Engineering Engineers, Part D: Journal of Automobile

Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering http://pid.sagepub.com/

A review of energy consumption, management, and recovery in automotive systems, with considerations of future trends Fabio Chiara and Marcello Canova Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering 2013 227: 914 originally published online 25 March 2013 DOI: 10.1177/0954407012471294 The online version of this article can be found at: http://pid.sagepub.com/content/227/6/914

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Review Article

A review of energy consumption, management, and recovery in automotive systems, with considerations of future trends

Proc IMechE Part D: J Automobile Engineering 227(6) 914–936 Ó IMechE 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954407012471294 pid.sagepub.com

Fabio Chiara and Marcello Canova

Abstract In response to the current and future energy and environment challenges, the automotive industry is strongly focusing on improving the fuel efficiency of vehicles. Although the electrification of automotive powertrains is clearly the principal path towards sustainable transportation, many opportunities still exist to improve the fuel economy of conventional vehicles. However, some of the technical solutions representing the state of the art in research and advanced development are difficult to benchmark in terms of their potential benefits for fuel consumption improvement. A greater understanding of the fuel energy utilization on the vehicle (here intended as a ‘system’ ) is therefore necessary in order to identify the readily available opportunities for efficiency improvements and, ultimately, to develop automobiles which are more fuel efficient. To this extent, this paper presents a review of the state of the art and technology trends in the field of energy management and recovery for automotive systems, with the primary focus on conventional powertrains. An understanding of the fuel energy utilization and dissipation associated with the vehicle subsystems (the engine, transmission, and chassis) is provided, as well as an overview of the opportunities and potential challenges in improving the fuel economy through system-level energy management, recovery, and harvesting. Finally, an overview of the most important solutions for managing energy dissipation, energy recovery, and harvesting is presented, discussing their potential for fuel economy improvement, technical readiness, and challenges. Wherever possible, projections on fuel economy improvements, based on either experimental data or simulations, are reported to provide opportunity for the assessment and comparison of current and future technologies.

Keywords Energy consumption, fuel consumption, energy management, energy recovery, internal combustion engine, transmission, chassis

Date received: 8 June 2012; accepted: 12 November 2012

Introduction Personal transportation is today highly dependent on the automobile. In the USA, there are approximately 240 3 106 light-duty vehicles (LDVs), including about 135 3 106 cars and 105 3 106 light-duty trucks, leading to an estimated fuel consumption of approximately 145 3 109 gal in 2008.1 According to the US Energy Information Administration (EIA), the fuel utilization by US cars and light-duty trucks accounts for approximately 44% of US oil consumption and some 10% of world oil consumption.1,2 Furthermore, the EIA estimates that more than 60% of liquid fuels used in the country will be imported during the next 25 years. Although recent fuel economy legislation and increased fuel prices have marginally lowered energy

use, aggregate fuel consumption data from LDVs had been projected2 to grow at a rate of 0.3% per year, presenting an extremely challenging energy and environment problem. The first efforts towards a reduction in the fuel consumption of passenger cars started to appear in the late 1970s, following the energy crisis in 1973–1974, and the introduction of the Energy Policy and Conservation

Center for Automotive Research, The Ohio State University, Columbus, Ohio, USA Corresponding author: Marcello Canova, Center for Automotive Research, The Ohio State University, 930 Kinnear Road, Columbus, OH 43212, USA. Email: [email protected]

Chiara and Canova Act of 1975, establishing mandatory fuel economy standards for automobiles and light-duty trucks.3 However, since the mid-1980s, the average fuel consumption of US LDVs has remained nearly constant. As shown in Figure 1, this apparent counterintuitive result can be explained by observing that the increase in the vehicle efficiency consequent to advances in vehicle technologies has been largely traded off against increases in other attributes such as the acceleration power, size, and mass of the vehicle.6 The overall trend in the car and light-duty truck performance in terms of the horsepower, mass, size, and acceleration in the last 40 years can be separated into four phases.7 The first phase (1977–1981) shows a modest deterioration in the vehicle performance and a substantial reduction in the fuel consumption (27% for passenger cars and 22% for light-duty trucks respectively), primarily as a consequence of the high fuel prices induced by the 1970s oil crisis. In the second phase (1982–1987), a considerable pushback in the reduction in the fuel consumption, in favor of performance improvement could be observed. The fuel consumption of new cars and light-duty trucks decreased by 7.5% and 5.4% respectively as the fuel prices began to decline. In the third phase (1988–2005), the market for a reduction in the fuel consumption experienced neither a pull through high fuel prices nor a push through more stringent corporate average fuel economy (CAFE) standards. Gains in the vehicle and powertrain efficiency have been used largely to increase the vehicle performance attributes such as the power and mass, while keeping the fuel consumption essentially constant. The same period has also seen a shift away from passenger cars towards light-duty trucks, particularly sport utility vehicles (SUVs) and minivans. The combined result of these trends translated into a

915 steady growth in US LDV fuel use since the late 1980s. The fourth phase (2006–2010) marks a radical change in the past trend, with a marked increase in the LDV fuel economy, consequent to the surge in the fuel costs followed by the 2008 economic crisis. This new direction for the industry and consumers is confirmed also by the sharp decrease in the sales of light-duty trucks and SUVs which occurred in the same period. In response to the above issues, the automotive industry is today oriented towards developing vehicles which are more fuel efficient. Often opportunities for fuel economy savings within vehicle systems are overlooked since their potential benefits are difficult to quantify. For this reason, an improved understanding of the sources of energy utilization in a vehicle system is necessary for developing vehicles which are more fuel efficient. This paper condenses a thorough literature review focusing on the state of the art and technology trends in the field of energy management and recovery for automotive systems. The objective is to develop an understanding of utilization of the vehicle energy, as well as the opportunities and potential challenges in improving the fuel economy through system-level energy management, recovery, and harvesting. While the main focus of this analysis is to describe and assess technologies for conventional powertrain systems, electrification and hybridization are also mentioned as part of the available opportunities for improvement in the fuel economy. The starting point of the paper is a thorough analysis of the major sources of energy dissipation for vehicles, with the objective of identifying the vehicle subsystems and components where most of the fuel economy improvements could be achieved through research and technology development.

Figure 1. (a) Trends in the US fuel economy and (b) comparison with other vehicle characteristics.4,5 MPG: miles/gal; mph: miles/h; $: US$.

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Proc IMechE Part D: J Automobile Engineering 227(6) where M and A are the mass and front area respectively of the vehicle, Iw and rw are the inertia and radius respectively of the wheel, and CD and Cr are the aerodynamic drag coefficient and the tire rolling resistance coefficient respectively. Based on the sign of the power at the wheel, three modes are identified for fuel consumption analysis, _ w . 0), the braking phase namely the traction phase (W _ w = 0). Assuming _ w \ 0), and the coasting phase (W (W that the parameters appearing in equation (1) are independent of the vehicle speed, the power at the wheel can be integrated over a driving cycle to obtain the vehicle’s energy consumption 0 1 0 1 ð ð B C B C E w = aa @ V3 dtA + ar @ V dtA cycle

0

Figure 2. (a) US EPA Urban Driving Schedule and (b) US EPA Highway Driving Schedule for evaluation of the fuel economy.4

1

ð

B + ai @

V

cycle

ð2Þ

dV C dtA dt

cycle

Then, a summary of currently available and future technologies for managing energy dissipation for conventional vehicle will be introduced, focusing primarily on the powertrain and chassis (braking and suspension system). Technologies for energy recovery and harvesting will be also reviewed in detail. Finally, the main technical challenges for energy recovery and harvesting will be summarized, together with suggestions on the potential for vehicle implementation.

