energies

Review Electric Boosting and Energy Recovery Systems for Engine Downsizing

Mamdouh Alshammari 1,2, Fuhaid Alshammari 2 and Apostolos Pesyridis 1,*

1 Centre of Advanced Powertrain and Fuels (CAPF), Department of Mechanical, Aerospace and Civil Engineering, Brunel University London, Middlesex UB8 3PH, UK; [email protected] 2 Department of Mechanical Engineering, University of Hai’l, Hail 55476, Saudi Arabia; [email protected] * Correspondence: [email protected]



Received: 31 October 2019; Accepted: 4 December 2019; Published: 6 December 2019


Abstract: Due to the increasing demand for better fuel economy and increasingly stringent emissions regulations, engine manufacturers have paid attention towards engine downsizing as the most suitable technology to meet these requirements. This study sheds light on the technology currently available or under development that enables engine downsizing in passenger cars. Pros and cons, and any recently published literature of these systems, will be considered. The study clearly shows that no certain boosting method is superior. Selection of the best boosting method depends largely on the application and complexity of the system.

Keywords: engine downsizing; electrically assisted turbocharger; electric supercharger; e-turbo; waste heat recovery; turbocharging; supercharging; turbocompounding; organic Rankine cycle

1. Introduction Although internal combustion engines are getting more efficient nowadays, still the major part of fuel energy is transformed into wasted heat. In terms of harmful exhaust emissions, the transportation sector is responsible for the one-third of CO2 emissions worldwide and approximately 15% of the overall greenhouse gas emissions [1]. Moreover, owing to the limited amount of fossil fuels, prices fluctuate significantly, with consistent general rising trends, resulting in economic issues in non-oil-producing countries. For example, fuel prices have continually increased, from 60 pence/litre in 1997 to 120 pence/litre in 2013 in the United Kingdom [2]. Reduction of exhaust gas emissions is gaining great attention. Recently, organic Rankine cycles (ORC) have been intensively applied [3]. Alshammari et al. [4,5] tested the ORC system as a bottoming cycle in heavy-duty diesel engines. The results were promising with 3% reduction in engine exhaust gases and 6 kW generated power. In a subsequent study, Alshammari and Pesyridis [6] improved the performance of the ORC by testing the coupled engine-ORC system at low-temperature cooling water and integrating the custom-designed radial inflow turbine detailed in [7]. Lowering the cooling water temperature resulted in higher enthalpy drop and hence higher electrical power (9 kW compared to 6 kW in the previous testing). However, the implementation of ORC in modern passenger cars requires additional features to achieve a compact integration and controllability in the engine. Moreover, vehicles gain extra weight when coupled to an ORC system, which results in higher fuel consumption [8]. Another waste heat recovery technology of note in internal combustion engines is thermoelectric generation (TEG). Thermoelectric generators convert some of the waste heat of an internal combustion engine (IC) into electricity using the Seebeck effect. However, this technology has three main challenges. Firstly, it has generally exhibited a substantially inferior efficiency, typically less than 4% [9]. The second challenge is the bigger size of the radiator and extended piping to the exhaust manifold [10]. Thirdly,

Energies 2019, 12, 4636; doi:10.3390/en12244636 www.mdpi.com/journal/energies

than 4% [9]. The second challenge is the bigger size of the radiator and extended piping to the exhaust Energies 2019, 12, 4636 2 of 33 manifold [10]. Thirdly, thermoelectric generators are not mature yet and some efficient materials are yet to be manufactured [11]. However, new nano‐crystalline or nano‐wire thermoelectric materials thermoelectricare currently generators in the development are not mature stage yet and to someimprove efficient the materials conversion are yetefficiency to be manufactured of thermoelectric [11]. However,generators. new nano-crystalline or nano-wire thermoelectric materials are currently in the development stage toTherefore, improve the automotive conversion vehicles, efficiency especially of thermoelectric passenger generators. cars, need more advanced and practical technologiesTherefore, in automotive order to improve vehicles, the especially engine performance passenger in cars, terms need of emissions more advanced and fuel and consumption. practical technologiesThe most incompetent order to improve emission the reduction engine performance technology in today terms is of engine emissions downsizing and fuel consumption. coupled with Theboosting most competent technology. emission Thirouard reduction et al. technology [12] defined today engine is engine downsizing downsizing as the coupled use of a with ‘small boosting‐capacity technology.engine operating Thirouard at ethigh al. [ 12specific] defined engine engine loads downsizing to achieve as the low use fuel of aconsumption’. ‘small-capacity The engine main operatingadvantages at high of downsizing specific engine technology loads to are achieve a significantly low fuel increased consumption’. power The and main torque advantages for the engine of downsizingwithout increasing technology the are capacity a significantly of the increasedengine. Moreover, power and fuel torque consumption for the engine is reduced without primarily increasing by thedecreasing capacity of the the friction engine. losses Moreover, associated fuel consumption with reduced is engine reduced size primarily and improving by decreasing the efficiency the friction of an lossesengine associated when running with reduced at high engine loads size [12]; and with improving small intake the e ffithrottling,ciency of pumping an engine losses when are running lessened. at highPetitjean loads [ 12et ];al. with [13] small described intake the throttling, latter aspect pumping as effectively losses are ‘moving lessened. the Petitjean best fuel et economy al. [13] described island [of thethe latter engine] aspect close as e fftoectively the steady ‘moving state the road best load fuel condition’, economy islandor alternately [of the engine] as avoiding close conducting to the steady the stateoperation road load in condition’,the area with or alternatelyhuge pumping as avoiding losses. With conducting regard the to operationfriction, sliding in the surface area with friction huge is pumpingtypically losses. reduced With by regard decreasing to friction, the piston sliding‐ring surface‐to‐cylinder friction contact is typically area reduced (associated by decreasing with a reduced the piston-ring-to-cylindernumber of cylinders contact and/or areadecreased (associated bore withand astroke) reduced and number the swept of cylinders area of and crankshaft/or decreased journal borebearings. and stroke) and the swept area of crankshaft journal bearings. ThisThis eff effectect is illustratedis illustrated in in Figure Figure1, which 1, which compares compares the the brake brake specific specific fuel fuel consumption consumption (BSFC) (BSFC) mapmap of of a a 2.6 2.6 L L naturally naturally aspirated aspirated gasolinegasoline engine and and a a 1.8 1.8 L L turbocharged turbocharged downsized downsized engine. engine. The TheBSFC BSFC of of downsized downsized engine engine is isconsistently consistently low low along along the the steady steady state state road road load load curve. curve.

FigureFigure 1. 1.Comparison Comparison of of brake brake specific specific fuel fuel consumption consumption (BSFC) (BSFC) maps maps of naturally of naturally aspirated aspirated and and downsizeddownsized engines engines [14 [14].]. In this example, full-load performance potential is typically maintained through pressure charging In this example, full‐load performance potential is typically maintained through pressure (supercharging) to facilitate downsizing [15–18]. In conjunction with turbocharging, direct fuel charging (supercharging) to facilitate downsizing [15–18]. In conjunction with turbocharging, direct injection and variable valve timing for inlet and exhaust valves can aid in the downsizing for gasoline fuel injection and variable valve timing for inlet and exhaust valves can aid in the downsizing for engines [15]. According to Turner et al. [19], gasoline direct injection (GDI) produces high compression gasoline engines [15]. According to Turner et al. [19], gasoline direct injection (GDI) produces high ratios for improved thermal efficiency because of its charge cooling effects, and variable valve timing compression ratios for improved thermal efficiency because of its charge cooling effects, and variable (inlet and exhaust) increases scavenging and reduces part-load throttling losses. These technologies valve timing (inlet and exhaust) increases scavenging and reduces part‐load throttling losses. These have been combined and adopted by various manufacturers, including Ford [20] and Romeo [21], to technologies have been combined and adopted by various manufacturers, including Ford [20] and reduce emissions through engine downsizing. Fiat has removed the throttle (throttling losses) and Romeo [21], to reduce emissions through engine downsizing. Fiat has removed the throttle (throttling its ‘MultiAir’ electrohydraulic valve actuation technology [22]. Other technologies synergistic with losses) and its ‘MultiAir’ electrohydraulic valve actuation technology [22]. Other technologies downsizing, including spray-guided direct injection [23], have been implemented by Mercedes [24] while variable compression ratio has not yet attained production [25,26].

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The basis for increased specific engine output, which is crucial to engine downsizing, is the definition of fundamental engine performance parameters. Heywood [27] derived the equation for specific power as follows:  F  P η f ηvSp−QHVρa,in A = Ap 4

2 where P is power (W), Ap is the total piston area (m ), ηf is fuel conversion efficiency, ηv is volumetric efficiency, and S−p is mean piston speed (m/s). QHV is fuel heating value (J/kg), ρa,in is the inlet air density, and (F/A) is fuel–air ratio. In this equation, the following factors directly affecting the performance of an engine are considered. Increasing any of them will also increase the engine performance (provided all other factors are equal):

1. Fuel conversion efficiency (inversely proportional to specific fuel consumption); 2. Volumetric efficiency; 3. Inlet air density; 4. Maximum fuel/air ratio that can be usefully burned in the engine; 5. Mean piston speed.

Engine downsizing targets points 2 and 3 in this list and point 1 of at least under ‘real-world’ (part load) driving conditions, if not full-load conditions. In this paper, the boosting requirements are discussed. In addition, the variety of boosting systems are presented and the suitability of each one is analysed in relation to the engine requirements and in comparison to the other boosting systems.

2. Downsizing Enablers This section briefly presents various types of turbines and compressors integrated in different boosting systems.

2.1. Compressor Technologies Two main configurations of compressors, namely, single-stage and multistage, are discussed in terms of their applications and operating conditions. In order to meet the requirements of continuously enhanced brake mean effective pressure (BMEP), the turbocharger boost pressure ratio should be increased as shown in Figure2. It is worth mentioning that BorgWarner has developed water-cooled compressor housing. For high specific power outputs, an extreme boost pressure and pressure ratio are required. However, a high-pressure ratio also leads to a high compressor outlet temperature. Therefore, the water-cooled compressor housing is integrated in order to reduce the compressor outlet temperature and ultimately to prevent the coking of oil in the compressor volute and the charge air cooler [28]. Energies 2019, 12, 4636 4 of 33

Figure 2. Required boosting pressure ratio at different BMEP [29]. Figure 2. Required boosting pressure ratio at different BMEP [29]. 2.1.1. Single Stage Compressor 2.1.2. Multistage Compressor Operating withFigure a high-pressure 2. Required ratio boosting in single pressure stage ratio compressor at different inBMEP Figure [29].3 results in transonic flow andThe strong multistage tip clearance configuration losses. Figure However, 3 is at two a pressure compressors ratio ofconnected 4, the aforementioned through a common challenges shaft 2.1.2. Multistage Compressor canwith be the overcome turbine. by This applying configuration stability is enhancementbeneficial since methods the system such operates as air injection with high at boost the impeller pressure inducerandThe low/di multistage ffrotationaluser inlet, configuration speed adjustable and inlet henceFigure guide lower3 is vanes,two stress compressors and and fluctuating higher connected efficiency. plate atthrough the However, compressor a common multistage outlet. shaft Thewithconfiguration aforementioned the turbine. has This drawbacks; methods configuration assist the difficulty inis beneficial decreasing of matching since the massthe betweensystem flow surge operates the forcompressors thewith compressor. high and boost extra pressure Another power methodandrequired low for rotationalby stability the turbine enhancementspeed in orderand henceto is thedrive treatmentlower the two stress of compressors. the and compressor higher efficiency. casing near However, the inducer. multistage Figure4 showsconfiguration a compressor has drawbacks; map with the casing difficulty treatments. of matching The figure between clearly the shows compressors the shift and of surge extra limit power at lowrequired speed by operation the turbine is more in order distinct to drive and the towards two compressors. higher mass flow rate.

