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International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 6, June 2018, pp. 999–1008, Article ID: IJMET_09_06_112 Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=9&IType=6 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication Scopus Indexed

APPROACHES TO IMPROVEMENT OF

TURBOCHARGERS FOR AUTOMOTIVE

ENGINES

Dmitry Anatolyevich Petrichenko, Viktor Sergeevich Korotkov, Roman Viktorovich

Stukolkin, Vsevolod Anatolyevich Neverov and Igor Arkadyevich Papkin

Moscow Polytechnic University, Bolshaya Semenovskaya str., 38, 107023, Moscow, Russia

ABSTRACT

This article discusses approaches to improvement of a boost system of internal

combustion engines. It reviews different existing and promising boost systems,

including positive-displacement and centrifugal compressors, turbochargers and

combined boost systems, which encapsulate mechanical and electrical compressors.

The article demonstrates that boost systems give a positive effect on the working

parameters of a power unit. A comparative analysis of the systems regarding occupied

volume, efficiency of its compressor and drive units, compression ratio was held. It

revealed advantages and disadvantages of different units as well as their influence on

the internal combustion engine characteristics. The performed analysis made it

possible to choose the most efficient and promising directions of further research,

aimed at improvement of the operation characteristics of vehicles by means of boost

systems.

Keywords: Internal combustion engine, Boost systems, Turbocharger, Combined propulsion system, combined charging, combined boost systems. Cite this Article: Dmitry Anatolyevich Petrichenko, Viktor Sergeevich Korotkov, Roman Viktorovich Stukolkin, Vsevolod Anatolyevich Neverov and Igor Arkadyevich Papkin, Approaches to Improvement of Turbochargers for Automotive Engines, International Journal of Mechanical Engineering and Technology, 9(6), 2018, pp. 999–1008 http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=6

1. INTRODUCTION

Application of superchargers in propulsion unit improves engine operation performances: power, flexibility, applicability and others. However, as any other engineering systems, superchargers in total and centrifugal compressors in particular are characterized by certain disadvantages. The most critical of them is response rate: turbo lag. Numerous modern researches are aimed at decrease in activation time of superchargers by various methods. In addition, there are other concerns: relatively low efficiency of turbine, increased heating of

Approaches to Improvement of Turbochargers for Automotive Engines


boost air, heavy operation conditions of centrifugal compressor parts, etc. This article considers modern approaches to improvement of centrifugal compressor.

2. CENTRIFUGAL COMPRESSOR ARRANGEMENT

Centrifugal compressor is an assembly which uses the energy of waste gases for increase in air pressure inside intake system. In general, centrifugal compressor arrangement can be subdivided into three parts: turbine, compressor, and encapsulating housing. Figure 1 illustrates such assembly in more details. The units and parts which can be improved are highlighted by frames. They are as follows:

• Wheels of turbine and compressor;

• Turbine housing;

• Controller or control system;

• Bearings;

• Oil supply and cooling system.

Figure 1 Arrangement of centrifugal compressor.

Analysis of these elements would permit to determine efficiency of this or that engineering approach.

2.1. Wheels of turbine and compressor

The main trends in design of compressor wheels of turbochargers are aimed at complex spatial shaping of blades curved counter rotation. Application of vaneless diffusors in centrifugal compressors significantly improves their performances.

Significant influence on turbocharger efficiency is exerted by roughness of wheels and pipelines. Thus, for the most responsible segments, such as diffusor channels and impeller passages, it is required to apply such engineering processes which guarantee roughness of at least Ra 1.0. The most well-known production technique of aluminum wheels, centrifugal molding, provides the surface roughness Ra 10…20. The required roughness can be obtained only by means of specialized cleaning. This permits to increase isentropic efficiency by 2…2.5% and to decrease the temperature of gases before turbine by 25…30°C [1].

The highest increase in compressor efficiency is achieved by the use of wheels with blades curved backward which reduces negative effect of reverse circulation at wheel outlet and improves flow conditions in the next diffusor. However, this also leads to decrease in wheel head capacity which should be compensated by increase either in rotation rate or in wheel diameter. The issue of improvement of compressor wheel capacity was discussed in [2]. One approach is comprised of improvement of impeller configuration as illustrated in Fig. 2. The pressure increased by about 15% from 2.13 to 2.48, and the isentropic efficiency increased by 3.5% from 76% to 79.5%. In addition, air mass flow increased by about 16%.