Analysis of the vehicle’s energy demand in regulatory driving cycles Published studies on the energy utilization and fuel consumption of passenger cars started to appear in the late 1970s. Among others, Evans et al.8 were first to evaluate the factors that mostly affect the fuel economy of vehicles in urban traffic, based on extensive experimental studies. More recently, Sovran and Blaser9 presented a thorough analysis of the physics of the fuel economy of motor vehicles in a concise and understandable form. It is well known that the fuel consumption of a vehicle is largely dependent on the driving profile. To this end, most fuel economy studies presented are based on the US Environmental Protection Agency (EPA) Urban and Highway Driving Schedules4, shown in Figure 2. In the work by Sovran and Blaser,9 the sources of the fuel consumption of vehicles are evaluated on the basis of a longitudinal dynamics model for a vehicle which computes the force and propulsive power required to follow the above driving schedules. The result is the road load equation, used to calculate the power at the wheel5 and given by   1 Iw dV 3 _ Ww = Cr MgV + CD ArV + M + 4 2 V 2 dt rw ð1Þ

where aa = 12rCD A ar = Cr Mg Iw ai = M + 4 2 rw

ð3Þ

The above formulation separates the vehicle parameters from the integral terms, which are solely dependent on the vehicle’s speed profile. This simple approach allows immediate evaluation of the impact of several vehicle parameters on the vehicle’s energy consumption and, ultimately, on the fuel economy. For instance, Sovran and Blaser9 indicated that the dominant source of the vehicle’s energy consumption in urban driving conditions is the vehicle inertia (58% of the tractive power at the wheel), owing to the several stop–start operations. Conversely, aerodynamic losses become predominant in highway driving (50% of the tractive power at the wheel), because of the higher average velocity. Other factors analyzed in the paper are the influence of the drivetrain efficiency, the efficiency of the internal-combustion (IC) engine, and the energy required to power the vehicle’s auxiliary loads. Representative values of the fuel consumption parameters are finally given for a midsize passenger car. A similar study was conducted by Baglione et al.,10 where an analysis simulation tool for the vehicle energy was developed to provide a breakdown of the various sources of energy consumption for a full-size all-wheeldrive pick-up truck over an urban driving cycle. Along the same lines, a comparative analysis of energy losses for a passenger car and light-duty truck was presented by Yanni and Venhovens,11 considering the cycles shown in Figure 2. The results presented in the paper show that the light-duty truck requires 60% (urban)

Chiara and Canova and 80% (highway) more energy than a passenger car to complete the same driving cycle. A sensitivity analysis was conducted to evaluate the effect of different parameters on the vehicle’s energy consumption. It was found that a 10% drop in the vehicle mass has, in most cases, the largest impact because it addresses the energy needs associated with the accelerating inertia and rolling resistance of the vehicle at the same time. Other studies provide insights into specific sources of the vehicle’s energy loss, including the aerodynamic resistance11,12 and the tire’s rolling resistance,13,14 as well as the propulsion, transmission, and driveline losses,15–19 quantifying their contribution through modeling and experimental approaches. Parasitic losses, introduced by the vehicle’s ancillary loads, are another source of fuel economy penalty.20 An account of the efforts conducted by the automotive industry in reducing the fuel consumption in LDVs was provided by Hochgraf,21 based on an analysis of the changes in the vehicle’s energy demand and powertrain efficiency of a wide spectrum of vehicles. Various vehicle chassis and powertrains are then ranked by the most efficient powertrains and lowest-energyconsumption vehicle chassis. A regression analysis of the vehicle’s fuel consumption versus the estimated test mass shows that 74% of the variation in the vehicle’s energy consumption can be explained by the mass of the vehicle and 25% is due to other factors such as the aerodynamic drag. In the paper by Hochgraf and Duoba,22 an extension of the above study was presented to provide a clear understanding of the contributions of hybrid and nonhybrid drivetrains to the vehicle’s fuel consumption. The study highlights the fact that hybrid powertrains are not the only way to reduce the vehicle’s fuel consumption significantly. In this sense, reductions in the vehicle mass and auxiliary energy demand are equally important. Similarly, Sovran and Blaser23 evaluated the impact of regenerative braking on the fuel consumption of a hybrid vehicle using the analysis tools which they had originally developed earlier.9 The conclusion that a mass reduction is a valuable path towards reducing the vehicle’s energy demand has been presented by several other researchers (for instance, see the studies by Wohleceker et al.,24 the National Research Council,25 and Stodolsky and Cuenca26). A detailed study on the impact of the vehicle mass on the fuel economy was presented by An and Santini,27 based on a comparison of 12 conventional and hybrid vehicles in an urban driving cycle. As expected, significant fuel consumption was found to be related to the vehicle inertia and idling losses, leading to a fuel efficiency for a vehicle of around 17% (based on the model year 2004). Along the same lines, Pagerit et al.28 investigated the impact of a reduction in the vehicle mass for several vehicle platforms and advanced powertrain technologies, also considering the effects of the driving cycles and the propulsion system’s efficiency.

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Analysis of energy conversion and dissipation in automotive systems In order to understand the potentials for managing and recovering energy in automotive systems, the basic principles of energy analysis are here introduced, focusing on the most relevant vehicle subsystems and components. This allows the ‘tank-to-wheel’ energy conversion process to be understood, and hence how the fuel energy is utilized in the vehicle. The analysis also allows identification of the primary sources of energy dissipation.

Internal-combustion engine system It is well known that the primary source of the fuel consumption of a vehicle is due to the energy conversion processes that occur in the IC engine. Studies leveraging the use of the first and second laws of thermodynamics to study IC engines have been published for over 40 years and are today available in the literature (for instance, see the book by Heywood,29 the book by Benson and Whitehouse,30 and the paper by Caton31 for a review of several technical contributions). To understand the root causes of energy dissipation, a simple energy balance can be formulated for an IC engine, considering the in-cylinder processes in steadystate operating conditions.32–34 With reference to the simplified block diagram shown in Figure 3, a simplified energy balance can be given as _ b +W _ mÞ ðm_ f QLHV + Q_ ht + Q_ cl Þ  ðW = m_ exh hexh  ðm_ a ha + m_ f hf Þ

ð4Þ

where QLHV ð . 0Þ is the fuel’s lower heating value (kJ/kg), Q_ ht ð \ 0Þ represents the wall’s heat transfer losses (kW), Q_ cl ð \ 0Þrepresents the losses due to _ b ð . 0Þ is the incomplete combustion products (kW), W _ m ð \ 0Þ represents the friction brake power (kW), W losses (kW), m_ is the mass flow rate (kg/s), and h is the sensible enthalpy (kJ/kg) associated with the air, fuel, and exhaust gases. Similarly, a second-law analysis can be conducted to identify the amount of exergy destruction, which can be used to identify the processes on which further studies must be concentrated for better energy source utilization.32,33,35–37 Starting from the same assumptions, the exergy balance for the engine system shown in Figure 3 is given as  X X T0 _ _ cv + m_ a ea W 0= 1 Qcv  Tj ð5Þ j j + m_ f ef  m_ exh eexh  E_ d where e = h  h0  T0 ðs  s0 Þ represents the exergy of the fluid and Tj is assumed to be a reference temperature at which the heat transfer processes listed in equation (4) occur.

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Figure 3. Block diagram for engine energy balance. IC: internal-combustion.

