Due to increased demands for improved fuel economy of passenger cars, low-end and part-load performance is of key importance for the design of automotive turbocharger turbines. In an automotive drive cycle, a turbine which can extract more energy at high pressure ratios and lower rotational speeds is desirable. In the literature it is typically found that radial turbines provide peak efficiency at speed ratios of 0.7, but at high pressure ratios and low rotational speeds the blade speed ratio will be low and the rotor will experience high values of positive incidence at the inlet. Based on fundamental considerations, it is shown that mixed flow turbines offer substantial advantages for such applications. Moreover, to prove these considerations an experimental assessment of mixed flow turbine efficiency and optimal blade speed ratio is presented. This has been achieved using a new semi-unsteady measurement approach. Finally, evidence of the benefits of mixed flow turbine behaviour in engine operation is given. Regarding turbocharged engine simulation, the benefit of wide-ranging turbine map measurement data as well as the need for reasonable turbine map extrapolation is illustrated.

Due to emission legislation, turbocharging of the automotive internal combustion engine is becoming common practice. This is not only the case for Diesel engines but also for gasoline engines. Turbocharging the internal combustion engine helps to achieve the required emission levels while maintaining suitable driving characteristics. The key requirements for new turbochargers are improved performance over a wide operating range while meeting increasingly strict packaging constraints.

To date radial flow turbines (RFTs) are mostly employed in turbocharger applications for automotive engines. This paper describes characteristics about mixed flow turbines (MFTs) leading to the conclusion that such turbines provide considerable advantages for fulfilling the demands in automotive turbocharger applications. In Figure

Turbine geometry definition.

A dominant role for the turbine performance is incidence, that is, the difference between rotor inlet flow angle and blade angle at the rotor leading edge. According to Japikse and Baines [

It is common knowledge that radial turbines have their optimum efficiency at a blade speed ratio (

Theoretically the rotor inlet flow angle can be reduced by increasing

Most of the radial turbines currently used have radial fibres due to mechanical constraints. The blade inlet angle is zero and the combination with a volute with small

Radial versus mixed flow turbine.

Mixed flow turbines offer the advantage of additional degrees of freedom for aero design compared to radial inflow turbines which usually adopt a radial stacking because of mechanical constraints. The blade inlet angle of mixed flow turbines can be nonzero even with radial blade sections. Therefore, with mixed flow turbines it is possible to realize more favourable efficiency characteristics compared to radial turbines with respect to automotive turbocharger applications. Mixed flow turbines can be designed having a lower inertia which positively contributes to transient response, yet still maintaining allowable stress limits. Stress levels in the turbine back disc are lower for a mixed flow design which supports higher allowable speeds.

Optimum incidence for mixed flow turbines occurs at lower blade speed ratios

The aforementioned considerations are valid for steady flow conditions. However, turbines for automotive turbocharger applications are subject to highly pulsating inlet flows. The concept of pulse turbocharging, which is becoming increasingly popular, is aiming at optimum utilization of exhaust pulses through minimum manifold volume. As a consequence, the instantaneous turbine inlet conditions vary over a wide range of flow rates. Therefore the development focus is not on achieving optimum design point efficiency but on achieving a turbine characteristic which offers a high efficiency over a wide range of flow conditions.

Looking at design point efficiencies would lead to the conclusion that radial turbines are superior compared to mixed flow turbines for the specific speeds relevant for automotive turbocharger applications (e.g., [

Throughout this work the nomenclature shown in Figure

Enthalpy-entropy diagram of an expansion process within a turbine.

Figure

Velocity triangles at turbine wheel inlet and outlet.

The equivalent (inlet) diameter of a MFT is defined by (

For the ideal case, no losses, no incidence, and negligible swirl at turbine outlet are assumed.

In absence of losses, the turbine efficiency equals unity:

No incidence, negligible swirl at turbine exit (

Radial flow turbine inflow characteristics for different incidence angles.

The definition of incidence is given by

Considering mixed flow turbines, the consequences are as follows. For simplification, the characteristics of a MFT are explained assuming a RFT with back sweep as depicted in Figure

Backswept RFT/MFT inflow characteristics for different incidence angles.

The optimum flow turning within the backswept rotor is achieved for negative incidence, which in fact means almost radial inflow. Thus

Optimum

Degrees of freedom for mixed flow and radial flow turbines.

The fact that optimum incidence is achieved at higher blade loading for mixed flow turbines has been reported by several authors. Even optimum blade loading values exceeding unity have been reported (e.g., [

For the radial rotor this means that due to the absence of an axial component in the vector approaching the rotor, the flow is by nature purely radial (red vector). The vector triangle described above is not established, and thus even when designing a nonzero rake angle, the “mixed-flow-effect” is not achievable.

