Hybrid Electric Vehicles: a Review of Existing Conﬁgurations and Thermodynamic Cycles

Home , Atkinson cycle, Hybrid vehicle, Miller cycle, Otto cycle

Review Hybrid Electric Vehicles: A Review of Existing Conﬁgurations and Thermodynamic Cycles

Rogelio León , Christian Montaleza , José Luis Maldonado , Marcos Tostado-Véliz * and Francisco Jurado

Department of Electrical Engineering, University of Jaén, EPS Linares, 23700 Jaén, Spain; [email protected] (R.L.); [email protected] (C.M.); [email protected] (J.L.M.); [email protected] (F.J.) * Correspondence: [email protected]; Tel.: +34-953-648580

Abstract: The mobility industry has experienced a fast evolution towards electric-based transport in recent years. Recently, hybrid electric vehicles, which combine electric and conventional combustion systems, have become the most popular alternative by far. This is due to longer autonomy and more extended refueling networks in comparison with the recharging points system, which is still quite limited in some countries. This paper aims to conduct a literature review on thermodynamic models of heat engines used in hybrid electric vehicles and their respective conﬁgurations for series, parallel and mixed powertrain. It will discuss the most important models of thermal energy in combustion engines such as the Otto, Atkinson and Miller cycles which are widely used in commercial hybrid electric vehicle models. In short, this work aims at serving as an illustrative but descriptive document, which may be valuable for multiple research and academic purposes.

Keywords: hybrid electric vehicle; ignition engines; thermodynamic models; autonomy; hybrid conﬁguration series-parallel-mixed; hybridization; micro-hybrid; mild-hybrid; full-hybrid

Citation: León, R.; Montaleza, C.; Maldonado, J.L.; Tostado-Véliz, M.; Jurado, F. Hybrid Electric Vehicles: A Review of Existing Conﬁgurations 1. Introduction and Thermodynamic Cycles. Thermo Nowadays, low pollutant mobility throughout the world is being substantially devel- 2021, 1, 134–150. https://doi.org/ oped and promoted, especially focusing on the automotive segment [1,2]. In this regard, 10.3390/thermo1020010 the transport sector represents one of the main sources of air pollution [3], especially by contributing carbon monoxide, unsmoked hydrocarbons and nitrogen oxides (NOx) [4,5]. Academic Editors: Ignazio Blanco Because of the increasing importance given to pollution concerns, many efforts have been and T M Indra Mahlia conducted in recent years to develop different techniques and approaches to minimize gas emissions caused by the mobility sector [6,7]. Thereby, the automotive industry is experi- Received: 10 June 2021 encing a deep transformation towards electric-based mobility [8,9]. This trend is provoking Accepted: 21 July 2021 Published: 22 July 2021 leading brands to be oriented to this new sector, by developing a plethora of novel vehicle models [10]. In this context, the launching of the Toyota Prius in December 1997 [11] was

Publisher’s Note: MDPI stays neutral a first milestone that was followed in 1999 with the Honda Insight, a semi-hybrid with with regard to jurisdictional claims in manual change or CVT of reduced size and weight, with a low fuel consumption and published maps and institutional affil- optimized aerodynamics [11,12]. At the same time, environmental anti-pollution laws are iations. evolving to address the incoming environmental issues in order to safeguard ecosystems and people’s health [13]. Such is the case of the Euro standards, which are regulating key aspects such as emission bounds since 1988 [14]. Consequently, unlike the Japanese and American automotive markets which have objectives of manufacturing gasoline hybrid vehicles, hybrid solutions with diesel engines have been profusely studied in the European Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. market by leading brands such as Citroën, Opel and Peugeot [12]. This article is an open access article According to the Society of Automotive Engineers, a hybrid vehicle can be defined distributed under the terms and as that vehicle with two or more energy storage systems which must provide power to conditions of the Creative Commons the propellant system either together or independently [15]. Similarly, the heavy duty Attribution (CC BY) license (https:// hybrid vehicles group indicates that a hybrid vehicle must have at least two energy storage creativecommons.org/licenses/by/ systems and energy converters. In practice, a hybrid electric vehicle (HEV) combines 4.0/). the great autonomy of conventional vehicles with spark ignition engines, compression

Thermo 2021, 1, 134–150. https://doi.org/10.3390/thermo1020010 https://www.mdpi.com/journal/thermo Thermo 2021, 1 135

ignition engines, fuel cells and solar panels with the speed, performance and environmental advantages of electric vehicles, obtaining an automobile with lower fossil fuel consumption and lower pollutant emissions to the atmosphere [16–18]. In general, pure electric vehicles have limited autonomy [19] (see Tables1 and2), due to the low energy density of batteries [20] (see Table3) compared with conventional liquid fuels for internal combustion engine (ICE) vehicles. The autonomy depends on the capacity of the batteries and the type of driving, but currently, with the advances of automotive technology, autonomies from 800 to 1000 km can be achieved [21,22], while the zero emissions autonomy (pure electric) ranges from 60 to 75 km. Most of commercial vehicles are plug-in (see Table2), because their larger battery capacity and simpler recharging process compared with conventional hybrids [23,24]. On-board electric storage is normally composed by Li-ion batteries with a capacity 13–18 kWh (see Table2). On the other hand, electric vehicles often have long recharging times. Nevertheless, fast and ultra-fast recharging technologies are emerged which enable recharging times about <30 min [25]. Nonetheless, this kind of infrastructures is still limited. In addition, many countries do not yet dispose of a sufficiently large network of recharging stations [17,18]. These limitations have supposed a formidable barrier to the widespread usage of pure electric vehicles. However, this trend is expected to be changed in a near future [26]. In this context, HEVs suppose a formidable alternative to pure electric vehicles. As commented, this development will be motivated by the implantation of more restrictive normative. For example, additional taxes on fuel consumption are being imposed in numerous European countries such as Spain [27]. In addition, there exists a continuous evolution on batteries technologies. For each generation, new technologies are developed for large-scale electric storage [28–31]. This research effort will have a direct impact on the large implantation of electric vehicles. As an example, one critical aspect in the automotive sector is the total weight of the storage system. This feature is strongly determined by the energy density of a battery technology, which is defined as the total weight (in kg) necessary to store 1 Wh. Figure1 shows the evolution of this characteristic in recent years and the expected trend over a 20-year horizon for different technologies. As seen, emerging technologies such as Li-ion batteries are expected to offer very attractive characteristics for the automotive sector [20,32,33]. Other promising research areas are focused on designing states or specific power trains configurations for HEVs [34–37].

Table 1. Summary of characteristics of various type of vehicles [19].

Driving Range (km) Annual Fuel Cost Fuel Compact and Small Mid and Large (USD) All electric 94–360 320–650 600–950 Plug-in hybrid 500–800 480–1000 750–2800 Fuel cell electric 570 600 NA Ethanol 450–660 480–860 1900–2900

Table 2. Characteristics of various commercial HEVs.

Vehicle Model Electric Autonomy (km) Batteries Type Mercedes Class A 250e 77 Li-ion 15.6 Plug-in Toyota RAV4 Plug-in Hybrid 75 Li-ion 18.1 Plug-in Suzuki Across 75 Li-ion 18.1 Plug-in Volkswagen Golf eHybrid 71 Li-ion 13.0 kWh Plug-in Mercedes B 250 e 70 Li-ion 15.6 kWh Plug-in Mercedes CLA 250 e 69 Li-ion 15.6 kWh Plug-in Range Rover Evoque P300e 66 Li-ion 15.0 kWh Plug-in Thermo 2021, 1, FOR PEER REVIEW 3

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Table 3. Comparison of energy densities of various fuels and battery technologies (adapted from [20]).Table 3. Comparison of energy densities of various fuels and battery technologies (adapted from [20]).

Fuel/TechnologyFuel/Technology Energy Density Energy (MJ/kg) Density (MJ/kg) Gasoline 45 Gasoline 45 Diesel Diesel45 45 HydrogenHydrogen 120 120 Lead-AcidLead-Acid 0.11–0.18 0.11–0.18 Li-ion Li-ion0.54–1.08 0.54–1.08 NiCd 0.11–1.26 NiCd 0.11–1.26

Energy density (Wh/kg)

400 Other Li-ion batteries Li-ion 300

200 NiH

100

Lead-Acid Year

FigureFigure 1. 1.EnergyEnergy density density feature feature for for various various typical typical battery battery technologies technologies in inautomotive automotive sector sector and and expectedexpected future future trend trend (adapted (adapted from from [33]). [33 ]).

