Vehicle Design and Technology for Reducing Fuel Consumption: Comparison
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In recent times, the significance of advancing road transportation technologies has notably increased. This is mainly driven by the escalating need for road transportation systems that are not only safe but also environmentally sustainable. Moreover, enhancing fuel efficiency in road vehicles (i.e., automobiles) holds the potential to contribute significantly to the reduction of a country’s economic vulnerability (i.e., improved energy security), by reducing the reliance on energy imports. 

  • automotive fuel consumption
  • fuel efficiency
  • sustainability

1. Introduction

In vehicular design, the mass is a major factor influencing fuel efficiency. The reduction of the vehicle’s mass brings about tangible benefits, such as the reduction of both traction power requirements and wheel rolling resistance during acceleration (therefore leading to fuel savings). The Vehicle-Specific Power (VSP) is an important metric in vehicle design and parameter optimization. It quantifies the power output relative to the vehicle’s mass; that is, a higher VSP indicates a more powerful engine relative to the vehicle’s weight. The VSP is commonly expressed in kilowatts per ton (kW/ton) for passenger cars, or in kilowatts per amount of passengers (kW/passenger) for public transportation systems.
Mathematically, the VSP is expressed as
𝑉𝑆𝑃=𝑣·(𝑎·(1+𝜀)+𝑔·𝑔𝑟𝑎𝑑𝑒+𝑔·𝐶𝑅)+𝜌𝐶𝐷𝐴𝑣32𝑚, V S P = v · a · 1 + ε + g · g r a d e + g · C R + ρ C D A v 3 2 m ,
where:
  • v: Vehicle speed.
  • a: Vehicle acceleration.
  • ε: Mass factor that considers the conversion of the inertia of rotating masses to translational mass
  • g: Gravity.
  • grade: Road grade (road inclination).
  • CR: Coefficient of rolling resistance.
  • ρ: Air density.
  • CD: Coefficient of aerodynamic resistance.
  • A: Frontal area.
  • m: Vehicle mass.
With progressive design efforts aimed at reducing the vehicle’s mass and enhancing its VSP, there have been noticeable improvements in fuel efficiency and (correspondingly) some reductions in environmental impact.
In vehicle design, engineers undertake predictive analyses to estimate operational metrics and fuel consumption, emphasizing VSP as an important design criterion. Advanced mathematical tools, such as the Powertrain System Analysis Toolkit (PSAT), developed by Argonne in the MATLAB/Simulink environment, allow the simulation of vehicular performance and fuel economy dynamics [16,17][1][2]. Leveraging these tools, engineers can survey diverse design configurations, powertrain alternatives, and operational scenarios, improving vehicle efficiency and fuel consumption even before the creation of a physical prototype. The utilization of such predictive platforms enhances the design process’s efficiency by preempting extensive physical testing and by ensuring that the resultant vehicle design is strategically tailored for optimal fuel efficiency, performance, and environmental sustainability.