Figure 3. Configurations of single‐stage and multistage compressors.

Figure 3. ConfigurationsConfigurations of single single-stage‐stage and multistage compressors.

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Figure 4. Measured performance of compressors with case treatment [30]. Figure 4. Measured performance of compressors with case treatment [30]. 2.1.2. Multistage Compressor 2.1.3. Variable Trim Compressor The multistage configuration Figure3 is two compressors connected through a common shaft with the turbine.A new This technology configuration is proposed is beneficial by sinceSwine the and system Engeda operates [31]to with modify high the boost impeller pressure design and by lowtrimming rotational the speed impeller. and hence This lowertechnology stress assists and higher in maintaining efficiency. However,the efficiency multistage of the compressor configuration high for longer time. This can be achieved by optimizing the hub and shroud control points in the Bezier has drawbacks; theFigure diffi culty4. Measured of matching performance between of compressors the compressors with case and treatment extra power [30]. required by the turbinepolynomial. in order Omidi to drive et the al. two[32] compressors.presented a comparison study between trimmed compressor and base 2.1.3.one Variable as can be Trim seen Compressor in Figure 5. The results showed that the trimmed compressor presented a higher 2.1.3.isentropic Variable efficiency Trim Compressor compared to the base one, with a maximum difference of 2%. A new technology is proposed by Swine and Engeda [31]to modify the impeller design by trimmingA new the technology impeller. This is proposed technology by Swineassists andin maintaining Engeda [31 the] to efficiency modify the of the impeller compressor design high by trimmingfor longer the time. impeller. This can This be technologyachieved by assists optimizing in maintaining the hub and the shroud efficiency control of the points compressor in the Bezier high forpolynomial. longer time. Omidi This et can al. be [32] achieved presented by a optimizing comparison the study hub andbetween shroud trimmed control compressor points in the and Bezier base polynomial.one as can be Omidi seen in et al.Figure [32] presented5. The results a comparison showed that study the trimmed between trimmedcompressor compressor presented and a higher base oneisentropic as can beefficiency seen in compared Figure5. The to the results base showed one, with that a maximum the trimmed difference compressor of 2%. presented a higher isentropic efficiency compared to the base one, with a maximum difference of 2%.

Figure 5. Comparison between trimmed (optimized) and base compressors [32].

2.2. Turbine Technologies The aim of turbine is either to generate electricity as in the electrical turbocharging systems or to drive the compressor as in the conventional turbochargers. Figure 5. Comparison between trimmed (optimized) and base compressors [32]. Figure 5. Comparison between trimmed (optimized) and base compressors [32]. 2.2.1. Variable Geometry Turbine (VGT) 2.2. Turbine Technologies

The aim of turbine is either to generate electricity as in the electrical turbocharging systems or to drive the compressor as in the conventional turbochargers.

2.2.1. Variable Geometry Turbine (VGT)

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2.2. Turbine Technologies

The aim of turbine is either to generate electricity as in the electrical turbocharging systems or to drive the compressor as in the conventional turbochargers. In VGT, Figure 6, the exhaust gas delivered by the engine is controlled and automatically 2.2.1. Variable Geometry Turbine (VGT) supplied to the turbine in order to permit turbine power to sufficiently provide energy, which drives the compressorIn VGT, Figure at 6the, the required exhaust boost gas delivered level wherever by the engine the engine is controlled is running and in automatically its range. This supplied can be toaccomplished the turbine inby ordervarying to the permit throat turbine area of power the turbine to su ffistatorciently in order provide to control energy, the which mass drives flow of the the compressorexhaust gas. at As the the required throat area boost decreases, level wherever the exhaust the engine pressure is running and the inrotational its range. speed This increase. can be accomplishedContrarily, increasing by varying the the throat throat area ofreduces the turbine the statorexhaust in orderpressure to controland, consequently, the mass flow the of theturbocharger exhaust gas. boost As thedecreases. throat area decreases, the exhaust pressure and the rotational speed increase. Contrarily, increasing the throat area reduces the exhaust pressure and, consequently, the turbocharger boost decreases.

Figure 6. Turbine with different stator positions (variable geometry turbine (VGT)), (a) closed, (b) mid, (cFigure) open 6. [33 Turbine]. with different stator positions (variable geometry turbine (VGT)), (a) closed, (b) mid, (c) open [33]. The configuration of the turbine can be axial, radial, or mixed flow. Axial and radial turbines are commonlyThe configuration applied in of turbocharging. the turbine can However, be axial, radial, small or axial mixed flow flow. turbines Axial su andffer radial from the turbines high tip are leakagecommonly at the applied rotor blade.in turbocharging. Similarly, radial However, turbines small have axial a limitedflow turbines operating suffer range from because the high of itstip inletleakage geometrical at the rotor requirements. blade. Similarly, Mixed radial flow turbines (whichhave a limited is a modified operating radial range turbine because configuration of its inlet asgeometrical can be seen requirements. in Figure7) have Mixed an flow advantage turbines of (which the cone is angle,a modified which radial is between turbine axial configuration and radial. as Ascan a result,be seen the in Figure design 7) of have mixed an flow advantage turbines of has the morecone angle, freedom which and, is consequently, between axial a and wider radial. range As of a operation.result, the This design enables of mixed the mixed flow flowturbines turbines has more to accomplish freedom higherand, consequently, mass-induced a masswider flow range rate of andoperation. optimum This effi enablesciency atthe lower mixed velocity flow turbines ratios, compared to accomplish to equivalent higher mass radial‐induced ones. mass flow rate and optimum efficiency at lower velocity ratios, compared to equivalent radial ones.

In VGT, Figure 6, the exhaust gas delivered by the engine is controlled and automatically supplied to the turbine in order to permit turbine power to sufficiently provide energy, which drives the compressor at the required boost level wherever the engine is running in its range. This can be accomplished by varying the throat area of the turbine stator in order to control the mass flow of the exhaust gas. As the throat area decreases, the exhaust pressure and the rotational speed increase. Contrarily, increasing the throat area reduces the exhaust pressure and, consequently, the turbocharger boost decreases.

Figure 6. Turbine with different stator positions (variable geometry turbine (VGT)), (a) closed, (b) mid, (c) open [33].

The configuration of the turbine can be axial, radial, or mixed flow. Axial and radial turbines are commonly applied in turbocharging. However, small axial flow turbines suffer from the high tip leakage at the rotor blade. Similarly, radial turbines have a limited operating range because of its inlet geometrical requirements. Mixed flow turbines (which is a modified radial turbine configuration as can be seen in Figure 7) have an advantage of the cone angle, which is between axial and radial. As a result, the design of mixed flow turbines has more freedom and, consequently, a wider range of Energiesoperation.2019, 12 This, 4636 enables the mixed flow turbines to accomplish higher mass‐induced mass flow7 of rate 33 and optimum efficiency at lower velocity ratios, compared to equivalent radial ones.

Figure 7. Configuration of mixed flow turbine [34], radial inflow turbine [35] and axial flow turbine [36]. Figure 7. Configuration of mixed flow turbine [34], radial inflow turbine [35] and axial flow turbine [36].

BaetsBaets et et al. al. [37 [37]] presented presented a direct a direct comparison comparison between between the effi ciencythe efficiency fixed and fixed variable and geometry variable mixedgeometry flow mixed turbines. flow As turbines. shown inAs Figure shown8, in the Figure e fficiency 8, the curve efficiency of the curve VGT of is slightlythe VGT lower is slightly because lower of sealingbecause losses of sealing and the losses geometry and the losses geometry in the losses nozzle in ring. the nozzle However, ring. Ishino However, and Ishino Bessho and [38] Bessho improved [38] theimproved geometry the of geometry the nozzle of ring the nozzle by applying ring by pivoting applying technique pivoting in technique which the in nozzle which vanes the nozzle are pivoted vanes atare the pivoted trailing at edge. the trailing Their resultsedge. Their showed results a 14% showed improvement a 14% improvement of the pivoted of VGT the comparedpivoted VGT to conventionalcompared to VGT.conventional VGT.

FigureFigure 8. 8. ComparisonComparison betweenbetween fixed fixed geometry geometry and and variable variable geometry geometry mixed mixed flow flow turbines. turbines. 2.2.2. Active Control Turbocharger 2.2.2. Active Control Turbocharger An interesting development to the VGTs in recent years has been the active control turbocharger An interesting development to the VGTs in recent years has been the active control turbocharger (ACT) developed by one of the co-authors and his research group at Imperial College London [39]. (ACT) developed by one of the co‐authors and his research group at Imperial College London [39]. This system has a significant theoretical potential and consists of a variable geometry mechanism (both This system has a significant theoretical potential and consists of a variable geometry mechanism in pivoting vane and sliding wall versions), which is opened and closed at the frequency of incoming (both in pivoting vane and sliding wall versions), which is opened and closed at the frequency of pressure pulses at the turbine inlet. It, therefore, required a more robust nozzle and actuation system incoming pressure pulses at the turbine inlet. It, therefore, required a more robust nozzle and and a high-power actuator to be able to achieve this type of operation. By optimizing the turbine throat actuation system and a high‐power actuator to be able to achieve this type of operation. By optimizing area throughout each exhaust gas pulse, it could recover significantly more energy than a conventional the turbine throat area throughout each exhaust gas pulse, it could recover significantly more energy VGT system. However, there are several practical issues with the control and reliability of operation of than a conventional VGT system. However, there are several practical issues with the control and the mechanism in a real engine environment and it has so far not seen production [40]. reliability of operation of the mechanism in a real engine environment and it has so far not seen production [40].

2.2.3. Twin and Double Turbine Scrolls Figure 9 illustrates the difference between the twin entry and double entry configurations in turbines. Two turbocharger turbine entries are used to ultimately increase the energy of the exhaust driving the turbine in which the turbine wheel is directly fed with the highly pulsating flow twin or double turbine scrolls. The double entry scroll delivers better efficiency at full load while the twin entry scroll presents better performance at partial load.

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2.2.3. Twin and Double Turbine Scrolls Figure9 illustrates the di fference between the twin entry and double entry configurations in turbines. Two turbocharger turbine entries are used to ultimately increase the energy of the exhaust driving the turbine in which the turbine wheel is directly fed with the highly pulsating flow twin or double turbine scrolls. The double entry scroll delivers better efficiency at full load while the twin entry scroll presents better performance at partial load.

Figure 9. (a) Twin entry [41]; (b) double entry scrolls turbines.

3. Supercharging Figure 9. (a) Twin entry [41]; (b) double entry scrolls turbines. According to Watson and Janota [42], ‘supercharging can be defined as the introduction of air (or air3./fuel Supercharging mixture) into an engine cylinder at a density greater than the ambience’. This process allows a proportionally huge amount of fuel to be burned, thus increasing the potential power output of the According to Watson and Janota [42], ‘supercharging can be defined as the introduction of air engine. Three basic methods, namely, turbocharging, pressure wave supercharging, and mechanical (or air/fuel mixture) into an engine cylinder at a density greater than the ambience’. This process supercharging, are used to achieve this condition. allows a proportionally huge amount of fuel to be burned, thus increasing the potential power output 3.1.of Turbochargingthe engine. Three basic methods, namely, turbocharging, pressure wave supercharging, and mechanical supercharging, are used to achieve this condition. A turbocharger is a device with a compressor and turbine on a single shaft, and the turbine is powered3.1. Turbocharging by energy in the engine’s exhaust gases to drive the compressor and increase the intake pressure. The design of such turbomachines is widely available in literature such as [43–46]. A turbocharger is a device with a compressor and turbine on a single shaft, and the turbine is A Sankey diagram for a typical 1.4 L four-cylinder spark ignition (gasoline) engine is shown in powered by energy in the engine’s exhaust gases to drive the compressor and increase the intake Figure 10. A maximum of one-third of fuel energy is converted into useful work, and an average of pressure. The design of such turbomachines is widely available in literature such as [43–46]. 15% of fuel energy is wasted as exhaust heat. One major benefit of a turbocharger is that it utilizes A Sankey diagram for a typical 1.4 L four‐cylinder spark ignition (gasoline) engine is shown in exhaust gas energy that would otherwise be wasted, thus leading to an overall improvement in Figure 10. A maximum of one‐third of fuel energy is converted into useful work, and an average of thermal efficiency. 15% of fuel energy is wasted as exhaust heat. One major benefit of a turbocharger is that it utilizes exhaust gas energy that would otherwise be wasted, thus leading to an overall improvement in thermal efficiency.

