Aerodynamic means of reducing the influence of warning accessories on drag and emissions, for intervention vehicles

Aerodynamic performance of passenger vehicles is one of the most efficient methods to further reduce the CO2 emissions and comply with the increasing constraints in the legislation. Previously performed studies on exterior warning accessories (systems with visual alerting signals that may be found on intervention vehicles e.g. ambulances), indicate a significant degradation of aerodynamic coefficient (Cd). Even if this category of vehicles does not have to comply with the standard emission rules, it is important to study and discover the means to reduce aerodynamic drag for this type of vehicles considering the general aim to reduce air pollution. The target would be to have a Cd level equal to that found in the base vehicles, before transformation (i.e. often, intervention vehicles are obtained by converting a vehicle available to general customers - multistage vehicle transformation). The current paper’s purpose is to continue the investigation on methods to improve the aerodynamic performance of intervention vehicles. This will be done by compensating the shape and dimensions of the warning devices with the addition of supplementary body elements or by adapting existing parts. The Cd value of the different configurations is assessed using computational fluid dynamics (CFD) software based on Lattice Boltzmann mathematical model. 3D models of different generic body type vehicles (sedan, hatch back, SUV) were created and immersed in a virtual wind tunnel. To simulate the real-world measurement method specific boundary conditions on walls, inlets, outlets were set while the fluid environment was defined with a network from fine (2mm length, near the studied model), to coarse (20mm length, near exterior walls). The results indicate that there is great potential in reducing the aerodynamic drag and, by extension, fuel consumption and CO2 emissions for all body types studied. Deviations could exist due to different make designs, or diversity of warning devices for example so the results obtained have a general nature. Nevertheless, the conclusion remains the same: if one’s aim is to improve air quality or prepare for new legislation (that will also apply to intervention vehicles), the aerodynamic optimization should be further scrutinized as it can prove to be a robust method to obtain the targeted results.


Introduction
Depollution of automotive manufacturing industry is a challenge that is a daily reality for any role in this domain, as it was implemented 3 decades ago to improve air quality and, by extension, health benefits [1].The CO2 emission can be linked with almost all branches of automotive industry, from cradle to grave, starting with conception and manufacturing of each part, logistics, vehicle assembly process, customer use and finally recycling [2].The current paper is part of a study concerning the reduction of aerodynamic resistance assessed by product between aerodynamic coefficient and frontal vehicle area (CdA), with the purpose of improving the CO2 emission level for intervention vehicles.The focus of the research is on vehicles destinated for general use, that are further converted to be used as intervention vehicles (transport of patients, auxiliary vehicles for various tasks like fire extinguish, disaster control, etc.) and for this purpose must be equipped with warning lights, as indicated in Figure 1.This extra equipment is most often mounted on the roof of the vehicles and has a direct impact in aerodynamic resistance and CO2 emissions.While current legislation exempts this category of vehicles from meeting specific CO2 requirements, the aim of the paper is to identify some means to reduce the aerodynamic resistance of the warning lights [3].The influence of a generic warning light equipment on arbitrary chosen vehicles designs, corresponding to sedan, hatch back and SUV body type, was studied as a first part of the research [5], with the help of Computational Fluid Dynamics (CFD) software.A brief synthesis is shown in Figure 2. Based on this previous study, it was concluded that the warning lights should be mounted in B pillar area and avoid extreme advanced or rear position to obtain the smallest degradation in aerodynamic resistance and CO2.Numerical results are presented in Table 1.