2. Advances in Internal Combustion Engines (ICEs)

Over the decades, technological innovations have substantially increased ICE efficiency. Contemporary high-power, low-speed diesel engines present efficiency values of up to 50% [18][3]. Notwithstanding these advances, many opportunities remain to refine ICE systems. Prospective advancements for diesel and gasoline engines, projected to decrease passenger cars’ fuel consumption by approximately 20% within the next decade, incorporate strategies such as friction reduction, enhanced compression ratios, comprehensive variable valve control, two-stage turbochargers, cylinder deactivation, direct fuel injection, accessory electrification, start/stop system integration, refined transmission systems, and reductions in vehicle weight, tire resistance, friction, and aerodynamic drag [19][4]. Such evolution in ICE technologies and vehicular architecture holds significant promise for augmented fuel efficiency and a diminished ecological footprint.
Recent advancements in gasoline engine combustion paradigms comprise [20][5] Compression Ignition (GCI), Homogeneous Charge Compression Ignition (HCCI), Partially Premixed Combustion (PPC), Low-Temperature Combustion (LTC), Octane-on-Demand (OOD), and Reactivity-Controlled Compression Ignition (RCCI). Such combustion paradigms have effectively and positively transformed contemporary engine efficiencies. Their distinctiveness lies in their unique fuel–air mixture preparation and their reliance on varying degrees of flameless combustion. Additionally, combustion strategies often combine technologies such as High-Pressure Direct Injection (HPDI), Cyclical Multi-Injection (CMI), Extensive Exhaust Gas Recirculation (EEGR), and Adaptive Valve Timing (AVT). Contemporary gasoline engines have been optimized to operate at elevated compression ratios, consequently increasing engine efficiency and reducing emission levels. Moreover, high compression ratios in compression ignition engines fueled with gasoline also help increase efficiency. Nevertheless, the attainment of such efficiencies typically requires the concurrent evolution of fuel formulations and engine designs, which underscores the importance of developing fuels and engines simultaneously.
Some modern innovations in engine technology include turbocharged gasoline direct injection compression ignition (GDCI) engines, which have notably delivered a 22% reduction in fuel consumption [16,21][1][6]. The use of gasoline direct injection (GDI) engines, supported by lean combustion techniques, might contribute to enhancements in fuel economy up to 15%. Furthermore, other super-lean combustion technologies are projected to reach thermal efficiency values of up to 45%.
When subjected to increased injection pressures, diesel engines exhibit enhanced fuel atomization, promoting improved air–fuel mixing, which subsequently increases the engine’s efficiency [22][7]. Thermal efficiency values up to 52.9% are being obtained in heavy-duty diesel engines, with improvements such as (approximately) 2% reductions in gas exchange losses, a 0.9% reduction in frictional losses, and a 0.6% reduction in accessory consumption. Concurrently, additional efforts are directed toward energy recovery: up to 2.5% from exhaust gases, and up to 1.3% from the thermal energy of the coolant. These endeavors support energy recovery systems that are based on the organic Rankine cycle (ORC) and incorporate advanced methodologies like turbocompounding [23][8], which produces work (or generates electricity) by utilizing the exhaust gas energy by means of a gas turbine, mirroring a cogeneration approach [24,25][9][10].
Energy recovery via complementary thermodynamic bottoming cycles presents an important resource for increasing the fuel efficiency of ICEs. Such cycles encompass the following: (a) Organic Rankine Cycle, (b) Kalina Cycle, (c) Trilateral Flash Cycles, (d) Supercritical CO2 Rankine Cycle, and (e) Gas-based Stirling and Brayton Cycles. Furthermore, the vaporization of LNG has been identified as a potential heat sink for the Rankine Cycle, as pointed out by Paanu [26][11]. Moreover, the Miller Cycle offers another way to enhance fuel efficiency. At an ICE technology level, innovative solutions such as increasing the mean effective pressure (MEP), supported by multistage turbochargers, and integrating turbocompounding are being explored.
Engine downsizing (ED), which consists of reducing engine displacement while sustaining (or even increasing) power and torque densities, has become a focal strategy among many ICE manufacturers [27][12]. The primary objective behind ED lies in the drive to operate engines within specific speed/mean-effective-pressure regimes, yielding diminished frictional and heat transfer losses, thereby optimizing efficiency [27][12]. In effect, ED can replace larger, multi-cylinder engines with smaller, fewer-cylinder engines, thereby minimizing friction. Such downsized engines can complement hybrid electric vehicles, whether in parallel or series configurations. In the former, the engine benefits from supplementary battery power; in the latter, it predominantly serves as a generator, providing electricity either for direct propulsion or battery replenishment. In this case, an electric machine propels the vehicle.
In the current market, a significant number of gasoline engines are categorized as downsized engines, which typically exhibit a reduction factor, also called downsizing factor (DF), of approximately 35–40%. The DF is a metric that quantifies the decrease in engine displacement (when compared to traditional engines) and can be defined by the following expression:
𝐷𝐹=𝑉𝑆𝑤𝑒𝑝𝑡𝑁𝐴−𝑉𝑆𝑤𝑒𝑝𝑡𝐷𝑜𝑤𝑛𝑠𝑖𝑧𝑒𝑑𝑉𝑆𝑤𝑒𝑝𝑡𝑁𝐴,
D F = V S w e p t N A − V S w e p t D o w n s i z e d V S w e p t N A ,