Dmitry Anatolyevich Petrichenko, Viktor Sergeevich Korotkov, Roman Viktorovich Stukolkin, Vsevolod Anatolyevich Neverov and Igor Arkadyevich Papkin


Figure 2 Comparison of baseline and optimized impellers.

Mitsubishi Heavy Industries Co., Ltd. (MHI) performs research attempting to increase efficiency of turbocharger wheels. The turbine wheel of this company in order to decrease inertia is made of special heat resistant titanium–aluminum alloys [3]. This allowed decreasing the wheel diameter with the increase in capacity. Nozzle blade shape was developed. In comparison with conventional S shape, the backward curved blades provide gas flow rate via blade clearance higher by 8.8%. Increase in compressor operation range by 12-14% was achieved due to shape optimization of air pipelines; however, this could be achieved only by close cooperation between MHI and engine manufacturers.

On the basis of simulation systems of gas flows, Cummins Engine improved geometry of compressor blades [4]. Air recirculation zones were detected, resistance against air flow as a function of position of compressed air outlet with regard to compressor wheel was determined.

Honeywell International Inc. in its DualBoost™ system replaced radial turbine with axial one [5]. Axial turbines are characterized by better efficiency at lower ratios of centrifugal compressor rate to that of inlet air as well as by lower inertia moments (up to 50%) than equivalent radial units. Combination of axial turbine with double sided compressor resolves a set of issues. Firstly, turbine operation is accelerated, secondly, axial loads are compensated, thirdly, inertia is lower.

The comparison tests of DualBoost™ turbocharger and conventional radial compressor were carried out with Ford petrol engine, 1.6 l with maximum torque of 280 Nm at 4500 rpm and maximum capacity of 132 kW at 4750 rpm. Both turbochargers were adjusted to equal air flow rate with pressure increase rate of 2.0. Experimental results demonstrated that upon steady engine operation, the torque of 180 Nm was achieved by 450 ms and 270 Nm by 600 ms earlier than with the conventional turbocharger. Maximum torque was achieved already at 1500 rpm. In addition of increased capacity, application of DualBoost™ increases economic efficiency. While simulating circulation via US06 system, the flow rate decreased by 2.7%, via NEDC system - by 2.6%. In total, the following results were achieved:

• the turbine was accelerated by 25% faster;

• the torque was by 20% higher after the first second of acceleration (Fig. 3);

Therefore, DualBoost™ makes it possible to increase power-to-volume ratio and to simplify production technique of compressor and turbine wheels.

Figure 3 Engine acceleration



2.2. Turbine housing

Variable geometry turbocharger (VGT) attracts attention of MHI [6–8]. From early 2000-s the VGT designs for passenger vehicles passed three generations. Improvements were achieved for the following parts of centrifugal compressors:

• turbine wheel;

• nozzle blades;

• Compressor.

Various technical approaches to variable geometry turbochargers are reviewed in [9]. It is revealed that despite the high cost of such devices in comparison with conventional centrifugal compressors, VGT provide high fuel efficiency, good transition performances, decrease in harmful emissions and improved torque. The trend of VGT activation is determined. Electric or hydraulic methods become more and more popular and provide more accurate control than pneumatic.

Aiming at improvement of turbocharging efficiency, MHI developed [10] twin-scroll turbine with two channels for outlet of waste gases. Cross section of twin-scroll centrifugal compressor is illustrated in Fig. 4. The performances are improved due to decrease in turbine flow loss upon pulsation of waste gases. However, this area of turbocharging is underdeveloped and should be studied later.

Figure 4 Twin-scroll centrifugal compressor.

2.3. Control system

Modern centrifugal compressors should be equipped with automatic control system (ACS). This protects engine against increased loads during release of accelerator pedal, decreases loses for pumping cycles of crank mechanism. Generally, ACS is comprised of one or several bypass valves. There are three types of valves:

• Wastegate valve – bypass of waste gases via turbine;

• Bypass valve – air bypass via compressor;

• Blow-off valve – pressure relief in charging system into atmosphere.

Honeywell International Inc. (USA) developed rotary electric actuator (REA) mounted on compressor housing and interconnected with wastegate [11]. While operating in combination with turbocharger, the REA controls air flow both in steady and in transient modes. In comparison with the pneumatic valve (average response time: 500 ms), the response time of electric valve is 150 ms. Since the REA is connected with engine only by electric channel, it is possible to eliminate necessity for separate channel with vacuum which simplifies the design.