It should be emphasized that with regard to mechanical stress constraints, the distinct advantage of a mixed flow wheel over a radial flow wheel is that this nonzero blade inlet blade angle is achieved without violation of radial stacking condition. In addition to this, mixed flow turbine wheels offer the chance to design turbine wheels with reduced inertia. One key benefit is that the back disk is clearly reduced in diameter.

An analytical relationship between cone angle (

Turbine efficiency as a function of blade speed ratio for radial and mixed flow turbines [

This characteristic of mixed flow turbines is desirable especially for the requirements of automotive turbocharger applications. Under pulsation conditions, the maximum exhaust gas enthalpy is available for high pressure ratios, occurring directly after exhaust valve opening. As turbine speed does only slightly change—if at all—during an engine cycle, the high pressure ratio results in low values of

In the current work the enthalpy-based definition of the degree of reaction is adopted:

Optimum

Figure

Turbine performance comparison: mixed flow versus radial flow turbine.

From the theoretical analysis performed above, there is sufficient reason for investigating the behaviour of turbines over a very wide operating range in more detail.

A potential procedure for doing so is described in the following Section.

This study was undertaken for a small mixed flow turbine for automotive gasoline engine application. In [

Mixed flow turbine efficiency contour plot [

The turbine pressure ratio is plotted versus blade speed ratio. The colour and the isolines separate areas of same total to static turbine efficiency. Optimum efficiency is achieved within a range of

Furthermore it can be seen that according to theory the optimum blade speed ratio where the turbine offers best efficiency is increasing with pressure ratio.

For a fixed geometry (wastegated) turbine, efficiency is only depending on flow coefficient, loading coefficient, and Reynolds’ number.

Based on the steady wide mapping results shown above, a new, instantaneous method for measurement of efficiency at very low values of

High inertia rotor (HIR) assembly.

By measuring the instantaneous speed and utilizing the known rotor inertia, the instantaneous turbine acceleration power is directly evaluated. This instantaneous acceleration power is compared against an almost constant isentropic enthalpy difference, generated by the hot gas burner of the test bench. As mass flow was measured and held constant, temperature at turbine inlet was controlled by setting the heating unit to constant power. This was re-checked by the temperature measurement. However, the applied thermocouples were not fast response and could not determine accurately the temperature with respect to time due to their thermal inertia. Pressure measurements were done with “fast response” pressure transducers to control whether pressure ratio varies during acceleration and to allow for a phase correction between the pressure before and after the turbine. This was done with calibrated piezo-resistive absolute pressure transducers [

Test setup for unsteady turbine performance measurement.

Turbine housing as well as all hot gas and measurement pipes as insulated to minimize heat transfer between the turbocharger and the test cell environment. Furthermore, turbine inlet temperature, oil conditioning, and water cooling were set to constant low values to minimize heat transfer. Initially, the HIR is locked and the desired turbine pressure ratio and turbine inlet temperature are set. After starting of the transient measurement system, the rotor is released and accelerates. An automatic shutdown procedure is applied to prevent overspeed of the HIR.

Regarding maximum rotational speed, two main aspects have to be considered.

Firstly maximum allowable speed must not be exceeded, to avoid any damage of the HIR itself as well as the bearing system. It is clear that the rotor dynamics of such a HIR system are very different from a conventional turbocharger rotor.

Secondly, the speed has to be low enough; that is, elastic deformation of the rotor does not change the moment of inertia. Otherwise, the speed signal could not be used for turbine net power measurement during acceleration of the rotor.

A typical result of the instantaneous measurements, for a constant

Typical result of an unsteady HIR measurement.

The calculated values for torque, power, and efficiency for very low values of

The evaluation procedure for instantaneous torque, power, and efficiency is given as follows.

The rotor acceleration is calculated by

Runaway speed measurements for two turbine inlet temperatures.

Comparison of steady and unsteady test results.

For a pressure ratio of 1.4 and a turbine inlet temperature of 20°C, a runaway blade speed ratio of about 1.04 was recorded.

From analysing Figure

However, some deviation exists which can be explained. The steady results have been collected by the so-called “turbine net efficiency approach” [

Regarding unsteady turbine operation, it is to note that due to the almost constant turbine pressure ratio during acceleration, no filling and emptying effects within the turbine scroll have to be expected. Thus, although this is an unsteady measurement, the problems that are encountered during efficiency measurement under pulsed conditions are overcome. Thus, the experimental approach presented here is labelled “semi-unsteady.”