where DF is the downsizing factor, 𝑉𝑆𝑤𝑒𝑝𝑡𝑁𝐴 is the swept volume of a naturally aspirated traditional engine with a given output power, and 𝑉𝑆𝑤𝑒𝑝𝑡𝐷𝑜𝑤𝑛𝑠𝑖𝑧𝑒𝑑

is the swept volume of a downsized alternative engine with a similar power output.
According to Turner [27][12], a reduction factor of 40% can result in an improvement in fuel economy of approximately 12%. This improvement is attributed to the reduction in the engine’s swept volume and the corresponding decrease in friction within the cylinders.
The optimization of modern engines across a wide range of operating regimes (i.e., speed–torque ranges) is now achievable through a comprehensive understanding of the factors influencing fuel consumption. Such factors include intake air flows, fuel atomization, combustion, and exhaust gas formation, as well as their interaction with other factors, such as chamber/piston geometry and fuel injection strategies. Currently, these interactions can be modeled with robust computational tools. Moreover, advanced ignition systems, such as High-Energy Inductive Systems (HEISs), Plasma, Corona, and Laser, have significantly reduced combustion variability. Furthermore, the development of Variable Valve Timing (VVT) and Variable Compression Ratio (VCR) strategies for gasoline engines, along with multiple-pulse injection (MPI) techniques, have enabled higher thermodynamic efficiency within a wide range of operating regimes. Engineers are also exploring the use of multiple pulses and multiple fuels in the same combustion cycle to further enhance engine performance.
Another approach considered for improving fuel efficiency in ICEs is the implementation of the start–stop system. This system automatically shuts off the engine when the vehicle comes to a stop (e.g., at traffic lights) [28,29][13][14]. During this idle period, all systems operate using battery power. In second-generation start–stop systems, the engine shuts off at a specific speed during deceleration, with the speed being a determining factor for the system’s efficiency. The engine remains off throughout the periods of zero speed in each cycle, contributing to energy savings and enhanced overall efficiency [30][15]. However, when the engine restarts (i.e., transient state), fuel consumption briefly increases above the reference at a specific operating temperature. This increase occurs because the generator is activated to compensate for the battery charge lost during engine startup. The overall benefit of the start–stop system is determined by balancing the energy saved during zero-speed operation and the energy required for engine restart. The final outcome depends on factors such as the duration of idleness in each cycle, the engine’s consumption rate during idleness, the additional energy needed to start the engine, and the specific strategy employed by the generator during operation. These factors account for variations in fuel consumption between different vehicles and driving cycles [31][16].
The landscape of vehicle powertrain technologies is undergoing a transformative shift in response to the stringent CO2 and fuel economy regulations set beyond 2020. Conventional powertrains and fuels alone are no longer sufficient to meet these everchanging standards. This fact has catalyzed an increasing diversification of powertrain technologies and energy carriers, highlighting engine rightsizing as a pivotal strategy for both conventional vehicles (i.e., powered by ICEs) and electric vehicles (EVs). Integral to this change is the integration of Connected Powertrains and Advanced Driver Assistance Systems (ADASa). These technologies are becoming increasingly prevalent in vehicle designs, offering significant improvements in fuel consumption, safety, and comfort. The development of these systems requires a comprehensive full life-cycle methodology, fostering the advancement of new engine technologies, sensors, and on-board computing. These innovations are central in enabling real-world implementations of high-efficiency engine technologies, like low-temperature combustion and high-speed controls. A prominent example of such advancements is the SuperTruck II program (by Cummins), recognized by the U.S. Department of Energy (DoE) for its pioneering research in heavy-duty diesel engine technology. It achieves a milestone of 55% brake thermal efficiency (BTE), which is the power taken by the engine crankshaft out of the total power generated by the ICE, through the integration of waste heat recovery systems. This initiative underscores the synergy between hardware optimization and sophisticated engine calibration. The SuperTruck II program (an extension of the SuperTruck I initiative) aims to further double efficiency and halve CO2 emissions, contributing significantly to global energy savings.
A study by Brooker et al. [32][17] investigated the effects of reducing vehicle weight (i.e., lightweighting) on fuel efficiency and cost for different powertrains: conventional gasoline vehicles, hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (EVs). Using the FASTSim tool, the study found that lightweighting has the most significant impact on conventional vehicles, reducing fuel consumption by about 0.58 L/100 km for a 150 kg mass reduction. While the impact on HEVs, PHEVs, and EVs is less pronounced in terms of fuel efficiency, lightweighting considerably reduces battery costs, particularly for PHEVs and EVs. The study concluded that the lightweighting practice is cost-effective only when it costs less than USD 6 for each kg of mass reduced and emphasizes its importance in lowering both component and fuel costs. The work of Dahham et al. [18][3] focused on improving thermal efficiency and reducing carbon emissions in ICEs. It reviewed advanced strategies for higher efficiency and cleaner combustion, considering thermodynamic cycles, gas exchange systems, advanced combustion strategies, and thermal and energy management.