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Figure 10. Sankey diagram showing energy balance for naturally aspirated spark ignition engine [47]. Figure 10. Sankey diagram showing energy balance for naturally aspirated spark ignition engine In other works (particularly in Watson and Janota[47]. [42] and Baines [48]), conventional automotive turbochargers use a centrifugal compressor and radial flow turbine. Such turbomachines have an optimumIn other operating works (particularly point and in ‘are Watson unsuitable and Janota for [42] operation and Baines under [48]), various conventional flow ranges’ automotive [42] becauseturbochargers of their use design a centrifugal and operating compressor principles. and Thisradial case flow is applicableturbine. Such for anturbomachines automotive engine.have an Emissionoptimum reduction operating technologies, point and ‘are such unsuitable as exhaust for operation gas recirculation under various and diesel flow ranges’ particulate [42] because filters, renderof their compressor design and and operating turbine matching principles. as This problematic case is applicable [49] Several for authors an automotive [17,42,48 ,engine.50,51] indicated Emission areduction fundamental technologies, agreement such when as matching exhaust agas turbocharger recirculation and and an diesel engine, particulate that is, torque filters, at render low speedcompressor and power and atturbine high enginematching speed. as problematic A large turbocharger [49 Several providesauthors [17,42,48,50,51] power at high speedindicated but a experiencesfundamental low-speed agreement performance when matching and transient a turbocharger response and because an engine, of the that lack is, of torque exhaust at low gas flowspeed rateand to power overcome at high the engine inertia speed. of the A system.large turbocharger A small turbocharger provides power provides at high improved speed but low-speedexperiences torquelow‐speed and transient performance response and because transient of theresponse reduced because inertia, of although the lack it of requires exhaust turbine gas flow bypassing rate to toovercome prevent excessive the inertia turbocharger of the system. speed A small at high turbocharger engine speeds, provides thus sacrificingimproved low the‐ espeedfficiency. torque Small and turbochargerstransient response generally because exhibit of low the ereducedfficiency inertia, because although of the increased it requires leakage turbine pressure bypassing losses to between prevent theexcessive turbine andturbocharger housing. This speed low-speed at high performance engine speeds, impairment thus is compoundedsacrificing the with efficiency. highly boosted Small enginesturbochargers [17,52]. Thegenerally drivability exhibit characteristics low efficiency of a comparable because of naturally the increased aspirated leakage unit arepressure the aim losses of a forcedbetween reduction the turbine engine, and and housing. various This solutions low‐speed have beenperformance introduced impairment to diminish is compounded the effects of thiswith turbocharginghighly boosted compromise. engines [17,52]. The drivability characteristics of a comparable naturally aspirated unitA are turbine the aim bypass—or of a forced ‘wastegate’—actualizes reduction engine, and various the correct solutions sizing have for been a turbocharger introduced under to diminish low enginethe effects speed of [ 48this]. Theturbocharging wastegate iscompromise. opened with an increase in speed, thus allowing a proportion of exhaustA gasturbine to bypass bypass—or the turbine, ‘wastegate’—actualizes limiting the boost pressure the correct and sizing preventing for a theturbocharger overspeeding under of the low turbocharger.engine speed However, [48]. The wastegate thermal effi isciency opened is relinquishedwith an increase because in speed, the bypassed thus allowing exhaust a proportion gas energy of isexhaust wasted. gas to bypass the turbine, limiting the boost pressure and preventing the overspeeding of the turbocharger.Variable geometry However, turbine thermal indicates efficiency that theis relinquished effective turbine because area the (or aspectbypassed ratio) exhaust can be gas matched energy tois the wasted. changing exhaust gas flow rate [53]. The concepts designed to achieve this condition can be classifiedVariable into two geometry categories turbine depending indicates on thethat adjustability the effective of turbine the geometry area (or of the aspect volute ratio) or nozzle. can be Accordingmatched to Matsurathe changing et al. exhaust [54], a turbocharger gas flow rate with [53]. variable The concepts geometry designed volute isto aachieve low-cost this alternative condition tocan variable be classified geometry into nozzle two categories arrangements, depending which on are the generally adjustability complex. of the geometry Despite experimentally of the volute or demonstratednozzle. According improvements to Matsura in transient et al. [54], response a turbocharger [54–56], onlywith a variable few variable geometry geometry volute volute is a designslow‐cost havealternative attained to commercial variable geometry production. nozzle Baines arrangements, [48] indicated which that this are condition generally may complex. be because Despite of theexperimentally aerodynamic inedemonstratedfficiency issues improvements relative to fixed in transient geometry response counterparts [54–56], and only the durabilitya few variable and performancegeometry volute deterioration designs in service.have attained Turbochargers commercial with variableproduction. nozzle Baines geometry [48] have indicated achieved that huge this commercialcondition success.may be Variablebecause nozzleof the devices aerodynamic have two inefficiency principal types,issues namely, relative pivoting to fixed vanes geometry and movingcounterparts sidewall and [48 ],the and durability can provide and (amongst performance other benefits)deterioration improved in service. transient Turbochargers performance [with54] andvariable low speed nozzle boost geometry [57]. By have using achieved a variable huge nozzle commercial turbocharger success. fitted Variable to 1.8 Lnozzle direct devices injection have diesel two engine,principal Hawley types, et namely, al. [58] pivoting increased vanes the torque and moving in the entiresidewall engine [48], speed and can range provide compared (amongst with other an

Energies 2019, 12, 4636 10 of 33 equivalent fixed geometry unit. Wijetunge et al. [17] and Matsura et al. [54] believed that variable geometry turbochargers rely on the build-up of exhaust gas energy and thus do not completely solve the problem of transient response, particularly at low engine speeds. Variable nozzle devices with huge proliferation (as opposed to variable geometry volute) shall be referred to as ‘variable geometry turbocharger’ (VGT). Various arrangements are used in multiple turbochargers and generally classified into three categories, namely; series, parallel, and sequential. Several factors limiting the pressure ratio, such as efficiency reductions at high-pressure ratios, mass flow range requirements, and temperature limits [48], can be overcome by a single compressor. A series turbocharged configuration may become viable with the increasing boost pressure (i.e., pressure ratio) requirements, which is the case in downsizing. Considering a two-stage system, two turbochargers are placed in series such that the exhaust gases undergo two expansion stages and the intake charge goes through two compression stages. Watson and Janota [42] indicated that ‘high overall pressure and expansion ratios may be developed by using conventional turbochargers’ without relinquishing efficiency or mass flow range. Series systems may include bypass valves (for turbines and/or compressors) for immense operation flexibility as shown in Figure 11 (reproduced from Pflüger [59]). A two-stage arrangement by Pfluger [59] on a 12 L commercial diesel engine showed increased torque at all engine speeds, increased rated power, improved air supply, reduced BSFC and smoke, and potential to reduce NOx emissions compared with an equivalent single-stage system. Although transient tests are not performed, the transient response could potentially be improved with a two-stage system. Baines [48] and Watson and Janota [42] highlighted the disadvantages of series turbocharging, such as the costs of extra turbocharger and intercooler (which is usually required), increased bulk and complexity of the system, and additional pressure losses. Baines [48] stated that the transient performance of a two-stage system is generally worse than that of an equivalent single-stage unit because the exhaust gas energy available to accelerate the two turbochargers is shared between them. In a computational investigation by Saulnier and Guilain [60], a 2.0 L diesel engine was downsized to 1.5 L by moving from single-stage to two-stage turbocharging; equivalent steady-state performance was easily achieved, but the low-speed transient response worsened. Energies 2019, 12, 4636 11 of 33

Figure 11. Schematic diagram of regulated series (two-stage) turbocharging system [59]. Figure 11. Schematic diagram of regulated series (two‐stage) turbocharging system [59]. In a parallel turbocharged arrangement, two (or four) turbochargers of equal size are used to In a parallel turbocharged arrangement, two (or four) turbochargers of equal size are used to replace a large single unit. Parallel turbocharging is typically used on engines with six or more replace a large single unit. Parallel turbocharging is typically used on engines with six or more cylinders by dividing the exhaust pipes from the cylinders into groups, which are favourable for cylinders by dividing the exhaust pipes from the cylinders into groups, which are favourable for exhaust pulse effects (pulse turbocharging is comprehensively studied by Watson and Janota [42]). In exhaust pulse effects (pulse turbocharging is comprehensively studied by Watson and Janota [42]). a system with two turbochargers, each turbine receives exhaust gases from half of the cylinders of In a system with two turbochargers, each turbine receives exhaust gases from half of the cylinders of the engine, and the compressors generally feed into a common intake plenum on the intake side as the engine, and the compressors generally feed into a common intake plenum on the intake side as shown in Figure 12. For the aforementioned exhaust pulse effects, the benefits of parallel turbocharging shown in Figure 12. For the aforementioned exhaust pulse effects, the benefits of parallel are reduced (combined) turbocharger inertia for improved transient response [61,62] and simplified turbocharging are reduced (combined) turbocharger inertia for improved transient response [61,62] packaging, particularly for V-type engines [42,61]. As discovered by Sommerhoff [61], the net gain and simplified packaging, particularly for V‐type engines [42,61]. As discovered by Sommerhoff [61], of a parallel system is controversial compared with that of a single turbocharger setup because of the net gain of a parallel system is controversial compared with that of a single turbocharger setup some factors, such as the high efficiency of large turbomachinery and an associated reduction in because of some factors, such as the high efficiency of large turbomachinery and an associated back pressure. reduction in back pressure.

