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Design and development of a turbo-expander for charge air cooling

C D Whelan and R A Richards

WDL Ltd

Dr S W T Spence Queen’s University Belfast

A Young

Young Calibration Ltd ABSTRACT The potential benefits of sub-ambient charge air cooling for internal combustion engines include improved performance, fuel economy and exhaust emissions. A high efficiency expander is critical to achieving good overall system performance. This paper presents the design, manufacture & testing of a turbo-expander using automotive turbocharger technology. A turbine design code is embedded in a 1D engine performance prediction model and used to predict system performance, perform optimisation studies and generate the 1D geometry of the turbine. The process was validated by making and testing a prototype turbo-expander with a new cold-air turbine replacing the exhaust turbine of a donor automotive turbocharger. 1 INTRODUCTION The concept of charge air-cooling by turbo-expansion applied to internal combustion engines was developed in the 1950s and 60s (1), (2). It was successfully applied to natural gas fuelled power generation engines, enabling useful power increases and providing protection from the detonation effects of varying fuel properties. In recent years, a number of projects and studies have been performed, all re-examining the basic turbo-expansion principle (3), (4). Today, the major technical, legislative and market drivers for improved fuel economy and emissions encourage a further examination of the potential for turbo-cooling. This potential was identified by the authors in 2003 who performed thermodynamic studies to evaluate the benefits, particularly when applied to ’down-sized’ boosted gasoline engines. These studies demonstrated that a charge air temperature reduction of over 30ºC could be achieved with no fuel consumption penalty, and were reported in 2006 (5).

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This in turn led to the need to develop tools and design techniques to enable efficient and cost-effective turbo-expanders to be designed and developed, which resulted in the work reported here. This was a project, code-named ACCESS, which was part-funded by the UK Government Technology Strategy Board under an energy-related ‘Advanced Design and Manufacture’ theme. 2 OBJECTIVES The key objectives of the ACCESS project were the design, testing and validation of a turbo-expander, suitable for application to a down-sized automotive engine and for use in other (non-automotive) applications. These focused on the possibility of applying current turbocharger designs and manufacturing methods to a turbo-expander. The critical issue here is the design of a high-efficiency, low-cost, turbine and housing to replace the existing turbine. Turbocharger turbines are designed for operation at high gas temperatures and have other constraints which trade off efficiency with a number of vehicle operational requirements. The turbo-expander turbine does not have these constraints, so its design can be optimised for efficiency. The overall project objectives in turn led to a number of specific objectives, namely:

- Developing a modelling tool for predicting the performance of an IC engine with a turbo-expansion system

- Design of a cold-air turbine to replace the exhaust gas turbine of a turbocharger

- Manufacture of a prototype turbo-expander - Test and validation of the turbine design and it's performance in the

operating turbo-expander 3 SCOPE The scope of work was defined by the ACCESS project agreed with and supported by the UK Government, and covered:

- 1D modelling of potential air-cycle systems - Project-specific 1D turbine aerodynamic design and off-design codes - Integration of 1D turbine design/off-design code into the system modelling - Component design, using 1D and 3D techniques - Prototype manufacture of a turbo-expander - Experimental test programme (rig and end–user system) - Validation of the design and system codes from the test results

4 METHODOLOGY The requirement to demonstrate a practical, cost-effective solution to turbo-expander design and manufacture had a major influence on the methodology of the project. Other workers in this field, including those working on non-automotive applications of turbo-expansion and air-cycle systems, have struggled to achieve both the predicted system performance and to deliver suitable hardware. Typically, the necessity of using mismatched hardware has led to disappointing results on completion of the work (6).

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With these issues in mind, the methodology selected was:

- Combined thermodynamic modelling of the turbo-expander and its application system, in order to predict the overall performance, matching conditions and operating range. The applications spanned a range of industries, all of which featured air-cycle cooling as a critical element.

- Selection of an available production turbocharger as the base unit for conversion to a turbo-expander, in order to demonstrate high volume manufacturing feasibility. The frame size was selected to match the required flowrates and the turbo-expander compression pressure ratio

- Based on this frame size, the turbine ‘design point’ was selected from performance optimisation using the system model.

