Aaron W. Costall

Pulse Performance Modeling of a Twin Entry Turbocharger Turbine Under Full and Unequal Admission

The pulsating nature of 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 submodel accuracy essential for virtual product development meeting U.S. 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 although unequal admission results indicate the need for further model development.

Assessment of Unsteady Behavior in Turbocharger Turbines

The flow in turbocharger turbines is highly unsteady in nature as it responds to the exhaust manifold of an internal combustion engine. This paper investigates the significance of unsteadiness by examining first its relevance to real engine situations and then its effect on turbocharger turbine operation. The engine simulations carried out show the relevance of the Strouhal number effect for real turbocharger applications, which has been demonstrated experimentally on a turbine stage test stand. Therefore, for realistic multiple-cylinder-engine configurations with different exhaust gas pipe lengths and firing frequencies the importance of the actual unsteady behavior needs careful assessment. The effect upon the turbine itself is examined by modeling the laboratory arrangement to replicate the test stand configuration and operation using a one-dimensional wave action code. The 1D model is validated against experimental results obtained using a new permanent magnet eddy-current dynamometer for a mixed flow turbine suitable for a medium-sized automotive application covering an equivalent speed range of 50–100%, U2 /Cis of 0.3–1.1 and a pulse frequency of 20–80 Hz. The turbine model has been refined using unsteady experimental data and so enables the capture of unsteady effects in engine design codes. The beneficial effect of the ability of this model to predict turbine mass flow is discussed.Copyright

Effect of volute geometry on the steady and unsteady performance of mixed-flow turbines

Abstract The steady and unsteady performance of two mixed-flow turbocharger turbine rotors has been investigated. These rotors differ mainly in their inlet blade angle geometry; one has a constant blade angle (rotor A) and the other a notionally constant incidence angle (rotor B). The results indicate that the constant blade inlet angle design offers improved efficiency characteristics over the constant incidence one with total-to-static efficiency at the design point ranging between 0.7 and 0.76. A detailed assessment of the influence of volute geometry on the turbine performance has been carried out, which confirmed that the geometry of volute plays a critical role in the overall performance of a turbine. The unsteady performance tests have indicated a substantial deviation from the performance and flow characteristics of equivalent steady-state tests, which is quantified and discussed.

Comparison between the steady performance of double-entry and twin-entry turbocharger turbines

Most boosting systems in internal combustion engines utilize ‘pulse turbocharging’ to maximize the energy extraction by the turbine. An internal combustion engine with more than four cylinders has a significant overlap between the exhaust pulses which, unless isolated, can decrease the overall pulse energy and increase the engine pumping loss. Thus, it is advantageous to isolate a set of cylinders and introduce the exhaust gases into two or more turbine entries separately. There are two main types of multiple entry turbines depending on the method of flow division: the twin-entry and the double-entry turbine. In the twin-entry design, each inlet feeds the entire circumference of the rotor leading edge regardless of inlet conditions. In contrast, the double-entry design introduces the flow from each gas inlet into the rotor leading edge through two distinct sectors of the nozzle. This paper compares the performance of a twin and double-entry mixed flow turbine. The turbines were tested at Imperial College for a range of steady-state flow conditions under equal and unequal admission conditions. The performance of the turbines was then evaluated and compared to one another. Based on experimental data, a method to calculate the mass flow under unequal admission from the full admission maps was also developed and validated against the test results.Copyright

Off-Road Diesel Engine Transient Response Improvement by Electrically Assisted Turbocharging

Turbocharged diesel engines are widely used in off-road applications including construction and mining machinery, electric power generation systems, locomotives, marine, petroleum, industrial and agricultural equipment. Such applications contribute significantly to both local air pollution and CO2 emissions and are subject to increasingly stringent legislation. To improve fuel economy while meeting emissions limits, manufacturers are exploring engine downsizing by increasing engine boost levels. This allows an increase in IMEP without significantly increasing mechanical losses, which results in a higher overall efficiency. However, this can lead to poorer transient engine response primarily due to turbo-lag, which is a major penalty for engines subjected to fast varying loads. To recover transient response, the turbocharger can be electrically assisted by means of a high speed motor/generator. When the engine load is increased, the electrical machine acts as a motor to accelerate the turbocharger so that the torque demand can be met rapidly. Conversely, when boost delivery exceeds demand the electrical machine can act as a generator to recover energy that would otherwise be wastegated. This paper presents a model for the transient response of the electrically-assisted turbocharged engine when subjected to a step increase of torque demand. The base model is representative of a 7-litre turbocharged intercooled diesel engine and has been implemented in Matlab-Simulink and calibrated against test bed data. The model is used for the analysis of the dynamic behaviour of the engine with different levels of electric assist to the turbocharger. The results show that while turbocharger response improves with electric assist, compressor surge can occur in generating mode and that limitations on electric assist power are present.