BesidesBesides the the electrical electrical part part of an of HEV, an HEV, the conventional the conventional combustion combustion layout layout is still ispre- still sentpresent in current in current HEV HEV models. models. The Thecombustion combustion components components are areexpected expected to tohave have a vital a vital importanceimportance in infuture future HEVs, HEVs, helping helping to toovercome overcome the the limitations limitations of ofpurely purely electric electric vehicles. vehicles. In Inthis this sense, sense, the the combustion combustion system system is isstill still necessary necessary for for extending extending the the autonomy autonomy offered offered byby the the electric electric counterpart. counterpart. In Inaddition, addition, electricity electricity cost cost in inmany many countries countries is isstill still quite quite high high andand few few competitive competitive with with traditional traditional fuel prices.prices. InIn thisthis context, context, vehicle vehicle manufacturers manufacturers and andresearchers researchers have have developed developed many many configurations configurations for for coupling coupling both both systems. systems. The The various var- iousconfigurations configurations with with the the application application of aof heat a heat engine engine and and an an electric electric traction traction in anin a HEVn HEV seek seeksignificant significant improvements improvement ins in autonomy, autonomy, as as the the heat heat engine engine has has the the mission mission of of rechargingrecharg- ingthe the batteries batteries in in a standarda standard configuration configuration and and provides provide thes the propulsion propulsion force force in in conditions condi- tionsof constantof constant running running and and overtaking. overtaking. In In this this context, context, this this work work provides provides a reviewa review of of the themost most typical typical configurations configurations applied applied in in industrial industrial HEVs. HEVs. In In a a second second stage, stage, the the state state of of art artof of the the thermodynamic thermodynamic models models considered considered in in such such applications applications are are reviewed. reviewed. This This way, way, this thispaper paper aims aims at at providing providing an an overall overall review review of theof the current current technological technological status status of HEVs,of HEVs, with withemphasis emphasis on on the the vehicle vehicle layout layout and and mathematical mathematical models. models. In the rest of this paper, Section2 reviews the most typical configurations currently In the rest of this paper, Section 2 reviews the most typical configurations currently contemplated in HEVs. Section3 develops the thermodynamic models of thermal cycles contemplated in HEVs. Section 3 develops the thermodynamic models of thermal cycles that are usually exploited for heat engines in HEVs. This paper is concluded with Section4. that are usually exploited for heat engines in HEVs. This paper is concluded with Section 4. 2. Most Typical Configurations for HEVs 2. Most Typical Configurations for HEVs In general, a powertrain system for a vehicle in general requires to meet a series of characteristicsIn general, a [ 17powertrain,18]: system for a vehicle in general requires to meet a series of characteristics [17,18]: • High performance.  • HighLow performance. pollutant emissions from fossil fuels.  • LowEnough pollutant energy emissions storage from on board fossil to fuels. cover adequate autonomy.  • EnoughSufficient energy power storage generation on board to to supply cover theadequate various autonomy. requirements in the driving and  Sufficientbehavior power of a vehicle. generation to supply the various requirements in the driving and behavior of a vehicle. The powertrain is defined as the junction of an energy source and the energy converter or also referred to as a power source. For example, gasoline and ICE, hydrogen-fuel cells

Thermo 2021, 1, FOR PEER REVIEW 4 Thermo 2021, 1, FOR PEER REVIEW 4

The powertrain is defined as the junction of an energy source and the energy con- Thermo 2021, 1 The powertrain is defined as the junction of an energy source and the energy con-137 verter or also referred to as a power source. For example, gasoline and ICE, hydrogen- verter or also referred to as a power source. For example, gasoline and ICE, hydrogen- fuel cells and an electric motor, batteries and electric motor, etc. Figure 2 schematically fuel cells and an electric motor, batteries and electric motor, etc. Figure 2 schematically shows the overall energy flows in a hybrid powertrain. In this case, there is the possibility shows the overall energy flows in a hybrid powertrain. In this case, there is the possibility andof operating an electric two motor, powertrains batteries according and electric to motor,load requirements. etc. Figure2 schematicallyIn the case of showsthe vehicle the of operating two powertrains according to load requirements. In the case of the vehicle overallwith gasoline energy ﬂowshybridization in a hybrid comprising powertrain. ICE, In thisbattery case, system there is and the electric possibility motor, of operating the path with gasoline hybridization comprising ICE, battery system and electric motor, the path two1 indicated powertrains in Figure according 2 is met. to loadThrough requirements. this energy In path, the casethe propulsion of the vehicle mode with with gasoline ICE is 1 indicated in Figure 2 is met. Through this energy path, the propulsion mode with ICE is hybridizationonly applied when comprising the batteries ICE, battery are almost system discharged and electric and motor, the ICE the is path not 1able indicated to charge in only applied when the batteries are almost discharged and the ICE is not able to charge Figurethem, 2or is also met. when Through the batteries this energy have path, been the fully propulsion charged and mode the with ICE ICEis able is only to supply applied the them, or also when the batteries have been fully charged and the ICE is able to supply the whenpower the demand batteries of arethe almostvehicle. discharged In contrast, and the the path ICE 2 iscorresponds not able to chargeto the purely them, orelectri alsoc power demand of the vehicle. In contrast, the path 2 corresponds to the purely electric whenmode, the in batterieswhich the have ICE been is switched fully charged off, andfor example, the ICE is at able low to speed supply or the in power zero pollutant demand mode, in which the ICE is switched off, for example, at low speed or in zero pollutant ofemission the vehicle. zones In [38]. contrast, The path the path 2 can 2 correspondsbe conducted to in the reverse purely mode electric when mode, the inbatteries which theare ICEemission is switched zones off, [38]. for The example, path 2 atcan low be speed conducted or in zeroin reverse pollutant mode emission when the zones batteries [38]. The are charged from the ICE. pathcharged 2 can from be conducted the ICE. in reverse mode when the batteries are charged from the ICE. Path 1 Path 1

Source 1 Converter 1 Source 1 Converter 1 + + Load Load + Source 2 Converter 2 + Source 2 Converter 2

Path 2 Path 2

FigureFigure 2.2. Energy flows in a hybrid powertrain. Figure 2.Energy Energy ﬂows flows in in a a hybrid hybrid powertrain. powertrain. TheThe transmissiontransmission ofof anan HEVHEV lackslacks aa conventional conventional gearbox. gearbox. InIn contrast,contrast, thethe centralcentral The transmission of an HEV lacks a conventional gearbox. In contrast, the central partpart inin thethe transmissiontransmission ofof suchsuch vehiclesvehicles isis thethe epicycloidalepicycloidal gear,gear, alsoalso calledcalled planetary,planetary, part in the transmission of such vehicles is the epicycloidal gear, also called planetary, fromfrom wherewhere thethe movementmovement toto the the intermediate intermediate sprockets sprockets is is transferred. transferred. TheThe movement movement from where the movement to the intermediate sprockets is transferred. The movement towardstowards the the differential differential assembly assembly is throughis through the the intermediate intermediate sprockets sprockets while while the backward the back- towards the differential assembly is through the intermediate sprockets while the back- movementward movement is achieved is achieved by reversing by reversing the direction the direction of the of electric the electric motor. motor. For For the the sake sake of ward movement is achieved by reversing the direction of the electric motor. For the sake clarity,of clarity, Figure Figure3 schematically 3 schematically illustrates illustrates a typical a typical transmission transmission for a for hybrid a hybrid vehicle. vehicle. of clarity, Figure 3 schematically illustrates a typical transmission for a hybrid vehicle.

Epicycloidal Electric ICE Generator Epicycloidal Electric ICE Generator gear motor gear motor

Differential Differential set set

FigureFigure 3.3.Schematic Schematic diagramdiagram ofof aa typicaltypical hybridhybrid transmissiontransmission system.system. Figure 3. Schematic diagram of a typical hybrid transmission system. ThroughoutThroughout thisthis section,section, thethe mostmost typicaltypical configurationsconfigurations withwith applicationsapplications onon HEVsHEVs Throughout this section, the most typical configurations with applications on HEVs areare reviewed.reviewed. InIn thethe literature,literature, thethe differentdifferent configurationsconfigurations inin HEVsHEVs areare broadlybroadly classifiedclassified are reviewed. In the literature, the different configurations in HEVs are broadly classified eithereither onon thethe basisbasis ofof thethe hybridizationhybridization levellevel oror accordingaccording to to its its architecture architecture [ 39[39,40].,40]. either on the basis of the hybridization level or according to its architecture [39,40]. 2.1. Classification by Vehicle Hybridization Level 2.1. Classification by Vehicle Hybridization Level 2.1. InClassification essence, the by hybridization Vehicle Hybridization concept Level refers to HEVs and mentions the level at which In essence, the hybridization concept refers to HEVs and mentions the level at which a vehicleIn essence, could be the considered hybridization as purely concept electric. refers Roughly to HEVs speaking, and mention the hybridizations the level at which level a vehicle could be considered as purely electric. Roughly speaking, the hybridization level determinesa vehicle could the importance be considered of the as pure electricly electric. and combustion Roughly systemsspeaking, of the an hybridization HEV. Thereby, level the determines the importance of the electric and combustion systems of an HEV. Thereby, higherdetermines level ofthe hybridization, importance of the the more electric important and combustion the electric systems system is.of Thean HEV. hybridization Thereby, the higher level of hybridization, the more important the electric system is. The hybridi- ofthe an higher HEV could level beof hybridization, conceived from the two more different important point the of views electric [41 system]: is. The hybridization of an HEV could be conceived from two different point of views [41]: •zationHybridization of an HEV could of the be propulsion conceived system:from two this different classification point of comprises views [41]: those vehicles that have at the same time an electric traction system and another based on a heat engine (usually combustion or compression engines). Therefore, both systems have the