Energies 2019, 12, 4636 12 of 33

Figure 12. Schematic diagram of a parallel twin turbocharging system. Figure 12. Schematic diagram of a parallel twin turbocharging system. In a sequential system, Figure 13, two (or more) turbochargers are arranged in parallel and In a sequential system, Figure 13, two (or more) turbochargers are arranged in parallel and supply charge air to a common intake manifold, similar to parallel turbocharging. Different from supply charge air to a common intake manifold, similar to parallel turbocharging. Different from a a purely parallel configuration, the turbines are driven by exhaust gases from a common exhaust purely parallel configuration, the turbines are driven by exhaust gases from a common exhaust manifold, and the number of turbochargers in operation varies depending on the systematic use manifold, and the number of turbochargers in operation varies depending on the systematic use of of flow control valves. The turbochargers used may be of equal (or similar) size. For a twin-turbo flow control valves. The turbochargers used may be of equal (or similar) size. For a twin‐turbo system, only one turbocharger is in operation during the first sequence (at low engine speeds), and system, only one turbocharger is in operation during the first sequence (at low engine speeds), and two turbochargers are used during the second sequence (high engine speeds). Tashima et al. [63] two turbochargers are used during the second sequence (high engine speeds). Tashima et al. [63] developed this system for a 1.3 L gasoline rotary (Wankel) engine. Alternatively, a small turbocharger developed this system for a 1.3 L gasoline rotary (Wankel) engine. Alternatively, a small turbocharger may be used for low engine speed operation, switching solely to a large turbocharger at high engine may be used for low engine speed operation, switching solely to a large turbocharger at high engine speeds. Hancock et al. [15] used this system but serially arranged the turbochargers on a highly speeds. Hancock et al. [15] used this system but serially arranged the turbochargers on a highly downsized 1.2 L three-cylinder GDI engine. These two cases have a similar purpose and result: downsized 1.2 L three‐cylinder GDI engine. These two cases have a similar purpose and result: Changing the effective turbine area to match the engine speed and exhaust gas flow and improve the Changing the effective turbine area to match the engine speed and exhaust gas flow and improve the low-speed boost, torque, and transient response. Baines [48] described the effect to ‘a stepwise variable low‐speed boost, torque, and transient response. Baines [48] described the effect to ‘a stepwise geometry scheme’. In the aforementioned work of Tashima et al. [63], boost pressure and torque were variable geometry scheme’. In the aforementioned work of Tashima et al. [63], boost pressure and substantially improved (by 200% and 36%, respectively) at low engine speeds compared with those of torque were substantially improved (by 200% and 36%, respectively) at low engine speeds compared a conventional turbocharger. In vehicle acceleration tests, transient response was markedly superior as with those of a conventional turbocharger. In vehicle acceleration tests, transient response was indicated by the 43% reduction in the time taken to reach maximum boost. Similar improvements were markedly superior as indicated by the 43% reduction in the time taken to reach maximum boost. exhibited in computational simulations of a 2.5 L four-cylinder gasoline engine conducted by Brüstle et Similar improvements were exhibited in computational simulations of a 2.5 L four‐cylinder gasoline al. [64]. For the disadvantages of sequential turbocharging, Tashima et al. [63] and Baines [48] indicated engine conducted by Brüstle et al. [64]. For the disadvantages of sequential turbocharging, Tashima the need for systematic matching of the turbochargers and engine speed with sequence transition, et al. [63] and Baines [48] indicated the need for systematic matching of the turbochargers and engine which is crucial in avoiding a drop in torque prior to the switch and compressor surge (and choking). speed with sequence transition, which is crucial in avoiding a drop in torque prior to the switch and For other multiturbocharger schemes, additional plumbing and associated potential pressure losses compressor surge (and choking). For other multiturbocharger schemes, additional plumbing and must be considered. In relation to VGTs, Wijetunge et al. [17] asserted that the transient response of associated potential pressure losses must be considered. In relation to VGTs, Wijetunge et al. [17] any of these turbocharged systems is ultimately limited by the available exhaust gas energy, and any asserted that the transient response of any of these turbocharged systems is ultimately limited by the transient response issues are multiplied in highly boosted (e.g., highly downsized) applications. available exhaust gas energy, and any transient response issues are multiplied in highly boosted (e.g., highly downsized) applications.

Energies 2019, 12, 4636 13 of 33

Figure 13. Schematic diagram of a sequential twin turbocharging system [65]. Figure 13. Schematic diagram of a sequential twin turbocharging system [65]. 3.2. Mechanical Supercharging 3.2. Mechanical Supercharging supercharging occurs when the increased charge air density is provided by a pump or compressor,Mechanical which supercharging is usually driven occurs fromwhen the the engine increased crankshaft charge viaair density a gear train is provided or belt and by a pulley pump system.or compressor, The term which ‘supercharger’ is usually driven is usually from reserved the engine for crankshaft mechanically via drivena gear train systems or belt [66 ],and and pulley this precedentsystem. The will term be maintained‘supercharger’ from is hereusually onwards. reserved Superchargers for mechanically can bedriven categorised systems based[66], and on thethis compressionprecedent will methods, be maintained such as positive from here displacement onwards. Superchargers and dynamic compressor. can be categorised based on the compressionA positive methods, displacement such as pump positive displaces displacement fluid inand a dynamic pipe system compressor. by cyclically trapping and dischargingA positive a fixed displacement amount of fluid. pump In an displaces automotive fluid context, in a pipe increased system charge by densitycyclically is accomplishedtrapping and bydischarging pumping the a airfixed into amount the intake of atfluid. a faster In rate an thanautomotive the engine context, would normallyincreased ingest. charge Considering density is thataccomplished the rate is fixed by pumping relative to the engine air speedinto the (with intake fixed at drive a faster ratio), rate positive than displacementthe engine would superchargers normally areingest. capable Considering of producing that athe constant rate is boostfixed pressure.relative to The engine mechanical speed (with drive fixed results drive in good ratio), transient positive response,displacement but the superchargers downside is are that capable power isof drawn producing from a the constant useful outputboost pressure. of the engine The rathermechanical than utilisingdrive results the ‘free’ in good exhaust transient gas energy response, similar but to the that downside in turbocharging. is that power Some ofis thedrawn energy from used the by useful the superchargeroutput of the are engine recovered rather as positive than pumpingutilising workthe ‘free’ on the exhaust pistons [gas66]; byenergy contrast, similar a turbocharger to that in raisesturbocharging. exhaust backpressure Some of the and energy thus used increases by the pumping supercharger losses are and recovered trapped residuals as positive [67 pumping]. Bhinder work [66] indicatedon the pistons that a major [66]; disadvantageby contrast, ofa positiveturbocharger displacement raises exhaust superchargers backpressure is that their and size thus and increases weight arepumping relative losses to the and boost trapped provided. residuals Many [67]. different Bhinder designs [66] ofindicated positive that displacement a major disadvantage superchargers, of includingpositive displacement roots, screw (such superchargers as Lysholm is that compressor), their size slidingand weight vane, are and relative scroll, to have the been boost developed provided. andMany received different varying designs degrees of positive of commercial displacement success. superchargers, Root-type superchargers including do roots, not provide screw internal(such as compressionLysholm compressor), compared with sliding other vane, types and listed scroll, here have and been thus havedeveloped a relatively and received low efficiency varying (Stone degrees [68] forof commercial details). Recent success. developments, Root‐type superchargers such as in Eaton’s do not twin provide vortices internal series compression (TVS) [69], compared with reduced with clearancesother types and listed improved here flowand characteristics,thus have a relatively have brought low aefficiency huge improvement. (Stone [68] Devices for details). with internal Recent compression,developments, such such as Lysholm as in Eaton’s compressor, twin providevortices large series volumetric (TVS) [69], and with isentropic reduced efficiencies clearances [70] butand requireimproved a high flow cost characteristics, for manufacturing have precision brought requirements a huge [improvement.66]. The analysis Devices of Stone with [68] revealedinternal thatcompression, these efficiency such as improvements Lysholm compressor, are rapidly provide eroded large when volumetric the internal and isentropic compression efficiencies ratio of the[70] devicebut require does nota high match cost the for overall manufacturing required pressureprecision ratio, requirements that is, during [66]. The external analysis compression of Stone [68] (or expansion).revealed that A these Root-type efficiency supercharger improvements was used are byrapidly Joyce eroded [71] on when a 4.0 L the six-cylinder internal compression gasoline engine ratio toof increase the device torque does and not powermatch the by 35–50%overall required and exceeded pressure the outputratio, that of ais, naturally during external aspirated compression 6.0 L V12. At(or part expansion). load (equal A torque),Root‐type the supercharger supercharged was engine used has by greater Joyce BSFC [71] thanon a the4.0 naturallyL six‐cylinder aspirated gasoline unit becauseengine to of increase parasitic torque losses andand reducedpower by compression 35%–50% and ratio. exceeded However, the BSFCoutput is of substantially a naturally lower aspirated for the6.0 supercharged L V12. At part engine load (equal than the torque), large-capacity the supercharged V12 under engine the same has conditionsgreater BSFC due than to the the increased naturally numberaspirated of valvesunit because (cylinders) of parasitic and hence losses higher and fuel reduced consumption. compression Transient ratio. performance However, wasBSFC not is testedsubstantially against naturallylower for aspirated the supercharged engines but engine was expected than the to slightly large‐capacity diminish. V12 Joyce under [71] explainedthe same thatconditions ‘there is due a finite to the time increased taken to number compress of the valves air in (cylinders) the volume and between hence the higher blower fuel and consumption. the engine. Thus,Transient the massperformance of air delivered was not by tested the supercharger against naturally is diff erentaspirated from engines that received but was by expected the engine’ to slightly diminish. Joyce [71] explained that ‘there is a finite time taken to compress the air in the

Energies 2019, 12, 4636 14 of 33 under changing speed. Engine performance is reduced in reference to the steady state maximum. This condition formed the basis of using a variable supercharger drive ratio, such as in a CVT. Stone [72] addressed the point of a continuously variable drive ratio but in relation to reducing the part-load throttling losses. Considering the performance differences between supercharging and turbocharging, Richter and Hemmerlein [73] performed a comparative study on a 2.5 L four-cylinder gasoline engine. The supercharged version showed a torque advantage of 50–70% at low to medium engine speeds, and torque was approximately identical at high engine speeds. However, turbochargers allow smaller engine displacements to produce much more power relative to their size and improve fuel economy. Joyce [71] and Richter and Hemmerlein [73] applied a bypass valve around the respective superchargers for part-load operation. Dynamic compressors include centrifugal (radial flow) and axial compressors, and the centrifugal type is more common of the two for automotive supercharger use. A centrifugal compressor works by accelerating the intake air to a high velocity, and this velocity is converted to pressure by means of diffusion [42]. Consequently, the produced pressure ratio increases with the compressor speed. Thus, the mechanically driven centrifugal supercharger boost increases with engine speed at a fixed ratio as reported by Bhinder [66], making it incompatible for automotive engines. This characteristic of centrifugal compressors is not a problem when it is used in a turbocharger because the latter’s speed independently varies from engine’s speed. Centrifugal superchargers are typically small, light, and capable of producing higher pressure ratios than their positive displacement counterparts. Although the increasing boost with engine speed attribute is a drawback, it also allows the use of a high compression ratio in gasoline engines. In this case, air flow can be improved at high engine speeds where volumetric efficiency would usually drop without significantly increasing cylinder pressures at low to medium engine speeds, thus avoiding auto ignition (knock). This benefit is relevant to applications where performance at high engine speeds is importance, especially on specialist high-performance car manufacturers Koenigsegg and Caterham [58]. Following on from his earlier line of reasoning, Bhinder [66] suggested that driving a centrifugal supercharger through a variable transmission would match the boost to to the engine requirements at any given speed and load. This idea is being developed by Rotrak [67], a joint venture between centrifugal supercharger manufacturer Rotrex and variable transmission specialist Torotrak. The Rotrak device combines a full-toroidal variator with a centrifugal supercharger, where the latter incorporates a compact, innovative epicyclical traction drive patented by Rotrex and has a single-stage step up ratio of approximately 13:1 to achieve the high speeds required by the compressor. No results for performance simulations or engine testing have been published, but the technologies are mature and have been proven individually. Listing the potential benefits of the Rotrak concept for gasoline engines, Stone [67] stated that the engine load via the boost pressure can be controlled (i.e., by controlling compressor speed), thereby reducing the throttling of the engine and the pumping losses.

