scroll, resulting in a minor enhancement of the overall pulse intensity. ASY also altered the relationship between flow parameters and expansion ratios, shifting toward lower flow parameters and higher expansion ratios at the larger scroll and the opposite at the smaller scroll. With increased ASY, turbine isentropic energy proportion decreased in the low blade speed ratio (U/C) region and increased in the high U/C region, enhancing turbine isentropic efficiency. Turbine efficiency at 1100 r/min was 3%–4% higher than at 800 r/min with minimal change in total isentropic power. Relative specific fuel consumption improved, and heat loss decreased, suggesting a larger ASY should be adopted when exhaust gas recirculation (EGR) capability is sufficient.