Equations (4) and (5) can be used effectively to provide a relatively accurate energy and exergy analysis of the engine. Simple ideal-gas models (with temperaturedependent properties) can be used to calculate the energy and exergy flows for the air, fuel, and combustion products. The loss due to incomplete combustion can be calculated from the analysis of engine-out emission data, by applying a mass and energy balance. The energy balance in equation (4) can then be applied to identify the rate of thermal energy rejected to the cylinder walls and, ultimately, to the coolant. The exergy balance can then be applied to complete the analysis. Examples of application of the first- and second-law analysis have been reported32–37 in order to investigate the effects of the operating parameters and design variables on the energetic and exergetic efficiencies of IC engines. In particular, Edwards et al.36 reported a thermodynamic analysis of a turbocharged diesel engine conducted through a high-fidelity gas dynamic simulation code calibrated on experimental data to obtain an accurate prediction of the engine friction, heat losses and temperatures of the cylinder wall. The results presented are useful in providing an insight into the thermodynamics of the engine’s energy conversion. Detailed explanation of the energy lost in the various components of the engine was also provided. Farrell et al.37 recently conducted a thermodynamic analysis of IC engine processes, through a predictive engine combustion model. The study compared the energetics of port fuel injection, lean-burn direct injection and ultra-lean-burn direct injection. The results show that a peak efficiency of 35–43% is achievable with state-of-the-art hardware. Although lean-burn operations lead to a higher engine thermal efficiency, owing to reduced heat transfer and exhaust losses, combustion irreversibilities tend to increase because of lower flame temperatures. In general, the studies focusing on first- and secondlaw analysis of IC engine processes tend to converge on the following remarks.34–36 1.

The energy dissipation in IC engines is predominantly caused by the heat transfer losses and by the energy carried by the exhaust gases. Friction,

2.

although marginal if compared with the former two causes, is another root cause for the reduction in the brake efficiency. The availability loss (i.e. the potential to do useful work) is caused principally by the irreversible combustion process. Furthermore, the heat transfer to the walls as well as the exhaust gas exergy flow are also significant.

In summary, the main contributions found in this field indicate that the thermal energy rejected to the coolant and to the exhaust gases is the predominant source of energy and availability losses in IC engines.35,38 This delineates a clear path towards the development of design and system integration solutions oriented towards mitigating engine energy dissipation.

Auxiliary loads The parasitic losses associated with the engine’s auxiliary loads, such as the electrical system, the airconditioning (AC) system, the engine cooling system, and the power steering system, represent another considerable fraction of the energy consumed by the vehicle.35,38 Since the auxiliary systems are generally driven by the engine crankshaft through a belt drive, they often work inefficiently, drawing power from the engine in operating conditions where the engine itself operates in far-from-optimal conditions. Few contributions have been proposed recently on the energy analysis of the vehicle’s auxiliary loads. A broad analysis of the contributions to the fuel consumption of vehicles caused by the auxiliary units has been proposed by Fenske et al.,20 Rumbolz et al.,39 and Pettersson and Johansson,40 with reference to both light-duty and heavy-duty applications. In particular, the experimental study conducted by Rumbolz et al.39 on a vehicle fleet reports that about 8% of the fuel energy is used to operate the auxiliaries. More recently, experimental studies on the impact of the parasitic loads on the fuel economy of vehicles have been reported by Campbell et al.41 and Muncrief et al.,42 with reference to a hybrid electric urban bus.

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Other studies, for instance those by Lyu et al.,43 Pirotais et al.,44 and Lin and Sunden,45 point towards the AC system and the engine cooling system as the predominant power consumers at the engine crankshaft, with a potential increase in the vehicle’s fuel consumption by up to 30% for US EPA Urban Driving Cycle conditions. Earlier studies conducted at the National Renewable Energy Laboratory reported an increase in the fuel consumption for different vehicles when the AC system is in use.46,47 It is seen that AC system operation resulted in a 20–25% increase in the fuel used over the Federal Testing Procedure (FTP) driving cycle. Measured power consumption values for automotive AC systems have also been reported,45,48–50 indicating that the AC compressor typically absorbed up to 4 kW, which in turn means a fuel consumption of up to 0.5 l/100 km. The power consumption related to the cooling system is also significant,51–53 with the coolant pump and radiator fan absorbing up to 1.5 kW on a mid-size passenger car.38,51,52 An energy analysis of different power steering systems, based on validated simulation models, has been presented,54,55 showing that conventional steering systems with a mechanically driven pump consume up to 1 kW (mid-size passenger car and urban driving). Note that the power steering power consumption is typically higher at idle and low-load conditions, where the engine is already operating inefficiently.54 Few studies characterize the energy consumption of vehicle electric loads. For example, Boulos et al.56 presented the simulation results on an electrical system for a luxury passenger car, with a detailed analysis of the power consumed by electrical loads (lighting, audio and navigation, heating, etc.).

Transmission system Energy losses occur in the vehicle’s transmission system, owing to friction of the moving parts, as well as the presence of auxiliary loads (pumps, actuators, and cooling system). Typically, the power loss in a transmission can be obtained from the efficiency data and the input power5 according to _ loss, tr = ð1  htr ÞW _ in W

ð6Þ

_ in is the engine’s brake power, determined from where W the engine’s energy balance in equation (4) minus the power consumed by the auxiliary loads. As a first approximation, the transmission efficiency could be considered primarily a function of the gear ratio. However, since most of the losses in transmission elements involve friction of the lubricated contacts, fluid churning, or fluidic coupling, the efficiency is also a strong function of the viscosity of the oil or lubricant, and hence of the operating temperature. A good starting point for the analysis of energy losses in transmission systems has been provided by

Table 1. Indicative efficiency value in each gear for a five-speed manual transmission and automatic transmission.57 Efficiency (%) for the following

Gear 1 Gear 2 Gear 3 Gear 4 Gear 5

Manual transmission

Automatic transmission

92–96 92–97 93–97 93–99 92–97

60–85 60–90 85–95 85–95 83–94

Kluger and Long,57 where typical efficiency values at different gears are compared for manual transmissions, automatic transmissions, and continuously variable transmissions (CVTs). Indications are also given regarding pathways for efficiency improvements. The efficiency values reported in the paper for typical fivespeed manual and automatic transmissions are summarized in Table 1. Similar results have been reported more recently,58 comparing the performance and potential for improvement in the fuel economy of different transmission technologies.

Vehicle chassis: braking and suspension systems Even though the chassis system does not directly contribute to fuel consumption, equation (1) shows a direct correlation between the vehicle mass and the power demand at the wheel. As a rule of thumb, about 3.5 g of carbon dioxide (CO2) per kilometer can be saved for every 100 kg mass reduction.59 The energy dissipation analysis related to the braking actuation system and the vehicle’s suspension system is targeted to investigate the potential of advanced technologies to improve the fuel economy. Results on the energy dissipation of conventional (passive) automotive suspensions have been given by Beno et al.60 and Soliman,61 indicating that the dissipation could be reduced by up to 15% through the use of active systems. Similar results have been shown for the hydraulic system of the brakes, justifying the introduction of ‘bywire’ technologies such as electrohydraulic or electromechanical systems.62–64

Summary Although most of the sources reviewed in the previous sections focus on very specific issues related to the vehicle’s energy utilization and dissipation, few studies have been able to provide a sufficiently broad analysis of the complete energy utilization for a vehicle. For instance, Baglione et al.,10 Fenske et al.,20 Patton et al.,65 Delorme et al.,66 and An and DeCicco67 have presented data on the energy consumption of passenger cars and light-duty trucks for US EPA regulatory cycles.

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Figure 4. Block diagram of power flow in a vehicle for fuel energy calculation. IC: internal-combustion.

Table 2. Summary of vehicle data for different classes. Data for the following

Engine type Transmission type Mass (kg) Front area (m2) Tire radius (m) US EPA city (miles/gal) US EPA highway (miles/gal)

Mid-size sedan

Light-duty truck

L4, 2.4 l, SI Five-speed automatic 1100 2.2 0.32 23 34

V6, 3.5 l, SI Six-speed automatic 1550 3.2 0.38 15 20

SI: spark ignition; US EPA: US Environmental Protection Agency.