- 1D design and off-design turbine performance prediction models written independently and then embedded into the system modelling code

- 3D design of the turbine, working from the 1D design point - Manufacture of a prototype turbine wheel and housing - Conversion of the selected turbocharger into a turbo-expander - Rig testing and performance validation of the turbo-expander

This methodology enabled the design requirements for flowrates, pressure ratio and efficiency to be rigorously determined early in the project and then to be tracked throughout the design, analysis, manufacture and testing phases of the work. 5 SYSTEM MODELLING

Figure 1 - AMESim model of turbocharged and turbo-cooled engine

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System modelling was performed using the AMESim modelling and simulation environment. Figure 1 shows a model of a highly boosted 1.4L gasoline engine with EGR, incorporating turbo-cooling to control air manifold temperature. Earlier work using this type of model had demonstrated the potential for improvements in power, fuel economy and emissions arising from this application of turbo-cooling (5). This type of system modelling enables rapid and effective assessment of the impact of the performance of an individual component on the entire system. One aim of this project was to improve and extend this capability to include changes in turbine design parameters. 6 1D TURBINE MODELLING AMESim provided an ideal platform for this work. An AMESim model is made up of linked sub-models. Each sub-model contains the set of equations defining the dynamic behaviour of an engineering system and its implementation as computer code. In addition to the built in libraries of sub-models, an interface is provided which enables the user to create custom sub-models by embedding FORTRAN or C code in a sub-model structure. For compressor and turbine modelling, AMESim includes sub-models which enable performance of a given machine to be described by means of flowrate/pressure ratio/efficiency maps in the usual way (Figure 2).

Figure 2 - AMESim turbo-expander model & turbine sub-model

Two replacement custom turbine sub-models were developed, using code based on well-known published turbine design and performance prediction programs (7), (8). The first of these sub-models performs design point analysis; the second predicts off-design performance.

Turbine SubmodelTurbine

Submodel

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6.1 Design Point Model Input design requirements include power, speed, mass flowrate, inlet temperature and pressure. Design variables include stator-exit angle, rotor exit tip to rotor inlet radius ratio, rotor exit hub to tip radius ratio, and the magnitude and radial distribution of rotor exit tangential velocity. An internal empirical model accounts for losses due to viscous forces in the rotor and stator, disc friction on the back of the rotor, clearance between rotor and casing, and the exit velocity loss. The design code was adapted to suit the input and output conventions of the AMESim turbine sub-model, and incorporated in a custom sub-model which could then be utilised within the AMESim environment as part of a complete system model. The result is a powerful tool which enables the exploration of the effect of turbine design variables on turbine performance, and, more importantly, on the system of which the turbine is a part. 6.2 Model Validation Initial validation of the design point modelling method was achieved using published test data from a series of turbocharger tests by Spence et al (9). The Spence design parameters were used as inputs to the model, and performance predicted by the model was compared to the Spence results. Good agreement was achieved. A sample of the results is given in Figure 3 below.

• Design point model:(continuous lines)– Pout: 1.1 bar– Design point PR: 2.5– Design point Pin: 2.75 bar

• Spence data:(single points □)– Specific speed 0.89– Blade speed ratio 0.69– Total-total effy 91%– Total-static effy 83%

blade speed ratio

specific speed

blade speed ratio

specific speed

total – total efficiency

total – static efficiency

Figure 3 - AMESim turbine design point model validation

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6.3 Off-Design Model The design model enables rapid selection of appropriate design parameters at a given design point. In many applications, off-design performance is equally important. To meet this need, a turbine sub-model for simulation of off-design behaviour was developed in the same way, again using established design codes as the basis. This model utilises the design point requirements and turbine geometry (from the design model) as inputs. Turbine performance is calculated for given values of variables including flowrate, pressure ratio and inlet temperature. Embedding this code in the AMESim turbine sub-model provides a quasi-static simulation of the turbine performance, with consequent improvement (over the steady flow map-based approach) in the prediction of behaviour under unsteady flow conditions. This model can also be used to study the effect of small changes in the design parameters to optimise off-design performance, where required by the application. By using these two sub-models within an application model, the design and performance of the turbine can be selected and optimised to meet the overall system requirements. 7 TURBINE 1D DESIGN Three potential applications requiring similar air mass flowrates were chosen, as follows:

- Charge air chilling of a highly boosted gasoline engine (1.4 litre) - Biogas fuel chilling for gas purification in a generator/CHP engine

(~300kW) - Conditioned air supply in a tri-generation system (diesel CHP engine

~200kW) A proprietary turbocharger with a suitable compressor was identified, and 1D system modelling was used to characterise the required turbine performance for the applications - using the turbine "design model" described above, with a target turbine efficiency not less than 80%. Figure 4 shows the response of the design model outputs (in this example, rotor diameters and predicted efficiencies) to the design point speed setting with mass flowrate fixed. These results show the relationship between speed and diameter, and that lower speed yields slightly higher total-static efficiency. This information enabled design point speed to be chosen to provide the best compromise between compressor and turbine performance. The outcome of this work was a target design point specification for the prototype high efficiency turbine, as shown in Table 1. Subsequent optimisation of the 1D design led to a reduction in turbine speed from 113,000 rpm to 107,000 rpm. The revised design point data is shown in the second column of Table 1.

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Design Point Model

• Response of design model outputs to the design point speed

• With mass flow fixedtotal – static efficiency

total – total efficiency

rotor exit diameter [mm]

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Figure 4 - Turbine design point model results

Table 1 Turbine design point

TARGET FROMSYSTEM MODELLING

FINAL DESIGN(SPEED REDUCED)

DESIGN POINT PERFORMANCE TARGET

Design point Tin (K) 321 321Pout (bar) 1.06 1.06Pressure Ratio (-) 3.04 3.04Pin (bar) 3.21 3.22Mass flow (kg/s) 0.224 0.223Speed (rev/min) 113000 107000Power (kW) 15.5Efficiency (%) 79% 80%

DESIGN POINT DIMENSIONS

Rotor inlet tip diameter (mm) 52.5 50Rotor exit tip diameter (mm) 41.5 43Rotor exit hub diameter (mm) 15.3 15Tip clearance at rotor exit (mm) 0.2 0.25Rotor inlet blade height (mm) 3.6 6No. of blades (-) 11 11

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Design point Tin (K) 321 321Pout (bar) 1.06 1.06Pressure Ratio (-) 3.04 3.04Pin (bar) 3.21 3.22Mass flow (kg/s) 0.224 0.223Speed (rev/min) 113000 107000Power (kW) 15.5Efficiency (%) 79% 80%

DESIGN POINT DIMENSIONS

Rotor inlet tip diameter (mm) 52.5 50Rotor exit tip diameter (mm) 41.5 43Rotor exit hub diameter (mm) 15.3 15Tip clearance at rotor exit (mm) 0.2 0.25Rotor inlet blade height (mm) 3.6 6No. of blades (-) 11 11

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8 TURBINE 3D DESIGN AND MANUFACTURE Having defined the 1D turbine rotor geometry and the desired turbine inlet flow angle, the detailed 3D design of the stage was undertaken. The 3D blade geometry was produced using the ANSYS Blademodeller software, with a conventional radially fibred blade design being adopted. A model of the complete rotor was produced using a solid modelling CAD package, and a single blade segment of the rotor was checked for stress levels using the ANSYS finite element analysis software. (Since the speeds and temperatures were comparatively low in this application, acceptable stress levels were easily achievable). The turbine housing comprised an asymmetrical scroll with a symmetrical trapezoidal shaped cross section. The design method for this followed the procedure described by Chapple et al. (10). A solid model of the housing flow path was developed using CAD software prior to grid generation for CFD modelling. The turbine housing had a throat area of 404 mm2 with the throat centre of area located at a radius of 40.7 mm. The results of the 3D design exercise confirmed the accuracy of the design mass flowrate predicted using the 1D model. The predicted efficiency value exceeded the design target, although it is acknowledged that while the rotor mesh was adequate for confirming the design mass flowrate, it may not have been adequately refined to achieve a mesh-independent prediction of stage efficiency. Nevertheless, the predicted rotor incidence, Mach number distributions and exit swirl values gave confidence that the turbine was operating close to its best efficiency point. Figure 5 shows the final turbine rotor and housing geometries manufactured at Queen’s University Belfast.