Means of reducing warning lights' influence on CdA
The two main factors that led to the present study were (i) anticipation of potential legislative changes that would also require intervention vehicles to meet specific CO2 emissions standards and (ii) the significant benefits brought by a cleaner, less polluted air in urban areas.
The target set at the beginning of the work was to achieve the same aerodynamic performance for the intervention vehicle as for the base vehicle, before transformation.This step represents the second phase of the work and, for traceability, will be continued with the same CFD tools and coordinates.The mounting position over pillar B was chosen as reference for the intervention vehicle considering that, in practice, this is frequently used but also it was proved to have the most reduced impact on aerodynamic drag.Considering the mentioned premises, the fallowing CdA values must be compensated to achieve the CdA target: 0.094m² for Sedan type, 0.074m² for Hatch back type, 0.059m² for SUV type.
To manage the CdA improvement the work hypothesis was to overcome the warning lights (equipment characteristics used for current CFD study are shown in Figure 3) degradation with auxiliary parts that can be easily mounted.Heavy modification of the vehicle was avoided for the purpose of the paper to ensure the application of the research also in real-life use -implementation of heavy modifications would be more difficult and thus there will be no significant overall CO2 level improvement if solutions found would not be applied.The parts that were decided to be modified/added in current study are: a. Replacing the underbody fairings with parts extended in surface and with improved flatness b.Adding an extra mask on wheel rim to reduce porosity, parameter directly linked to CdA reduction (brake cooling is not considered to be an issue, as the emergency braking is validated at full load and with wheels that have similar characteristics as the proposed model [6]) Performed CFD simulations on each body type, with above detailed technical evolutions indicate a various influence of added parts on the CdA overall performance, as shown in Table 2.If for Sedan body type the modifications made reduce the influence of the warning lights with 52.13% and for SUV with 102.2%, for the hatch back body type the gain is almost inexistent with just 1.2%.The CdA values obtained are a direct consequence of the rear wake of the three studied cases, as indicated in Figure 6.To achieve the purpose of reducing the warning lights influence, rear wake balance must be seeked.A balanced rear wake can be visualized with the help of streamlines and must be horizontal and reduced in dimension on X axis.

Hatch back body type intervention vehicle SUV body type intervention vehicle
By analyzing the results obtained, several conclusions may be drawn.Firstly, as expected, a newly introduced element will not have the same effect on different body types.This is due to a different shape of the vehicle and air flow speed above and under the body (giving a specific rear wake shape and dimensions).Consequently, for obtaining better results, specific customization per body is required.Secondly, the aerodynamic resistance of warning light can be fully compensated (SUV case -balanced rear wake) or reduced to half (Sedan case -rear wake slightly unbalanced in downward direction) by using a relatively simple body kit for intervention vehicles.
When it comes to hatch back body type, further studies will need to be performed to identify a proper kit to match the wake of the base vehicle, as the current setup has an unbalanced rear wake, with big dimensions.Some options that will be tested to see if they improve the wake for the hatch back body type intervention vehicles are: partial coverage of underbody, addition of deflectors on underbody fairings or extension of rear upper diffuser.

Conclusion
The specific warning lights on intervention vehicles have a direct influence on aerodynamic resistance and CO2 emissions.While current legislation grants a special status to this category of vehicles, their impact on overall CO2 measured levels, especially in urban areas, remains a reality.This paper demonstrates, by using CFD simulations, that the impact of warning lights on aerodynamic resistance can be compensated to a certain extent with the modification of accessories such as underbody fairings and wheel rim porosity.
The results obtained are generic and show that CO2 emissions may be reduced only by adding specific accessories -which should not be that difficult also from a practical perspective.The specific parts to be modified, their shape and position should be studied for each particular case to have accurate results.It must also be considered that CFD simulations incorporates a calculation error.The final CO2 levels can be validated with physical measurements.CFD method, as used in the current paper, can be applied in the analysis phase to identify best technical solutions, followed by the validation phase that is focused on final fine-tunning iterations before the official homologation process.

Figure 2 .
Figure 2. CFD analysis for air flow behaviour near warning lights to determine best mounting position.

Figure 3 .
Figure 3. 'Warning light' shape and dimension, used for current study.

Figure 6 .
Figure 6.Rear wake balance for Sedan, Hatch back and SUV studied cases.

Table 1 .
CdA results for base vehicle and warning lights in advanced, above pilar B and rear position.

Table 2 .
CdA results for optimised technical definition, compared to base and intervention vehicles.