3.3. Combined Charging Systems At low engine speeds, a pressure charging system would incorporate the transient response and torque, which can be provided by a mechanical supercharger with high efficiency and the part-load flexibility of a turbocharger. One solution to this dilemma is to simply combine a declutchable supercharger with a conventional turbocharger in a sequential series arrangement, and such systems have been investigated, particularly for heavy duty diesel applications [52,54,74]. This configuration shall hereafter be referred to as a ‘combined charging system’. Schmitz et al. [52] investigated a combined charging system for a 10.9 L V6 commercial diesel engine with a positive displacement supercharger (Wankel-type, which has internal compression) that is upstream of a fixed geometry turbocharger. An electromagnetic clutch was fitted to the supercharger drive pulley for the supercharger to disengage (in conjunction with an intake bypass valve) at high engine speeds or low loads and maintain efficiency. Compared with the purely turbocharged engine, the combined charging system provided significantly increased low speed boost and torque with only a small BSFC penalty. Available Energies 2019, 12, 4636 15 of 33 engine braking power was vastly improved. For transient performance, the time taken to reach maximum engine speed from idle was reduced by more than 30% during the simulations of loaded vehicle acceleration. Matsura et al. [54] tested a similar system with a Wankel-type supercharger on an 11 L six-cylinder commercial diesel engine. The base engine was fitted with a VGT, and the supercharger was downstream of the turbocharger and intercooler. Low-speed torque was increased by 50–60%, and loaded vehicle acceleration was reduced remarkably by 65%. Smoke and BSFC could be reduced whilst achieving the same torque curve because of the improved air supply. Tomita et al. [74] investigated a combined charging system similar to the two previous systems on an 8.8 L six-cylinder heavy duty diesel engine. A Lysholm screw-type supercharger (positive displacement, with internal compression) was fitted downstream of the turbocharger and prior to the intercooler, and the turbocharger featured a wastegate. The addition of the supercharger be substantially increased the low-speed torque whist improving the air–fuel ratio (AFR), thereby reducing the smoke levels. BSFC was comparable for the two systems with some improvement at low engine speed. Approximately 20–55% of the power used to drive the supercharger was recovered as positive pumping work, and the amount recovered decreased with the increase in engine speed. For transient response, loaded vehicle acceleration tests showed that acceleration times were reduced whilst simultaneously improving the transient boost pressure and AFR. The idea of the combined charging system has not been explored by the heavy-duty diesel sector. Volkswagen has successfully implemented this technology on a 1.4 L GDI engine [75]. A Root-type supercharger was used in conjunction with a fixed geometry turbocharger. As reported by Whitworth [76] at the time the ‘twincharged’ engine was released to market, Volkswagen claimed that it ‘delivers the equivalent performance of a naturally aspirated 2.3 L four-cylinder engine, but with a significant drop in fuel consumption’, thus costing less than a modern turbocharged HSDI diesel engine and is equally reliable. For passenger car diesel engines, Cantore et al. [77] downsized a 2.5 L four-cylinder turbocharged HSDI diesel engine to 1.8 L by adopting a combined charging system from a computer model without reduction in steady state or transient performance. Approximately one-third of the power consumed by the supercharger is recovered as pumping work, and the downsized engine significantly delivers better part-load fuel consumption than the baseline for points along a typical driving cycle. One of the authors of the previous work, Mattarelli [78] conducted a computational study for a 2.8 L four-cylinder HSDI diesel engine by comparing the performance of a combined charging system with a series sequential turbocharged arrangement and a baseline engine, which is fitted with a VGT. The turbocharger of the combined system was a VGT, which was the high-pressure turbocharger of the sequential system. The two configurations provided increased steady state torque (up to 14%) and slightly reduced BSFC over the baseline. In simulated vehicle accelerations from 70 km/h to 120 km/h, the sequential and combined systems were approximately 14% and 25% quicker than the baseline, respectively. For reference, a sequential system with fixed geometry for the two turbochargers provides smaller improvement than the baseline single VGT system in a steady state and performed only marginally better than the baseline (~3%) in the acceleration test. A positive displacement supercharger was used in each of these examples of combined charging systems.

3.4. Existing Electric Forced Induction Systems In this section, the operations of five different technologies of electric forced induction systems (EFIS) are presented as shown in Figure 14. The five existing technologies of electric turbochargers and superchargers are:

(1) Electrically assisted turbocharger (EAT); (2) Electric compressor (EC); (3) Electrically split turbocharger (EST); (4) Turbocharger with an additional electrically driven compressor—upstream (TEDC);

(5) Turbocharger with an additional electrically driven compressor—downstream (TEDC) fluids.

3.4.1. Electrically assisted turbocharger The electrically assisted turbocharger (EAT, also referred as hybrid turbocharger) topology is shown in Figure 14a. Between the compressor and the turbine, a high‐speed electric machine is fitted [79], see Figure 15. At low engine speed, additional torque is provided to the compressor, as the electric machine operates as a motor; thus the boost pressure is increased, which results in the reduction of the transient response [80]. On the other hand, at high engine speed, power is generated and transmitted to the energy storage as the electric machine functions as a generator. EnergiesRicardo2019, 12 et, 4636 al. [79] stated that investigation on the electrically assisted turbochargers is needed16 of as 33 they are beneficial to eliminate the turbo‐lag. Moreover, another advantage of the hybrid turbochargers(5) Turbocharger is that with the an turbocharger additional electricallyspeed is maintained driven compressor—downstream during gearshifts. In addition (TEDC) to fluids. that, at high engine speeds, the turbocharger speed is reduced [79].

Figure 14. Cont.

Energies 2019, 12, 4636 17 of 33

Figure 14. Schematic layout of electric forced induction system (EFIS) topologies (black line (thin)— Figure 14. Schematic layout of electric forced induction system (EFIS) topologies (black line (thin)—powerpower flow, black flow, line black (thick)—air line (thick)—air flow path flow with path valve, with blue valve, line—engine blue line—engine inlet air flow, inlet red air line— flow, redexhaust line—exhaust gas flow). gas (a) flow). Electrically (a) Electrically assisted turbocharger assisted turbocharger (EAT). (b (EAT).) Electric (b )compressor Electric compressor (EC). (c) (EC).Electrically (c) Electrically split turbocharger split turbocharger (EST). (d (EST).) Turbocharger (d) Turbocharger with an with additional an additional electrically electrically driven drivencompressor—upstream compressor—upstream (TEDC). (TEDC).(e) Turbocharger (e) Turbocharger with an additional with an electrically additional driven electrically compressor— driven compressor—downstreamdownstream (TEDC). (TEDC).

3.4.1. Electrically assisted turbocharger The electrically assisted turbocharger (EAT, also referred as hybrid turbocharger) topology is shown in Figure 14a. Between the compressor and the turbine, a high-speed electric machine is fitted [79], see Figure 15. At low engine speed, additional torque is provided to the compressor, as the electric machine operates as a motor; thus the boost pressure is increased, which results in the reduction of the transient response [80]. On the other hand, at high engine speed, power is generated and transmitted to the energy storage as the electric machine functions as a generator.

Energies 2019, 12, 4636 18 of 33

Figure 15. Integration of electric machine with turbocharger.

RicardoSimilarly, et al.a recent [79] stated paper that by investigation Lee et al. [81] on supported the electrically that assistedelectrically turbochargers assisted turbochargers is needed as theyprovide are beneficialhigher boosting to eliminate at low the RPM turbo-lag. than a conventional Moreover, another turbocharger advantage and of the the power hybrid output, turbochargers which isis thatconsumed the turbocharger by the motor speed and inverter is maintained is low. during On the gearshifts. other hand, In the addition shaft inertia to that, is increased at high engine since speeds,the electric the turbochargermachine is interconnected speed is reduced between [79]. the turbine and the compressor [81], thus for the turbochargerSimilarly, operation, a recent papermore power by Lee is et required. al. [81] supportedFurthermore, that another electrically drawback assisted of this turbochargers topology is providethe high higher‐temperature boosting effect at low on RPM the than electric a conventional machine, hence, turbocharger an additional and the powercooling output, is needed. which To is consumedmitigate the by high the‐temperature motor and inverter effect, Lee is low. et al. On [81] the presented other hand, some the topological shaft inertia approaches. is increased First, since the theelectric electric machine machine can isbe interconnected connected or disconnected between the to turbine the shaft and with the compressor the usage of [81 clutches,], thus for so thethe turbochargermachine can operation,be installed more outside power of isthe required. turbocharger. Furthermore, Second, another a large drawback airgap permanent of this topology magnet is themachine high-temperature could be used, effect which on the can electric be located machine, outside hence, of an the additional turbocharger. cooling This is needed. electric To machine mitigate thedecreases high-temperature the temperature effect, effect Lee etas al.the [81 airgap] presented is used some as inlet topological air path. approaches. First, the electric machine can be connected or disconnected to the shaft with the usage of clutches, so the machine can be installed3.4.2. Electric outside Supercharger of the turbocharger. Second, a large airgap permanent magnet machine could be used, which can be located outside of the turbocharger. This electric machine decreases the temperature Electric supercharger is also referred as hybrid supercharger or electric compressor or e‐booster, effect as the airgap is used as inlet air path. see Figure 16. The solely difference between electric supercharger and turbocharger is the omission 3.4.2.of the Electric turbine Supercharger [79] as is shown in Figure 14b. This is beneficial as not only is no additional inertia to the system from the turbine and the turbine shaft, but also because the backpressure effect is Electric supercharger is also referred as hybrid supercharger or electric compressor or e-booster, eliminated. Similarly, Villegas et al. [82] stated that the friction losses are decreased since there are no see Figure 16. The solely difference between electric supercharger and turbocharger is the omission of mechanical connections between the engine and the e‐compressor. Furthermore, in this topology, the the turbine [79] as is shown in Figure 14b. This is beneficial as not only is no additional inertia to the boost pressure is increased at the transient response at low engine speeds. system from the turbine and the turbine shaft, but also because the backpressure effect is eliminated. Moreover, the compressor is electrically driven by an electric machine which makes the control Similarly, Villegas et al. [82] stated that the friction losses are decreased since there are no mechanical more flexible [81]. However, due to the incapability of this topology to generate energy itself, connections between the engine and the e-compressor. Furthermore, in this topology, the boost pressure electrical energy has to be provided to the storage by an integrated starter generator (ISG) or the is increased at the transient response at low engine speeds. regenerative braking system [83]. In addition, according to Lee et al. [81], the main drawback of this topology is the low system efficiency. Also, the development of the e‐ compressor has faced a challenge since cars are being designed to work on 12V architecture [84]. However, some automobile makers have been considered to use a 48V battery. According to a simulation that was conducted by Nishiwaki et al. [85] for a 48V battery system, the regenerative electric power and the motor power were risen as the boosting pressure was increased. This was due to the higher power produced by

the 48V. Consequently, higher boost pressure and fast response time were resulted. Therefore, the electric power that is required for the e‐booster operation can be covered by the regenerative energy. Finally, the results of the simulation have shown a significant improvement in the transient response Energies 2019, 12, 4636 19 of 33 at low engine speeds [85].

