Role of Aerodynamics and Thermal Management in the Vehicles of Tomorrow

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1 Role of Aerodynamics and Thermal Management in the Vehicles of Tomorrow Lennart Löfdahl Prologue Approximately 30 % of the world oil production is today consumed by road going vehicles, and from an environmental perspective a huge quantity of emissions is produced by the transport sector. Reduced fuel consumption is an overall target for decreasing emissions; therefore, comprehensive research on energy efficient vehicles is in progress all over the world. Promising vehicle concepts with significantly improved energy effectiveness, as compared to earlier generations, have been launched. However, these vehicles also have to obey the laws of physics, and future propulsion systems are required to be capable of overcoming the so-called driving resistance. Vehicle aerodynamics and Thermal management are two strongly linked topics which play key roles in developing, controlling and minimizing the driving resistance. The subject of this paper is to clarify the role of Vehicle aerodynamics and Thermal management in the energy optimization process, and also to elucidate some areas where they are and possibly can be used for significant drag reduction in the foreseeable future. 1. Introduction A relevant starting point for discussions on energy efficiency is the general energy flux of a typical vehicle. In figure 1 such a flow chart is shown. It is intended that the figure should reflect everyday use of a vehicle, and although the figure is quite universal it may be observed that approximately 70% of the supplied total energy content disappears as heat losses. The remaining 30% is used for the mechanical energy required to overcome the driving resistance.

2 Figure 1: Energy flow diagram for a road going vehicle, figures given are approximate In Figure 2 the mechanical energy losses are shown and split into parts: aerodynamic drag, rolling resistance, gravitation and inertial forces which are dependent on load case and infrastructure. Although the figure shows a passenger car, the picture is basically the same for all road going vehicles. Note also that energy consumption owing to auxiliary equipments and tentative inertial momentum because of accelerations are not included. It is noteworthy that the aerodynamic drag is a function of the vehicle shape and velocity, while the three remaining terms all include the vehicle mass. In figure 3 the magnitudes of the different mechanical energy terms are shown for a typical passenger car, from Hucho/Woll [1]. From the figure some interesting observations may be made; comparing the rolling resistance and the aerodynamic drag they are of approximately the same magnitude at about 70 km/h. Above this velocity the aerodynamic drag increases with the square of the velocity so at 240 km/h the drag is about five times as large as the rolling resistance in spite of the fact that the latter has double its value. This means of course that the aerodynamic drag is important at higher velocities; however, with current oil prices it is also very significant at lower velocities. For a more comprehensive discussion on the influence of acceleration and slope, see Hucho/Woll [1].

3 Figure 2: Sketch of the principal components composing the so-called the driving resistance The force diagram of figure 2 is a cornerstone in discussions on fuel consumptions; however, to mimic the fuel consumption of a typical vehicle used in everyday transport a number of driving cycles have been developed. These cycles are always exposed to some criticism, but in Europe the NEDC (New European Driving Cycle) has been generally accepted by most vehicle manufacturers and transport groups. Using the equation in figure 2 and the NEDC it has been shown by Wiedemann [2] that for a light vehicle, up to 40 % of the total drag resistance may be derived directly to the aero drag. In addition, it is also obvious from Figure 2 that the acceleration term is most interesting since under certain conditions this term might become negative (braking), and this can be used for improving the energy efficiency by regeneration of energy. So a low aerodynamic drag is important here as well, since it does not only contribute to low driving resistance but also to a more efficient energy recovery.

4 Figure 3: Magnitude of the different terms in figure 2, from Hucho/Woll [1] To summarize, because of a radical change in the design philosophy, low aerodynamic drag is a key parameter for improved energy efficiency; and in the optimizing process interplay between Vehicle aerodynamics and Thermal management it is a pre-requisite. In figure 4 an attempt is made to classify the two connected topics. The vehicle aerodynamic parts are clearly oriented towards performance in terms of top speed, acceleration and braking. Safety issues like stability, dirt distribution and splash and spray also fall in the aerodynamic category, while thermal management is oriented more towards the vehicle operation. However, the areas are strongly linked and cannot exist independently. A few words on the governing equations; in the vehicle aerodynamic part the fundamental equations are the equation of continuity and the derivatives of the Navier-Stokes equations, for instance the Bernoulli equation. Including thermal effects further complicates the situation since the energy equation has to be added. The problems are quite cumbersome to solve, and to produce reliable solutions interplay between numerical methods (Computational Fluid Dynamics (CFD)) and properly designed wind tunnel experiments is required. For full details on the governing equations and for their derivations reference is made to more comprehensive text books in the fields, for instance Crowe & Roberson [3], Panton [4], Bachelor [5] or Eckert & Drake [6].

5 Figure 4: Classification of linked topics Vehicle Aerodynamics and Thermal Management 2. Vehicle Aerodynamics In figure 5 it is illustrated how the three main contributions to the total drag of a passenger car may be divided. It must be remarked that the streamlined Bugatti in the figure is only an image and the magnitudes of the different drag contributions given apply much better to a modern passenger cars. Furthermore, the magnitudes specified should be looked on as guidelines, and a tolerance must be accepted for each value. In spite of this, it is evident that the major drag contribution may be derived to the under-body, wheel and wheel houses, = 55%, and approximately 45% originate from the vehicle shape. During recent years, stylists and aerodynamicists have successfully worked together to reduce the drag caused by the vehicle shape. For aerodynamicists there is, however, much work left to be done in the wheel area and along the under-body, since it is in these areas where the largest paybacks are expected to be found. Much new technology, engineering work, active and re-active flow control are needed. In this section some characteristic areas, which have a potential for aerodynamic drag reduction, are elucidated and discussed. However, it should be remarked that in a short paper like this only a fraction of these areas can be covered, and for a more comprehensive discussions reference is again made to the work of Hucho [7].