Fundamental Characterization of Turbocharger Turbine Unsteady Flow Behavior

Fluid flow in the volute of a turbocharger turbine can be decidedly unsteady due to the pulsating nature of the exhaust gas in the manifolds of an internal combustion engine. Despite this it is conventional to use a quasi-steady or “filling and emptying” technique to model the turbine in one-dimensional turbocharged engine simulations. Depending on the inherent level of unsteadiness, this approach may be insufficient to capture the true turbine operation since neither method is able to resolve unsteady effects due to the presence of any wave action in the flow. Building on previously reported work, this paper aims to establish a measure of unsteadiness that takes account of the attributes of engine exhaust gas flow that give rise to gas dynamic unsteadiness. This characterization is achieved by decomposing the pulse into its constituent frequencies using Fourier analysis. A one-dimensional wave action code, featuring a bespoke boundary condition that permits application of a pressure pulse in Fourier series form, is used to investigate the effect of the contributing variables for some simplified cases. This allows the construction of the correct form of dimensionless parameter. Finally, the new dimensionless measures, the Fourier series Strouhal and acoustic Strouhal numbers (FSt and FaSt respectively), are evaluated at different test conditions to establish criteria for the transition from a filling and emptying mode to gas dynamic operation. The analysis suggests limiting values of FSt≤0.15, and FaSt≤0.02, to be used as an approximate guide for turbine model selection.Copyright

Electric Turbo Assist as an Enabler for Engine Downspeeding

Electric Turbo Assist (ETA) is a novel air system technology that provides an extra degree of freedom in engine-turbocharger matching for optimal system performance and emissions. This paper presents simulation results from a research project to develop and demonstrate ETA, an integrated turbocharger-motor/generator, for a particular off-road machine duty cycle. Engine cycle simulation tools have been employed to investigate the potential of ETA to reduce fuel consumption through a combined downspeeding and exhaust energy regeneration strategy while meeting the required emissions levels and maintaining the desired engine response. Results show that ETA enables a flexible air system that can meet and even exceed the required air-fuel ratio and exhaust gas recirculation targets when the engine is downspeeded, while providing a useful fuel saving over the non-ETA baseline engine calibration.Copyright

Detailed Study of Pulsating Flow Performance in a Mixed Flow Turbocharger Turbine

Automotive turbocharger turbines experience a highly unsteady and pulsating flow field due to the abrupt operation of the exhaust valves in a reciprocating internal combustion engine. Previous work has demonstrated and validated against experiment a computational model of a turbine stage under such conditions. The same model is used in the present paper to examine in greater detail the complex flow characteristics observed. The pulsating inlet condition results in a highly disturbed flow field in the turbine stage, the main features of which have already been identified. The effect of the passing of the blades at the volute tongue is observed, and the fluctuating velocity field in the blade passages is seen to lead to poor flow direction control at the turbine inlet and exit. The turbine geometry, calculated for steady flow, is forced to operate away from design conditions for most of the pulse period. Through a detailed analysis of the intricate flow field features at varying instants during the pulse period, this paper highlights areas of the blade geometry and periods in the pulse profile that should be investigated further, such that the integrated performance across the entire pulse cycle can be improved.© 2005 ASME

Turbocharger Matching Method for Reducing Residual Concentration in a Turbocharged Gasoline Engine

In a turbocharged engine, preserving the maximum amount of exhaust pulse energy for turbine operation will result in improved low end torque and engine transient response. However, the exhaust flow entering the turbine is highly unsteady, and the presence of the turbine as a restriction in the exhaust flow results in a higher pressure at the cylinder exhaust ports and consequently poor scavenging. This leads to an increase in the amount of residual gas in the combustion chamber, compared to the naturally-aspirated equivalent, thereby increasing the tendency for engine knock. If the level of residual gas can be reduced and controlled, it should enable the engine to operate at a higher compression ratio, improving its thermal efficiency. This paper presents a method of turbocharger matching for reducing residual gas content in a turbocharged engine. The turbine is first scaled to a larger size as a preliminary step towards reducing back pressure and thus the residual gas concentration in-cylinder. However a larger turbine causes a torque deficit at low engine speeds. So in a following step, pulse separation is used. In optimal pulse separation, the gas exchange process in one cylinder is completely unimpeded by pressure pulses emanating from other cylinders, thereby preserving the exhaust pulse energy entering the turbine. A pulse-divided exhaust manifold enables this by isolating the manifold runners emanating from certain cylinder groups, even as far as the junction with the turbine housing. This combination of appropriate turbine sizing and pulse-divided exhaust manifold design is applied to a Proton 1.6-litre CamPro CFE turbocharged gasoline engine model. The use of a pulse-divided exhaust manifold allows the turbine to be increased in size by 2.5 times (on a mass flow rate basis) while maintaining the same torque and power performance. As a consequence, lower back pressure and improved scavenging reduces the residual concentration by up to 43%, while the brake specific fuel consumption improves by approx. 1%, before any modification to the compression ratio is made.

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.