Figure 5 - Turbine rotor and one half of turbine housing showing flow path

geometry 9 TURBO-EXPANDER MANUFACTURE A key objective of the project was to demonstrate that a high performance turbo-expander could be developed using the same design and manufacturing process as a mass-production automotive turbocharger. For this reason the turbo-expander was based as closely as possible on an available commercial turbocharger. The unit selected, from one of the major manufacturers, was sized to provide the target turbo-expander performance required for the proposed systems applications. The wheel diameters were 60mm for the compressor and 53mm for the exhaust gas turbine.

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The selected turbocharger utilised ball bearing shaft support. Axial thrust forces in a turbo-expander are greater than in a standard turbocharger, and the ball bearing system can withstand these higher axial thrust loads. Re-designing a mass-production sleeve-bearing turbocharger to take higher axial loads was outside the scope of the project. For some non-automotive turbo-expander applications where the oil flowrate and leakage are critical, the ball bearing shaft system would, in any case, be the preferred solution. The change from exhaust gas turbine to cold-air turbine was made by machining off the original steel alloy turbine, leaving a stub shaft similar to the compressor end of the shaft, then interference fitting the aluminium cold air turbine. The aluminium alloy housing for the new cold-air turbine was designed to directly replace the cast nickel-iron alloy exhaust gas housing of the original turbocharger. The target rotor-shroud clearances were achieved without the need for selective assembly. Figure 6 shows the turbine design, the volute, the turbo-expander assembly and shaft.

Figure 6 - Turbo-expander components and assembly

10 TESTING AND VALIDATION 10.1 Test rig A test rig was designed and made specifically to test the turbo-expander. The main features were an independent ‘intake boost’ system, to pressurise the turbo-expander, inter- and after-cooling, and the ability to measure and control the humidity of the air passing through the turbo-expander. A downstream throttle controlled the air mass flowrate. A simple control loop maintained the target intake pressure over the mass flowrate range.

COLD AIR TURBINE

COLD AIR TURBINE WHEEL

ASSEMBLED TURBO-EXPANDER AND SHAFT

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The instrumentation consisted of pressure and temperatures at all key locations, relative humidity in and out of the turbo-expander, turbo-expander shaft speed and air mass flowrate. All the measured parameters were data-logged. The operating range of the rig was: - Intake air pressure: 0 ~ 3.0 barA - Intake air temperature: ambient +5 ~ 140°C - Air mass flow: 0 ~ 300g/s The layout of the principal components can be seen in the photograph below (Figure 7).

AFTERCOOLER

TURBINE

HUMIDITY SENSOR

MOISTURE TRAP

COMPRESSOR

AFTERCOOLER

TURBINE

HUMIDITY SENSOR

MOISTURE TRAP

COMPRESSOR

Figure 7 - Turbo-expander test rig

10.2 Testing method and plan A standard test procedure was used, which consisted of a mass flowrate ‘sweep’ at a fixed intake pressure, repeated over a range of intake pressures. The test plan consisted of back-to-back measurement of the performance of the original donor turbocharger then the dedicated turbo-expander. 10.3 Results The results obtained for the rig testing are summarised and compared to the model predictions in Figure 8. Three sets of turbine performance curves are shown: 1. Measured performance from the donor exhaust gas turbine 2. Predicted performance from the embedded off-design cold air turbine model 3. Measured performance of the cold air turbine As expected, efficiency of the turbocharger turbine was relatively poor, for reasons discussed above. The cold air turbine design was predicted to achieve up to 85% total-static efficiency and the test results confirm this.