3. Advances in Alternatives to Petroleum-Based Fuels

In the field of ICEs, traditionally powered by fuels such as gasoil (i.e., diesel) and gasoline (i.e., petrol), recent studies have highlighted the potential of the use of diverse alternative fuels. Hydrogen stands out for its high energy efficiency and clean combustion, emitting only water vapor, but faces challenges in efficient storage and sustainable production methods. Other fuels like biofuels (biodiesel and bioethanol) offer sustainability and lower emissions, despite the fact that their production can impact food resources and land use. Natural gas, with its cleaner burn compared to traditional fuels, struggles with storage and refueling infrastructure. Synthetic fuels (e-fuels), compatible with existing ICEs and infrastructure, are hindered by high production costs and energy-intensive manufacturing. Each fuel presents a unique set of advantages and challenges, indicating that a multi-faceted approach is required for sustainable transportation solutions.
Bedar et al. [34][18] showed the effectiveness of the use of simarouba glauca biodiesel in a diesel engine for improving brake thermal efficiency and reducing emissions of carbon monoxide (CO), hydrocarbon (HC), and smoke opacity, especially when combined with exhaust gas recirculation (EGR). A study by Vadivelu et al. [35][19] investigated the performance of a diesel engine using biodiesel derived from cashew nut shell liquid, augmented with hydrogen and ethanol. This combination was found to significantly boost brake thermal efficiency and reduce CO and HC emissions, suggesting a promising pathway for reducing diesel engine pollution [35][19].
Hydrogen’s unique combustion characteristics are being harnessed to develop reduced-emissions spark-ignition engines, with dynamic performances comparable to that of conventional gasoline engines but achieving efficiencies comparable to that of diesel engines. Hydrogen’s particular properties, such as low flammability limit and high flame speed, promote stable combustion in spark-ignition engines, resulting in reduced NOx formation. This characteristic enables efficient engine operation, particularly at low- and medium-torque regimes. Innovative engine design improvements (e.g., increased combustion pressure and engine downsizing) further enhance the efficiency and power densities of hydrogen-fueled engines, surpassing those of conventional hydrocarbon-fueled engines [36][20].
Shivaprasad et al. [37,38][21][22] investigated the impact of hydrogen blending with gasoline in spark-ignition engines, indicating that a 20% hydrogen blend can improve engine efficiency and reduce emissions. While such blending has been shown to reduce HC and CO emissions, contributing to improved combustion efficiency, it is also associated with an increase in nitrogen oxide (NOx) emissions. Collectively, these studies illuminate the potential of biofuels and hydrogen blends in transitioning toward more sustainable and efficient internal combustion engine technologies. While alternative fuels like hydrogen present promising benefits in terms of efficiency and reduced certain emissions, they necessitate careful management and further technological innovation to mitigate trade-offs, particularly concerning increased nitrogen oxide emissions.
In investigating the performance of hydrogen/diesel reactivity-controlled compression ignition (H2/diesel RCCI) engines, a study by Duan et al. [39][23] reveals that the indicated thermal efficiency (ITE) is significantly enhanced by employing a double direct injection (DDI) strategy over a single direct injection (DI) strategy, primarily due to improved fuel distribution and combustion conditions. The ITE increases notably when intake pressure is raised from approximately 100 to 120 kPa but decreases with further pressure enlargements due to reduced combustion efficiency and higher combustion temperatures.