ability, either independently or in combination, to propel the automobile. Propulsion Thermo 2021, 1 138

system hybridization vehicles are composed of a heat and electric motor, but the latter is only used for starting and keeping the vehicle at low speeds over short distances. • Hybridization of the power supply system: in this case, the vehicles have more than one type of energy system, which could be either production or storage, being at least one of them purely electric. Intuitively, this configuration must count with at least an electric motor. The hybridization system with power supply combines an electrical system and a fuel that serves to increase the autonomy, but the tractor system will be electric, being the function of the heat engine to recharge the batteries when they are running out. This model is also valid for fuel-cell electric vehicles, in which the electric energy is produced through fuel cells that convert hydrogen to electrical energy [42]. According to [43], the above classification can be further subdivided into four levels. In that sense, the electric part in a hybrid vehicle is increased as it reaches a significant percentage of hybridization. That is, with the increase in the rate of hybridization, the environmental impact continues to decrease, but the level of complexity (power control, coupling, energy diversification) of the vehicle system continues to increase until there is no longer the need of a heat engine. This classification approach is described in subsequent sections.

2.1.1. Micro-Hybrid The vehicles in this classiﬁcation encompass the alternator and the start button in the same set. A small electric motor is other key feature of this kind of vehicles. In this sense, the engine only serves to charge the battery system as much as possible during braking phases, besides providing the so-called ‘Stop and Start’ service, which is devoted on restoring the heat engine before starting the running. In this sense, any automobile that provides such kind of capability could be encompassed into this category. One highlight of the ‘Stop and Start’ service is the moment during which the engine is put on below 6 km/h, and starts automatically with the help of the electric motor when it needs to accelerate again. Finally, it is worth remarking that vehicles within this category usually present a petrol economy between 5% and 8%.

2.1.2. Mild-Hybrid In the second category, the mild-hybrid vehicles have a more powerful electric motor and are usually equipped with a higher capacity battery system. This configuration allows the electric system supports the heat engine even during acceleration. However, the electric counterpart is still only able to partially fulfill the function of ICE, because it lacks the sufficiently capacity for propelling the vehicle by itself. In this kind of HEV, the electric system is also used to start the propulsion of the vehicle and initialize the whole traction system. This type of hybridization system allows to recover the kinetic energy of the vehicle through the braking phase with reversible electrical components, in the same way the gasoline economy is favorable because it typically ranges from 20% to 25%.

2.1.3. Full-Hybrid In this category, the vehicles typically encompass a heat engine (MEP-MEC) and an electric motor both connected to the transmission. Additionally, this scheme incorporates a generator and a high capacity battery. The full hybrid vehicles usually can operate under pure-electric mode up to 30 or 40 km/h, beyond that, the electric system needs to be supported by the heat engine. During acceleration, the electric motor supports the thermal system, whereas in braking and retention the kinetic energy is transformed into electricity and stored in batteries. The electric motor is also useful during starting processes, which makes this conﬁguration very suitable for long trips with frequent start-stop transitions. Both systems are mechanically connected with the wheels, allowing to circulate in electric mode, being the most efﬁcient solution allowing to reduce the gasoline consumption by ~45%. The Toyota Prius supposes a notorious example of this kind of hybridization level. This vehicle uses a permanent electric motor coupled to the transmission and a petrol Thermo 2021, 1, FOR PEER REVIEW 6

system, whereas in braking and retention the kinetic energy is transformed into electricity and stored in batteries. The electric motor is also useful during starting processes, which makes this configuration very suitable for long trips with frequent start-stop transitions. Both systems are mechanically connected with the wheels, allowing to circulate in electric Thermo 2021, 1 139 mode, being the most efficient solution allowing to reduce the gasoline consumption by ~45%. The Toyota Prius supposes a notorious example of this kind of hybridization level. This vehicle uses a permanent electric motor coupled to the transmission and a petrol engineengine that that gives gives movement movement toto a a generator, generator, also also incorporating incorporating a a 200 200 V V battery battery located located at at thethe rear. rear. The The size size of of the the heat heat engine engine can can be be reduced, reduced, applying applying the the concept concept of of downsizing downsizing andand large large capacity capacity battery. battery. Figure Figure4 shows4 shows a typicala typical architecture architecture of of a fulla full hybrid hybrid vehicle. vehicle.

Wheels

Heat Electric Generator engine motor

Batteries

FigureFigure 4. 4.Typical Typical architecture architecture of of an an HEV HEV with with full-hybridization full-hybridization level. level.

2.1.4.2.1.4. Plug-in-HybridPlug-in-Hybrid TheThe vehicles vehicles within within this this category category present present an an architecture architecture very very similar similar to the to full-hybridthe full-hy- level.brid level. However, However, the plug-in-hybrid the plug-in-hybrid has the has capability the capability of being of being connected connected to an to upscale an up- electricscale electric grid. This grid. way, This the way, battery the battery system system can be rechargedcan be recharged from the from traction the traction system during system breakingduring breaking stages or stages directly or fromdirectly the from electric the system. electric Onesystem. interesting One interesting feature of feature this kind of this of vehiclekind of is vehicle the possibility is the possibility of exploiting of theexploiting on-board the storage on-board system stor forage grid system supporting for grid tasks. sup- Forporting example, tasks. the For vehicle example, batteries the vehicle could batteries be exploited could as be storage exploited facilities as storage in smart facilities homes in throughsmart homes bidirectional through chargers, bidirectional thus chargers, supporting thus the supporting labor of onsite the labor renewable of onsite generators renewa- onble pursuing generators a more on pursuing efﬁcient energya more management efficient energy in dwellings management [43]. Thisin dwellings principle [ is43]. called This vehicle-to-homeprinciple is called and vehicle is illustrated-to-home in and Figure is illustrated5. in Figure 5.

Bidirectional charger

FigureFigure 5. 5.Vehicle-to-home Vehicle-to-home capability. capability.

2.2.2.2. ClassificationClassification by by Architecture Architecture InIn thethe literatureliterature (e.g.,(e.g., see [40]), [40]), the the HEVs HEVs are are often often classified classified attending attending to totheir their archi- ar- chitecture.tecture. This This classification classification attends attends to the to theon-board on-board system system layout, layout, components components and inter- and interconnectionconnection and andenabled enabled energy energy paths paths among among them. them. Subsequent Subsequent sections sections describe describe the thedif- differentferent categories categories within within this this classification classification approach. approach.

2.2.1. Series Configuration These kinds of vehicles are also called vehicles of extended autonomy. In this case, the vehicle is driven entirely by the electric motor, which is moved by a heat engine with fuel supply. Figure6 depicts a schematic diagram of this architecture. The mechanical output of the ICE is first converted into electricity by a generator; then, this energy could be destined to charge the batteries or propel the wheels through the electric motor, which also allows to capture the energy during braking. Thereby, the ICE is mechanically decoupled to the transmission system. This configuration allows to operate the ICE efficiently, since Thermo 2021, 1, FOR PEER REVIEW 7 Thermo 2021, 1, FOR PEER REVIEW 7

2.2.1. Series Configuration 2.2.1. Series Configuration These kinds of vehicles are also called vehicles of extended autonomy. In this case, These kinds of vehicles are also called vehicles of extended autonomy. In this case, the vehicle is driven entirely by the electric motor, which is moved by a heat engine with the vehicle is driven entirely by the electric motor, which is moved by a heat engine with fuel supply. Figure 6 depicts a schematic diagram of this architecture. The mechanical fuel supply. Figure 6 depicts a schematic diagram of this architecture. The mechanical output of the ICE is first converted into electricity by a generator; then, this energy could output of the ICE is first converted into electricity by a generator; then, this energy could Thermo 2021, 1 be destined to charge the batteries or propel the wheels through the electric motor, which140 be destined to charge the batteries or propel the wheels through the electric motor, which also allows to capture the energy during braking. Thereby, the ICE is mechanically decou- also allows to capture the energy during braking. Thereby, the ICE is mechanically decoupled to the transmission system. This configuration allows to operate the ICE efficiently, pled to the transmission system. This configuration allows to operate the ICE efficiently, since its torque and speed are independent of the mechanical demand of the vehicle. sinceits torque its torque and speed and speed are independent are independent of the of mechanical the mechanical demand demand of the vehicle.of the vehicle. Hence, Hence, the ICE can be operated at any point of its characteristics. This way, the vehicle Hence,the ICE the can ICE be operatedcan be operated at any pointat any of point its characteristics. of its characteristics. This way, This the way, vehicle the generally vehicle generally works at those operating points by which consumption and emissions are min- generallyworks at works those operatingat those operating points by points which by consumption which consumption and emissions and emissions are minimal. are min- By imal. By this configuration, the battery system acts an accumulator facility that can store imal.this conﬁguration,By this configuration, the battery the system battery acts system an accumulator acts an accumulator facility that facility can store that thecan excessstore the excess of energy thus allowing to disconnect the ICE momentarily. This principle is theof energyexcess of thus energy allowing thus toallowing disconnect to disconnect the ICE momentarily. the ICE momentarily. This principle This principle is normally is normally handled by energy management programs, which continuously control the state normallyhandled handled by energy by management energy management programs, programs, which continuously which continuously control the control state ofthe charge state of charge of the batteries on pursuing a fuel consumption reduction [44,45]. ofof charge the batteries of the batteries on pursuing on pursuing a fuel consumption a fuel consumption reduction reduction [44,45]. [44,45].