The above information can be integrated with the model-based analysis of the vehicle energy to provide a comprehensive overview of the typical energy utilization for LDVs in urban and highway conditions.5,68,69 Figure 4 provides a typical power flow representation of a vehicle, which, combined with equations (1) to (6), allows the sources of energy dissipation to be isolated. To this extent, a complete energy analysis of a midsize passenger car and a light-duty truck is here proposed with reference to the US EPA City and Highway Driving Cycles (shown in Figure 2). The data were distilled from the results collected in the literature review and integrated with the results of simulation tools for the vehicle energy,68–72 using the vehicle data summarized in Table 2. The results obtained are intended to provide a qualitative representation of the vehicle’s energy consumption for a specific vehicle category. The results of the analysis are summarized in Figures 5 to 8, normalized on the basis of the fuel energy. The results obtained agree with the work by Ricardo, Inc.,73 where simulation of the vehicle energy was conducted for a mid-size sedan. Figure 5 summarizes the energy utilization for a mid-size sedan on the US EPA City Driving Cycle. In urban driving conditions, the brake energy produced by the engine is about 21% of the fuel energy, where 65% is rejected to the coolant and the exhaust gases. A significant portion (12%) is also lost because of pumping (i.e. throttling) and friction, while a minor portion is spent for the auxiliary loads (coolant pump and oil pump). Note that, according to the US EPA

Figure 5. Energy utilization on a US EPA City Driving Cycle for a mid-size sedan.

specifications, the energy losses due to the AC system are not considered. A significant fraction of the brake energy is then dissipated in the transmission, dividing the remaining between the vehicle resistance and drag, idling, coasting, and braking. Note that the idling and braking energy losses, which account for about 6% of the fuel energy, can be potentially recovered through a mild hybridization of the powertrain, enabling engine stop–start and regenerative braking.5,7,23 The breakdown of the energy usage changes slightly for the US EPA Highway Driving Cycle, as in Figure 6. Here, the engine operates more closely to the peak brake efficiency region, leading to decreases in the pumping and friction losses. The energy rejected to the exhaust also decreases, because of the decreases in

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Figure 7. Energy utilization on a US EPA City Driving Cycle for a light-duty truck. Figure 6. Energy utilization on a US EPA Highway Driving Cycle for a mid-size sedan.

the idling and acceleration. However, the energy rejected to the coolant shows a marginal increase, for the same reason as above. At the vehicle level, it can be observed that the majority of the brake energy is spent to overcome the aerodynamic resistance and tire resistance. On the other hand, the fraction of brake energy that could be potentially recovered in urban driving is decreased significantly, showing how the benefits of vehicle hybridization become limited in highway conditions. A summary of the energy performances of light-duty trucks is shown in Figure 7 and Figure 8. In urban driving conditions, the fuel energy breakdown is rather similar to that of a mid-size car. This can be explained by the fact that the slight increase in the peak brake efficiency of the engine that comes from the increased engine displacement (mostly through improvement in the combustion efficiency due to friction reduction) is balanced by the more significant part-load operations of the engine and by the increased accessory loads. At the vehicle level, the marginal improvement in the transmission efficiency is largely absorbed by the increased rolling and aerodynamic losses. The effects of the larger inertia of the vehicle are also visible in the increased coasting and braking losses. On the other hand, a more significant fraction of the vehicle energy could be recovered through hybridization, justifying

Figure 8. Energy utilization on a US EPA Highway Driving Cycle for a light-duty truck.

922 the large number of full-size hybrid powertrains among SUVs and trucks.

Managing energy dissipation: state of the art and technology trends A large portion of the sources referenced in the section on the analysis of the vehicle energy contain indications of potential paths towards a reduction in the energy dissipation.13,18,36,45,49–51,55,59–61,65–67 The objective of this section is to provide an overview of the current and future technologies aimed at improving the vehicle’s fuel economy through a reduction in and management of the energy dissipation of different vehicle systems.

Managing the engine’s energy dissipation Several technologies for IC engines have been examined in the context of their potential benefits in reducing energy dissipation and classified on the basis of the mechanisms by which fuel consumption benefits are realized. They include technologies for improving the engine thermodynamics, friction reduction, and engine thermal management. Each of the above-mentioned topics are described briefly in the following paragraphs. Improving the engine thermodynamics. Improvements to the thermodynamic cycle of the engine and, in a broad sense, to the air-path, fueling, and combustion system, are typically oriented towards increasing the thermal efficiency of the engine. It is well known that a higher in-cylinder pressure and temperature, which are the result of fast-burn combustion systems, lead to a higher thermal efficiency of SI engines as a result of the higher fuel burn rate.74 High fuel burn rates are made possible by developing more turbulence in the air–fuel mixing process and obtaining a more uniform charge formation. This is the concept, for example, behind homogeneous charge compression ignition (HCCI) engines. HCCI combustion involves a premixed homogeneous charge of air, fuel, and residual gases which is combusted by autoignition.75–77 The autoignition process results from tight control of the pressure and temperature conditions within the cylinder. Fuel economy benefits of over 16% over a US EPA City Driving Cycle with HCCI implementation on a baseline 2.3 l direct-injection (DI) engine with twin-independent variable valve timing (VVT) and exhaust gas recirculation (EGR) have been demonstrated.75 Several other studies have provided insight into the fuel economy benefits of implementing HCCI technology.74,76–80 Controlled autoignition (CAI) is another promising combustion technology that allows the full potential of HCCI to be harnessed but with fewer risks in terms of pre-ignition and engine knock.80–83 It is worth noting that HCCI, CAI and lowtemperature combustion (LTC) modes at large lead to

Proc IMechE Part D: J Automobile Engineering 227(6) reduced exhaust gas temperatures, hence reducing the availability of the exhaust gas energy. This could potentially prevent the use of waste-heat recovery technologies for exhaust thermal energy harvesting. A new innovation in the combustion systems of an engine is the introduction of DI systems. DI systems have evolved with the aid of flexible and reliable common-rail fuel injection systems. In the paper by Andriesse and Ferrari,84 a gasoline direct-injection (GDI) system implemented on an engine has been demonstrated to achieve fuel economy benefits up to 10% compared with the baseline results from the same engine but without GDI. Other work85,86 also provided simulation results as well as results from test data on vehicles showing reduced fuel consumption, high torque and better power characteristics compared with those of conventional port fuel injection engines. Other combustion-related technologies leading to a better engine performance and a lower fuel consumption are worth mentioning for their benefits, in particular, deceleration fuel shut-off,87 load regulation,88 cylinder deactivation,89–93 LTC,94 swirl control methods,95,96 and premixed charge compression ignition.97–99 Modifications to the designs of the engine’s air-path system are typically required when making changes to the combustion systems. A description of techniques has been provided by Guzzella et al.100 and Lancefield,101 such as charge mixture control, adaptation during transient operations, and increased compression ratio, together with tuned intake and exhaust systems and EGR which are used in modern engines. Such engines provide characteristics with a performance that is better than or the same as the baseline while reducing the fuel consumption by up to 15%. Downsized engines are being increasingly deployed in production vehicles. Engine downsizing by 30% is shown to improve the fuel economy by 8–10% while improving the torque and acceleration performances. Furthermore, downsizing by 40% has been estimated100–102 to improve the fuel economy by more than 20%. To avoid penalizing the fuel economy, downsizing and turbocharging are typically accompanied by a combination of technologies such as dual-cam phasing, VVT, and DI.101–103 These systems allow utilization of the complete potential of a boosted and downsized engine for improving the fuel economy of a powertrain by operating in the most efficient regions of the engine map. To aid engine operation in most efficient regions of the engine map, Honda introduced a variable valve actuation (VVA) system for the first time in a production vehicle capable of simultaneously varying the opening instant and lift of the intake and exhaust valves.103 The system, which is still in production today, is known as variable valve timing and lift electronic control (VTEC). The timings for intake valve closing (IVC) (just before the advent of the compression stroke) and exhaust valve opening (EVO) (just

