The present study experimentally investigates three different automotive turbochargers of varying turbine/wastegate combination: two of similar turbine housing (BorgWarner) but different bypass throat diameter (20 and 26 mm), and a third of both different housing (Honeywell) and throat size from the former two (21 mm). The effects of turbine housing flow passage design and bypass throat size on open wastegate turbine performance and wastegate flow efficiency were examined at discrete wastegate valve openings: 5¿¿, 10¿¿, 20¿¿, and 40¿¿. The study also analyzes the effects of these geometrical differences on change in flow rate through the wastegate when examined independent of or in parallel with the rotor. Steady flow experiments were performed on a flow bench and cold-flow turbocharger experimental stand. A semi-empirical physical model has also been developed in this study for 1-D engine simulation codes to better characterize the parallel flow paths through the turbines and thus improve predictive accuracy of open wastegate performance. Measured turbine characteristics with closed wastegate were extrapolated and interpolated with a custom preprocessor and input to the code for the rotor, while wastegate flow was simulated in two ways: as an effective orifice area applied at the rotor (default approach) or as a physical parallel path (proposed alternative).
As the wastegates were opened under parallel flow, total turbine mass flow parameter (MFP) increased in proportion to wastegate size at 5¿¿ and 10¿¿ positions for similar total-to-static expansion ratio (ER-ts) and speed parameter. At larger openings, the combined effect of housing design and wastegate flow efficiency are of greater effect; the Honeywell turbine exhibited a similar increase in total MFP from 20¿¿ to 40¿¿ as from 10¿¿ to 20¿¿, while the BorgWarner turbines showed a substantially diminished gain in total MFP from 20¿¿ to 40¿¿ for a given ER-ts and speed parameter.
Wastegate-alone flow efficiency experiments revealed that the Honeywell wastegate discharge coefficient is higher than both BorgWarner wastegates at 40¿¿ for fixed ER-ts, and it was either equivalent to or slightly greater than the 26 mm BorgWarner wastegate at smaller openings. The discharge coefficient for the 20 mm BorgWarner wastegate was greater than the 26 mm variant for all fixed openings and ER-ts. All wastegates exhibited a slight increase in discharge coefficient with ER-ts. Estimation of combined rotor-and-wastegate flow by adding wastegate-alone and rotor-alone flows resulted in significant over-estimation of total MFP for the BorgWarner turbines. The error increased with bypass opening and, with few exceptions, was greater for the 26 mm than the 20 mm wastegate turbine for fixed opening degree and ER-ts. For the Honeywell turbine, estimated total turbine MFP was generally within ¿¿5% of measured quantities for all openings and ER-ts.
Using rotor-alone MFP characteristics and wastegate-alone flow efficiency measurements, the predictive accuracy of open wastegate turbine flow in a 1-D code was improved by physically modeling the parallel bypass path. The rotor-and-wastegate flow predictions for the 26 mm wastegate BorgWarner turbine were particularly more accurate, most significantly at ER-ts>1.4: error ranged from -0.50 to 22.94% under the default approach, whereas in the proposed model the error was confined to -4.62 to 3.21%.