Tomasz Duda

Investigation of Mechanical Loss Components and Heat Transfer in an Axial-Flux PM Machine

This paper investigates components of mechanical loss together with heat transfer effects in an axial-flux permanent-magnet motor. The mechanical loss components generated within electrical machines are well known; however, their prediction or derivation has not been widely reported in the literature. These, together with the electromagnetic loss sources and heat transfer effects, are crucial and must be accounted for when considering high-power-density, high-speed, and/or compact machine designs. This research is focused on separating the mechanical loss components to gain a more in-depth understanding of the effects and their importance. Both experimental and theoretical techniques have been employed in the analysis of a machine demonstrator. In particular, hardware tests with dummy rotors have been performed to measure the bearing and windage/drag loss components. These have been supplemented with computational fluid dynamics analysis to theoretically evaluate the aerodynamic effects occurring within the mechanical air gap accounting for loss and heat transfer. It has been identified that the analyzed hardware demonstrator suffered bearing loss significantly higher than that suggested by the bearing manufacturer. This has been attributed to design of the mechanical assembly accommodating bearings, which resulted in inappropriate bearing preload. The excessive bearing loss had a significant detrimental effect on the machine thermal behavior. In contrast, the aerodynamic effects have been found to have less pronounced effects here, due to fully enclosed and naturally cooled machine construction.

Investigation of mechanical loss and heat transfer in an axial-flux PM machine

This paper investigates components of the mechanical loss together with heat transfer effects in an axial-flux PM motor. The mechanical loss components generated within electrical machines are well known, however, their prediction or derivation has not been widely reported in the literature. These, together with the electromagnetic loss sources and heat transfer effects are crucial and must be accounted for when considering high-power density, highspeed and/or compact machine design. This research is focused on separating the mechanical loss components to gain a more in depth understanding of the effects and their importance. Both experimental and theoretical techniques have been employed in the analysis. In particular, hardware tests with dummy rotors have been performed to measure the bearing and windage/drag loss components. These have been supplemented with CFD analysis to theoretically evaluate the aerodynamic effects occurring within the mechanical air-gap accounting for loss and heat transfer. Further to these, a 3D lumped parameter thermal model of the axial-flux PM demonstrator has been developed to validate predictions of the mechanical loss components and heat transfer mechanisms. The theoretical findings show good agreement with experimental data. Moreover, the research outcomes suggest that the mechanical and aerodynamic effects require careful consideration when a less conventional machine design is considered.

Lumped Capacitance and Three-Dimensional Computational Fluid Dynamics Conjugate Heat Transfer Modeling of an Automotive Turbocharger

One dimensional wave-action engine models have become an essential tool within engine development including stages of component selection, understanding system interactions and control strategy development. Simple turbocharger models are seen as a weak link in the accuracy of these simulation tools and advanced models have been proposed to account for phenomena including heat transfer. In order to run within a full engine code, these models are necessarily simple in structure yet are required to describe a highly complex 3D problem. This paper aims to assess the validity of one of the key assumptions in simple heat transfer models, namely, that the heat transfer between the compressor casing and intake air occurs only after the compression process. Initially a sensitivity study was conducted on a simple lumped capacity thermal model or a turbocharger. A new partition parameter was introduced alpha, which divides the internal wetted area of the compressor housing into pre and post compression. The sensitivity of heat fluxes to alpha was quantified with respect to the sensitivity to turbine inlet temperature (TIT). At low speeds, the TIT was the dominant effect on compressor efficiency whereas at high speed alpha had a similar influence to TIT. However, modelling of the conduction within the compressor housing using an additional thermal resistance caused changes in heat flows of less than 10%. Three dimensional CFD analysis was undertaken using a number of cases approximating different values of alpha. It was seen that when considering a case similar to alpha=0, significant temperature could build up in the impeller area of the compressor housing, indicating the importance of the pre-compression heat path. The 3D simulation was used to estimate a realistic value for alpha which was suggested to be between 0.15 and 0.3. Using a value of this magnitude in the lumped capacitance model showed that at low speed there would be less than 1% point effect on apparent efficiency which would be negligible compared to the 8% point seen as a result of TIT. In contrast, at high speeds, the impact of alpha was similar to that of TIT, both leading to approximately 1% point apparent efficiency error.