Combustion Combustionengine Generator engine Generator

Electric motor Batteries Electric motor Deposit Batteries Deposit

Figure 6. Series configuration of an HEV. FigureFigure 6. 6. SeriesSeries configuration configuration of of a ann HE HEV.V. 2.2.2. Parallel Configuration 2.2.2.2.2.2. Parallel Parallel Configuration Configuration By this configuration, both the heat and electric engines can propel the transmission ByBy this this configuration, configuration, both both the the heat heat and and electric electric engines engines can can propel propel the the transmission transmission systems. An electric hybrid vehicle with the parallel configuration has the ICE and electric systems.systems. An An electric electric hybrid hybrid vehicle vehicle with with the the parallel parallel configuration configuration has has the the ICE ICE and and electric electric motormotor coupled coupled to to the the final final drive drive axle of thethe wheelswheels viavia clutches. clutches. Moreover, Moreover, this this configuration configura- motor coupled to the final drive axle of the wheels via clutches. Moreover, this configura- tionallows allows the the ICE ICE and and electric electric motor motor to to supply supply power power to to drive drive the the wheels wheels in in combined combined or tion allows the ICE and electric motor to supply power to drive the wheels in combined orisolated isolated modes. modes. ThisThis way,way, bothboth systemssystems workwork in parallel. Figure Figure 7 shows the the schematic schematic or isolated modes. This way, both systems work in parallel. Figure 7 shows the schematic diagramdiagram of of the the parallel parallel configurat configurationion for for HEVs. HEVs. This This configuration configuration supposes supposes a aremarkable remarkable diagram of the parallel configuration for HEVs. This configuration supposes a remarkable simplificationsimplification of of the the series series architecture architecture as as the the electric electric generator generator is is no no longer longer necessary necessary [44]. [44 ]. simplification of the series architecture as the electric generator is no longer necessary [44]. InIn this this sense, sense, the the electric electric motor motor could could work work in in reverse reverse mode, mode, converting converting the the kinetic kinetic energy energy In this sense, the electric motor could work in reverse mode, converting the kinetic energy ofof the the trans transmissionmission system system to to electricity electricity which which is is stored stored in in batteries. batteries. Generally, Generally, ICE ICE size size of the transmission system to electricity which is stored in batteries. Generally, ICE size cancan be be reduced reduced reduce reduce with with respect respect to to (w.r.t.) (w.r.t.) the the series series configuration configuration [45]. [45]. can be reduced reduce with respect to (w.r.t.) the series configuration [45].

Combustion Combustionengine engine

Deposit Deposit

Electric motor Batteries Electric motor Batteries

Figure 7. Parallel configuration of an HEV. FigureFigure 7. 7. ParallelParallel configuration configuration of of a ann HEV. HEV. 2.2.3. Mixed Configuration In essence, this configuration combines both previous architectures on a whole. This

way, the vehicle could be propelled in this case through the ICE, the electric motor or both systems at once [45]. The heat engine is directly connected to the transmission system and is mechanically coupled to the electric system through a differential set, which mechanically couples both electric and heat systems. The electric system is composed by a motor and a generator, which allows to convert the excess of energy produced by the ICE during Thermo 2021, 1, FOR PEER REVIEW 8

2.2.3. Mixed Configuration In essence, this configuration combines both previous architectures on a whole. This way, the vehicle could be propelled in this case through the ICE, the electric motor or both systems at once [45]. The heat engine is directly connected to the transmission system and Thermo 2021, 1 141 is mechanically coupled to the electric system through a differential set, which mechanically couples both electric and heat systems. The electric system is composed by a motor and a generator, which allows to convert the excess of energy produced by the ICE during brbrakingaking into into electricity electricity to to be be stored stored in in batteries. batteries. By By far, far, this this configuration configuration is is more more complex complex thanthan the the others, others, but but allows allows to to gather gather all all the the advantages advantages obtained obtained with with series series and and parallel parallel configurations.configurations. Figure Figure 88 presentspresents a a diagram diagram of of the the mixed mixed configuration configuration f foror HEVs. HEVs.

Combustion engine

Deposit Generator

Batteries

Electric motor FigureFigure 8. 8. MixedMixed configuration conﬁguration of of an an HEV. HEV. 2.2.4. Summary of Architectures for HEVs 2.2.4. Summary of Architectures for HEVs For the sake of summarizing, Table4 provides a summary of the main advantages of For the sake of summarizing, Table 4 provides a summary of the main advantages of disadvantages of the different architectures for HEVs described in the previous subsections. disadvantages of the different architectures for HEVs described in the previous subsections.Table 4. Summary of the advantages and disadvantages of the different architectures for HEVs.

TableArchitecture 4. Summary of the advantages Advantages and disadvantages of the different Disadvantagesarchitectures for HEVs. Architecture • EfficientAdvantages operation of the ICE. • ManyDisadvantages components. Series • Good performance at low speeds. • Bad performance at high speeds.  Efficient• Simpler operation control of of the the ICE. ICE.  •ManyLow compone efficiency.nts. Series  Good performance at low speeds.  Bad performance at high speeds. • No necessity of electric generator. • Control of the ICE is more Parallel  Simpler control of the ICE.  Low efficiency. • ICE reduced size. complex.  No necessity of electric generator.  Control of the ICE is more com- Parallel • Great flexibility. • Very complex architecture Mixed  ICE• reducedHigh efficiency. size. plex.involving many components.  Great flexibility.  Very complex architecture in- Mixed 3. Thermodynamic High Models efficiency. for HEVs volving many components. In this section, the most common thermodynamic processes in HEVs and the mathe- 3. Thermodynamic Models for HEVs matical formulations usually considered for modeling them are reviewed. In this sense, thermodynamicIn this section, processes the most in common an HEV thermodynamic are frequently modeledprocesses by in thermalHEVs and cycles, the mathe- thus al- maticallowing formulations a simplification usually of their considered conceptualization for modeling making them their are analysis reviewed. and In optimization this sense, thermodynamicsignificantly simpler process [46es–49 in]. a Inn HEV addition, are frequently the analysis modeled of the thermodynamic by thermal cycles cycles, thus will al- al- lowinglow an a understanding simplification ofof thetheir fundamental conceptualization trends formaking the design their analysis of the engines. and optimization Subsequent significantlysections describe simpler the [46 most–49]. usual In addition, thermodynamic the analysis cycles of inthe HEVs, thermodynamic i.e., Otto, Atkinson cycles will and allowMiller an cycles. understanding of the fundamental trends for the design of the engines. Subse- quent sections describe the most usual thermodynamic cycles in HEVs, i.e., Otto, Atkinson and3.1. Miller Nomenclature cycles. The nomenclature used through this section is listed below for simplicity. • Q Heat • W Work • T Temperature • S Entropy • p Pressure • V Volume • η Thermal efficiency • U Energy . • m Mass flow The subscripts denote the different stages of the thermodynamic cycles, as they are labelled in the corresponding figures. Thermo 2021, 1, FOR PEER REVIEW 9

3.1. Nomenclature The nomenclature used through this section is listed below for simplicity.  푄 Heat  푊 Work  푇 Temperature  푆 Entropy  푝 Pressure  푉 Volume  휂 Thermal efficiency  푈 Energy  푚̇ Mass flow Thermo 2021, 1 142 The subscripts denote the different stages of the thermodynamic cycles, as they are labelled in the corresponding figures.

3.2.3.2. Otto Otto Cycle Cycle ThisThis cycle cycle is is associated associated with with the the ignition ignition process process that that happens happens in in the the heat heat engine. engine. In In thethe literature, literature, this cyclecycle isis alsoalso called called Spark Spark ignition. ignition. Figure Figure9 shows 9 shows the thep/ V푝/scheme푉 scheme of theof theOtto Otto cycle cycle whose whose main main steps steps are: are: • AdiabaticAdiabatic compression compression (1 (1–2):–2): compression compression of the of the working working fluid, ﬂuid, the pi theston piston has to has per- to formperform the work the work 푊1.W 1. • ContributionContribution of of heat heat at at constant constant volume volume (2 (2–3):–3): instantaneous instantaneous introduction introduction of of the the heat heat Q 푄11 isis provided. provided. • Adiabatic expansion (3–4): expansion, which is corresponding to the W work, per-  Adiabatic expansion (3–4): expansion, which is corresponding to the 푊2 2work, per- formedformed by by the the working working fluid. ﬂuid. • Extraction of heat at constant volume (4–5): instantaneous extraction of Q heat.  Extraction of heat at constant volume (4–5): instantaneous extraction of 푄22 heat.