Chiara and Canova after the end of the power stroke) greatly affect the torque and power characteristics of an engine. Using VVA systems, the IVC and EVO can be optimized. This is because, depending on when the intake valve closes, the volumetric efficiency and therefore the amount of fresh charge entering the engine change. In addition, by appropriately modulating the times at which the intake and exhaust valves close and open, pumping losses of the engine can be reduced, leading the engine to operate in higher-efficiency conditions. Various automotive companies have introduced VVA systems with newer additions to their fleet as successful implementation of VVT and VVA technologies with promises74 for an improvement in the fuel consumption of the order of 8–9%. Several other reports have provided details on the benefits of VVA technology,101–103 and others have outlined the obstacles to global implementation of this technology.103–105 Reduction in the engine friction. The contribution of friction to engine losses has been explained in detail by Weinrich and Bargende,34 Hoshi,106 Kurbet and Malagi,107 Nakada,108 and Rundo.109 Of the total frictional losses, about 53% is contributed by the piston shaft and liner. The cam shaft and valves, as well as the crankshaft, together contribute about 26% to the rest of the friction losses.34 One method that is suggested to reduce the friction losses is to increase the operating temperature of engines. Also, the friction during warmup was found to be twice the value of an engine already warmed up.110 Other ways of reducing friction includes refining the piston shape, reducing the diameter and width of bearing in the crankshaft and connecting-rod system, using lightweight valves and rocker arms, decreasing the loads on the valve spring, and substituting multi-belts for conventional V-belts.106 Lowering the viscosity of the lubricating oil can also influence the friction losses.109 However, Nakada108 suggested that the reduction in friction can present associated problems such as increased wear, increased oil consumption, and poor reliability. Engine thermal management. Engine cooling strategies have recently evolved with the use of electronic components in the cooling system.112–116 Electric pumps, thermostats, and fans play major roles in decreasing the losses. The contribution of friction towards the engine losses was explained in the previous topic. The measures to reduce friction, such as increases in the coolant and lubricant temperatures, will also help in the thermal management of the engine. Use of electric pumps115 and hybrid coolant pumps116 reduces the parasitic losses from auxiliary components and allows better control of the engine temperature for the best engine efficiency.117 The thermal management system (TMS) developed by Ribeiro et al.115 improved the compactness of the

923 cooling module by adjusting the spacing of the radiator fins. This system also used the waste heat from the engine to raise the temperature of the transmission oil. An electric radiator fan was also used for improved performance. More recently, Laboe and Canova118 presented a system-oriented approach to recover waste heat from the powertrain to reduce the warm-up time and to increase the operating temperatures of the transmission and engine oils. The test data presented for the vehicle show an improvement in the fuel economy of up to 4.4% on regulatory cycles. Another improvement is the introduction of components having ‘on-demand’ operational capabilities. Electric coolant pump, electronic thermostats, active grille shutters, and variable-speed fans are some of these components.113–116,119 An active grille shutter allows the flow of air through the grille based on the demand of the cooling system on the AC system and also improves the fuel efficiency of the vehicle by reducing the aerodynamic drag. Wind tunnel tests on a vehicle at 105 km/h showed a 3% reduction in drag using an active grille shutter.119

Managing powertrain energy dissipation Improvements to the existing powertrain setup and introduction of new technologies are analyzed in this section. Since transmission technologies directly influence the fuel economy and drivability of the vehicle, greater emphasis is given to them. Transmission system. The most immediate approach towards improving the efficiency of transmission systems is to increase the number of gears. Recently, eightspeed120 and nine-speed121 automatic transmissions have been put in production, with a potential for up to 16% fuel economy improvement compared with a sixspeed automatic transmission. Similarly, dual-clutch transmissions (DCTs) have been shown to improve the fuel economy by up to 15% compared with conventional transmissions with planetary gear sets.122,123 A recent example is the Volkswagen seven-speed DCT, which shows a significantly higher efficiency in lowtorque and low-speed conditions and incorporates a low-capacity transmission oil pump together with an accumulator to reduce the pump’s parasitic loads.124 Similarly, CVTs,125,126 automated manual transmissions,58 and infinitely variable transmissions125 have also been proposed in the past few years. Several companies have recently introduced advanced torque converters with improved hydraulic circuits, torque converter lock-up clutch, and pendulum dampers to reduce the primary firing order torsional vibrations. The addition of pendulum dampers permits lower lock-up speeds to be achieved, thereby decreasing the torque converter losses and helping to improve the fuel economy.127,128

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Table 3. Transmission technologies and estimated reduction in the fuel consumption. Technology

Reduction in the fuel consumption (%)

Five-speed automatic transmission Six-speed automatic transmission Seven-speed automatic transmission Eight-speed automatic transmission Six-speed DCT CVTs

2–3 3–5 5–7 6–8 6–9 1–7

DCT: dual-clutch transmission; CVT: continuously variable transmission. Source: reproduced with permission from Committee on the Assessment of Technologies for Improving Light-Duty Vehicle Fuel Economy, National Research Council.74

The effect of an increased gear number for automatic transmissions was studied previously.74 When compared with a six-speed automatic transmission, a state-of-the-art eight-speed transmission shows up to 8% reduction in the fuel consumption over regulatory cycles. Table 3 (data from previous work74) summarizes the potential for a reduction in the fuel consumption for various transmission technologies, when compared with an SI-engine vehicle with a four-speed automatic transmission of similar output. Even though the increase in the number of gears improves the drivability and transmission performance, the complexity and cost of most of these technologies can be limiting factors to their deployment. Other modifications to transmission systems have been proposed to reduce friction. One example was the introduction of torque converter to a CVT which uses a wet-type multi-plate clutch.129 This enhancement helped to lower the CVT friction by 8% and to increase the gear ratio by 14%. Introduction of lightweight, high performance, and robust bearings into a transmission system can also improve the energy efficiency.130,131 Finally, reducing the viscosity of the transmission fluid (for instance, through thermal management) has been shown to improve the efficiency of a transmission system.118,125 Electrification of the auxiliary loads. An increase in the auxiliary load on a vehicle will have an impact on its fuel economy. In a conventional vehicle, the AC system is the largest auxiliary load. The contribution46 of AC systems to the total emissions is about 30%. Automotive AC power consumption is split between the compressor’s mechanical power and the air fan’s electric power. In this sense, electrically driven compressors and fans offer the opportunity to decrease the load on the engine by enabling on-demand operations. In addition, the cycling of the compressor can be optimized to exploit the crankshaft energy during deceleration of the vehicle.132 Other solutions, for instance the introduction of ventilated seats, have been proposed as a means for fuel consumption improvement.133

Another component where electrification can benefit energy consumption is the steering system. The conventional hydraulic-power-assisted steering could be replaced by electrohydraulic-power-assisted steering and electrically assisted power steering. Electrohydraulicpower-assisted steering replaces the pump on the combustion engine with a pump driven by an electric motor, therefore enabling on-demand use. With electrically assisted power steering, future steering requirements, such as park assistant, lane-keeping assistant, and sidewind compensation, can be easily implemented.134 Other potentials of ancillary load reduction include optimizations of the alternator and a reduction in the masses of the front-end accessory drive systems.135–137 With increasing electrification needs, the voltage of the electrical system might be increased in the near future. For instance, 36 V and 42 V systems have been proposed, also in conjunction with starter–alternator technologies.138–141