4. Other Technologies Related to Fuel Economy

The entire planet is linked by a massive transportation infrastructure, which is largely based on the use of the internal combustion engine (ICE). It would take decades and tremendous financial expense to replace ICEs, so it is probable that they continue to be the central powertrain technology used for road transportation all over the world in the coming years. Reitz [41][24] indicated the obstacles still faced by current alternatives to ICEs, such as electric vehicles (EVs) powered by batteries. Currently, electric batteries have high cost, weight, and other limitations. Furthermore, they are expected to be charged using energy obtained from renewable sources, such as wind and solar. Nevertheless, these sources still represent a small fraction of the world’s energy supply. The reduction of fuel consumption in road transportation needs to be addressed by a mix of solutions, involving battery and hybrid electric vehicles (BEVs and HEVs), fuel cell electric vehicles (FCEVs), and conventional vehicles (ICEs), depending on the consumer, the country, and the specific application considered.
Energy consumption during fuel production and transportation is closely related to fuel economy. In this sense, the life cycle assessment (tank-to-wheels) is an adequate measure of fuel performance. The selection of the feedstock and the fuel production pathways have a great impact on the overall energy efficiency of road vehicles. BEVs have a higher in-use energy efficiency as compared to HEVs; BEVs can convert 70–90% of the energy stored in the battery into movement, whereas the theoretical peak efficiency of HEVs is only 40%. Some of the in-use efficiency advantages of BEVs over HEVs are offset by the conversion losses occurring during electricity generation from fossil fuels and by losses during transmission and charging. Collectively, this can add up to around 60% of the total energy use.
Another factor affecting the energy efficiency of BEVs is the electricity consumption of auxiliary systems, like heating and air conditioning. To provide energy for A/C systems, BEVs must use energy from the battery, while HEVs can make use of waste heat from the engine. Energy consumption varies across different electric vehicle sizes, and the average BEV is heavier than the average HEV. The extra weight of BEVs, imputable to the weight of the battery and the associated secondary weight required to strengthen the vehicle body, reduces the energy advantage of EVs over HEVs in terms of energy consumption. The reduced mass of vehicle components, obtained by replacing existing materials with lighter ones, must be balanced with the energy expenses during vehicle production. On the other side, it seems that road transportation will always have some energy externalities, even if all ICEs were replaced by EVs. Although the energy efficiency of electric motors is far larger than ICEs, the energy required to produce electrical energy for EVs is far higher than that required to produce fossil fuels. Moreover, the production of batteries is an energy-demanding activity. The reduction of the mass of EVs would contribute to lower energy required per km. Additionally, the recycling of batteries is also an energetic and environmental task to be solved.
Modern control systems, characterized by their adaptability, are important in increasing a vehicle’s efficiency, especially in managing the engagement of engine cylinders. Some manufacturers have pioneered the Cylinder on Demand technology, aiming to improve fuel efficiency through selective cylinder deactivation. Concurrently, Dynamic Skip Fire systems have been introduced. This innovative approach intelligently deactivates distinct cylinders based on immediate operational demands, thus ensuring proficient torque delivery while maintaining optimal noise, vibration, and harshness levels (i.e., NVH levels). Such cylinder deactivation strategies introduce unprecedented versatility and precision, promoting holistic enhancements in vehicular performance.

5. Control Signals Transmission

In contemporary vehicular power architectures, the integration of modern communication interfaces, which compute the measured operational variables to control powertrain modules, can contribute significantly to fuel efficiency. A synergistic interplay between the engine and the transmission allows for the crankshaft rotational velocity optimization. Specifically, by transferring data points to a computer for the calculation of torque and power requirements in a real-time context, vehicle dynamics, ride quality, and fuel efficiency can be improved. The central role of the Controller Area Network (CAN) protocol is to promote better communication between engine subsystems and the Electronic Engine Control Module (EECM). Modern controllers and algorithms are continuously improved, enhancing communication between powertrain modules and other vehicular systems. These improvements allow for adjustments in engine configuration and calibration and are reinforced by automated shift strategies and agile software, which can recalibrate in response to fluctuations in load demands, terrains, velocities, and engine torque outputs. Moreover, the integration of communication interfaces with navigation infrastructures and satellite-derived data further enhances fuel economy. Such integrations empower vehicles to judiciously modulate speeds during curvilinear trajectories or to preemptively accelerate before uphill gradients, tailoring operations to improve fuel efficiency.
Thermocoasting is an innovative energy-conservation mechanism, which is electronically activated during regenerative processes in ICEs and employs engine braking without reducing (significantly) the vehicle’s momentum. This mechanism is initiated when the accelerator is released, allowing the vehicle to roll. This strategy not only facilitates energy preservation during downhill trajectories but also mitigates drastic temperature variations, often observed in engines without energy recovery systems.