Figure 16. ElectricElectric supercharger. Moreover, the compressor is electrically driven by an electric machine which makes the control 3.4.3. Electrically Split Turbocharger more flexible [81]. However, due to the incapability of this topology to generate energy itself, electrical energyAs has shown to be in provided Figure 14c, to in the the storage electrically by an split integrated turbocharger, starter generatorthe turbine (ISG) and orthe the compressor regenerative are notbraking connected system to [ 83the]. same In addition, shaft. However, according the to mechanical Lee et al. [81 energy], the main from drawback the exhaust of gas this is topology converted is tothe electrical low system and stored efficiency. in the Also, energy the storage. development This energy of the is e- used compressor to drive the has compressor faced a challenge for boosting since [81].cars are being designed to work on 12V architecture [84]. However, some automobile makers have beenThe considered main advantage to use a 48V of battery. this system According is that to athe simulation compressor that wasand conductedthe turbine by can Nishiwaki rotate independently;et al. [85] for a 48V hence, battery they system, can have the different regenerative speeds. electric Moreover, power since and thethe motorcompressor power and were turbine risen areas the separated, boosting the pressure temperature was increased. effect is decreased This was compared due to the to higheran electrically power producedassisted turbocharger by the 48V. [81].Consequently, On the other higher hand, boost Lee pressureet al. [81] andstated fast that response due to timethe high were power resulted. output Therefore, consumption the electric of the powermotor, thatgenerator, is required and for inverters, the e-booster the system’s operation cost can bewill covered be increased. by the regenerative Furthermore, energy. there Finally, is an additionalthe results energy of the simulation conversion have loss shownas two inverters a significant are improvementrequired, for the in thesystem transient operation. response at low engine speeds [85]. 3.4.4. Turbocharger with an Additional Electrically Driven Compressor—Upstream and Downstream3.4.3. Electrically (TEDC) Split Turbocharger InAs Figure shown 14d,e, in Figure the topology14c, in the of electrically a turbocharger split with turbocharger, an additional the turbine electrically and thedriven compressor compressor are (TEDC),not connected located to upstream the same shaft. and downstream, However, the respectively, mechanical energyis shown. from In the this exhaust turbocharger, gas is converted the electric to machineelectrical andis connected stored in theto the energy electrically storage. driven This energy compressor is used toand drive is not the affected compressor by forthe boosting exhaust [gas81]. drivenThe turbine. main advantage Thus, since of thisthe electric system ismachine that the drives compressor solely and the the compressor, turbine can the rotate transient independently; response ishence, improved they can for have low dienginefferent speed speeds. as Moreover, the boost sincepressure the compressor build‐up is and made turbine faster are [83]. separated, Moreover, the accordingtemperature to Lee effect et isal. decreased [81], the benefits compared of the to topology an electrically are the assisted low power turbocharger consumption [81]. of On the the electric other motorhand, Leeand et inverter al. [81] as stated well that as the due improvement to the high power of the output steady consumption state performance. of the motor, It is obvious generator, that and the TEDCinverters, downstream the system’s has cost faster will transient be increased. response Furthermore, compared to there the isupstream an additional topology energy since conversion the latter hasloss to as compress two inverters a larger are required,volume. for the system operation. In a recent paper by Nishiwaki et al. [85], the topology in Figure 14e is referred to as a two‐stage turbocharger3.4.4. Turbocharger system. with In anaddition, Additional in the Electrically paper, a TEDC Driven system Compressor—Upstream on a 1.5‐litre gasoline and engine was Downstream (TEDC) examined by GT‐Power, an engine simulation tool, and engine bench test. Results obtained by In Figure 14d,e, the topology of a turbocharger with an additional electrically driven compressor (TEDC), located upstream and downstream, respectively, is shown. In this turbocharger, the electric machine is connected to the electrically driven compressor and is not affected by the exhaust gas driven turbine. Thus, since the electric machine drives solely the compressor, the transient response is improved for low engine speed as the boost pressure build-up is made faster [83]. Moreover, according Energies 2019, 12, 4636 20 of 33 to Lee et al. [81], the benefits of the topology are the low power consumption of the electric motor and inverter as well as the improvement of the steady state performance. It is obvious that the TEDC downstream has faster transient response compared to the upstream topology since the latter has to compress a larger volume. In a recent paper by Nishiwaki et al. [85], the topology in Figure 14e is referred to as a two-stage turbocharger system. In addition, in the paper, a TEDC system on a 1.5-litre gasoline engine was examined by GT-Power, an engine simulation tool, and engine bench test. Results obtained by Nishiwaki et al. [85] suggest that the TEDC system’s response time at 1500 rpm was improved by Nishiwaki et al. [85] suggest that the TEDC system’s response time at 1500 rpm was improved by 43%, 43%, compared with a normal two‐stage turbocharger. compared with a normal two-stage turbocharger. On the other hand, the main disadvantage of the TEDC is that the electric energy required to On the other hand, the main disadvantage of the TEDC is that the electric energy required to drive the compressor needs to be generated by generator/alternator powered by the engine shaft or drive the compressor needs to be generated by generator/alternator powered by the engine shaft or regenerative braking system [83]. Furthermore, another major drawback that should be considered regenerative braking system [83]. Furthermore, another major drawback that should be considered is is the low system efficiency of the topology [81]. the low system efficiency of the topology [81].

3.4.5.3.4.5. Superturbocharging Superturbocharging TheThe combination combination (electrical (electrical or or mechanical) mechanical) of of superchargers superchargers and and turbochargers turbochargers in in a a system system is is referredreferred as as Superturbocharging Superturbocharging or or “Superturbo”. “Superturbo”. Superturbocharging Superturbocharging has has two two principal principal versions versions (electrical(electrical/mechanical)/mechanical) as as follows: follows:

(i)(i) TurboSuperGeneratorTurboSuperGenerator TurboSuperGeneratorTurboSuperGenerator (TSG), (TSG), FigureFigure 1717,, is a a super super-turbo‐turbo system system that that not not only only benefits benefits from from the theadvantages advantages of ofsupercharging supercharging and and turbocharging, turbocharging, but but also also benefits benefits fromfrom thethe multi-stagemulti‐stage turbochargingturbocharging [79 [79].]. In In the the TSG, TSG, the the engine engine alternator alternator is is replaced replaced as as boost boost is is provided provided by by supercharger supercharger andand turbocharger. turbocharger. Moreover, Moreover, the ditheff erencedifference with thewith other the super-turboother super systems‐turbo systems is that in is TSG that topology in TSG thetopology turbocharger the turbocharger is placed before is placed the electric before supercharger,the electric supercharger, hence the hybrid hence supercharger the hybrid supercharger is the high- pressureis the high compressor‐ pressure stage compressor [84]. According stage [84]. to According Baderman [to84 Baderman], integral Powertrain[84], integral (IP) Powertrain has developed (IP) has a TSG,developed named a SuperGen, TSG, named which SuperGen, allow an which aggressive allow downsizing an aggressive without downsizing compromising without oncompromising drivability. Furthermore,on drivability. IP Furthermore, has proved that IP has the accelerationproved that the time acceleration of TSG downsized time of TSG engine downsized was shorter engine than was a normal-sized,shorter than a naturally normal‐sized, aspirated, naturally or a sequentially aspirated, or turbocharged a sequentially engine turbocharged [84]. engine [84].

Figure 17. Configuration of Turbosupergenerator (TSG) [79]. Figure 17. Configuration of Turbosupergenerator (TSG) [79].

(ii) Mechanical Superturbocharger It is an empowering topology for engine downsizing with no loss of vehicle transient response and peak power. As shown in Figure 18, the main elements of the technology are a traction drive, compressor, and turbine all being attached to a single crankshaft and subsequently connected to a continuously variable transmission. This technology consists of two modes. The first one boosts engine power via turbocompounding while the second transmits power from the engine to a supercharger in order to boost the transient response at lower speeds.

Energies 2019, 12, 4636 21 of 33

(ii) Mechanical Superturbocharger It is an empowering topology for engine downsizing with no loss of vehicle transient response and peak power. As shown in Figure 18, the main elements of the technology are a traction drive, compressor, and turbine all being attached to a single crankshaft and subsequently connected to a continuously variable transmission. This technology consists of two modes. The first one boosts engine power via turbocompounding while the second transmits power from the engine to a supercharger in order to boost the transient response at lower speeds.

Figure 18. Configuration of Superturbocharger [86]. Figure 18. Configuration of Superturbocharger [86]. 4. Turbocompounding 4. Turbocompounding As discussed in Section 2.1, a turbocharger uses an exhaust-driven turbine to power a compressor and consequentlyAs discussed increase in Section the density 2.1, ofa theturbocharger air entering uses the engine.an exhaust Harnessing‐driven theturbine power to produced power a bycompressor the turbine and in a diconsequentlyfferent manner, increase such asthe by density directly of adding the air it entering to the useful the engine. output Harnessing of the engine, the maypower be a produced beneficial approach.by the turbine According in a different to Watson manner, and Janota such [42 as], aby turbocompounded directly adding it engine to the is useful one withoutput ‘some of mechanical the engine, linkage may andbe a power beneficial transmission approach. between According the exhaust-gas to Watson driven and Janota turbine [42], and a crankshaftturbocompounded of the engine’. engine However, is one with this term‘some also mechanical describes linkage a system and in whichpower the transmission turbine drives between an electricalthe exhaust generator,‐gas driven and the turbine exhaust and energy crankshaft is recovered of the as engine’. electrical However, power. Mechanical this term also and describes electrical a turbocompoundingsystem in which the arrangements turbine drives are exploredan electrical in the generator, following and section. the exhaust energy is recovered as electrical power. Mechanical and electrical turbocompounding arrangements are explored in the 4.1.following Mechanical section. Turbocompounding Four basic mechanical turbocompounding arrangements with individual particular advantages and4.1. disadvantages Mechanical Turbocompounding have been developed (Figure 19, reproduced from Baines [32]). Four basic mechanical turbocompounding arrangements with individual particular advantages and disadvantages have been developed (Figure 19, reproduced from Baines [32]).

Energies 2019, 12, 4636 22 of 33

FigureFigure 19. 19.Turbocompounding Turbocompounding schemes—( schemes—(aa)) directlydirectly coupled turbocharger; turbocharger; (b (b) )separately separately coupled coupled turbocharger;turbocharger; (c )(c separate) separate power power turbine, turbine, seriesseries arrangement;arrangement; ( (dd) )separate separate power power turbine, turbine, parallel parallel arrangementarrangement [48 [48].].

InIn the the first first type, type, the the engine engine shafts shafts and and turbochargerturbocharger are are directly directly linked linked (Figure (Figure 19a). 19a). However, However, a a fixedfixed transmission transmission ratio ratio that that corresponds corresponds to to the the engineengine and turbocharger turbocharger speeds speeds at at all all speed speed and and load load conditions,conditions, with with particular particular problems problems at at part part load, load, is is di difficultfficult to to choose choose for for this this arrangement arrangement similarsimilar to theto centrifugal the centrifugal supercharger supercharger [42,48]. [42,48]. Brockbank Brockbank [87] discussed [87] discussed the use of the CVT use with of this CVT arrangement, with this a modificationarrangement, that a independently modification that controls independently the turbocompounding controls the turbocompounding level of the engine level speed. of Furthermore,the engine thespeed. turbocharger Furthermore, could the intentionally turbocharger be could driven intentionally by the engine be driven by manipulating by the engine the by CVT manipulating ratio when insutheffi cientCVT boostratio occurs.when insufficient Hence, the turbochargerboost occurs. effHence,ectively the acts turbocharger as a supercharger. effectively A system acts as of thisa typesupercharger. (with a CVT) A wassystem utilised of this on type the well-known(with a CVT) Napier was utilised Nomad on aircraft the well engine‐known of Napier the 1950s Nomad [42,48 ]. Accordingaircraft engine to Baines of the [48 1950s], such [42,48]. machine According (and other to Baines contemporary [48], such machine turbocompounded (and other contemporary engines) shows potentialturbocompounded but is notoriously engines) complex shows potential and quickly but is superseded notoriously by complex the advancing and quickly development superseded of by gas turbinethe advancing engines. Chadwelldevelopment and of Walls gas [turbine18] investigated engines. Chadwell the VanDyne and SuperTurbocharger, Walls [18] investigated a modern the VanDyne SuperTurbocharger, a modern counterpart for the automotive sector that offers counterpart for the automotive sector that offers supercharging, turbocharging, and turbocompounding supercharging, turbocharging, and turbocompounding in one device. The SuperTurbocharger in one device. The SuperTurbocharger consists of a turbocharger that transmits power to and from the consists of a turbocharger that transmits power to and from the engine crankshaft via a high‐speed engine crankshaft via a high-speed traction drive with an integral reduction ratio, a set of reduction traction drive with an integral reduction ratio, a set of reduction gears, a traction drive CVT, and a gears, a traction drive CVT, and a belt-and-pulley system (Figure 20, reproduced from VanDyne et belt‐and‐pulley system (Figure 20, reproduced from VanDyne et al. [86]). In the computational study al.of [86 Chadwell]). In the and computational Walls [18], studythe SuperTurbocharger of Chadwell and was Walls used [18 ],to the downsize SuperTurbocharger a typical 3.2‐litre was V6 used tonaturally downsize aspirated a typical gasoline 3.2-litre engine V6 naturally to a 2‐litre aspirated four‐cylinder gasoline unit. engine The downsized to a 2-litre engine four-cylinder produced unit. Thesuperior downsized steady engine state produced torque but superior had less steady satisfactory state torque full‐load but BSFC. had less The satisfactory authors attribute full-load such BSFC. Theoutcomes authors to attribute the fuel suchenrichment outcomes and toretarded the fuel ignition enrichment timing and necessary retarded to maintain ignition the timing turbine necessary inlet to maintaintemperature the limit turbine of inlet950 °C. temperature Nevertheless, limit part of 950‐load◦C. fuel Nevertheless, efficiency was part-load increased, fuel eandfficiency a 17% was increased,reduction and in afuel 17% consumption reduction in over fuel the consumption NEDC was overachieved. the NEDC Simulated was achieved.transient tip Simulated‐in tests suggest transient tip-inthat tests the downsized suggest that engine the downsizedhas satisfactory engine transient has satisfactory response; however, transient it response; has not been however, compared it has notwith been the compared baseline engine. with the In this baseline work, engine. a compressor In this map work, width a compressor enhancement map technique width was enhancement used to techniquefacilitate was extreme used downsizing. to facilitate extremeBypassing downsizing. some of the Bypassing compressed some intake of the air compressedto the upstream intake of the air to the upstream of the turbine’s increased mass flow through the compressor at low engine speeds can Energies 2019, 12, 4636 23 of 33