P 3

푄1

2 W 4 푄2

0 1

Figure 9. p V Figure 9. 푝//푉 diagramdiagram of of the the Otto Otto cycle. cycle. In the engines of four strokes (4T), the extraction of heat occurs in the exhaust phase, In the engines of four strokes (4T), the extraction of heat occurs in the exhaust phase, from the opening of the exhaust valve (4–1–0). In addition, the mixture of the air–fuel ﬂuid from the opening of the exhaust valve (4–1–0). In addition, the mixture of the air–fuel fluid is introduced into the intake stroke (0–1). This process is graphically represented in the is introduced into the intake stroke (0–1). This process is graphically represented in the horizontal dashed line of the p/V diagram of Figure9. Theoretically the processes (1–0 horizontal dashed line of the 푝/푉 diagram of Figure 9. Theoretically the processes (1–0 and 0–1) between them are cancelled out, in such a way that a loss or gain of zero heat is andgenerated, 0–1) between so that them the p /areV diagramcancelled of out, the in ideal such Otto a way cycle that only a loss considers or gain the of closedzero heat cycle. is generated, so that the 푝/푉 diagram of the ideal Otto cycle only considers the closed cycle. Similarly, the work in this phase is zero, W2−3 = 0 the heat supply is performed at constant Similarly, the work in this phase is zero, 푊2−3 = 0 the heat supply is performed at con- Q1 volume [41,50]. stant From푄1 volume the p/ [41,50].V diagram of the Otto cycle in Figure9 one can establish the following relationships:From the 푝/푉 diagram of the Otto cycle in Figure 9 one can establish the following relationships: V2 = V3, V4 = V1 (1)

While the compression (CR) and pressure푉2 = 푉3, 푉 (4PR= )푉1 ratios for this cycle are given by [46(1]:)

V CR = 1 (2) V3 p PR = 3 (3) p1 The thermal efﬁciency of the Otto cycle can be established as [51]:

T ηOtto = 1 − 1 (4) T2 The ﬁrst law can be applied to the adiabatic process (1–2), obtaining [46]:

T V 1−K 1 = 1 (5) T2 V2 Thermo 2021, 1, FOR PEER REVIEW 10

While the compression (퐶푅) and pressure (푃푅) ratios for this cycle are given by [46]:

푉1 퐶푅 = (2) 푉3

푝3 푃푅 = (3) 푝1 The thermal efficiency of the Otto cycle can be established as [51]:

푇1 휂푂푡푡표 = 1 − (4) 푇2 Thermo 2021, 1 143 The first law can be applied to the adiabatic process (1–2), obtaining [46]: 1−퐾 푇1 푉1 = ( ) (5) where K is the specific heat ratio defined푇 as:2 푉2 퐾 where is the specific heat ratio defined as: c = p K 푐푝 (6) 퐾 =cv (6) 푐푣 where cv and cp are the specific heat of the gas at constant volume and pressure, respectively. 푐 푐 Thiswhere way, 푣 theand Equation 푝 are the (4) specific can be rewritten:heat of the gas at constant volume and pressure, respectively. This way, the Equation (4) can be rewritten: ηOtto = 1 − CR1−K (7) 휂푂푡푡표 = 1 − 퐶푅1−퐾 (7) By the mathematical model above, thethe performanceperformance ofof anan OttoOtto cyclecycle onlyonly dependsdepends onon the valuesvalues ofofK 퐾and andCR 퐶푅, and, and therefore therefore it is it not is affectednot affected by the by amount the amount of heat of provided heat provided or the degreeor the degree of explosion of explosion [50]. The [50]. influence The influence of such of parameters such parameters on the on thermal the thermal efficiency efficiency of the Ottoof the cycle Otto is cycle shown is shown in Figure in Figure10[ 48,49 10]. [48,49]. For a given For a value given of valueK, thermal of 퐾, thermal efficiency efficiency notably increasesnotably increases with increasing with increasingCR at low 퐶푅 values at low of the values compression of the compression ratio. This trend ratio. changes This trend for higherchanges values for higher of the values volumetric of the compression volumetric compression ratio (approximately ratio (approximatelyCR = 10). Beyond 퐶푅 = this10). threshold,Beyond this thermal threshold, efficiency thermal curves efficiency flatten out, curves lessening flatten the out, benefits lessening of working the benefits with high of pressures.working with high pressures.

푂푡푡표 휂 퐾 = 1

퐾 = 1

퐶푅 Figure 10.10. Thermal efficiencyefficiency of of the the Otto Otto cycle cycle as as function function of of the the specific-heat specific-heat and and compression compression ratios. ratios. The mathematical model described above for the Otto cycle responds to ideal conditions.The In practice,mathematical performing model described of heat engines above mayfor the be Otto far away cycle to responds these ideal to ideal conditions condi- mainlytions. In due practice, to the followingperforming reasons of heat [52 engines]: may be far away to these ideal conditions •mainlyHeat due loss to the through following the walls,reasons caused [52]: by the need to have a cooling system for the  ignitionHeat loss engines through organs. the walls, caused by the need to have a cooling system for the • Needignition to engines anticipate organs. ignition with respect to death point, because combustion is not  instantaneousNeed to anticipate and aignition certain timewith isrespect needed. to death point, because combustion is not • Exhaustinstantaneous opening and advance, a certain due time to is the needed. inertia of the valves and gas masses. • LossExhaust of pumping opening workadvance, during due theto the exhaust inertia and of intakethe valves stroke. and gas masses.  Loss of pumping work during the exhaust and intake stroke. 3.3. Atkinson Cycle The vast majority of hybrid electric vehicles use the Atkinson cycle for internal combustion engines because its higher efficiency [51]. This cycle is characterized by having a longer expansion stroke than the compression one, thus largely managing the energy available in the injected fuel [53]. In this way, the mixture has more time to expand and produces within the combustion chamber a greater amount of work [54]. The Atkinson cycle is relatively ideal for a hybrid vehicle since the internal combustion engine with this cycle has greater efficiency in thermal energy, but at the cost of low power [55]. In this regard, an electric motor is generally needed to complement the heat engine [56]. Figure 11 depicts the p/V diagram of the Atkinson cycle, which comprises an adiabatic compression (1–2), addition of isochoric heat (2–3), an adiabatic expansion (3–4) and finally isobaric heat extraction. Thermo 2021, 1, FOR PEER REVIEW 11

3.3. Atkinson Cycle The vast majority of hybrid electric vehicles use the Atkinson cycle for internal combustion engines because its higher efficiency [51]. This cycle is characterized by having a longer expansion stroke than the compression one, thus largely managing the energy available in the injected fuel [53]. In this way, the mixture has more time to expand and produces within the combustion chamber a greater amount of work [54]. The Atkinson cycle is relatively ideal for a hybrid vehicle since the internal combustion engine with this cycle has greater efficiency in thermal energy, but at the cost of low power [55]. In this regard, an electric motor is generally needed to complement the heat engine [56]. Figure 11 depicts the 푝/푉 diagram of the Atkinson cycle, which comprises an adiabatic compres- Thermo 2021, 1 sion (1–2), addition of isochoric heat (2–3), an adiabatic expansion (3–4) and finally iso-144 baric heat extraction.

푄1

2 W

Intake stroke 0 Exhaust stroke 4 1 푄2 V

Figure 11. p/V diagram of the Atkinson cycle. Figure 11. 푝/푉 diagram of the Atkinson cycle. From the p/V showed in Figure 11, the following relations are deduced: From the 푝/푉 showed in Figure 11, the following relations are deduced:

V2 = V3, p1 = p4 (8) 푉2 = 푉3, 푝1 = 푝4 (8) LetLet us us define deﬁne the the compression, compression, temperature temperature ( (푇푅TR)) and and pressure pressure ratios ratios for for this this cycle cycle as as followsfollows [46]: [46]: V4 CR = 푉4 (9) 퐶푅 =V3 (9) 푉3 T TR = 3 (10) T푇13 푇푅 = (10) p푇 PR = 31 (11) p4 푝3 The amount of heat supplied and rejected푃푅 = can be deduced as follows: (11) 푝4 . The amount of heat supplied andQ1 rejected= mcv(T can3 − beT2 )deduced as follows: (12) . 푄1 = 푚̇ 푐푣(푇3 − 푇2) (12) Q2 = mcp(T4 − T1) (13)