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Figure 8 - Turbine predicted and measured performance

The (non-dimensional) mass flowrate versus pressure ratio results show excellent agreement with the values predicted by the off-design modelling. The efficiency results also showed good agreement with the prediction, albeit with some over-prediction at the lower pressure ratios. Given the known difficulty of accurately predicting efficiency with 1D simulation, these results are highly encouraging. 11 CONCLUSIONS The key conclusions of the work were:

- An effective 1D tool for the design and performance prediction of a turbo-expander in a system application has been successfully developed

- A high performance, high-efficiency turbo-expander has been produced, derived from a mass-production turbocharger

- The turbine design and off-design performance predictions have been validated on a dedicated test rig

- The potential to design and develop efficient, cost-effective, turbo-expanders for a range of industry applications has been demonstrated

12 THE FUTURE With the now-proven ability to design and develop effective turbo-expanders, future work will be focused on validation of the potential benefits for automotive boosting system applications, as previously demonstrated (5). Planned activities include:

- Application of the current prototype turbo-expander to a running engine. The target engine is a high performance turbocharged engine, which will be used to investigate charge air refrigeration as a means of reducing high load fuel enrichment, leading to fuel economy improvements

Access Cold Air Turbine

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- Investigating charge-air refrigeration in down-sized boosted engines which employ EGR at high load. Turbo-expansion offers the potential to reduce intake and in-cylinder gas temperatures significantly, thereby reducing both boost pressure requirements and improving combustion conditions.

- Extending the integrated turbine/engine model into turbocharger turbine

transient performance prediction. This is a spin-off activity from the current project, in which the turbine model, embedded into an engine simulation code, will allow calculation of unsteady turbine performance; an improvement on the current practice of instantaneous ‘look-up’ on a steady-state performance map. This is particularly relevant to low speed high boost operation where the prediction of the turbocharger turbine matching and engine torque is currently very difficult.

13 ACKNOWLEGEMENTS The authors are grateful to ANSYS Inc. for the use of CFD and grid generation software in this research study. © Authots 2010 14 REFERENCE LIST (1) Crooks, W. R., "Combustion Air Conditioning Boosts Output 50 Percent"

CIMAC 1959 a15, p475 (2) Helmich, M.J., "Development of Combustion Air Refrigeration System

Enabling Reliable Operation at 220 psi BMEP for a Large Four-Cycle Spark-Ignited Gas Engine" ASME 66-DGEP-7

(3) R C Meyer and S M Shahed, "An Intake Charge Cooling System for Application to Diesel, Gasoline and Natural Gas Engines" SAE910420, 1991

(4) R W G Turner, R J Pearson, M D Bassett and J Oscarsson, "Lotus: Performance and Fuel Economy Enhancement of Pressure Charged SI Engines through Turbo-expansion – An Initial Study’" SAE2003-01-0411, 2003

(5) C D Whelan and R A Richards, "Turbo-Cooling applied to Light Duty Vehicle Engines" IMechE 8th Conference on Turbochargers and Turbocharging, May 2006

(6) Spence, S.W.T., W.J. Doran, D.W. Artt and G. McCullough, "Performance Analysis of a Feasible Air-Cycle Refrigeration System for Road Transport" International Journal of Refrigeration, Vol 28(3), 2005, pp. 381-388

(7) Arthur J Glassman, "Computer Program for Design Analysis of Radial-Inflow Turbines" NASA Technical Note TN D-8164, February 1976

(8) Charles A Wasserbauer and Arthur J Glassman, "Fortran Program for Predicting Off-Design Performance of Radial-Inflow Turbines" NASA Technical Note TN D-8063, September 1975

(9) S W T Spence, R S E Rosborough, D Artt, G McCullough, "A Direct Performance Comparison of Vaned and Vaneless Stators for Radial Turbines" ASME Journal of Turbomachinery January 2007 Vol. 129

(10) Chapple, P. M., Flynn, P. F. and Mulloy, J.M., 1980, "Aerodynamic design of fixed and variable geometry nozzleless turbine casings" Trans. ASME J. Eng. Pwr., Vol. 102, pp. 141-147.