Managing vehicle and chassis energy dissipation Usage of lightweight material for the vehicle body, tire materials, and streamlining can contribute to the management of energy dissipation in vehicles. In addition, management techniques for energy dissipation in the chassis are available for deployment, such as brake-bywire and active suspension systems. Reduction in vehicle mass. The introduction of lightweight materials has led to a reduction in the mass of the vehicle by 30% over the past 35–40 years.25–27 The two primary design trends for achieving a reduction in the masses of vehicles are the use of lightweight materials and the modification of the chassis and frame design. Materials such as aluminum and magnesium alloys, high-strength steels, and composites are some ways of reducing the mass. These materials also provide a better performance for cost in terms of the fuel economy.142,143 In recent years, changes in the vehicle’s body structure have allowed the introduction of lightweight materials into the manufacturing process. While unibody design is today the primary approach for passenger cars, space frame and monocoque designs have also been proposed, as such designs offer more opportunities to incorporate lightweight materials.74 However, these approaches involve a significant increase in the costs. In addition to the direct benefit of a reduction in the structural mass, lightweight vehicles allow downsizing of the engine and transmission components. A general relationship between the mass reduction and the fuel consumption has been give by Carpenter,144 where for every 10% reduction in the mass (primary and secondary included), the fuel consumption is reduced by 6–8%. Table 4 summarizes the results of a study where the average fuel consumption reduction values were collected for different vehicle types and driving cycles.145

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Table 4. Fuel economy benefits over various cycles using thermoelectric devices (data from Ricardo, Inc.145). Cycle

LA4 Highway SC03 US06

Improvement in the fuel economy (%)

Reduction in CO2 (g/km)

0.13 0.36 0.15 0.08

0.2 0.42 0.24 0.14

CO2: carbon dioxide.

The fuel consumption gains shown make mass reduction the single most effective way to improve the fuel economy. Improvements in the vehicle aerodynamics. As shown in equation (1), improvements in the vehicle aerodynamics are possible only if either the front area of the vehicle or the drag coefficient is reduced.11,12 Reducing the aerodynamic drag coefficient is possible through several approaches, for instance the introduction of front and rear spoilers.11 In the paper by Kim et al.,146 a rear spoiler was installed on a lightweight vehicle, showing that the drag coefficient can be reduced by up to 5%. Better streamlining of the vehicle body can also reduce the aerodynamic resistance. For example, the use of an active grille shutter119 contributes to a reduction in the aerodynamic drag as it controls the air flow through the grille based on the demand of the cooling system and the AC system. Control of the position of the active grille shutter is designed to maximize the aerodynamic benefits and to minimize the additional power consumption by the cooling fan. An active suspension system can also be considered as a means to reduce the aerodynamic resistance, namely by adjusting the vehicle height.147 A study conducted by Argonne National Laboratory74 suggests that a 10% reduction in aerodynamic drag would result in a 0.25% decrease in the vehicle’s fuel consumption for an urban cycle and a 2.15% decrease in the fuel consumption for a highway cycle, owing to the higher speeds. It must be noted, however, that reducing Cd to below 0.25 for most passenger cars generally requires modifications that are impractical or too costly for production vehicles, implying that reductions in the vehicle mass and frontal area are the only possible directions to achieve considerable fuel savings. Rolling resistance reduction. The National Research Council has suggested that a reduction of 10% in the rolling resistance will lead to a 1–2% improvement in the fuel economy.148 Energy-efficient tires with a low rolling resistance can be used for this purpose.13,14 The reduction in the vehicle mass will also lead to a reduction in the rolling resistance.21,67 Another new

technology that helps to reduce the rolling resistance is the use of active suspension systems.147 Electrification of brake system. Like many other components, the brake system in vehicles is also undergoing changes, with the conventional system being changed to a by-wire system through electrification. The bestknown types of by-wire system are electrohydraulic brakes and electromechanical brakes. While the former type uses conventional hydraulic calipers, the latter type has electromechanical components.63,64 An electronic wedge brake is a form of electromechanical brake system which has the potential to improve the vehicle’s chassis safety, the mass, the reliability, and the space requirements. Further, the on-demand technology allows a reduction in the energy demand on the engine.149,150 Active suspension systems. The operation of a conventional suspension system is almost fully dependent on the road surface. An active suspension system makes use of electromagnetic actuators, sensors, controllers, and capacitors in place of conventional shock absorbers. These systems have the capability to recover, store, and manage energy.147 Active suspension systems also help to lower the center of gravity of the vehicle and to improve the vehicle dynamics, contributing to reducing the aerodynamic and tire rolling resistances.

Final remarks While the aforementioned technologies aim at providing means to improve the fuel economy of vehicles, it is important to observe that improvements in the efficiencies of components may not always manifest themselves as an overall improvement in the fuel consumption, owing to secondary issues. To this extent, the technologies reviewed above carry information on their potential for fuel economy improvement, with the understanding that trade-offs will be necessary when each technology is integrated. With this in mind, the fuel economy estimates presented should be interpreted as an upper bound, namely a qualitative indication of the maximum potential of each technology to improve the vehicle’s fuel economy which can be used as a criterion for technology sorting during the design phase. In addition, it is important to mention that nearly all the systems described above require the presence of electronic control in order to optimize their performance and coordination with other components and subsystems present in a vehicle. While the role of automotive controls in achieving fuel economy targets is not explicitly mentioned in this review, interested readers should refer to the work by Guzzella,151 which outlines some challenges and opportunities in this field.

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Energy harvesting and recovery: state of the art and technology trends Engine waste heat recovery Owing to the considerable amount of energy and availability that is normally dissipated through the coolant and the exhaust gas systems of automotive powertrains, there is a considerable interest in using waste heat recovery technologies for converting thermal energy into mechanical or electrical energy. The following sections outline the most relevant technologies that are today the state of the art. Organic Rankine cycles. The organic Rankine cycle (ORC) is a closed thermodynamic cycle which uses the waste heat streams to heat a working fluid and then to expand the fluid through a turbine to produce the shaft power. The shaft power is used to rotate a generator which converts the mechanical energy into electrical energy. For hybrid vehicles, this electrical energy can be used to charge the battery packs. This leads to an increase in the overall brake thermal efficiency of the vehicle.152–156 The choice of the working fluid is based on the application. Commonly used working fluids are ethanol, R-245ca, iso-pentane, and water. An ORC which uses iso-pentane or R-245ca as a working fluid is able to recover 4–5% of the fuel energy.157 A review of ORC technologies for automotive applications has been presented by Wang et al.158 and Quoilin and Lemort.159 Recently, BMW,160,161 Honda,162,163 and Ford164 have worked on the implementation of Rankine cycles with steam and organic fluids for passenger cars, while Cummins,165 Caterpillar,166 Daimler Trucks,167 and Volvo168 have carried out similar work for trucks. The interest of manufacturers is justified by the announced reductions in the fuel consumption ranging from 5% to 10%, depending on the system and the driving cycle. Skarke et al.169 presented a feasibility analysis on the application of ORCs for exhaust waste heat recovery in automotive engines, with application to a prototype plug-in hybrid vehicle. Simulation results considering both urban and highway driving conditions show that the system is able to recover up to 10% of the engine waste heat in highway driving conditions, corresponding to a 7% improvement in the fuel economy. Recuperative thermal management systems. With approximately 65% of the energy produced in the combustion chamber being dissipated to the environment (as suggested in Figures 5 to 8), recuperation and harvesting of part of this wasted energy to increase the fluid temperature help to reduce the vehicle’s fuel consumption. To this extent, TMSs focus on balancing the needs of multiple subsystems and components which may require heat for operation, heat rejection, or operation within specified temperatures.118 In this sense,

