Pulse performance modeling of a twin entry turbocharger turbine under full and unequal admission

The pulsating nature of the gas flow within the exhaust manifold of an internal combustion engine is not well captured by the quasi-steady techniques typically employed by cycle simulation programs for turbocharger modeling. This problem is compounded by the unequal admission conditions imposed on the turbine by the use of multiple entry housings, installed as standard on pulse turbocharged diesel engines. This unsteady behavior presents the simulation engineer with a unique set of difficulties when modeling turbocharger turbines. It is common for experienced analysts to accommodate multiple entries by splitting the flow across duplicate components and by tuning the level of interference between volute entries, but this necessarily bespoke approach is limited to upstream modifications that cannot capture true turbine unsteady operation. This paper describes recent simulation code development work undertaken at Caterpillar to improve machine sub-model accuracy essential for virtual product development meeting US nonroad Tier 4 emission standards. The resulting turbine performance model has been validated against experimental data for a twin entry turbocharger suitable for heavy duty nonroad applications, obtained using a permanent magnet eddy-current dynamometer and pulse flow test facility. Comparison between experiment and prediction demonstrates good agreement under full admission in terms of both instantaneous flow capacity and turbine actual power, though unequal admission results indicate the need for further model development.