6. Improved Fuel Economy of Vehicles by Reducing Friction Losses

Friction is an inherent and, in many instances, requisite phenomenon. It plays fundamental roles in the operation of the components in a vehicle. An automobile comprises numerous tribological components, including bearings, pistons, transmissions, clutches, gears, windshield wipers, tires, and electrical contacts. Understanding and applying tribological principles to vehicle design is crucial for promoting the reliability of road vehicles. The mass production and utilization of automobiles have propelled significant advancements in the field of tribology. Specifically, the rising demands for automotives with better tribological performance have driven notable progress in lubrication technology, and the development of advanced tribological surfaces. These advancements have played an important role in enhancing the overall performance and durability of vehicles.
Friction reduction in the tribological pairs of engine systems has a significant impact on fuel consumption across all operating regimes. An optimized tribological design of engine systems (e.g., the powertrain, the valvetrain) can be achieved through improved clearances, low-friction surfaces, and decreased contact areas. Increasing the specific output of the engine without altering its geometric dimensions further reduces specific friction losses. The ongoing development of the piston ring/cylinder assembly plays a central role in achieving friction reduction. For instance, a topographical optimization analysis during the piston’s design phase can help identify areas where less material can be utilized without compromising the piston’s strength. Additionally, optimizing tribological pairs in the engine allows for the use of low-viscosity oils, which in turn reduces the thermal load in the oil circuit, as less cooling oil is required to lubricate the pistons. Controlling the oil pump based on specific power demand at each operating regime can lead to further fuel savings, particularly in real-world driving conditions [42][25].
Numerous research efforts that are aimed at mitigating friction’s deleterious impact on energy consumption in specific engine systems or components (e.g., piston groups, bearings). Potential reductions in friction losses could reach up to 40%, consequentially reducing the global primary energy consumption by as much as 8.6% [43][26]. Therefore, while the complete eradication of friction remains unattainable, the pursuit of substantial reductions in frictional losses is important for reducing fuel consumption. Holmberg et al. [44][27] have conducted calculations of friction dissipation in different systems (and sub-systems) in passenger cars, comprising the engine, transmission, tires, and brakes. They estimated that one-third of the global fuel consumption is utilized to overcome friction (excluding brake friction) in these components in passenger cars. Efforts to reduce friction losses hold the potential to triple fuel economy, as it would additionally decrease exhaust and cooling energy losses. Several measures are being explored to reduce friction in passenger vehicles. These measures include (a) the implementation of advanced coatings and surface-finishing technology in engine and transmission components to reduce friction losses and control wear processes, (b) the utilization of new low-viscosity lubricants and additives to perform better in extreme operating conditions, and (c) the development of tire designs that minimize rolling friction [43][26].
In an urban driving cycle, a relatively small fraction of the energy of the fuel is applied to propelling the wheels (in a medium-sized passenger car). This highlights the significant potential for efficiency improvements in vehicle design and operation. By examining mechanical losses (predominantly from friction) and their relation to fuel consumption, opportunities for substantial enhancements in energy utilization can be identified. A particular focus is given to the predominant contributors to these losses, including the piston-ring/cylinder assembly and various drivetrain components.
Moreover, understanding the intricacies of bearing tribology and valvetrain dynamics is crucial in addressing these losses. These components can significantly affect overall vehicle efficiency under different operational conditions. The multifaceted nature of mechanical losses is related to various factors such as lubricant quality, thermal conditions, and material properties. Furthermore, it is crucial not to overlook the auxiliary components, as they can account for 20% (or more) of the mechanical friction losses [45][28].
Initiatives have been undertaken to reduce boundary-type lubricated friction in bearings and other tribological systems in ICEs. A notable example includes the incorporation of friction-reducing additives into engine lubricants, such as molybdenum dithiocarbamate (MoDTC) [46][29]. On the other hand, the valvetrain presents a diverse array of tribological challenges. Its complexity includes several tribological interfaces: cam/follower, valve-guide/stem-seals, valve-heads/valve-seat, and other tribological systems, such as hydraulic lash adjusters, lifter guides, pivots, camshaft bearings, and both belt and chain drives. Refinement of these components can help ensure the valvetrain’s efficient functionality.