2.4. Bearing assemblies

Bearing assembly determines efficiency, operability and lifetime of overall centrifugal compressor. Modern turbochargers are mainly equipped with forced oil supply from engine oiling system. Two types are applied more often: with floating rotary sleeves and with floating non-rotary single sleeve. Ball bearings become more and more popular in addition to the above-mentioned types. Such bearing can decrease fuel consumption by 2%.

Modern bearing assemblies of centrifugal compressors are discussed in details in [12]. Bearing were tested for developed friction force using special testing facility. It was established that for radial and thrust bearings the oil temperature was a critical factor for minimization of friction loss which decreased due to increase in oil temperature, this was especially noticeable at low rotation velocities. The influence of oil pressure can be determined at high velocities. Friction moment decreases at high velocities upon the decrease in pressure. Axial force applied to floating bearing significantly increases friction moment (Fig. 5a). This is especially seen at low engine rpm. Friction moment decreases with the increase in rotation velocity. When comparing three types of bearings at oil temperature of 90°C and oil pressure of 3 bar without axial load, semi-floating and floating bearings have nearly similar curves of friction moment, and the curve of friction moment of ball bearing is characterized by lower performances (Figs. 5b and 5c).

Due to the trend of replacement of sliding bearings with ball bearings, special models are required for optimization of variables of rolling bodies and materials. Cummins Engine Co. (USA) developed dynamic model for analysis of rolling bearings taking into account flexibility of bearing shell, rigidity of balls, effect of supplied oil viscosity. This allowed to determine contact stresses of balls and to analyze bearing state in transient processes [13].

Figure 5 Speed-torque characteristic: a – under the influence of the axial force, floating plain bearings at 90°C; b – with influence of the thrust load in a semi-floating and full-floating plain bearing at 90°C;

c – with influence of thrust load, in a ball bearing at 90°C.

2.5. Oil supply and cooling system

Modern turbochargers are highly intensive units in terms of temperature. Its overheating can lead to destruction of bearings, oil coking, high thermal deformation, weakening of materials. Usual arrangement applied in most turbochargers stipulates only cooling by the oil used for lubrication of bearings. Garrett [14] solves this issue by installation of forced cooling by coolant or by oil (Fig. 6). Design of such turbochargers is based on special cavities used in internal combustion engines. Cooling jacket surrounds central portion of turbocharger housing which encapsulates bearings, shaft, oil cavities and other parts. The researches demonstrated that systems with forced cooling made it possible to decrease oil temperature at turbocharger outlet.



2.6. Combined arrangements

Volkswagen Group applies TSI system in modern engines. It can be presented in two embodiments: either one-stage charging by centrifugal compressor, or two-stage charging by centrifugal compressor and mechanic supercharger. Speed characteristics of TSI engines are illustrated in Fig. 7.

Figure 6 Cooling cavities. Figure 7 Speed characteristics of TSI engines with one-stage (a) and two-stage (b) charging.

Aiming at decrease in turbo lag, Volvo Group (Sweden) proposed Power Pulse™ system (Fig.8) [15]. The essence of this system is accumulation of compressed air in separate cylinder. When sharp acceleration is required, the special valve opens and additional air together with main portion is supplied to compressor. Thus, the turbocharger shaft is rotated faster, the pressure in inlet collector increases, the time of engine acceleration at lower rotation frequencies decreases. Reserve of compressed air in cylinder is rapidly recovered by electric compressor, and the system is ready for next acceleration.

Figure 8 PowerPulse™

One of the recent projects of BorgWarner Turbo Systems GmbH (Germany) is eBooster® system (Fig. 9) comprised of an electric compressor installed before or after centrifugal compressor [16]. Contrary to the systems where electric motor directly rotates turbocharger, this system operates in two stages, that is, as serial turbochargers. Upon operation at one speed without necessity of acceleration, the electric compressor is deactivated. When accelerator is maximally pedaled, ordinary and electric compressors are accelerated simultaneously. However, turbocharger requires more time to reach nominal capacity than electric compressor (see Fig. 10). When the capacity of waste gases is sufficient for the required dynamics, the electric compressor is deactivated.



Figure 9 eBooster®: 1 – compressor, 2, 4 – bypass valves, 3 – electric compressor, 5 – radiator.