Proc IMechE Part D: J Automobile Engineering 227(6) recuperative TMSs offer the ability to recover some of the exhaust heat to aid rapid warm-up of the engine and powertrain fluids.118,170 An example of a recuperative TMS was proposed by Kuze et al.,171 where a heat storage tank is used to store warm coolant using a controlled heat storage system, after on-road vehicle operation. The stored thermal energy is routed to the engine block and head at coldstart conditions to improve the engine thermal efficiency and to lower the emissions. An advanced TMS was introduced by Agarwal et al.170 This TMS scheme makes use of an exhaust-tocoolant heat exchanger for waste exhaust heat recovery. The coolant can be routed to a combination of heat exchangers such as a transmission oil heater and an engine oil cooler for rapid fluid warm-up. The coolant can also be routed to the cabin heater to improve passenger comfort in cold conditions. Other examples of exhaust heat recovery systems and their benefits can be seen elsewhere.172–184 Engine and powertrain TMSs typically consist of a combination of devices such as heat exchangers, flow control devices, electric fans and pumps, etc., which are used for temperature conditioning. Instances of use of these devices are discussed in the following sections. Compact heat exchangers have seen a steady improvement in their design in the past few years, leading to lighter, smaller, and more effective heat exchangers.180,181 The improvement in the design of radiators, cabin heaters, condensers, evaporators, oil coolers, and charge air coolers has led to a mass reduction of 25– 30% and improvements in the effectiveness. Other recent examples of advanced compact heat exchangers have been shown in other studies.181–184 Electronic thermostats have been introduced to control the flow of coolant actively through various heat exchangers in a TMS, leading to rapid warm-up of the powertrain fluids and improved temperature control.185 Instances of the use of an electronic thermostat have been mentioned in other work.38,40,117,185 Choukroun and Chanfreau186 tested a new coolant-routing approach for the purpose of reducing the warm-up time and increasing the fuel economy. Through those studies, the time for the coolant to reach 100 °C is reduced by about 50% compared with the case when no electronic thermostat or electric pump is used. The improvement in the fuel economy is 2–3% over the baseline over a European driving cycle. Electric coolant pumps allow on-demand cooling of the engine block and head as well as the other fluids present in the engine and powertrain.117,187 The use of electric pumps reduces the parasitic loads to the engine crankshaft and the duration of the cold-start phase and aids in operating the engine under optimal thermal conditions. Electric cooling pumps have been introduced in recent vehicles, as for instance shown by Kavanagh.188 The fuel economy benefits of electric cooling pumps have been shown38,188 to be in the range 1.5–2%.

Chiara and Canova Variable-speed radiator fans have been electrified to contribute to reducing the parasitic losses. In addition, variable-speed electric radiator fans can further reduce the power consumption by drawing only the required amount of power and also by avoiding over-cooling of the coolant.189–191 Active grille shutters are used to limit the heat transfer by forced convection which occurs when ambient air is drawn through the engine compartment at a high vehicle velocity. This reduces the amount of heat loss to the ambient (useful during cold-start conditions) and also the aerodynamic drag on the vehicle. The result192 is a reduction in the vehicle’s fuel economy by up to 2.6%. Most of the estimates for reduction in the fuel consumption are due to the reduced air resistance because of closed grille shutters; nevertheless there are also benefits derived from reducing the engine’s heat loss.193 Turbocompounding. Compounding refers to the combination of more than one source to produce a single output. Turbocompounding uses the exhaust gases to run a turbine, which can be mechanically coupled to the engine to produce a single output. Electric turbocompounding firsts converts the waste exhaust energy into shaft work, which is then coupled back to the engine electrically.153,160,194 Thermoelectrics. Thermoelectric devices produce electricity by exploiting the Peltier effect to convert directly the energy present in thermal gradients into electrical energy.195 Applications of thermoelectric materials to automotive waste heat recovery have been developed in the past few years. For instance, Crane et al.196 presented a modified radiator that uses such thermoelectric modules and showed that almost 1 kW could be harvested using this system. Even though thermoelectrics can also be used at the exhaust side, because of their low efficiency (1–3%) they are considered impractical. Moreover, inserting thermoelectric modules in the radiator presents numerous challenges such as the cost, the added system mass, the power consumption of accessories, and the vehicle drag. Mori et al.197 investigated the potential for improvement in the fuel economy of thermoelectric devices specifically when recovering waste heat from the engine’s exhaust system. Table 5 summarizes the fuel economy gains from the use of thermoelectric modules over various driving cycles. The reduction in the overall CO2 emissions over the same driving cycles are also listed in this table.

Powertrain energy harvesting and hybridization The electrification of automotive powertrains is a promising solution for fuel energy savings. As mentioned above, several components of a vehicle powertrain, such as the ancillary loads, present significant

927 Table 5. Impact of mass reduction options on the fuel consumption. Reduction (%) in the mass from the following baselines

Reduction in the fuel consumption (vehicle mass only) Reduction in the fuel consumption (with resized engine)

5%

10%

20%

1–2

3–4

6–8

3–3.5

6–7

11–13

Source: reproduced with permission from Mori et al.197

opportunities for fuel economy improvement through electrification and on-demand operations. A further step consists of hybridizing the vehicle powertrain, namely by introducing a secondary energy converter and energy storage system that offers the ability to store and release energy on demand. Therefore, hybrid vehicles can use multiple power sources to provide propulsion either directly or indirectly. While the level of hybridization can vary considerably for various powertrains, the general consensus tends to distinguish mild hybrids and full hybrids. In full hybrids, the fraction of electrical power installed on the vehicle is of the order of 30–50% of the overall propulsion power. In mild hybrids, the fraction of electric power is less than 25%, offering less flexibility for fuel economy improvement.23,147 Depending on the degree of hybridization, hybrid powertrains offer several options to improve the vehicle’s energy efficiency, such as regenerative braking, idling reduction, improvement in the engine efficiency through downsizing, and displacing of operating conditions. Several architectures for hybrid vehicles are available to implement the functions described above. A review of opportunities for energy savings from vehicle hybridization has been presented by Sovran and Blaser.23 A more detailed discussion of the hybridization of automotive powertrains is not within the scope of this paper. The interested reader should refer to the large body of literature that has been published in this field, particularly the books by Guzzella and Sciaretta,9 Miller,198 and Mi et al.199 as starting points. It is worth mentioning that idling reduction through the engine’s stop–start system is a functionality which, originally developed for mild hybrids and full hybrids, is being extended also to non-hybrid vehicles. The advent of the crankshaft-mounted integrated starter– generator has allowed a reduction in the engine idling and the electrification of auxiliary loads. In addition to allowing the start–stop feature in vehicles, an integrated starter–generator can permit the transmission to operate more frequently in lockup mode, with the opportunity to allow elimination of the torque converter when

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combined with advanced transmissions. While fuel savings of between 3% and 5% have been shown through the implementation of stop–start systems for the FTP cycle, the potential for fuel consumption improvement increases by up to 10% in real-world driving conditions, e.g. in congested city driving.74

opportunities for energy harvesting may exist at a chassis level, principally in the new design and control solutions for braking systems and suspension systems. While the amount of energy recovered through the following solutions is significantly low, it is still worth mentioning such opportunities for completeness.