Review of Turbocharger Mapping and 1D Modelling Inaccuracies with Specific Focus on Two-Stag Systems

The adoption of two stage serial turbochargers in combination with internal combustion engines can improve the overall efficiency of powertrain systems. In conjunction with the increase of engine volumetric efficiency, two stage boosting technologies are capable of increasing torque and pedal response of small displacement engines. In two stage serial turbocharges, a high pressure (HP) and a low pressure (LP) turbocharger are connected by a series of ducts. The former can increase charge pressure for low air mass flow typical of low engine speed. The latter has a bigger size and can cooperate with higher mass flows. In serial configuration, turbochargers are packaged in a way that the exhaust gases access the LP turbine after exiting the HP turbine. On the induction side, fresh air is compressed sequentially by LP and HP compressors. By-pass valves and waste-gated turbines are often included in two stage boosting systems in order to regulate turbochargers operations. One-dimensional modelling approaches are considered for investigating the integration of boosting systems with internal combustion engines. In this scenario, turbocharger behaviour are input in the powertrain models through previously measured compressor and turbine maps in turbocharger gas stands. However, this procedure does not capture all the effects that occur on engine application such as heat transfer, friction and flow motion that influence the turbochargers operations. This is of particular importance for two stage serial turbochargers where the LP compressor may induce a swirling motion to the flow at the entry of the HP compressor. In addition, flow non-uniformities caused by bends between the two compressors can make the HP compressor perform differently. In this paper, a review of the available literature containing approaches to quantify the effects of heat transfer on turbocharger efficiency and the flow influence in the prediction of two stage serial turbochargers performance is explored.

Lumped capacitance and 3D CFD conjugate heat transfer modelling of an automotive turbocharger

One dimensional wave-action engine models have become an essential tool within engine development including stages of component selection, understanding system interactions and control strategy development. Simple turbocharger models are seen as a weak link in the accuracy of these simulation tools and advanced models have been proposed to account for phenomena including heat transfer. In order to run within a full engine code, these models are necessarily simple in structure yet are required to describe a highly complex 3D problem. This paper aims to assess the validity of one of the key assumptions in simple heat transfer models, namely, that the heat transfer between the compressor casing and intake air occurs only after the compression process.Initially a sensitivity study was conducted on a simple lumped capacity thermal model of a turbocharger. A new partition parameter was introduced αA, which divides the internal wetted area of the compressor housing into pre and post compression. The sensitivity of heat fluxes to αA was quantified with respect to the sensitivity to turbine inlet temperature (TIT). At low speeds, the TIT was the dominant effect on compressor efficiency whereas at high speed αA had a similar influence to TIT. However, modelling of the conduction within the compressor housing using an additional thermal resistance caused changes in heat flows of less than 10%.Three dimensional CFD analysis was undertaken using a number of cases approximating different values of αA. It was seen that when considering a case similar to αA=0, meaning that heat transfer on the compressor side is considered to occur only after the compression process, significant temperature could build up in the impeller area of the compressor housing, indicating the importance of the pre-compression heat path. The 3D simulation was used to estimate a realistic value for αA which was suggested to be between 0.15 and 0.3. Using a value of this magnitude in the lumped capacitance model showed that at low speed there would be less than 1% point effect on apparent efficiency which would be negligible compared to the 8% point seen as a result of TIT. In contrast, at high speeds, the impact of αA was similar to that of TIT, both leading to approximately 1% point apparent efficiency error.Copyright

Experimental and analytical investigation of implementing a ball bearing turbocharger on a production diesel engine

Ball bearing turbocharger technology has started to be adopted for mass-production engines due to the potential benefit in transient performance and fuel consumption. Compared to the conventional journal bearing, the low friction of the ball bearing allows the turbocharger to accelerate faster so that the engine can be supplied with boost pressure more quickly following a transient torque request and under steady state offers reduced engine back pressure, which can reduce engine fuel consumption. In this study, the benefits of using a ball bearing turbocharger compared to a conventional journal bearing turbocharger were identified first in simulation and then validated in a back to back comparison of two otherwise identical turbochargers through extensive experimental analysis.