Figure 10 General view of electric compressor

Hybrid turbocharger developed by MHI (Japan) [17] is comprised of consecutively connected compressor, high speed electric machine (electric motor and generator), and turbine. Upon operation at high engine rotations, the excessive energy produced by turbine is consumed for generator operation which charges electric battery. The use of generator requires for special invertor for conversion of high frequency alternate current. Upon operation at low rotation frequencies, the energy is supplied in reverse order. The current from battery via invertor is supplied to the electric machine which now operates as electric motor.

Simulation of the processes on test bench with prototype (constant load 0.16 Nm, equivalent to 120 000 rpm and capacity of 2 kW, 72 V battery) demonstrated decrease in time consumption for turbine acceleration to operation frequency by 33% and increase in torque by 17% in the range from 1000 to 1200 rpm in comparison with variable geometry centrifugal compressors.

However, such system is based on application of high speed electric machines. One relevant project is discussed in [18]. High rotation speeds can be achieved by hi-tech materials and methods of their processing. Thus, rotor is made of mechanically strengthened neodymium permanent magnet (Nd-Fe-B). Stator is manufactured in the form of laminated core with extremely thin (0.15 mm) electric steel plates in order to reduce losses. Experiments and computer simulation demonstrated that the best properties were achieved by six-pole machine with invertor of pseudoconstant current capable to operate continuously with invertor for 2.2 kW at 120 000 rpm and to provide 220 000 rpm at peak loads.

Another approach to increase of turbocharging pressure [19] is application of electric compressor in combination with centrifugal compressor. The use of electric compressor makes it possible to separate it from hot turbine which has positive effect on system operation and decreases temperature of charged air. The tests demonstrated the time to nominal capacity of compressor in idle mode equaling to 1.0 s (acceleration from 10 000 to 140 000 rpm). In addition, simulation results of 1.4 l petrol engine demonstrated decrease in transient mode by 35%.



In addition, the company developed completely electric compressor powered by vehicle electric circuit. This allows using it as auxiliary device which boosts the main unit under insufficient capacity at low rotations [20].

3. CONCLUSION

Therefore, designers and manufacturers of centrifugal compressors are highly interested in continuous improvement of their performances. Improvements are related to nearly all units and parts of centrifugal compressors: wheels of turbines and compressors, their housings, bearing assemblies, oil supply and cooling systems, control systems, etc. Finally, some recommendations can be given corresponding to current trends in development:

• To decrease roughness of wheels below Ra 1.0. However, further increase in surface finish characteristics may not provide high increase in efficiency whereas labor consumptions and production expenses are higher by an order of magnitude.

• To simulate wheels in appropriate software, thus optimizing blade profile and shape.

• For diesel engines, application of centrifugal compressors with variable configuration is very important. For petrol engines, as shown in [21], application of twin-scroll turbines can significantly improve specific power, flexibility, and other performances of internal combustion engines.

• Modernization of control system using bypass valves makes it possible to improve reliability of supercharger eliminating compressor operation in surging modes. In addition, electronic control system facilitates flow adjustment of boost air and waste gases.

• Conversion from slide bearings to roller bearings decreases resistance against oil flow, thus allowing not increasing capacity of oil pump. Cooling cavities decrease heat intensity of parts of centrifugal compressor and make it easier to engine preheating after start-up.

• Multistage charging makes it possible to decrease sizes of certain assemblies upon increase of total system performance. However, this approach increases total cost of propulsion unit and decreases reliability. It would be more reasonable to apply electric machines providing good acceleration characteristics. In addition, it becomes possible to recover excessive energy which in conventional centrifugal compressors is recovered via valves of control system.

The above recommendations make it possible to improve significantly performances of centrifugal compressors as well as performances of combined propulsion system. Herewith, some approaches can be characterized by low economic effect: expenses for improvement are too high in comparison with the improved property. Thus, further decrease in roughness of wheels can increase the production costs by several times, whereas increase in efficiency is a few percent. As a consequence, it is required to analyze economic effect for considered approaches. However, since many developments are at the stage of prototype, it is difficult to obtain information about the involved expenses of implementation. Then, it is possible to perform indirect estimations: on the basis of a number of publications devoted to the considered approach. Therefore, the modern vector of development of centrifugal compressors becomes obvious: hybridization and improvement of wheel geometry.



ACKNOWLEDGMENTS

This work was financially supported by the Ministry of Education and Science of the Russian Federation within the subsidiary grant agreement No. 14.577.21.0213 (September 29, 2016; unique identifier of applied research and experimental development RFMEFI57716X0213).

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