|Fig. 1: More than 60% of the energy released during combustion of fuel in a gasoline-powered vehicle is lost in its engine like this one shown above.(Source: Wikimedia Commons)|
Automobiles weren't widely available to the public until the early 20th century once the Ford manufacturing line grew.  Although steam-powered, and electric powered vehicles existed prior to the automobile boom, the majority of the vehicles were gasoline powered due to advancements in internal combustion (IC) technology, and the growing petroleum infrastructure.  Since then, there have been many advancements in technology within the automotive industry that have improved fuel efficiency while meeting the needs and desires of their consumers. This paper will focus on automobiles and the energy efficiencies of driving.
Automobiles have helped develop the transportation sector in the US, and currently consume a notable amount of energy. According to a US Department of Transportation 2013 survey, the transportation sector within the U.S. is responsible for about 28% (27 quadrillion Btu) of the primary energy consumption.  From the modes of transportation defined, the modes accounted by highway vehicles consumed ~81% (~22 quadrillion Btu consumed) of all energy consumption within the transportation sector in the US in 2013.  On the other hand, non-highway and military use vehicles consumed ~16% (4.2 quadrillion Btu) and ~3% (0.67 quadrillion Btu) of all energy consumed within the transportation sector respectively. If the US would like to lower the amount of energy consumed in the transportation sector, highway-vehicle manufacturers would be able to make an impact if they work towards improving the fuel efficiency for any of their future vehicle models.
Energy efficiency η is defined as the amount of usable energy out divided by the energy in: η = Eout/Ein. Thermodynamic principles state that η is always less than the Carnot efficiency ηcarnot = 1 - Tcold/Thot, where Thot and Tcold are the absolute (Kelvin) temperatures of the heat source and sink, respectively. Although other metrics are available in determining the efficiency of vehicles (i.e. torque curves, compression ratios, thrust-to-weight ratios) these thermodynamic equations are useful in determining the engine efficiency of a vehicle with an internal combustion engine (ICE), such as the engine shown in Fig. 1. 
In order to compare vehicles with ICE within the transportation sector, the energy efficiency of the engines can first be compared per ICE vehicle on the highway. The efficiencies of electric motors (85-90%) will not be determined since thy require an external power source (often a heat engine at a power plant). [5-7] First we must assume that all of the fuel added to the vehicle is ignited in the vehicle's ICE. This ignited fuel manifests itself as high temperature and pressure acting on the engines pistons. Since ICE engines are primarily heat engines, their theoretical efficiency can be calculated if we assume they operate as thermodynamic cycles where all chemical energy is converted into useful mechanical energy for transportation. However, this idealized case cannot be applied since ideal conditions are rarely met (i.e. frictionless world, ideal gases). 
The energy efficiency of driving a vehicle can be found after identifying the energy efficiencies of its components have been found by conducting energy balances throughout the vehicle. In an ICE vehicle over 62.4% of the energy released from combustion is lost as heat due to friction (i.e. while pumping air in and out of engine) and the remainder energy is used for work done outside the engine.  Conducting an energy balance on the remaining components reveal that 10 to 25% of energy inputted is consumed for power to the wheels, 4 to 6% is consumed due to parasitic losses, 5 to 6% is consumed to drive terrain losses, and 3% due to idleness (accounted for in friction acting on engine, and parasitic/appliance losses). [9-12]
As previously mentioned, over 68 to 76% of energy is lost in an ICE vehicle as a result of friction. [11,13] In order to mitigate these engine losses vehicle manufactures have invested in, technologies that improve valve timing, air injection, and fuel chemistry (diesel engines).  The recovered energy from the engine is then transferred to other vehicle components for utilization.