turbine’s increased mass flow through the compressor at low engine speeds can simultaneously simultaneouslyprevent surges preventand generate surges a and high generate boost. aUsing high boost.the intake Using air the bypass intake at air high bypass engine at high loads engine also loadsrenders also fuel renders cooling fuel unnecessary. cooling unnecessary. Thus, a torque Thus, curve a torque comparable curve comparable with a 4.2 with‐litre a V8 4.2-litre was attained V8 was attainedfrom the from 2.0‐litre the 2.0-litreengine engineand produced and produced better betterfull‐load full-load BSFC BSFCcurve curve than thanthe previous the previous one. one. As Asexpected, expected, its part its part-load‐load fuel fuel economy economy was was also also vastly vastly superior superior with with a 36% a 36% improvement improvement over over that that of ofthe the NEDC. NEDC. During During full full-load‐load operation, operation, up up to to 10 10 kW kW of of power power was was transmitted transmitted to to the the device (for supercharging) at low engine speeds, and up to 12 kW was recovered by turbocompounding at high engine speeds. However, the potential of turbocompounding during part-loadpart‐load conditions was not discussed, and transient performance waswas notnot compared.compared.

Figure 20. The Van Dyne superturbocharger [[86].86].

It isis worth worth mentioning mentioning that that in termsin terms of driving of driving cycles, cycles, hybrid hybrid boosting boosting systems systems would be would favoured be iffavoured the more if relevantthe more and relevant recent and WLTP recent cycle WLTP is used. cycle The is used. worldwide The worldwide harmonized harmonized light-duty light vehicles‐duty testvehicles procedure, test procedure, or WLTP, or is aWLTP, worldwide is a worldwide standard for standard testing passengerfor testing vehicles passenger and vehicles light commercial and light vehicles.commercial As ofvehicles. 1 September As of 2017, 1 September it aims to provide2017, it more aims realistic to provide consumption more realistic specifications consumption with its considerablyspecifications more with dynamic its considerably testing parameters more dynamic than previous testing cycles parameters like NEDC. than As previous a result, cycles the benefit like ofNEDC. hybrid As boosting a result, the is enhanced benefit of in hybrid WLTP boosting mode, because is enhanced of the in hybridWLTP mode, boosting because systems’ of the ability hybrid to respondboosting more systems’ rapidly ability and to effi respondciently to more the more rapidly frequent and efficiently and more dynamicto the more driver frequent demand and changes more evidentdynamic in driver WLTP, demand compared changes to conventional evident in boostingWLTP, compared devices and to conventional in particular turbochargers.boosting devices and in particularThe second turbochargers. type of mechanical turbocompounding involves a connection of the compressor and turbineThe to second the engine type of via mechanical separate transmissions turbocompounding (Figure involves 19b). The a connection advantage of ofthe this compressor setup is thatand theturbine compressor to the engine and turbine via separate can operate transmissions at different (Figure speeds, 19b). and The the advantage turbomachinery of this setup can be is matched that the separately.compressor However,and turbine the can same operate problems at different encountered speeds, by and the the previous turbomachinery system will can occur be matched unless variableseparately. speed However, drives arethe employed.same problems A positive encountered displacement by the compressor previous system (and possibly will occur expander unless asvariable well) canspeed be drives used to are overcome employed. the A low-speed positive displacement performance compressor deficit, but (and the problempossibly ofexpander part-load as operationwell) can willbe used persist to overcome [42], and the the relative low‐speed ineffi performanceciency of such deficit, devices but renders the problem the potential of part gains‐load negligibleoperation will [48]. persist Wallace [42], [88 and] developed the relative the inefficiency concept of aof ‘di suchfferential devices compound renders the engine’ potential (DCE), gains in whichnegligible the [48]. compressor Wallace is [88] driven developed by the the engine concept through of a an‘differential epicyclic gearcompound system, engine’ and the (DCE), turbine in transmitswhich the power compressor to the engineis driven via by a fixed the speedengine ratio. through This an design epicyclic was initiallygear system, applied and for the two-stroke turbine

Energies 2019, 12, 4636 24 of 33 transmits power to the engine via a fixed speed ratio. This design was initially applied for two‐stroke diesel engines. Later Later iterations iterations [89] [89] were were adapted adapted for for heavy heavy-duty‐duty four four-stroke‐stroke engines and used a variable geometry turbine transmitting power through a CVT to ensure optimum turbine efficiency efficiency (see Figure 2121,, reproduced from Wallace et al. [[89]).89]). Although the DCE presented numerous benefits benefits over conventional turbocharged diesel engines particularly in terms of steady state and transient performance, fuel economy, and refinement refinement [90], [90], its commercial production has has not not been been attained. attained. Baines [46] [46] reasonably attributed this outcome toto thethe great complexitycomplexity ofof thethe system.system.

Figure 21. DifferentialDifferential compound engine schematic [[89].89].

The third type of mechanical turbocompounding uses a conventional turbocharger with a second turbine turbine mounted mounted in in series series with with and and downstream downstream of ofthe the turbocharger turbocharger turbine. turbine. Power Power from from the secondthe second turbine—known turbine—known as a ‘power as a ‘power turbine’—is turbine’—is mechanically mechanically transmitted transmitted to the crankshaft to the crankshaft (Figure (Figure19c). Baines 19c). [48] Baines and [Patterson48] and Patterson et al. [91] et claimed al. [91] claimedthat this thatapproach this approach is the most is the common most common form of turbocompoundingform of turbocompounding and is the and first is the to first be commercially to be commercially used usedfor the for heavy the heavy-duty‐duty vehicle vehicle sector sector by Scaniaby Scania and and the the turbocharger turbocharger manufacturer manufacturer Cummins Cummins Turbo Turbo Technologies Technologies (formerly (formerly Holset) Holset) [92]. [92]. Baines [48] [48] indicated indicated that that ‘the ‘the power power turbine, turbine, because because it itis isdealing dealing with with a gas a gas of lower of lower density, density, can can be largerbe larger and and rotate rotate more more slowly slowly than than the the turbocharger, turbocharger, and and this this eases eases the the transmission transmission problem’. problem’. However, Watson, andand JanotaJanota [[42]42] posited that the additional turbine in the exhaust stream increases backpressure due to the increase in the outlet pressure pressure (and thus reducing reducing power) of the the turbocharger turbocharger and the large large engine engine pumping pumping losses. losses. These These effects effects partly partly offset offset the the benefit benefit of ofturbocompounding turbocompounding in relationin relation to tooverall overall engine engine efficiency. efficiency. Nevertheless, Nevertheless, Walsham Walsham [93] [93 ]tested tested and and compared various turbocharging technologies technologies and and argued argued that that the the modified modified exhaust exhaust flow flow characteristics characteristics are are caused caused by theby thepower power turbine turbine result result in the in turbocharger, the turbocharger, which which behaves behaves similarly similarly to a small to a turbine small turbine at low engine at low speedsengine speeds(i.e., with (i.e., low with mass low massflows) flows) and andto a to large a large turbine turbine at athigh high engine engine speeds. speeds. This This condition increases boost and transient torque over a conventionally turbocharged engine and improves the

Energies 2019, 12, 4636 25 of 33 full-load BSFC. By contrast, the transient response does not match that of a comparable VGT system. The effective turbine area of the VGT can be actively controlled, and the apparent turbine size of the turbocharger in a turbocompounded system is a passive effect that is dependent on engine speed and load. Nevertheless, the turbocompounded system remains superior in terms of BSFC. Wallace [94] conducted a computational study to ascertain the ultimate performance potential of turbocompounded heavy-duty diesel engines by comparing single- and two-stage turbocharging systems with and without additional power turbine for a nominal 8-litre six-cylinder diesel engine. Similar limiting torque curves were achieved with and without turbocompounding for both systems. The two-stage systems have higher BMEP levels that correspond to the greater boost compared with the single-stage system. Conversely, the single-stage systems have similar full-load BSFC values. The greatest efficiency of the turbocompounded engine is skewed towards high engine speeds, in which the power turbine maximally recovers energy. At low loads, the turbocompounded engine loses in relation to efficiency. Under these conditions, the engine must drive the power turbine at a loss because recoverable exhaust energy is minimal, and pumping losses are high due to the increased backpressure caused by the power turbine. For the two-stage systems, the turbocompounded engine has a favourable full-load BSFC curve, but the optimal values are even more skewed towards high engine speeds than those with the single-stage system. The non-turbocompounded engine has the advantage in terms of part-load BSFC for the same reasons stated above. Wallace [94] concluded that turbocompounding ‘becomes technically and economically viable only in units operating at very high levels of boost and BMEP’. This conclusion is affirmed by Baines [48], who asserted that turbocompounding ‘is not really suitable for application to passenger car engines, for example, which spend most of their time at part load’. The fourth type of mechanical turbocompounding has a power turbine that is arranged in parallel with the turbocharger instead of in a series (Figure 19d). Baines [48] argued that this configuration may be preferable for large diesel engines, and the power turbine necessary for an equivalent series system would be prohibitively large and expensive.

