Meng Soon Chiong

Unsteady Performance Prediction of a Single Entry Mixed Flow Turbine Using 1-D Gas Dynamic Code Extended With Meanline Model

Turbochargers are widely regarded as one of the most promising enabling technology for engine downsizing, in the aim to achieve better specific fuel consumption, thermal efficiency and most importantly carbon reduction. The increasing demand for higher quality engine-turbocharger matching, leads to the development of computational models capable of predicting the unsteady behaviour of a turbocharger turbine when subjected to pulsating inlet flow. Due to the wide range of engine loads and speed variations, an automotive turbocharger turbine model must be able to render all the frequency range of a typical exhaust pulse flow. A purely one-dimensional (1-D) turbine model is capable of good unsteady swallowing capacity prediction, provided it is accurately validated. However, the unsteady turbine power evaluation still heavily relies on the quasi-steady assumption. On the other hand, meanline model is capable of resolving the turbine work output but it is limited to steady state flow due to its zero dimensional nature.This paper explores an alternative methodology to realize turbine unsteady power prediction in 1-D by integrating these two independent modelling methods. A single entry mixed-flow turbine is first modelled using 1-D gas dynamic method to solve the unsteady flow propagation in turbine volute while the instantaneous turbine power is subsequently evaluated using a mean-line model. The key in the effectiveness of this methodology relies on the synchronization of the flow information with different time-scales. In addition to the turbine performance parameters, the common level of unsteadiness was also compared based on the Strouhal number evaluations. Comparison of the quasi-steady assumption using the experiment results was made in order to further understand the strength and weaknesses of corresponding method in unsteady turbine performance prediction. The outcomes of the simulation showed a good agreement in the shape and trend profile for the instantaneous turbine power. Meanwhile the predicted cycle-averaged value indicates a positive potential of the current turbine model to be expanded to a whole engine simulation after few minor improvements.Copyright

Assessment of Cycle Averaged Turbocharger Maps Through One Dimensional and Mean-Line Coupled Codes

Downsizing the internal combustion engine has been shown to be an effective strategy towards CO2 emissions reduction, and downsized engines look set to dominate automotive powertrains for years to come. Turbocharging has been one of the key elements in the success of downsized internal combustion engine systems. The process of engine-turbocharger matching during the development stage plays a significant role towards achieving the best possible system performance, in terms of minimizing fuel consumption and pollutant emissions. In current industry practice, engine modeling in most cases does not consider the full unsteady analysis of the turbocharger turbine. Thus, turbocharged engine performance prediction is less comprehensive, particularly under transient load conditions. Commercial one-dimensional engine codes are capable of satisfactory engine performance predictions, but these typically assume the turbocharger turbine to be quasi-steady, hence the inability to fully resolve the pulsating flow performance. On the other hand, a one-dimensional gas dynamic turbine model is capable of simulating the pressure wave propagation in the model domain, thus serving as a powerful tool to analyze the unsteady performance. In addition, a mean-line model is able to compute the turbine power and efficiency through the conservation method and Euler’s Turbomachinery Equation. However, none of these modeling methods have been widely implemented into commercial one-dimensional engine codes thus far. The objective of this paper is to assess the possibility of numerically producing the steady equivalent cycle averaged turbocharger turbine maps, which could be used in commercial engine codes for performance prediction. The cycle-averaged maps are obtained using a comprehensive turbocharged engine model including accurate pulsating exhaust flow performance prediction. The model is validated against experimental results and effects of flow frequency on the maps are discussed in detail.

Assessment of Partial-Admission Characteristics in Twin-Entry Turbine Pulse Performance Modelling

One-dimensional modelling of a twin-entry turbine usually considers only the full-admission characteristic in their analysis. However, due to out-of-phase exhaust flow, the actual operating environment is constantly under the intermittent combination of full, unequal and partial-admission conditions. This leads to unsatisfactory on-engine performance prediction, even though some present models have already accounted the finite twin-entry volute and interaction between the volute entries. This paper explores the potential improvement in twin-entry pulse flow model by including the partial-admission characteristics through the established one-dimensional model domain. The predicted results are validated against the experimental data obtained from the Imperial College pulse-flow testing facility. In addition, influences of the quality of partial-admission performance map are also analysed. The mathematical prediction methodology, which was revised from literature works, derived the twin-entry turbine partial-admission characteristics from the known full-admission performance. This study is intended to outline the importance of twin-entry turbine partial-admission characteristics in pulse performance modelling. In comparison to the literature findings, current model has satisfactorily resolved the twin-entry out-of-phase pulse flow performance, particularly the instantaneous actual power. The model prediction shows a twin-entry turbine unsteady swallowing capacity is mostly encapsulated within the quasi-steady full and partial-admission characteristic lines. On the other hand, the unsteady actual power hysteresis curve is found beyond quasi-steady characteristic lines at most instant throughout the pulse cycle.Copyright

Assessment of Turbocharger Turbine Unsteady Flow Modelling Methodology on Engine Performance

Although it is well known that the flow entering a turbine of a turbocharger engine is highly unsteady, engine manufacturers prefer to use turbine performance predictions that are based on steady-state performance maps, which inherently lead to inaccuracies in the turbines behavior and mismatches between turbocharger turbines and engines. The reason for this preference is due to the turbocharger turbine design software that are generally available to engine manufacturers being based on and compatible with steady-state performance maps and this fact led researchers to investigate how the inaccuracies of this steady-state treatment of the turbine can be alleviated. To this effect, this paper investigates how modelling techniques on Ricardo Wave, a 1D gas dynamics engine simulation software, gives rise to more accurate turbine swallowing curve predictions using steady-state maps. In particular, the turbine being investigated is that of Szymko [1], which is a twin nozzleless mixed-flow turbine that is being powered by a 10 litre, 6 cylinder 4 stroke diesel engine with an operating range from 800-2000 RPM for which 800, 1200 and 1600 engine RPM relate to 40, 60 and 80Hz exhaust gas pulse frequencies at the turbine. The main investigation in this paper is to demonstrate the capability of the engine simulation software to deal with unsteady flows and specifically to show the significant effect of accounting for the volute design in the single turbine wheel entry model. The data obtained in this investigation were compared with those of Szymko [1], which offered a validated set of data to compare against.