Dividing (13) by (12) and simplifying푄2 = 푚 onė 푐푝( obtains:푇4 − 푇1) (13) Dividing (13) by (12) and simplifyingQ one(T /obtains:T ) − 1 2 = K 4 1 (14) Q (T /T ) − (T /T ) 1푄2 3 (푇14⁄푇1) −2 1 1 = 퐾 (14) Now, observing the adiabatic푄1 processes(푇3⁄ 1–2푇1) − and(푇2 3–4,⁄푇1) one has: Now, observing the adiabatic processes 1–2 and 3–4, one has: T V 1−K V T 1−K T 1−K 2 = 2 = 3 4 = CR−1 4 (15) T1 V1 V4 T1 T1

1−K T4 V4 − = = CR1 K (16) T3 V3 The Equation (16) could be used to determine the useful ratio T4 , as follows: T1 T 4 = TR·CR1−K (17) T1 Next, replacing (17) into (15) one obtains:

T 1−K 2 = TR·CR−K (18) T1 Thermo 2021, 1 145

The expression for the thermal efﬁciency of the Atkinson cycle can now be obtained by substituting (17) and (18) into (14), which yields:

1−K Atkinson TR·CR − 1 η = 1 − K − (19) TR − (TR·CR−K)1 K

From (9) and (11) the following relationship can be established in the Atkinson cycle:

1 CR = PR K (20)

Equation (20) allows to derive an alternative expression for the thermal efﬁciency as a function of the pressure and temperature ratios, as follows:

1 −K TR·PR K − 1 Atkinson = − η 1 K 1−K (21) TR TR − PR

3.4. Miller Cycle The Miller cycle is used in those hybrid vehicles in which the lack of torque from the engine is compensated by the addition of the electric motor. In the Miller cycle a larger cylinder than usual is usually used. The compression stroke is shortened in conjunction with the expansion stroke, where, as the piston moves upwards in the compression stroke the load is pushed out of the normally closed valve [57]. Delaying the closing time of the intake valves causes the temperature of the mixture to be reduced as well as the load mixture fraction. Consequently, the indicated effective average pressure and net cyclic work is reduced [58]. To solve this issue, a variable valve timing mechanism could be used to increase the efﬁciency of the Miller cycle [59,60]. Figure 12 plots the P/V diagram of the Thermo 2021, 1, FOR PEER REVIEW Miller cycle which comprises an adiabatic compression (1–2), addition of isochoric heat13 (2–3), an adiabatic expansion (3–4), elimination of isochoric heat (4–5) and elimination of isobaric heat (5–1).

P 3

푄1

W 4 2 푄2 ′ 푄2 1 5 V

Figure 12. p/V diagram of the Miller cycle. Figure 12. 푝/푉 diagram of the Miller cycle From the Figure 12 one can directly deduces: From the Figure 12 one can directly deduces:

V2 = V3, V4 = V5, p5 = p1 (22) 푉2 = 푉3, 푉4 = 푉5, 푝5 = 푝1 (22) WhereasWhereas the the compression, compression, temperature temperature and and pressure pressure ratios ratios are are respective respectivelyly defined deﬁned forfor this this cycle cycle as as follows follows [46]: [46]: V5 CR푉=5 (23) 퐶푅 = V3 (23) 푉3 T TR = 3 (24) 푇3 T1 푇푅 = (24) 푇1

푝3 푃푅 = (25) 푝5 The thermal efficiency of the Miller cycle is determined by the relationship between the total and supplied heats, as follows [46]: ′ 푄2 + 푄2 푐푣(푇4 − 푇5) + 푐푝(푇5 − 푇1) 휂푀푖푙푙푒푟 = 1 − = 1 − (26) 푄1 푐푣(푇3 − 푇2) The Equation (26) can be rewritten as a function of the specific heat ratio straightfor- ward as:

(푇4⁄푇1) + (퐾 − 1)(푇5⁄푇1) − 퐾 휂푀푖푙푙푒푟 = 1 − (27) 푇푅 − (푇2⁄푇1) By using the relations of the adiabatic process, the following relations must hold for the Miller cycle: 1−퐾 1−푘 푇4 푉4 푉5 = ( ) = ( ) = 퐶푅1−퐾 (28) 푇3 푉3 푉3

−퐾 푉4 −퐾 푝4 = 푝3 ( ) = 푝3퐶푅 (29) 푉3 푇 The ratio 4 can be deduced from Equation (28): 푇1

푇4 = 푇푅 ∙ 퐶푅1−퐾 (30) 푇1 For the isochoric process 4–5, the following relation holds:

Thermo 2021, 1 146

p PR = 3 (25) p5 The thermal efﬁciency of the Miller cycle is determined by the relationship between the total and supplied heats, as follows [46]:

0 Q + Q cv(T4 − T5) + cp(T5 − T1) ηMiller = 1 − 2 2 = 1 − (26) Q1 cv(T3 − T2)

The Equation (26) can be rewritten as a function of the speciﬁc heat ratio straightfor- ward as: (T /T ) + (K − 1)(T /T ) − K ηMiller = 1 − 4 1 5 1 (27) TR − (T2/T1) By using the relations of the adiabatic process, the following relations must hold for the Miller cycle: T V 1−K V 1−k 4 = 4 = 5 = CR1−K (28) T3 V3 V3 −K V4 −K p4 = p3 = p3CR (29) V3 The ratio T4 can be deduced from Equation (28): T1 T 4 = TR·CR1−K (30) T1 For the isochoric process 4–5, the following relation holds: p5 T5 = T4 (31) p4

By using the relations (30) and (31) one can deduce:

T T p p CR 5 = 4 5 = · 1−K 5 = TR CR −K TR (32) T1 T1 p4 p3CR PR

For the isobaric process 5–1 the following relation holds: T1 V1 = V5 (33) T5

Combining (33) and the relationship valid for 1–2, the following expression can be easily derived:

− − − − T V 1 K V T 1 K 1 CR 1 K TR 1 K 2 = 2 = 3 5 = TR = (34) T1 V1 V5T1 CR PR PR

Finally, one can insert the result (34) into (27) to obtain:

1−K PR PR·CR + (K − 1)CR − K TR Miller = − η 1 K (35) PR PR − TR

3.5. Finite Time Thermodynamics Finally, this paper briefly introduces the Finite Time Thermodynamics (FTT) concept. FTT is a branch of thermodynamics theory, which aims at modeling those processes which take place during a finite time rate [61,62]. In contrast to conventional theory, FTT is conceived from a macroscopic point of view with heat conductances, friction coefficients, Thermo 2021, 1 147

overall reaction rates, etc [63]. The very first steps of FTT took place in the mid-1970s, when Curzon and Ahlborn [64] derived the efficiency term of a Carnot engine considering finite heat transfer between the working fluid and heat reservoir. This idea can be extended to any other endoreversible system (i.e., internally reversible) such as Otto [65], Atkinson [66] and Miller cycles [67]. Nowadays, FTT has a crucial importance for studying optimum performances and configurations of internal combustion engines. Recent progresses are mainly devoted to further analyzing the optimum performances of air standard cycles, determining the optimal path of internal combustion cycles, assessing the limits of the cycles, and development of advanced simulation frameworks [62]. An illustrative example of the importance of FTT, is the analysis of the specific heat of working fluid. Traditionally, this parameter was considered constant. However, as pointed out in [62], this assumption does not reflect the reality. For practical, cycles, properties of working fluids change due to combustion reactions. These variations of the base parameters may introduce significant inaccuracies in the study of thermodynamic cycles. To solve this issue, the reference [68] introduced the concept of variable specific heat. In its simplest model, the specific heat varies linearly with the temperature, as follows [69]:

cp = αp + K·T (36)

cv = αv + K·T (37) where α’s are constant parameters. Another important aspect in which traditional theories are not applicable is the con- sideration of heat transfer losses, for which the cycle maximum temperature should not be considered ﬁxed [70]. On the other hand, other studies consider the quantum characteristics of the working ﬂuid [71], in contrast to classical techniques which are based on phenomenological law and equilibrium statistical mechanics.

4. Conclusions This paper has presented an illustrative but descriptive review of the most typical configurations and architectures for hybrid electric vehicles. Accordingly, the different configurations have been classified attending to different criteria contemplated in the literature. The main advantages and disadvantages of each architecture have been identified and highlighted with the aim of serving as a valuable contribution to related research and academic purposes. On the other hand, the usual thermodynamic cycles that are typically exploited in HEVs have been developed and reviewed. Thus, Otto, Atkinson and Miller cycles have been mathematically elaborated and descriptively illustrated. The review presented in this paper has been performed as illustratively as possible, so as to be of interest for the wider public. Additionally, a variety of key references have been provided with the aim of serving as a bibliographic benchmark for the ease of further expanding the scope of this paper.