|Fig. 2:The Chevrolet Suburban extended-length SUV weighs more than a compact vehicle, and thus requires more energy to overcome inertial forces. (Source: Wikimedia Commons)|
The second area with the second greatest losses of energy is when power is transmitted to the wheels. Between 10 to 25% of energy inputted into a vehicle is lost when power is transmitted to wheels. [11,12] Approximately 9 to 12% of energy is lost due to wind resistance, while 5 to 7% is lost due to rolling resistance and 5 to 7% is lost due to braking. [11,12] Braking contributes to energy loss, since the kinetic energy of vehicle previously obtained from combusting fuel is now lost as heat generated while braking. As a result, braking can consume a significant amount of energy if the kinetic energy needed to overcome the vehicle's inertia is large, and if the vehicle is being driven in traffic with multiple stops. Air resistance is another force opposing the vehicle's motion due to changes in velocity for a laminar flow, and the squared velocity of turbulent flow for two objects (i.e. vehicle and air) that result in a loss of energy. The air resistance acting on the vehicle is impacted by the size, shape, and velocity of the vehicle and results in a 9% to 12% loss in total input energy, and is usually lower if the vehicle is operated at slower velocities.  Rolling resistance is another resistive force for when energy is delivered to power the vehicle's wheels that also consumes 5 to 7% of energy inputted. Rolling resistance is the resistive force against tires that develops from the physical deformation of tires. This resistive force can be mitigated if material and design of the tire are changed. [11,12] Another way to reduce the amount of rolling resistance a vehicle experiences is to reduce the vehicle's weight. Vehicle manufacturers can produce large vehicles that meet consumer needs , such as the Chevrolet Suburban in Fig. 2, and work towards reducing rolling resistance by reducing the amount of mass within the vehicle, or produce compact vehicles which already weigh less and thus experience less rolling resistance.
A third area that contributes to reducing energy efficiency is drivetrain losses, which can consume ~4% energy.  Energy is consumed in this facet of energy consumption for both ICE and electric vehicles due to friction within the drivetrain components. The last area that contributes to energy loss are losses due to additional accessories. Accessories that utilize the energy produced by the engine (e.g. power steering, water pump, air conditioner, etc.) can result in a 4 to 6% loss in energy consumed. [11,14] Although these losses aren't as significant as the previously mentioned losses, they provide vehicle manufactures an area to improve on.
|Fig. 3:Pictured above are two of world's most popular electric vehicles, Nissan Leaf (left), and Tesla Model S (right). Electric vehicle's experience similar resistive forces as ICE vehicle's with the exception of energy losses due to their electric motors and charging inefficiencies.(Source: Wikimedia Commons)|
Together, these losses represent the amount of energy lost for an ICE vehicle being driven in the city and highway. Although electric vehicles, such as those shown in Fig. 3, weren't covered in this paper, they should experience the same resistive forces with the exception of any resistive forces involving the ICE, and additional resistive forces involved in charging. If charging losses are taken into account for electric vehicles, electric vehicles would be noted as losing about 16% of the energy inputted would be dissipated due to overcoming the battery's resistance to charging to full capacity. Although electric vehicles are efficient, as previously noted, this paper did not dive into the energy efficiency of electric vehicles since the energy used to charge the battery was obtained from an external source and not by a component designed by the vehicle manufacturer.
In addition, new technologies and their impact on energy efficiencies were not thoroughly discussed in this paper since new technologies are often unique per vehicle manufacturer (i.e. regenerative braking, automated vehicles, hybrids, variable valve timing and lift).  As a result, only technologies common to all vehicles were taken a look at. If a more in depth comparison were to be developed, more information unique to each vehicle (i.e. model, use) would have to be collected.
The transportation sector accounts for a significant percentage of energy consumption in the US. Within the sector, energy consumption by highway vehicles constitutes more than 75% of energy consumption. In order to elucidate why so much energy is consumed, the energy efficiencies of driving were taken a look at for ICE vehicles by applying thermodynamic principles in a component by component basis. Since vehicles never operate in a thermodynamically ideal world, vehicles experience energy loss in many areas. First ICE vehicles and electric vehicles were distinguished from each other. Vehicles with an ICE lose 68 to 76% of the energy inputted as fuel, while electric vehicles lose 16% as the battery is charged to full capacity. [7,11] Then an energy analysis for ICE vehicles allowed us to identify energy losses occurring in components where power is delivered to wheels, accessory components, drivetrain components, and inefficiencies during operation (idleness). Although these efficiencies were conducted for gasoline ICE vehicles, they are also applicable to electric vehicles with the exception of losses associated with the ICE. However, these efficiencies should be updated if new technology should be taken into consideration. Together, these energy efficiencies elucidate areas that vehicle manufacturers can work on in order to improve the energy efficiency of their vehicles, lower fuel consumption, while meeting the needs of their consumers.
© Joel Dominguez. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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