The dual objectives of obtaining more sustainable designs of vehicles and (specifically) the development of stringent regulatory standards advocating for efficient engines with augmented power-to-weight ratios require tribological components to operate at the minimum oil film thickness. A prevailing trend leans toward the adoption of engine oils with lower viscosities, such as SAE 5 W-20 and 0 W-20, targeting enhanced fuel economy. While this strategy reduces frictional losses, it also raises concerns regarding engine longevity, accentuating the importance of the surface technology in engine components in relative motion. These lubrication intricacies underscore the necessity of consistent oil replacements and the utilization of premium engine oils, which are formulated as a response to the requirements imposed by diverse driving scenarios. In this regard, appropriate maintenance protocols serve to anticipate problems stemming from degraded oil, safeguarding the engine from potential wear and corrosion. The tribological community that specializes in engine systems is faced with a substantial challenge when trying to reconcile the complex tribological effects affecting engine components with the (also) complex chemical interactions and degradation processes exhibited by engine lubricants, both in their fluidic state and on component surfaces. The general objective is to engineer lubricants that extend the components’ longevity, while concurrently optimizing engine performance.
During cold starts and brief journeys of an ICE, engine oil may not achieve sufficient operating temperatures, hindering the lubrication of systems and subsystems. Persistent low oil temperatures can generate problems such as the accumulation of fuel, combustion residuals, and water. Concurrently, the sedimentation of engine oil additives at the sump base, due to moisture contamination, can potentially neutralize the oil’s corrosion inhibitors, thereby risking the engine’s integrity against corrosive elements. Specifically in gasoline-fueled vehicles, fuel filtration into the oil can substantially reduce its viscosity, diverging from anticipated values at given temperatures. In such driving conditions, the oil’s natural thickening at decreased temperatures is somewhat counterbalanced by reduced viscosity from fuel incorporation during a cold start of the engine [47][30]. During this period, it is crucial to focus on increasing the rate of heating of the cylinder liner. This helps to improve combustion, resulting in reduced emissions. Moreover, a faster warming of the cylinder liner reduces friction levels against the piston ring, leading to improved fuel efficiency. In addition to this, it is advisable to enhance the rate of lubricant heating to minimize losses.
In nanotribological research, several pressing requirements have been delineated. These include (a) the design of nanostructured materials with higher strength and hardness and with self-repairing capabilities and safety features; (b) the incorporation of carbon and ceramic structural materials, with strength values superior to that of steel (by as much as ten times); (c) the evolution of polymer components that not only possess three times the strength of currently used materials but also have melting temperatures surpassing 100 °C; (d) the quest for multifunctional materials for mitigating friction, wear, and corrosion; (e) the exploration of nanoparticles’ incorporation to reinforce aluminum alloys against wear; (f) the application of nanocoatings on metallic substrates to achieve higher hardness values and reduce friction; (g) the innovative utilization of nanoparticle-reinforced materials, as potential replacements for metallic elements in vehicular contexts; and (h) the development of predictive models and simulations leveraging multi-scale computations to produce materials (bulk and surface) with superior tribological attributes. Such advancements are aligned with a future where materials exhibit enhanced performance and longevity, especially in industries such as automotive engineering, harmonizing with sustainability objectives.
While tribology plays a significant role in increasing the efficiency of road vehicles (and other industries), it has, until recently, remained somewhat sidelined in climate change discourses [43][26]. According to the BP Energy Outlook 2019 [1][31], the total primary energy supply (TPES) required was quantified at 584 exajoules (EJ). Leveraging proficient tribological strategies could have caused a medium- to long-term CO2 emission reduction of 6% to 11% in 2019, translating to global savings of 35–64 EJ. Importantly, forecasts predict an increase in TPES to 961 EJ by 2050 [48,49,50][32][33][34]. Given such increasing energy demands, refining tribological technologies and strategies becomes crucial.