Author Contributions: Conceptualization, R.L., C.M. and J.L.M.; methodology, R.L., C.M. and J.L.M.; software, R.L., C.M. and J.L.M.; validation, R.L., C.M. and J.L.M.; formal analysis, M.T.-V.; investigation, R.L., C.M., J.L.M. and M.T.-V.; resources, M.T.-V. and F.J.; data curation, M.T.-V. and F.J.; writing—original draft preparation, R.L., C.M. and J.L.M.; writing—review and editing, M.T.-V. and F.J.; visualization, M.T.-V.; supervision, M.T.-V. and F.J.; project administration, M.T.-V. and F.J.; funding acquisition, F.J. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The icons used in this article were developed by Freepik, Flat icons and dDara from www.ﬂaticon.com (accessed on 21 July 2021). Thermo 2021, 1 148

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Choma, E.F.; Evans, J.S.; Hammitt, J.K.; Gómez-Ibáñez, J.A.; Spengler, J.D. Assessing the health impacts of electric vehicles through air pollution in the United States. Environ. Int. 2020, 144, 106015. [CrossRef] 2. Cárcel-Carrasco, J.; Pascual-Guillamón, M.; Salas-Vicente, F. Analysis on the Effect of the Mobility of Combustion Vehicles in the Environment of Cities and the Improvement in Air Pollution in Europe: A Vision for the Awareness of Citizens and Policy Makers. Land 2021, 10, 184. [CrossRef] 3. Yang, X.; Lin, W.Q.; Gong, R.X.; Zhu, M.Z.; Springer, C. Transport decarbonization in big cities: An integrated environmental co-benefit analysis of vehicles purchases quota-limit and new energy vehicles promotion policy in Beijing. Sustain. Cities Soc. 2021, 71, 102976. [CrossRef] 4. Xie, Y.; Wu, D.; Zhu, S. Can new energy vehicles subsidy curb the urban air pollution? Empirical evidence from pilot cities in China. Sci. Total Environ. 2021, 754, 142232. [CrossRef] 5. Pini, F.; Piras, G.; Garcia, D.A.; Di Girolamo, P. Impact of the different vehicle fleets on PM10 pollution: Comparison between the ten most populous Italian metropolitan cities for the year 2018. Sci. Total Environ. 2021, 773, 145524. [CrossRef][PubMed] 6. Zhao, J.; Xi, X.; Na, Q.; Wang, S.; Kadry, S.N.; Kumar, P.M. The technological innovation of hybrid and plug-in electric vehicles for environment carbon pollution control. Environ. Impact Assess. Rev. 2021, 86, 106506. [CrossRef] 7. Breuer, J.L.; Samsun, R.C.; Stolten, D.; Peters, R. How to reduce the greenhouse gas emissions and air pollution caused by light and heavy duty vehicles with battery-electric, fuel cell-electric and catenary trucks. Environ. Int. 2021, 152, 106474. [CrossRef] [PubMed] 8. Alla, S.A.; Bianco, V.; Tagliafico, L.A.; Scarpa, F. Pathways to electric mobility integration in the Italian automotive sector. Energy 2021, 221, 119882. [CrossRef] 9. Ajanovic, A.; Haas, R.; Schrodl, M. On the Historical Development and Future Prospects of Various Types of Electric Mobility. Energies 2021, 14, 1070. [CrossRef] 10. Torreglosa, J.P.; Garcia-Trivino, P.; Vera, D.; Lopez-Garcia, D.A. Analyzing the Improvements of Energy Management Systems for Hybrid Electric Vehicles Using a Systematic Literature Review: How Far Are These Controls from Rule-Based Controls Used in Commercial Vehicles? Appl. Sci. 2020, 10, 8744. [CrossRef] 11. Clifford, J. History of the Toyota Prius. Toyota UK Mag. 2015. Available online: https://mag.toyota.co.uk/history-toyota-prius/ (accessed on 2 June 2021). 12. Kim, N.; Rousseau, A. Thermal impact on the control and the efficiency of the 2010 Toyota Prius hybrid electric vehicle. Proc. Inst. Mech. Eng. Part D J. Automob. Eng. 2016, 230, 82–92. [CrossRef] 13. Jin, Y.N.; Andersson, H.; Zhang, S.Q. Air Pollution Control Policies in China: A Retrospective and Prospects. Int. J. Environ. Res. Public Health 2016, 13, 1219. [CrossRef][PubMed] 14. Tostado-Véliz, M.; Bayat, M.; Ghadimi, A.A.; Jurado, F. Home Energy Management in off-grid Dwellings: Exploiting Flexibility of Thermostatically Controlled Appliances. J. Clean. Prod. 2021, 310, 127507. [CrossRef] 15. Society of American Engineers. Hybrid Electric Vehicle (HEV) & Electric Vehicle (EV) Terminology; J1715_200802; Society of American Engineers: Warrendale, PA, USA, 2008. 16. Hannan, M.A.; Azidin, F.A.; Mohamed, A. Hybrid electric vehicles and their challenges: A review. Renew. Sustain. Energy Rev. 2014, 29, 135–150. [CrossRef] 17. Alegre, S.; Míguez, J.V.; Carpio, J. Modelling of electric and parallel-hybrid electric vehicle using Matlab/Simulink environment and planning of charging stations through a geographic information system and genetic algorithms. Renew. Sustain. Energy Rev. 2017, 74, 1020–1027. [CrossRef] 18. Kumar, P.R.; Shankar, C.G.; Uthirasamy, R.; Vijayalakshmi, V.J. A Review of Electric Vehicle Technologies. In Proceedings of International Conference on Artificial Intelligence, Smart Grid and Smart City; Kumar, L.A., Jayashree, L.S., Manimegalai, R., Eds.; Springer: New York, NY, USA, 2019. 19. U. S. Department of Energy. Fuel Economy Guide. 2021. Available online: https://www.fueleconomy.gov/feg/pdfs/guides/ FEG2021.pdf (accessed on 15 July 2021). 20. IDAE. Guía para la Promoción del Vehículo Eléctrico en las Ciudades; IDAE: Madrid, Spain, 2011. Available online: https:// www.movilidad-idae.com/sites/default/files/2019-06/Gu%C3%ADaPromoci%C3%B3nVECiudades_2011.pdf (accessed on 2 June 2021). 21. Adhikari, M.; Ghimire, L.P.; Kim, Y.; Aryal, P.; Khadka, S.B. Identification and Analysis of Barriers against Electric Vehicle Use. Sustainability 2021, 12, 4850. [CrossRef] 22. Endesa, X. 7 Differences between an Electric Vehicle and a Plug-In Hybrid. Available online: https://www.endesax.com/en/ resources/stories/cirve-iberian-fast-charging-corridors1 (accessed on 15 July 2021). 23. Cao, J.F.; He, H.W.; Wei, D. Intelligent SOC-consumption allocation of commercial plug-in hybrid electric vehicles in variable scenario. Appl. Energy 2021, 281, 115942. [CrossRef] 24. Hajipour, E.; Mohiti, M.; Farzin, N.; Vakilian, M. Optimal distribution transformer sizing in a harmonic involved load environment via dynamic programming technique. Energy 2017, 120, 92–105. [CrossRef] Thermo 2021, 1 149