Non-electrified hybrid technologies. In recent times, nonelectrified hybrid vehicle technologies have been explored in an effort to offer a more cost-effective solution to hybrid electric vehicles. To this extent, shortterm energy storage solutions have been recently proposed to enable a conventional powertrain to achieve regenerative braking and torque assist functionalities while avoiding the issues of integrating a high-voltage battery and electric motors.72,200–208 Mechanical energy storage systems, using a flywheel as the storage element, have been proposed recently for short-term energy recovery in a parallel hybrid configuration.72,200 For instance, several European automotive makers have recently announced a flywheel kinetic energy recovery system (KERS) which has the potential of reducing the fuel consumption by 20% in urban driving.201,209 The flywheel is placed inside a vacuum containment to reduce the friction losses due to the air viscosity and it rotates on high-quality bearings. Toroidal CVT transmissions have also been used to maximize the efficiency in the energy conversion processes.202,203 Recent simulation studies have shown how a properly designed flywheel system can enable improvement to be made in the vehicle’s fuel consumption up to 12% in regulatory and real-world driving conditions.72 Similarly, hydraulic energy storage systems recuperate energy by pumping and storing hydraulic fluid in a high-pressure tank. The pump is then operated as a motor and converts the pressure into kinetic energy.72,204,205,210,211 In this case, the lower efficiency of the hydraulic components (particularly the pump and/or motor operating in partial displacement conditions) limits the potential for fuel economy improvement72 to about 10%. Key characteristics in the definition of an energy storage system for vehicle applications are its specific energy and specific power. In terms of hybrid vehicles, the specific energy is directly related to the range, while the specific power is directly related to the performance. As shown by Holm et al.,212 flywheels and hydraulic accumulators possess a significant specific power but have a limited energy storage capability. For this reason, range requirements indicate that flywheels and hydraulic accumulators are solutions limited to shortterm energy storage.

Regenerative suspension system. Regenerative suspension system can convert the linear motion of the suspension into electrical energy, for instance through the use of piezoelectric crystals or smart materials. The harvested energy (in electrical form) can then be used to charge a battery or to run auxiliary devices, thereby reducing the load on the engine. Experimental results have shown that the power generated by regenerative suspensions is of the order of 100–400 W, the higher values obtained with an increase in the tire stiffness.213,214 Regeneration, together with active control of the damping, also helps to improve the ride comfort of the vehicles.

Chassis energy harvesting Besides the functionalities described above for managing energy dissipation and energy recovery, limited

Regenerative braking system. For hybrid electric vehicles, regenerative braking is one of the most important functionalities for energy recovery. In this sense, conventional by-wire braking systems could offer limited regeneration capabilities, if an electric generator could be integrated in the brake design. On the other hand, the bus voltage and the high speed of the wheels limit significantly the maximum power of the generator. The cost of the design and reliability issues further discourage this opportunity from practical implementations.

Conclusions This paper presented a survey of the state of the art and main technology trends for energy management and recovery for automotive systems, with primary focus on conventional powertrains. Fuel energy utilization and dissipation associated with the vehicle subsystems have been investigated, offering a view for potential opportunities and challenges to improve the vehicle’s fuel consumption through system-level energy management and harvesting of the available thermal energy and kinetic energy. The first section of the paper focuses on investigating the vehicle’s energy demand in typical driving cycle conditions. The main indications are that the dominant sources of the vehicle’s energy demand in urban and highway driving conditions are the vehicle inertia (58% of the tractive power at the wheel) and aerodynamic loads (50% of the tractive power at the wheel) respectively. The second section presents an analysis of the energy conversion and the dissipation sources in an automotive system. Building on the results obtained in the previous part of the work, the third section identifies the main state of the art and technology trends to manage the vehicle’s energy dissipation sources. The last section is dedicated to identifying and discussing

Chiara and Canova the most promising technologies and systems for recovering the thermal energy and kinetic energy that are normally lost in a conventional automotive system. With this in mind, a thorough review of the main waste heat recovery systems available in the state of the art is presented, focusing on ORCs, recuperative TMSs, turbocompounding systems, and thermoelectric devices. Moreover, powertrain energy harvesting by means of electrified and non-electrified solution is discussed. Based on the review conducted, the main sources of fuel energy loss in a vehicle were identified and quantified. These can be summarized as follows. 1.

2.

3.

The thermal energy rejected to the engine coolant and to the exhaust gases is the predominant source of energy and availability losses in IC engines. Among the auxiliary loads, the AC system and the engine cooling system are the main sources of mechanical energy consumption. Transmission losses are another source of energy losses in vehicles; their extent is, however, largely dependent on the system technology, the number of gears, as well as the operating temperature.

As a conclusion of this study, a summary of directions to manage the energy dissipation and to implement forms of energy harvesting on conventional vehicles is proposed as follows. 1.

2.

3.

4.

Increasing the engine thermal efficiency, for instance through downsizing and the use of advanced fuel injection systems and air-path management systems (variable-valve actuation system and boosting), is currently the primary avenue towards improvement in the vehicle’s fuel economy, having the highest potential for energy savings. In conjunction with improvement in the engine technology, advanced transmissions with an increased number of gears will play a significant role in optimization of the powertrain system, allowing the engine to operate more frequently close to its highest-efficiency area. As the engine’s heat transfer losses are a considerable source of energy and exergy loss, the adoption of waste heat recovery solutions or a smart engine TMS has a high potential for achieving significant fuel economy gains. Other relevant areas for improvement in the vehicle’s fuel economy can be found in the electrification of ancillary loads to allow on-demand operation, as well as introducing design changes which aim at reducing the vehicle mass and improving its aerodynamics.

Based on the review conducted, it appears evident that engine- and transmission-related technologies appear as the most immediate solutions to be implemented, owing to the higher benefits that can be

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Appendix 1 Notation A CD Cr ea eexh ef E_ d Ew g ha hexh hf Iw m_ a m_ exh m_ f M Q_ cl Q_ cv Q_ ht QLHV rw sa sexh sf T0 Tj V Wb Win Wloss,tr Wm

surface area (m2) aerodynamic drag coefficient rolling resistance coefficient exergy of air (kW) exergy of the exhaust gases (kJ/kg) exergy of the fuel (kJ/kg) power dissipated (kW) energy at the wheel (kJ) acceleration due to gravity (m/s2) enthalpy of air (kJ/kg) enthalpy of the exhaust gases (kJ/kg) enthalpy of the fuel (kJ/kg) mass moment of inertia of the wheel (kg m2) mass flow of air (kg/s) mass flow of the exhaust gases (kg/s) mass flow of the fuel (kg/s) mass (kg) power loss due to the combustion inefficiency (kW) heat transfer at the boundary of the control volume (kW) heat transfer with the wall (kW) lower heating value of the fuel (kJ/kg) radius of the wheel (m) entropy of air (kJ/kg) entropy of the exhaust gases (kJ/kg) entropy of the fuel (kJ/kg) ambient temperature (K) reference temperature for the heat transfer process (K) velocity (m/s) brake power (kW) input power (kW) power loss in the transmission (kW) power loss due to mechanical friction (kW)

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Proc IMechE Part D: J Automobile Engineering 227(6)

Ww

power available at the wheel (kW)

htr r

transmission efficiency density (kg/m3)

Abbreviations AC CAFE CAI CVT DCT DI EGR EIA EVO FTP

air-conditioning corporate average fuel economy controlled autoignition continuously variable transmission dual-clutch transmission direct injection exhaust gas recirculation Energy Information Administration exhaust valve opening Federal Testing Procedure

GDI HCCI IVC KERS LDV LTC ORC SI SUV TMS US EPA VTEC VVA VVT

gasoline direct injection homogeneous charge compression ignition inlet valve closing kinetic energy recovery system light-duty vehicle low-temperature combustion organic Rankine cycle spark ignition sport utility vehicle thermal management system US Environmental Protection Agency variable valve timing and lift electronic control variable valve actuation variable valve timing