7. Improved Fuel Economy by Reducing Rolling Resistance (Tire Technology)

In a vehicle, tire dynamics play a critical role in energy efficiency. The rolling resistance (RR), a result of the tire’s interaction with the road, significantly impacts the vehicle’s fuel consumption. Understanding the mechanisms behind rolling resistance helps in developing tires that contribute to fuel economy and environmental sustainability. This involves exploring various factors from hysteresis in the tire materials to aerodynamic effects and the role of tire pressure. Contemporary advancements in tire technology, including material composition and tire labeling systems, offer promising avenues for reducing rolling resistance and, consequently, fuel consumption.
Furthermore, if the tire pressure is 40% lower than the recommended level, fuel consumption can rise up to 8%. It is important to mention that tire pressure can naturally be reduced monthly by one to two psi during normal operating conditions. Additionally, ambient temperature fluctuations have a direct effect on tire pressure: a 5 °C temperature variation corresponds to a 2% change in pressure. A series of studies [51,54,55,56,57][35][36][37][38][39] have focused on the intricate relationship between tire pressure (P) and rolling resistance (RR), considering different proportionality relations, such as (1/P), (1/P)0.5, and linear and quadratic relations involving the vehicles’ service load (Z), speed (V), and pressure (P). The proposed correlation is expressed as RR ∝ Pα, where the value of the α index typically ranges between −0.3 and −0.5 for contemporary radial tires.
The use of nitrogen (instead of regular air) in tires offers the advantage of reducing tire pressure loss [55][37]. Laboratory tests have shown that new tires inflated with nitrogen experience approximately two-thirds less pressure loss compared to those inflated with air, both under static and dynamic loads. This difference can be attributed to the higher diffusivity ratio of oxygen through the rubber, making nitrogen a more effective option for maintaining tire pressure over time.

8. Improved Fuel Economy by Improving Thermal Management in Automobiles

Recent innovations in thermal management technology offer ways to mitigate parasitic thermal losses across diverse vehicular systems, thereby increasing efficiency in engines, transmissions, and equipment for heating, ventilation, and air conditioning (HVAC). Regarding thermal management systems, several innovative solutions are emerging. Notably, among these are intelligent coolant pumps, electronic coolant control mechanisms, proactive engine heating strategies, transmission oil bypass valves, and exhaust heat recovery systems. Moreover, new concepts like thermoelectric generators, the Organic Rankine Cycle (ORC) for optimized heat recovery, and innovative thermal energy storage methodologies are being explored [58][40]. Other thermal management constructive alternatives include residual heat recovery, insulated glazing, solar reflective paint, active seat and cabin ventilation, active transmission heating, solar panels for battery charging and cabin ventilation, active aerodynamics, and engine start–stop systems (with or without heater circulation).
It is important to mitigate heat absorption in vehicles, also known as heat soak or thermal soak, when exposed to prolonged sun exposure [59][41]. For instance, enhancing the treatment of glass and windshields’ surface and composition (e.g., providing higher transmissivity during cold weather and lower transmissivity during hot weather) plays an important role in reducing thermal soak and cabin temperatures, leading to reduced fuel consumption by air conditioning systems. By incorporating infrared reflective coatings and advanced glass technologies, vehicles can effectively reduce thermal loads and contribute to overall fuel efficiency improvements. Osborne’s research [58][40] covers thermal management technologies designed to optimize vehicle performance and reduce fuel consumption, in consideration of cost factors. These technologies comprise active engine heating (AEH), active seat ventilation (ASV), and cooled exhaust gas recirculation (EGR). Efficient thermal management can lead to a fuel consumption reduction of approximately 2% to 7.5%, depending on the vehicle’s underlying thermal management characteristics.
Other thermal management methods are being investigated, including independent cooling for different cabin zones, automatic climate control (ACC), and air quality management. Implementing techniques to decrease thermal loads and improve heating, ventilation, and air conditioning (HVAC) systems plays a significant role in achieving sustainable objectives [60,61][42][43]

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