25. Zhao, Y.; He, X.; Yao, Y.; Huang, J.J. Plug-in electric vehicle charging management via a distributed neurodynamic algorithm. Appl. Soft Comput. 2019, 80, 557–566. [CrossRef] 26. España 2050: Aumento de Impuestos a la Gasolina y al Diésel y una Tasa por el uso Del Vehículo. Heraldo updated 27 May 2021. Available online: https://www.heraldo.es/noticias/nacional/2021/05/20/gobierno-subira-impuestos-gasolina-diesel-creara- tasa-uso-medio-vehiculo-1493577.html (accessed on 2 June 2021). 27. Arévalo, P.; Tostado-Véliz, M.; Jurado, F. A Novel Methodology for Comprehensive Planning of Battery Storage Systems. J. Energy Storage 2021, 37, 102456. [CrossRef] 28. Hilgers, M.; Achenbach, W. Alternative Powertrains and Extensions to the Conventional Powertrain; Springer Vieweg: Wiesbaden, Germany, 2021; pp. 17–41. 29. Harper, G.; Sommerville, R.; Kendrick, E.; Driscoll, L.; Slater, P.; Stolkin, R.; Walton, A.; Christensen, P.; Heidrich, O.; Lambert, S.; et al. Recycling lithium-ion batteries from electric vehicles. Nature 2019, 575, 75–86. [CrossRef] 30. Zu, C.-X.; Li, H. Thermodynamic analysis on energy densities of batteries. Energy Environ. Sci. 2011, 4, 2614–2624. [CrossRef] 31. Van Noorden, R. The rechargeable revolution: A better battery. Nature 2014, 507, 26–28. [CrossRef][PubMed] 32. Cano, Z.P.; Banham, D.; Ye, S.; Hitennach, A.; Lu, J.; Fowler, M.; Chen, Z. Batteries and fuel cells for emerging electric vehicle markets. Nat. Energy 2018, 3, 279–289. [CrossRef] 33. Dimitrova, Z. Predictive and holistic energy distribution for hybrid electric vehicles. MATEC Web Conf. 2017, 133, 02002. [CrossRef] 34. Dimitrova, Z. Performance and economic analysis of an organic Rankine Cycle for hybrid electric vehicles. IOP Conf. Ser. Mater. Sci. Eng. 2019, 664, 012009. [CrossRef] 35. Capata, R.; Scuibba, E. Study, Development and Prototyping of a Novel Mild Hybrid Power Train for a City Car: Design of the Turbocharger. Appl. Sci. 2020, 11, 234. [CrossRef] 36. Capata, R. Preliminary Analysis of a New Power Train Concept for a City Hybrid Vehicle. Designs 2021, 5, 19. [CrossRef] 37. Mapelli, F.L.; Tarsitano, D. Energy control for Plug-In HEV with ultracapacitors lithium-ion batteries storage system for FIA Alternative Energy Cup Race. In Proceedings of the 2010 IEEE Vehicle Power and Propulsion Conference, Lille, France, 1–3 September 2010; pp. 1–6. [CrossRef] 38. Center for Advanced Automotive Technology. HEV Levels. Available online: http://autocaat.org/Technologies/Hybrid_and_ Battery_Electric_Vehicles/HEV_Levels/#:~{}:text=Vehicles%20%3E%20HEV%20Levels-,HEV%20Levels,the%20function%20 of%20regenerative%20braking (accessed on 2 June 2021). 39. Zhang, F.; Wang, L.; Coskun, S.; Pang, H.; Cui, Y.; Xi, J. Energy Management Strategies for Hybrid Electric Vehicles: Review, Classification, Comparison, and Outlook. Energies 2020, 13, 3352. [CrossRef] 40. Ge, Y.; Chen, L.; Sun, F.; Wu, C. Thermodynamic simulation of performance of an Otto cycle with heat transfer and variable specific heats of working fluid. Int. J. Therm. Sci. 2005, 44, 506–511. [CrossRef] 41. Tostado-Véliz, M.; Arévalo, P.; Jurado, F. A Comprehensive Electrical-Gas-Hydrogen Microgrid Model for Energy Management Applications. Energy Convers. Manag. 2021, 228, 113726. [CrossRef] 42. Guzzella, L.; Sciarretta, A. Vehicle Propulsion Systems, 3rd ed.; Springer: Berlin/Heidelberg, Germany, 2012. 43. Tostado-Véliz, M.; León-Japa, R.S.; Jurado, F. Optimal Electrification of Off-grid Smart Homes Considering Flexible Demand and Vehicle-to-Home Capabilities. Appl. Energy 2021, 298, 117184. [CrossRef] 44. Onori, S.; Serrao, L.; Rizzoni, G. Hybrid Electric Vehicles: Energy Management Strategies; Springer: Berlin/Heidelberg, Germany, 2016. 45. Miller, J.M. Propulsion Systems for Hybrid Vehicles; The Institution of Electrical Engineers: London, UK, 2004; Volume 45. 46. Haseli, Y. Most efficient engine. In Entropy Analysis in Thermal Engineering Systems; Haseli, Y., Ed.; Academic Press: Cambridge, MA, USA, 2020. [CrossRef] 47. Naber, J.D.; Johnson, J.E. Internal combustion engine cycles and concepts. In Alternative Fuels and Advanced Vehicle Technologies for Improved Environmental Performance; Folkson, R., Ed.; Woodhead Publising: Cambridge, MA, USA, 2014. 48. Balmer, R.T. Vapor and Gas Power Cycles. In Modern Engineering Thermodynamics; Balmer, R.T., Ed.; Academic Press: Cambridge, MA, USA, 2011. [CrossRef] 49. Dincer, I.; Demir, M.E. Gas Turbine Cycles. In Comprehensive Energy Systems; Dincer, I., Ed.; Elsevier: Oxford, UK, 2018. 50. Gathadi, P.; Bhole, A. Electric Motors and Rotor Position Detection Techniques of Integrated Starter Alternator for HEV: A Review. In Proceedings of the 2018 International Conference on Current Trends towards Converging Technologies (ICCTCT), Coimbatore, India, 1–3 March 2018; pp. 1–6. [CrossRef] 51. Chandra, M.; Dan, P.K.; Bhattacharjee, D.; Mandol, S.; Patra, P. Devising Product Design Architecture Strategies: Case of HEV Powertrain. In Research into Design for a Connected World; Chakrabarti, A., Ed.; Springer Nature: Singapore, 2019; Volume 134. [CrossRef] 52. Park, H.G.; Kwon, Y.J.; Hwang, S.J.; Lee, H.D.; Kwon, T.S. A Study for the estimation of temperature and thermal life of traction motor for commercial HEV. In Proceedings of the 2012 IEEE Vehicle Power and Propulsion Conference, Seoul, Korea, 9–12 October 2012; pp. 160–163. [CrossRef] 53. Zhou, X.; Qin, D.; Rotella, D.; Cammalleri, M. Hybrid Electric Vehicle Powertrain Design: Construction of Topologies and Initial Design Schemes. In Advances in Italian Mechanism Science; Carbone, G., Gasparetto, A., Eds.; Springer Nature: Cham, Switzerland, 2019; Volume 68, pp. 49–60. [CrossRef] Thermo 2021, 1 150

54. Maggetto, G.; Van, J. Electric vehicles, hybrid electric vehicles and fuel cell electric vehicles: State of the art and perspectives. Ann. Chim. Sci. Mater. 2001, 26, 9–26. [CrossRef] 55. Capata, R. Urban and Extra-Urban Hybrid Vehicles: A Technological Review. Energies 2018, 11, 2924. [CrossRef] 56. Capasso, C.; Veneri, O. Experimental analysis on the performance of lithium based batteries for road full electric and hybrid vehicles. Appl. Energy 2014, 136, 921–930. [CrossRef] 57. Tate, E.D.; Harpster, M.O.; Savagian, P.J. The electrification of the automobile: From conventional hybrid, to plug-in hybrids, to extended-range electric vehicles. SAE Int. J. Passeng. Cars Electron. Electr. Syst. 2008, 1, 156–166. [CrossRef] 58. Kulikov, I.A.; Lezhnev, L.Y.; Bakhmutov, S.V. Comparative Study of Hybrid Vehicle Powertrains with Respect to Energy Efficiency. J. Mach. Manuf. Reliab. 2019, 48, 11–19. [CrossRef] 59. León, R.S.; Maldonado, J.L.; Contreras, R.W. Prediction of CO and HC emissions in Otto motors through neural networks. Ingenius 2020, 23, 30–39. [CrossRef] 60. Contreras, R.W.; Maldonado, J.L.; León, R.S. Application of feed-forward backpropagation neural network for the diagnosis of mechanical failures in engines provoked ignition. Ingenius 2019, 21, 32–40. [CrossRef] 61. Andresen, B. Current Trends in Finite-Time Thermodynamics. Angew. Chem. Int. Ed. 2011, 50, 2690–2704. [CrossRef] 62. Ge, Y.L.; Chen, L.G.; Sun, F.R. Progress in finite time thermodynamic studies for internal combustion engine cycles. Entropy 2016, 18, 139. [CrossRef] 63. Andresen, B. Finite-time thermodynamics and thermodynamic length. Rev. Générale Therm. 1996, 35, 647–650. [CrossRef] 64. Curzon, F.L.; Ahlborn, B. Efficiency of a Carnot engine at maximum power output. Am. J. Phys. 1975, 43, 22. [CrossRef] 65. Ge, Y.L.; Chen, L.G.; Qin, X.Y. Effect of specific heat variations on irreversible Otto cycle performance. Int. J. Heat Mass Transf. 2018, 122, 403–409. [CrossRef] 66. Shi, S.S.; Ge, Y.L.; Chen, L.G.; Feng, F.J. Four objective optimization of irreversible Atkinson cycle based on NSGA-II. Entropy 2020, 22, 1150. [CrossRef] 67. Wu, Z.X.; Chen, L.G.; Feng, H.J. Thermodynamic optimization for an endoreversible Dual-Miller cycle (DMC) with finite speed of piston. Entropy 2018, 20, 165. [CrossRef][PubMed] 68. Rocha-Martinez, J.A.; Navarrete-Gonzalez, T.D.; Pava-Miller, C.G.; Ramirez-Rojas, A.; Angulo-Brown, F. A simplified irreversible Otto engine model with fluctuations in the combustion heat. Int. J. Ambient Energy 2006, 27, 181–192. [CrossRef] 69. Rocha-Martinez, J.A.; Navarrete-Gonzalez, T.D.; Pava-Miller, C.G.; Ramirez-Rojas, A.; Angulo-Brown, F. Otto and Diesel engine models with cyclic variability. Rev. Mex. Física 2002, 48, 228–234. 70. Klein, S.A. An explanation for observed compression ratios in international combustion engines. J. Eng. Gas Turbine Power 1991, 113, 511–513. [CrossRef] 71. Wang, H.; Liu, S.; Du, J. Performance analysis and parametric optimum criteria of a regeneration Bose–Otto engine. Phys. Scr. 2009, 79, 055004. [CrossRef]