Due to the exhaust gas pulse from the intermittent operation of the reciprocating engine, the turbocharger turbine operates under unsteady admittance. In Figure

On-engine turbine operation.

Thus, the correct prediction of engine steady operation, as well as unsteady operation or even vehicle acceleration behavior, is strongly depending on sensible and correct extrapolation.

A wide map measurement of turbine data can help to avoid the need for extrapolation. However, usually a wide map measurement is not available. A wide map measurement can be used to develop improved extrapolation algorithms. In general the turbine efficiency data to be extrapolated should not contain friction influence from the bearing system. Friction is not related to aerodynamic parameters but to real shaft speed as well as to thrust load.

However, standard hot gas stand turbine efficiency data usually contains friction data due to the measurement method. Thus, modelling of bearing friction depending on shaft speed, and thrust load can also be a source of error for extrapolation.

In this section it is investigated how various turbine characteristics and designs affect the on-engine operation. From the above sections it is known that the efficiency characteristics of mixed flow turbines can be advantageous for automotive turbocharger applications.

Figure

Increase of boost pressure versus time depending on different turbine configurations.

This simulation program is capable of predicting turbocharged engine performance under steady and transient conditions. ITES is focused on the detailed modelling and numerical description of the turbocharger. The study shown in Figure

All results are compared to a mixed flow turbine rotor made from conventional nickel-base alloy. This is referred to as the base configuration.

The effects that have been investigated are inertia and turbine efficiency. A clear advantage can be seen when comparing the base configuration with a turbine variant made from gamma titanium aluminide (

Comparison of nickel-base alloy and

The

Please note that the simulation results also depend on the investigated engine load step and especially the starting conditions of the load step for example, acceleration from stand still profits more from reduced inertia.

The effect of modifying the characteristics of the turbine map, the main topic of the current work, was also investigated. Compared to the base configuration, a variant with reduced

Due to increasingly higher demands for improved fuel economy of passenger cars, low-end and part-load performance is of key importance for the design of automotive turbocharger turbines. In an automotive drive cycle, a turbine which can extract more energy at high pressure ratios and lower rotational speed is desirable.

It is commonly quoted that a radial turbine provides peak efficiency at blade speed ratios of about 0.7, but at high pressure ratios and low rotational speeds the blade speed ratio will be low and the rotor will experience high values of positive incidence at the inlet. The present study shows that even for radial turbines the blade speed ratio where optimum efficiency is reached is usually lower than the commonly quoted blade speed ratio of 0.7. The present work gives theoretical justification and experimental evidence that for mixed flow turbines optimum efficiency can be obtained at even lower blade speed ratios. This can be attributed to a more favourable inlet blade angle, swallowing capacity and inertia when compared to a radial design.

The present study shows that mixed flow turbines have key advantages for automotive turbocharger applications as they have improved performance at low blade speed ratios. This means that a significant portion of the pulse energy available in the exhaust gas can be utilized. The behaviour of a mixed flow turbocharger turbine was investigated by steady-state wide mapping and also by employing a new, semi-unsteady measurement approach. It was found that the unsteady approach shows very good agreement with the steady and runaway measurements. It was theoretically derived that the blade speed ratio for optimum efficiency of a mixed flow turbine is far below the commonly cited value of 0.7. This was also proven experimentally. Finally an investigation of how this could improve on-engine behaviour was described. The benefit of low-inertia mixed flow turbocharger turbine wheels has been clearly demonstrated.

Static pressure (Pa)

Stagnation pressure (Pa)

Blade speed ratio (—)

Pressure ratio (—)

Velocity in stationary frame (m/s)

Velocity in rotating, relative frame (m/s)

Blade speed (m/s)

Enthalpy difference (J/kg)

Compressor

Diameter

Isentropic spouting velocity (m/s)

Degree of reaction (-)

Incidence (deg,

Mass flow rate (kg/s)

Rotational velocity (1/s)

Time difference (s)

Time (s)

Power (W)

Torque (Nm)

Enthalpy (J/(kg K))

Entropy (J/(kg K)).

Computational fluid dynamics

Mass flow parameter

Radial flow turbine

Mixed flow turbine.

Cone angle (deg)

Rake or camber angle (deg)

Blade angle (deg)

Relative flow angle (deg)

Absolute flow angle (deg)

Loading coefficient (—)

Simplified loading coefficient (—)

Rotational speed (rad/s).

Optimum

Static

Total

Axial, in

Radial

Isentropic

Static to static

Total to total

Total to static

Turbine stage inlet

Turbine wheel inlet

Turbine (wheel) exit

Meridional

Turbine

Circumferential

Acceleration.