4.2. Electrical Turbocompounding The various arrangements of electrical turbocompounding systems essentially reflect the four types detailed under Section 3.1. Hence, the first type is fundamentally similar to the electrically assisted turbocharger (EAT), which is described in the section on electrical pressure charging systems. With an appropriate control scheme, the integrated motor of an EAT can function as a generator to recover exhaust gas energy as electrical power. Panting et al. [95] stated that this approach eliminates the need for a wastegate because the turbocharger speed can be controlled by adjusting the power generation level. This observation was corroborated by Katrašnik et al. [96]. Hopmann and Algrain [97] developed an EAT of this type and focused on maximising the fuel efficiency of a 14.6-litre six-cylinder heavy-duty diesel engine. Modifications to the turbocharger included the redesign of the bearing housing and turbine housing interface to insulate the shaft-mounted motor from extreme thermal gradients. A second motor-generator was mounted on the engine crankshaft to provide a two-way energy transfer between the EAT and the engine. Engine performance was simulated for several steady state operating points. The overall efficiency of the electrical system was estimated as 85% for the simulation. Fuel consumption reduction of 2.5% to 10% was predicted with the maximum potential benefit at rated power (i.e., high speed and load). The authors also claimed that this device offers more efficient and flexible operation compared with fixed ratio mechanical turbocompounding systems. Millo et al. [98] investigated the possibility of replacing a VGT with an EAT with turbocompounding capability for an 8-litre six-cylinder diesel engine for urban bus application. They identified the driving cycle of the application as a critical factor of the potential benefit of turbocompounding systems, namely, the proportion of time during which energy can be recovered. This feature was especially pertinent for the currently investigated system because recovered energy is stored in supercapacitors, which could hold sufficient energy for up to six assisted accelerations only. However, the supercapacitors could also be charged from and discharged to the 24 V vehicle electrical system. Thus, the potential Energies 2019, 12, 4636 26 of 33 efficiency of this system is manifested through the reduced alternator load. Several driving cycles were simulated, and the fuel consumption for the EAT system was maximally reduced at approximately 6% during free-flowing highway (extra-urban) driving conditions. Nevertheless, the efficiency benefit decreased to approximately 1% when the proportion of congested traffic conditions increased due to the increasing power requirement for accelerating the EAT and the few opportunities for energy recovery. Predicted improvements in transient response were highly substantial. The EAT achieved the boost target 30% faster than the VGT during simulated tip-in tests. Hountalas et al. [99] conducted a computational study comparing a mechanical turbocompounding system (with a series power turbine) and an EAT with turbocompounding capability for a nominal 10.3-litre six-cylinder heavy-duty diesel engine. Both systems reduced the primary engine output because of the increased exhaust backpressure and pumping losses. Both also generated a net increase in output because the recovered power through turbocompounding was greater than these reductions. This outcome is consistent with previously findings. With an assumed power turbine efficiency of 80%, the mechanical system provided a maximum BSFC reduction of 4.5% at full load. The corresponding value for the electrical system was 2%. These values decreased to 0.5% and 0.2% at a 25% engine load. The use of high-efficiency turbomachinery greatly affected the recovery potential of the EAT and increased the BSFC reductions to 6.5% at full load and 3.3% at part load. The considerable sensitivity of turbocompounding effectiveness to EAT turbine efficiency was also explored by Millo et al. [98]. The second type of electrical turbocompounding system comprises an electrically driven compressor (EDC) as described under Section 3.4.2 on electrical pressure charging systems with an exhaust gas turbine driving a separate electrical generator. This characteristic facilitates the independent control of the speeds of the separate turbomachines. Each machine can be optimally matched to engine speed and load. Aeristech [100] developed an example of this concept, but performance testing results are currently lacking. The third type of electrical turbocompounding is similar to the series power turbine system detailed under Section 3.1, but the power turbine drives an electrical generator instead of mechanically transmitting the power to the crankshaft. Patterson et al. [91] performed computer simulations to compare this system with an equivalent mechanical power turbine system for a typical heavy-duty diesel engine. The energy recovered by the electrical power turbine could either directly power a flywheel-mounted motor or be stored in the vehicle electrical system. The authors acknowledged the trade-off between the work implemented by the power turbine and the increase in exhaust backpressure. Such trade-off is unfavourable for the turbocharger and engine efficiencies. They also asserted the need for high turbomachinery efficiency to maximise the benefits of turbocompounding. Simulation results reveal that that both turbocompounding systems improve fuel efficiency, but the electrical power turbine offers greater fuel efficiency benefits at all load points (particularly at part load) where the speed of the mechanical power turbine is constrained by its fixed transmission ratio. Different from the mechanical unit in Section 3.1 with reference to Wallace et al. [94], the electrical power turbine will consume no power at low loads and idle engine. The fourth type of electrical turbocompounding is the equivalent of the parallel power turbine arrangement listed under Section 3.1, but the power turbine drives an electrical generator. Different from its mechanical system counterpart, the electrical power turbine does not require constant operation [101]. Odaka et al. [102] employed this functionality by using the exhaust gases from the turbocharger wastegate valve to drive the power turbine. The base performance of the engine was unaffected, the exhaust backpressure was not increased, and the exhaust energy that would otherwise be wasted was recovered instead. The parallel power turbine was evaluated on an 8-litre six-cylinder commercial diesel engine in steady-state and transient conditions. Similar to all the other turbocompounding arrangements reviewed, the greatest benefit was achieved at high load and engine speeds—a region in which a typical vehicle engine seldom operates. Hence, the energy recovered at low speed and load was negligible, and the overall effect on fuel efficiency was not predicted. Energies 2019, 12, 4636 27 of 33

5. Thermal Efficiency Comparison of the Various Boosting Systems The different boosting systems are directly compared with a baseline engine in terms of thermal The different boosting systems are directly compared with a baseline engine in terms of thermal efficiencyefficiency as as shown shown in in Figure Figure 22. 22 As. As can can be beseen seen in the in the figure, figure, mechanical mechanical super super turbo turbo demonstrates demonstrates thethe highest highest thermal thermal efficiency efficiency of of 30% 30% followed followed by bythe theelectrical electrical supercharging, supercharging, which which presents presents 24.45% 24.45% thermalthermal effiefficiency.ciency. Series Series sequential sequential turbocharger, turbocharger, turbo-super turbo generator,‐super electricalgenerator, turbocompounding, electrical turbocompounding,and active control turbocharger and active control offer similarturbocharger performance offer similar of 20%. performance The figure of also 20%. confirms The figure a better alsoperformance confirms a of better the VGT performance over that of of the fixed VGT geometry over that turbine. of fixed geometry turbine. ItIt is worthworth mentioningmentioning that that the the comparison comparison shown shown in Figurein Figure 22 is22 based is based on estimated on estimated and modelling and modellingresults rather results than rather experimental than experimental results [79 results]. [79].

FigureFigure 22. 22. ThermalThermal efficiency efficiency of different of different boosting boosting systems systems [79]. [79]. 6. Conclusions 6. Conclusion Although a variety of boosting methods are available, selection of an optimum method is still Although a variety of boosting methods are available, selection of an optimum method is still difficultdifficult since since this this selection selection depends depends largely largely on on the the applications. applications. Increased Increased specific specific power, power, improved improved partpart-load‐load BSFC, BSFC, and and transient transient performance performance as asclose close to a to naturally a naturally aspirated aspirated characteristic characteristic as possible as possible areare the the most most critical critical factors factors for for a downsized a downsized engine engine to be to beeffective. effective. SeveralSeveral turbocharging turbocharging systems systems were were assessed, assessed, including including VGTs VGTs and anddifferent different arrangements arrangements with with multiplemultiple turbochargers. turbochargers. Most Most of ofthese these systems systems showed showed enhanced enhanced specific specific engine engine power power and fuel and fuel efficiency,efficiency, andand fasterfaster transient transient response response in in comparison comparison with with conventional conventional turbocharging. turbocharging. Many Many authors, authors,nonetheless, nonetheless, insisted insisted that the that low-speed the low‐ torquespeed torque and transient and transient response response of these of these systems systems is limited is in limitedthe long in run the long by the run available by the available exhaust exhaust gas energy, gas energy, and these and problemsthese problems worsen worsen in the in highlythe highly boosted boostedengines engines [15,17,54 [15,17,54].]. Mechanical Mechanical supercharging supercharging avoids avoids both of both the disadvantages,of the disadvantages, but it isbut considered it is consideredwith less effiwithciency less than efficiency turbocharging than turbocharging because of thebecause parasitic of the power parasitic losses. power Supercharging’s losses. Supercharging’ssupporters maintain supporters that, itmaintain offers positivethat, it offers pumping positive work pumping while turbochargingwork while turbocharging incurs pumping incurslosses pumping because oflosses the exhaustbecause backpressureof the exhaust causedbackpressure by the caused turbine-consequently, by the turbine‐consequently, turbocharging is turbochargingnot free energy is not recovery free energy [66,67 recovery]. However, [66,67]. in However, turbocharged in turbocharged gasoline engines gasoline with engines variable with valve variabletiming system, valve timing it is possiblesystem, it to is achievepossible positiveto achieve pumping positive pumping at low speed at low and speed high and loads. high loads. Combined Combined charging systems, featuring both a declutchable supercharger and a turbocharger, were charging systems, featuring both a declutchable supercharger and a turbocharger, were found to found to provide the transient response and low‐speed torque of supercharging with the overall provide the transient response and low-speed torque of supercharging with the overall efficiency efficiency and part‐load flexibility of turbocharging. Each of the combined charging systems and part-load flexibility of turbocharging. Each of the combined charging systems discussed used a discussed used a positive displacement supercharger; using a more efficient centrifugal supercharger positive displacement supercharger; using a more efficient centrifugal supercharger alternatively may alternatively may produce additional benefits. Moreover, driving the supercharger through a CVT wouldproduce provide additional better benefits. flexibility Moreover, of operation. driving the supercharger through a CVT would provide better flexibilityElectrical of operation. boosting systems—namely electrically driven compressors (EDCs) and electrically assistedElectrical turbochargers boosting (EATs)—were systems—namely also considered. electrically Combined driven compressorscharging systems, (EDCs) which and involve electrically anassisted electrically turbochargers driven compressor (EATs)—were in place also of considered.the mechanical Combined supercharger, charging have systems, been evaluated which by involve some authors, providing similar performance benefits. It is suggested, nonetheless, that a mechanical

Energies 2019, 12, 4636 28 of 33 an electrically driven compressor in place of the mechanical supercharger, have been evaluated by some authors, providing similar performance benefits. It is suggested, nonetheless, that a mechanical arrangement has significant advantages over electrical boosting systems, as the latter are generally limited by problems such as electrical heating and battery depletion or need upgrading of the standard vehicle electrical architecture. Various turbocompounding arrangements (mechanical and electrical) were discussed. The general agreement of the authors is that it is workable only for high-load operating applications [94], as it offers a small amount of energy recovery at low speed and load. Similarly, Baines [48] declares that turbocompounding ‘is not really suitable for application to passenger car engines, for example, which spend most of their time at part load’. Nevertheless, a significant promise has been shown by a particular configuration (the Van Dyne superturbocharger) where the turbocharger shaft and engine crankshaft are mechanically linked via a CVT, and based on the CVT ratio used, provides turbocharging, supercharging, or turbocompounding. Significant levels of downsizing are obviously achievable, whilst attaining the three critical factors mentioned at the beginning of this section. In conclusion, then, two pressure charging concepts stand out as applicable for providing considerable levels of effective downsizing, and as such are worthwhile of further research:

A combined charging system consisting of a conventional turbocharger in conjunction with a • declutchable supercharger driven through a CVT; A turbocharger that is mechanically linked to the engine crankshaft via a CVT, so providing • potential for turbocharging, supercharging, and turbocompounding.

Author Contributions: M.A. was the research student that conducted the detailed study and wrote the paper assisted by F.A. who also edited the paper. A.P. conceived of the project, created the layout of the investigations, and checked the analysis and subsequent discussion. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

Notation

DCE differential compound engine S−p mean piston speed (m/s) EAT electrically assisted turbocharger BMEP brake mean effective pressure TEDC Turbocharger with an additional electrically driven compressor in inlet (F /A) fuel-air ratio a air 2 Ap total piston area (m ) ρ density (kg/m3) P power (W) ηf fuel conversion efficiency BSFC brake specific fuel consumption ηv volumetric efficiency VGT variable geometry turbocharger GDI gasoline direct injection CVT continuously variable transmission QHV fuel heating value (J/kg) EAT Electrically assisted turbocharger EC Electric compressor EST Electrically split turbocharger Energies 2019, 12, 4636 29 of 33

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